Clinical Applications of 23Na-MRI
Matilde Inglese1

1Icahn School of Medicine at Mount Sinai, United States

Synopsis

Over the last few decades, the advent of high and ultra-high field MRI and the growing availability of novel sequence acquisition schemes have led to substantial advances in sodium 23 (23Na) imaging and have rekindled interest in clinical applications. Advantages and disadvantages associated with the use of 23Na MRI will be discussed. The contribution of 23Na MRI to a better understanding of the pathophysiology of neurological diseases and the potential clinical impact of sodium measurements in terms of diagnosis and prognosis will also be discussed. Finally, future directions to pursue in this research field will be identified

Over the last few decades, the advent of high and ultra-high field MRI and the growing availability of novel sequence acquisition schemes have led to substantial advances in sodium 23 (23Na) imaging and have rekindled interest in clinical applications (1). From a biological point of view, the 23Na ion has a critical role in several cellular functions such as mitosis, cellular proliferation, generation and propagation of action potentials and cell volume regulation (2). To ensure the maintenance of tissue homeostasis and the preservation of intracellular structures and processes, 23Na concentration is strictly controlled by the ATP-driven Na/K pump; pathological changes that determine an expansion of the extracellular space (e.g. tissue injury, edema or necrosis) or functional impairment of the Na/K pump are therefore expected to result in an increased tissue 23Na concentration (2). From an MRI point of view, 23Na yields the second strongest nuclear magnetic resonance (NMR) signal among all biologically relevant NMR-active nuclei. However, the average concentration of Na ions in brain and the sensitivity of the 23Na nucleus is much lower than that of protons. In most biological tissues, sodium can be separated in two compartments: extra-cellular and intra-cellular. The extracellular sodium concentration (ESC) is ~ 140 mmol/L and the intracellular sodium concentration (ISC) is ~ 15 mmol/L. The ability of the cell to maintain a sodium gradient across the cell membrane, sodium ion homeostasis, can be used as an operational definition of tissue viability. The observed average sodium concentration is composed of the weighted average of ESC and ISC in the brain tissue, which is around 20% and 80%, respectively (3-7). Measurements of total sodium concentration (TSC) obtained using 23Na MRI can be useful in measuring tissue viability in patients with stroke (7) and cell infiltration in patients with brain tumors (8). More recently, it has been shown that, in patients with epilepsy, a chronic TSC elevation is associated with the epileptogenic region even during the interictal period (9). Finally, our and other groups have shown that 23Na MRI is feasible in patients with multiple sclerosis (MS) and have reported the presence and clinical relevance of TSC abnormalities in MS patients with different phenotypes (10-14). Since TSC is an average of ESC and ISC, SQ 23Na MRI does not allow discrimination between the two concentrations, especially the ISC which is physiologically most relevant (15). Advantages and disadvantages associated with the use of 23Na MRI will be discussed. The contribution of 23Na MRI to a better understanding of the pathophysiology of neurological diseases and the potential clinical impact of sodium measurements in terms of diagnosis and prognosis will also be discussed. Finally, future directions to pursue in this research field will be identified. This information will benefit a large and multidisciplinary community of neuroscientists, clinicians, physicists and bioengineers.

Acknowledgements

NIH NINDS R01NS099527

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Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)