31P spectroscopy in human calf muscle at 7 tesla using a balanced double-quadrature proton- phosphorus RF coil

Introduction. Heteronuclear spectroscopy benefits from the increases in both sensitivity and spectral resolution at high fields. 31P experiments on calf muscle form one of the most common type of experiments to study energy metabolism in vivo. The aim of this work is to design a high sensitivity double-tuned 1H/31P coil for calf studies at 7T. The basic geometry is a double-quadrature double-tuned co-planar coil pair. Although the sensitivity of the proton channel could be sacrificed if only shimming and scout imaging are to be performed, the potential use of nOe enhancement and proton decoupling makes it desirable to have as high an efficiency on the proton channel as possible. In the co-planar overlapping quadrature surface coil design, counter-rotating currents lead to reduced proton sensitivity. Alecci [1] and Dabirzadeh [2] have shown that simple trap insertion into the heteronuclear coil can prevent these countercurrents. However, in a multi-segmented coil, insertion of a single trap-circuit reduces the coil symmetry significantly. Although multiple trap circuits could be introduced, a better solution is to incorporate a second-order lumped-element circuit which produces an open circuit at the proton frequency and a capacitance equivalent to the other series capacitors at the heteronuclear frequency. The design and performance of such a coil is shown here.