Quantitative sodium MR imaging: A review of its evolving role in medicine

Abstract

Sodium magnetic resonance (MR) imaging in humans has promised metabolic information that can improve medical management in important diseases. This technology has yet to find a role in clinical practice, lagging proton MR imaging by decades. This review covers the literature that demonstrates that this delay is explained by initial challenges of low sensitivity at low magnetic fields and the limited performance of gradients and electronics available in the 1980s. These constraints were removed by the introduction of 3T and now ultrahigh (≥7T) magnetic field scanners with superior gradients and electronics for proton MR imaging. New projection pulse sequence designs have greatly improved sodium acquisition efficiency. The increased field strength has provided the expected increased sensitivity to achieve resolutions acceptable for metabolic interpretation even in small target tissues. Consistency of quantification of the sodium MR image to provide metabolic parametric maps has been demonstrated by several different pulse sequences and calibration procedures. The vital roles of sodium ion in membrane transport and the extracellular matrix will be reviewed to indicate the broad opportunities that now exist for clinical sodium MR imaging. The final challenge is for the technology to be supplied on clinical ≥3T scanners.

Measuring tissue sodium concentration: Cross‐vendor repeatability and reproducibility of 23Na‐MRI across two sites

“…Previous authors have demonstrated increased TSC within CNS neoplasms such as high‐grade glioma, and that 23 Na MRI can be used as a predictive marker of both tumor grade and prognosis . Within glioma, cellular proliferation and changes in cell membrane ion transporters contribute to elevated intracellular sodium concentrations, but these tumors also display increases in the sodium‐rich extravascular–extracellular space secondary to necrosis and loss of normal cellular packing . Evidence from in vivo animal studies of temporal 23 Na MRI changes in glioma following chemotherapy treatment suggest that TSC may also serve as an early marker of progression and treatment response in this tumor group .…”

“…Several studies have shown intracellular sodium volume to be significantly higher in malignant tumors compared with benign lesions due to increased cell density, altered metabolism, and changes in cell membrane ion transporters . In addition to this increase in intracellular sodium, many malignant tumors also show an increase in the sodium‐rich extravascular–extracellular space, which in some cases may be secondary to an increase in necrosis . Combining information on sodium concentration from both the intracellular and the extracellular compartments provides complementary information that can be used to probe tissue structure, cell density, and membrane integrity in more detail, which could be used in the assessment of tumor progression and treatment response .…”

X‐nuclei imaging: Current state, technical challenges, and future directions

“…23 Na MRI has been frequently applied to investigate diseases such as stroke, tumor, multiple sclerosis, and cartilage degradation . This focus is attributed to the high prevalence of these diseases and the connection of Na + concentration to pathophysiology.…”

“…The exact onset time is often unknown, albeit being crucial for treatment decisions. A prospective biomarker to determine the onset time and to characterize tissue viability is the tissue sodium concentration (TSC) threshold …”

Zero‐gradient‐excitation ramped hybrid encoding (zGRF‐RHE) sodium MRI

“…Disturbances of this delicate balance provide a sensitive, early indicator for cell breakdowns and give an insight into cell integrity and tissue viability. With sodium nuclei providing the second strongest MR‐signal in biological tissue, sodium MRI offers promising potential as a clinical biomarker with capabilities ranging from early diagnosis to therapy response monitoring as well as the detection of diseases without anatomical abnormalities . Because of hardware advancements, the number of sodium MRI studies is steadily increasing ranging across a variety of diseases encompassing stroke, tumors, neurodegenerative diseases, and musculoskeletal pathologies …”