We present a new-generation optical-resolution confocal photoacoustic microscope, consisting of a 0.25-numerical aperture optical microscope objective and a 75-MHz center-frequency spherically focused ultrasonic transducer. Experiments verified that this microscope has a transverse resolution of 2 µm, which is the highest to our knowledge among all photoacoustic imaging systems. In situ imaging of mouse ears shows the feasibility of resolving individual red blood cells in microvessels using the current system.
We show that it is possible to obtain high optical contrast photoacoustic images of tissue with 0.55 μm transverse
resolution. To achieve high sensitivity, we used a high NA (0.85), 125 MHz spherically focused ultrasonic transducer in
a confocal arrangement with a high resolution optical objective (NA=0.6). Laser pulses of a few nJ in pulse energy with
durations of 1.5 ns at a 20 KHz pulse repetition rate were used to generate photoacoustic waves. Although the
penetration depth is limited to hundreds of microns by both optical scattering and ultrasonic absorption, the developed
technique can compete with optical microscopy, for example, in quantitative spectral measurements, in microcirculation
research, or in nanoparticle detection.
We developed a novel concept of using a negative acoustic lens to increase the acceptance angle of an unfocused large-area ultrasonic transducer (detector), leading to more than twofold improvement of the tangential resolution in both thermoacoustic and photoacoustic tomography. In both thermoacoustic and photoacoustic tomography, for a given transducer bandwidth, the aperture size of the detector affects the tangential resolution greatly when the object of interest is near the detector surface. We were able to overcome such tangential resolution deterioration by attaching an acoustic concave lens, made of acrylic in front of the flat detector surface. We then quantified the tangential resolution improvement using phantom images. We also showed that the use of the negative lens preserves the shape of an object after the image is reconstructed.
We report here a novel breast cancer scanner using microwave and light excitation and ultrasound detection. This
combined thermoacoustic and photoacoustic tomography scanner is a nonionizing low cost system that can potentially
provide high-resolution, dual contrast (microwave and light absorption) three dimensional images of the breast. Front
breast compression will be used in this scanner to alleviate patient discomfort, experienced in side breast compression
during traditional X-ray mammography. This scanner will use dry instead of gel ultrasonic coupling. We have also
developed a carbon nanotube-based contrast agent for both thermoacoustic and photoacoustic imaging. In the future,
targeted molecular photoacoustic and thermoacoustic imaging should be possible using this contrast agent.
Photoacoustic imaging and optical coherence tomography have complementary imaging contrasts.
Photoacoustic imaging is sensitive to optical absorption, thus is able to generate detailed maps of deep
microvasculature in vivo. Optical coherence tomography exploits the optical scattering contrast, and can
provide real-time, micrometer-resolution imaging of tissue. We integrate an optical-resolution
photoacoustic microscopy and a spectral-domain optical coherence tomography into a single system. Our
preliminary experiments showed that it could be a valuable imaging tool for microcirculation studies in
vivo.
Because the wavelengths of radio-frequency (RF) waves used in thermoacoustic tomography (TAT) are comparable
with the size of detected objects, RF diffraction plays important roles in TAT. The RF diffraction affects
not only the global distribution of the RF field in the tissue, but also local RF energy deposition. In this paper,
we discussed these two major effects. Both numerical simulations and phantom experiments are done to demonstrate
these phenomena. We also provide a partial correction method for the image distortion and a calibration
algorithm for image calibration.
Photoacoustic tomography (PAT), also referred to optoacoustic tomography, is a hybrid imaging technique that
combines an optical contrast mechanism and ultrasonic detection principles. The laser-induced photoacoustic
signals in PAT are broadband in nature, but only a bandpass approximation of the signal is recorded by use
of a conventional ultrasonic transducers due to its limited bandwidth. To circumvent this, a PAT system
has been developed that records photoacoustic signals by use of multiple ultrasonic transducers that possess
different central frequencies. In this work, we investigate a sensor fusion methodology for combining the multiple
measurements to obtain an estimate of the true photoacoustic signal that is superior to that obtainable by use
of any single transducer measurement. From the estimated photoacoustic signals, three-dimensional images of
the optical absorption distribution are reconstructed and are found to possess improved accuracy and statistical
properties as compared to the single transducer case. Preliminary computer-simulation studies are presented to
demonstrate and investigate the proposed method.
