Double-tuned four-ring birdcage resonators for in vivo 31P-nuclear magnetic resonance spectroscopy at 11.75 T.

A high signal-to-noise ratio (SNR) in 31P-nuclear magnetic resonance (31P-NMR) spectroscopy can be obtained only with good B0-field homogeneity and optimal coil sensitivity. This demands double-tuned coils with a highly sensitive 31P channel and an additional 1H channel for 1H-magnetic resonance imaging, shimming, 1H decoupling, and nuclear Overhauser enhancement (NOE). For studies on an 11.75 T magnet, we built coils derived from the four-ring birdcage design originally described by Murphy-Boesch. A comparison with conventional, single-tuned coils shows that, in spite of double tuning, there is no significant loss in 31P sensitivity while the 1H channel provides the requested performance. The coil design offers the advantage of circular polarization on both channels.