Design and use of a folded four-ring double-tuned birdcage coil for rat brain sodium imaging at 9.4 T

A folded four-ring quadrature birdcage coil was designed and constructed with a double-tune configuration of an outer high-pass coil for ¹H (400 MHz) and inner low-pass coil for ²³Na (105.72 MHz at 9.4 T). The coil was evaluated on the bench and in the scanner, comparing its performance with that of single-tuned coils and a large four-ring coil. All coils were tuned and matched and the isolation between two quadrature ports was found to be better than -13.7 dB for ¹H and -27 dB for ²³Na. Signal-to-noise ratios (SNRs) were calculated and ²³Na flip angle maps were acquired. ²³Na SNR of the folded four-ring reached ∼93% of that obtained with the single-tuned coil. A set of in vivo¹H and ²³Na axial images to cover the whole rat brain were obtained. The performance of the folded four-ring coil and its benefit for ²³Na imaging experiments have been demonstrated. This proposed four-ring coil could avoid length restrictions, e.g. the shoulders, by folding the outer rings vertically. This facilitates the construction of double-tuned four-ring birdcage coils just to fit the head, leading to higher filling factors and better SNR.