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This PDF file contains the front matter associated with SPIE Proceedings Volume 8223, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.
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Optoacoustic imaging has enabled the visualization of optical contrast at high resolutions in deep tissue. Our
Multispectral optoacoustic tomography (MSOT) imaging results reveal internal tissue heterogeneity, where the
underlying distribution of specific endogenous and exogenous sources of absorption can be resolved in detail. Technical
advances in cardiac imaging allow motion-resolved multispectral measurements of the heart, opening the way for studies
of cardiovascular disease. We further demonstrate the fast characterization of the pharmacokinetic profiles of lightabsorbing
agents. Overall, our MSOT findings indicate new possibilities in high resolution imaging of functional and
molecular parameters.
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We combined photoacoustic ophthalmoscopy (PAOM) with autofluorescence imaging for simultaneous in vivo imaging
of dual molecular contrasts in the retina using a single light source. The dual molecular contrasts come from melanin and
lipofuscin in the retinal pigment epithelium (RPE). Melanin and lipofuscin are two types of pigments and are believed to play opposite roles (protective vs. exacerbate) in the RPE in the aging process. We successfully imaged the retina of
pigmented and albino rats at different ages. The experimental results showed that multimodal PAOM system can be a
potentially powerful tool in the study of age-related degenerative retinal diseases.
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Anti-cancer drugs typically exert their pharmacological effect on tumors by inducing apoptosis, or programmed cell
death, within the cancer cells, with PCD occurring as soon as 4 hours after treatment. Detection of apoptosis in patients
could decisively report a response to treatment days or even weeks before MRI, CAT, and ultrasound indicate
morphological changes in the tumor. Here we developed a novel
near-infrared dye based imaging probe to directly detect
apoptosis with high specificity in cancer cells by utilizing a
non-invasive photoacoustic imaging technique. Nude mice
bearing head and neck tumors received cisplatin chemotherapy were imaged by PAI after tail vein injection of the
contrast agent. In vivo PAI indicated a strong apoptotic response to chemotherapy on the peripheral margins of tumors,
whereas untreated controls showed no contrast enhancement by PAI. The apoptotic status of the mouse tumor tissue was
verified by immunohistochemical techniques staining for cleaved caspase-3 p11 subunit. The results demonstrated the
potential of this imaging probe to guide the evaluation of chemotherapy treatment.
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Functional detection in primate brains has particular advantages because of the similarity between non-human
primate brain and human brain and the potential for relevance to a wide range of conditions such as stroke and
Parkinson's disease. In this research, we used photoacoustic imaging (PAI) technique to detect functional changes
in primary motor cortex of awake rhesus monkeys. We observed strong increases in photoacoustic signal amplitude
during both passive and active forelimb movement, which indicates an increase in total hemoglobin concentration
resulting from activation of primary motor cortex. Further, with PAI approach, we were able to obtain depthresolved
functional information from primary motor cortex. The results show that PAI can reliably detect primary
motor cortex activation associated with forelimb movement in rhesus macaques with a minimal-invasive approach.
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Photoacoustic and thermoacoustic phantom images obtained with a multi-channel breast scanner designed for breast
cancer screening are presented here. A tunable laser system (OPOTEK Vibrant 355 I, Calsbad,CA) with a pulse duration
of 5 ns was used for photoacoustic irradiation, and a 3.0 GHz microwave source with a pulse width of 0.3-1 μs was used
for thermoacoustic tomography. Multiple (>=16) 2.25 MHz
single-element unfocused ultrasonic transducers at different
depths were scanned simultaneously for a full 360° to obtain a full data set for three-dimensional (3D) tomography.
Negative acoustic lenses were attached to these unfocused transducers to increase their acceptance angles. An ultrasound
receiving system with 64 parallel receiving channels (Verasonics Inc. Redmond, WA) was used for data acquisition. A
filtered backprojection algorithm was used to reconstruct
two-dimensional (2D) and 3D images. Different phantoms
were imaged to evaluate the performance of the scanner. A lateral resolution of less than 1 mm and an elevational
resolution of less than 5 mm were achieved. The phantom studies demonstrate that this scanner can potentially provide
high-resolution, dual-modality, three-dimensional images and can potentially be used for human breast cancer screening.
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Two-dimensional optoacoustic imaging with a hand-held probe operated in backward mode is being developed
for diagnostic imaging of breast cancer to evaluate the feasibility of a dual-modality optoacoustic plus ultrasonic
system that maps functional information of anatomical tissue structures with ultrasonic resolution. Tissue is
illuminated at 757nm and 1064nm for optical contrast between hypoxic blood of breast carcinomas and normally
oxygenated blood in benign masses. The system is optimized and calibrated in phantoms for a pilot clinical
study of patients with breast masses suspected for malignancy. Capability of the non-invasive system to improve
detection and diagnosis of breast tumors is discussed.
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Current imaging modalities are often not able to detect early stages of breast cancer with high imaging contrast.
Visualizing malignancy-associated increased hemoglobin concentrations might improve breast cancer diagnosis.
Photoacoustic imaging can visualize hemoglobin in tissue with optical contrast and ultrasound resolution, which makes it
potentially ideal for breast imaging. The Twente Photoacoustic Mammoscope (PAM) has been designed specifically for
this purpose. Based on a successful pilot study in 2007, a large clinical study using PAM has been started in December
2010. PAM uses a pulsed Q-switched Nd:YAG laser at 1064 nm to illuminate a region of interest on the breast.
Photoacoustic signals are detected with a 1MHz, unfocused ultrasound detector array. Three dimensional data are
reconstructed using an acoustic backprojection algorithm. Those reconstructed images are compared with conventional
imaging and histopathology. In the first phase of the study, the goal was to optimize the visualization of malignancies.
We performed sixteen technically acceptable measurements on confined breast malignancies. In the reconstructed
volumes of all malignancies, a confined high contrast region could be identified at the expected lesion depth. After ten
successful measurements, the illumination area was increased and the fluence was substantially decreased. This caused a
further significant increase in PAM lesion contrast.
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We developed a new optoacoustic microangiography system (OmAS) intended for in-vivo vascular imaging of a
human finger. The system employs an arc-shaped acoustic array that is rotated 360 degrees around the finger
providing optoacoustic data necessary for tomographic reconstruction of the three-dimensional images of a finger. A
near-infrared Q-switched laser is used to generate optoacoustic signals with increased contrast of blood vessels. The
laser is coupled through two randomized fiberoptic bundles oriented in orthogonal optoacoustic mode. To
demonstrate OmAS capabilities, we present a time-series of optoacoustic images of a human finger taken after the
hypothermia stress test. The images show a detailed vascular anatomy of a finger down to the capillary level. A
series of quick 30s scans allowed us to visualize the thermoregulatory response within the studied finger as it was
manifested via vasomotor activity during the hypothermia recovery. We propose that the developed system can be
used for diagnostics of various medical conditions that are manifested in change of the peripheral (finger) blood
flow. Examples of the medical conditions that could be diagnosed and staged using the OmAS include the peripheral
arterial disease (PAD), thrombosis, frostbite, and traumas.
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The development and performance of a cw, TE-cooled DFB quantum cascade laser based sensor for
quantitative measurements of ammonia (NH3) and nitric oxide (NO) concentrations present in
exhaled breath will be reported. Human breath contains ~ 500 different chemical species, usually at
ultra low concentration levels, which can serve as biomarkers for the identification and monitoring
of human diseases or wellness states. By monitoring NH3 concentration levels in exhaled breath a
fast, non-invasive diagnostic method for treatment of patients with liver and kidney disorders, is
feasible. The NH3 concentration measurements were performed with a 2f wavelength modulation
quartz enhanced photoacoustic spectroscopy (QEPAS) technique, which is suitable for real time
breath measurements, due to the fast gas exchange inside a compact QEPAS gas cell. A Hamamatsu
air-cooled high heat load (HHL) packaged CW DFB-QCL is operated at 17.5°C, targeting the
optimum interference free NH3 absorption line at 967.35 cm-1 (λ~10.34 μm), with ~ 20 mW of
optical power. The sensor architecture includes a reference cell, filled with a 2000 ppmv NH3 :N2
mixture at 130 Torr, which is used for absorption line-locking. A minimum detection limit (1σ) for
the line locked NH3 sensor is ~ 6 ppbv (with a 1σ; 1 sec time resolution of the control electronics).
This NH3 sensor was installed in late 2010 and is being clinically tested at St. Luke's Hospital in
Bethlehem, PA.
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Circulatory shock is lethal, if not promptly diagnosed and effectively treated. Typically, circulatory shock resuscitation is
guided by blood pressure, heart rate, and mental status, which have poor predictive value. In patients, in whom early goaldirected
therapy was applied using central venous oxygenation measurement, a substantial reduction of mortality was reported
(from 46.5% to 30%). However, central venous catheterization is invasive, time-consuming and often results in
complications. We proposed to use the optoacoustic technique for noninvasive, rapid assessment of central venous
oxygenation. In our previous works we demonstrated that the optoacoustic technique can provide measurement of blood
oxygenation in veins and arteries due to high contrast and high resolution. In this work we developed a novel
optoacoustic system for noninvasive, automatic, real-time, and continuous measurement of central venous oxygenation.
We performed pilot clinical tests of the system in human subjects with different oxygenation in the internal jugular vein
and subclavian vein. A novel optoacoustic interface incorporating highly-sensitive optoacoustic probes and standard
ultrasound imaging probes were developed and built for the study. Ultrasound imaging systems Vivid i and hand-held
Vscan (GE Healthcare) as well as Site-Rite 5 (C.R. Bard) were used in the study. We developed a special algorithm for
oxygenation monitoring with minimal influence of overlying tissue. The data demonstrate that the system provides
precise measurement of venous oxygenation continuously and in real time. Both current value of the venous oxygenation
and trend (in absolute values and for specified time intervals) are displayed in the system. The data indicate that: 1) the
optoacoustic system developed by our group is capable of noninvasive measurement of blood oxygenation in specific
veins; 2) clinical ultrasound imaging systems can facilitate optoacoustic probing of specific blood vessels; 3) the
optoacoustic system provides noninvasive monitoring during rapid changes in blood oxygenation.
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We report herein a novel three-dimensional photoacoustic computed tomography (PACT) system for small-animal
whole-body imaging. The PACT system, based on a 512-element
full-ring ultrasonic transducer array, was cylindrically
focused and capable of forming a two-dimensional image in 1.6 seconds. The pulsed laser could either illuminate
directly from the top or be reshaped to illuminate the sample from the side. Top illumination was mainly used for mouse
brain and mouse embryo imaging. Side illumination provided in vivo anatomical images of an adult mouse. By
translating the mouse along the elevational direction, the system provided serial cross-sectional images.
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The ability to detect molecular probes in deep tissue, based on optical signatures, has been limited by tissue scattering,
which reduces the spatial resolution and complicates quantification. To address this challenge, multispectral optoacoustic
tomography (MSOT) has been recently introduced, a hybrid technology that capitalizes on the optoacoustic effect to
combine rich optical contrast with the high spatial resolution and real-time operation of ultrasound. Using
multiwavelength illumination MSOT can visualize molecular probes based on their distinct optical absorption spectra
through several millimeters to centimeters of tissue. Herein we present a whole body multi-spectral optoacoustic
tomography system and report on spectral processing techniques for detection of molecular probes in living mice.
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In this paper we introduce a novel optical resolution photoacoustic micro-endoscopy system using GRIN-lens
focusing. This real-time imaging system takes advantage of an image guide fiber consisting of 100,000 individual
single-mode fibers in a 1.4-mm-diameter bundle, and a 532-nm fiber laser with repetition-rates as high as 600 kHz.
Fast scanning mirrors were used to scan the entire image guide fiber allowing for the maximum field-of-view. The
ability of the proposed system is demonstrated using both phantom and in vivo studies. The system offers <7 μm
lateral spatial resolution and several volumetric/C-scans per second frame-rate. The proposed setup can be inserted
into the body due to the flexible nature of the image guide and micron-scale footprint of the apparatus.
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Using the method of 3D optoacoustic tomography, we studied changes in tissues of the whole body of nude mice as the
changes manifested themselves from live to postmortem. The studies provided the necessary baseline for optoacoustic
imaging of necrotizing tissue, acute and chronic hypoxia, and reperfusion. They also establish a new optoacoustic model
of early postmortem conditions of the whole mouse body. Animals were scanned in a 37°C water bath using a three-dimensional
optoacoustic tomography system previously shown to provide high contrast maps of vasculature and organs
based on changes in the optical absorbance. The scans were performed right before, 5 minutes after, 2 hours and 1 day
after a lethal injection of KCl. The near-infrared laser wavelength of 765 nm was used to evaluate physiological features
of postmortem changes. Our data showed that optoacoustic imaging is well suited for visualization of both live and
postmortem tissues. The images revealed changes of optical properties in mouse organs and tissues. Specifically, we
observed improvements in contrast of the vascular network and organs after the death of the animal. We associated these
with reduced optical scattering, loss of motion artifacts, and blood coagulation.
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A photoacoustic tomography (PAT) system using a virtual point ultrasonic transducer was developed for transcranial
imaging of monkey brains. The virtual point transducer provided a 10 times greater field-of-view (FOV) than finiteaperture
unfocused transducers, which enables large primate imaging. The cerebral cortex of a monkey brain was
accurately mapped transcranially, through up to two skulls ranging from 4 to 8 mm in thickness. The mass density and
speed of sound distributions of the skull were estimated from adjunct X-ray CT image data and utilized with a timereversal
algorithm to mitigate artifacts in the reconstructed image due to acoustic aberration. The oxygenation saturation
(sO2) in blood phantoms through a monkey skull was also imaged and quantified, with results consistent with
measurements by a gas analyzer. The oxygenation saturation (sO2) in blood phantoms through a monkey skull was also
imaged and quantified, with results consistent with measurements by a gas analyzer. Our experimental results
demonstrate that PAT can overcome the optical and ultrasound attenuation of a relatively thick skull, and the imaging
aberration caused by skull can be corrected to a great extent.
