Optimized quadrature surface coil designs.

BACKGROUND: Quadrature surface MRI/MRS detectors comprised of circular loop and figure-8 or butterfly-shaped coils offer improved signal-to-noise-ratios (SNR) compared to single surface coils, and reduced power and specific absorption rates (SAR) when used for MRI excitation. While the radius of the optimum loop coil for performing MRI at depth d in a sample is known, the optimum geometry for figure-8 and butterfly coils is not. MATERIALS AND METHODS: The geometries of figure-8 and square butterfly detector coils that deliver the optimum SNR are determined numerically by the electromagnetic method of moments. Figure-8 and loop detectors are then combined to create SNR-optimized quadrature detectors whose theoretical and experimental SNR performance are compared with a novel quadrature detector comprised of a strip and a loop, and with two overlapped loops optimized for the same depth at 3 T. The quadrature detection efficiency and local SAR during transmission for the three quadrature configurations are analyzed and compared. RESULTS: The SNR-optimized figure-8 detector has loop radius r8 approximately 0.6d, so r8/r0 approximately 1.3 in an optimized quadrature detector at 3 T. The optimized butterfly coil has side length approximately d and crossover angle of > or = 150 degrees at the center. CONCLUSIONS: These new design rules for figure-8 and butterfly coils optimize their performance as linear and quadrature detectors.