Thermal imaging with ultrasound reflex transmission methods

Results were recently presented [1] demonstrating the feasibility of applying an ultrasound phase-contrast method for determining temperature change in water from a thermal plume in a through-transmission arrangement. Here we present results from reflex transmission measurements of steady-state heating from a HIFU transducer in a tissue phantom. A 1.68-MHz focused transducer interrogated the phantom and a near-acoustically transparent hydrophone was scanned throughout the sound field to obtain the reflection from the back surface of the phantom. The output from a 1.088-MHz HIFU transducer was interleaved with the diagnostic field to induce a steady-state temperature change in the phantom. Two hydrophone scans were performed to obtain the reflex transmission signal before and during heating. The sound fields were numerically backprojected to the region of heating, integrated along the propagation axis, and tomographically reconstructed to obtain the phase change, from which the sound speed and temperature change was determined. Temperature rises of 42degC were tested. The reconstructed temperature rise was within 2degC of the temperature rise obtained with magnetic resonance thermal imaging techniques. Due to the lengthy acquisition time, the temperature rise was necessarily steady-state, but the method is adaptable to dynamic heating measurements. Thermal lensing effects are visible in the images, but may be corrected for by applying a moving average filter. The result is a method which can visualize the temperature change over a spatial region sufficient for monitoring thermal ablation treatments.