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Magnetic Resonance Spectroscopy
Last reviewed: 18.10.2021
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Magnetic resonance spectroscopy (MP-spectroscopy) allows noninvasive information about brain metabolism. Proton 1H-MR spectroscopy is based on a "chemical shift" - a change in the resonance frequency of protons that make up various chemical compounds. This term was introduced by N. Ramsey in 1951 to denote the differences between the frequencies of individual spectral peaks. The unit of measurement of the "chemical shift" is a millionth part (ppm). We present the main metabolites and the corresponding chemical shift values, the peaks of which are determined in vivo in the proton MR spectrum:
- NAA-N-acetylaspartate (2.0 ppm);
- Cho - choline (3,2 ppm);
- Cr - creatine (3.03 and 3.94 ppm);
- ml - myoinositol (3.56 ppm);
- Glx - glutamate and glutamine (2,1-2,5 ppm);
- Lac - lactate (1.32 ppm);
- Lip - lipid complex (0,8-1,2 ppm).
Currently, two main methods are used in proton MP-spectroscopy - one-voxel and multi-shift (chemical shift imaging) MP-spectroscopy - a one-time detection of spectra from several areas of the brain. In practice, now began to include multi-nuclear MP-spectroscopy based on the MP signal from the nuclei of phosphorus, carbon and some other compounds.
For single-site 1H-MR spectroscopy, only one (brain voxel) site is selected for analysis . Analyzing the composition of frequencies in the spectrum recorded from this voxel, a distribution of certain metabolites is obtained on the chemical shift scale (ppm). The ratio between the peaks of metabolites in the spectrum, the decrease or increase in the height of individual spectral peaks allow non-invasive assessment of biochemical processes occurring in tissues.
With multi-pixel MP-spectroscopy, MP spectra are obtained for several voxels at once, and the spectra of individual sections in the study zone can be compared. The processing of data from multi-shot MP spectroscopy makes it possible to construct a parametric cut-off map, in which the concentration of a particular metabolite is marked with color, and visualize the distribution of metabolites in the cut, i.e. To obtain an image weighted by the chemical shift.
Clinical application of MR-spectroscopy. MP-spectroscopy is now quite widely used to evaluate various volumetric brain formations. The data of MP spectroscopy do not allow to predict the histological type of neoplasm with confidence, however, most researchers agree that tumor processes as a whole are characterized by a low NAA / Cr ratio, an increase in the Cho / Cr ratio and, in some cases, the appearance of a lactate peak. In most MP studies, proton spectroscopy was used in differential diagnosis by astrocytoma, ependyma and primitive neuroepithelial tumors, presumably determining the type of tumor tissue.
In clinical practice, it is important to use MP-spectroscopy in the postoperative period to diagnose continued growth of the neoplasm, relapse of the tumor or radiation necrosis. In complex cases, 1H-MR spectroscopy becomes a useful additional method in differential diagnostics along with obtaining perfusion-weighted images. In the spectrum of radiation necrosis, a characteristic feature is the presence of the so-called dead peak, a wide lactate-lipid complex in the range 0.5-1.8 ppm against the background of complete reduction of peaks of other metabolites.
The next aspect of the use of MR spectroscopy is the delineation of newly discovered primary and secondary lesions, their differentiation with infectious and demyelinating processes. The most revealing are the results of diagnosing brain abscesses using diffusion-weighted images. In the abscess spectrum, the peak of the lipid-lactate complex and peaks specific for the abscess content, such as acetate and succinate (anaerobic bacteria glycolysis products), the amino acid valine and leucine (the result of proteolysis) were noted in the abscess spectrum against the background of the absence of peak metabolite peaks.
In the literature, the information content of MR-spectroscopy in epilepsy is also widely studied, in the evaluation of metabolic disorders and degenerative lesions of white matter in children, in traumatic brain injury, brain ischemia and other diseases.
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