![]() MRI is the gold standard for investigation of most diseases of the central nervous system (CNS)and as such the indications are wide, including stroke, temporal lobe epilepsy, tumour, infection, inflammation, multiple sclerosis, dementia, metabolic disorders, post-trauma, congenital malformations, nerve palsies, internal auditory canal masses, vascular diseases and pituitary pathology. The risk of NSF per injection rises from 0.1% in stage 4 disease (15–30 ml/min/1.73 m 2) to 1% in stage 5 (<15 ml/min/1.73 m 2), 5 and most MRI centres will avoid gadolinium in these groups. Individuals with stage 3 kidney disease (eGFR 30–60 ml/min/1.73 m 2) have a very low risk of NSF. 4 Patients on haemodialysis should not receive gadolinium-based contrast. Phagocytosis by macrophages is believed to induce cytokine release, which results in fibrosis and the irreversible and debilitating clinical syndrome of contractures and pain. In severe renal disease, biological half-life is increased and the gadolinium chelate can dissociate, resulting in binding of free gadolinium in tissues by displacing endogenous metals, including iron and calcium. Gadolinium is chelated to allow safe excretion, as free gadolinium is highly toxic to tissues. The most common contrast medium used in MRI is gadolinium, a paramagnetic metal ion that shortens T1 relaxation time, resulting in increased T1 signal. MRI contrast and nephrogenic systemic fibrosis (NSF) ![]() Pregnancy is no longer a relative contraindication but pregnant patients should not receive gadolinium. Patients with severe claustrophobia and children under the age of 6 years will generally need sedation under the supervision of an anaesthetist. Some systems offer a wider bore that is also better tolerated by claustrophobic patients. Patients of very high body mass index may not fit within the bore of a conventional magnet. 3.0 tesla (T) scanners) and may need to be scanned at lower-field strength (1.5T). Many devices have not been fully tested in higher-field strength systems (eg. Some pacemakers are now MRI compatible but need to be switched off by a cardiac technologist before scanning, requiring coordination with the patient’s cardiologist. Death has occurred when patients with cardiac pacemakers were placed in MRI systems. Some devices may be safe to scan after appropriate investigation even when an ‘absolute contraindication’ is present. GPs should contact their MRI service for advice before denying a patient an MRI scan, for example, because of a pacemaker. *Need to be assessed as safe before patient enters MRI suite Stents, including abdominal aortic aneurysm, recently inserted.Contraindications to MRI 3 Absolute contraindications Patients are screened for the presence of these devices before entering the MRI suite and some examinations may need to be cancelled or deferred it is very useful for the MRI service to be aware of these devices before the patient’s appointment, to avoid delays to the patient. 3 Interference can occur outside the scanner as the magnetic field extends beyond the scan room. Table 1 gives a guide to common contraindications and precautions to MRI scanning. Implants containing metal may move in the MRI system and the function of implants can be permanently altered. An injection of contrast through an intravenous cannula may be required. The patient must wear headphones to suppress noise and these are used to play the patient’s favourite music or radio station streamed via the internet. The patient is still able to see out and can speak to the MRI technologist at all times. ![]() The head and neck are enclosed within a rigid radiofrequency coil this coil does not touch the patient but is close to the face. The patient needs to remove any metal and lie flat within the bore or ‘tunnel’ of the magnet. The examination is relatively long and takes 15–30 minutes to complete. The reader is referred to the November 2012 AFP article ‘MRI of the knee’ for a general discussion of MRI. ![]()
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