MR spectroscopy provides a measure of brain chemistry. The most common nuclei that are used are 1H (proton), 23Na (sodium), 31P (phosphorus). Proton spectroscopy is easier to perform and provides much higher signal-to-noise than either sodium or phosphorus. MR spectroscopy (MRS) allows tissue to be interrogated for the presence and concentration of various metabolites as lactate, lipids, alanine, N-acetylaspartate (NAA), glutamine / glutamate, GABA, citrate, creatine, choline, myo-inositol peak.
It can be used to serially monitor biochemical changes in tumors, stroke, epilepsy, metabolic disorders, infections, and neurodegenerative diseases. The MR spectra do not come labeled with diagnoses. They require interpretation and should always be correlated with the MR images before making a final diagnosis.
Evaluation of Spectra
- Glioma: MRS can help increase our ability to predict grade. As the grade increases NAA and creatine decrease and choline, lipids and lactate increase.
- Non-glial tumors: May be difficult but in general non-glial tumours will not have little if any NAA. In the setting of gliomas, choline will be elevated beyond the margins contrast enhancement in keeping with cellular infiltration.
- Radiation effects: Distinguishing radiation change and tumour recurrence can be problematic. In recurrent tumour choline will be elevated, whereas in radiation change,
- NAA, choline and creatine will all be low.
- Ischemia and infarction: Lactate will increase as the brain switches to anaerobic metabolism. When infarction takes place then lipids are released and peaks appear.
- Infection: As in all processes which destroy normal brain tissue, NAA is absent. Within bacterial abscess cavities, lactate, alanine, cytosolic acid and acetate are elevated / present.
- White matter diseases: progressive multifocal leukoencephalopathy (PML) may demonstrate elevated myo-inositol.
- Hepatic encephalopathy: Markedly reduced myo-inositol, and to a lesser degree choline. Glutamine is increased.
- Mitochondrial disorders: Leigh syndrome, elevated choline, reduced NAA and occasionally elevated lactate