⁵⁵Mn-based fiducial markers for rapid and automated RF coil localization for hyperpolarized ¹³C MRI

Purpose: To use fiducial markers containing manganese 55 to rapidly localize carbon 13 (¹³C) RF coils for correcting images for B₁ variation. Methods: Hollow high-density polyethylene spheres were filled with 3M sodium permanganate and affixed to a rectangular ¹³C-tuned RF coil. The relative positions of the markers and coil conductors were mapped using CT. Marker positions were measured by MRI using a series of 1D projections and automated peak detection. Once the coil location was determined, coil sensitivity was estimated using a quasi-static calculation. Simulations were performed to determine the minimum number of projections required for robust localization. Phantom experiments were used to confirm the accuracy of marker localization as well as the calculated coil sensitivity. Finally, in vivo validation was performed using hyperpolarized ¹³C pyruvate in a rat model. Results: In simulations, our algorithm was accurate in determining marker positions when at least 6 projections were used (RMSE 1.4 ± 0.9 mm). These estimates were verified in phantom experiments, where markers locations were determined with an RMS accuracy of 1.3 mm. A minimum SNR of 4 was required for automated detection to perform accurately. Computed coil sensitivity had a median error of 17% when taken over the entire measured area and 5.7% over a central region. In a rat, correction for nonuniform reception and flip angle was able to normalize the signals arising from asymmetrically positioned kidneys. Conclusion: Manganese 55 fiducial markers are an inexpensive and reliable method for rapidly localizing ¹³C RF coils and correcting ¹³C images for B₁ variation without user intervention.