Full waveform inversion for ultrasound computed tomography with high-sensitivity scan method

For breast cancer imaging by ultrasound computed tomography (CT) without dependence on patient breast size, we previously developed a high-sensitivity scan method in which a virtual fan-beam (vfan-beam) is generated from ultrasound waves emitted from 128 sources with unique delay times. Full waveform inversion (FWI) with multiple sound sources has not been previously applied to ultrasound CT using a ring transducer array. We have now developed a FWI calculation process that enables a vfan-beam to generate accurate sound speed images. A vfan-beam is accurately modeled by positioning the 128 sources and considering the delay times. The performance of the FWI calculation process with a vfan-beam was evaluated using a prototype ultrasound CT. For a circular phantom, the spatial resolution of a FWI image obtained with a vfan-beam was better than that of a filtered back projection (FBP) image. The image contrast of the FWI calculation process with a vfan-beam was comparable to that of the process with a conventional fan-beam generated from a single source. For a high-attenuation ellipse phantom, the sound speed image obtained with a conventional fan-beam had severe artifacts due to the low signal to noise ratio (SNR). Using a vfan-beam reduced the number of artifacts in the images due to the higher SNR. The FWI calculation process with a vfan-beam visualized a 3 mm inclusion more clearly than the FBP process. A measurement study demonstrated that the FWI process with a vfan-beam with a high SNR reduced the number of artifacts in the sound speed images and improved the spatial resolution for a high-attenuation breast.