KEYWORDS: Transducers, Acoustics, Acquisition tracking and pointing, Signal detection, Ultrasonography, Photoacoustic tomography, Signal generators, Reconstruction algorithms, Signal to noise ratio, Image quality
Although a small point ultrasound transducer has a wide acceptance
angle, its signal-to-noise (SNR) is low due to the high
thermal-noise-induced electric voltages in the transducer, which is
a result of its small active area. By contrast, a finite size flat
transducer has high sensitivity (good SNR), but the acceptance angle
is generally small, which limits its application in
reconstruction-based photoacoustic tomography (PAT). In this paper,
we report a negative lens concept to increase the acceptance angle
for a flat transducer. We also provide phantom experiments that
demonstrate this concept can greatly increase the detection region
for PAT and without losing sensitivity.
In this paper, we present new Adaptive and Robust Techniques (ART) for microwave-based thermoacoustic
tomography (TAT) and laser-based photo-acoustic tomography (PAT), and study their performances for breast
cancer detection. TAT and PAT are emerging medical imaging techniques that combine the merits of high
contrast due to electromagnetic or laser stimulation and high resolution offered by thermal acoustic imaging.
The current image reconstruction methods used for TAT and PAT, such as the widely used Delay-and-Sum
(DAS) approach, are data-independent and suffer from low resolution, high sidelobe levels, and poor interference
rejection capabilities. The data-adaptive ART can have much better resolution and much better interference
rejection capabilities than their data-independent counterparts. By allowing certain uncertainties, ART can
be used to mitigate the amplitude and phase distortion problems encountered in TAT and PAT. Specifically,
in the first step of ART, RCB is used for waveform estimation by treating the amplitude distortion with an
uncertainty parameter. In the second step of ART, a simple yet effective peak searching method is used for phase
distortion correction. Compared with other energy or amplitude based response intensity estimation methods,
peak searching can be used to improve image quality with little additional computational costs. Moreover, since
the acoustic pulse is usually bipolar: a positive peak, corresponding to the compression pulse, and a negative
peak, corresponding to the rarefaction pulse, we can further enhance the image contrast in TAT or PAT by using
the peak-to-peak difference as the response intensity for a focal point. The excellent performance of ART is
demonstrated using both simulated and experimentally measured data.
The absorption of electromagnetic energy causes thermal expansion and induces acoustic waves in biological tissues. Various tissues present particular characteristics in their absorption spectra. To acquire both photoacoustic and thermoacoustic images with multiple contrasts that reflect the absorption of electromagnetic energy, biological tissues are stimulated using laser and microwave pulses, respectively. Muscles with a rich blood supply strongly absorb green optical radiation, which provides excellent optical contrast. High water content tissues, such as connective tissue and muscle tissue, display high contrast to fatty tissues when imaged using microwave radiation. Most cancerous tissues have higher water and ionic concentrations, two characteristics that are also associated with angiogenesis and hemoglobin oxygen saturation. Therefore, cancer diagnosis based on information from tissue properties over an electromagnetic spectrum from microwave to optical bands can be more accurate than was previously available.
