NUMERICAL FDTD MODELS OF ELECTROMAGNETIC FIELD GENERATED BY THE RF COIL OF AN MRI SCANNER: COMPARISON AMONG DIFFERENT SOLUTIONS

摘要

Numerical models represent a very useful approach to investigate the effects of MRI on implanted medical devices. The finite-difference time-domain (FDTD) method was used to model the radiofrequency (RF) field generated by a birdcage resonator. We developed four numerical models which differ in terms of RF excitations and computational complexity, but which all produce a circular-polarized magnetic field at 64 MHz: a realistic 16-leg birdcage tuned by capacitors; a 16-leg birdcage with leg currents fixed to a specified value; a 2-plane waves excitation model and a 4-plane waves excitation model. Our interest was to evaluate the computational efficiency of the models, and to compare the resulting field pattern, as well as the induced specific absorption rate (SAR). It is shown that the assumption of fixed current inside the coil's leg does not significantly affect the results only when the study involved homogeneous domain, but not in the case of implanted objects. Moreover, by using a superposition of four plane waves, it is possible to obtain a field distribution surprisingly similar to the one of a fully modelled birdcage, drastically reducing the computation effort required by the simulation.