Nuclei of atoms with an odd number of protons or neutrons absorb or emit electromagnetic radiation when placed in a magnetic field. Hydrogen (protons), phosphorus 31P, sodium 23Na and carbon 13C nuclei have been studied; 31P spectroscopy is used to measure concentrations of adenosine triphosphate, phos-phocreatine (PCr) and intracellular pH in muscle, and neonatal brain metabolism. Repeated examinations, e.g. of tumour PCr, may indicate progression or remission of disease.
NMR imaging uses information on differences in relaxation times of nuclei. The contrast between grey and white matter in the brain is readily apparent, and excellent delineation is provided of pathologies such as demyelination and tumours in inaccessible sites. In the evaluation of lesions produced by multiple sclerosis or vascular lesions, findings are not pathognomonic but must be assessed, as with all ancillary investigation techniques, together with clinical signs. As with computed tomographic (CT) scanning, contrast enhancement may be used. The hazards associated with anaesthesia for NMR are discussed on (p. 607)
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