Using electromagnetic field calculations to understand the complexity of magnetic resonance imaging (MRI) at high magnetic field strength

Electromagnetic field calculations have often been utilized, even for fundamental problems, in magnetic resonance imaging (MRI). For such calculations to be truly useful, however, they must fulfill two criteria. First, they must be accurate descriptions of the problem. For MRI this can be difficult due to complex electrical behavior of RF coils and the presence of the human subject. Second, the results must be expressed in a form that has some meaning in the MR experiment. Drawing from several sets of calculation and experiments that we have performed, we describe the methods that we use to perform calculations and relate results to the MR experiment. We show that even very complicated signal intensity patterns seen in experimental images at 300 MHz in a head-sized phantom can be explained with Maxwell's equations and current MR theory when the theory of reciprocity is applied correctly. The methods and results presented demonstrate, unequivocally, that flip angle and coil sensitivity are related to circularly-polarized components of the B1 field with opposite polarization. We have also demonstrated the importance of considering the experimental methods used in defining the RF excitation pulse in order to relate results to experiment accurately.