MR spectroscopy (MRS)

• Magnetic resonance spectroscopy is a non-invasive in vivo method that allows the investigation of biochemical changes of intracellular metabolism in cerebral ischemia.
• The concentrations of metabolites detected by MRS are relatively low (2–20 mM) compared to the concentration of water (83.4 M) detected by MRI.
• This sequence is very sensitive to local magnetic field inhomogeneity.

N-acetyl aspartate (NAA)

NAA acts as a neuronal marker as it is almost exclusively confined to neurons in the human brain, where it is found predominantly in the axons and nerve processes. It has a sharp peak at 2.01 parts per million (ppm).

Lactate

Lactate is not normally detected within the brain. It is the end product of anaerobic glycolysis during ischemic conditions. It has a doublet peak at 1.33 ppm.

Normal cerebral tissue spectrum. Resonance peaks are: glutamate and glutamine (Glx) at 3.8 and 2.1± 2.45 parts per million (ppm), myo-inositol (mI) at 3.56 ppm, choline containing compounds (Cho) at 3.22 ppm, creatine and phosphocreatine (Cr/PCr) methyl singlet at 3.03 ppm, N-acetyl aspartate (NAA) methyl singlet at 2.01 ppm, and lactate doublet (Lac) at 1.33 ppm.	The figure shows decreased concentration of NAA and elevated lactate in the infarct center.

Metabolic progression of stroke

• Lactate is elevated in the infarct core due to ischemic necrosis and in the penumbra due to hypoxia in the at risk tissue.
• NAA is reduced in the infarction core due to neuronal damage. With infarction progression, this reduction will spread to the penumbra.
• The extent of NAA reduction correlates with the infarct age (see right) because of ongoing neuronal necrosis and replacement of neuronal tissue with gliosis.

MRS in stroke

The DWI image (above) reveals acute infarction involving the right basal ganglia.

The MRS (above) demonstrates elevated lactate peak and depressed NAA peak in the area of infarction, indicating ongoing neuronal necrosis.