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 Telsa 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.Magn Reson Med 41:268–275, 1999. © 1999 Wiley‐Liss, Inc.