Investigation of an elevation-focused transducer model to enable accurate three-dimensional full-waveform inversion in ultrasound computed tomography

Ultrasound computed tomography (USCT) is an emerging imaging modality that holds great promise for breast cancer diagnosis. Full-waveform inversion (FWI)-based image reconstruction methods can produce high spatial resolution images of the acoustic properties of the breast tissues. A practical design of breast USCT systems employs a circular elevation-focused ring-array of ultrasonic transducers, where the data is acquired by translating the ring-array orthogonally to the imaging plane. This design allows a fast slice-by-slice (SBS) reconstruction approach by stacking together two-dimensional (2D) images reconstructed for each position of the transducer array. However, the SBS approach assumes 2D propagation physics and usually employs simplified transducer models. Because of this, the reconstructed images can sometimes contain significant artifacts. Thus, three-dimensional (3D) imaging models that incorporate the focusing effects of transducers are needed to improve image quality in ring-array-based USCT. To address this, a 3D elevation-focused transducers model for use within a 3D time-domain pseudospectral wave propagation method was developed. The proposed method uses a stylized (line source) geometry of the transducer and achieves elevational focusing by applying a spatially varying delay to the ultrasound pulse (emitter mode) and recorded signal (receiver mode). The proposed transducer model was validated against a semi-analytical method based on the Rayleigh-Sommerfield diffraction integral solution. In addition, a virtual imaging study that utilizes a 3D anatomical numerical breast phantom and the proposed transducer model is presented. The results demonstrate that 3D FWI-based reconstruction methods that incorporate the proposed transducer model hold promise for improving image quality in ring-array-based USCT.