Double Tuned1H-23Na Birdcage Coils for MRI at 7 T : Performance evaluation through electromagnetic simulations

The aim of the present study is the electromagnetic evaluation of a 'four-ring' double-tuned (DT) birdcage coil designed for head imaging. The model consists of an inner Low-Pass (LP) birdcage coil and two externally nested High-Pass (HP) birdcage-like coils. LP and HP structures are tuned at the23Na and1H Larmor frequency at 7 T, respectively. Dimensional parameters are varied in order to optimize the radio frequency (RF) B1+field homogeneity at both Larmor frequencies. Results show that B1+field homogeneity at the1H Larmor frequency can be considerably increased by optimizing the mutual distance between the inner and outer coil structure length. This result is particularly significant in case of MRI at 7 T, to correct for the 'intrinsically' inhomogeneous B1+field due to the short wavelength.