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Two human tumour cell lines (K562, 293T) were stably transfected to achieve the genetic expression of tyrosinase,
which is involved in the production of the pigment eumelanin. The cells were injected subcutaneously into nude mice to
form tumour xenografts, which were imaged over a period of up to 26 days using an all-optical photoacoustic imaging
system. 3D photoacoustic images of the tumours and the surrounding vasculature were acquired at excitation
wavelengths ranging from 600nm to 770nm. The images showed tumour growth and continued tyrosinase expression
over the full 26 day duration of the study. These findings were confirmed by histological analysis of excised tumour
samples.
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pH is a tightly regulated indicator of metabolic activity. In mammalian systems, imbalance of pH regulation may result
from or result in serious illness. Even though the regulation system of pH is very robust, tissue pH can be altered in many
diseases such as cancer, osteoporosis and diabetes mellitus. Traditional high-resolution optical imaging techniques, such
as confocal microscopy, routinely image pH in cells and tissues using pH sensitive fluorescent dyes, which change their
fluorescence properties with the surrounding pH. Since strong optical scattering in biological tissue blurs images at
greater depths, high-resolution pH imaging is limited to penetration depths of 1mm. Here, we report photoacoustic
microscopy (PAM) of commercially available pH-sensitive fluorescent dye in tissue phantoms. Using both opticalresolution
photoacoustic microscopy (OR-PAM), and acoustic resolution photoacoustic microscopy (AR-PAM), we
explored the possibility of recovering the pH values in tissue phantoms. In this paper, we demonstrate that PAM was
capable of recovering pH values up to a depth of 2 mm, greater than possible with other forms of optical microscopy.
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Photoacoustic imaging has been widely used in structural and functional imaging. Because of its safety, high resolution,
and high imaging depth, it has great potential for a variety of medical studies. Capillaries are the smallest blood vessels
and enable the exchange of oxygen and nutrients. Noninvasive flow speed measurement of capillaries in vivo can benefit
the study of vascular tone changes and rheological properties of blood cells in capillaries. Recently, there has been a
growing interest in photoacoustic velocimetry, such as photoacoustic Doppler and M-mode photoacoustic flow imaging.
Methods capable of high-resolution imaging and low-speed flow measurement are suitable to measure blood speeds in
capillaries. Previously we proposed photoacoustic correlation spectroscopy (PACS) and shown its feasibility for lowspeed
flow measurement. Here, in vivo measurement of blood speeds in capillaries in a chick embryo model by PACS
technique is demonstrated. The laser-scanning photoacoustic microscopy system is used for fast imaging acquisition and
high-resolution imaging. The measured speed in capillaries is similar to those found in literatures, which confirm the
feasibility of the PACS method for blood velocimetry. This technique suggests a fairly simple way to study blood flow
speeds in capillaries.
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For deep tissue photoacoustic imaging, piezoelectric ultrasound detectors with large element sizes (>1mm) and relatively
low centre frequencies (<5MHz) are generally used, as they can provide the required high sensitivity to achieve imaging
depths of several centimetres. However, these detectors are generally not optimised in terms of element size and
bandwidth. To identify these parameters in order to improve SNR and spatial resolution, two models were employed.
The first was a numerical model and was used to investigate the effect of varying the detector element size on the
amplitude and SNR of photoacoustic images. The second model was used to optimise the detector bandwidth. For this,
the frequency content of simulated photoacoustic signals were studied for a range of depths and acoustic source sizes.
The model was based on an analytical solution to the wave equation for a cylindrical source and incorporated the effects
of frequency dependent acoustic attenuation. These models provide a new framework for optimising the design of
photoacoustic scanners for breast and other deep tissue imaging applications.
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A numerical inversion scheme for recovering a map of the absolute conductivity from the absorbed power density map
that is conventionally reconstructed in thermacoustic imaging is described. This offers the prospect of obtaining an
image that is more closely related to the underlying tissue structure and physiology. The inversion scheme employs a full
3D full wave model of electromagnetic propagation in tissue which is iteratively fitted to the measured absorbed power
density map using a simple recursive method. The reconstruction is demonstrated numerically using three examples of
absorbers of varying geometries, tissue realistic complex permittivity values and noise. In these examples, the
reconstruction is shown to rapidly converge to within good estimates of the true conductivity in less than 20 iterations.
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Tyrosinase, a key enzyme in the production of melanin, has shown promise as a reporter of genetic activity.
While green fluorescent protein has been used extensively in this capacity, it is limited in its ability to provide
information deep in tissue at a reasonable resolution. As melanin is a strong absorber of light, it is possible
to image gene expression using tyrosinase with photoacoustic imaging technologies, resulting in excellent resolutions
at multiple-centimeter depths. While our previous work has focused on creating and imaging MCF-7
cells with doxycycline-controlled tyrosinase expression, we have now established the viability of these cells in a
murine model. Using an array-based photoacoustic imaging system with 5 MHz center frequency, we capture
interleaved ultrasound and photoacoustic images of
tyrosinase-expressing MCF-7 tumors both in a tissue mimicking
phantom, and in vivo. Images of both the tyrosinase-expressing tumor and a control tumor are presented
as both coregistered ultrasound-photoacoustic B-scan images and
3-dimensional photoacoustic volumes created
by mechanically scanning the transducer. We find that the
tyrosinase-expressing tumor is visible with a signal
level 12dB greater than that of the control tumor in vivo. Phantom studies with excised tumors show that the
tyrosinase-expressing tumor is visible at depths in excess of 2cm, and have suggested that our imaging system is
sensitive to a transfection rate of less than 1%.
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Photoacoustic (PA) and thermoacoustic (TA) effects are based on the generation of acoustic waves after tissues absorb
electromagnetic energy. The amplitude of the acoustic signal is related to the temperature of the absorbing target tissue.
A combined photoacoustic and thermoacoustic imaging system built around a modified commercial ultrasound scanner
was used to obtain an image of the target's temperature, using reconstructed photoacoustic or thermoacoustic images. To
demonstrate these techniques, we used photoacoustic imaging to monitor the temperature changes of methylene blue
solution buried at a depth of 1.5 cm in chicken breast tissue from 12 to 42 °C. We also used thermoacoustic imaging to
monitor the temperature changes of porcine muscle embedded in 2 cm porcine fat from 14 to 28 °C. The results
demonstrate that these techniques can provide noninvasive real-time temperature monitoring of embedded objects and
tissue.
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As two hallmarks of cancer, angiogenesis and hypermetabolism are closely related to increased blood flow.
Volumetric blood flow measurement is important to understanding the tumor microenvironment and developing new
means to treat cancer. Current photoacoustic blood flow estimation methods focus on either the axial or transverse
component of the flow vector. Here, we propose a method to compute the total flow speed and Doppler angle by
combining the axial and transverse flow measurements. Both the components are measured in M-mode. Collating
the A-lines side by side yields a 2D matrix. The columns are Hilbert transformed to compare the phases for the
computation of the axial flow. The rows are Fourier transformed to quantify the bandwidth for the computation of
the transverse flow. From the axial and transverse flow components, the total flow speed and Doppler angle can be
derived. The method has been verified by flowing bovine blood in a plastic tube at various speeds from 0 to 7.5
mm/s and at Doppler angles from 30 to 330°. The measurement error for total flow speed was experimentally
determined to be less than 0.3 mm/s; for the Doppler angle, it was less than 15°. In addition, the method was tested
in vivo on a mouse ear. The advantage of this method is simplicity: No system modification or additional data
acquisition is required to use our existing system. We believe that the proposed method has the potential to be used
for cancer angiogenesis and hypermetabolism imaging.
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The concept of pure optical photoacoustic microscopy(POPAM) was proposed based on optical rastering of a focused
excitation beam and optically sensing the photoacoustic signal using a microring resonator fabricated by a
nanoimprinting technique. After some refinedment of in the resonator structure and mold fabrication, an ultrahigh Q
factor of 3.0×105 was achieved which provided high sensitivity with a noise equivalent detectable pressure(NEDP) value
of 29Pa. This NEDP is much lower than the hundreds of Pascals achieved with existing optical resonant structures such
as etalons, fiber gratings and dielectric multilayer interference filters available for acoustic measurement. The featured
high sensitivity allowed the microring resonator to detect the weak photoacoustic signals from micro- or submicroscale
objects. The inherent superbroad bandwidth of the optical microring resonator combined with an optically focused
scanning beam provided POPAM of high resolution in the axial as well as both lateral directions while the axial
resolution of conventional photoacoustic microscopy (PAM) suffers from the limited bandwidth of PZT detectors.
Furthermore, the broadband microring resonator showed similar sensitivity to that of our most sensitive PZT detector.
The current POPAM system provides a lateral resolution of 5μm and an axial resolution of 8μm, comparable to that
achieved by optical microscopy while presenting the unique contrast of optical absorption and functional information
complementing other optical modalities. The 3D structure of microvasculature, including capillary networks, and even
individual red blood cells have been discerned successfully in the proof-of-concept experiments on mouse bladders ex
vivo and mouse ears in vivo. The potential of approximately GHz bandwidth of the microring resonator also might allow
much higher resolution than shown here in microscopy of optical absorption and acoustic propagation properties at
depths in unfrozen tissue specimens or thicker tissue sections not now imageable with current optical or acoustic
microscopes of comparable resolution.
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Small animal models, such as zebrafish, drosophila, C. elegan, is considered to be important models in comparative
biology and diseases researches. Traditional imaging methods primarily employ several optical microscopic imaging
modalities that rely on fluorescence labeling, which may have potential to affect the natural physiological progress. Thus
a label-free imaging method is desired. Photoacoustic (PA) microscopy (PAM) is an emerging biomedical imaging
method that combines optical contrast with ultrasonic detection, which is highly sensitive to the optical absorption
contrast of living tissues, such as pigments, the vasculature and other optically absorbing organs. In this work, we
reported the whole body label-free imaging of zebrafish larvae and drosophila pupa by PAM. Based on intrinsic optical
absorption contrast, high resolution images of pigments, microvasculature and several other major organs have been
obtained in vivo and non-invasively, and compared with their optical counterparts. We demonstrated that PAM has the
potential to be a powerful non-invasive imaging method for studying larvae and pupa of various animal models.
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Recently we have reported in vivo near-real-time volumetric
optical-resolution photoacoustic microscopy (OR-PAM)
using a high pulse-repetition-rate (PRR) nanosecond fiber-laser to realize 2 volumetric image frames per second (fps)
within 1mm × 1mm field of view (FOV). Based on our previous
OR-PAM system, we are developing a label-free realtime
OR-PAM system in reflection mode for higher frame-rates. The system permits imaging of microcirculation
hemodynamics, and helps make the technology easier to use for biologists, providing real-time feedback for focusing and
positioning. Using a nanosecond-pulsed 532-nm fiber laser combined with fast-scanning mirrors, our proposed system
demonstrated its capability of sustained in vivo imaging of horizontal and vertical translation at 0.5 fps for 1mm × 1mm
FOV (400 × 400 pixels). Also, real-time in vivo imaging of blood flow at 30 fps for 250μm × 250μm FOV (100 × 100
pixels) was demonstrated. It is anticipated that the real-time nature of the system should prove important in clinical and
preclinical adaption, and may prove useful for functional brain imaging studies.
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In this report we introduce a novel experimental design for non-invasive scanning optoacoustic microscopy that utilizes
an off-axis parabolic mirror. Such reflector provides ideal and lossless conversion of a spherical wavefront into a plane
wave and enables diffraction-limited ultrasound focusing. We have designed and build a custom broadband transducer
with 0-19 MHz bandwidth and nominal sensitivity of 15 μV/Pa. With 17 dB amplification and noise level of ~ 1.6 mV
the estimated sensitivity limit of our experimental setup is 15 Pa. Using the reflector with numerical aperture of 0.5, we
have demonstrated lateral resolution limit of ~ 100 micrometers in test phantoms.
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We developed multi-contrast photoacoustic microscopy (PAM) for in vivo anatomical, functional, metabolic, and
molecular imaging. This technical innovation enables comprehensive understanding of the tumor microenvironment.
With multi-contrast PAM, we longitudinally determined tumor vascular anatomy, blood flow, oxygen saturation of
hemoglobin, and oxygen extraction fraction.
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The acquisition speed of previously reported mechanically-scanned Optical-Resolution Photoacoustic
Microscopy (OR-PAM) systems has been limited by both laser pulse repetition rate and mechanical
scanning speed. In this paper we introduce a mosaicing scheme wherein a grid of small sub-mm-scale
field-of-view (FOV) patches are acquired in 0.5s per patch, and a
3-axis stepper-motor system is used to
mechanically move the object to be imaged from patch-to-patch in less than 0.5s. Patch images are aligned
and stitched to generate a large FOV image composite. This system retains the SNR-advantages of focused-transducer
OR-PAM systems, and is a hybrid approach between optical-scanning and mechanical scanning.
With this strategy we reduce the data acquisition time of previously reported large-FOV systems by a factor
of around 23. SCID hairless mice are imaged. The wide-FOV,
high-speed data acquisition OR-PAM
system broadens the potential applications of the imaging modality.
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We present a spectrally encoded photoacoustic microscope based on a digital mirror device (DMD). It enables fast
spatially resolved spectral measurements of optical absorption. The imaging system can quickly tune the laser
illumination spectrum at the laser pulse repetition rate of 2 kHz. To demonstrate multi-wavelength absorption
measurements, we imaged optically absorbing solution phantom. Compared with spectral scanning, spectral encoding
recovers chromophore absorption spectra with improved accuracy by enhancing photoacoustic amplitude signal-to-noise
ratio.
