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

摘要

Thermal dose is the most generally accepted concept for estimating temperature-related tissue damage thresholds. However, another approach based on the intensity-time product of the form D = It~m has been used, where D is a tissue-dependent damage threshold, I is the spatial-peak, temporal-average intensity, t is time, and m is an exponent not greater than one. In this study these two relationships were compared analytically by substituting a well-known soft tissue solution for temperature vs. time into the thermal dose equation. The commonly accepted value for t_(43) of 240 minutes was used as a thermal dose threshold for necrosis. Then power law fits were found for I vs. t for various frequencies and beam diameters. Using published values for soft tissue acoustic and thermal properties, typical high intensity focused ultrasound (HIFU) frequencies and beam diameters, and exposure times from 1-20 s, the exponent m was found to fall between about 0.3 and 0.8. The value for m increased with increasing diameter but was independent of frequency. Thus, for the parameters examined, the intensity-time product relationship is equivalent to the thermal dose formulation over certain practical ranges of HIFU exposure duration.