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.

In view of potential thermal hazards, there is a need to determine the specific absorption rate (SAR) distributions associated with radiofrequency coils used in magnetic resonance imaging (MRI) (typically 10-120 MHz). Electric field (E-field) distributions in tissue-equivalent phantoms may be determined using a probe comprising a dipole antenna and a detector. The geometry of the dipole dictates the sensitivity of the device, thus two designs are discussed in this paper. Both probes are compact, have a spatial resolution of 2.5 cm3, operate at MR frequencies and have a response independent of the dielectric characteristics of the phantom material. Calibration of these probes requires a system capable of producing a known E-field both in air and in a tissue-like medium at frequencies between 10 and 120 MHz. Transverse electromagnetic wave (TEM) cells answering these specifications are described and the calibration procedure outlined. Accurately calibrated E-field probes can make field measurements in phantoms which can be used to verify predictions from numerical models. These numerical techniques may then be used to predict E-fields, and hence SAR, in patients.

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