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Dependencies of the optoacoustic (OA) transformation efficiency on tissue temperature were obtained for the application
in OA temperature monitoring during thermal therapies. Accurate measurement of the OA signal amplitude versus
temperature was performed in different ex-vivo tissues in the temperature range 25°C - 80°C. The investigated tissues
were selected to represent different structural components: chicken breast (skeletal muscle), porcine lard (fatty tissue)
and porcine liver (richly perfused tissue). Backward mode of the OA signal detection and a narrow probe laser beam
were used in the experiments to avoid the influence of changes in light scattering with tissue coagulation on the OA
signal amplitude. Measurements were performed in heating and cooling regimes. Characteristic behavior of the OA
signal amplitude temperature dependences in different temperature ranges were described in terms of changes in
different structural components of the tissue samples. Finally, numerical simulation of the OA temperature monitoring
with a linear transducers array was performed to demonstrate the possibility of real-time temperature mapping.
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The cell differentiation and proliferation of the central nervous system (CNS) are closely related to vascular
development. An imaging protocol that integrated optoacoustics (OA) with high-frequency ultrasound (HFU)
was developed for in vivo imaging of brain ventricles and vasculature in mouse embryos. A 40-MHz, co-polymer,
5-element annular-array transducer with a geometric focus of 12 mm was modified to accommodate free-beam,
coaxial illumination. Three-dimensional (3-D) data sets were acquired by raster scanning the transducer-optics
assembly in 50-μm increments. A single intact conceptus from an anesthetized mouse was surgically exposed
into PBS-filled Petri-dish. An 800-μm spot illumination from a pulsed, 532-nm, Nd-YAG laser was synchronized
with a high-voltage impulse excitation of the central array element to facilitate simultaneous and spatially coregistered
OA and HFU data acquisition. The resulting OA and HFU signals from each scan location were
recorded on all five array channels and post-processed using a synthetic-focusing algorithm to enhance the depth
of field (DOF). Dual-modality images were acquired from mouse embryos at E11.5, E12.5, and E13.5 days of
gestation. The extended DOF allowed morphologically accurate visualization of the embryonic head. The brain
ventricles were segmented from the HFU data and rendered in 3-D. The OA data provided visualization of the
vascular plexus as well as individual blood vessels. Feasibility of spatially co-registered, low-cost dual-modality
in vivo imaging of mouse embryos was demonstrated.
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The long-term goal of our research is to develop photoacoustic and Doppler ultrasound imaging methods for noninvasive
estimation of the oxygen consumption rate (MRO2) in vivo. Previously, we have demonstrated a combined
photoacoustic and high-frequency Doppler ultrasound system and shown the feasibility of flow velocity and oxygen
saturation (sO2) estimation using double-ink flow phantoms. In this work, the results of in vitro sheep blood experiments
are presented. Blood oxygen flux has been estimated at different sO2 levels and mean flow speeds, and the uncertainty of
the measurement has been quantified. In vivo experiments have been performed on Swiss Webster mice to provide coregistered
photoacoustic and Doppler flow images with imaging depths of ~2mm. Doppler bandwidth broadening
technique has been used to obtain transverse flow velocity. The diameter of the blood vessel is ~500μm and the mean
flow speed is 15cm/s. We are working towards sO2 estimation in vivo and 3D oxygen consumption imaging of tumors at
depths beyond OR-PAM.
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Photoacoustic endoscopy (PAE) provides unique functional information with high spatial resolution at super depths.
The provision of functional information is predicated on its strong spectroscopic imaging ability, and its deep imaging
capability is derived from its ultrasonic detection of diffused photon absorption. To accurately image functional
physiological parameters, it is necessary to rapidly alternate laser pulses of sufficient energy and different wavelengths.
In this study, we developed peripheral optical systems for PAE based on two identical pulsed-laser systems to achieve
the fast laser wavelength switching that is essential for accurate functional imaging. Each laser system was comprised of
a tunable dye laser pumped by a solid-state, diode-pumped Nd:YLF laser. Both systems deliver adequate energy at the
scanning head of the endoscope for imaging biological tissue in the optically diffusive regime (~0.3-0.6 mJ per pulse
with a repetition rate of ~1 kHz). In this paper, we introduce the employed laser systems and design of the light delivery
optics, and present results from an ex vivo animal imaging experiment that validates the system's multi-wavelength
functional imaging capability.
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This paper investigates the use of thin-film optical transmitters to generate focused ultrasound, aiming to develop highamplitude
focused ultrasound. Composite films were used as the optoacoustic sources, which consist of carbonnanotubes
(CNTs) and elastomeric polymers. As the nano-composites work as excellent optical absorbers and efficient
heat converters, thermo-elastic volume deformation within the composites produces strong optoacoustic pressure. These
films were formed on concave substrates for optoacoustic generation of the focused ultrasound. A focal waveform was
measured using a single-mode fiber-optic hydrophone. A peak positive pressure of ~4 MPa was achieved.
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We present a new micro-endoscopy system combining real-time C-scan optical-resolution photoacoustic
micro-endoscopy (OR-PAME), and a high-resolution fluorescence
micro-endoscopy system for visualizing
fluorescently labeled cellular components and optically absorbing microvasculature simultaneously. With a
diode-pumped 532-nm fiber laser, the OR-PAM sub-system is capable of imaging with a resolution
of ~ 7μm. The fluorescence sub-system consists of a diode laser with 445 nm-centered emissions as the light
source, an objective lens and a CCD camera. Proflavine, a FDA approved drug for human use, is used as
the fluorescent contrast agent by topical application. The fluorescence system does not require any
mechanical scanning. The scanning laser and the diode laser light source share the same light path within
an optical fiber bundle containing 30,000 individual single mode fibers. The absorption of Proflavine at 532
nm is low, which mitigates absorption bleaching of the contrast agent by the photoacoustic excitation
source. We demonstrate imaging in live murine models. The system is able to provide cellular morphology
with cellular resolution co-registered with the structural and functional information given by OR-PAM.
Therefore, the system has the potential to serve as a virtual biopsy technique, helping researchers and
clinicians visualize angiogenesis, effects of anti-cancer drugs on both cells and the microcirculation, as well
as aid in the study of other diseases.
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Photoacoustic (PA) imaging has been investigated for intravascular applications. One of the main challenges
is that the imaging frame rate is limited by the pulse repetition frequency (PRF), thus making real-time
imaging difficult with most high-power solid-state pulse lasers. The goal of this study is to combine
omni-directional optical excitation with a ring array transducer for high-frame-rate imaging, so that the image
frame rate is the same as the laser PRF. In the preliminary study, we developed a real-time integrated
IVUS/IVPA imaging system by modifying an IVUS system in combination with a high-speed Nd:YLF pulsed
laser. In addition, an optical fiber with axicon-like distal tip is designed for omni-directional excitation. In this
design, a PA image is acquired without rotating the laser light. The imaging frame rate of this integrated
imaging system is 19 fps. Both US and PA images are recorded at the same time and co-registered in the
fusion image. The US/PA images of tungsten wire, black tube and rabbit's atherosclerotic aorta were acquired
with this integrated system to evaluate its imaging performance. The lateral/axial -6 dB resolution of US
image is 2.56°/62.4μm. Resolution of PA imaging is 3.76°/91.5μm. The imaging system was also utilized to
acquire IVUS/IVPA images of atherosclerotic rabbit's aorta in ex vivo study.
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We present a new photoacoustic (PA) cell, which is sealed on the sample side with a 163 μm thick chemical vapor
deposition (CVD) diamond window. The investigation of samples containing volatile compounds with an openended
PA cell leads to varying conditions in the PA chamber (changing light absorption or relative humidity)
and thus causes unstable signals. In contrast the diamond cover ensures stable conditions in the PA chamber and
thereby enables sensitive measurements. This is particularly important for the investigation of biological samples
with a high water content. Due to the high thermal conductivity of CVD diamond (1800 W/mK) strong PA
signals are generated and the broad optical transmission range (250 nm to THz) renders the cell useful for various
applications. The performance of the cell is demonstrated by tracking glucose in aqueous keratinocyte solutions
with an external-cavity quantum cascade laser (1010-1095 cm-1). These measurements yield a detection limit of
100 mg/dl (SNR=3). Although glucose measurements within the human physiological range (30-500 mg/dl) are
feasible, further improvements are needed for non-invasive glucose monitoring of diabetes patients. First in vivo
measurements at the human forearm show an additional PA signal induced by blood pulsation at a frequency
around 1 Hz and a steadily increasing relative humidity in the PA chamber due to transepidermal water loss if
the cell is neither closed with a diamond window nor ventilated with N2.
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The problem of how to effectively deliver light dynamically to a small volume inside turbid media has been intensively
investigated for imaging and therapeutic purposes. Most recently, a new modality termed Time-Reversed Ultrasonically
Encoded (TRUE) optical focusing was proposed by integrating the concepts of ultrasound modulation of diffused light
with optical phase conjugation. In this work, the diffused photons that travel through the ultrasound focal region are
"tagged" with a frequency shift due to the ultrasound modulation. Part of the tagged light is collected in reflection mode
and transmitted to a photorefractive crystal, forming there a stationary hologram through interference with a coherent
reference optical beam. The hologram is later read by a conjugated optical beam, generating a phase conjugated wavefront
of the tagged light. It is conveyed back to the turbid medium in reflection mode, and eventually converges to the ultrasound
focal zone. Optical focusing effects from this system are demonstrated experimentally in tissue-mimicking phantoms and
ex vivo chicken breast tissue, achieving effective round-trip optical penetration pathlength (extinction coefficient
multiplied by round-trip focusing depth) exceeding 160 and 100, respectively. Examples of imaging optical inclusions with
this system are also reported.
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Photoacoustic tomography (PAT) and ultrasonography (US) of biological tissues usually rely on ultrasonic transducers
for the detection of ultrasound. For an optimum sensitivity, transducers require a physical contact with the tissue using a
coupling fluid (water or gel). Such a contact is a major drawback in important potential applications such as surgical
procedures on human beings and small animal imaging in research laboratories. On the other hand, laser ultrasonics (LU)
is a well established optical technique for the non-contact generation and detection of ultrasound in industrial materials.
In this paper, the remote optical detection scheme used in industrial LU is adapted to allow the detection of ultrasound in
biological tissues while remaining below laser exposure safety limits. Both non-contact PAT (NCPAT) and non-contact
US (NCUS) are considered experimentally using a high-power single-frequency detection laser emitting suitably shaped
pulses and a confocal Fabry-Perot interferometer in differential configuration. It is shown that an acceptable sensitivity is
obtained while remaining below the maximum permissible exposure (MPE) of biological tissues. Results were obtained
ex vivo on chicken breast specimens with embedded inclusions simulating blood vessels optical properties. Sub-mm
inclusions are readily detected at depths approaching 1 cm. The method is expected to be applicable to living tissues.
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The temperature dependence of photoacoustic generation is utilized for monitoring the temperature in flowing blood. A
phantom blood vessel is probed with photoacoustic (PA) excitation from a 830nm laser diode whose intensity is
sinusoidally modulated at ultrasound frequencies. A second laser diode at the same wavelength is used to photothermally
(PT) induce sinusoidal temperature fluctuations in the probed volume. The temperature oscillations lead to modulation
sidebands in the PA response. Measurement of the sidebands amplitude as a function of the PT modulation frequency,
for different flow rates, reveals a strong dependence of the PT modulation frequency response (MFR) on the flow rate.
This is attributed to the thermal properties of the volume under test, and in particular to the heat clearance rate, which is
strongly affected by the flow. A simplified lumped model based on the similarity between the system temporal behavior
and that of an RC circuit is used to analyze the resulting MFR's. With the addition of an appropriate calibration protocol
and by using multispectral PA and/or PT excitation the proposed approach can be used for simultaneous in-vivo
measurement of both flow and oxygenation level.
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Photoacoustic imaging (PAI) combines high ultrasound resolution with optical contrast. Laser-generated ultrasound
is potentially beneficial for cancer detection, blood oxygenation imaging, and molecular imaging. PAI
is generally performed using solid state Nd:YAG lasers in combination with optical parametric oscillators. An
alternative approach uses laser diodes with higher pulse repetition rates but lower power. Thus, improvement
in signal-to-noise ratio (SNR) is a key step towards applying laser diodes in PAI. To receive equivalent image
quality using laser diodes as with Nd:YAG lasers, the lower power must be compensated by averaging, which can
be enhanced through coded excitation. In principle, perfect binary sequences such as orthogonal Golay codes
can be used for this purpose when acquiring data at multiple wavelengths. On the other hand it was shown for
a single wavelength that sidelobes can remain invisible even if imperfect sequences are used. Moreover, SNR can
be further improved by using an imperfect sequence compared to Golay codes. Here, we show that pseudorandom
sequences are a good choice for multispectral photoacoustic coded excitation (MSPACE). Pseudorandom
sequences based upon maximal length shift register sequences (m-sequences) are introduced and analyzed for the
purpose of use in MSPACE. Their gain in SNR exceeds that of orthogonal Golay codes for finite code lengths.
Artefacts are introduced, but may remain invisible depending on SNR and code length.
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The method proposed in this work combines the advantages of optical detection (optical and acoustical transparency)
with 2D slice imaging, using an optical interferometer combined with an acoustic reflector. The concave reflector has the
shape of an elliptical cylinder and concentrates the acoustic wave generated around one focal line in the other one, where
an optical beam probes the temporal evolution of acoustic pressure. This yields line projections of the initial acoustic
pressure sources at distances corresponding to the time of flight. Image reconstruction from the signals recorded while
rotating the sample about an axis perpendicular to the optical detector requires only the application of the inverse Radon
transform. The resolution and sensitivity of the detection system were investigated in experiments on phantom samples.
Furthermore, the imaging system was tested on a real biological sample.
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In this paper we report on remote photoacoustic imaging using an interferometric technique. By utilizing a two-wave
mixing interferometer ultrasonic displacements are measured without any physical contact to the sample.
This technique allows measurement of the displacements also on rough surfaces. Mixing a plane reference beam
with the speckled beam originating from the sample surface is done in a Bi12SiO20 photorefractive crystal.
