A technique for double resonant operation of birdcage imaging coils

The type of radio-frequency (RF) coil known as a high-pass birdcage consists of a set of N wires arranged axially on the surface of a cylinder and connected by capacitors at each end. Such coils are widely used for NMR imaging because of the high degree of field homogeneity they provide. It is shown how to create homogeneous fields at two frequencies, using an unequal distribution of capacitance. A theoretical analysis which uses the discrete Fourier transform of the currents with respect to the angular positions of the N wires is presented. A perturbation theory analysis indicates a small sacrifice in homogeneity. The root-mean-square deviation of field magnitude around a circle is proposed as a measure of field inhomogeneity. For the case of double resonance at proton and fluorine frequencies, the loss of homogeneity is at worst 1% and is small compared to the natural inhomogeneity for an N=8 wire coil for radii up to one half the coil radius. The presence of a conducting shield degrades the homogeneity. The theoretical ideas were confirmed in a computer simulation of one particular coil design. A working coil was constructed and images obtained of proton and fluorine phantoms