Abstract
Pulsed microwaves above specific energy thresholds have been reported to cause brain injury in animal models. The actual physical mechanism causing brain damage is unexplained while the clinical reality of these injuries remains controversial. Here we propose mechanisms by which pulsed microwaves may injure brain tissue by transduction of microwave energy into damaging acoustic phonons in brain water. We have shown that low intensity explosive blast waves likely initiate phonon excitations in brain tissues. Brain injury in this instance occurs at nanoscale subcellular levels as predicted by physical consideration of phonon interactions in brain water content. The phonon mechanism may also explain similarities between primary non-impact blast-induced mild Traumatic Brain Injury (mTBI) and recent clinical and imaging findings of unexplained brain injuries observed in US embassy personnel possibly due to directed radiofrequency radiation. We describe experiments to elucidate mechanisms, RF frequencies and power levels by which pulsed microwaves potentially injure brain tissue. Pathological documentation of nanoscale brain blast injury has been supported experimentally using transmission electron microscopy (TEM) demonstrating nanoscale cellular damage in the absence of gross or light microscopic findings. Similar studies are required to better define pulsed microwave brain injury. Based upon existing findings, clinical diagnosis of both low intensity blast and microwave-induced brain injury likely will require diffusion tensor imaging (DTI), a specialized water based magnetic resonance imaging (MRI) technique.
Highlights
Swanson et al [1] examined 24 US Cuban embassy personnel exposed to an unknown directed energy source
Watanabe et al [24] employed finite differential analysis to model the effect of 1 mW/cm2, 915 MHz single 20 μs wide square pulses incident upon the back of realistic human head models. These workers found that thermoelastic coupling of microwave energy into the brain occurred near the brain surface, launching an acoustic wave propagating to the opposite side of the head at the speed of sound in water and reverberating up to several times
Igarashi et al [27] showed 50% mortality with extensive gross brain damage in rats directly exposed at close range to a single high pulse of 3 kW, 2.45 GHz microwaves for 0.1 s
Summary
Swanson et al [1] examined 24 US Cuban embassy personnel exposed to an unknown directed energy source. Watanabe et al [24] employed finite differential analysis to model the effect of 1 mW/cm 915 MHz single 20 μs wide square pulses (rise time 400 ns) incident upon the back of realistic human head models These workers found that thermoelastic coupling of microwave energy into the brain occurred near the brain surface, launching an acoustic wave propagating to the opposite side of the head at the speed of sound in water and reverberating up to several times. Acoustic shock could excite the lowest acoustic phonon in water with a 7.5 GHz frequency [13,14,15] With sufficient power, such energy could initiate a damage mechanism as occurs in explosive shock with skull bone rippling, Figure 3B schematically depicts this effect. The efficacy of these mechanisms to produce brain damage depends upon microwave frequency as shown in Figure 4 along with microwave pulse rise-time
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