KEYWORDS: Ultrasonography, Prostate, Prototyping, Tissues, Transducers, Imaging systems, Fluctuations and noise, Data acquisition, Ultrasound tomography, Signal to noise ratio
Plane-wave, fan-beam and wide-beam ultrasound can transmit higher ultrasound energy compared to synthetic-aperture ultrasound, leading to improved signal-to-noise ratios in ultrasound reflection/scattering signals. This is particularly useful for transrectal ultrasound imaging using end-firing transrectal ultrasound probes. We conduct a phantom study to evaluate the capabilities of plane-wave, fan-beam and wide-beam ultrasound for prostate imaging. The penetration depth decreases from plane-wave to fan-beam to wide-beam ultrasound, with increasing imaging areas. We use a transrectal ultrasound prototype consisting of a 256-channel Verasonics Vantage system and a GE intracavitary curved linear array to form plane-wave, fan-beam and wide-beam ultrasound. Our imaging results of a tissue-mimicking prostate phantom show that wide-beam ultrasound produces the best imaging among the three different beams of ultrasound when using the same number of ultrasound incident angles.
Ultrasound tomography is to reconstruct tissue mechanical properties using ultrasound signals for cancer characterization. We study the capability of plane-wave ultrasound-waveform inversion to reconstruct sound-speed values of prostate tumors. Our ultrasound-waveform inversion algorithm iteratively fits synthetic ultrasound waveforms with recorded ultrasound waveforms starting from an initial model. We verify the algorithm using synthetic ultrasound data for numerical prostate phantoms consisting of multiple tumors in homogeneous and heterogeneous background prostate tissues. Our reconstruction results demonstrate that our new plane-wave transrectal ultrasound-waveform tomography has the potential to accurately reconstruct the sound-speed values of prostate tumors for cancer characterization. In addition, we build a new transrectal ultrasound tomography prototype using a 256-channel Verasonics Vantage system and a GE intracavitary curved linear array to acquire plane-wave ultrasound reflection data for transrectal ultrasound tomography.
Minimum variance beamforming (MVBF) is an adaptive beamforming technique, which aims to improve the lateral resolution by computing and applying signal-dependent apodization rather than predetermined apodization as typically done in conventional delay-and-sum (DAS) beamforming. Although studies have shown that the improvement in lateral resolution associated with MVBF is significant, the axial resolution remains unaffected. In this work, we combine MVBF and spiking deconvolution to improve both lateral and axial resolutions in synthetic aperture ultrasound imaging. We implement our new method and evaluate its performance using experimental datasets from a tissue-mimicking phantom. Our results show that our new method yields improved axial and lateral resolutions as well as image contrast.
Ultrasound attenuation of breast tumors is related to their types and pathological states, and can be used to detect and characterize breast cancer. Particularly, ultrasound scattering attenuation can infer the margin properties of breast tumors. Ultrasound attenuation tomography quantitatively reconstructs the attenuation properties of the breast. Our synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays records both ultrasound reflection and transmission signals. We develop an ultrasound attenuation tomography method using ultrasound energy-scaled amplitude decays of ultrasound transmission signals and conduct ultrasound attenuation tomography using a known sound-speed model. We apply our ultrasound transmission attenuation tomography method to a breast phantom dataset, and compare the ultrasound attenuation tomography results with conventional beamforming ultrasound images obtained using reflection signals. We show that ultrasound transmission attenuation tomography complements beamforming images in identifying breast lesions.
Frequency-space prediction filtering (FXPF), also known as FX deconvolution, is a technique originally developed for random noise attenuation in seismic imaging. FXPF attempts to reduce random noise in seismic data by modeling only real signals that appear as linear or quasilinear events in the aperture domain. In medical ultrasound imaging, channel radio frequency (RF) signals from the main lobe appear as horizontal events after receive delays are applied while acoustic clutter signals from off-axis scatterers and electronic noise do not. Therefore, FXPF is suitable for preserving only the main-lobe signals and attenuating the unwanted contributions from clutter and random noise in medical ultrasound imaging. We adapt FXPF to ultrasound imaging, and evaluate its performance using simulated data sets from a point target and an anechoic cyst. Our simulation results show that using only 5 iterations of FXPF achieves contrast-to-noise ratio (CNR) improvements of 67 % in a simulated noise-free anechoic cyst and 228 % in a simulated anechoic cyst contaminated with random noise of 15 dB signal-to-noise ratio (SNR). Our findings suggest that ultrasound imaging with FXPF attenuates contributions from both acoustic clutter and random noise and therefore, FXPF has great potential to improve ultrasound image contrast for better visualization of important anatomical structures and detection of diseased conditions.
