Sodium MRI (also known as
23 Na-MRI) is a specialised
magnetic resonance imaging technique that uses strong
magnetic fields, magnetic field gradients, and
radio waves to generate images of the distribution of sodium in the body, as opposed to more common forms of
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
that utilise
protons (or hydrogen) present in water (
1H-MRI).
Like the proton, sodium is naturally abundant in the body, so can be imaged directly without the need for
contrast agent
A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
s or
hyperpolarization. Furthermore, sodium
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s play a role in important biological processes via their contribution to concentration and
electrochemical gradient
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and ...
s across
cellular membranes, making it of interest as an imaging target in health and disease.
In contrast to conventional MRI of the proton, Sodium MRI is complicated by the low concentrations of Na
nuclei relative to concentration of H
2O molecules in biological tissues
(10-45 mM) and the lower
gyromagnetic ratio of the
23Na nucleus as compared to a
1H nucleus,.
This causes low
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
sensitivity and the requirement for a stronger magnetic field for equivalent
spatial resolution
In physics and geosciences, the term spatial resolution refers to distance between independent measurements, or the physical dimension that represents a pixel of the image. While in some instruments, like cameras and telescopes, spatial resolut ...
. The quadrupolar
23Na nucleus also has a faster
transverse relaxation rates and multiple
quantum coherence
In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e., temporally or spatially constant) interference. It contains several distinct concepts ...
s as compared to the
1H nucleus,
requiring specialized and high performance
MRI sequences to capture information before the
contrast used to image the body is lost.
Biological significance
Tissue sodium concentration (TSC) is tightly regulated by healthy cells and are altered by energy status and cellular integrity, making it an effective marker for disease states.
Cells maintain a low intracellular Na
+ concentration by actively pumping Na ions out via the Na
+/K
+ ATPase channel, and any challenge to the cell’s metabolism causing low ATP supply or compromise of the cell’s membrane integrity will drastically increase intracellular Na
+ concentrations. After exhaustive exercise, for example,
23Na MRI can detect Na
+ levels in tissues rising sharply, and can even visualize a sodium-rich meal in a patient’s stomach.
Malignant tumor
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal ble ...
s in particular alter their metabolism drastically, often to account for
hypoxic
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the t ...
intratumor conditions, leading to an decrease in
cytosolic pH. To compensate, Na
+ ions from the extracellular space are exchanged for protons in the Na
+/H
+ antiport,
the loss of which often attenuates cancer growth. Therefore,
23Na MRI is a useful clinical tool for detecting a number of disease states, including heart disease and cancer, as well as monitoring therapy. For example,
23Na MRI has been shown to measure cellularity in ovarian cancer. Tissue damage in stroke patients can also be evaluated using
23Na MRI, with one study showing that a change of 50% higher TSC than the TSC in healthy brain tissue is consistent with complete infarction,
and therefore can be used to determine tissue viability and treatment options for the patient. Tumor malignancy can also be evaluated based on the increases in TSC of rapidly proliferating cells. Malignant tumors have approximately 50-60% increased TSC relative to that of healthy tissues
– however, increases in TSC cannot be determined to be due to changes in extracellular volume, intracellular sodium content or neovascularization. Another interesting use of
23Na MRI is in evaluating multiple sclerosis, wherein accumulation of sodium in axons can lead to axon degeneration. Preliminary studies have shown that there is a positive correlation between elevated TSC and disability.
Uses in Prostate Cancer
Recently, work has been undertaken to assess the utility of using sodium-MRI to characterize prostate cancer lesions in men. In this study, patients were imaged with sodium MRI prior to surgical removal of the prostate. TSC was extracted from the images and compared to the Gleason score of imaged lesions. This work showed statistically significant increases in TSC as prostate cancer increased in aggression. This preliminary study suggests that sodium MRI can accurately characterize the stage of prostate cancer. This suggests the potential use of sodium-MRI to better management and stage patients with prostate cancer into treatment schemes.
Advantages
23Na MRI measures cellular
metabolic rate
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ce ...
as well as disease-related change in tissues and organs. It had improved from 45min length to only 15 mins at 1.5T.
Unlike other MRI scanning, sodium MRI captures only sodium signals inside bodies. For cartilage degeneration,
proteoglycan degrades with negative charge, and positively charged sodium ion bond with proteoglycan.
Both proteoglycan and sodium level decrease, so less signals are observed by sodium MRI.
23Na MRI is very sensitive and specific to change in proteoglycan, so it is good to use for monitoring of proteoglycan degeneration in cartilage.
References
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Magnetic resonance imaging