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Cocherova, Elena, Gabriel Malicky, Peter Kupec, Vladimir Stofanik, & Jozef Pucik. "Evaluation of Resonance Properties of the Human Body Models Situated in the RF Field." Advances in Electrical and Electronic Engineering[Online], 10.5 (2012): 345 - 349. Web. 7 Mar. 2013
Application of radiofrequency (RF) is becoming increasingly common in the development of medical therapies. One of the main concerns is the possibility for unwanted side effects. In this regard, temperature effects are perhaps the most commonly described. These effects are mainly attributed to the absorption of the RF field by human tissues. Estimating experimentally the impact of RF fields on bulky materials is generally a major task. As an alternative, scientists and engineers have implemented simulation strategies capable of mimicking such an environment quite precisely. The main objective of the simulation is to calculate the specific absorption rate (SAR) of RF, which is the velocity of RF assimilation upon contact with the tissue. The SAR mainly depends on the internal electric field (E). The human body is then approximated to the geometry of ellipsoids and cylinders. In such geometry, vectors for the electric and magnetic fields (H) and the rate of energy transfer (p) can configure differently leading to polarizations. If the E vector aligns parallel to the longitudinal axis of the body an E-polarization is obtained. Similarly if the H or p vectors align parallel to the longitudinal axis of the body, H- and p- polarizations are obtained, respectively. The SAR depends on each particular polarization and for instance, in the E-polarization SAR increases rapidly for frequencies below 30 MegaHz, reaches a maximum in the range between 30 and 300 MegaHz, decays from 400 MegaHz to 3000 MegaHz, and remains constant above 3000 MegaHz. The E-polarization is of most interest for RF studies considering that resonance is highly multiplied in this arrangement. Properties of muscle tissue were incorporated into the simulation to represent the human body as best as possible. Simulations for children’s bodies were achieved by simply reducing the height of the model ellipsoid or cylinder without changing the ratio of height to width of 3 to 1. A closer approximation to the geometry of the human body was accomplished by running then simulations on a tridimensional reconstruction of a woman’s body. The simulations allowed determining SAR distribution profiles on the model bodies and suggested that the higher the frequency the closer the maximum gets to surface of the body. Also, the shorter the body the higher the SAR and maximum frequency (resonance) will be. A direct comparison of SAR profiles for ellipsoids cylinders showed that resonance is higher for ellipsoids while the total SAR (area under the curve) is lower. Simulations for the woman’s model resulted in similar SAR profiles to those observed for cylinders and ellipsoids with the same height. Maximum values for frequencies or resonance values can be predicted theoretically when the height is exactly half of the wavelength of incident radiation. This estimate predicts values that are higher than those observed with the conducted simulation. So, new expressions for the frequency of resonance were proposed. Similarly, empirical expressions for the total SAR of ellipsoids and cylinders were derived.
The simulations allowed a very precise estimation of SAR impact on the human body through simple geometries such as ellipsoids and cylinders. Relatively small variations with respect to a woman’s body model were attributed to the lack of additional body shape parameters in the model. The resonance frequency defines a maximum for SAR that is explained in light of the penetration depth, which is a measure of how deep can the electromagnetic radiation penetrate into a material. So, at lower frequencies the penetration depth is considerable and only a small fraction of the incident radiation is absorbed, which is reflected in small values of SAR. On the contrary, at frequencies above the resonance frequency, the penetration depth is small and all incident radiation is absorbed and that’s the reason why the SAR is no longer increasing.