Design and evaluation of a radio frequency coil for nuclear magnetic resonance imaging of fluorine and protons

A frequency-switchable, homogeneous-field rf saddle coil has been designed for imaging both protons (1H, 59.1 MHz) and fluorine (19F, 55.6 MHz) on a 1.4-T superconducting small-bore nuclear magnetic resonance imager. Frequency and impedance switching is accomplished by external capacitance and cable length changes; these operations permit imaging of both nuclei without perturbing the sample. The coil is optimized for 19F operation, yet performs better at the proton frequency than does the unswitched 19F coil. The angular distribution of the coil's wires and the use of distributed capacitors are designed to optimize field homogeneity and Q. A quantitative image of field homogeneity is presented. The coil is suitable for imaging small animals (7-cm-diam bore) and couples far better to small samples than does our standard receiver coil (15.2 cm in diameter). Images of phantoms and rats injected with a perfluorinated blood substitute are presented.