Sodium Imaging

Can't sodium also be used for spectroscopy?  

²³Na MR image of the brain.

Sodium (Na) is the principal electrolyte in the human body, found predominantly in body fluids and in the tissue extracellular space. Nearly all body sodium exists as the ²³Na isotope and is NMR-active. Sodium has no natural chemical shift dispersion and is not suited to spectroscopy, but can be used for MR imaging. Sodium's gyromagnetic ratio is 11.26 MHz/T, giving it a Larmor frequency approximately one-fourth that of ¹H. This much lower resonant frequency means that specially tuned RF-coils and electronics are needed to perform ²³Na imaging. 

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²³Na has a nuclear spin I = 3/2, so it possesses (2I + 1) = 4 quantized energy levels and a non-spherical (quadrupolar) charge distribution. Its quadrupolar magnetic moment results in very short relaxation times whose exact values depend on the local environment:

Quadrupolar relaxation due to nonspherical (ellipsoidal) charge distribution that interacts with nearby electrical fields. Even though the mean distance (d) between the quadrupolar nucleus and the lone charge (+) does not change, nuclear orientation effects the energy levels and causes relaxation.

• Sodium in solution. Very rapid tumbling of the sodium ion produces random changes in the orientation of its electric field gradient, with “averaging” of the quadrupolar interaction to zero.  Both T1 and T2 undergo mono-exponential decay, with values each of about 50-60 ms.
• Sodium in tissues.  Here molecular motion is slowed and relaxation is dominated by quadrupolar interactions. Relaxation times are bi-exponential and much shorter than those in fluids. Typical short and long T2 values are on the order of 0.8 ms and 20 ms, while the two T1 components each have relaxation times of about 25 ms.

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Because of rapid T2 decay, pulse sequences used for ²³Na imaging require very short TE values (< 0.5 ms if quantification is desired). This mandates the use of rapid gradient echo or FID acquisition, typically with 3D radial or spiral data collection. 

Although not yet a part of standard clinical practice, Na imaging shows potential value in several dimensions:

1. Musculoskeletal. The highest levels of sodium in the body can be found in articular cartilage, where Na+ ions are drawn toward the negatively charged side chains of proteoglycans. Sodium level in cartilage measured by MRI can serve as an index of cartilage health.
2. Ischemia. Total sodium concentration increases by up to 50% in stroke and by over 200% in cardiac ischemia with reperfusion.
3. Oncologic. Malignant tumors exhibit increased intracellular and extracellular concentrations of sodium. Sodium MRI may also be used to monitor the effect of cytotoxic therapies, where a massive increase of sodium signal reflects apoptosis (dead and dying cells).

Advanced Discussion (show/hide)»

Although ²³Na has no natural chemical shift dispersion, it is possible to use chemical shift reagents to generate spectroscopic type information. An example of such a reagent is the lanthanide chelate Dy(TTHA)³⁻. This compound is thought not to penetrate cell membranes and will therefore create a frequency offset for ²³Na nuclei in the extracellular space. Chemical shifts on the order of 20-40 ppm can be generated. Unfortunately current chemical shift reagents are all toxic to humans and hence remain confined to experimental use in animals.

The signal from sodium in solution can be suppressed using inversion recovery techniques. Multiple quantum filtered (MQF) methods may allow separation of tissue sodium signals into isotropic and anisotropic (e.g, constrained by collagen or myelin) environments. Diffusion-based sequences may also allow separation of sodium signal between those arising in the intracellular and extracellular spaces.

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References
      Borthakur A, Mellon E, Niyogi S, et al. Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage. NMR Biomed. 2006; 19:781–821.
     Hagiwara A, Bydder M, Oughourlian TC, et al. Sodium MR neuroimaging. AJNR Am J Neuroradiol 2021; 42:1920-26.  [DOI LINK]
     Hancu I, Boada FE, Shen GX. Three-dimensional triple-quantum-filtered ²³Na imaging of in vivo human brain. Magn Reson Med. 1999; 42:1146–54.
     Madelin G, Lee J-S, Regatte RR, Jerschow A. Sodium MRI: Methods and applications. Prog NMR Spectr 2014; 79:14-47. (very comprehensive technical and clinical review)
     Nielles-Vallespin S, Weber MA, Bock M, et al. 3D radial projection technique with ultrashort echo times for sodium MRI: clinical applications in human brain and skeletal muscle. Magn Reson Med. 2007; 57:74–81.
     Qian Y, Boada FE. Acquisition-weighted stack of spirals for fast high-resolution three-dimensional ultra-short echo time MR imaging. Magn Reson Med. 2008; 60:135–45.
     Ouwerkerk R. ²³Na MRI: from research to clinical use. J Am Coll Radiol 2007; 4:739-741.
     Ouwerkerk R, Bleich KB, Gillen JS, et al. Tissue sodium concentration in human brain tumors as measured with ²³Na MR imaging. Radiology 2003; 227:529–537.
      Ra JB, Hilal SK, Oh CH. An algorithm for MR imaging of the short T2 fraction of sodium using the FID signal. J Comput Assist Tomogr 1989; 13:302–309.
     Zaric O, Juras V, Szomolanyi P, et al. Frontiers of sodium MRI revisited: from cartilage to brain imaging. J Magn Reson Imaging 2021; 54:58-75.  [DOI LINK]
      Zbyn S, Mlynarik V, Juras V, et al. Sodium MR imaging of articular cartilage pathologies. Curr Radiol Rep 2014; 2:41.

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Related Questions
     What are the causes of T1 and T2 relaxation?
     What nuclei besides hydrogen can be investigated with MRS? 

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