MR spectrum – normal

The normal MR spectrum

• observed spectra varies with echo time (TE) used in acquiring them.
• short (20-30 ms), intermediate (135-144 ms) or long (270-288 ms).
• more peaks in short TE.
• long TE typically detects peaks for 4 compounds
  1. NAA (N-acetylaspartate)
  2. choline
  3. creatine (Cr)
  4. lactate
• different areas of brain have different concentrations of metabolites.
• concentration varies with brain maturation.
• Infants
  • high choline, myoinositol
  • low Cr, NAA

N-acetyl aspartate (NAA)

• most obvious peak
• at 2.01 ppm
• reference for chemical shift determination
• contributions from NAA, N-acetylaspartate glutamate, glycoproteins, amino acid residues in peptides.
• two functions in adult brain
  • precursor of brain lipids
  • involved in coenzyme A interactions
  • alternate suggestion of it being an osmolyte – neurotransmitter / neuromodulator precursor
• higher in cortex than in white matter
• located in neurons and its branches
• low concentration in mature glia
• reduced in neurodegenerative diseases
• absolute or relative decrease in relation to Cr peak suggests axonal / neuronal damage
• transient reduction – energy depletion as synthesis of NAA occurs in mitochondria.
• in infants – concentration is same in gray and white matter
  • indicator of normal oligodendroglial development

Choline

• trimethylamine peak
• 3.21 ppm
• choline, betaine, carnitine, myo-inositol and taurine
• contains phosphoryl choline, phosphorylethanolamine, glycerophosphorylchlorine, glycerophosphorylethanolamine and free choline
• phosphorylethanolamine dominates in neonates; decreases with age with phosphoryl choline
• glycerophosphorylchlorine, glycerophosphorylethanolamine increase with age
• choline reflects structural components of cell membranes (myelin sheaths)
• membrane bound choline not visualized in MR
• increased in highly cellular processes – high grade tumors, neurodegenerative disorders
• focal inflammation -> marked local cellularity -> cell membrane breakdown -> elevated choline
• seen higher in thalami and cerebellum > cerebral white matter > cerebral cortex

Creatine (Cr)

• 3.03 ppm
• methyl protons of creatine and phosphocreatine
• minor contributions from gamma-amino butyrate, lysine and glutathione
• second smaller peak at 3.94ppm
• maintenance of energy dependent systems in all brain cells
• cerebellum > thalami > basal ganglia > cortical gray matter > cortical white matter
• stable in most situations; hence used as a standard for other metabolites

Myo-inositol

• two peaks 3.56 ppm and 4.06 ppm
• storage pool for membrane phosphoinositides – involved in hormonal systems and enzyme regulation
• essential growth factor – precursor of phosphatidylinositol
• located primary in glial cells – specific marker
• osmoregulation, cell nutrition, detoxification
• 3.56 ppm peak gets minor contributions from glycine and inositol-1-phosphate

Scyllo-inositol

• singlet peak at 3.35 ppm
• not from taurine as previously described

Cerebral glucose

• singlet peak at 3.43 ppm with short TE
• area under peak used as gross measurement of glucose concentration in brain

Lactate

• doublet peak 1.3 ppm (1.27 and 1.36 ppm)
• trace amounts in appropriate for gestational age term neonates
• more than trace amounts in first few hours of life indicate some form of brain injury
• normal findings in CSF of premature and term neonates; concentration up to 2.7 mm/L
• exclusion of CSF important to rule out false positivity in encephalopathy
• propan-1,2-diol mimics doublet peak – it’s a solvent used in anti epileptics

Glutamine-glutamate

• 2.1 – 2.4 ppm region
• second peak at 3.75 ppm alpha -CH moiety
• excitatory neurotransmitter
• seen in hypoxia, seizures, possible trauma
• difficult to identify as it overlaps with NAA and Cr peaks; improves in higher magnetic fields

Macromolecular peaks

• typical in short echo spectra in infants
• between 0.5 and 1.0 ppm and between 1.0 and 1.6 ppm
• methylene and methyl protons
• no clinical significance if less than half peak of NAA
• when large peaks – degenerative process
• inborn errors of metabolism, child abuse in normal infants

Phosphodiester and phosphomonoesters

• progressive decrease – good marker of infant brain development
• high in preterm infants

Key spectral variations in different brain regions

• NAA/Cr
  • brain stem and cerebellum > cerebral hemispheres
  • cerebral white matter > basal nuclei
  • frontal white matter > parietal white matter
• Choline/Cr
  • Choline > Cr in white matter
  • Cr > choline in gray matter

 

Brain maturation and spectral variations

• decreasing phosphomonoester peak
• increasing phosphocreatine and phosphodiester peaks
• increase in NAA relative to choline and creatine phosphocreatine peaks
• diminishing myo-inositol peak
• scyllo-inositol highest at birth

*adapted from pediatric neuroimaging textbook – Barkowich