A method to separate conservative and magnetically-induced electric fields in calculations for MRI and MRS in electrically-small samples.

This work presents a method to separately analyze the conservative electric fields (E(c), primarily originating with the scalar electric potential in the coil winding), and the magnetically-induced electric fields (E(i), caused by the time-varying magnetic field B1) within samples that are much smaller than one wavelength at the frequency of interest. The method consists of first using a numerical simulation method to calculate the total electric field (E(t)) and conduction currents (J), then calculating E(i) based on J, and finally calculating E(c) by subtracting E(i) from E(t). The method was applied to calculate electric fields for a small cylindrical sample in a solenoid at 600MHz. When a non-conductive sample was modeled, calculated values of E(i) and E(c) were at least in rough agreement with very simple analytical approximations. When the sample was given dielectric and/or conductive properties, E(c) was seen to decrease, but still remained much larger than E(i). When a recently-published approach to reduce heating by placing a passive conductor in the shape of a slotted cylinder between the coil and sample was modeled, reduced E(c) and improved B1 homogeneity within the sample resulted, in agreement with the published results.