THE DESIGN AND TEST OF A NEW VOLUME COIL FOR HIGH-FIELD IMAGING

A major problem in the development of high field (>100 MHz) large volu me (>6000 cm(3)) MR coils is the interaction of the coil with the subj ect as well as the radiation loss to the environment. To reduce subjec t perturbation of the coil resonance modes, a volume coil that uses an array of freely rotating resonant elements radially mounted between t wo concentric cylinders was designed for operation at 170 MHz. Substan tial electromagnetic energy is stored in the resonant elements outside the sample region without compromising the efficiency of the overall coil. This stored energy reduces the effect of the subject on the circ uit and maintains a high Q, facilitating the tuning and matching of th e coil. The unloaded Q of the coil is 680; when loaded with a head, it was 129. The ratio of 5.3 of the unloaded to loaded Q supports the no tion that the efficiency of the coil was maintained in comparison with previous designs. The power requirement and signal-to-noise performan ce are significantly improved. The coil is tuned by a mechanism that i mparts the same degree of rotation on all of the elements simultaneous ly, varying their degree of mutual coupling and preserving the overall coil symmetry. A thin radiofrequency shield is an integral part of th e coil to reduce the radiation effect, which is a significant loss mec hanism at high fields. MR images were collected at 4T using this coil design with high sensitivity and B-1 homogeneity.