Sodium MRI in human heart: a review

This paper offers a critical review of the properties, methods and potential clinical application of sodium (Na-23) MRI in human heart. Because the tissue sodium concentration (TSC) in heart is about similar to 40 mu mol/g wet weight, and the Na-23 gyromagnetic ratio and sensitivity are respectively about one-quarter and one-11th of that of hydrogen (H-1), the signal-to-noise ratio of Na-23 MRI in the heart is about one-6000th of that of conventional cardiac H-1 MRI. In addition, as a quadrupolar nucleus, Na-23 exhibits ultra-short and multi-component relaxation behavior (T-1 similar to 30ms; T-2 similar to 0.5-4ms and 12-20ms), which requires fast, specialized, ultra-short echo-time MRI sequences, especially for quantifying TSC. Cardiac Na-23 MRI studies from 1.5 to 7T measure a volume-weighted sum of intra- and extra-cellular components present at cytosolic concentrations of 10-15mM and 135-150mM in healthy tissue, respectively, at a spatial resolution of about 0.1-1ml in 10min or so. Currently, intra- and extra-cellular sodium cannot be unambiguously resolved without the use of potentially toxic shift reagents. Nevertheless, increases in TSC attributable to an influx of intra-cellular sodium and/or increased extra-cellular volume have been demonstrated in human myocardial infarction consistent with prior animal studies, and arguably might also be seen in future studies of ischemia and cardiomyopathies - especially those involving defects in sodium transport. While technical implementation remains a hurdle, a central question for clinical use is whether cardiac Na-23 MRI can deliver useful information unobtainable by other more convenient methods, including H-1 MRI. Copyright (c) 2015 John Wiley & Sons, Ltd.