Impact of Starting Model on Waveform Inversion in Ultrasound Tomography

The convergence of waveform inversion in ultrasound tomography is heavily impacted by the choice of starting model. Ray tomography is often used as the starting model for waveform inversion; however, artifacts resulting from ray tomography can continue to persist during waveform inversion. On the other hand, a homogeneous starting model for waveform inversion may result in cycle skipping artifacts if the frequency of the transmitted waveform is too high or the error between the starting model and ground-truth is too large. Clinical in vivo breast data suggests that waveform inversion from a homogeneous starting model is sufficient for an accurate reconstruction of the speed of sound if the starting model is close enough to the average speed of sound in the medium and the starting frequency for waveform inversion is sufficiently low to avoid cycle skipping. Comparing the results of waveform inversion using ray tomography and a homogeneous sound speed as initial models, the homogeneous starting model avoids oscillatory artifacts produced by ray tomography at the edges of the breast. Although the RMS error between the two waveform inversion results is 29.6 m/s, most of the error is the result of reconstruction artifacts at the edges of the breast. When the RMS error is measured inside the breast away from its boundaries, this RMS error drops down to 11.5 m/s.