KEYWORDS: Tumors, Molecular imaging, Spectroscopy, Brain, Imaging spectroscopy, Acquisition tracking and pointing, In vivo imaging, Photoacoustic spectroscopy, Neuroimaging, Near infrared
Molecular imaging is a newly emerging field in which the modern tools of molecular and cell biology have been
married to state-of-the-art technologies for noninvasive imaging. The study of molecular imaging will lead to better
methods for understanding biological processes as well as diagnosing and managing disease. Here we present
noninvasive in vivo spectroscopic photoacoustic tomography (PAT)-based molecular imaging of αvβ3 integrin in a
nude mouse U87 brain tumor. PAT combines high optical absorption contrast and high ultrasonic resolution by
employing short laser pulses to generate acoustic waves in biological tissues through thermoelastic expansion.
Spectroscopic PAT-based molecular imaging offers the separation of the contributions from different absorbers based
on the differences in optical absorption spectra among those absorbers. In our case, in the near infrared (NIR) range,
oxy-heamoglobin (O2Hb), deoxy-heamoglobin (HHb) and the injected αvβ3-targeted peptide-ICG conjugated NIR
fluorescent contrast agent are the three main absorbers. Therefore, with the excitation by multiple wavelength laser
pulses, spectroscopic PAT-based molecular imaging not only provides the level of the contrast agent accumulation in
the U87 glioblastoma tumor, which is related to the metabolism and angiogenesis of the tumor, but also offers the
information on tumor angiogenesis and tumor hypoxia.
Simultaneous transcranial imaging of two functional parameters, the total concentration of hemoglobin and the hemoglobin oxygen saturation, in the rat brain in vivo is realized noninvasively using laser-based photoacoustic tomography (PAT). As in optical diffusion spectroscopy, PAT can assess the optical absorption of endogenous chromophores, e.g., oxygenated and deoxygenated hemoglobins, at multiple optical wavelengths. However, PAT can provide high spatial resolution because its resolution is diffraction-limited by photoacoustic signals rather than by optical diffusion. Laser pulses at two wavelengths are used sequentially to acquire photoacoustic images of the vasculature in the cerebral cortex of a rat brain through the intact skin and skull. The distributions of blood volume and blood oxygenation in the cerebral cortical venous vessels, altered by systemic physiological modulations including hyperoxia, normoxia, and hypoxia, are visualized successfully with satisfactory spatial resolution. This technique, with its prominent sensitivity to endogenous contrast, can potentially contribute to the understanding of the interrelationship between neural, hemodynamic, and metabolic activities in the brain.
KEYWORDS: Tumors, Luminescence, Acquisition tracking and pointing, In vivo imaging, Brain, Photoacoustic tomography, Neuroimaging, Near infrared, Head, Signal detection
We present a dual modality imaging technique by combining photoacoustic tomography (PAT) and near-infrared (NIR) fluorescence imaging for the study of animal model tumors. PAT provides high-resolution structural images of tumor angiogenesis, and fluorescence imaging offers high sensitivity to molecular probes for tumor detection. Coregistration of the PAT and fluorescence images was performed on nude mice with M21 human melanoma cell lines with αvβ3 integrin expression. An integrin αvβ3-targeted peptide-ICG conjugated NIR fluorescent contrast agent was used as the molecular probe for tumor detection. PAT was employed to noninvasively image the brain structures and the angiogenesis associated with tumors in nude mice. Coregistration of the PAT and fluorescence images was used in this study to visualize tumor location, angiogenesis, and brain structure simultaneously.
Based on the multi-wavelength laser-based photoacoustic tomography, non-invasive in vivo imaging of functional parameters, including the hemoglobin oxygen saturation and the total concentration of hemoglobin, in small-animal brains was realized. The high sensitivity of this technique is based on the spectroscopic differences between oxy- and deoxy-hemoglobin while its spatial resolution is bandwidth-limited by the photoacoustic signals rather than by the optical diffusion as in optical imaging. The point-by-point distributions of blood oxygenation and blood volume in the cerebral cortical venous vessels, altered by systemic physiological modulations including hyperoxia, normoxia and hypoxia, were visualized successfully through the intact skin and skull. This technique, with its prominent intrinsic advantages, can potentially accelerate the progress in neuroscience and provide important new insights into cerebrovascular physiology and brain function that are of great significance to the neuroscience community.
