Optimization of multi-element transverse field radio frequency surface coils

Abstract

In several MRI and MRS applications it is necessary to select a well-defined region of interest (ROI) with optimized signal-to-noise ratio and radio frequency (RF) field homogeneity. We have shown, theoretically and experimentally, that transverse field RF surface coils comprising a multitude of parallel linear elements are characterized by improved B1 spatial homogeneity and sensitivity in the central ROI, with respect to the standard square loop design. Finite-element modelling of RF coils tuned to 64.0 MHz was used to study the effect of the number of linear current elements and relative positioning with respect to B1 field homogeneity and sensitivity in the central ROI, both when empty and in the presence of a homogeneous tissue model. Workbench B1 field calibrations obtained with RF coil prototypes tuned to 64.0 MHz are in good agreement with our theoretical finite-element modelling. We have shown that the number of linear current elements and their relative positioning can be carefully selected to optimize either the B1 field homogeneity or sensitivity in the central ROI. We have shown that a number of multi-element RF coil geometries can be chosen to obtain an optimized RF field distribution and/or field amplitude. For example, we found that a six-element RF coil with 6 mm element separation is a good compromise for optimizing the B1 sensitivity (up to a factor 2) and the spatial homogeneity (about 30 mm with ΔB1/B1 ⩽ 20%) in the central ROI, with respect to the standard square loop or the two-element FO8 design. MRI images obtained at 2.35 T (proton frequency 100.34 MHz) with standard GE pulse sequences showed that the six-element FO8 prototype significantly increases the RF field homogeneity and RF field amplitude, with respect to the SL coil design.