After data acquisition the structure of the specimen is reconstructed using a Fourier-domain synthetic focusing
aperture technique. We show three-dimensional imaging on
tissue-mimicking phantoms and biological samples.
Furthermore, we show remote photoacoustic measurements on a human forearm in-vivo.
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The recently developed vibrational photoacoustic (VPA) microscopy allows bond-selective imaging of deep tissues
by taking advantage of intrinsic contrast from harmonic vibration of C-H bonds. Due to the spectral similarity of
molecules in the overtone vibration region, the compositional information is not available from VPA images acquired by
single wavelength excitation. Here we demonstrate that lipids and collagen, two critical markers in many kinds of
diseases, can be distinguished by hyperspectral VPA imaging. A phantom consisted of rat tail tendon (collagen) and fat
tissue (lipids) was constructed. Wavelengths between 1650 and 1850 nm were scanned to excite the first
overtone/combination vibration of C-H bond. B-scan hyperspectral VPA images, in which each pixel contains a
spectrum, was analyzed by a Multivariate Curve
Resolution - Alternating Least Squares (MCR-ALS) algorism to
recover the spatial distribution of two chemical components in the phantom.
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We report the development of a novel frequency-domain biomedical photoacoustic (PA) system that utilizes a
continuous-wave laser source with a custom intensity modulation pattern for spatially-resolved imaging of biological
tissues. The feasibility of using relatively long duration and low optical power laser sources for spatially-resolved PA
imaging is presented. We demonstrate that B-mode PA imaging can be performed using an ultrasonic phased array
coupled with multi-channel correlation processing and a
frequency-domain beamforming algorithm. Application of the
frequency-domain PA correlation methodology is shown using
tissue-like phantoms with embedded optical contrast,
tissue ex-vivo samples and a small animal model in-vivo.
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Optical ultrasound detection has become an attractive alternative to piezoelectric ultrasound detection for photoacoustic
imaging. The favorable properties of optical detection are high resolution, complete optical and acoustical transparency.
Recently, it has been shown that optical phase contrast full field detection in combination with a CCD-camera can be
used to record acoustic fields. This allows to obtain
two-dimensional photoacoustic projection images in real-time. The
present work shows an extension of the technique towards full
three-dimensional photoacoustic tomography. The
specifications of the detection system, resolution and sensitivity, are 66μm and 3.4kPa. The reconstruction of the initial
three dimensional pressure distribution is a two step process. First of all, projection images of the initial pressure
distribution are acquired. This is done by back propagating the observed wave pattern in frequency space. In the second
step the inverse Radon transform is applied to the obtained projection dataset to reconstruct the initial three dimensional
pressure distribution. Simulations and experiments are performed to show the overall applicability of this technique for
real-time photoacoustic imaging.
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Traumatic brain injury (TBI) and spinal cord injury are a major cause of death for individuals under 50 years of age. In
the USA alone, 150,000 patients per year suffer moderate or severe TBI. Moreover, TBI is a major cause of combatrelated
death. Monitoring of cerebral venous blood oxygenation is critically important for management of TBI patients
because cerebral venous blood oxygenation below 50% results in death or severe neurologic complications. At present,
there is no technique for noninvasive, accurate monitoring of this clinically important variable. We proposed to use
optoacoustic technique for noninvasive monitoring of cerebral venous blood oxygenation by probing cerebral veins such
as the superior sagittal sinus (SSS) and validated it in animal studies. In this work, we developed a novel, medical grade
optoacoustic system for continuous, real-time cerebral venous blood oxygenation monitoring and tested it in human
subjects at normal conditions and during hyperventilation to simulate changes that may occur in patients with TBI. We
designed and built a highly-sensitive optoacoustic probe for SSS signal detection. Continuous measurements were
performed in the near infrared spectral range and the SSS oxygenation absolute values were automatically calculated in
real time using a special algorithm developed by our group. Continuous measurements performed at normal conditions
and during hyperventilation demonstrated that hyperventilation resulted in approximately 12% decrease of cerebral
venous blood oxygenation.
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Cerebral ischemia after birth and during labor is a major cause of death and severe complications such as cerebral palsy.
In the USA alone, cerebral palsy results in permanent disability of 10,000 newborns per year and approximately 500,000
of the total population. Currently, no technology is capable of direct monitoring of cerebral oxygenation in newborns.
This study proposes the use of an optoacoustic technique for noninvasive cerebral ischemia monitoring by probing the
superior sagittal sinus (SSS), a large central cerebral vein. We developed and built a multi-wavelength, near-infrared
optoacoustic system suitable for noninvasive monitoring of cerebral ischemia in newborns with normal weight (NBW),
low birth-weight (LBW, 1500 - 2499 g) and very low birth-weight (VLBW, < 1500 g). The system was capable of
detecting SSS signals through the open anterior and posterior fontanelles as well as through the skull. We tested the
system in NBW, LBW, and VLBW newborns (weight range: from 675 g to 3,000 g) admitted to the neonatal intensive
care unit. We performed single and continuous measurements of the SSS blood oxygenation. The data acquisition,
processing and analysis software developed by our group provided real-time, absolute SSS blood oxygenation
measurements. The SSS blood oxygenation ranged from 60% to 80%. Optoacoustic monitoring of the SSS blood
oxygenation provides valuable information because adequate cerebral oxygenation would suggest that no therapy was
necessary; conversely, evidence of cerebral ischemia would prompt therapy to increase cerebral blood flow.
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Near-field Radio-frequency Thermoacoustic Imaging (NRTI) is an imaging modality that was recently introduced to
generate thermoacoustic signals using ultra-short high energy impulses. Because it allows for a higher energy coupling
within an ultra-short time, it can achieve higher resolutions and higher signal to noise ratio, compared to traditional
thermoacoustic tomography based on radiating sources at single frequencies. As for traditional thermoacoustic imaging
the contrast comes from the conductivity and the dielectric properties of the tissues, while the resolution depends on the
measured acoustic waves. Since NRTI depends on the efficient generation of high energy short impulses, the ability to
control their time width and pulse shape is of high importance. We present here a methodology for generating such
impulses based on transmission lines. The ability of such generators to generate impulses in the range of tens of nanoseconds
enables high resolution images in the range of tens of microns to hundreds of microns without compromising the
amount of the energy coupled. Finally the pulser is used to generate high resolution images of small absorbing insertions,
of phantoms with different conductivities and of ex-vivo mouse images. From the phantoms it is possible to see both the
capabilities of the system to accurately image small insertions as well as the high quality images generated from imaging
phantoms, from ex-vivo mouse images it is possible to see several anatomical characteristics, such as the mouse
boundary, the spine and some other characteristics in the mouse abdomens.
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Combined intravascular photoacoustic (IVPA) and intravascular ultrasound (IVUS) imaging has been previously
established as a viable means for imaging atherosclerotic plaques using both endogenous and exogenous contrast. In this
study, IVUS/IVPA imaging of an atherosclerotic rabbit aorta following injection of gold nanorods (AuNR) with peak
absorbance within the tissue optical window was performed. Ex-vivo imaging results revealed high photoacoustic signal
from localized AuNR. Corresponding histological cross-sections and digital photographs of the artery lumen confirmed
the presence of AuNR preferentially located at atherosclerotic regions and in agreement with IVPA signal. Furthermore,
an integrated IVUS/IVPA imaging catheter was used to image the AuNR in the presence of luminal blood. The results
suggest that AuNR allow for IVPA imaging of exogenously labeled atherosclerotic plaques with a comparatively low
background signal and without the need for arterial flushing.
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Trapping and manipulation of micro-scale objects mimicking metastatic cancer cells in a flow field have been
demonstrated with magnetomotive photoacoustic (mmPA) imaging. Coupled contrast agents combining gold nanorods
(15 nm × 50 nm; absorption peak around 730 nm) with 15 nm diameter magnetic nanospheres were targeted to 10 μm
polystyrene beads recirculating in a 1.6 mm diameter tube mimicking a human peripheral vessel. Targeted objects were
then trapped by an external magnetic field produced by a dual magnet system consisting of two disc magnets separated
by 6 cm to form a polarizing field (0.04 Tesla in the tube region) to magnetize the magnetic contrast agents, and a
custom designed cone magnet array with a high magnetic field gradient (about 0.044 Tesla/mm in the tube region)
producing a strong trapping force to magnetized contrast agents. Results show that polystyrene beads linked to
nanocomposites can be trapped at flow rates up to 12 ml/min. It is shown that unwanted background in a photoacoustic
image can be significantly suppressed by changing the position of the cone magnet array with respect to the tube, thus
creating coherent movement of the trapped objects. This study makes mmPA imaging very promising for differential
visualization of metastatic cells trafficking in the vasculature.
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Normally, urine flows down from kidneys to bladders. Vesicoureteral reflux (VUR) is the abnormal flow of urine from
bladders back to kidneys. VUR commonly follows urinary tract infection and leads to renal infection. Fluoroscopic
voiding cystourethrography and direct radionuclide voiding cystography have been clinical gold standards for VUR
imaging, but these methods are ionizing. Here, we demonstrate the feasibility of a novel and nonionizing process for
VUR mapping in vivo, called photoacoustic cystography (PAC). Using a photoacoustic (PA) imaging system, we have
successfully imaged a rat bladder filled with clinically being used methylene blue dye. An image contrast of ~8 was
achieved. Further, spectroscopic PAC confirmed the accumulation of methylene blue in the bladder. Using a laser pulse
energy of less than 1 mJ/cm2, bladder was clearly visible in the PA image. Our results suggest that this technology
would be a useful clinical tool, allowing clinicians to identify bladder noninvasively in vivo.
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Cytochrome c is a heme protein normally bound to mitochondria and is important for mitochondrial electron transport
and apoptosis initiation. Since cytochrome c is nonfluorescent, it is always labeled with fluorescent molecules for
imaging, which, however, may affect normal cellular functions. Here, label-free photoacoustic microscopy (PAM) of
mitochondrial cytochrome c was realized for the first time by utilizing the optical absorption around the Soret peak.
PAM was demonstrated to be sensitive enough to image mitochondrial cytochrome c at 422 nm wavelength.
Mitochondrial cytochrome c in the cytoplasm of fixed fibroblasts was clearly imaged by PAM as confirmed by
fluorescent labeling. By showing mitochondrial cytochrome c in various cells, we demonstrated the feasibility of PAM
for label-free histology of mouse ear sections. Therefore, PAM can sensitively image cytochrome c in unstained cells at
422 nm wavelength and has great potential for functional imaging of cytochrome c in live cells or in vivo.
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Ultraviolet photoacoustic microscopy (UVPAM) can image cell nuclei in vivo with high contrast and resolution
noninvasively without staining. Here, we used UV light at wavelengths of 210-310 nm for excitation of DNA and RNA
to produce photoacoustic waves. We applied the UVPAM to in vivo imaging of cell nuclei in mouse skin, and obtained
UVPAM images of the unstained cell nuclei at wavelengths of 245-282 nm as ultrasound gel was used for acoustic
coupling. The largest ratio of contrast to noise was found for the images of cell nuclei at a 250 nm wavelength.
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Filtered backprojection (FBP) algorithms are commonly employed for image reconstruction in optoacoustic tomography
(OAT). A limitation of FBP algorithms is that they require the measured acoustic data to be densely
sampled, which necessitates expensive ultrasound arrays that possess a large number of elements or increased
data-acquisition times if mechnical scanning is employed. Additionally, FBP algorithms are based on idealized
imaging models that do not accurately model the response of the transducers and fail to exploit the statistical
characteristics of noisy measurement data to minimize noise levels in the reconstructed images. Iterative image
reconstruction algorithms can circumvent these difficulties. However, to date, iterative reconstruction algorithms
have not been successfully applied to three-dimensional (3D) OAT. In this work we investigate the use of an
iterative image reconstruction method in 3D OAT. The large computational burden of 3D iterative image reconstruction
is circumvented by implementing the reconstrution algorithm with graphics processing units (GPUs).
The ability of the reconstruction algorithm to mitigate artifacts due to incomplete data is demonstrated.
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In photoacoustic imaging, upon short laser pulse irradiation, absorbers generate N-shaped pulses which can be detected
by ultrasound transducers. Radio frequency signals from different spatial locations are then reconstructed taking into
account the ultrasound transducer angular response. Usually, the directivity is part of the "a priori" characterization of the
transducer and it is assumed to be constant in the reconstruction algorithm.
This approach is valid in both transmission and reflection ultrasound imaging, where any echo resembles the transducer
frequency response. Center frequency and bandwidth of any echo are almost the same, and the ultrasound transducer
collect signals with the same "fixed" acceptance angle. In photoacoustics, instead, absorbers generate echoes whose time
duration is proportional to the absorber size. Large absorbers generate low frequency echoes, whereas small absorber
echoes are centered at higher frequencies. Thus for different absorber sizes, different pulse frequencies are obtained and
different directivities need to be applied.
For this purpose once a radio-frequency signal is aquired, it is pre-processed with a sliding window: every segment is
Fourier transformed to extract the central frequency. Then, a proper directivity can be estimated for each segment.
Finally signals can be reconstructed via a backprojection algorithm, according to the system's geometry. Echoes are
backprojected over spheres with the angular extension being adapted to the frequency content of the photoacoustic
sources.
Simulation and experimental validation of this approach are discussed showing promising results in terms of image
contrast and resolution.
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We report on an investigation of the role of shear waves in transcranial PAT brain imaging. Using a recently
developed PAT image reconstruction method for use with layered media, we quantify the extent to which accounting
for shear waves in the reconstruction method can improve image quality. The effects of shear waves
propagating in the solid layer on the ability to estimate Fourier components of the object are investigated as a
function of the thickness of the layer supporting shear waves as well as the incidence angle of the field in the
planewave representation. These results clarify the role of shear waves in transcranial PAT image formation and
indicate that further research is warranted to develop reconstruction algorithms that account for shear waves.
