Spatial distribution of high-frequency electromagnetic energy in human head during MRI: numerical results and measurements.

Finite Element Method (FEM) using 26-node isoparametric finite elements was applied for modeling saddle-shaped head coils used in Magnetic Resonance Imaging (MRI) generating linearly polarized radiofrequency (RF) pulses at 64 MHz. The human head was modeled from MR scans of a volunteer and additional information were taken from Atlas of Sectional Human Anatomy. The physical dimensions of the head coil and the head permit a calculation of the outside magnetic field by a quasistatic approach. Of course, a full-wave approach was applied within the head. Values of specific energy--specific absorption (SA)--as well as of specific power--specific absorption rate (SAR)--were calculated by the method, simulating the real exposure conditions during MRI. Although the results of the used numerical method were compared previously to the results of the analytical solution with homogeneous sphere and to the results of RF measurements on heterogeneous phantom, a comparison between the numerical results of the modeled human head and in vivo measurements performed on the human head of the volunteer was made once more. Since the results are in excellent agreement, they argue for the correctness of the numerical method. The "worst-case" temperature elevations delta theta of the "hot-spots" were calculated, as well. Finally, the results of SA, SAR, and delta theta are compared to the existing recommendations.