Analysis of ICNIRP 2020 Basic Restrictions for Localized Radiofrequency Exposure in the Frequency Range above 6 GHz.

ABSTRACT: ICNIRP 2020 guidelines have defined a practical temperature elevation threshold for human health effects, namely the operational adverse health effect threshold that forms the basis of the absorbed power and energy density basic restrictions. These basic restrictions for localized exposures at frequencies above 6 GHz were evaluated by comparing numerically computed temperature rise against the target temperature rise of 2.5 o C, which is the operational adverse health effect threshold divided by the occupational safety factor of 2. The numerical model employs the maximum absorbed power and energy density levels allowed by the occupational basic restriction for both pulsed and continuous wave exposures. These analyses were performed considering 3- and 4-tissue layer models and a variety of beam diameters, frequencies, and exposure durations. The smallest beam diameters were based on a study of theoretically achievable beam widths from half-wave resonant dipoles and show the impact of the averaging area on the computed temperature elevation. The results demonstrated that ICNIRP's assumed occupational safety factors in the frequency range above 6 GHz were not sufficiently maintained for all exposure scenarios and particularly for short pulse exposures at frequencies of 30 GHz or higher with small beam diameters. Worst-case tissue temperature elevations were estimated to be as much as 3.6 times higher than ICNIRP's target temperature increases. Consequently, the authors suggest a small modification in the application of the ICNIRP 2020 localized basic restrictions, thereby limiting the worst-case tissue temperature increases to 1.4 times the target value.

Steady state temperature rise in multilayered tissue due to arbitrary periodic SAR using finite difference FFT and transfer function method.

Steady state (SS) and transient temperature-rise in tissue from radiofrequency exposure forms the underlying basis for limits in international exposure guidelines. Periodically pulsed or intermittent exposures form a special case of having both peak and average levels, producing temperature-rise oscillations in the SS. Presented here is a method for determining tissue temperature-rise for periodic specific absorption rate (SAR) modulation having arbitrary waveform. It involves the finite difference solution of a form of the Pennes Bioheat Transfer equation (BHTE) and uses the concept of the transfer function and the Fast Fourier Transform (FFT). The time-dependent BHTE is converted to a SS harmonic version by assuming that the time-dependent SAR waveform and tissue temperature can both be represented by Fourier series. The transfer function is obtained from solutions of the harmonic BHTE for an assumed SAR waveform consisting of periodic impulses. The temperature versus time response for an arbitrary periodic SAR waveform is obtained from the inverse FFT of the product of the transfer function and the FFT of the actual SAR waveform. This method takes advantage of existing FFT algorithms on most computational platforms and the ability to store the transfer function for later re-use. The transfer function varies slowly with harmonic number, allowing interpolation and extrapolation to reduce the computational effort. The method is highly efficient for the case where repeated temperature-rise calculations for parameter variations in the SAR waveform are sought. Examples are given for a narrow, circularly symmetric beam incident on a planar skin/fat/muscle model with rectangular, triangular and cosine-pulsed SAR modulation waveforms. Calculations of temperature-rise crest factor as a function of rectangular pulse duty factor and pulse repetition frequency for the same exposure/tissue model are also presented as an example of the versatility of the method.

Model of Steady-state Temperature Rise in Multilayer Tissues Due to Narrow-beam Millimeter-wave Radiofrequency Field Exposure.

The assessment of health effects due to localized exposures from radiofrequency fields is facilitated by characterizing the steady-state, surface temperature rise in tissue. A closed-form analytical model was developed that relates the steady-state, surface temperature rise in multilayer planar tissues as a function of the spatial-peak power density and beam dimensions of an incident millimeter wave. Model data was derived from finite-difference solutions of the Pennes bioheat transfer equation for both normal-incidence plane waves and for narrow, circularly symmetric beams with Gaussian intensity distribution on the surface. Monte Carlo techniques were employed by representing tissue layer thicknesses at different body sites as statistical distributions compiled from human data found in the literature. The finite-difference solutions were validated against analytical solutions of the bioheat equation for the plane wave case and against a narrow-beam solution performed using a commercial multiphysics simulation package. In both cases, agreement was within 1-2%. For a given frequency, the resulting analytical model has four input parameters, two of which are deterministic, describing the level of exposure (i.e., the spatial-peak power density and beam width). The remaining two are stochastic quantities, extracted from the Monte Carlo analyses. The analytical model is composed of relatively simple functions that can be programmed in a spreadsheet. Demonstration of the analytical model is provided in two examples: the calculation of spatial-peak power density vs. beam width that produces a predefined maximum steady-state surface temperature, and the performance evaluation of various proposed spatial-averaging areas for the incident power density.