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As for any other imaging technique spatial resolution and sensitivity are important features for a photoacoustic imaging
device. It is already well known that spatial resolution depends on the size and the bandwidth of the detectors. Therefore
for photoacoustic image reconstruction usually small point-like and broadband detectors are assumed, which measure the
pressure as a function of time on a detection surface around the sample. But in reality point-like detectors are not ideal at
all: because of the small detector volume the thermodynamic fluctuations (= noise) get high and the signal amplitude is
low, which results in a bad signal-to-noise ratio (SNR). For a bigger detector volume the fluctuations are less and the
signal amplitude is better, which gives a better SNR. But on the other hand the photoacoustic pressure signal is averaged
over the whole detector volume, which results in blurring and a reduced spatial resolution if reconstruction algorithms
for point-like detectors are used. To characterize this trade-off between spatial resolution and sensitivity for a varying
detector volume in a quantitative way the pressure is described by a random variable having the measured pressure as a
mean value and noise as random fluctuations around that mean value ("stochastic process"). For a PVDF detector the
optimum for the detector size is given.
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In optoacoustic tomography (OAT), also known as photoacoustic tomography, a variety of analytic reconstruction
algorithms, such as filtered backprojection (FBP) algorithms, have been developed. Analytic algorithms are
typically computationally more efficient than iterative image reconstruction algorithms but possess disadvantages
that include the inabilty to accurately compensate for the response of the measurement system and stochastic
noise. While these shortcomings can be circumvented by use of iterative image reconstruction methods, threedimensional
(3D) iterative reconstruction is computationally burdensome. In this work, we present a novel datarestoration
method that seeks to recover an accurate estimate of the pressure data with reduced noise levels from
knowledge of the experimentally acquired transducer output data. From knowledge of the "restored" pressure
data, a computationally efficient analytic algorithm can be applied for image reconstruction. Accordingly, this
approach combines the advantages of an iterative reconstruction algorithm with the computational efficiency
of an analytic algorithm. Curvelet-based data-space restoration is demonstrated by use of computer-simulation
studies.
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Ultrasound Modulated Optical Tomography I: Joint Session with Conference 8272
In optical scattering media such as biological tissue, light propagation is randomized by multiple scattering. Beyond one
transport mean free path, where photon propagation is in the diffusive regime, direct light focusing becomes infeasible.
The resulting loss of light localization poses serious challenge to optical imaging in thick scattering media. Ultrasound
modulated optical tomography (UOT) combines high optical contrast and good ultrasonic resolution, and is therefore an
ideal imaging modality for soft biological tissue. A variety of detection techniques have been developed in UOT in an
effort to discriminate the ultrasonically encoded diffused light as the imaging signal. We developed a photorefractive
crystal based detection system, which has the ability to image both the optical and acoustic properties of biological
tissues. With the improved photorefractive crystal based detection, tissue-mimicking phantom samples as thick as 9.4 cm
can be imaged. We further exploit the virtual source concept in UOT and combine it with optical time reversal to achieve
diffusive light focusing into scattering media. Experimental implementation of this new technology is presented.
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Near infrared spectroscopy is a widely adopted optical sensing technique to measure tissue oxygenation non-invasively
in human tissues such as the brain and muscle. However, in many situations, the region of interest is beneath a
superficial layer, e.g., muscle overlaid by a superficial layer of skin and fat, which can affect the accuracy of the optical
measurement. By applying focused ultrasound in the region of interest, acousto-optic (AO) sensing techniques can
potentially provide a measurement less susceptible to physiological changes in the superficial layer. In this work, a
digital correlator based AO system has been used to perform a series of phantom experiments to assess the sensitivities
of the AO and optical measurements (both in reflection modes) to absorption changes (μa = 0.0235, 0.05, 0.1, 0.2, and
0.3 cm-1) in three different locations, including one deep location (23 mm away from the surface) and two superficial
locations (8 mm away from the surface). Our results show that the AO measurements have a higher sensitivity factor to
the absorption change in the deeper location (177 % cm) than the optical measurements (16 % cm). For the two more
superficial locations, the AO measurements have lower sensitivity factors (-5 % cm and 56 % cm) than the optical
measurements (194 % cm and 101 % cm). This study has shown the potential of the AO technique for physiological
monitoring, which requires a technique to be sensitive to oxygenation changes in the deeper layer while minimizing any
contamination from the superficial layer.
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Ultrasound Modulated Optical Tomography II: Joint Session with Conference 8272
Ultrasound imaging has benefited from non-linear approaches to improve image resolution and reduce the effects of
side-lobes. A system for performing second harmonic ultrasound modulated optical tomography is demonstrated which
incorporates both pulsed optical illumination and acoustic excitation. A pulse acoustic inversion scheme is employed
which allows the second harmonic ultrasound modulated optical signal to be obtained while still maintaining a short
pulse length of the acoustic excitation. For the experiments carried out the method demonstrates a reduction in the
effective line spread function from 4mm for the fundamental to 2.4mm for the second harmonic. The first use of pulsed
ultrasound modulated optical tomography in imaging fluorescent targets is also discussed. Simple experiments show that
by changing the length of the acoustic pulse the image contrast can be optimized. The modulation depth of the detected
signal is greatest when the length of the object along the acoustic axis is an odd number of half wavelengths and is
weakest when the object is an integer multiple of an acoustic wavelength. Preliminary ultrasound modulated imaging
results are also presented where the target generates light within the medium without the use of an external light source.
Although signal to noise ratio is likely to be a major challenge, this result highlights a potentially useful application of
ultrasound modulated optical tomography in bio- or chemi-luminescence imaging.
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Adequate capillary blood flow is a critical parameter for tissue vitality.
We present a novel non-invasive method for measuring blood flow based on the acousto-optic effect, using ultrasound
modulated diffused light. The benefits of the presented method are: deep tissue sampling (> 1cm), continuous real time
measurement, simplicity of apparatus and ease of operation.
We demonstrate the ability of the method to measure flow of scattering fluid using a calibrated flow phantom model.
Fluid flow was generated by a calibrated syringe pump and the phantom's sampled volume contained millimeter size
flow channels. Results demonstrate linear dependence of flow as measured by the presented technique (CFI) to actual
flow values with R2=0.91 in the range of 0 to 2 ml/min, and a linear correlation to simultaneous readings of a laser
Doppler probe from the same phantom.
This data demonstrates that CFI readings provide a non-invasive platform form measuring tissue microcirculatory blood
flow.
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We demonstrate the feasibility of quantitative optical absorption imaging by using Monte Carlo simulation of combined
photoacoustic tomography (PAT) and ultrasound-modulated optical tomography (UOT). Our simulation results show
that the optical fluence on initial photoacoustic (PA) pressure waves can be accurately compensated for recovering
exact optical absorption coefficients by using a fluence map acquired from UOT signals. Further, when the optical
heterogeneities of a sample were varied, the recovered optical absorption coefficients from a target remained constant
while PA amplitudes fluctuated. In practice, PAT and UOT systems can be potentially combined because both imaging
systems can easily share light illumination and ultrasound application patterns.
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We report on experimental demonstration of focused photoacoustic (PA) imaging for simultaneous recovery of the
acoustic and optical properties of absorber in homogeneous media. The PA signals are reconstructed from tissue-like
phantom experiments using Hilbert transform (HT) algorithm coupled with a focused PA imaging system. The results
demonstrate that the HT-based PA signal occurs at the edge of heterogeneous sample. The average acoustic velocity
could be obtained by the size dividing the traveling time. In addition, the absorption coefficient of absorber could be
reconstructed by the intensity of the HT-based PA signal at the edge of sample based on the theoretical analysis.
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Perfluorocarbon droplets containing optical absorbing nanoparticles have been developed for use as theranostic agents
(for both imaging and therapy) and as dual-mode contrast agents. Droplets can be used as photoacoustic contrast agents,
vaporized via optical irradiation, then the resulting bubbles can be used as ultrasound imaging and therapeutic agents.
The photoacoustic signals from micron-sized droplets containing silica coated gold nanospheres were measured using
ultra-high frequencies (100-1000 MHz). The spectra of droplets embedded in a gelatin phantom were compared to a
theoretical model which calculates the pressure wave from a spherical homogenous liquid undergoing thermoelastic
expansion resulting from laser absorption. The location of the spectral features of the theoretical model and experimental
spectra were in agreement after accounting for increases in the droplet sound speed with frequency. The agreement
between experiment and model indicate that droplets (which have negligible optical absorption in the visible and
infrared spectra by themselves) emitted pressure waves related to the droplet composition and size, and was independent
of the physical characteristics of the optical absorbing nanoparticles. The diameter of individual droplets was calculated
using three independent methods: the time domain photoacoustic signal, the time domain pulse echo ultrasound signal,
and a fit to the photoacoustic model, then compared to the diameter as measured by optical microscopy. It was found the
photoacoustic and ultrasound methods calculated diameters an average of 2.6% of each other, and 8.8% lower than that
measured using optical microscopy. The discrepancy between the calculated diameters and the optical measurements
may be due to the difficulty in resolving the droplet edges after being embedded in the translucent gelatin medium.
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The laminar myocardial sheet architecture and its dynamic change play a key role in myocardial wall thickening.
Histology, confocal optical microscopy (COM), and diffusion tensor MRI (DTI) have been used to unveil the structures
and functions of the myocardial sheets. However, histology and COM require fixation, sectioning, and staining
processes, which dehydrate and deform the sheet architecture. Although DTI can delineate sheet architecture
nondestructively in viable hearts, it cannot provide cellular-level resolution. Here we show that photoacoustic
microscopy (PAM), with high resolution (~1 μm) and label-free detection, is appropriate for imaging 3D myocardial
architecture. Perfused half-split mouse hearts were also imaged by PAM in vitro without fixation, dehydration, nor
staining. The laminar myocardial sheet architecture was clearly visualized within a 0.15 mm depth range. Two
populations of oppositely signed sheet angles were observed. Therefore, PAM promises to access dynamic changes of
myocardial architectures in ex vivo perfused-viable hearts.
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Direct optical measurements in scattering media offer poor resolution due to the high scattering. Ultrasound is scattered
orders of magnitude less in tissue compared with light and therefore offers good resolution. Photoacoustics and acoustooptics
are both relatively new hybrid techniques that enable measurements of optical properties in scattering media by
combining ultrasound and light. Quantified measurements of the fluence and absorption coefficient however are desired
and can not be performed by these separate techniques. A new approach to achieve this goal is to combine both hybrid
techniques. By combining photoacoustic and acousto-optic measurements there is sufficient information to calculate the
absorption coefficient and fluence at the ultrasound focus used for the acousto-optics. We require knowledge on the
interaction of light and sound inside tissue, so the size of the so called tagging volume can be determined. This tagging
volume is defined by the size and shape of the ultrasound focus used in the acousto-optic measurements. A stochastic
model for acousto-optics is under development that used existing knowledge on the in the interaction between light and
sound. By separating light transport and the interactions of light and sound and writing this interaction as a probability
density function it is possible to find the effective geometrical properties of the tagging volume. At the moment multiple
interaction mechanisms of sound and light are added to this model. In the future this model will be validated in phantoms
and biological tissue.
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We demonstrate the ability of a novel device employing ultrasound modulation of near infrared light (referred as
"Ultrasound tagged light" or UTL) to perform non-invasive monitoring of blood flow in the microvascular level in tissue.
Monitoring microcirculatory blood flow is critical in clinical situations affecting flow to different organs, such as the
brain or the limbs. . However, currently there are no non-invasive devices that measure microcirculatory blood flow in
deep tissue continuously.
Our prototype device (Ornim Medical, Israel) was used to monitor tissue blood flow on anesthetized swine during
controlled manipulations of increased and decreased blood flow. Measurements were done on the calf muscle and
forehead of the animal and compared with Laser Doppler (LD). ROC analysis of the sensitivity and specificity for
detecting an increase in blood flow on the calf muscle, demonstrated AUC = 0.951 for 23 systemic manipulations of
cardiac output by Epinephrine injection, which is comparable to AUC = 0.943 using laser Doppler. Some examples of
cerebral blood flow monitoring are presented, along with their individual ROC curves.
UTL flowmetry is shown to be effective in detecting changes in cerebral and muscle blood flow in swine, and has merit
in clinical applications.
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We successfully encapsulated ICG in an ultrasound-triggerable perfluorocarbon double emulsion that prevents ICG from
binding with plasma proteins. Photoacoustic spectral measurements on point target as well as 2-D photoacoustic images
of blood vessels revealed that the photoacoustic spectrum changes significantly in blood when the ICG-loaded emulsion
undergoes acoustic droplet vaporization (ADV), which is the conversion of liquid droplets into gas bubbles using
ultrasound. Other than providing a new photoacoustic contrast agent, the ICG encapsulated double emulsion, when
imaged with photoacoustic tomography, could facilitate spatial and quantitative monitoring of ultrasound initiated drug
delivery.
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Photoacoustic tomography (PAT) suppresses speckles by prominent boundary buildups. We theoretically study the
dependence of PAT speckles on the boundary roughness, which is quantified by the root-mean-squared (RMS) value and
the correlation length of the height. The speckle visibility and the correlation coefficient between the reconstructed and
actual boundaries are quantified as a function of the boundary roughness. The statistics of PAT speckles is studied
experimentally.
