Experimental verification of numerically predicted electric field distributions produced by a radiofrequency coil

There are safety issues regarding energy deposition within tissues due to radiofrequency fields used in some magnetic resonance (MR) procedures. Procedures should be compliant with guidelines that specify limits to temperature elevation and specific absorption rate (SAR). In general, direct measurement of these quantities in patients is impractical and an alternative approach is to determine SAR from the electric field (E-field) distributions predicted by numerical models. In this initial study the E-field distribution in a tissue-simulating phantom due to a square coil driven at 31 MHz is predicted using a finite-difference time domain (FDTD) solution to Maxwell's equations. An experimental arrangement of the same problem was constructed and the resulting E-field distribution was measured using a calibrated minimally perturbing E-field probe. A comparison between experimentally and theoretically derived data showed that the numerically predicted E-fields were within +/-1 dB of the fields measured with the E-field probe in the phantom material. The results provide confidence in the use of the FDTD algorithm to determine quantitatively accurate E-field distributions arising from square radiofrequency (RF) coils used in MR procedures. The accuracy of numerical models of other coil designs such as bird cages, saddles and surface coils can be investigated in the same manner. Future studies will evaluate the exposure of patients to these RF fields.