William D. Hurt

Multiterm Debye Dispersion Relations for Permittivity of Muscle

A computer program was developed to fit multiple-term Debye-type expressions to published data of permittivity for muscle. The number of terms in this expression was varied and tested for significance. Based on these results, closed form expressions for dielectric permittivity and conductivity are suggested for use as estimators over the 10 Hz-100 GHz range.

Variability in EMF permittivity values: implications for SAR calculations

Digital anatomical models of man and animals are available for use in numerical calculations to predict electromagnetic field (EMF)-induced specific absorption rate (SAR) values. To use these models, permittivity values are assigned to the various tissues for the EMF frequencies of interest. There is, as yet, no consensus on what are the best permittivity data. This study analyzed the variability In published permittivity data and investigated the effects of permittivity values that are proportional on SAR calculations. Whole-sphere averaged and localized SAR values along the diameter of a 4-cm sphere are calculated for EMF exposures in the radio frequency range of 1 MHz to 1 GHz. When the dimensions of a sphere are small compared to the wavelength (i.e., wavelength inside the material is greater than ten times the dimensions of the object), the whole-sphere averaged SAR is inversely proportional to the permittivity of the material composing the sphere. However, the localized SAR values generally do not have the same relation and, as a matter of fact, vary greatly depending on the location within the sphere. These results indicate that care must be taken In choosing the permittivity values used in calculating SAR values and some estimate of the dependence of the calculated SAR values on variability in permittivity should be determined.

Recent Advancements in Dosimetry Measurements and Modeling

Whole-body specific absorption rate (SAR) values provide useful information about energy deposition resulting from exposure to radio frequency radiation (RFR). However, whole-body SAR values do not reveal possible localized “hot spots”. Although differences in regional temperatures have been measured in animals during RFR exposure [1–3], the use of temperature probes to make empirical measurements of these “hot spots” can be extremely time consuming and are invasive in nature [4].

Ultra-wideband electromagnetic pulses: Lack of effects on heart rate and blood pressure during two-minute exposures of rats †

Exposure to fast-rise-time ultra-wideband (UWB) electromagnetic pulses has been postulated to result in effects on biological tissue (including the cardiovascular system). In the current study, 10 anesthetized Sprague-Dawley rats were exposed to pulses produced by a Sandia UWB pulse generator (average values of exposures over three different pulse repetition rates: rise time, 174-218 ps; peak E field, 87-104 kV/m; pulse duration, 0.97-0.99 ns). Exposures to 50, 500 and 1000 pulses/s resulted in no significant changes in heart rate or mean arterial blood pressure measured every 30 s during 2 min of exposure and for 2 min after the exposure. The results suggest that acute UWB whole-body exposure under these conditions does not have an immediate detrimental effect on these cardiovascular system variables in anesthetized rats.

A Comparison of Sar Values Determined Empirically and by FD-TD Modeling

Specific absorption rate (SAR) is defined by the National Council on Radiation Protection and Measurements as “…the time derivative of the incremental energy absorbed by (dissipated in) an incremental mass contained in a volume of a given density” (NRCP, 1981). The whole-body and partial-body SAR form the basis of permissible exposure limits for radio frequency radiation (RFR). The whole-body SAR provides very useful information regarding the influence of frequency, polarization, and orientation on RFR absorption[1,2]. However, RFR is not absorbed uniformly throughout a biological system. Numerous factors contribute to the heterogeneity of SAR values in biological system, including differences in electrical properties of different tissues, impedance mismatches at tissue boundaries, and the complex geometry of individual organs and structures [1,2]. Due to these factors, local SAR must be used to reveal the distribution of RFR absorption within the animal. Without this information, bioeffects data obtained from one species cannot be meaningfully extrapolated to another.