3 The technologies
3.1 Structural neuroimaging involves non-invasive visualisation of the anatomical structure of the brain, in contrast to functional neuroimaging, which involves visualisation of the neurophysiological function of the brain. Two structural neuroimaging techniques that are currently used in the NHS are magnetic resonance imaging (MRI) and computed axial tomography (CT) scanning. MRI exploits the nuclear magnetic resonance phenomenon while CT scanning is based on a series of X-rays.
3.2 MRI is considered to be the preferred option for neuroimaging because it provides higher image resolutions than CT scans. It is also better able to picture the soft tissues of the brain whereas CT scanning is more effective for picturing bone and hard tissues. MRI is generally a safe diagnostic technique and few safety concerns are reported in practice. Safety concerns usually relate to interactions of MRI scanners with magnetic objects (for instance, pacemakers) and patients may be subjected to noise, hyperthermia and peripheral nerve stimulation causing muscle twitching. There is a refusal rate in the general patient population of 5–10% because of anxiety and claustrophobia (this rate may be much higher for people with psychosis). MRI scanning results in a number of false positive tests. In a retrospective study of 1000 healthy volunteers, 82% of MRI results were completely normal, and 1.1% required urgent referral. The remaining 16.9% may therefore have been unduly worried by a false positive MRI result of no medical consequence.
3.3 CT scanning can only detect differences in tissue density; lesions that have the same density as adjoining tissues will not be detected. However, in this case, an iodine-based contrast dye may be used for better visualisation. Contrast dyes may cause allergic reactions in some people, and in others may impair renal function. In some situations, MRI scanning may also require contrast enhancement. However, it is not generally expected that contrast enhancement would be required for evaluation of first-episode psychosis. One disadvantage of CT scanning is the dose of radiation absorbed during the process.
3.4 The acquisition cost of an MRI scanner is £1–2 million and that of a CT machine is approximately £500,000. Other costs associated with an MRI scanner include the space that the scanner and other computerised equipment occupy. Additional costs associated with both technologies include regular maintenance, additional clinical support, and staff costs and training to use the technologies. The costs of individual MRI and CT scans are estimated at £244 and £78, respectively (2005–2006 NHS Reference costs, codes RBF1 and RBC5, respectively).