| The finite difference time domain (FDTD) method is used to evaluate the radiofrequency (RF) magnetic (B1) field in heterogeneous models of the human body as would be produced during MRI, and also to evaluate the energy absorbed by the body due to the generation of the B 1 field in human tissues. Evaluations of (1) model spatial resolution necessary to evaluate compliance with regulatory limits on Specific energy Absorption Rate (SAR) and (2) the agreement of the calculated B1 field distribution with experimentally measured fields for similar cases are performed. Methods for calculating experimentally valuable measures of B1 homogeneity, signal-to-noise ratio (SNR), and SAR are presented, and methods for determining what imaging parameters can be used without exceeding SAR level limits are demonstrated. Finally, calculations are performed at several frequencies as high as 345 MHz in both a surface coil over the chest and an idealized birdcage coil over the head. The method by which FDTD models of human anatomies were created using the Visual Human Project of the National Libraries of Medicine is also presented. Pertinent values from these calculations are presented so that in future experiments using such coils in these regions, estimates of expected image B 1 homogeneity, SNR, and SAR for specific imaging parameters can be derived. It is shown that B1 homogeneity and coil sensitivity decrease with increasing B1 frequency. Calculated SAR levels generally increase significantly with B1 frequency for a given set of imaging parameters. Most importantly, it is shown that trends in the B1 field pattern, SNR, and SAR for a specific human geometry and RF coil can be calculated with fairly good agreement with experiment using the methods presented here. While further experiments and calculations are suggested to better assess the accuracy of the calculations and to improve that accuracy, the methods presented here could potentially be very useful in designing coils for use in MRI at high B1 frequencies and in designing experiments at high B1 frequencies that will not exceed standard limits on SAR. |