KEYWORDS: Ultrasonography, Ultrasound tomography, Breast, Transducers, Breast cancer, Data acquisition, Tomography, Algorithm development, Signal attenuation, Design for manufacturability, Prototyping, Mammography, In vivo imaging
Breast ultrasound tomography is an emerging imaging modality to reconstruct the sound speed, density, and ultrasound attenuation of the breast in addition to ultrasound reflection/beamforming images for breast cancer detection and characterization. We recently designed and manufactured a new synthetic-aperture breast ultrasound tomography prototype with two parallel transducer arrays consisting of a total of 768 transducer elements. The transducer arrays are translated vertically to scan the breast in a warm water tank from the chest wall/axillary region to the nipple region to acquire ultrasound transmission and reflection data for whole-breast ultrasound tomography imaging. The distance of these two ultrasound transducer arrays is adjustable for scanning breasts with different sizes. We use our breast ultrasound tomography prototype to acquire phantom and in vivo patient ultrasound data to study its feasibility for breast imaging. We apply our recently developed ultrasound imaging and tomography algorithms to ultrasound data acquired using our breast ultrasound tomography system. Our in vivo patient imaging results demonstrate that our breast ultrasound tomography can detect breast lesions shown on clinical ultrasound and mammographic images.
Ultrasound waveform tomography with the total-variation regularization could improve reconstructions of tumor margins,
but the reconstructions usually contain unwanted blocky artifacts. We develop a new ultrasound waveform tomography
method with a second-order total-generalized-variation regularization scheme to improve tomographic reconstructions
of breast tumors and remove blocky artifacts in reconstruction results. We validate our new method using numerical
phantom data and real phantom data acquired using our synthetic-aperture breast ultrasound tomography system with
two parallel transducer arrays. Compared to reconstructions of ultrasound waveform tomography with modified total-variation
regularization, our new ultrasound waveform tomography yields accurate sound-speed reconstruction results
with significantly reduced artifacts.
The sound-speed distribution of the breast can be used for characterizing breast tumors, because they typically have a higher sound speed than normal breast tissue. This is understood to be the result of remodeling of the extracellular matrix surrounding tumors. Breast sound-speed distribution can be reconstructed using ultrasound bent-ray tomography (USRT). We have recently demonstrated that USRT, using arrival times of both transmission and reflection data, significantly improves image quality. To further improve the robustness of tomographic reconstructions, we develop a USRT method using a modified total-variation (MTV) regularization scheme. Regularization is often used in solving inverse problems by introducing restrictions such as for smoothness. Tikhonov regularization is a widely used regularization scheme that tends to smooth tomographic images, but oversmoothing can obscure critical diagnostic detail such as tumor margins. Total-variation (TV) regularization is another common regularization scheme that preserves tumor margins, but at the cost of increased image noise. Our new USRT with MTV regularization is a Tikhonov-TV hybrid, reducing image noise while preserving margins. We apply our new method to ultrasound transmission data from numerical phantoms, and compare the results with those obtained using Tikhonov regularization.
KEYWORDS: Ultrasonography, Transducers, Breast, Ultrasound tomography, Imaging systems, Data acquisition, Tomography, Prototyping, Mammography, Breast cancer
Ultrasound tomography has great potential to provide quantitative estimations of physical properties of breast tumors for accurate characterization of breast cancer. We design and manufacture a new synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays. The distance of these two transducer arrays is adjustable for scanning breasts with different sizes. The ultrasound transducer arrays are translated vertically to scan the entire breast slice by slice and acquires ultrasound transmission and reflection data for whole-breast ultrasound imaging and tomographic reconstructions. We use the system to acquire patient data at the University of New Mexico Hospital for clinical studies. We present some preliminary imaging results of in vivo patient ultrasound data. Our preliminary clinical imaging results show promising of our breast ultrasound tomography system with two parallel transducer arrays for breast cancer imaging and characterization.
Ultrasound transmission tomography usually generates low-resolution breast images. We improve sound-speed reconstructions using ultrasound waveform tomography with both transmission and reflection data. We validate the improvement using computer-generated synthetic-aperture ultrasound transmission and reflection data for numerical breast phantoms. Our tomography results demonstrate that using both transmission and reflection data in ultrasound waveform tomography greatly enhances the resolution and accuracy of tomographic reconstructions compared to ultrasound waveform tomography using either transmission data or reflection data alone. To verify the capability of our novel ultrasound waveform tomography, we design and manufacture a new synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays for clinical studies. The distance of the two transducer arrays is adjustable for accommodating different sizes of the breast. The parallel transducer arrays also allow us to easily scan the axillary region to evaluate the status of axillary lymph nodes and detect breast cancer in the axillary region. However, synthetic-aperture ultrasound reflection data acquired by firing each transducer element sequentially are usually much weaker than transmission data, and have much lower signal-to-noise ratios than the latter. We develop a numerical virtual-point-source method to enhance ultrasound reflection data using synthetic-aperture ultrasound data acquired by firing each transducer element sequentially. Synthetic-aperture ultrasound reflection data for a breast phantom obtained using our numerical virtual-point-source method reveals many coherent ultrasound reflection waveforms that are weak or invisible in the original synthetic-aperture ultrasound data. Ultrasound waveform tomography using both transmission and reflection data together with numerical virtual-point-source method has great potential to produce high-resolution tomographic reconstructions in clinical studies of breast ultrasound tomography.
