Proton MR spectroscopy (1H-MRS)
05 November 2004
Proton MR spectroscopy (1H-MRS) is sensitive to within-individual changes in the concentration of brain metabolites over time on the order of 1 mmol/L, permitting a volume of interest (VOI) of between 1 to 8 cm3. Phosphorous MRS (31P-MRS) can study the high-energy phosphate metabolites, yet is only about 5% as sensitive, and so requires a much larger VOI, between 15 and 40 cm3. MRS thus presents a new opportunity for assessment of the biochemical composition of pathologic and healthy brain tissue in vivo.
Careful selection of pulse sequence and echo time is required as each can affect the range of biochemicals measured in the MRS scan. The particular method of quantitation is also important for interpretation of results: relative methods have used the physiologic water signal or the cell creatine signal as a reference, but this technique has the disadvantage of not definitively distinguishing between numerator or denominator meta-bolite changes. Absolute quantitation methods are more accurate in this respect, but results from between labs have not been consistent and clinical implementation of current techniques is impractical.
An alternative to consider may be a two-fold relative quantitation system. Concordant findings based on referencing of metabolite peaks to both the CSF-corrected water signal and the creatine signal are likely to reflect real biochemical change. Discordant findings may require further investigation with absolute techniques.
Proton MR spectroscopy (1H-MRS) spectra of frontal white matter (2 x 2 x 2 cm) in a 74-year-old woman. Acquisition was with the STEAM pulse sequence; repetition time, 1500 msec; echo time, 30 msec. Major metabolites are labeled: N-acetylaspartate (NAA), free cholines (Cho), creatine plus phosphocreatine (Cr), and glutamate plus glutamine (Glx). Chemical shift is represented on the x-axis in parts per million (ppm). The NAA/Cr ratio in this example was 1.15. (B) Axial slice T1-weighted MRI shows the location of frontal lobe MRS voxel. T1-weighted MRI. Left side of figure = right side of brain.
do the metabolite peaks signify?
1. N-acetylaspartate (chemical shift [d] = 2.02 and 2.6 parts per million [ppm]). N-acetylaspartate (NAA) exists in the brain at an approximate concentration of 12 mmol/L and has been found elevated in Canavan’s disease and decreased in areas of focal neurologic pathology. Given that NAA is predominately intraneuronal, it has been widely used as a marker of neuronal density. Reliable in vivo assay of NAA is suggested by strong correlations between in vitro estimates and MRS estimates in rat models and its excellent signal characteristics. Although the precise physiologic role remains uncertain, a recent in vitro report suggested that NAA may reflect myelination processes in the adult human. NAA has been found to undergo reversible changes in patients with relapsing MS, on recovery from brain injury, and in some patients with AIDS dementia complex after drug therapy. A highly significant correlation has furthermore been found in vitro between mitochondrial phosphorylation and rates of NAA synthesis. NAA may therefore be a useful in vivo marker of neurometabolic fitness, reflecting a level of neural viability that can recover after insult.
2. Choline compounds (d= 3.2 ppm). Choline is a rate limiting precursor in the synthesis of acetylcholine and a precursor to cell membrane phosphatidylcholine. The choline MRS peak measures total levels of mobile choline, which include free choline, acetylcholine (present in relatively minute quantities), glycerophosphorylcholine (a byproduct of phosphatidylcholine breakdown), and phosphocholine (a phosphatidylcholine precursor). Membrane phospha-tidylcholine is invisible on MRS. Correlations between regional choline levels of brain tumors and biopsy analysis show a significant association. On the other hand, repeatability studies have found poor test–retest reliability.
3. Myo-inositol (d= 3.6 and 4.0 ppm). Myo-inositol (MI) is a largely mysterious sugar-alcohol whose structure is similar to that of glucose. It is estimated that 70% of the MI peak comes from free MI directly and 15% from MI phosphate. MI may act as a marker of glial cell numbers, an osmoregulator, intracellular messenger, or detoxification agent in the brain as in the liver.
4. Creatine plus phosphocreatine (Cr) (d= 3.0 and 3.9 ppm). The phosphocreatine–creatine equilibrium reaction acts as a reserve for high energy phosphates and buffers cellular ATP/ADP ratios. The combined "Cr" signal thus reflects the health of systemic energy use and storage. As mentioned, Cr has been used as a reference metabolite to quantify other neurochemicals. Repeatability studies have shown it to be stable over the course of months within an individual. Factors known to affect Cr include age and white matter disease.
5. Glutamate–glutamine complex (d= 2.1 to 2.4 ppm). The complex chemical structures of glutamate and glutamine mean that their peaks are hard to distinguish and are commonly labeled "Glx." Glutamate functions as the major excitatory neurotransmitter in the brain. Glutamine may be important to brain function, serving a role in detoxification and regulation of its precursor glutamate within the astrocyte body.
Oxidative metabolism in the brain, as in muscle tissue, is notable for its high levels of phosphocreatine–creatine activity and a high steady state of mitochondrial respiration. 31P-MRS allows assessment of these different high-energy chemicals. The major peak assignments in 31P-MRS include: 1)a, ß, and d nucleotide triphosphates, which reflect ATP levels; 2) phosphocreatine, a key indicator of oxidative meta-bolism (along with ATP); 3) phosphodiesters (PDE), comprised of resonances from glycerophosphorylcholine and other elements of the phospholipid bilayer and therefore reflecting levels of cell membrane breakdown products; 4) phosphomonoesters (PME), mainly composed of signal from phosphocholine and other key precursors of membrane phospholipids; and 5) inorganic phosphate, used to estimate intracellular pH.