Most MRI is done on the 1H nucleus, but other nuclei can be imaged too. Sodium (23Na) can be observed with MRI scanners if they have the right electronics and software.
Below you will see some samples of 23Na imaging.
Introduction
Tissue sodium levels are altered by cellular integrity and energy status in living animal cells. A low intracellular sodium concentration, [Na]in, is maintained by actively pumping sodium out of the cell with the Na+/K+ATP-ase pump against a concentration gradient formed by the much higher extracellular sodium concentration, [Na]out. If ATP supply is insufficient because the cellular energy metabolism is impaired or challenged, or if the cell membrane integrity is compromised, the intracellular sodium levels rise sharply.
Sodium (23Na) Magnetic Resonance Imaging (MRI) can detect elevated tissue sodium signal levels after exhaustive exercise, or in a variety of diseases such as myocardial infarction or cancer. Also, some therapies such as High Intensity Focussed Ultrasound (HIFUS) ablation as used to remove uterine fibroids, can increase toe sodium level in the targeted tissues. This allows a visualization of the therapy effect with an 23Na MRI scan after the treatment.
Methods
We use a twisted projection imaging (TPI) method originally developed by Fernando Boada of the Univ of Pittsburgh.
F. E. Boada, J. S. Gillen, G. X. Shen, S. Y. Chang and K. R. Thulborn, " Fast Three Dimensional Sodium Imaging ", Magnetic Resonance in Medicine, 37: 706-715, 1997
The movie below shows the k-space trajectories of the TPI method compared with the more common rectangular k-space scanning method.
Only half the TPI projections are shown for clarity. The paths are spirals on cones. In TPI the spirals start with a straight projection before they start twisting.
Rectangular k-space scanning (left) and TPI (right)
For a description of the method read the 1997 paper by Boada or read my paper on quantitative 23Na MRI in human hearts.
This shows sagittal 1H MRI and 23Na MRI of a patient with MI.
The contours of the MI are posterior and briefly visible in frames 2,3 and 4 (counting from top left) superimposed on both 1H and 23Na images.
Level contours of the 1H MRI are projected over the colored 23Na images.
Coronal view of a heart.
What is the very intense signal blob below?
A stomach full of sauerkraut! Sauerkraut contains typical 9 g/l NaCl whereas plasma has about 0.9g/l NaCl equivalent.
Movie with sagittal views of the same subject
Physiological and biochemical changes associated with proliferating malignant tumors, lead to an increase in tissue sodium. We measure tissue sodium concentration with 23Na MRI and combined this with Gd contrast enhancement 1H MR imaging (1H MRI) to examine the potential of 23Na MRI to improve the specificity of MRI breast exams. In patients undergoing presurgical chemotherapy we can use 23Na MRI to monitor the effect of the chemotherapy.
Methods: Registered 23Na and 1H images were recorded on a 1.5T GE Signa scanner with a 23Na coil inserted in a 1H phased array breast coil. 1H images consisted of sagittal T1-weighted (T1w), T2-weighted (T2w) images and 3D T1w images, pre- and post-administration of GdDTPA. T1w images were acquired with a Gradient Recalled Echo (GRE) sequence (TR/T1=250/4 ms). T2w images (Fast Spin Echo sequence, TR/TE = 5700/99 ms) and Pre- and post-Gd contrast images (3D-GRE sequence, TR/TE = 23/4 ms) were recorded with fat suppression. 23Na images (TR/TE = 100/0.4 ms) were acquired using adiabatic excitation with twisted projection imaging in 12 min for a0.2 ml voxel size. An embedded coil phantom with a NaCl solution served as fiducial marker and external reference for receive sensitivity corrections.
Post Gd contrast 1H MR image and 23Na MRI and image of a patient with advanced localized breast cancer. The bright area in the image on the left is the very large malignant tumor.
23Na MRI of patient before and after chemotherapy
