In B-mode imaging of the dependent or compressed breast, wave incidence at steep angles can change propagation directions and induce areas of signal dropout. To evaluate the image anomalies in reasonable simulation times, we performed full-wave studies for center frequencies of 1 and 4 MHz. Speed of sound and density of skin, typical coupling gel, and adipose tissue were assigned to the test couplant. Compared with commercial gel, skin-like couplant reduced the dropout area at 1 and 4 MHz by 57.1% and 96.7%, respectively, consistent with a decreased average beam deflection in the breast. Conversely, the adipose-like couplant increased the dropout area from that of simulated commercial gel by 26.5% and 36.7% at 1 and 4 MHz, respectively. In addition, the skin-like couplant resulted in the greatest beam deflection inside the breast among all couplants. The findings could aid the use of three-dimensional simulations to design ultrasound couplants for beam passage through tissue boundaries at steep angles to improve corrections of signal dropout and defocusing and in compound imaging.
In B-mode images from dual-sided ultrasound, it has been shown that by delineating structures suspected of being
relatively homogeneous, one can enhance limited angle tomography to produce speed of sound images in the same view
as X-ray Digital Breast Tomography (DBT). This could allow better breast cancer detection and discrimination, as well
as improved registration of the ultrasound and X-ray images, because of the similarity of SOS and X-ray contrast in the
breast. However, this speed of sound reconstruction method relies strongly on B-mode or other reflection mode
segmentation. If that information is limited or incorrect, artifacts will appear in the reconstructed images. Therefore, the
iterative speed of sound reconstruction algorithm has been modified in a manner of simultaneously utilizing the image
segmentations and removing most artifacts. The first step of incorporating a priori information is solved by any nonlinearnonconvex
optimization method while artifact removal is accomplished by employing the fast split Bregman method to
perform total-variation (TV) regularization for image denoising. The proposed method was demonstrated in simplified
simulations of our dual-sided ultrasound scanner. To speed these computations two opposed 40-element ultrasound linear
arrays with 0.5 MHz center frequency were simulated for imaging objects in a uniform background. The proposed speed
of sound reconstruction method worked well with both bent-ray and full-wave inversion methods. This is also the first
demonstration of successful full-wave medical ultrasound tomography in the limited angle geometry. Presented results
lend credibility to a possible translation of this method to clinical breast imaging.
Recent efforts in medical imaging have shown that mechanical stimulation of tissues and a suitable imaging modality can be used to interrogate elastic properties of human tissues. Malignant tissues can have elastic properties that allow the physician to separate them from benign counterparts or plaque in arteries can be characterized in regard to its age by measuring its elastic properties. Our system consists of: (1) an acoustic source to induce <i>tissue</i> displacement, (2) a tissue mimicking phantom, and (3) MRI as a method for imaging and measuring the induced shear wave in the phantom. Agar was used to construct a tissue mimicking phantom. A modified spin echo sequence was written to trigger the acoustic system and phase encode the displacement information with magnetic field gradients. A series of images was obtained from the modified multi-slice, spin-echo sequence. Images showed z-axis displacement created by the radiation force. Additional experiments recorded the x and y displacement and allowed for a full 3D vector reconstruction of shear wave propagation. MRI provides a method to record displacements created by radiation force. Acoustical sources can be used to induce shear waves, which in turn can be imaged with MRI methods to quantify and display this wave in a 3D fashion.