Proton traps for multi-nuclear RF coils: design analysis and practical implementation for 13C MRS in humans at 7T

Introduction: Multi-nuclear MRS (C, P) and MRI (Na) are of particular interest at ultra high field. Trapped coil designs have been proposed which allow the operation of lower-frequency coils in the presence of a proton coil. In this work we give an analysis of useful parameter ranges for practical design of a trap circuit with two capacitors, and present first results headed towards a dual loop quadrature coil for acquisition of C signals in humans at a 7T. The trap blocks current induced in the lowfrequency coil at the H frequency but passes RF at the second nucleus’ frequency. Blocking circuits proposed consist of an inductor (LTr) either in parallel with a single capacitor [1–3] or a combination of a series and a parallel capacitor (Cs,Cp), see insert in Fig. 1 [4]. Methods: At higher fields, when using a single capacitor trap design, the required inductance may become impractically small. The second order trap design gives an additional degree of freedom in choosing inductance and capacitance values, opening the possibility to match the trap reactance at the low frequency to the capacitor it replaces (Ccoil). We analytically solved the conditions for the trap capacitances and inductance (Fig. 1), yielding a pair of solutions for both Cs and Cp, for a given LTr (Fig. 2). Results: The low frequency coil has two resonances: the original of the untrapped coil at low frequency and a higher frequency “trap mode”. Care must be taken to resonate the trap mode well above the proton frequency. Otherwise both coils couple strongly at the proton frequency, making the proton coil unusable. The frequency of the trap mode is controlled by