Accelerating ultrasound speed of sound tomography through reflection and transmission imaging

Imaging of speed of sound (SOS) holds great promise for medical detection and diagnosis, since it is a quantitative, physical property of tissues not imaged by other means. The full Waveform Inversion (FWI) based method can build speed of sound images with high spatial resolution but it is computationally demanding. The iterative Algebraic Reconstruction Technique (ART) is a favored choice for fast projection reconstruction problems, but its convergence, like that of other algorithms, is strongly limited when used for limited angle tomography, i.e. tomography without full 360° projection data. Therefore, we propose a fast SOS imaging method with good quantitative and spatial fidelity. In our study, two linear ultrasound transducer arrays with the center frequency of 5 MHz are placed at the opposite side of the imaging target. First, we apply a computed ultrasound tomography method in classic pulse echo mode (CUTE), using various steering angles with transmit beamforming. Differences in time-of-flight along different transmission directions are reflected in echo-phase information which gives an approach to a very fast reconstruction of SOS images. Yet, this first step maintains substantial directional blurring lateral to the central beam direction and streak artifacts. Then, we use this poor quality SOS image as an initial result for the iterative optimization implemented in ART. With an approximately correct initial SOS map, blurring from limited angle ART in the axial direction is alleviated and the image is much improved quantitatively. We conduct a 2-D numerical simulation by simulating two opposite 128-element linear ultrasound transducers. The simulation results show the axial artifacts caused by limited angle tomography due to the lack of full information is obviously reduced. With optimization, the proposed fast SOS imaging method might help future breast cancer detection and diagnosis.