Diffraction-limited spatial resolution using synthetic focus time-of-flight ultrasound tomography

Ultrasound tomography (UT) reconstructs an image of speed-of-sound (SoS) based on analysing ultrasound (US) transmitted through tissue from a variety of angles. In ray tomography (RT), the time-of-flight (ToF) of US is related to integrals of SoS along acoustic rays. It is perceived that, while allowing a numerically efficient reconstruction, RT has the disadvantage of low spatial resolution and geometric distortions due to diffraction and refraction. We propose a modification to RT where, for each angle, the detected signal is synthetically backpropagated into the tissue sample and the ToF is analysed in a spatially resolved way, resulting in a 2D (for planar systems) map of ToF values. These maps are then accumulated and ramp-filtered as in FBP to obtain the SoS. We demonstrate in simulations and in phantoms that this technique inherently reduces the influence of the wave nature of US propagation on the final SoS image, allowing diffraction-limited resolution without the need for a computationally expensive full-wave inversion.