A NUMERICAL STUDY OF RADIOFREQUENCY DEPOSITION IN A SPHERICAL PHANTOMUSING SURFACE COILS

The electromagnetic fields induced by a surface coil in a spherical ph antom, having a wide range of electrical properties, is studied using numerical methods of calculation. The specific absorption fate (SAR), radiofrequency magnetic field (B-1), magnetic field energy within the phantom (E-B), and the volume-averaged SAR (<SAR>) are calculated at 1 0, 63, and 200 MHz. They are analyzed with respect to dielectric const ant, wavelength, and skin depth effects, which become increasingly imp ortant in high field magnetic resonance imaging (MRI) where safety and field homogeneity issues need further study. Particular attention is given to solutions representing neural tissue at each frequency. In ge neral, the <SAR> data at high field strengths have local maxima, with a quasi-harmonic behavior, when the following two resonant conditions are satisfied: 1) skin depth becomes comparable to, or larger than, th e sample diameter Ds; and 2) Ds is near an integral multiple of the wa velength. These are also the solutions,vith maximum E-B values and the least homogeneous B-1. Samples undergoing resonance at 200 MHz are sh own to have important off-axis B-1 maxima (affecting field homogeneity ) and large <SAR> values. Some non-resonating 200-MHz phantoms, includ ing simulations consistent with neural tissue, contain larger SAR maxi ma than the resonating samples, posing safety concerns in high field i maging of biologic tissue. (C) 1998 Elsevier Science Inc.