Thermoacoustic tomography (TAT) is an ultrasound-mediated biophotonic imaging modality with great potential for a wide range of
biomedical imaging applications. In this work, we demonstrate that half-time reconstruction approaches for TAT can mitigate image artifacts due to heterogeneous acoustic properties of an object.
We also discuss how half-time reconstruction approaches permit
explicit control of statistically complementary information in the measurement data, which can facilitate the reduction of image variances.
We report a preliminary study of breast cancer imaging by microwave-induced thermoacoustic tomography. In this study, we built a prototype of breast cancer imager based on a circular scan mode. A 3-GHz 0.3~0.5-μs microwave is used as the excitation energy source. A 2.25-MHz ultrasound transducer scans the thermoacoustic signals. All the measured data is transferred to a personal computer for imaging based on our proposed back-projection reconstruction algorithms. We quantified the line spread function of the imaging system. It shows the spatial resolution of our experimental system reaches 0.5 mm. After phantom experiments demonstrated the principle of this technique, we moved the imaging system to the University of Texas MD Anderson Cancer Center to image the excised breast cancer specimens. After the surgery performed by the physicians at the Cancer Center, the excised breast specimen was placed in a plastic cylindrical container with a diameter of 10 cm; and it was then imaged by three imaging modalities: radiograph, ultrasound and thermoacoustic imaging. Four excised breast specimens have been tested. The tumor regions have been clearly located. This preliminary study demonstrated the potential of microwave-induced thermoacoustic tomography for applications in breast cancer imaging.
Photoacoustic tomography (PAT) in a circular scanning configuration was developed to image the deeply embedded optical heterogeneity in biological tissues. Based on the intrinsic contrast between blood and chicken breast muscle, an embedded blood object that was 5 cm deep in the tissue was detected using pulsed laser light at a wavelength of 1064 nm. Compared with detectors for flat active surfaces, cylindrically focused ultrasonic transducers can reduce the interference generated from the off-plane photoacoustic sources and make the image in the scanning plane clearer. While the optical penetration was optimized with near-infrared laser pulses of 800 nm in wavelength, the optical contrast was enhanced by indocyanine green (ICG) whose absorption peak matched the laser wavelength. This optimized PAT was able to image fine objects embedded at a depth of up to 5.2-cm, which is 6.2 times the 1/e optical penetration depth, in chicken breast muscle, at a resolution of < ~750 microns with a sensitivity of <7 pmol of ICG in blood. The resolution was found to deteriorate slowly with increasing imaging depth.
Photoacoustic tomography employs short laser pulses to generate acoustic waves. The photoacoustic image of a test sample can be reconstructed using the detected acoustic signals. The reconstructed image is characterized by the convolution of the sample structure in optical absorption, the laser pulse, and the impulse response of the ultrasonic transducer used for detection. Although laser-induced ultrasonic waves cover a wide spectral range, a single transducer can receive only part of the spectrum because of its limited bandwidth. To systematically analyze this problem, we constructed a photoacoustic tomographic system that uses multiple ultrasonic transducers, each at a different central frequency, to simultaneously receive the induced acoustic waves. The photoacoustic images associated with the different transducers were compared and analyzed. The system was used to detect the vascular system of the rat brain. The vascular vessels in the brain cortex were revealed by all of the transducers, but the image resolutions differed. The higher frequency detectors with wider bandwidths provided better image resolution.
