Currents and fields in shielded RF resonators for NMR/MRI

A new method for calculating the current densities in shielded RF probes for NMR imaging and spectroscopy is presented. From these current densities, accurate field distributions can be calculated and the effects of shielding the probe deduced. Without accurate calculations of the current density in the shielded coil conductors, the performance of such coils cannot be reliably predicted. Shielded coils are shown not only to drastically reduce external flux leakage but also to improve the homogeneity of the coil structure. Preliminary experimental results for circular cross section RF resonators confirm the accuracy of the calculated fields. The method of current density calculation is applicable to coils of various cross sectional shapes. As an example, the current density and fields generated by a shielded ellipsoidal RF coil are calculated.