Modern neuroimaging has transformed the investigation of head injury. Skull radiographs are now rarely performed if magnetic resonance imaging ( MRI) or CT brain scanning is available.

MRI scans cannot be performed if there is any magnetic material present either in the body (e.g. a pacemaker) or attached to the body. Therefore on the trauma unit CT brain scanning is the preferred investigation, with its faster acquisition time and good visualization of subdurals and extradurals.

However, in the postacute setting MRI is the better instrument.(!.2) Often cerebral contusions are found near the bone-brain interface (see above) where the image quality of CT is poor because of imaging artefacts from the adjacent bone. MRI has no such limitation. MRI is also able to detect, on 7"2-weighted images, changes in signal associated with a diffuse axonal injury when the white matter appears normal on CT brain imaging. The better image resolution of MRI is also in its favour.

Despite its greater sensitivity a normal MRI does not rule out significant brain injury. On the other hand, particularly in the elderly, MRI may detect abnormalities unrelated to the head injury.

The MRI scan can be normal and yet functional imaging of cerebral metabolism using single-photon emission computed tomography or positron-emission tomography will detect abnormalities/1.. !4) In general, changes on functional imaging correlate better with neuropsychological test performance than do lesions found on structural imaging/!5) However, abnormalities on functional imaging are not necessarily due to brain injury. Hypometabolism may be seen in mental illness without brain injury, for example in depression. There is a recent report of marked hypometabolism on positron-emission tomography imaging in a man with cognitive impairment occurring immediately after a psychological traumaA6,) He had sustained no head injury.

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