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Spectral analysis of photoacoustic (PA) molecular imaging (PMI) of ferritin expressed in human melanoma cells
(SK-24) was performed in vitro. Ferritin is a ubiquitously expressed protein which stores iron that can be detected by PA
imaging, allowing ferritin to act as a reporter gene. To
over-express ferritin, SK-24 cells were co-transfected with plasmid
expressing Heavy chain ferritin (H-FT) and plasmid expressing enhanced green fluorescent protein (pEGFP-C1) using
LipofectamineTM 2000. Non-transfected SK-24 cells served as a negative control. Fluorescent imaging of EGFP
confirmed transfection and transgene expression in co-transfected cells. To detect iron accumulation in SK-24 cells, a
focused high frequency ultrasonic transducer (60 MHz, f/1.5), synchronized to a pulsed laser (<20mJ/cm2), was used to
scan the PA signal from 680 nm to 950 nm (in 10 nm increments) from the surface of the 6-well culturing plate. PA
signal intensity from H-FT transfected SK-24 cells was not different from that of non-transfected SK-24 cells at
wavelengths less than 770 nm, but was over 4 dB higher than
non-transfected SK-24 cells at 850 ~ 950 nm. Fluorescent
microscopy indicates significant accumulation of ferritin in H-FT transfected SK-24 cells, with little ferritin expression
in non-transfected SK-24 cells. The PA spectral analysis clearly differentiates transfected SK-24 cells from nontransfected
SK-24 cells with significantly increased iron signal at 850 ~ 950 nm, and these increased signals were
associated with transfection of H-FT plasmid. As such, the feasibility of ferritin as a reporter gene for PMI has been
demonstrated in vitro. The use of ferritin as a reporter gene represents a new concept for PA imaging, and may provide
various opportunities for molecular imaging and basic science research.
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An image reconstruction formula is presented for photoacoustic computed tomography (PCT) that is valid for a
layered medium in which some of the layers may be solids and detection is performed on a planar measurement
surface. It is assumed that the optical absorber is embedded in a single fluid layer and any elastic solid layers
present are separated by one or more fluid layers.
Computer-simulation studies are used to validate the proposed
reconstruction formula.
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In this paper we present a method to visualize the pressure field of an ultrasound beam in a single shot of the CCD and
to image the shear wave propagation based on acousto-optic laser speckle contrast analysis. The contrast images show
features in the near field, far field and central region of the ultrasound beam and the pressure profile fits with that
measured with a hydrophone. The shear wave propagation was acquired by changing the imaging delay time after the
ultrasound burst. This method can be used to study the shear wave properties of common tissue phantoms to guide
experiments on tissue.
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structural and functional imaging. Image reconstruction of PAI requires the solution of an inverse source
problem, where the source represents the optical energy absorption distribution in the object. PAI in
spherical or circular geometry gives good image resolution yet is slow in signal acquisition and image
formation. Reducing the number of detection angles can ameliorate such issues. Besides, it is almost
impossible to cover the entire surface of tissue. This will restrict it in the medical application with
incomplete projection data. To resolve such limiting factors, in this thesis, a preconditioned conjugate
gradient method is applied to the normal equations (PCCGNR method) for reconstructing the absorption
distribution. Under the common assumption, a zero-mean Gaussian noise is added to the projection signals
and a computer simulated has been used for the evaluation. This algorithm works well in rectification of the
measurement and converges quickly onto an accurate estimate of the distribution of absolute absorption. It
not only runs much faster than the FBP algorithm, but also shows stronger robustness in that it provides
better image quality with detection data. We observed that diagonal preconditioners offer some
improvement in convergence rate for image reconstruction, and reconstructed image preconditioning
with ω = 0 (diagonal scaling) is closer to the true image than with ω> 0 . In addition, a physical experiment
that will be done with our experiment equipment system further demonstrates the potential of the proposed
algorithm in practical applications.
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Co-registered Ultrasound and Photoacoustic images provide complimentary structure and functional information for
cancer diagnosis and assessment of therapy response. In SPIE Photonics West 2011, we reported a system that acquires
from 64 channels and displays up to 1 frame per second (fps) ultrasound pulse-echo images, 5 fps photoacoustic images,
and 0.5 fps co-registered images. In this year, we report an upgraded system which acquires from 128 channels and
displays up to 15 fps co-registered ultrasound and photoacoustic images limited by our laser pulse repetition rate. The
system architecture is novel and it provides real-time
co-registration of images, the ability of acquiring the channel RF
data for both modalities, and the flexibility of adjusting every parameter involved in the imaging process for both
modalities.
The digital signal processor board is upgraded to an FPGA-based PCIe board that collects the data from the
acquisition modules and transfers them to the PC memory at 2.5GT/s rate through an x8 DDR PCIe bus running at
100MHz clock frequency. The modules FPGA code is also upgraded to form a beam line in 90 microseconds and to
communicate through ultrafast differential tracks with the PCIe board. Furthermore, the printed circuit board (PCB)
design of the system was adjusted to provide a maximum of 80dB signal-to-noise ratio at 60dB gain, which is
comparable to some commercial ultrasound machines.
The real-time system allows capturing co-registered US/PAT images free of motion artifacts and also provides
ultrafast dynamic information when a contrast agent is used. The system is built for clinical use to assist the diagnosis of
ovarian cancer. However, the hardware is still under testing and evaluation stage, experimental and clinical results will
be reported later.
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Optical resolution photoacoustic microscopy (OR-PAM) has been shown as a promising tool for label-free microvascular
and single-cell imaging in clinical and bioscientific applications. However, most OR-PAM systems are realized
by using a bulky laser for photoacoustic excitation. The large volume and high price of the laser may restrain the
popularity of OR-PAM. In this study, we develop a low-cost and compact OR-PAM system based on a commercially
available DVD pickup head. We showed that the DVD pickup head have the required laser energy and focusing optics
for OR-PAM. The firmware of a DVD burner was modified to enable its laser diode to provide a 13-ns laser pulse with
1.3-nJ energy at 650 nm. Two excitation wavelengths at 650 and 780 nm were available. The laser beam was focused
onto the target after passing through a 0.6-mm thick DVD transparent polycarbonate coating, and then aligned to be
confocal with a 50-MHz focused ultrasonic transducer in forward mode. To keep the target on focus, a scan involving
auto-tracking procedure was performed. The lateral resolution was verified via cross-sectional imaging of a 6-μm carbon
fiber. The measured -6 dB width of the carbon fiber was 6.66 μm which was in agreement with optical diffraction limit.
The proposed OR-PAM has potential as an economically viable and compact blood screening tool available outside of
large laboratories due to its low cost and portability. Furthermore, a better spatial resolution could be provided by using a
blue ray DVD pickup head.
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Properties of excitation laser are the important parameters that affect the photoacoustic image quality. As for the pulse
width, it is closely related to signal strength and image resolution, which reported as a result of an experiment using a
laser diode that can control the pulse width easily1. However, though a solid-state laser is promising for a medical
application due to its high pulse energy creating high photo acoustic signal, its influence on waveform or the image
quality has not been discussed in detail because the pulse width is hardly changeable in a solid-state laser.
We use two kinds of solid-state lasers, i.e., Q-switched Nd:YAG and Ti-Sapphire Laser, in this study and generate
different pulse width between 4.5 and 45 ns by changing wavelength and excitation energy. These laser pulses are
entered into a silicon tube composed of carbon-particle suspension as absorber whose wavelength dependence for
absorption is small. We detect the generated laser-induced photoacoustic waves by hydrophone.
The photoacoustic temporal waveform shows sharper as the pulse width is shorter, which also indicates high frequency
signal components increase. The width of the first peak on the temporal waveform is corresponding to the pulse width.
Additionally, as a result of the photoacoustic imaging experiment preformed with 192-channel PZT linear array probe to
image a thin wire, the modulation transfer function shows that the narrower the pulse width, the slightly better the image
resolution.
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In photoacoustic imaging, an adaptive beamforming method with coherence factor (ABF-CF) was previously
introduced for improving spatial resolution and signal-to-noise ratio (SNR) over a conventional delay-and-sum
beamforming method (DAS). However, the ABF-CF method is not suitable for being used in practical diagnosis since it
is overly-sensitive for off-axis interferences and noises. In this paper, a new adaptive beamforming method with
spatially-smoothed coherence factor (ABF-SSCF) is presented for ultrasound and photoacoustic combined imaging to
enhance the contrast and spatial resolution while preserving the target information by applying a spatial-smoothing
technique into CF coefficients from multiple sub-arrays within an array probe. To evaluate the ABF-SSCF method,
computer simulation and ex vivo experiments were conducted. For the computer simulation, 64-channel radio-frequency
(RF) data with one channel amplitude-varying off-axis interference was generated. Also, The ex vivo experiments were
conducted where 128-channel pre-beamformed RF data were captured from a microcalcification-contained breast core
specimen with a commercial ultrasound system equipped with a research package by using a 7-MHz linear array probe
(SonixTouch, Ultrasonix Corp., BC, Canada) and an Nd:Yag laser excitation system (Surelite III-10 and Surelite OPO
Plus, Continuum Inc., Santa Clara, CA, USA). From the simulation and ex vivo experiments, the proposed ABF-SSCF
method provides better contrast and spatial resolutions comparable than the DAS method. Also, compared to the ABFCF
method, image information is clearly presented without being degraded by off-axis interferences. These results
indicate that the proposed ABF-SSCF method can simultaneously enhance the image quality and efficacy of the ABF
method for ultrasound and photoacoustic combined imaging.
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Near-infrared spectroscopy (NIRS) can provide an estimate of the mean oxygen saturation in tissue. This technique is
limited by optical scattering, which reduces the spatial resolution of the measurement, and by absorption, which makes
the measurement insensitive to oxygenation changes in larger deep blood vessels relative to that in the superficial tissue.
Acousto-optic (AO) techniques which combine focused ultrasound (US) with diffuse light have been shown to improve
the spatial resolution as a result of US-modulation of the light signal, however this technique still suffers from low
signal-to-noise when detecting a signal from regions of high optical absorption. Combining an US contrast agent with
this hybrid technique has been proposed to amplify an AO signal. Microbubbles are a clinical contrast agent used in
diagnostic US for their ability to resonate in a sound field: in this work we also make use of their optical scattering
properties (modelled using Mie theory). A perturbation Monte Carlo (pMC) model of light transport in a highly
absorbing blood vessel containing microbubbles surrounded by tissue is used to calculate the AO signal detected on the
top surface of the tissue. An algorithm based on the modified Beer-Lambert law is derived which expresses intravenous
oxygen saturation in terms of an AO signal. This is used to determine the oxygen saturation in the blood vessel from a
dual wavelength microbubble-contrast AO measurement. Applying this algorithm to the simulation data shows that the
venous oxygen saturation is accurately recovered, and this measurement is robust to changes in the oxygenation of the
superficial tissue layer.
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Multi-Spectral Optoacoustic Tomography (MSOT) offers real time imaging that simultaneously exploits high ultrasound
resolutions and strong optical contrast. It allows visualizing absorbers in tissue by using their distinct spectral absorption
profiles. This work presents a non-invasive in vivo study of kinetics involved in the clearance of carboxylated dye in
mouse kidneys. The distinctio
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We investigate laser-induced ultrasound generated in a plane semi-transparent layered polymer structure. The
scope is to study relations between generated ultrasound, as e.g. amplitude, and centre frequency and bandwidth
of its frequency spectrum, and properties of the polymer layers, like thickness and absorption. This knowledge can
then be used when designing polymer film based, semi-transparent ultrasonic devices specifically for photoacoustic
applications. The experimental study is set-up as a factorial experiment with a completely randomised design.
In the experiments, the light source is a pulsed Nd:YAG laser. As absorber, a semi-transparent, non-conductive
polymer film in a plane layered structure of one or more layers on a glass substrate is used. The frequency spectra
of the generated ultrasound spans 2 to 20 MHz, which is recorded by a broadband PVDF ultrasonic transducer.
The results show that an increased thickness of the polymer layer structure relate to a lower center frequency
and a lower bandwidth, and that an increased optical absorption and a decreased layer structure thickness is
related to a higher ultrasound amplitude.
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In photo-acoustic (PA) imaging, valuable medical applications based on optical absorption spectrum such as contrast
agent imaging and blood oxygen saturation measurement have been investigated. In these applications, there is an
essential requirement to determine optical absorption coefficients accurately. In present, PA signal intensities have been
commonly used to determine optical absorption coefficients. This method achieves practical accuracy by combining with
radiative transfer analysis. However, time consumption of radiative transfer analysis and effects of signal generation
efficiencies were problems of this method. In this research, we propose a new method to determine optical absorption
coefficients using continuous wavelet transform (CWT). We used CWT to estimate instantaneous frequencies of PA
signals which reflects optical absorption distribution. We validated the effectiveness of CWT in determination of optical
absorption coefficients through an experiment. In the experiment, planar shaped samples were illuminated to generate
PA signal. The PA signal was measured by our fabricated PA probe in which an optical fiber and a ring shaped P(VDFTrFE)
ultrasound sensor were coaxially aligned. Optical properties of samples were adjusted by changing the
concentration of dye solution. Tunable Ti:Sapphire laser (690 - 1000 nm) was used as illumination source. As a result,
we confirmed strong correlation between optical absorption coefficients of samples and the instantaneous frequency of
PA signal obtained by CWT. Advantages of this method were less interference of light transfer and signal generation
efficiency.
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The specificity of the hemodynamic response function (HRF) is determined spatially by the vascular
architecture and temporally by the evolution of hemodynamic changes. Here, we used functional photoacoustic
microscopy (fPAM) to investigate the spatiotemporal evolution of the HRFs of hemoglobin concentration (HbT),
cerebral blood volume (CBV) and hemoglobin oxygen saturation (SO2) in single cerebral vessels to rat left-forepaw
stimulation. The HRF changes in specific cerebral vessels responding to different stimulation intensities and durations
were bilaterally imaged with 36 × 65-μm spatial resolution. Various electrical stimulations were applied with stimulation
intensities at 1, 2, 6 and 10-mA combined with 5-s and 15-s stimulation durations, respectively. Our main findings were
as follows: 1) the functional HbT and SO2 increased sub-linearly with increasing stimulus intensities and 2) the results
suggested that the CBV changes are more linearly correlated with arterioles than HbT and SO2 within a limited dynamic
range of stimulation intensities and duration. The findings in this study indicate that the regulation of hemodynamic
changes in single cerebral vessels can be reliable studied by the fPAM technique without the use of contrast agents.
