Comparison of Thermal Response for RF Exposure in Human and Rat Models

Abstract

In the international guidelines/standards for human protection against electromagnetic fields, the specific absorption rate (SAR) is used as a metric for radio-frequency field exposure. For radio-frequency near-field exposure, the peak value of the SAR averaged over 10 g of tissue is treated as a surrogate of the local temperature elevation for frequencies up to 3–10 GHz. The limit of 10-g SAR is derived by extrapolating the thermal damage in animal experiments. However, no reports discussed the difference between the time constant of temperature elevation in small animals and humans for local exposure. This study computationally estimated the thermal time constants of temperature elevation in human head and rat models exposed to dipole antennas at 3–10 GHz. The peak temperature elevation in the human brain was lower than that in the rat model, mainly because of difference in depth from the scalp. Consequently, the thermal time constant of the rat brain was smaller than that of the human brain. Additionally, the thermal time constant in human skin decreased with increasing frequency, which was mainly characterized by the effective SAR volume, whereas it was almost frequency-independent in the human brain. These findings should be helpful for extrapolating animal studies to humans.

SAR optimization for multi-dipole antenna array with regard to local hyperthermia

“…Fig. 5 illustrates a heat accumulation in a small sphere that is directly due to the increased SAR value in this area, according to the Pennes equation (6). Moreover, the temperature in the target region does not exceed therapeutic level 42°C after an exposure time 30 min, which creates good conditions for localized HT treatment.…”

“…The challenge of modern hyperthermia is to design such HT systems that selectively heat only cancerous tissue, keeping intact healthy tissues [4]. Developing research connected with the EM energy usage results in the emergence of novel radiating elements [5] that allow for constant and controlled EMF exposure conditions [6,8]. Importantly, the use of high frequencies limits the area of tissue penetration [1].…”

“…According to bioheat equation, the requested temperature levels of given tissue are connected with corresponding specific absorption rate (SAR) values [14]. Very often, for locoregional tissue heating the single dipole antenna [6], their arrays [15], as well as more complex antenna matrixes [9,11,13,16] are successfully utilized.…”

Special Issue: “Electric, Magnetic, and Electromagnetic Fields in Biology and Medicine: From Mechanisms to Biomedical Applications”

“…A couple of other relevant papers are collected in this special issue that address the topic of the extrapolation of the outcome of animal studies to humans [13], and the identification and description of methods using non-ionizing radiation (NIR) such as EMF and optical radiation in Swedish health care [14]. Specifically, Kodera and Hirata computationally estimated the thermal time constants of temperature elevation in human head and rat models exposed to dipole antennas at 3–10 GHz [13], while Hansson Mild and coworkers identified three applications in Swedish health care (magnetic resonance imaging (MRI), transcranial magnetic stimulation (TMS), and electrosurgery) where acute effects at existing exposure levels could not be ruled out [14].…”

“…Specifically, Kodera and Hirata computationally estimated the thermal time constants of temperature elevation in human head and rat models exposed to dipole antennas at 3–10 GHz [13], while Hansson Mild and coworkers identified three applications in Swedish health care (magnetic resonance imaging (MRI), transcranial magnetic stimulation (TMS), and electrosurgery) where acute effects at existing exposure levels could not be ruled out [14]. …”