The Development of a Reverberation Chamber for the Assessment of Biological Effects of Electromagnetic Energy Absorption in Mice

Abstract

In this paper, we present the design, RF-EMF performance, and a comprehensive uncertainty analysis of the reverberation chamber (RC) exposure systems that have been developed for the use of researchers at the University of Wollongong Bioelectromagnetics Laboratory, Australia, for the purpose of investigating the biological effects of RF-EMF in rodents. Initial studies, at 1950 MHz, have focused on investigating thermophysiological effects of RF exposure, and replication studies related to RF-EMF exposure and progression of Alzheimer's disease (AD) in mice predisposed to AD. The RC exposure system was chosen as it allows relatively unconstrained movement of animals during exposures which can have the beneficial effect of minimizing stress-related, non-RF-induced biological and behavioral changes in the animals. The performance of the RCs was evaluated in terms of the uniformity of the Whole-Body Average-Specific Absorption Rate (WBA-SAR) in mice for a given RF input power level. The expanded uncertainty in WBA-SAR estimates was found to be 3.89 dB. Validation of WBA-SAR estimates based on a selected number of temperature measurements in phantom mice found that the maximum ratio of the temperature-derived WBA-SAR to the computed WBA-SAR was 1.1 dB, suggesting that actual WBA-SAR is likely to be well within the expanded uncertainties.

Keywords: SAR validation; computational modeling; reverberation chamber; uncertainty.

Figure 1

One of the RCs at UoWBL shows top and bottom shelving within the RC and associated equipment rack. Also visible are four of the six antennas, one of the three stirrers (bottom back right), with a video monitor placed on top of the RC. An antenna is placed approximately centrally on each of the sides of the RC, except for the front, where an antenna is placed on the panel to the right of the door.

Figure 2

Schematic diagram of the UoWBL reverberation chamber (RC) exposure system showing key elements of the monitoring and control system. Signal cables: USB (dotted line) and analog signals (dashed and solid line).

Figure 3

Typical distributions at measurement points. CDFs from one of the 39 measurement points show distributions of individual components of the field and a Rayleigh distribution (K = 0) for comparison. Fitting a Rician distribution to the components yields the following K‐factors and stirring efficiencies (%): 0.1622 (86%), 0.1682 (85.6%), and 0.2514 (79.9%) for |E _x |, |E _y |, and |E _z |, respectively.

Figure 4

(a and b) Computational models, showing the two shelves of mice with four mice per cage. The different colors of the Swinburne and IT'IS mice are not meaningful and is an artefact of the different input data formats and does not affect the calculations. (a) Shows mice arranged randomly to mimic typical behavior, while (b) shows a uniform arrangement that matches the tubes shown in Figure 5.

Figure 5

Hybrid method. In the computational model (left side, showing a scenario where there are four phantom mice in each cage), computations were performed to determine the mean square field <|E _mod | ² > on the center plane across 13 sample points and the average WBA‐SAR across all N phantom mice. In the reverberation chamber (RC; right side), measurements were performed to determine the mean square field <|E _meas | ² > across 13 sample points on the center plane for a constant P _in to the RC antennas. The computation (left side) shows the center plane distribution of the magnitude of the E‐field with a 1 dB difference between red (the peak) and orange colors. (The data was generated through user‐written code, calculating the average over the 52 plane wave simulations of the square of the magnitude of the E‐field at each point. The square root of these values was then fed back into XFDTD to create the plot.).

Figure 6

Example of measurement of the temperature rise (T_meas) in a horizontal mouse phantom and the exponential curve fit (T_exp_fit). The total number of phantoms in the reverberation chamber was 32 with a combined total mass of 0.76 kg. The RF was turned ON at 0 h and OFF after 4 h.