AN ANALYSIS AND OPTIMIZATION OF ELLIPTIC RF PROBES USED IN MAGNETIC-RESONANCE-IMAGING

In magnetic resonance imaging of the entire body, it is often desirabl e to use an elliptical RF probe, rather than a circular one. As an ell ipse more closely conforms to the anatomical cross section of the huma n thorax and head, better filling factors and therefore improved signa l-to-noise ratios may be achieved by the use of elliptical RF coils. T he probe is usually of bird-cage type, but the rungs are of finite wid th due to the high-frequency signals involved. This paper presents a m ethod for computing the magnetic fields produced inside elliptical pro bes, and the current distributions on the rungs. A slotted shield is a ssumed to surround the probe, and its influence on field homogeneity i s studied. In particular, the currents in a 16-runged unshielded ellip tical coil of practical interest were determined optimally in one case , using simulated annealing to optimize the homogeneity of the magneti c field within the probe. The effects of a segmented shield of both el liptical and circular cross section on this coil are discussed, and th e results are confirmed by experiment.