Directly detected ⁵⁵Mn MRI: Application to phantoms for human hyperpolarized ¹³C MRI development

In this work we demonstrate for the first time directly detected manganese-55 (⁵⁵Mn) magnetic resonance imaging (MRI) using a clinical 3T MRI scanner designed for human hyperpolarized ¹³C clinical studies with no additional hardware modifications. Due to the similar frequency of the ⁵⁵Mn and ¹³C resonances, the use of aqueous permanganate for large, signal-dense, and cost-effective "¹³C" MRI phantoms was investigated, addressing the clear need for new phantoms for these studies. Due to 100% natural abundance, higher intrinsic sensitivity, and favorable relaxation properties, ⁵⁵Mn MRI of aqueous permanganate demonstrates dramatically increased sensitivity over typical ¹³C phantom MRI, at greatly reduced cost as compared with large ¹³C-enriched phantoms. A large sensitivity advantage (22-fold) was demonstrated. A cylindrical phantom (d=8cm) containing concentrated aqueous sodium permanganate (2.7 M) was scanned rapidly by ⁵⁵Mn MRI in a human head coil tuned for ¹³C, using a balanced steady state free precession acquisition. The requisite penetration of radiofrequency magnetic fields into concentrated permanganate was investigated by experiments and high frequency electromagnetic simulations, and found to be sufficient for ⁵⁵Mn MRI with reasonably sized phantoms. A sub-second slice-selective acquisition yielded mean image signal-to-noise ratio of ~60 at 0.5cm³ spatial resolution, distributed with minimum central signal ~40% of the maximum edge signal. We anticipate that permanganate phantoms will be very useful for testing HP ¹³C coils and methods designed for human studies.