Ultrasound computed tomography (USCT) holds great promise for improving the detection and management of breast cancer. Because they are based on the acoustic wave equation, waveform inversion-based reconstruction methods can produce images that possess improved spatial resolution properties over those produced by ray-based methods. However, waveform inversion methods are computationally demanding and have not been applied widely in USCT breast imaging. In this work, source encoding concepts are employed to develop an accelerated USCT reconstruction method that circumvents the large computational burden of conventional waveform inversion methods. This method, referred to as the waveform inversion with source encoding (WISE) method, encodes the measurement data using a random encoding vector and determines an estimate of the sound speed distribution by solving a stochastic optimization problem by use of a stochastic gradient descent algorithm. Both computer simulation and experimental phantom studies are conducted to demonstrate the use of the WISE method. The results suggest that the WISE method maintains the high spatial resolution of waveform inversion methods while significantly reducing the computational burden.

/pdf/waveform-inversion-with-source-encoding-for-breast-sound-c2tignlcgw.pdf

Waveform inversion with source encoding for breast sound speed reconstruction in ultrasound computed tomography

Application of the frequency domain acoustic wave equation on data acquired from ultrasound tomography scans is shown to yield high resolution sound speed images on the order of the wavelength of the highest reconstructed frequency. Using a signal bandwidth of 0.4-1 MHz and an average sound speed of 1500 m s(-1), the resolution is approximately 1.5 mm. The quantitative sound speed values and morphology provided by these images have the potential to inform diagnosis and classification of breast disease. In this study, we present the formalism, practical application, and in vivo results of waveform tomography applied to breast data gathered by two different ultrasound tomography scanners that utilize ring transducers. The formalism includes a review of frequency domain modeling of the wave equation using finite difference operators as well as a review of the gradient descent method for the iterative reconstruction scheme. It is shown that the practical application of waveform tomography requires an accurate starting model, careful data processing, and a method to gradually incorporate higher frequency information into the sound speed reconstruction. Following these steps resulted in high resolution quantitative sound speed images of the breast. These images show marked improvement relative to commonly used ray tomography reconstruction methods. The robustness of the method is demonstrated by obtaining similar results from two different ultrasound tomography devices. We also compare our method to MRI to demonstrate concordant findings. The clinical data used in this work was obtained from a HIPAA compliant clinical study (IRB 040912M1F).

/pdf/frequency-domain-ultrasound-waveform-tomography-breast-6oio2dzfd5.pdf

Frequency domain ultrasound waveform tomography: breast imaging using a ring transducer.

Photoacoustic computed tomography (PACT) and ultrasound computed tomography (USCT) are emerging modalities for breast imaging. As in all emerging imaging technologies, computer-simulation studies play a critically important role in developing and optimizing the designs of hardware and image reconstruction methods for PACT and USCT. Using computer-simulations, the parameters of an imaging system can be systematically and comprehensively explored in a way that is generally not possible through experimentation. When conducting such studies, numerical phantoms are employed to represent the physical properties of the patient or object to-be-imaged that influence the measured image data. It is highly desirable to utilize numerical phantoms that are realistic, especially when task-based measures of image quality are to be utilized to guide system design. However, most reported computer-simulation studies of PACT and USCT breast imaging employ simple numerical phantoms that oversimplify the complex anatomical structures in the human female breast. We develop and implement a methodology for generating anatomically realistic numerical breast phantoms from clinical contrast-enhanced magnetic resonance imaging data. The phantoms will depict vascular structures and the volumetric distribution of different tissue types in the breast. By assigning optical and acoustic parameters to different tissue structures, both optical and acoustic breast phantoms will be established for use in PACT and USCT studies.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-22/issue-4/041015/Generation-of-anatomically-realistic-numerical-phantoms-for-photoacoustic-and-ultrasonic/10.1117/1.JBO.22.4.041015.pdf

Generation of anatomically realistic numerical phantoms for photoacoustic and ultrasonic breast imaging.

Photoacoustic computed tomography (PACT), also known as optoacoustic tomography, is an emerging imaging technique that holds great promise for biomedical imaging. PACT is a hybrid imaging method that can exploit the strong endogenous contrast of optical methods along with the high spatial resolution of ultrasound methods. In its canonical form that is addressed in this article, PACT seeks to estimate the photoacoustically-induced initial pressure distribution within the object. Image reconstruction methods are employed to solve the acoustic inverse problem associated with the image formation process. When an idealized imaging scenario is considered, analytic solutions to the PACT inverse problem are available; however, in practice, numerous challenges exist that are more readily addressed within an optimization-based, or iterative, image reconstruction framework. In this article, the PACT image reconstruction problem is reviewed within the context of modern optimization-based image reconstruction methodologies. Imaging models that relate the measured photoacoustic wavefields to the sought-after object function are described in their continuous and discrete forms. The basic principles of optimization-based image reconstruction from discrete PACT measurement data are presented, which includes a review of methods for modeling the PACT measurement system response and other important physical factors. Non-conventional formulations of the PACT image reconstruction problem, in which acoustic parameters of the medium are concurrently estimated along with the PACT image, are also introduced and reviewed.

pdf/a-survey-of-computational-frameworks-for-solving-the-3umr8oss26.pdf

A survey of computational frameworks for solving the acoustic inverse problem in three-dimensional photoacoustic computed tomography.