Since optical contrast is sensitive to functional parameters, including the hemoglobin oxygen saturation and the total concentration of hemoglobin, imaging based on optical contrast has been widely employed for the real-time monitoring of tissue oxygen consumption and hemodynamics in biological tissues. However, due to the overwhelming scattering of light in tissues, traditional optical imaging modalities cannot provide satisfactory spatial resolution. Functional photoacoustic tomography is a novel technique that combines high optical contrast and high ultrasonic resolution. Here, we present our study of a laser-based photoacoustic technique that, for the first time to our knowledge, monitors blood oxygenation in the rat brain in vivo. The cerebral blood oxygenation in the rat brain was imaged by photoacoustic detection at two wavelengths. The change in the hemoglobin oxygen saturation in the brain vessels as a result of the alternation from hyperoxia status to hypoxia status was visualized successfully with satisfactory spatial resolution. This work demonstrates that photoacoustic technique, based on the spectroscopic absorption of oxy- and deoxy-hemoglobin, can provide accurate functional imaging of cerebral blood oxygenation in the small-animal brain non-invasively with the skin and skull intact.
Optical contrast agents, such as indocyanine dyes, nano-particles and their functional derivatives, have been widely applied to enhance the sensitivity and specificity of optical imaging. However, due to the overwhelming scattering of light in biological tissues, the spatial resolution of traditional optical imaging degrades drastically as the imaging depth increases. For the first time to our knowledge, non-invasive in vivo photoacoustic imaging of an optical contrast agent, distributed in the rat brain, was implemented with near-infrared light. Injection of indocyanine green polyethylene glycol, a contrast agent with a high absorption at the 805-nm wavelength, into the circulatory system of a rat enhanced the absorption contrast between the blood vessels and the background brain tissues. Because near-infrared light can penetrate deep into the brain tissues through the skin and skull, we were able to successfully reconstruct the vascular distribution in the rat brain from the detected photoacoustic signals. The dynamic concentration of this contrast agent in the brain blood after the intravenous injection was also studied. This work proved that the distribution of an exogenous contrast agent in biological tissues can be imaged clearly and accurately by photoacoustic tomography. This new technology has high potential for application in dynamic and molecular medical imaging.
Scanning thermoacoustic tomography based on microwave- induced thermoacoustic waves was studied. 2D images of approximately 50-mm thick biological tissue samples were obtained experimentally. The thermoacoustic signals were also simulated theoretically. The image resolution was significantly improved compared with purely microwave imaging.
Two imaging techniques combining ultrasound and light are reviewed. The motivation is to combine the advantages of optical information and acoustic imaging resolution. The first technique is sonoluminescence tomography, where a sonoluminescence signal generated internally in the media by continuous-wave ultrasound is used. 2D images can be produced for objects embedded in turbid media by raster scanning the media. The second technique is ultrasound-modulated optical tomography, where a frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. Multiple 1D images obtained at various positions perpendicular to the ultrasonic axis were composed to obtain a 2D tomographic image of the medium.
A novel chirped ultrasound-modulated optical tomography technique was developed to image turbid media. Frequency analysis was employed to obtain spatial resolution along the ultrasonic axis. 2D images from scattering medium were obtained. The chirped ultrasound modulated signal was detected in chicken breast tissue.
Two imaging techniques combining ultrasound and light are reviewed. The motivation is to combine the advantages of optical information and acoustic imaging resolution. The first technique is sonoluminescence tomography, where a sonoluminescence signal generated internally in the media by continuous-wave ultrasound is used. Two-dimensional images can be produced for objects embedded in turbid media by raster scanning the media. The second technique is ultrasound- modulated optical tomography, where a frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. Multiple 1D images obtained at various positions perpendicular to the ultrasonic axis were composed to obtain a 2D tomographic image of the medium.
The rule of the distortion introduced by pressure and thermal stresses in the active cooling mirror were proved experimentally. In order to minimize the distortion of a mirror the active cooing mirror should be consisted of a thin faceplate layer, a good heat exchanger layer and a hard rigidity substrate. According to this requirement of a superthin multilayer mirror were designed. Its maximum distortion was only 0.10 micrometers , when the 8.69kw laser beam was incident on the surface of mirror and the coolant pressure was 0.13MPa.
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