Ultrasound tomography fundamentally relies on low-frequency data to avoid cycle skipping in full-waveform inversion (FWI). In the absence of sufficiently low-frequency data, we can extrapolate low-frequency content from existing high-frequency signals by using the same approach used in frequency-difference beamforming. This low-frequency content is then used to kickstart FWI and avoid cycle skipping at higher frequencies. In simulations, the structural similarity index measure and peak signal-to-noise ratio of the reconstructed image improve by 0.28 and 8.6 dB, respectively, as a result of frequency differencing. Experiments show that internal structures can be seen with greater clarity because of frequency differencing.
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