Design Considerations for Radiofrequency Whole-Body and Head Coils

It is shown that the transverse-plane B-1(+) homogeneity in TEM whole-body and head RF coils, which is responsible for acquiring uniformly illuminated MR images, is frequency independent and solely depends on achieving high excitation purity of the fundamental m = 1 mode. This necessitates that the capacitive coupling introduced by the interconnecting lumped capacitors should maximize the separation between the resonance frequency of the fundamental mode and that of all other higher order modes. Alternatively, full-orthogonal excitation of the coil, if possible, would mitigate this design challenge, especially if high-Q operation cannot be guaranteed. In this contribution, a field theoretical technique for the optimization of the capacitive coupling is introduced and applied to the birdcage configuration as a representative of TEM coils. A fairly general analysis of these coils is also introduced and/or revisited, which underlies the proposed technique and offers simple analytic design equations. Simulation results are presented to validate the considered approach. The presented analysis deals with the homogeneously filled coils and therefore does not consider the B-1(+) inhomogeneities caused by the loading. The latter will be the subject of future publications.