Evaluation of MR-induced hot spots for different temporal SAR modes using a time-dependent finite difference method with explicit temperature gradient treatment.

An investigation of magnetic resonance (MR)-induced hot spots in a high-resolution human model is performed, motivated by safety aspects for the use of MR tomographs. The human model is placed in an MR whole body resonator that is driven in a quadrature excitation mode. The MR-induced hot spots are studied by varying the following: (1) the temporal specific absorption rate (SAR) mode ("steady imaging", "intermittent imaging"), (2) the simulation procedure (related to given power levels or to limiting temperatures), and (3) different thermal tissue properties including temperature-independent and temperature-dependent perfusion models. Both electromagnetic and thermodynamic simulations have been performed. For the electromagnetic modeling, a commercial finite-integration theory (FIT) code is applied. For the thermodynamic modeling, a time-domain finite-difference (FD) scheme is formulated that uses an explicit treatment of temperature gradient components. This allows a flux-vector-based implementation of heat transfer boundary conditions on cubical faces. It is shown that this FD scheme significantly reduces the staircase errors at thermal boundaries that are locally sloped or curved with respect to the cubical grid elements.