Ultrasound waveform tomography is a promising tool for breast cancer characterization. However, the method is very time-consuming for large datasets acquired using a synthetic-aperture ultrasound tomography system consisting of hundreds to thousands of transducer elements. We introduce a data blending approach to ultrasound waveform tomography to greatly improves the computational efficiency. This method simultaneously simulates ultrasound waves emitted from multiple transducer elements. A random phase is applied to each source to distinguish the effect of different sources. The random phase helps eliminate the unwanted cross interference produced by different sources. This approach greatly reduces the computation time of ultrasound waveform tomography to one tenth of that for the original ultrasound waveform tomography.
KEYWORDS: Breast, Ultrasound tomography, Ultrasonography, Transmitters, Tomography, Receivers, Ray tracing, Reconstruction algorithms, Transducers, Breast cancer
Breast ultrasound tomography is a promising imaging modality that has the potential to improve the diagnosis and screening of breast cancer. We develop a bent-ray ultrasound tomography algorithm to reconstruct sound-speed images of the breast. We investigate the acceleration of the algorithm using graphical processing units (GPUs). We adapt the algorithmic steps of ultrasound bent-ray tomography to a GPU cluster, and use multi-GPU scaling to speed up the computation. Our results show that it is very promising to use a GPU cluster with multiple GPU cards to achieve nearly real-time tomographic reconstruction.
Regularization is often needed in breast ultrasound waveform tomography to improve tomographic reconstructions. A global regularization parameter may lead to either over-regularization or under-regularization in different regions in the imaging domain. We develop a new ultrasound waveform tomography method with spatially-variant regularization. Our new method employs different regularization parameters in different regions of the breast so that each regularization parameter is optimal for the local region. Our numerical examples demonstrate the improvement of ultrasound waveform tomography using the spatially-variant modified total-variation regularization for sound-speed reconstructions of large and small breast tumors, particularly when their sizes are significantly different from one another.
Ultrasound bent-ray tomography can produce the sound-speed distribution of the breast for detection and diagnosis of breast cancer. However, the conventional ultrasound ray tomography uses only transmission data, leading to low-resolution images. We develop a new ultrasound bent-ray tomography technique using both transmission and reflection data to improve sound-speed reconstructions. We employ an ultrasound reflection imaging technique, Kirchhoff migration, to obtain the locations of reflectors for calculating arrival times of ultrasound refection signals. We use both first-arrival times (time-of-flights) of ultrasound transmission data and arrival times of ultrasound reflection data for sound-speed reconstructions. Our numerical studies show that our new ultrasound bent-ray tomography using both transmission and reflection data significantly improves the image resolution and sound-speed reconstructions compared to the conventional ultrasound ray tomography using only transmission data.
Early detection of breast cancer is the key to reducing the cancer mortality rate. With increasing computational power, waveform inversion becomes feasible for high-resolution ultrasound tomography. Because of the limited measurement geometry, ultrasound waveform tomography is usually ill-posed, which requires certain computational methodologies to stabilize waveform inversion. We develop a new ultrasound waveform tomography method using a modified total-variation regularization for detecting and characterizing small breast tumors. To solve the minimization problem, we use an alternating-minimization algorithm in which the original optimization is equivalently decomposed
into two simple subproblems. We use numerical breast-phantom data to demonstrate the improved capability of our new tomography method for accurately reconstructs the sound-speed values and shapes of small tumors.
Imaging breast microcalcifications is crucial for early detection and diagnosis of breast cancer. It is challenging for current clinical ultrasound to image breast microcalcifications. However, new imaging techniques using data acquired with a synthetic-aperture ultrasound system have the potential to significantly improve ultrasound imaging. We recently developed a super-resolution ultrasound imaging method termed the phase-coherent multiple-signal classification (PC-MUSIC). This signal subspace method accounts for the phase response of transducer elements to improve image resolution. In this paper, we investigate the clinical feasibility of our super-resolution ultrasound imaging method for detecting breast microcalcifications. We use our custom-built, real-time synthetic-aperture ultrasound system to acquire breast ultrasound data for 40 patients whose mammograms show the presence of breast microcalcifications. We apply our super-resolution ultrasound imaging method to the patient data, and produce clear images of breast calcifications. Our super-resolution ultrasound PC-MUSIC imaging with synthetic-aperture ultrasound data can provide a new imaging modality for detecting breast microcalcifications in clinic without using ionizing radiation.