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Breast calcification is one of the most important indicators for early breast cancer detection. In this study, based on a
medical ultrasound array imaging platform, we attempt to develop a real-time and high penetration photoacoustic (PA)
array imaging system for visualization of breast calcifications. Phantom studies were used to verify the imaging
capability and penetration depth of the developed PA array system for calcification imaging. Intralipid gelatin phantoms
with different-sized hydroxyapatite (HA) particles - major chemical composition of the breast calcification associated
with malignant breast cancers - embedded were imaged. Laser at 750 nm was used for photoacoustic excitation and a
custom-made 5-MHz photoacoustic array transducer with linear light guides was applied for photoacoustic signal
detection. Experimental results demonstrated that this system is capable of calcification imaging of 0.3-0.5 mm HA
particles. For the 0.5-mm HA particles, the imaging contrast was about 34 dB and the achievable penetration was 20 mm
where the axial, lateral, and elevational resolution of this PA array imaging system is 0.39 mm, 0.38 mm, and 1.25 mm,
respectively. The highest frame rate was 10 frames/sec limited by the laser pulse rate. Overall, our results demonstrate
that it is promising for PA imaging as a real-time diagnosis and biopsy guidance tool of breast micro-calcifications
outside mass lesion. Future work will focus on optimization of the photoacoustic transducer to further improve the
penetration depth and development of photoacoustic and ultrasound dual-modal imaging to enhance the calcification
imaging capability.
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The photoacoustic (PA) signal attenuation was affected by many factors in an imaging system. In this presentation, the
factors lead to the signal attenuation and their characters were discussed based on tissue optics, acoustic transportation
and detection in a long-focal-zone PA imaging system. A method to recover the detected PA signals was presented and
employed to image a thyroid sample in vitro. The experimental results demonstrated that the method could be used to
improve the imaging depth and quality in the PA system.
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For photoacoustic imaging detectors which provide high spatial resolution while being highly sensitive are essential.
Integrating line detectors made of single mode polymer fibers achieve these requirements. In this paper
several approaches and preliminary experiments for single mode polymer fiber line detectors are presented. Operation
point stabilization by utilizing a fiber-based phase shifter is shown as well as results using different fiber
couplers in the setup.
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Detection of acoustic waves is the cornerstone of photoacoustic tomography (PAT). Detection has conventionally
been performed mechanically using ultrasonic transducers, or optically by interferometric techniques.
We propose an interferometric detection scheme using low coherence interferometry (LCI) and discuss the
challenges, advantages and limitations of applying this technique to photoacoustics.
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Photoacoustic signals originating from weak sources can be hard to discriminate from higher intensity signals
resulting from the photoacoustic background. In order to reveal these weaker signals we propose a method
where the expected background signal is subtracted from the actual signals. In this method an ultrasound image
provides the geometry for the pressure distribution used in a photoacoustic wave field simulation. The simulated
photoacoustic signals are subtracted from the actual recordings and the residual is used for image reconstruction.
This method was successfully validated experimentally with a vessel phantom containing three optical absorption
irregularities within the vessel wall.
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Genetically encoded probes powerfully and non-invasively target specific tissues, cells, and subcellular
locations. iRFP, a novel near-infrared fluorescent protein with low quantum yield whose absorption and
fluorescence maxima are located at wavelengths longer than the
Q-band of hemoglobin absorption, is
ideal for PAT. Here, we report on an in vitro comparison of iRFP with other far-red fluorescent proteins,
and its use in imaging a mouse tumor xenograft model. In an in vivo experiment, we stably transfected
iRFP into MTLn3 adenocarcinoma cells and injected them into the mammary fat pad of female SCID/NCr
mice, then imaged the resulting tumors two and three weeks post injection. The contrast increase from
the protein expression was high enough to clearly separate the tumor region from the rest of the
animal.
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We have investigated different types of optical diffusers for the image quality assessment of
photoacoustic tomography (PAT). PAT has been adapted in many biomedical research efforts over the past decade,
however, studies on image quality of PAT have not been performed as much as that for photoacoustic microscopy.
We developed a simple imaging phantom using strings of red plastic embedded in gelatinous base. Using a 532 nm
Nd:YAG laser and focused/unfocused transducers, we reconstructed PAT images of the phantom with various types
of optical diffusers placed on top of phantoms. Our initial results showed that the uniformity of the diffuser did not
affect the PAT image quality, while the degree of light scattering contributed relatively more to the image quality.
Image quality of biological samples will be presented and discussed.
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A custom-made first prototype of a linear array ultrasound transducer for breast imaging is presented. Large active area
transducer elements (5 mm × 5 mm) with 1 MHz resonance frequency are chosen to obtain a relatively high sensitivity.
Acoustic lenses are used to enlarge the narrow acceptance angle of such transducer elements. The minimum detectable
pressure, frequency bandwidth and electrical impedance of the transducer elements are characterized. The results show
the transducer has a minimum detectable pressure of 0.8 Pa, which is superior than the transducers used in the Twente
Photoacoustic Mammoscope system previously developed in our group. The bandwidth of the transducer is relative
small, however it can be improved when using optimized matching layer thickness in future. We also observed a strong
lateral resonance at 330 kHz, which may cause problems in various aspects for a photoacoustic imaging system. We
discuss the future improvement and plans for the transducer optimizations.
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Red blood cells (RBCs) aggregate in the presence of increased plasma fibrinogen and low shear forces during blood
flow. RBC aggregation has been observed in deep vein thrombosis, sepsis and diabetes. We propose using
photoacoustics (PA) as a non-invasive imaging modality to detect RBC aggregation. The theoretical and experimental
feasibility of PA for detecting and characterizing aggregation was assessed. A simulation study was performed to
generate PA signals from non-aggregated and aggregated RBCs using a frequency domain approach and to study the PA
signals' dependence on hematocrit and aggregate size. The effect of the finite bandwidth nature of transducers on the PA
power spectra was also investigated. Experimental confirmation of theoretical results was conducted using porcine RBC
samples exposed to 1064 nm optical wavelength using the Imagio Small Animal PA imaging system (Seno Medical
Instruments, Inc., San Antonio, TX). Aggregation was induced with Dextran-70 (Sigma-Aldrich, St. Louis, MO) and the
effect of hematocrit and aggregation level was investigated. The theoretical and experimental PA signal amplitude
increased linearly with increasing hematocrit. The theoretical dominant frequency content of PA signals shifted towards
lower frequencies (<30 MHz) and 9 dB enhancements in spectral power were observed as the size of aggregates
increased compared to non-aggregating RBCs. Calibration of the PA spectra with the transducer response obtained from
a 200 nm gold film was performed to remove system dependencies. Analysis of the spectral parameters from the
calibrated spectra suggested that PA can assess the degree of aggregation at multiple hematocrit and aggregation levels.
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Photoacoustic imaging is a hybrid imaging modality capable of producing images based on optical contrast, but with
depth penetration and resolution similar to ultrasound imaging. In this work, a staring, sparse approach to 3D
photoacoustic imaging was used to image a number of objects. The photoacoustic system described in this paper
improved upon a previous generation that contained 30 commercial transducers, by incorporating 60 custom-built
transducers into a compact hemispherical array. Imaging was performed by acquiring an experimental estimate of the
imaging operator and solving a linear system model to provide an estimate of the object. The imaging operator contained
18,000 voxels, each at 0.5-mm isotropic resolution. The dimensions of the imaging operator were 30 mm × 30 mm × 2.5
mm. In the first experiment, a black wire was arranged in a triangular shape and imaged in a 0.3% IntralipidTM solution.
The second experiment utilized a rotating human hair, where the hair was imaged at different angular positions. Both
objects were successfully captured with reasonable accuracy, though image artifacts were present in both sets of images.
The experimental results demonstrated that objects of substantial geometrical complexity could be reconstructed using
measurements from only 60 transducers with prior knowledge of the imaging operator.
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Photoacoustic Doppler Flowmetry has several potential advantages over its purely acoustical counterpart. The key ones
are better inherent contrast and potential molecular information. It is therefore highly desired to continue to develop this
modality into a viable complementary tool alongside with Doppler Ultrasound flowmetry. Working towards this goal we
have constructed a Photoacoustic Doppler setup based on a combined pair of laser diodes at 830nm and a 10MHz
focused acoustical transducer. Using tone-burst intensity modulation, depth-resolved Doppler spectrograms of a phantom
vessel containing flowing suspension of carbon particles, were obtained. In order to investigate the conditions required
for successful photoacoustic Doppler measurement in blood a k-space photoacoustic simulation was performed. It tested
the photoacoustic response which is obtained for moving random spatial distributions of red blood cells and the effect of
several parameters, such as particles density, ultrasonic frequency and optical spot size.
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Photoacoustic microscopy (PAM) has been shown to be a valuable tool for quantifying hemoglobin oxygenation within
single vessels. Recently, optical-resolution PAM was developed to achieve higher resolution by reducing the laser beam
diameter, which increased the light intensity. As intensity increases, saturation of the optical absorption and multiphoton/
multi-step absorption can occur, which, together with the temperature dependence of thermal expansion, result in
a non-linear dependence of the photoacoustic signal on the excitation pulse fluence. For hemoglobin, the major absorber
in tissue for photoacoustic imaging, these non-linear phenomena have strong wavelength dependence. To enable
quantitative photoacoustic measurements at different wavelengths in the presence of nonlinearity, a careful wide range
analysis of the intensity-dependent absorption is required. Here, we built a photoacoustic spectrometer, using a tunable
nanosecond optical parametric oscillator that operates between 410 nm and 2400 nm as our light source. To reduce
uncertainty in our measurements due to inhomogeneous spatial distribution of the optical fluence, we used a flat-top
beam illumination and a flat transducer which was mounted in reflection mode, effectively reducing quantitative
measurements to a one dimensional problem. Intensity-dependent non-linear spectra of the photoacoustic signals of oxyand
deoxy-hemoglobin were obtained. These measurements have the potential to contribute significantly to quantitative
functional PAM.
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We discuss a method for using the pseudoinverse of the system matrix to perform photoacoustic image reconstruction. In
our method, the regularization levels are set and the pseudoinverse matrices are calculated just once for all possible objects,
so the reconstruction step consists only of a matrix-vector multiplication, which is very fast. We expect this method to
work well in photoacoustic imaging because the dominant noise mechanism is usually transducer thermal noise, which is
object independent. We find that this reconstruction method offers improvements in ideal observer signal-to-noise ratio,
resolution, and the length of streak artifacts compared to standard filtered backprojection.
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Radiofrequency ablation (RFA) procedures are used to destroy abnormal electrical pathways in the heart that can cause
cardiac arrhythmias. Current methods relying on fluoroscopy, echocardiography and electrical conduction mapping are
unable to accurately assess ablation lesion size. In an effort to better visualize RFA lesions, photoacoustic (PA) and
ultrasonic (US) imaging were utilized to obtain co-registered images of ablated porcine cardiac tissue. The left
ventricular free wall of fresh (i.e., never frozen) porcine hearts was harvested within 24 hours of the animals' sacrifice. A
THERMOCOOLR Ablation System (Biosense Webster, Inc.) operating at 40 W for 30-60 s was used to induce lesions
through the endocardial and epicardial walls of the cardiac samples. Following lesion creation, the ablated tissue samples
were placed in 25 °C saline to allow for multi-wavelength PA imaging. Samples were imaged with a VevoR 2100
ultrasound system (VisualSonics, Inc.) using a modified 20-MHz array that could provide laser irradiation to the sample
from a pulsed tunable laser (Newport Corp.) to allow for
co-registered photoacoustic-ultrasound (PAUS) imaging. PA
imaging was conducted from 750-1064 nm, with a surface fluence of approximately 15 mJ/cm2 maintained during
imaging. In this preliminary study with PA imaging, the ablated region could be well visualized on the surface of the
sample, with contrasts of 6-10 dB achieved at 750 nm. Although imaging penetration depth is a concern, PA imaging
shows promise in being able to reliably visualize RF ablation lesions.
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We have developed a 2.5-mm outer diameter photoacoustic endoscopic mini-probe to use in the instrument channel
(typically 2.8 or 3.7 mm in diameter) of standard video endoscopes. To achieve adequate signal sensitivity, we
fabricated a focused ultrasonic transducer using a highly-sensitive PMN-PT piezoelectric material. We quantified the
PMN-PT transducer's operating parameters and validated the
mini-probe's endoscopic imaging capability through an ex
vivo imaging experiment on a rat colon.
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As a new class of sentinel lymph node (SLN) tracers for photoacoustic (PA) imaging, Au nanocages offer the advantages
of noninvasiveness, strong optical absorption in the near-infrared region (for deep penetration), and accumulation in
higher concentrations than the initial injected solution. By monitoring the amplitude changes of PA signals in an animal
model, we quantified the accumulations of nanocages in SLNs over time. Based on this method, we quantitatively
evaluated the kinetics of gold nanocages in SLN in terms of concentration, size, and surface modification. We could
detect the SLN at an Au nanocage injection concentration of 50 pM and a dose of 100 μL in vivo. This concentration is
about 40 times less than the previously reported value. We also investigated the influence of nanocages' size (50 nm and
30 nm in edge length), and the effects of surface modification (with positive, or neutral, or negative surface charges).
The results are helpful to develop this AuNC-based PA imaging system for noninvasive lymph node mapping, providing
valuable information about metastatic cancer staging.
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Pressure changes caused by an ultrasound pattern inside tissue will modify its density and hence its refractive index. We
present an integrated computational imaging approach to minimize multiple-scattered reflected light from tissue. It is
based on optimum modulation of the refractive index of tissue using an ultrasound pattern. We examine issues related
the design of such pattern using COMSOL Multiphysics. An optimum ultrasound pattern could be used to design and
implement an integrated- computational optical coherence tomography (IC-OCT) system with extended depth of
imaging.
