Dual-frequency, dual-quadrature, birdcage RF coil design with identical B1 pattern for sodium and proton imaging of the human brain at 1.5 T.

Rapid quantification of tissue metabolites in vivo by MRS or MRI can be achieved using dual-frequency RF coils with identical B1 field distributions at the observation frequencies of the metabolites and tissue water protons. Tissue sodium is used as an example for optimizing the dual-frequency, dual-quadrature RF coils for such measurements in humans. In the setting of sodium imaging, the challenge of dual-quadrature birdcage configurations is to decouple the sodium and proton channels because the fourth harmonic of the sodium frequency is very close to the proton frequency. A generalizable method for effectively decoupling these two RF frequencies is presented in this paper. The method is demonstrated with the design of an EPI compatible, dual-quadrature, double-tuned, 23Na/1H birdcage coil. The performance of the RF probe is reported at 1.5 Tesla in terms of signal-to-noise ratio, B1 homogeneity and image quality.