Ultrasound waveform tomography using the conjugate gradient method produces images with different qualities in different regions of the imaging domain, partly because the ultrasound wave energy is dominant around transducer elements. In addition, this uneven distribution of the wave energy slows down the convergence of the inversion. Using the Hessian matrix to scale the gradients in waveform inversion can reduce the artifacts caused by the geometrical spreading and the defocusing effect resulting from the incomplete data coverage. However, it is computationally expensive to calculate the Hessian matrix. We develop a new ultrasound waveform tomography method that weights the gradient with the ultrasound wave energies of the forward and backward propagation wavefields. Our new method balances the wave energy distribution throughout the entire imaging domain. This method scales the gradients using the square root of the wave energy of forward propagated wavefields from sources and that of backpropagated synthetic wavefields from receivers. We numerically demonstrate that this new ultrasound waveform tomography method improves sound-speed reconstructions of breast tumors and accelerates the convergence of ultrasound waveform tomography.
Time-reversal with Multiple Signal Classification (TR-MUSIC) is an ultrasound imaging algorithm
for detecting small targets embedded in a medium. This technique can produce images with subwavelength
resolution when the targets are pointlike, and when the number of targets is fewer than
the number of transducer elements used to image the medium. In this experimental study, we evaluate
the performance of the TR-MUSIC algorithm when the interrogated medium contains extended
targets that cannot be considered as point scatterers. We construct tissue-mimicking phantoms embedded
with distributed glass spheres. We show that the quality of the phantom images obtained using
the TR-MUSIC algorithm decreases with increasing sphere size. However, significant improvement
is achieved when the image plane is divided into sub-regions, where each sub-region is imaged separately.
The windowed TR-MUSIC algorithm accurately locates the spheres (extended targets), but the
images do not provide quantitative information about the shape and reflectivity of the spheres.
Ultrasound could be an attractive imaging modality for detecting breast microcalcifications, but it requires significant
improvement in image resolution and quality. Recently, we have used tissue-equivalent phantoms to demonstrate that
synthetic-aperture ultrasound has the potential to detect small targets. In this paper, we study the in vivo imaging capability
of a real-time synthetic-aperture ultrasound system for detecting breast microcalcifications. This LANL's (Los Alamos
National Laboratory's) custom built synthetic-aperture ultrasound system has a maximum frame rate of 25 Hz, and is one
of the very first medical devices capable of acquiring synthetic-aperture ultrasound data and forming ultrasound images in
real time, making the synthetic-aperture ultrasound feasible for clinical applications. We recruit patients whose screening
mammograms show breast microcalcifications, and use LANL's synthetic-aperture ultrasound system to scan the regions
with microcalcifications. Our preliminary in vivo patient imaging results demonstrate that synthetic-aperture ultrasound is
a promising imaging modality for detecting breast microcalcifications.
Ultrasound waveform tomography takes wave propagation effects into account during image reconstruction,
and has the potential to produce accurate estimates of the sound speeds of small breast tumors.
However, waveform tomography is computationally time-consuming for large datasets acquired
using a synthetic-aperture ultrasound tomography system that consists of hundreds to thousands of
transducer elements. We introduce a source encoding approach to ultrasound waveform tomography
to significantly improve the computational efficiency. The method simultaneously simulates ultrasound
waveforms emitted from multiple transducer elements. To distinguish the effect of different
sources, we apply a random phase to each source. The random phase helps eliminate the unwanted
cross interferences produced by different sources. This approach greatly reduces the computational
time of ultrasound waveform tomography to one tenth of that for the original waveform tomography,
and makes it feasible for ultrasound waveform tomography in clinical applications.
Waveform tomography has the potential to quantitatively reconstruct the sound speed values of breast
tumors. It is difficult to obtain quantitative values of the sound speed of breast tumors when their
sizes are in the order of, or smaller than, the ultrasound wavelength. Because of the ill-posedness
of the full-waveform inversion, regularization techniques are usually used to improve reconstruction.
We develop an ultrasound waveform tomography method with the total-variation regularization to
improve sound-speed reconstructions of small breast tumors. Our numerical examples demonstrate
that our ultrasound waveform tomography with the total-variation regularization is a promising tool
for quantitative estimation of the sound speed of small breast tumors.
It is difficult for ultrasound to image small targets such as breast microcalcifications. Synthetic aperture ultrasound imaging
has recently developed as a promising tool to improve the capabilities of medical ultrasound. We use two different tissueequivalent
phantoms to study the imaging capabilities of a real-time synthetic aperture ultrasound system for imaging
small targets. The InnerVision ultrasound system DAS009 is an investigational system for real-time synthetic aperture
ultrasound imaging. We use the system to image the two phantoms, and compare the images with those obtained from
clinical scanners Acuson Sequoia 512 and Siemens S2000. Our results show that synthetic aperture ultrasound imaging
produces images with higher resolution and less image artifacts than Acuson Sequoia 512 and Siemens S2000. In addition,
we study the effects of sound speed on synthetic aperture ultrasound imaging and demonstrate that an accurate sound speed
is very important for imaging small targets.
