HIFU-induced changes in optical scattering and absorption of tissue over nine orders of thermal dose

The optical properties of tissue change during thermal ablation. Multi-modal methods such as acousto-optic (AO) and photo-acoustic (PA) imaging may provide a real-time, direct measure of lesion formation. Baseline changes in optical properties have been previously measured over limited ranges of thermal dose for tissues exposed to a temperature-controlled water bath, however, there is scant data for optical properties of lesions created by HIFU. In this work, the optical scattering and absorption coefficients from 400-1300 nm of excised chicken breast exposed to HIFU were measured using an integrating sphere spectrophotometric technique. HIFU-induced spatiotemporal temperature elevations were measured using an infrared camera and used to calculate the thermal dose delivered to a localized region of tissue. Results obtained over a range of thermal dose spanning 9 orders of magnitude show that the reduced scattering coefficient increases for HIFU exposures exceeding a threshold thermal dose of CEM₄₃  =  600  ±  81 cumulative equivalent minutes. HIFU-induced thermal damage results in changes in scattering over all optical wavelengths, with a 2.5-fold increase for thermal lesions exceeding 70 °C. The tissue absorption coefficient was also found to increase for thermally lesioned tissue, however, the magnitude was strongly dependent on the optical wavelength and there was substantial sample-to-sample variability, such that the existence of a threshold thermal dose could not be determined. Therapeutic windows, where the optical penetration depth is expected to be greatest, were identified in the near infrared regime centered near 900 nm and 1100 nm. These data motivate further research to improve the real-time AO and PA sensing of lesion formation during HIFU therapy as an alternative to thermometry.