Heat Transport in Tissues and Temporal and Spatial Averaging of Laser and Microwave Exposures

K. R. Foster¹ and E. R. Adair^{2 1}Department of Bioengineering, University of Pennsylvania, Philadelphia PA 19104, USA. ²Air Force Research Laboratory, Human Effectiveness Directorate, Directed Energy Division, Brooks Air Force Base, Texas 78235, USA

The microwave standards setting community has long struggled with the problem of setting limits for partial body exposure, and for time-dependent exposures to RF energy. Moreover, in some respects the exposure standards for microwaves in the millimeter wave range, and for exposure to infrared energy (from lasers) differ, even though the energy penetration depths of these two forms of energy in the body are similar. There is a perceived need in the standards setting community to bring these two standards into congruence.

Assuming that the limiting hazards are thermal, principles of heat transfer and thermal modeling can shed much light on these issues. I review the implications of two different kinds of thermal models for estimating the thresholds for injury from RF and laser heating. The first model is the bioheat equation, which is an extension of the heat conduction equation with an additional term to model the effects of blood flow in dissipating energy. The second model is the Hardy-Stolwijk model, a lumped-parameter model for the body which includes heat exchange with the environment and thermoregulatory mechanisms.

Both models yield a range of thermal response times and estimates of heat transport within the body, that can be used to assess the thermal effects of RF and laser exposure. The bioheat equation is useful for studying the details of heat transport in limited regions of tissue, particularly as a result of spatially nonuniform SAR and for short-term exposures. The Hardy-Stolwijk model is a useful and apparently very successful model for predicting the thermophysiological responses of the body to microwave exposure.