KEYWORDS: Breast, Ultrasonography, Super resolution, Transducers, Image resolution, Mammography, Reflection, Diffraction, Chemical elements, In vivo imaging
Ultrasound image resolution and quality need to be significantly improved for breast microcalcification detection. Super-resolution
imaging with the factorization method has recently been developed as a promising tool to break through the
resolution limit of conventional imaging. In addition, wave-equation reflection imaging has become an effective method
to reduce image speckles by properly handling ultrasound scattering/diffraction from breast heterogeneities during image
reconstruction. We explore the capabilities of a novel super-resolution ultrasound imaging method and a wave-equation
reflection imaging scheme for detecting breast microcalcifications. Super-resolution imaging uses the singular value decomposition
and a factorization scheme to achieve an image resolution that is not possible for conventional ultrasound
imaging. Wave-equation reflection imaging employs a solution to the acoustic-wave equation in heterogeneous media
to backpropagate ultrasound scattering/diffraction waves to scatters and reconstruct images of heterogeneities. We construct
numerical breast phantoms using in vivo breast images, and use a finite-difference wave-equation scheme to generate
ultrasound data scattered from inclusions that mimic microcalcifications. We demonstrate that microcalcifications can
be detected at full spatial resolution using the super-resolution ultrasound imaging and wave-equation reflection imaging methods.
Ultrasound tomography is an attractive imaging method for the detection of breast cancer. The complex anatomy
of the breast with its different spatial scales and material property contrasts make accurate reconstructions very
challenging. This paper proposes a hybrid approach whereby Travel-of-Flight and Diffraction Tomography are combined together to achieve high-resolution and high-accuracy sound-speed reconstructions. The method is validated with several numerical phantoms.
Ultrasound is commonly used as an adjunct to mammography for diagnostic evaluation of suspicions arising
from breast cancer screening. As an alternative to conventional sonography that uses hand-held transducers,
toroidal array probes that encircle the breast immersed in a water bath have been investigated for ultrasound
tomography. This paper introduces a new method for three-dimensional synthetic aperture diffraction tomography
that maximizes the resolution in the scanning direction and provides quantitative reconstructions of the
acoustic properties of the object. The method is validated by means of numerical simulations.
KEYWORDS: Breast, Ultrasonography, Ultrasound tomography, Tomography, Tissues, In vivo imaging, Inverse problems, Reconstruction algorithms, Cancer, Breast cancer
Breast ultrasound tomography is a rapidly developing imaging modality that that has the potential to impact breast
cancer screening and diagnosis. A new ultrasound breast imaging device (CURE) with a ring array of transducers has
been designed and built at Karmanos Cancer Institute, which acquires both reflection and transmission ultrasound
signals. To extract the sound-speed information from the breast data acquired by CURE, we have developed an iterative
sound-speed image reconstruction algorithm for breast ultrasound transmission tomography based on total-variation
(TV) minimization. We investigate applicability of the TV tomography algorithm using in vivo ultrasound breast data
from 61 patients, and compare the results with those obtained using the Tikhonov regularization method. We
demonstrate that, compared to the Tikhonov regularization scheme, the TV regularization method significantly improves
image quality, resulting in sound-speed tomography images with sharp (preserved) edges of abnormalities and few
artifacts.
We report and discuss clinical breast imaging results obtained with operator independent ultrasound tomography. A
series of breast exams are carried out using a recently upgraded clinical prototype designed and built on the principles of
ultrasound tomography. The in-vivo performance of the prototype is assessed by imaging patients at the Karmanos
Cancer Institute. Our techniques successfully demonstrate in-vivo tomographic imaging of breast architecture in both
reflection and transmission imaging modes. These initial results indicate that operator-independent whole-breast imaging
and the detection of cancerous breast masses are feasible using ultrasound tomography techniques. This approach has
the potential to provide a low cost, non-invasive, and non-ionizing means of evaluating breast masses. Future work will
concentrate on extending these results to larger trials.
The development of ultrasound tomography for the detection of breast cancer could have a major impact on the
effectiveness of current diagnostic tools. Here, the potential of ultrasound tomography is investigated by means
of a new generation of toroidal ultrasound arrays that can measure both the signals reflected and transmitted
through human breast, simultaneously. Experiments performed on phantoms and human breast in vivo are
used to compare continuous wave (CW) insonification versus wideband (WB) excitation. It is shown that while
transmission diffraction tomography has little benefit from WB excitation, reflection tomography is greatly
improved due to the low signal-to-noise ratio of reflection measurements.
Ultrasound attenuation parameters of breast masses are closely related to their types and pathological states, therefore, it
is essential to reliably estimate attenuation parameters for quantitative breast tissue characterization. We study the
applicability of three different attenuation tomography methods for ultrasound breast imaging using a ring transducer
array. The first method uses the amplitude decays of signals transmitted through the breast to reconstruct attenuation
coefficients. The second method employs the spectral ratios between the pulse propagating through the breast and that
through water to obtain attenuation parameters. The third method makes use of the complex energy ratios estimated
using the amplitude envelopes of transmitted signals. We use in vitro and in vivo breast data acquired with a clinical
ultrasound breast imaging system (CURE) to compare these tomography methods. Our results show that the amplitude
decay method yields attenuation coefficients with more artifacts than the other two methods. There is bias and
variability in the estimated attenuation using the spectral ratio due to its sensitivity to different temporal band-widths and
signal-to-noise-ratios of the data. The method based on the complex signal energy ratio is more robust than the other
two methods and yields images with fewer artifacts.
