HEATING OF TISSUES BY MICROWAVES - A MODEL ANALYSIS

We consider the thermal response times for heating of tissue subject t o nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau(1)) pertains to heat conv ection by blood flow, and is of the order of 20-30 min for physiologic ally normal perfusion rates; the second (tau(2)) characterizes heat co nduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiat ion with an exponentially decreasing specific absorption rate, which m odels a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform sp ecific absorption rate, which models localized exposure. In both cases , the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau( 1) and tau(2). In appropriate limits of the ratio of these time consta nts, the local temperature rise is dominated by conductive or convecti ve heat transport. Predictions of the block model agree well with rece nt data for the thresholds for perception of warmth or pain from expos ure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to m icrowave radiation, to limit the temperature rise in tissue from radia tion by pulsed sources. We compare the ANSI exposure standards for mic rowaves and infrared laser radiation with respect to the maximal incre ase in tissue temperature that would be allowed at the maximal permiss ible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard. Bioelectromagnetics 19 :420-428, 1998. (C) 1998 Wiley-Liss, Inc.