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The penetration depth of ballistic optical imaging technologies is limited by light scattering. To study the effect of
scattering on optical-resolution photoacoustic microscopy (OR-PAM), we divided the signals in OR-PAM into two
classes: one is from the target volume defined by the optical resolution cell (Class I); the other is from the rest of the
acoustic resolution cell (Class II). We developed a way to simulate the point spread function (PSF) of our OR-PAM
system considering both optical illumination and acoustic detection, then used the PSF to calculate the contributions of
each class of signal to the total signal at different focal depths. Our simulation results showed that: 1) The Class II
signal decays much more slowly than the Class I signal; 2) The full width at half maximum (FWHM) of the PSF for the
focal depth of 0.9 transport mean free path (TMFP) is not broadened much (~10%) compared with that for a clear
medium; 3) Image contrast is degraded with increasing depth when there is a uniform absorption background.
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We have developed dichroism optical-resolution photoacoustic microscopy, capable of imaging polarization-dependent
optical absorption (i.e., dichroism) with excellent specificity. This technical innovation enriches molecular photoacoustic
contrasts and holds particular potential for detecting
amyloid-associated neurodegenerative and cardiovascular diseases.
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We have recently developed Transient Absorption Ultrasonic Microscopy (TAUM) as an ultrahigh-resolution
photoacoustic microscopy technique. The amplitude of the multiphoton pump-probe interaction is dependent on the
interpulse delay between the pump and probe pulses. Measuring the interpulse delay dependent TAUM amplitude maps
out the ground state recovery time of the chromophore. The ground state recovery time is a molecular signature that may
be used to differentiate multiple chromophores, analogous to fluorescence lifetime. We have used TAUM to measure the
ground state recovery time of Rhodamine 6G to be 3.65 ns, which matches well with known literature values. Whole
blood is also investigated, with measured ground state recovery times of 3.74 ns for oxygenated blood and 7.9 ns for
deoxygenated blood. The distinct difference in lifetimes for the oxidized and reduced forms suggests the feasibility of
subcellular SO2 images maps in future iterations of TAUM.
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Massive small bowel resection (SBR) results in villus angiogenesis and intestinal adaptation. The exact mechanism
that causes intestinal villus angiogenesis remains unknown. We hypothesize that hemodynamic changes within the
remnant bowel after SBR will trigger intestinal angiogenesis. To validate this, we used photoacoustic microscopy
(PAM) to image the microvascular system of the intestine in C57B6 mice and to measure blood flow and oxygen
saturation (sO2) of a supplying artery and vein. Baseline measurements were made 6 cm proximal to the ileal-cecal
junction (ICJ) prior to resection. A 50% proximal bowel resection was then performed, and measurements were
again recorded at the same location immediately, 1, 3 and 7 days following resection. The results show that arterial
and venous sO2 were similar prior to SBR. Immediately following SBR, the arterial and venous sO2 decreased by
14.3 ± 2.7% and 32.7 ± 6.6%, respectively, while the arterial and venous flow speed decreased by 62.9 ± 17.3% and
60.0 ± 20.1%, respectively. Such significant decreases in sO2 and blood flow indicate a hypoxic state after SBR.
Within one week after SBR, both sO2 and blood flow speed had gradually recovered. By 7 days after SBR, arterial
and venous sO2 had increased to 101.0 ± 2.9% and 82.7 ± 7.3% of the baseline values, respectively, while arterial
and venous flow speed had increased to 106.0 ± 21.4% and 150.0 ± 29.6% of the baseline values, respectively. Such
increases in sO2 and blood flow may result from angiogenesis following SBR.
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We have previously demonstrated that tyrosinase is a reporter gene for photoacoustic imaging since tyrosinase is the
rate-limiting step in the synthesis of melanin, a pigment capable of producing strong photoacoustic signals. We
previously created a cell line capable of inducible tyrosinase expression (important due to toxicity of melanin) by stably
transfecting tyrosinase in MCF-7 Tet-OnR cell line (Clontech) which expresses a doxycycline-controlled transactivator.
Unfortunately, Clontech provides few Tet-On Advanced cell lines making it difficult to have inducible tyrosinase
expression in cell lines not provided by Clontech. In order to simplify the creation of cell lines with inducible expression
of tyrosinase, we created a single plasmid that encodes both the transactivator as well as tyrosinase. PCR was used to
amplify both the transactivator and tyrosinase from the Tet-OnR Advanced and pTRE-Tight-TYR plasmids,
respectively. Both PCR products were cloned into the pEGFP-N1 plasmid and the newly created plasmid was transfected
into ZR-75-1, MCF-7, and MIA PaCa-1 cells using lipofectamine. After several days, brown melanin was only observed
in cells incubated with doxycycline, suggesting that the newly created single plasmid allowed inducible tyrosinase
expression in many different cells lines.
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Ultrasound array systems with conventional delay-and-sum beamformers are often adopted for high frame rate
photoacoustic imaging. In this report, we propose a coherence factor (CF) based imaging approach to further improve the
image contrast of such array systems by suppressing sidelobes of the acoustic diffraction pattern. Specifically, minimum
variance based coherence factor (CFMVDR) is used. Furthermore, because CF-based weighting is susceptible to
variations in signal-to-noise-ratio (SNR), we also adopt a Wiener filter approach to alleviate this problem so that the
method can perform well under all SNR conditions. This is of particular interest as the SNR in photoacoustic imaging is
typically low. To test this method, a human hair and a graphite phantom were used as test subjects. The imaging system
consisted of a 523nm pulsed laser, and a 128-channel linear array (bandwidth from 5 to 10MHz) for photoacoustic signal
detection. It is demonstrated that the beam widths (i.e., lateral resolution) can be effectively improved and the noise
background is suppressed by 20 dB. The contrast improvement is also evident.
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Laser thermal therapy involves heating tissue using light to temperatures between 55 °C and 95 °C for several minutes
resulting in coagulation and cell death. This treatment method has been under investigation for use as a minimally
invasive method for treating solid tumors and cancer cells. Heating tissues results in highly variable outcomes and
challenges; for example, ensuring complete coagulation of the target tissue while avoiding damage to surrounding
healthy tissues. Overcoming such challenges requires precise and real-time monitoring. Optoacoustic imaging has been
proposed as a real-time, noninvasive method for monitoring laser thermal. Ex-vivo porcine tenderloin samples were
heated using a 1000 μm core optical fiber coupled to an 810 nm diode laser at a constant power of 7 W for 10 minutes.
Lesions (6-7 mm diameter) were scanned using a prototype
reverse-mode optoacoustic system consisting of a pulsed
laser which operates at 1064 nm coupled to a bifurcated fibre bundle, and an 8 element annular array wideband
ultrasound transducer with a central frequency ~5 MHz. Scanning was done across native and coagulated tissue with an
energy of 6.5 mJ at a 1064 nm wavelength. Three lesions of similar size, shape and coagulation state were chosen for
analysis. Thermal coagulation effects were analyzed using optoacoustic signal amplitude and spectral analysis of the
optoacoustic RF data. Results show that the signal amplitude and the intercept and midband fit of the power spectrum
exhibit interesting differences between native and coagulated tissue states.
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An Interferometric Polymer Optical Waveguide Sensor (IPOWS) for optoacoustic signal detection has been fabricated
by UV-imprinting method. The sensor has been characterized in sensitivity, dynamic range and frequency bandwidth.
The noise equivalent pressure (NEP) of the sensor is around 100 Pa for a bandwidth range of 20 MHz. We have
compared experimentally the performance of the IPOWS with a piezoelectric ultra wideband sensor and other optical
fiber sensors based on single-mode silica and polymer optical fibers. All sensors are designed for the detection of
optoacoustic wave sources with a frequency bandwidth that exceed 10MHz.
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Gold nanorods (GNR) with a peak absorption wavelength of 760 nm were prepared using a seed-mediated method. A
novel protocol has been developed to replace hexadecyltrimethylammonium bromide (CTAB) on the surface of GNR
with 16-mercaptohexadecanoic acid (MHDA) and metoxy-poly(ethylene glycol)-thiol (PEG), and the monoclonal
antibodies: HER2 or CD33. The physical chemistry property of the conjugates was monitored through optical and zetapotential
measurements to confirm surface chemistry. The plasmon resonance is kept in the near infrared area, and
changes from strong positive charge for GNR-CTAB to slightly negative for GNR-PEG-mAb conjugates are observed.
The conjugates were investigated for different cells lines: breast cancer cells and human leukemia lines in vivo
applications. These results demonstrate successful tumor accumulation of our modified PEG-MHDA conjugates of GNR
for HER2/neu in both overexpressed breast tumors in nude mice, and for thermolysis of human leukemia cells in vitro.
The conjugates are non-toxic and can be used in pre-clinical applications, as well as molecular and optoacoustic imaging,
and quantitative sensing of biological substrates.
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We have applied optical-resolution photoacoustic microscopy (OR-PAM) for longitudinal monitoring of cerebral
metabolism through the intact skull of mice before, during, and up to 72 hours after a 1-hour transient middle cerebral
artery occlusion (tMCAO). The high spatial resolution of OR-PAM enabled us to develop vessel segmentation
techniques for segment-wise analysis of cerebrovascular responses.
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A new design of light illumination scheme for deep tissue photoacoustic (PA) imaging, a light catcher, is proposed and
evaluated by in silico simulation. Finite element (FE)-based numerical simulation model was developed for
photoacoustic (PA) imaging in soft tissues. In this in silico simulation using a commercially available FE simulation
package (COMSOL MultiphysicsTM, COMSOL Inc., USA), a short-pulsed laser point source (pulse length of 5 ns) was
placed in water on the tissue surface. Overall, four sets of simulation models were integrated together to describe the
physical principles of PA imaging. Light energy transmission through background tissues from the laser source to the
target tissue or contrast agent was described by diffusion equation. The absorption of light energy and its conversion to
heat by target tissue or contrast agent was modeled using bio-heat equation. The heat then causes the stress and strain
change, and the resulting displacement of the target surface produces acoustic pressure. The created wide-band acoustic
pressure will propagate through background tissues to the ultrasound detector, which is governed by acoustic wave
equation. Both optical and acoustical parameters in soft tissues such as scattering, absorption, and attenuation are
incorporated in tissue models. PA imaging performance with different design parameters of the laser source and energy
delivery scheme was investigated. The laser light illumination into the deep tissues can be significantly improved by up
to 134.8% increase of fluence rate by introducing a designed compact light catcher with highly reflecting inner surface
surrounding the light source. The optimized parameters through this simulation will guide the design of PA system for
deep tissue imaging, and help to form the base protocols of experimental evaluations in vitro and in vivo.
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High-energy and short-duration outputs from lasers are desirable to improve the photoacoustic image quality when
imaging deeply-seated lesions. In many clinical applications, optical fibers are used to couple the high-energy laser pulse
to tissue. These high peak intensity pulses can damage an optical fiber input face if the damage threshold is exceeded. It
is necessary to reduce the peak intensity to minimize the fiber damage and to delivery sufficient light for imaging. In this
paper, a laser-pulse-stretching technique is introduced to reduce the peak intensity of laser pulses. To demonstrate the
technique, an initial 17ns pulse was stretched to 37ns by a ring-cavity laser-pulse-stretching system, and the laser peak
power reduced to 42%. The stretched pulse increased the fiber damage threshold by 1.5-fold. Three ultrasound
transducers centered at 1.3MHz, 3.5MHz, 6MHz frequencies were simulated and the results showed that the
photoacoustic signal of 0.5mm-diameter target obtained with 37ns pulse was about 98%, 91% and 80% respectively
using the same energy as with the 17ns pulse. Simulations were validated using a broadband hydrophone. Quantitative
comparisons of photoacoustic images obtained with three corresponding ultrasound transducers showed that the image
quality was not affected by stretching the pulse.
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Traditional Photoacoustic tomography provides the distribution of absorbed optical
energy densities which are the products of the optical absorption coefficients and fluences.
However, the absorption coefficient is the only functional parameter that is related to disease
diagnosis, such as cancer. In this paper, we report the experimental investigation of an improved
fitting procedure which can quantitatively characterize optical absorption coefficients of multiple
targets. The original fitting procedure was proposed by us and used for a single target. The fitting
procedure included a complete photoacoustic forward model, which incorporated an analytical
model of light transport and a model of acoustic propagation. Using the target information from
the PAT images and the background information from diffuse optical measurements (DOM), the
fitting method minimizes the photoacoustic measurements and forward model data and recovers
the target absorption coefficient quantitatively. The fitting errors in the absorption coefficients
can reach 20% to 100% if the original fitting procedure is directly used on multiple targets. In
our improved fitting method, the ratio between the photoacoustic intensities is introduced and
served as extra input to the fitting procedure. As a result, the total number of unknown
parameters is reduced and fitting accuracy is improved. The hybrid system used in the
experiment combines a 64-channel photoacoustic system with a frequency-domain diffused
optical system. The experiment was performed in the reflection geometry suitable for breast
imaging. Phantom experiments include the combination of high contrast and low contrast targets
with absorption coefficients ranging from 0.07 to 0.28 cm-1 and with different spatial separations.
The phantoms were inserted into a chicken breast tissue. The fitting errors of multiple targets
were reduced to less than 20% for both high and low contrast targets. These results illustrate the
potential application of this quantitative DOM-assisted photoacoustic fitting procedure to image
and diagnose breast cancer having multiple and complex tumor distribution.
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Photoacoustic (PA) tomography (PAT) can image optical absorption contrast with ultrasonic spatial resolution in the
optical diffusive regime. Multi-wavelength PAT can noninvasively monitor hemoglobin oxygen saturation (sO2) with
high sensitivity and fine spatial resolution. However, accurate quantification in PAT requires knowledge of the optical
fluence distribution, acoustic wave attenuation, and detection system bandwidth. We propose a method to circumvent
this requirement using acoustic spectra of PA signals acquired at two optical wavelengths. With the acoustic spectral
method, the absorption coefficients of an oxygenated bovine blood phantom at 560 and 575 nm were quantified with
errors of ><5%.
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