Ultrasound reflection imaging is a promising imaging modality for detecting small, early-stage breast cancers. Properly
accounting for ultrasound scattering from heterogeneities within the breast is essential for high-resolution and high-quality
ultrasound breast imaging. We develop a globally optimized Fourier finite-difference method for ultrasound reflectivity
image reconstruction. It utilizes an optimized solution of acoustic-wave equation and a heterogeneous sound-speed distribution
of the breast obtained from tomography to reconstruct ultrasound reflectivity images. The method contains a
finite-difference term in addition to the split-step Fourier implementation, and minimizes ultrasound phase errors during
wavefield inward continuation while maintaining the advantage of high computational efficiency. The accuracy analysis indicates
that the optimized method is much more accurate than the split-step Fourier method. The computational efficiency
of the optimized method is one to two orders of magnitude faster than time-reversal imaging using a finite-difference
time-domain wave-equation scheme. Our new optimized method can accurately handle ultrasound scattering from breast
heterogeneities during reflectivity image reconstruction. Our numerical imaging examples demonstrate that the optimized
method has the potential to produce high-quality and high-resolution ultrasound reflectivity images in combination with a
reliable ultrasound sound-speed tomography method.
To improve clinical breast imaging, a new ultrasound tomography imaging device (CURE) has been built at the
Karmanos Cancer Institute. The ring array of the CURE device records ultrasound transmitted and reflected ultrasound
signals simultaneously. We develop a bent-ray tomography algorithm for reconstructing the sound-speed distribution of
the breast using time-of-flights of transmitted signals. We study the capability of the algorithm using a breast phantom
dataset and over 190 patients' data. Examples are presented to demonstrate the sound-speed reconstructions for different
breast types from fatty to dense on the BI-RADS categories 1-4. Our reconstructions show that the mean sound-speed
value increases from fatty to dense breasts: 1440.8 m/ s (fatty), 1451.9 m/ s (scattered), 1473.2 m/ s(heterogeneous), and 1505.25 m/ s (dense). This is an important clinical implication of our reconstruction. The mean
sound speed can be used for breast density analysis. In addition, the sound-speed reconstruction, in combination with
attenuation and reflectivity images, has the potential to improve breast-cancer diagnostic imaging. The breast is not
compressed and does not move during the ultrasound scan using the CURE device, stacking 2D slices of ultrasound
sound-speed tomography images forms a 3D volumetric view of the whole breast. The 3D image can also be projected
into a 2-D "ultrasound mammogram" to visually mimic X-ray mammogram without breast compression and ionizing
radiation.
KEYWORDS: Multiple scattering, Fermium, Frequency modulation, Scattering, Sensors, Signal to noise ratio, Ultrasound tomography, Image resolution, Distortion, Super resolution
Ultrasound tomography attempts to retrieve the structure of an object by exploiting the interaction of acoustic
waves with the object. A fundamental limit of ultrasound tomography is that features cannot be resolved if they
are spaced less than λ/2 apart, where λ is wavelength of the probing wave, regardless of the degree of accuracy of
the measurements. Therefore, since the attenuation of the probing wave with propagation distance increases as
λ decreases, resolution has to be traded against imaging depth. Recently, it has been shown that the λ/2 limit is
a consequence of the Born approximation (implicit in the imaging algorithms currently employed) which neglects
the distortion of the probing wavefield as it travels through the medium to be imaged. On the other hand, such a
distortion, which is due to the multiple scattering phenomenon, can encode unlimited resolution in the radiating
component of the scattered field. Previously, a resolution better than λ/3 has been reported in these proceedings
[F. Simonetti, pp. 126 (2006)] in the case of elastic wave probing. In this paper, we demonstrate experimentally
a resolution better than λ/4 for objects immersed in a water bath probed by means of a ring array which excites
and detects pressure waves in a full view configuration.
Waveform tomography results are presented from 800 kHz ultrasound transmission scans of a breast phantom, and from an in vivo ultrasound breast scan: significant improvements are demonstrated in resolution over time-of-flight reconstructions. Quantitative reconstructions of both sound-speed and inelastic attenuation are recovered. The data were acquired in the Computed Ultrasound Risk Evaluation (CURE) system, comprising a 20 cm diameter solid-state ultrasound ring array with 256 active, non-beamforming transducers.
Waveform tomography is capable of resolving variations in acoustic properties at sub-wavelength scales. This was verified through comparison of the breast phantom reconstructions with x-ray CT results: the final images resolve variations in sound speed with a spatial resolution close to 2 mm.
