Experimentally verified, theoretical design of dual-tuned, low-pass birdcage radiofrequency resonators for magnetic resonance imaging and magnetic resonance spectroscopy of human brain at 3.0 Tesla.

A new theoretical method is presented for designing frequency responses of double-tuned, low-pass birdcage coils. This method is based on Kirchhoff's equations through a nonsymmetric matrix algorithm and extended through a modification of the corresponding eigenvalue system from a single-tuned mode. Designs from this method are verified for sodium/proton, dual-tuned, double-quadrature, low-pass birdcage coils at 1.5 Tesla and 3.0 Tesla and then are used to design dual-tuned, double-quadrature, lithium/proton and phosphorus/proton birdcage coils for 3.0 Tesla. All frequencies show experimental deviations of less than 3% from theory under unloaded conditions. The frequency shifts caused by loading and radiofrequency shielding are less than 1 MHz and can be compensated readily by adjustment of variable capacitors. Applications to human neuroimaging and spectroscopy are demonstrated.