MR Spectroscopy
MR spectroscopy (MRS) measures the chemical composition of tissue non-invasively. Instead of an image, the scan produces a spectrum of peaks, each representing a metabolite whose position (chemical shift, in ppm) and height show its identity and concentration.
Proton (hydrogen-1) spectroscopy of the brain is the most widely used. It is done on a 1.5 or 3 tesla scanner after the anatomical MRI. Two approaches are used.
Single voxel spectroscopy samples a cube of tissue (typically 2 x 2 x 2 cm) with short or long echo time, taking 3 to 8 minutes.
Multivoxel chemical shift imaging samples a grid across a slice or slab and produces metabolite maps, taking 8 to 15 minutes.
Phosphorus-31 spectroscopy (energy metabolites in muscle and liver), carbon-13 and hyperpolarised methods are used in research and specialist centres. Prostate, breast, liver and muscle spectroscopy have limited clinical use.
The main proton metabolites in the brain are N-acetylaspartate at 2.0 ppm (neuronal marker), choline at 3.2 ppm (membrane turnover), creatine at 3.0 ppm (energy reference), myo inositol at 3.5 ppm (glial marker, short echo time), lactate as a doublet at 1.3 ppm (anaerobic metabolism), lipids at 0.9 to 1.3 ppm (necrosis), glutamate and glutamine at 2.1 to 2.5 ppm, and, with special techniques, 2-hydroxyglutarate.
Brain tumours are the commonest clinical indication. MRS distinguishes tumour from non-neoplastic lesions with a sensitivity and specificity of about 80 to 90%, and it adds to the MRI for grading, biopsy targeting and the separation of recurrence from radiation necrosis.
Indications.
- Characterisation of a brain mass: glioma grade, lymphoma, abscess, metastasis, tumefactive demyelination.
- Separation of recurrent tumour from radiation necrosis or treatment effect.
- Choice of a biopsy site in a heterogeneous tumour.
- Metabolic and mitochondrial disease: lactate in Leigh disease and mitochondrial encephalopathy, N-acetylaspartate loss in Canavan disease, and creatine deficiency.
- Neonatal hypoxic ischaemic encephalopathy: lactate and N-acetylaspartate for prognosis.
- Inherited metabolic disease and leukodystrophy.
- Hepatic encephalopathy and Wernicke encephalopathy (research and specialist use).
- Epilepsy: lateralisation of temporal lobe epilepsy (reduced N-acetylaspartate).
- Neurodegenerative and psychiatric disease (research).
- Isocitrate dehydrogenase mutant glioma detection with 2-hydroxyglutarate (specialist).
- Prostate and breast cancer (adjunct in some centres).
Not for routine screening, and not a stand alone test. It is always interpreted with the MRI and clinical context.
Contraindications and precautions.
- The usual MRI safety screen.
- Gadolinium is not needed for spectroscopy. Contrast given before can slightly change line shape, so spectroscopy is often done first or the contrast effect is accounted for.
- Patients must lie still for several minutes. Motion distorts the spectrum.
- Voxels placed near the skull, fat, air, blood or metal are unreliable.
- Sedation in children, with monitoring.
- Pregnancy: acceptable if clinically necessary.
Preparation and technique.
No preparation is needed. The patient lies supine with the head in the coil. The anatomical scan sets the voxel position. The operator places the voxel within the solid, enhancing part of the lesion and avoids necrosis, cysts, blood and bone.
A matching voxel in the opposite normal appearing brain is used for comparison.
The magnetic field is adjusted over the voxel (shimming) to a narrow water linewidth, and water suppression is applied. Short echo time (30 to 35 ms) shows more metabolites including myo inositol, glutamate and lipids.
Intermediate echo time (135 to 144 ms) gives a flatter baseline and shows lactate as an inverted doublet.
Spectra are processed and fitted to give peak areas, ratios (choline to N-acetylaspartate, choline to creatine, N-acetylaspartate to creatine) and, where possible, absolute concentrations.