Waveform tomography overcomes the resolution limit of time-of-flight methods caused by finite frequency (diffraction) effects. The method is a combination of time-of-flight tomography, and 2-D acoustic waveform inversion of the transmission arrivals in ultrasonic data. For selected frequency components of the waveforms, a finite-difference simulation of the visco-acoustic wave equation is used to compute synthetic data in the current model, and the data residuals are formed by subtraction. The residuals are used in an iterative, gradient-based scheme to update the sound-speed and attenuation model to produce a reduced misfit to the data. Computational efficiency is achieved through the use of time-reversal of the data residuals to construct the model updates. Lower frequencies are used first, to establish the long wavelength components of the image, and higher frequencies are introduced later to provide increased resolution.
Ultrasonic reflection imaging has the potential to produce higher image resolution than transmission tomography, but
imaging resolution and quality still need to be further improved for early cancer detection and diagnosis. We present an
ultrasound reflection image reconstruction method using the split-step Fourier propagator. It is based on recursive inward
continuation of ultrasonic wavefields in the frequency-space and frequency-wavenumber domains. The inward continuation
within each extrapolation interval consists of two steps. In the first step, a phase-shift term is applied to the data in
the frequency-wavenumber domain for propagation in a reference medium. The second step consists of applying another
phase-shift term to data in the frequency-space domain to approximately compensate for ultrasonic scattering effects of
heterogeneities within the breast. We use synthetic ultrasound pulse-echo data recorded around a ring for heterogeneous,
computer-generated, numerical breast phantoms to study the imaging capability of the method. The phantoms are derived
from an experimental breast phantom and a sound-speed tomography image of an in-vivo ultrasound breast data collected
using a ring array. The heterogeneous sound-speed models used for pulse-echo imaging are obtained using a computationally
effcient, first-arrival-time (time-of-flight) transmission tomography method. Our studies demonstrate that reflection
image reconstruction using the split-step Fourier propagator with heterogeneous sound-speed models significantly improves
image quality and resolution. We also numerically verify the spatial sampling criterion of wavefields for a ring
transducer array.
Sound-speed tomography images can be used for cancer detection and diagnosis. Tumors have generally higher sound speeds than the surrounding tissue. Quality and resolution of tomography images are primarily determined by the insonification/illumination aperture of ultrasound and the capability of the tomography method for accurately handling heterogeneous nature of the breast. We investigate the capability of an efficient time-of-flight tomography method using transmission ultrasound from a ring array for reconstructing sound-speed images of the breast. The method uses first-arrival times of transmitted ultrasonic signals emerging from non-beamforming ultrasound transducers located around a ring. It properly accounts for ray bending within the breast by solving the eikonal equation using a finite-difference scheme. We test and validate the time-of-flight transmission tomography method using synthetic data for numerical breast phantoms containing various objects. In our simulation, the objects are immersed in water within a ring array. Two-dimensional synthetic data are generated using a finite-difference scheme to solve acoustic-wave equation in heterogeneous media. We study the reconstruction accuracy of the tomography method for objects with different sizes and shapes as well as different perturbations from the surrounding medium. In addition, we also address some specific data processing issues related to the tomography. Our tomography results demonstrate that the first-arrival transmission tomography method can accurately reconstruct objects larger than approximately five wavelengths of the incident ultrasound using a ring array.
Ultrasonic imaging has the potential to enhance our capability to detect and diagnose breast cancers, but its imaging quality and resolution need to be significantly improved. We make use of the principle of the time-reversal mirror to develop an image-reconstruction method for ultrasonic breast imaging. It reconstructs images of scatterers (e.g., tumors) that generate/scatter ultrasonic waves by backpropagating measured ultrasonic signals into a heterogeneous breast model on computers using the principle of time-reversal mirror. We use solutions of the (two-way) full wave equation and one-way wave equation in heterogeneous media for backpropagation. We found that the one-way wave-equation-based imaging method can produce higher-resolution images than the two-way propagation-based imaging method when the data acquisition aperture is limited (for a linear transducer array). With a full aperture, our imaging results demonstrate that imaging with time-reversed ultrasound can produce high-quality images of the breast.
Reflection imaging has the potential to produce higher-resolution breast images than transmission tomography; however, the current clinical reflection imaging technique yields poor-quality breast images due to speckle. We present a new ultrasonic breast imaging method for obtaining high-resolution and clear breast images using ultrasonic reflection data acquired by a new ultrasonic scanning device that provides a better illumination of targets of interest than the clinical B-scan. The new imaging method is based on the solution of the wave equation in Cartesian coordinates and is implemented using Fast Fourier Transform algorithms. We apply the new ultrasonic breast imaging method to two ultrasonic data sets obtained using an experimental ultrasound scanner recently developed by the Karmanos Cancer Institute. One data set was acquired for a "cyst" phantom using 360 transmitter positions and 321 receiver positions along a 20-cm diameter ring. Another data set was collected with 180 transmitter positions and 1601 receiver positions along a 30-cm diameter ring with the breast specimen located at the center of the ring. We report on the breast imaging results for these two data sets using the new breast imaging method. The results demonstrate that the wave-equation-based ultrasonic breast imaging has the potential to produce high-resolution breast images.
