SAR and Temperature Distributions in Canonical Head Models Exposed to Near- and Far-Field Electromagnetic Radiation at Different Frequencies

Abstract

This article presents numerical computations of the specific absorption rate (SAR) distribution and temperature elevation in a canonical model of the human head—homogeneous spherical models exposed to near-field dipoles and far-field plane waves at 400 MHz and 900 MHz. Two homogeneous spheres filled with average brain equivalent materials are investigated: 18 cm approximating an adult head and 10 cm for child-head sizes. The distribution of SAR and temperature elevation computed for the spherical models are analyzed and compared. The computed SAR and temperature distributions inside the head models are very different for the near-field dipole and far-field plane wave. The highest SAR in both models occurred near the surface, directly beneath the feed point of the dipole. The decrease in SAR was faster at 900 MHz than at 400 MHz for the spherical head in the near field. In contrast, the maximum SAR shifted to inside the spheres for far-field plane waves, and was characterized by resonant oscillations, in both cases. The results indicated that 400 and 900 MHz RF energy from near-field 1/4 λ dipole antennas can penetrate about 100 and 80%, respectively, deeper into the smaller head model. In the homogeneous spherical head models, the induced temperature patterns were correlated closely with SAR distributions. However, the distribution and location of maximum temperature rise was different from that of SAR. The results suggest that 400 and 900 MHz RF fields from near-field 1/4λ dipole antennas could produce temperature elevations inside the smaller head model—at greater distances—about 70% deeper than the larger model.