SAR AND B-1 FIELD DISTRIBUTIONS IN A HETEROGENEOUS HUMAN HEAD MODEL WITHIN A BIRDCAGE COIL

Calculations of radiofrequency magnetic (B-1) field and specific energ y absorption rate (SAR) distributions in a sphere of tissue and a mult i-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, 2 00, and 300 MHz. Plots of B-1 field magnitude and SAR distributions, a verage SAR, maximum local SAR, and measures of B-1 field homogeneity a nd signal-to-noise ratio are given. SAR levels for arbitrary pulse seq uences can be estimated from the calculated data. Maximum local SAR le vels are lower at lower frequencies, in quadrature rather than in line ar coils, and in linear fields oriented posterior-to-anterior rather t han left-to-right in the head. It should be possible to perform many e xperiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.