Microwave-Induced Pressure Waves in Mammalian Brains

This paper presents direct measurements of acoustic pressure waves in brains of rats, cats, and guinea pigs irradiated with pulsed 2.450 and 5.655 GHz microwaves. A smal disk hydrophone transducer was surgically implanted in brains of anesthetized animals. Rectangular pulses (3 kW peak, 2.5 and 5.5 , us wide at 2.450 GHz and 200 kW peak, 0.5 μs wide at 5.655 GHz) were applied through horns, waveguides, and direct contact antennas. The results clearly indicate that pulsed microwaves induce acoustic pressure waves in the brain, confirming earlier theoretical predictions. Furthermore, hydrophone output waveforms and on-line analyzed spectra show that fundamental and second harmonics were nearly identical to those predicted by the thermoelastic theory. However, the hydrophone records show complex sequences of higher order vibrational modes which deviate from predictions based on a homogeneous spherical model of the head.