Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle α with respect to B0. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 μm). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at α=0°, 45°, and 90°. As α increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B0 without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.