OPTIMIZED SMALL-BORE, HIGH-PASS RESONATOR DESIGNS

The radiofrequency field homogeneity and quality factor (Q) of a numbe r of high-pass resonator designs are compared theoretically and experi mentally for small-bore coils. It was found that for high-pass resonat ors with the number of distributed capacitors greater than eight, a si gnificant and unacceptable decrease in Q resulted. A number of schemes are proposed and investigated to improve the homogeneity of eight-cap acitor high-pass resonators, thus balancing Q and homogeneity. These d esigns include variation in rung width, interrung feeding, a new hybri d coil design, and the use of internal guard rings. Eleven high-pass r esonators were modeled and built to test the designs. The current dens ity in each conductor was calculated using a novel numerical technique based on the inverse finite Hilbert transform which implicitly includ es the effects of RF eddy currents. It was found that the effect of su ch eddy currents is important in coils of small geometry. RF fields we re calculated from the current densities by using a retarded-potential solution to the free-space Maxwell equations. The interrung-fed hybri d coil design with guard rings was found to be superior to conventiona l designs. (C) 1995 Academic Press, Inc.