Ultrasound transmission tomography is a promising modality for breast cancer diagnosis. For image reconstruc- tion approximations to the acoustic wave equation such as straight or bent rays are commonly used due to their low computational complexity. For sparse apertures the coverage of the volume by rays is very limited, thereby requiring strong regularization in the inversion process. The concept of fat rays reduces the sparseness and includes the contributions to the measured signal originating from the first Fresnel zone. In this work we investi- gate the application of the fat ray concept to ultrasound transmission tomography. We implement a straight ray, bent ray and fat ray forward model. For the inversion process a least squares solver (LSQR), a simultaneous al- gebraic reconstruction technique (SART) and a compressive sensing based total variation minimization (TVAL3) is applied. The combination of forward models and inversion processes has been evaluated by synthetic data. TVAL3 outperforms SART and LSQR, especially for sparse apertures. The fat ray concept is able to decrease the error with respect to the ground truth compared to the bent ray method especially for SART and LSQR inversion, and especially for very sparse apertures.
For Ultrasound Tomography reflectivity imaging Synthetic Aperture Focusing Technique (SAFT) is often applied. Phase aberration correction is required to achieve images with high resolution and high contrast, for which a sound speed map is required. For USCT systems these sound speed maps are usually reconstructed using the transmission data from the raw data set, which is also used for reflection tomography. We compare straight and bent ray phase aberration correction SAFT algorithms with respect to different reconstruction algorithms to derive the sound speed map. Evaluations are carried out based on a simulated phantom and measured data from the Multimodal Ultrasound Breast Imaging System (MUBI). Phase aberration correction enables recovering the contrast of the image, while without SAFT results in considerably unfocused inner structures. By applying a reconstructed sound speed map however the local contrast cannot be fully recovered compared to the ideal case. Introducing bent ray transmission reconstruction approaches based on the Fast Marching or B´ezier curve method in all cases improves the results over the straight ray transmission tomography.