Richard G. Olsen

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 ¿s 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.

Microwave Pulse-Induced Acoustic Resonances in Spherical Head Models

Microwave-induced acoustic pressures in the spherical models of human and animal heads are measured using a small hydrophone transducer. The measured acoustic frequencies that correspond to mechanical resonance of the head model agree with those predicted by the thermoelastic theory of interaction. Further, a three-pulse burst applied at the appropriate pulse repetition frequencies could effectively drive the model to respond in such a manner that the microwave-induced pressure amplitude would be increased by threefold or more.

Partial-body absorption resonances in a sitting rhesus model at 1.29 GHz

SummaryMicrowave absorption at 1.29 GHz in an upright sitting rhesus model was studied using a gradient-layer, envelope calorimeter. Various body members of the model such as the arms, legs, head and neck, etc. were individually studied in separate experiments. Results of these experiments showed a major fraction of the overall microwave absorption to be occurring in the extremities rather than in the central trunk of the model. These results have corroborated previous dosimetric information obtained with a nonperturbing temperature probe. The overall average specific absorption rate was about twice that predicted from available theoretical methods, and a multibody interaction resonance in the lower legs, and to some extent the arms, is believed to have produced the enhanced absorption. Based on these results it is predicted that a relatively strong leg absorption would occur in a sitting human for similar irradiation conditions at frequencies between 500 and 700 MHz.

Evidence for Microwave-Induced Acoustical Resonances in Biological Matérial

Pulse bursts of microwave energy were used to stimulate resonant-type acoustical response in a rectangular muscle-equivalent model and in a spherical brain-equivalent model. The rectangular model was irradiated using a military radar transmitter at 5.655 GHz, 200 kW peak, and the spherical model was irradiated using a pulse-type cavity oscillator at 1.10 GHz, 4 kW peak. Hydrophone transducers were implanted in the models to record the microwave-induced mechanical vibrations. Four properly timed radar pulses produced a threefold increase in the acoustical amplitudes in the muscle model. In the spherical model, a pulse train of three properly timed microwave pulses doubled the stress wave amplitudes as recorded by the implanted hydrophone. These results show that certain pulse parameters of microwave irradiation can be adjusted to increase the intensity of induced mechanical vibrations in both rectangular and spherical tissue equivalent models.

Specific absorption rate and radiofrequency current-to-ground in human models exposed to near-field irradiation

To expand our knowledge of near-field radiofrequency energy absorption in occupationally exposed workers, we used coffin-sized calorimeters to measure specific absorption rate in full-size human models. The models were subjected to near-field irradiation at two frequencies at an outdoor groundplane facility. We also measured radiofrequency current-to-ground in the models to supplement a previous study at 29.9000 MHz. The results have enabled us to construct a frequency-independent mathematical relationship between specific absorption rate and radiofrequency current for the given exposure system. Moreover, the results show a favorable comparison to radiofrequency radiation dosimetry handbook predictions of average specific absorption rate when only the vertical electric field (E-field) component is used to normalize specific absorption rate. Once determined on a case-by-case basis, the use of specific absorption rate vs. radiofrequency current curves for any exposure system or condition could be a simple and quick method to determine onsite compliance with specific absorption rate-based exposure standards.

Coaxial Nonmetallic Thermocouple with Electronic Ice Point for Dosimetric Use in Electromagnetic Environments

AbstractA refined version of the nonmetallic thermocouple (NMT) is presented along with results of o microwave dosimetric testing of the device in a practical laboratory environment, A smaller device was achieved by using coaxial conductors of a dissimilar nature, and a slightly modified, commercially available electronic ice point provided temperature compensation of the secondary junctions adequate for normal laboratory use. Results of practical dosimetric comparisons in tissue-equivalent material between the coaxial NMT and a widely used, more sophisticated temperature probe showed mean specific absorption rate (SAR) to vary between 13.6% and 31%, but no statistically significant difference was observed in instances where the probes were compared at identical locations during irradiation.

Microwave Thermoelastic Tissue Imaging

A microwave-induced thermoelastic tissue imaging system is suggested as a new and promising imaging modality. It appears to possess some unique features that may allow it to become as useful as these other methods and to permit non-invasive tissue imaging of tissue characteristics which are not identifiable by other techniques. It uses nonionizing radiation and relies on a beam of impinging microwave to launch an acoustic wavefront into tissue. There is a direct relation between the pattern of absorbed microwave pulses and the induced thermoelastic pressure waves in biological tissues. Moreover, regions of differing permittivity would exhibit differential absorption. This thermoelastic wave of pressure would propagate through the tissue and be detected by a two-dimensional array of piezoelectric transducers positioned on the body surface to give an image of the intervening tissue structure. Signals from the outputs of this transducer array are then amplified and band-limited at a signal conditioning stage. A computer-controlled data acquisition system samples and converts them to digital form for further processing. A hybrid parralel/serial design of dividing the array into segments and collecting data from each segment sequentially is used. Image processing algorithms are applied to digitized pictures for enhancing the images. The processed two-dimensional image is displayed on a color monitor. An example showing the image of a human hand model illustrates the potential usefulness of microwave-induced thermoelastic tissue imaging.