Combined ²³ Na and ¹³ C imaging at 3.0 Tesla using a single-tuned large FOV birdcage coil

Purpose: An unmet need in carbon-13 (¹³ C)-MRI is a transmit system that provides uniform excitation across a large FOV and can accommodate patients of wide-ranging body habitus. Due to the small difference between the resonant frequencies, sodium-23 (²³ Na) coil developments can inform ¹³ C coil design while being simpler to assess due to the higher naturally abundant ²³ Na signal. Here we present a removable ²³ Na birdcage, which also allows operation as a ¹³ C abdominal coil.

Methods: We demonstrate a quadrature-driven 4-rung ²³ Na birdcage coil of 50 cm in length for both ²³ Na and ¹³ C abdominal imaging. The coil transmit efficiencies and $B_1^{+}$ maps were compared to a linearly driven ¹³ C Helmholtz-based (clamshell) coil. SNR was investigated with ²³ Na and ¹³ C data using an 8-channel ¹³ C receive array within the ²³ Na birdcage.

Results: The ²³ Na birdcage longitudinal FOV was > 40 cm, whereas the ¹³ C clamshell was < 32 cm. The transmit efficiency of the birdcage at the ²³ Na frequency was 0.65 µT/sqrt(W), similar to the clamshell for ¹³ C. However, the coefficient of variation of ²³ Na- $B_1^{+}$ was 16%, nearly half that with the ¹³ C clamshell. The 8-channel ¹³ C receive array combined with the ²³ Na birdcage coil generated a greater than twofold increase in ²³ Na-SNR from the central abdomen compared with the birdcage alone.

Discussion: This ²³ Na birdcage coil has a larger FOV and improved $B_1^{+}$ uniformity when compared to the widely used clamshell coil design while also providing similar transmit efficiency. The coil has the potential to be used for both ²³ Na and ¹³ C imaging.