Temperature-Time Profile in Rats Subjected to Selective Microwave Irradiation of the Brain

Real-time cortical and colonic temperature changes in rats irradiated for 20 min by a 2450 MHz contact radiator at the animals head were measured. Ten μs pulses were delivered to the radiator at pulse repetition rates of 25-500 pulses/s and at energy rates of 0.3 to 1325 mW that corresponded to incident power densities of 0.18 to 875 mW/cm2. Lower power densities yielded a small cortical temperature increase below that for the body core whereas higher power densities produced a greater cortical temperature rise. Even at an incident power density as high as 875 mW/cm2 that resulted in a specific absorption rate of 70 mW/g in the brain, the cortical temperature increased by less than 3.0°C while the colonic temperature rose by 0.5°C. The cortical temperature rapidly rose during the initial transient period of irradiation. It soon became nonlinear and achieved a steady-state level around 39°C. From these observations we concluded that the brain temperature was lowered by thermoregulatory processes involving blood circulation, temperature, and mass differences between the brain and the nonirradiated body of the anesthetized rat, and furthermore, the irradiated brain received significant protection from the nonirradiated body.