Less Commonly Observed Brain Specta
Alanine (Ala) is a small amino acid whose major spectral peak is a doublet from its methyl group at 1.46 ppm. This doublet overlaps with the lactate doublet, and like lactate, inverts at intermediate TE values (~144 ms). Ala also has a second smaller peak at 3.77 ppm. Ala levels may be elevated in meningiomas, central neurocytomas, and primitive neuroectodermal (PNET) tumors. Elevations are also reported in demyelination and bacterial abscesses.
Acetate (Ac), the anion of acetic acid, commonly accumulates in bacterial abscesses due to enhanced glycolysis and fermentation by the infecting organism. It produces a singlet peak at 1.91 ppm. Acetate is also elevated in the in Canavan's disease due to deficiency of the enzyme aspartoacyclase.
Branched Chain Amino Acids (leucine, isoleucine, valine) may be identified in patients with various inborn errors of metabolism such as maple syrup urine disease. They are also seen in pyogenic and fungal (but not tuberculous) abscesses. Their principal multiplet peaks lie in the 0.9 – 1.0 range and may invert when intermediate TE values (~144 ms) are used.
Ethanol, the primary ingredient in alcoholic beverages, may be detected in intoxicated patients, characterized by its main triplet peak at 1.19 ppm.
GABA (γ-aminobutyric acid) is the principal inhibitory neurotransmitter in the central nervous system. It has three relatively equal-sized peaks at 1.90, 2.30, and 3.02 ppm. Due to its low concentration compared to other more prominent overlapping (NAA and Cho) peaks, special MRS methods such as spectral editing are needed to identify and resolve GABA resonances.
Galactose, a monosaccharide sugar, and its metabolites galactitol and galactonate, have resonances around 3.7 that may be detected in patients with the inborn error of metabolism, galactosemia.
Glucose, the principal monosaccharide circulating as blood sugar, may be detected during extreme hyperglycemic states such as diabetic ketoacidosis. Principal resonances are at 3.43 and 3.8 ppm.
Glycine (Gly), the smallest the amino acids, has a primary resonance at 3.56 ppm. It is elevated in many brain tumors (glioblastomas, ependymomas, medulloblastomas) and is especially prominent in central neurocytomas. It may also be seen in the spectra of the brain and other organs in nonketotic hyperglycinemia (an inborn error of Gly metabolism).
Ketone bodies, including acetone (2.22 ppm), acetoacetate (2.26 and 3.46 ppm) and β-hydroxybutyrate (doublet peaks at 1.15 and 1.25 ppm) can be detected in patients on ketogenic diets and in those with diabetic ketoacidosis.
Mannitol, an exogenously administered sugar alcohol used for treatment of increased intracranial pressure and low output renal failure, produces a detectable peak at 3.8 ppm.
Phenylalanine, an aromatic amino acid, has its prominent resonances in the upfield part of the spectrum (i.e., to the left of water) in the 7.31-7.43 range, that are seen in the inborn error of metabolism, phenylketonuria.
Succinate, the anion of succinic acid, is a component of the citric acid cycle. Its singlet peak at 2.4 ppm is often seen together with that of acetate (1.9) and is a marker for anaerobic pyogenic infections as well as cysticercosis.
Taurine, a ubiquitous amino-sulfonic acid, is inhibits neural transmission and stabilizes cell membranes. It has two peaks (one at 3.25 ppm, usually obscured by Cho) and a second at 3.42 ppm, just on the left shoulder of Cho. It is elevated in a wide range of disorders, but especially in primitive neuroectodermal tumors (PNET/medulloblastoma/retinoblastoma) and metastatic renal cell cancer.
Additional notes about normal ¹H spectra outside the brain
In the liver only a broad peak from cholesterol (δ = 6.5 to 8.0) is commonly detected. Liver-related amino acids are also seen (glutamine and aspartate, δ = 2.0 to 3.0). Creatine is not found in the liver (or kidney) so checking for a Cr peak in the spectra of these organs provides a method of quality control for evaluating the adequacy of spatial localization.
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