A Green's function approach to local rf heating in interventional MRI

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf co ils that are being increasingly used in interventional MRI. This work prese nts a two-step model for rf heating in an interventional MRI setting: (1) t he spatial distribution of power in the sample from the rf pulse (Maxwell's equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equat ion). The tissue bioheat equation is approximated as a linear, shift-invari ant system in the case of local rf heating and is fully characterized by it s Green's function. Expected temperature distributions are calculated by co nvolving (averaging) transmit coil specific absorption rate (SAR) distribut ions with the Green's function. When the input SAR distribution is relative ly slowly varying in space, as is the case with excitation by external rf c oils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (DeltaT). However, for h ighly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Green's function method predicts heating t hat is significantly different from the averaging method in current regulat ions. In our opinion, the Green's function method is a better predictor sin ce it is based on a physiological model. The Green's function also elicits a time constant and scaling factor between SAR and DeltaT that are both fun ctions of the tissue perfusion rate. This emphasizes the critical importanc e of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body he ating. (C) 2001 American Association of Physicists in Medicine.