Reverse-Time Migration for Ultrasound Tomography
Reverse-time migration (RTM) is how the oil industry images the subsurface: propagate the recorded wavefield backward in time, the source wavelet forward, and apply an imaging condition at each step. It handles two-way propagation, multiple arrivals and complex velocity structure far better than the ray-tracing methods common in medical ultrasound. Seismic imaging and ultrasound computed tomography (USCT) share the same physics, so this paper adapts RTM to breast USCT, where the goal is to image tumors by their velocity contrast with healthy tissue.
Method
The wavefield is extrapolated with a phase-shift-plus-interpolation (PSPI) propagator: exact phase shifts for several reference velocities, interpolated in the frequency-space domain to the local velocity. The main change from the seismic setting is geometry. Seismic data is recorded on a flat surface, while USCT places sources and receivers on a ring around the organ, so the propagators start from a circular datum.
Synthetic data came from finite-difference modeling with a 9-point Laplacian on a 0.25 m × 0.25 m model at 0.5 mm sampling, with a 500 kHz wavelet, using a synthetic breast (1600 m/s tissue, 1700 m/s tumors, in 1500 m/s fluid) and then a velocity model of a real cancerous breast.
Results
- Circular acquisition gave better illumination than a flat datum, especially along the diagonals.
- Shot density matters: the 4-shot image shows significant migration noise, while the 72-shot image is far cleaner, because stacking suppresses unfocused energy.
- On the real breast model, the tumor is imaged even with 4 shots and clearly resolved with 72.
Limitations
All data is synthetic and 2D. RTM images impedance boundaries, so sharp contrasts like tumors image well but gradual tissue transitions do not, and it needs an accurate velocity model from tomography first.