A new strategy to overcome cycle skipping: frequency-difference waveform inversion

Most ultrasound transducer designs are driven towards high frequencies by the need to maximize the resolution achievable by B-mode reflectivity imaging. Because full-waveform inversion (FWI) requires low frequencies to overcome cycle skipping, it is often difficult to apply FWI to the same high-frequency transducer hardware. Recent work on time-domain adaptive waveform inversion (AWI) addresses this based on a deconvolution approach to overcome cycle skipping. However, most clinical implementations of ultrasound tomography rely on frequency-domain FWI to quickly reconstruct images on clinically relevant time scales. Although the broadband nature of AWI makes it difficult to translate to the frequency domain, we can approximate the properties of AWI by using a frequency-differencing approach. We develop and describe both a model-domain and a data-domain method for frequency-differencing. Simulations and phantom experiments show that each frequency-differencing approach helps overcome cycle skipping by providing a sufficiently accurate starting model for FWI.