Brain

Abstract

The role of nuclear medicine in investigating diseases of the central nervous system is currently in flux. Older techniques which demonstrated regional abnormalities in capillary permeability have been almost entirely superseded by x-ray computed tomography (CT) and magnetic resonance imaging (MRI), both of which relate pathology more precisely to adjacent anatomical structures. Tracers which demonstrate the distribution of specific neuroreceptors within the central nervous system are becoming available, but their clinical applications are not yet established. Currently interest is focused mainly on imaging regional cerebral metabolism. The effects of ischaemia become apparent rapidly in the brain because, despite its high rate of oxidative metabolism, it has effectively no reserves either of oxygen or of glucose and a very limited capacity for anaerobic metabolism. It is therefore critically dependent on maintenance of an adequate blood supply for its continued function. Oxygen and glucose consumption can be measured directly with positron emission tomography (PET) tracers but, because cerebral metabolism is dependent on immediate availability of both oxygen and glucose, blood flow and perfusion are clinically useful surrogates which can be measured with single-photon tracers. Blood volume and perfusion reserve can also be determined. All of these measurements can be made either globally or regionally and expressed either in absolute units or relative to other parts of the brain.