In Vivo Magnetic Resonance Spectroscopy - Applications of MRS

Applications of MRS

In (1H) Magnetic Resonance Spectroscopy each proton can be visualized at a specific chemical shift (peak position along x-axis) depending on its chemical environment. This chemical shift is dictated by neighboring protons within the molecule. Therefore, metabolites can be characterized by their unique set of 1H chemical shifts. The metabolites that MRS probes for have known (1H) chemical shifts that have previously been identified in NMR spectra. These metabolites include:

1.) N-acetyl Aspartate (NAA): with its major resonance peak at 2.02ppm, decrease in levels of NAA indicate loss or damage to neuronal tissue, which results from many types of insults to the brain. Its presence in normal conditions indicates neuronal and axonal integrity.

2.) Choline: with its major peak at 3.2ppm, choline is known to be associated with membrane turnover, or increase in cell division. Increased choline indicates increase in cell production or membrane breakdown, which can suggest demyelination or presence of malignant tumors.

3.) Creatine & phosphocreatine: with its major peak at 3.0ppm, creatine marks metabolism of brain energy. Gradual loss of creatine in conjunction with other major metabolites indicates tissue death or major cell death resulting from disease, injury or lack of blood supply. Increase in creatine concentration could be a response to cranialcerebral trauma. Absence of creatine may be indicative of a rare congenital disease.

4.) Lipids: with their major aliphatic peaks located in the 0.9-1.5ppm range, increase in lipids is seen is also indicative of necrosis. These spectra are easily contaminated, as lipids are not only present in the brain, but also in other biological tissue such as the fat in the scalp and area between the scalp and skull.

5.) Lactate: reveals itself as a doublet (two symmetric peaks in one) at 1.33ppm. Normally lactate is not visible, for its concentration is lower that the detection limit of MRS, however presence of this peak indicates glycolysis has been initiated in an oxygen deficient environment. Several causes of this include ischemia, hypoxia, mitochondrial disorders, and some types of tumors.

6.) Myo-inositol: with its major peak at 3.56ppm, an increase in Myo-inositol has been seen in patients with Alzheimer’s, dementia, and HIV patients.

7.) Glutamate and Glutamine: these amino acids are marked by a series of resonance peaks between 2.2 and 2.4ppm. Hyperammonemia, hepatic encephalopathy are two major conditions that result in elevated levels of glutamine and glutamate. MRS, used in conjunction with MRI or some other imaging technique, can be used to detect changes in the concentrations of these metabolites, or significantly abnormal concentrations of these metabolites.