  Ultrasound computed tomography (USCT) is an emerging medical imaging modality
that holds great promise for improving human health. Full-waveform inversion
(FWI)-based image reconstruction methods account for the relevant wave physics
to produce high spatial resolution images of the acoustic properties of the
breast tissues. A practical USCT design employs a circular ring-array comprised
of elevation-focused ultrasonic transducers, and volumentric imaging is
achieved by translating the ring-array orthogonally to the imaging plane. In
commonly deployed slice-by-slice (SBS) reconstruction approaches, the
three-dimensional (3D) volume is reconstructed by stacking together
two-dimensional (2D) images reconstructed for each position of the ring-array.
A limitation of the SBS reconstruction approach is that it does not account for
3D wave propagation physics and the focusing properties of the transducers,
which can result in significant image artifacts and inaccuracies. To perform 3D
image reconstruction when elevation-focused transducers are employed, a
numerical description of the focusing properties of the transducers should be
included in the forward model. To address this, a 3D computational model of an
elevation-focused transducer is developed to enable 3D FWI-based reconstruction
methods to be deployed in ring-array-based USCT. The focusing is achieved by
applying a spatially varying temporal delay to the ultrasound pulse (emitter
mode) and recorded signal (receiver mode). The proposed numerical transducer
model is quantitatively validated and employed in computer-simulation studies
that demonstrate its use in image reconstruction for ring-array USCT
