Conductor geometry and capacitor quality for performance optimization of low-frequency birdcage coils

Birdcage coils are used widely in nuclear magnetic resonance (NMR) applications for their ability to operate in transmit/receive mode with high signal-to-noise ratio (SNR) and wide held homogeneity. When used at low frequencies, a number of drawbacks arise that drastically reduce their overall performances. In this article we show how the choice of the conductors cross-geometry and capacitor quality affects the coil overall performance. Several birdcage coils, tuned at 7.66 MHz, were designed and built, using conductors with different shapes and capacitors with different quality factors. Realized coils were tested and characterized by laboratory workbench in order to extract several quality indices. In addition, experiments were done in a real magnetic resonance (MR) scanner to evaluate the field homogeneity. We show that an optimized choice of conductor shape and capacitors allows us to obtain high-quality birdcage coils tuned at frequencies in the megahertz range. Our results are applicable in several situations as MR imaging (MRI) in low-field scanners, imaging of nuclear species at intermediate fields, Overhauser systems, and prepolarized NMR. (C) 2004 Wiley Periodicals, Inc.