Cryoablation temperature monitoring with dense ultrasonic speed-of-sound shift imaging

OBJECTIVE: To accurately monitor temperature during cryoablation, a minimally invasive technique that destroys tissue locally by forming an ice ball around an inserted cryoprobe. Approach: We present a dense slowness-shift imaging method that estimates local speed-of-sound changes from ultrasound B-mode images using optical flow. This single-transducer, image-based approach enables mapping of spatial temperature change without requiring additional hardware. Cryoablation experiments were conducted in a tissue-mimicking phantom and ex vivo turkey breast. Main results: Slowness deviation increased with decreasing temperature. In the phantom, the dependence was linear (αₐ = -20.70 ηs·m⁻¹C°⁻¹), while in turkey breast it followed an exponential relationship (αₜ = 34.04×exp(0.075(-ΔT)-1) ηs·m⁻¹C°⁻¹). The algorithm detected sub-degree temperature variations and accurately tracked cooling down to -39.4 ± 5.6 °C. Significance: Accurate temperature monitoring is essential for effective and safe cryoablation. This work demonstrates the feasibility of ultrasound-based, noninvasive temperature monitoring during cryoablation, providing a scalable, real-time alternative to existing invasive or high-cost thermal assessment techniques.