Use of tailored higher modes of a birdcage to design a simple double‐tuned proton/phosphorus coil for human calf muscle studies at 7 T

A large number of designs exist for double‐ or multiple‐tuned radiofrequency (RF) coils, based largely upon the use of trap circuits (either bandstop or bandpass) or two circuits with significant mutual inductive coupling. An alternative approach is to use an RF coil, which intrinsically has several resonant modes. As is well‐known, the homogeneity of the N /2 resonant modes of an N leg birdcage decreases significantly with N . However, here we show that at high field (7 T) driving even a very high mode produces sufficient signal throughout the calf muscle to allow anatomical proton imaging, Dixon‐based fat/water separation and localized B 0 shimming for 31 P MR spectroscopy. The full sensitivity of the 31 P coil is maintained which is important for applications in muscle diseases such as muscular dystrophies, where fat infiltration leads to significant reduction in muscle tissue. In addition to showing 1 H and 31 P data sets from several volunteers, empirical formulae for estimating the frequencies of higher birdcage modes are derived, and electromagnetic simulations of coil performance presented.