D. H. Gultekin and P. H. Siegel, “Absorption of 5G Radiation in Brain Tissue as a Function of Frequency, Power and Time,” in IEEE Access, vol. 8, pp. 115593-115612, 2020, doi: 10.1109/ACCESS.2020.3002183.
Page(s): 115593 – 115612
Date of Publication: 12 June 2020
Electronic ISSN: 2169-3536 DOI: 10.1109/ACCESS.2020.3002183
Funding Agency: THz Global;
The rapid release of 5G wireless communications networks has spurred renewed concerns regarding the interactions of higher radiofrequency (RF) radiation with living species. We examine RF exposure and absorption in ex vivo bovine brain tissue and a brain simulating gel at three frequencies: 1.9 GHz, 4 GHz and 39 GHz that are relevant to current (4G), and upcoming (5G) spectra. We introduce a highly sensitive thermal method for the assessment of radiation exposure, and derive experimentally, accurate relations between the temperature rise ( ΔT ), specific absorption rate (SAR) and the incident power density ( F ), and tabulate the coefficients, ΔT/ΔF and Δ (SAR)/ ΔF , as a function of frequency, depth and time. This new method provides both ΔT and SAR applicable to the frequency range below and above 6 GHz as shown at 1.9, 4 and 39 GHz, and demonstrates the most sensitive experimental assessment of brain tissue exposure to millimeter-wave radiation to date, with a detection limit of 1 mW. We examine the beam penetration, absorption and thermal diffusion at representative 4G and 5G frequencies and show that the RF heating increases rapidly with frequency due to decreasing RF source wavelength and increasing power density with the same incident power and exposure time. We also show the temperature effects of continuous wave, rapid pulse sequences and single pulses with varying pulse duration, and we employ electromagnetic modeling to map the field distributions in the tissue. Finally, using this new methodology, we measure the thermal diffusivity of ex vivo bovine brain tissue experimentally.
Limitations of the Study
In this study, we use RF waveguide sources directly irradiating ex vivo bovine brain inside plastic containers to quantify the temperature rise and the specific absorption rate in the brain tissue as a function of frequency, incident power density and depth. However, this study does not take into account the layers of skin, fat, muscle and skull containing the brain in a realistic head model. The actual power density and the heat diffusion reaching the surface of the brain in a real head will obviously not be the same as the values we show in this study, where our power is incident directly on the brain tissue. However, the derived thermal coefficients and linear behavior are properties of the tissue and can be used to predict temperature changes as a function of incident RF power density. In addition, the behavior of skin, fat, muscle and bone can be roughly extrapolated within the same incident power regimes given a priori measurements of the dielectric and thermal diffusion constants. It is also interesting to note that the current safety limits of 28.76 and 143.8 W/m2 (public and occupational) applied to our directional RF waveguide source at 39 GHz would correspond to only 1.7 and 8.5 mW incident power levels, which is an extremely limited amount of power availability for a broad distribution communications system.
The accurate measurement of SAR by thermal methods such as ours near the surface and at low power density can be affected by the thermal diffusion time, and extra care must be taken. The diffusion length, d=2Dt−−−√ , must be shorter than the depth (d<z ) at which the SAR is being measured. This requires exposure to the radiation over a fairly short time for characterizing shallower depths (for example, t<2 s for d<1 mm) during the initial temperature rise and for fitting the ΔT/Δt and SAR =C(ΔT/Δt) . Although, short time exposures can easily be used for higher power densities and at higher frequencies, the accuracy of fitting the ΔT/Δt decreases at lower power density and frequency. For purposes of covering the greatest power density range in our bovine brain tissue, we used fitting times that yielded a highly linear slope of ΔT/Δt . Using Δt of 40, 30 and 20 s for 1.9, 4 and 39 GHz respectively, and the thermal diffusion coefficient of D=1.23⋅10−7m2 /s as measured in this study, we calculated the diffusion length (d ) as 4.4, 3.8 and 3.1 mm in brain tissue. This shows that SAR measurements at depths from 6 to 21 mm are free of thermal diffusion effects, whereas SAR measurements at 1 mm depth are somewhat affected by thermal diffusion over these fitting times.
In this paper, we present for the first time, a simple, highly accurate test system for measuring the temperature rise and the specific absorption rate in tissue samples and liquid or gel simulants as a function of frequency, RF exposure power and time – pulsed and CW. We use this setup to make, and compare, carefully calibrated measurements of bovine brain tissue and a gel simulant, Triton X and water, at both 4G (1.9 GHz) and newly allocated 5G frequency bands (4 GHz – 39 GHz). We show the effects of beam concentration, focusing, absorption and heat diffusion at all three frequencies and delineate a linear range over which we can derive highly accurate coefficients (ΔT/ΔF and Δ (SAR)/ΔF ) that can be used to predict the temperature rise and the specific absorption rate at prescribed depths and exposure times within the tissue or gel at power levels that go down to detectable limits (<1 mW). This method may be used to evaluate a wide range of RF radiation sources, tissues and simulants.
We also note that the impact of relatively modest incident RF power (1 W) and short exposure times (6 minutes CW and 30 second pulsed) at 39 GHz using a single mode waveguide source for the exposure, results in extremely large power density (16.5 kW/m2) and temperature rise (=60°C for CW, =35°C for 30 s pulse) in both bovine brain tissue and gel. This same temperature rise can be expected on skin (which has very similar dielectric properties) when such large surface power densities are present in very close proximity to the RF source or antenna, perhaps emanating from millimeter-wave base stations, handsets, or wireless-enabled appliances or kiosks. Although, current safety limits of 28.76 and 143.8 W/m2 for power density in unrestricted (public) and restricted (occupational) environments, respectively should prevent such exposures, the resulting limits on RF power generation of only 1.7 to 8.5 mW from a directional RF source, such as our waveguide at 39 GHz, in the vicinity, will greatly limit the application potential for any such communications system.
In the USA, the FCC and FDA are overseeing the implementation of millimeter wave technology in the public realm and more studies are needed to help guide the science, technology and policy. Our experimental method can provide threshold temperature and SAR values for both occupational and public exposures to millimeter waves with surface power densities from 16.5 W/m2 to 16.5 kW/m2 and exposure times from 1 second to 30 minutes.
Finally, we use our new data and this RF method to derive a thermal diffusivity coefficient for the ex vivo bovine brain tissue that is consistent with our prior measurements using an MRI. This is the first time that the thermal diffusivity of ex vivo bovine brain tissue has been directly measured by this thermal RF method , , , .