Creating a clinical platform for carbon-13 studies using the sodium-23 and proton resonances

Purpose: Calibration of hyperpolarized ¹³C-MRI is limited by the low signal from endogenous carbon-containing molecules and consequently requires ¹³C-enriched external phantoms. This study investigated the feasibility of using either ²³Na-MRI or ¹H-MRI to calibrate the ¹³C excitation.
Methods: Commercial ¹³C-coils were used to estimate the transmit gain and center frequency for ¹³C and ²³Na resonances. Simulations of the transmit B1 profile of a Helmholtz loop were performed. Noise correlation was measured for both nuclei. A retrospective analysis of human data assessing the use of the ¹H resonance to predict [1-¹³C]pyruvate center frequency was also performed. In vivo experiments were undertaken in the lower limbs of 6 pigs following injection of hyperpolarized ¹³C-pyruvate.
Results: The difference in center frequencies and transmit gain between tissue ²³Na and [1-¹³C]pyruvate was reproducible, with a mean scale factor of 1.05179 ± 0.00001 and 10.4 ± 0.2 dB, respectively. Utilizing the ¹H water peak, it was possible to retrospectively predict the ¹³C-pyruvate center frequency with a standard deviation of only 11 Hz sufficient for spectral–spatial excitation-based studies.
Conclusion: We demonstrate the feasibility of using the ²³Na and ¹H resonances to calibrate the 13C transmit B₁ using commercially available ¹³C-coils. The method provides a simple approach for in vivo calibration and could improve clinical workflow.