SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil. Specific energy absorption rate.

Calculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi-tissue human head model in a 12-element birdcage coil are presented. The coil model is driven in linear and quadrature modes at 63, 175, 200, and 300 MHz. Plots of B1 field magnitude and SAR distributions, average SAR, maximum local SAR, and measures of B1 field homogeneity and signal-to-noise ratio are given. SAR levels for arbitrary pulse sequences can be estimated from the calculated data. Maximum local SAR levels are lower at lower frequencies, in quadrature rather than in linear coils, and in linear fields oriented posterior-to-anterior rather than left-to-right in the head. It should be possible to perform many experiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.