Magnetic Field Reference Levels for Arbitrary Periodic Waveforms for Prevention of Peripheral Nerve Stimulation.

Guidelines for prevention of peripheral nerve stimulation from exposure to low frequency magnetic fields have been developed by standard-setting bodies. Exposure limits or reference levels (RLs) are typically set in terms of the maximum root-mean-square amplitude of a sinusoidal waveform; however, environmental flux densities are often periodic, non-sinusoidal waveforms. This work presents a procedure for deriving RLs for any generalized periodic waveform using the empirical nerve-stimulation threshold data obtained from human volunteer MRI experiments. For this purpose, the "Law of Electrostimulation" (LOE), which sets forth conditions of a waveform necessary to trigger the action potential required to depolarize cell membranes, is applied to various waveforms. The results of the LOE analysis are waveform-specific, amplitude thresholds of stimulation that are found in terms of the empirically-derived rheobase threshold time-rate-of-change flux density and chronaxie from trapezoidal pulse MRI experiments. The thresholds are converted to amplitude RLs in two asymptotic frequency regimes as per the usual practice in standard setting. The resulting RLs have the same frequency dependence as in existing standards (i.e., inverse-frequency below a transition frequency and flat above). It is shown that the transition frequency is dependent only on the shape of the waveform. Both sinusoidal and non-sinusoidal waveforms have identical peak-to-peak amplitude RLs above their respective transition frequencies. Below these frequencies, all peak-to-peak amplitude RLs have the same functional dependence on frequency when the frequency is normalized to the waveform-specific transition frequency. This results in simple criteria for testing the amplitude of any arbitrary periodic waveform against potential for stimulation. These criteria are compared to guidance given for non-sinusoidal waveforms in the ICNIRP 1 Hz-100 kHz exposure standard.

Cylindrical waveguide electromagnetic exposure system for biological studies with unrestrained mice at 1.9 GHz.

This paper presents the development of an in vivo exposure system for exposing small rodents. The system consists of four identical cylindrical waveguide chambers, each with a plastic cage for housing the animal. The chamber is fed by circularly polarized radiofrequency power in the 1.9 GHz cellular frequency band and is vertically mounted so that the long axis of the animal is co-planar with the rotating incident electric field. Power sensors were used along with directional or hybrid couplers and a digital voltmeter for data acquisition for real-time dose rate monitoring. The system was tested to evaluate its dose rate performance when a mouse phantom or a mouse cadaver was inside the cage. The dose rate was quantified in terms of whole-body-average (WBA) specific absorption rate (SAR) per input power using both measurement and computational methods. The exposures of the mouse phantom and cadaver were evaluated for various possible postures and positions. The measurement results showed that the highest WBA-SAR was 16.9 W kg per 1 W incident power when the cadaver was lying prone against the cage wall and the lowest WBA-SAR was 10.4 W kg per 1 W incident power when the cadaver was standing upright in the cage center. These results were found to be in good agreement with those obtained from the computational method.

Dosimetry evaluation of a cylindrical waveguide chamber for unrestrained small rodents at 1.9 GHz.

An exposure system, consisting of four identical cylindrical waveguide chambers, was developed for studying the effects of radiofrequency (RF) energy on laboratory mice at a frequency of 1.9 GHz. The chamber was characterized for RF dose rate as a function of animal body mass and dose rate variations due to animal movement in the cage. Dose rates were quantified in terms of whole-body average (WBA) specific absorption rate (SAR), brain average (BA) SAR and peak spatial-average (PSA) SAR using measurement and computational methods. Measurements were conducted on mouse cadavers in a multitude of possible postures and positions to evaluate the variations of WBA-SAR and its upper and lower bounds, while computations utilizing the finite-difference time-domain method together with a heterogeneous mouse model were performed to determine variations in BA-SAR and the ratio of PSA-SAR to WBA-SAR. Measured WBA-SAR variations were found to be within the ranges of 9-23.5 W/kg and 5.2-13.8 W/kg per 1 W incident power for 20 and 40 g mice, respectively. Computed BA-SAR variations were within the ranges of 3.2-10.1 W/kg and 3.3-9.2 W/kg per 1 W incident power for 25 and 30 g mouse models, respectively. Ratios of PSA-SAR to WBA-SAR, averaged over 0.5 mg and 5 mg tissue volumes, were observed to be within the ranges of 6-15 and 4-10, respectively.