Study type: Medical/biological study (experimental study)

Response, thermal regulatory threshold and thermal breakdown threshold of restrained RF-exposed mice at 905 MHz med./bio.

By: Ebert S, Eom SJ, Schuderer J, Apostel U, Tillmann T, Dasenbrock C, Kuster N
Published in: Phys Med Biol 2005; 50 (21): 5203-5215
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Aim of study (acc. to author)

Determination of the thermal regulatory and the thermal breakdown thresholds for in-tube restrained mice exposed to radiofrequency electromagnetic fields at 905 MHz.

Background/further details

Three regions of thermal response are distinguished: 1) a non-thermal regulatory region, where no measurable temperature response occurs, 2) a thermal regulatory region, where the regulatory system is able to compensate the absorbed energy (to achieve a stable body temperature), and 3) a thermal breakdown region, where the organism is not able to compensate the temperature increase.
For B6C3F1 mice, the thermal response was investigated for three different weight groups (20 g, 24 g, 29 g), both genders and for pregnant mice. In addition, NMRI mice with a weight of 36 g were examined for an interstrain comparison.

Endpoint

Exposure

Exposure Parameters
Exposure 1: 905 MHz
Exposure duration: continuous for 2 h
  • SAR: 20 W/kg average over mass (whole body) (2, 5, 7.2, 10, 12.6, 20 W/kg)

Exposure 1

Main characteristics
Frequency 905 MHz
Type
Charakteristic
  • guided field
Exposure duration continuous for 2 h
Additional info Taylor B N and Kuyatt C E ,1994, Guidelines for evaluating and expressing the uncertainty of NIST measurement results, Nat. Inst. Stand. & Technol. (Washington, DC), p 26.
Modulation
Modulation type unspecified
Exposure setup
Exposure source
Distance between exposed object and exposure source 111.5 mm
Chamber The mice were exposed inside a 'Mini Wheel Setup', the concept of which was derived from the 'Ferris Wheel Setup' [Balzano et al., 2000]. The mini wheel setup was designed as a mono-mode cavity resonant waveguide structure of two parallel, circularly shaped metallic plates (diameter 332 mm, separation 120 mm) shorted with metallic bars along the outer edge and with an isotropic antenna at the centre. The system was matched for resonance (return loss ≤ -18 dB) using a triple stub tuner.
Setup The mice were circularly positioned in restrainer tubes at a radial distance of 111.5 mm from the antenna with their body axis oriented parallel to the E-field.
Sham exposure A sham exposure was conducted.
Additional info Groups of eight mice were exposed to a series of increasing SAR levels on consecutive days (one exposure per mouse group per day). Four additional animals of the same breed, gender and weight group were always sham exposed in a second setup.
Parameters
Measurand Value Type Method Mass Remarks
SAR 20 W/kg average over mass measured and calculated whole body 2, 5, 7.2, 10, 12.6, 20 W/kg
Measurement and calculation details

Forward and reflected powers as well as the electric field inside the cavity were measured for exposure control. Power measurements were made using a dual directional coupler and a dual head power meter. The incident E-field inside the wheel was measured by two monopole sensors connected to diode detectors. A detailed numerical and experimental dosimetry and a comprehensive uncertainty assessment of the mini wheel setup were performed. The numerical dosimetry was based on FDTD simulations with high resolution mouse models for different animal sizes (20 g, 25 g, 30 g, 35 g). The numerical results were verified by experimental dosimetry subjecting mouse dummies (conical bottles filled with tissue simulating liquid) to precise E-field (using a near-field scanner DASY4) and temperature measurements. In addition to the whole-body SAR, detailed organ/tissue specific SAR values were determined and shown in a table. The uncertainty budget was assessed according to Taylor and Kuyatt [1994], taking all the occurring variabilities and variations into account. An uncertainty assessment for the whole-body and organ averaged SAR was performed according to Kuster et al [2006, publication 13770].

Reference articles

  • Balzano Q et al. (2000): An efficient RF exposure system with precise whole-body average SAR determination for in vivo animal studies at 900 MHz

Exposed system:

Methods Endpoint/measurement parameters/methodology

Investigated system:
Time of investigation:
  • before exposure
  • during exposure
  • after exposure

Main outcome of study (acc. to author)

The thermal regulatory threshold of in-tube restrained mice was revealed at SAR levels between 2 W/kg and 5 W/kg, whereas the breakdown of regulation was determined at 10.1 +/- 4.0 W/kg for B6C3F1 mice and 7.7 +/- 1.6 W/kg for NMRI mice. The thresholds show a clear dependence upon the metabolic rate and weight. NMRI mice were more sensitive to thermal stress and responded at lower SAR values with regulation and breakdown.
The data suggest that the thermal breakdown for restrained mice, exposed to radiofrequency fields, may occur at SAR levels of 6 W/kg at laboratory conditions.

Study character:

Study funded by

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