Ultrasound computed tomography (USCT) holds great promise for breast cancer screening. Waveform inversion-based image reconstruction methods account for higher order diffraction effects and can produce high-resolution USCT images, but are computationally demanding. Recently, a source encoding technique has been combined with stochastic gradient descent (SGD) to greatly reduce image reconstruction times. However, this method bundles the stochastic data fidelity term with the deterministic regularization term. This limitation can be overcome by replacing SGD with a structured optimization method, such as the regularized dual averaging method, that exploits knowledge of the composition of the cost function. In this paper, the dual averaging method is combined with source encoding techniques to improve the effectiveness of regularization while maintaining the reduced reconstruction times afforded by source encoding. It is demonstrated that each iteration can be decomposed into a gradient descent step based on the data fidelity term and a proximal update step corresponding to the regularization term. Furthermore, the regularization term is never explicitly differentiated, allowing nonsmooth regularization penalties to be naturally incorporated. The wave equation is solved by the use of a time-domain method. The effectiveness of this approach is demonstrated through computer simulation and experimental studies. The results suggest that the dual averaging method can produce images with less noise and comparable resolution to those obtained by the use of SGD.

Regularized Dual Averaging Image Reconstruction for Full-Wave Ultrasound Computed Tomography

Conventional sonography, which performs well in dense breast tissue and is comfortable and radiation-free, is
not practical for screening because of its operator dependence and the time needed to scan the whole breast.
While magnetic resonance imaging (MRI) can significantly improve on these limitations, it is also not
practical because it has long been prohibitively expensive for routine use. There is therefore a need for an
alternative breast imaging method that obviates the constraints of these standard imaging modalities. The
lack of such an alternative is a barrier to dramatically impacting mortality (about 45,000 women in the US per
year) and morbidity from breast cancer because, currently, there is a trade-off between the cost effectiveness
of mammography and sonography on the one hand and the imaging accuracy of MRI on the other. This paper
presents a progress report on our long term goal to eliminate this trade-off and thereby improve breast cancer
survival rates and decrease unnecessary biopsies through the introduction of safe, cost-effective, operatorindependent
sonography that can rival MRI in accuracy.
The objective of the study described in this paper was to design and build an improved ultrasound
tomography (UST) scanner in support of our goals. To that end, we report on a design that builds on our
current research prototype. The design of the new scanner is based on a comparison of the capabilities of our
existing prototype and the performance needed for clinical efficacy. The performance gap was quantified by
using clinical studies to establish the baseline performance of the research prototype, and using known MRI
capabilities to establish the required performance. Simulation software was used to determine the basic
operating characteristics of an improved scanner that would provide the necessary performance. Design
elements focused on transducer geometry, which in turn drove the data acquisition system and the image
reconstruction engine specifications. The feasibility of UST established by our earlier work and that of other
groups, forms the rationale for developing a UST system that has the potential to become a practical, low-cost
device for breast cancer screening and diagnosis.

Breast ultrasound tomography: Bridging the gap to clinical practice

A system and method for imaging a volume of tissue comprising: a modular transducer array, configured to substantially surround the volume of tissue, emit acoustic waveforms toward the volume of tissue, and receive acoustic waveforms scattered by the volume of tissue, comprising a first and a second modular transducer subarray configured to couple to one another; a controller configured to control acoustic signals emitted by the first and the second modular transducer subarrays; an electronic subsystem, coupled to the modular transducer array, comprising a multiplexor and beam-forming elements and configured to receive a set of acoustic data from the first and the second modular transducer subarrays; and a processor configured to analyze the set of acoustic data, determine the distribution of at least one acoustomechanical parameter within the volume of tissue, and render an image of the volume of tissue based on the acoustomechanical parameter.

System and method for imaging a volume of tissue

Ultrasound Tomography Systems for Medical Imaging

For women with dense breast tissue, who are at much higher risk for developing breast cancer, the performance of mammography is at its worst. Consequently, many early cancers go undetected when they are the most treatable. Improved cancer detection for women with dense breasts would decrease the proportion of breast cancers diagnosed at later stages, which would significantly lower the mortality rate. The emergence of whole breast ultrasound provides good performance for women with dense breast tissue, and may eliminate the current trade-off between the cost effectiveness of mammography and the imaging performance of more expensive systems such as magnetic resonance imaging. We report on the performance of SoftVue, a whole breast ultrasound imaging system, based on the principles of ultrasound tomography. SoftVue was developed by Delphinus Medical Technologies and builds on an early prototype developed at the Karmanos Cancer Institute. We present results from preliminary testing of the SoftVue system, performed both in the lab and in the clinic. These tests aimed to validate the expected improvements in image performance. Initial qualitative analyses showed major improvements in image quality, thereby validating the new imaging system design. Specifically, SoftVue’s imaging performance was consistent across all breast density categories and had much better resolution and contrast. The implications of these results for clinical breast imaging are discussed and future work is described.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8675/86750K/Breast-imaging-with-the-SoftVue-imaging-system-first-results/10.1117/12.2002513.pdf

Breast imaging with the SoftVue imaging system: first results

We describe the technical design and performance of SoftVue, a breast imaging device based on the principles of ultrasound tomography. SoftVue's imaging sensor is a ring shaped transducer operating at 3 MHz and consisting of 2048 elements. Data acquisition is achieved through 512 receive channels. The transducer encircles the breast, which is immersed in warm water while the patient lies in a prone position. The transducer is translated vertically to acquire data from the entire breast. The acquired data are used to reconstruct images using tomographic inversions. The reconstruction engine is based around a blade-server design that houses multiple CPUs and GPUs. Separate algorithms are used to reconstruct reflection, sound speed, and attenuation images. A patient scan generates a stack of each type of image. The system was designed with the clinical goal of detecting and characterizing breast masses based on their biomechanical and morphological properties. Ongoing clinical studies are being used to assess the performance of the system under realistic clinical settings. Results of the clinical assessment are presented. [The authors acknowledge research support from the National Cancer Institute (5 R44 CA165320-03). Neb Duric and Peter Littrup also acknowledge that they have financial interests in the SoftVue technology.]

Clinical breast imaging with ultrasound tomography: A description of the SoftVue system

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