Triple-tuned birdcage and single-tuned dipole array for quadri-nuclear head MRI at 7 T

The purpose of this work was to design and build a coil for quadri-nuclear MRI of the human brain at 7 T.

We built a transmit/receive triple-tuned (45.6 MHz for ${\hspace{0.17em}}^2$ H, 78.6 MHz for ${\hspace{0.17em}}^{23}$ Na, and 120.3 MHz for ${\hspace{0.17em}}^{31}$ P) quadrature four-rod birdcage that was geometrically interleaved with a transmit/receive four-channel dipole array (297.2 MHz for ${\hspace{0.17em}}^1$ H). The birdcage rods contained passive, two-pole resonant circuits that emulated capacitors required for single-tuning at three frequencies. The birdcage assembly also included triple-tuned matching networks, baluns, and transmit/receive switches. We assessed the performance of the coil with quality factor (Q) and signal-to-noise ratio (SNR) measurements, and performed in vivo multinuclear MRI and MR spectroscopic imaging (MRSI).

Q measurements showed that the triple-tuned birdcage efficiency was within 33% of that of single-tuned baseline birdcages at all three frequencies. The quadri-tuned coil SNR was 78%, 59%, 44%, and 48% lower than that of single or dual-tuned reference coils for ${\hspace{0.17em}}^1$ H, ${\hspace{0.17em}}^2$ H, ${\hspace{0.17em}}^{23}$ Na, and ${\hspace{0.17em}}^{31}$ P, respectively. Quadri-nuclear MRI and MRSI was demonstrated in brain in vivo in about 30 min.

While the SNR of the quadruple tuned coil was significantly lower than dual- and single-tuned reference coils, it represents a step toward truly simultaneous quadri-nuclear measurements.