Double Tuned 1H-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 the
<sup>23</sup>
Na and
<sup>1</sup>
H Larmor frequency at 7 T, respectively. Dimensional parameters are varied in order to optimize the radio frequency (RF) B
<sub>1</sub>
<sup>+</sup>
field homogeneity at both Larmor frequencies. Results show that B
<sub>1</sub>
<sup>+</sup>
field homogeneity at the
<sup>1</sup>
H 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 B
₁
⁺
field due to the short wavelength.