Conductors geometry and capacitors quality for performance optimization of low frequency birdcage coils

Birdcage coils are widely used in Nuclear Magnetic Resonance (NMR) applications for their ability to operate in transmit/receive mode with high signal to noise ratio and wide field homogeneity. When used at low frequencies, a number of drawbacks arise that drastically reduce their overall performances. In this paper we show how the choice of the conductors cross-geometry and capacitors 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. Experiments were also done in real MR scanner to evaluate the field homogeneity. We demonstrate that an optimized choice of conductors shape and capacitors allows to obtain high quality birdcage coils tuned at frequencies in the MHz range. Our results are applicable in several situations as MR imaging in low field scanners, imaging of nuclear species at intermediate fields, Overhauser systems and prepolarized NMR.