Cavity Resonators - Theory & Possibilities for MRI

Introduction The majority of RF coils are formed from inductive lengths of copper in various geometries, which are tuned to resonance by incorporation of appropriate capacitors. Volume coils are based on geometries which have a (co)-sinusoidal variation in azimuthal current distribution to produce a homogeneous transverse RF field [1]. An alternative approach is to design a cavity resonator, in which the resonance frequency is determined by the material properties (in particular the relative permittivity) and the dimensions of the structure. Advantages of such designs include simplicity and robustness, the lack of multiple lumped elements, and very low losses. Such devices have found much use in electron paramagnetic resonance [2], but only with the advent of very high magnetic fields for human/animal/biochemical studies have the dimensions of cavity resonators become relevant for magnetic resonance. Historically, it is interesting to note that the probe used in the very first NMR experiments was a cavity resonator [3].