A Comparison of the Thermal-Dose Equation and the Intensity-Time Product, It m, for Predicting Tissue Damage Thresholds

Thermal dose is the most generally accepted concept for estimating temperature-related tissue damage thresholds in high-intensity focused ultrasound (HIFU) procedures. However, another approach based on the intensity-time product I t m = D has been used, where D is a tissue-dependent damage threshold, I is the spatial-peak, temporal-average intensity and t is time. In this study, these two approaches were compared analytically by substituting a well-known soft-tissue solution for temperature vs. time into the thermal dose equation. From power law fits of I vs. t, m was found to fall between about 0.3 and 0.8. In terms of the intensity required for cell death for a given exposure time, the standard deviation of the error between the full thermal-dose formulation and the I t m = D prediction based upon the power-law fit was less than 5% for focal beam diameters up to 3 mm. Thus, for the practical range of HIFU parameters examined, the intensity-time product relationship is equivalent to the thermal dose formulation. (E-mail: gerald.harris@fda.hhs.gov)