Two efficient Fourier migration methods termed the extended local Born Fourier (ELBF) method and the extended Rytov Fourier (ELRF) method have been developed recently for imaging complex 3D structures. They are recursive methods based on local applications of Born and Rytov approximations within each extrapolation interval. The ELBF method becomes unreliable when the lateral slowness variations are large and/or the frequency is high, while the ELRF method is reliable for such cases. However, the ELRF method is approximately 30-40% slower than the ELBF method because the ELRF method requires one more computational step where exponentials of complex numbers are calculated than the ELBF method and propose an implementation scheme using variable extrapolation intervals to make the ELBF method reliable for all lateral slowness variations and frequencies. The size of the extrapolation interval depends on the lateral slowness variations within a given extrapolation region and the frequency, and consequently, the computational time of the ELBF method with variable extrapolation intervals increases with the lateral slowness variation and frequency. To take advantage of the faster computational speed of the ELBF method compared to the ELRF method and the better stability of the ELRF method compared to the ELBF method, we propose a hybrid local Born/Rytov Fourier migration method. In the hybrid method, the ELBF method is used for regions with small lateral slowness variations and/or low frequencies, otherwise, the ELRF method is used. Migrations of two synthetic datasets for complex structures using the ELBF method with variable extrapolation intervals and the hybrid method demonstrate that the quality of images obtained using these two methods is comparable to that of images obtained using the ELRF method. Comparison of computational times for migrations using different methods shows that the ELBF method with variable extrapolation intervals takes much more computational time than the ELRF method but the hybrid method saves more than 10% of the computational time required by the ELRF method.
Strongly heterogeneous media such as those with fine layers and/or aligned heterogeneities and/or empty pores can pose problems for classical finite-difference methods to simulate wave propagation. This is due to the difficulties of handling sharp interfaces in these media and total reflections from boundaries of empty pores in a porous medium. The phononic lattice solid by interpolation (PLSI) is a microscopic approach to P wave propagation in strongly heterogeneous media. The method is capable of handling sharp interfaces and, therefore, provides a powerful tool to simulate wave propagation in such media. Numerical simulations by the PLSI to simulate P wave propagation in these media are presented. Anisotropy induced by fine layers and aligned heterogeneities is observed. Numerical results demonstrate that the scattering effect of empty pores is much stronger than non-empty heterogeneities. Ultimately, the approach could enable numerical experiments to be conducted to study the microscopic mechanisms responsible for anisotropy and attenuation of seismic waves. This would require the approach to be extended to the elastic case.
KEYWORDS: 3D modeling, Acoustics, Wave propagation, Receivers, Data modeling, 3D image processing, Fourier transforms, Ray tracing, Stereoscopy, Interfaces
The acoustic pseudo-screen propagator is a kind of one-way wave propagator implemented in the wavenumber-space domain (dual-domain). It takes into account lateral velocity and density variations. We have recently used it for modeling primary reflected waves. In this paper, we use it as recursive downward wave extrapolators to develop a 3-D prestack depth migration method for common-shot data. In the method, the transversal Laplacian operators in the acoustic wave equation for heterogeneous media are applied exactly. The method has fast computational speed due to the use of fast Fourier transform algorithm and capability of handling wide-angle downward wave propagation/backpropagation in laterally heterogeneous media. Another main advantage of the method is huge memory saving relative to finite-difference or ray tracing based methods. Therefore, the 3-D acoustic pseudo-screen prestack depth migration method may enable 3-D prestack migration for large real 3-D data sets to be implemented within a reasonable CPU time on a supercomputer. Numerical examples of migrating synthetic reflected data generated by a finite-difference algorithm for 2-D and 3-D models are presented to demonstrate the feasibility of the method.
A fast modeling method based on multiple-forescattering single-backscattering (MFSB) approximation, i.e. the De Wolf approximation for calculating reflected (or backscattered) wave fields in 3D heterogeneous acoustic media is introduced. The method is much faster than full wave finite difference or finite element methods. The formulation is especially suitable for the configuration of surface reflection surveying. When discontinuities in a medium are not very sharp or parameter perturbations of heterogeneities are not very strong, reverberations between heterogeneities or resonance scattering can be neglected. However, for large volume heterogeneous media or long propagation distances the accumulated effect of multiple forward scattering becomes very important for both forward modeling and inverse problems. In such cases, the Born approximation is not valid while the De Wolf approximation can be applied. After renormalizing the multiple scattering series of the Lipmann-Schwinger equation, a MFSB approximation for acoustic waves is derived and a fast dual-domain computation scheme is presented, in which the multi-screen one-way wave propagator is used. Finally numerical examples are given to demonstrate the validity of the method.
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