Development and Implementation of a PIN-Diode Controlled, Quadrature-Enhanced, Double-Tuned RF Coil for Sodium MRI

Sodium (Na-23) MRI provides complementary cellular and metabolic information. However, the intrinsic MR sensitivity of Na-23 is considerably lower compared with that of the proton, making it difficult to measure MR-detectable sodium signals. It is therefore important to maintain the signal-to-noise ratio (SNR) of the sodium signal as high as possible. Double-tuned coils are often employed in combination with a H-1 coil, providing structural images and B-0 shimming capability. The double-tuned coil design can be achieved with the use of two geometrically decoupled coils whose B-1 field directions are perpendicular to each other. This can be used to design quadrature-driven, single-nucleus coils to improve SNR, and additionally, this coil can also be utilized as a linear-driven double-resonant mode. Here, we have developed and evaluateda quadrature-enhanced, double-tuned coil. The novel coil uses PIN-diode switches, inserted only in the loop coil, to shift the resonance frequency between H-1 and Na-23 so that Na-23 signals can be acquired in quadrature and the capability of using H-1 function remains. Consequently, the Na-23 SNR values obtained with the double-tuned coil are nearly 33% and 17% higher in comparison with geometrically identical single-tuned coils. SNR plots also show the superiority of double-tuned coil in Na-23.