Magnetic resonance imaging (MRI)

Summary

Magnetic resonance imaging (MRI) is a cross-sectional imaging technique which allows excellent soft-tissue definition and has an extensive and expanding role in radiology. The examination is expensive and can be quite lengthy, depending on the number of sequences required. The bore of the magnet is quite narrowand claustrophobic patients are often not able to tolerate the examination. Small children and confused or demented patients may not be able to remain still and sometimes sedation is required. This needs to be pre-arranged with the MRI department and the anaesthetic team.

Although MRI does not use ionizing radiation there are other safety considerations arising from the use of the strong magnetic field; these should be considered in terms of electromagnetic and missile effects.

Both patients and staff are required to fill out a safety questionnaire before entering the scanner.

Contraindications to MRI

1. Pacemaker/implantable defibrillator device: deactivated by magnetic field; fatal.

2. Intra-cranial aneurysm clips: dislodge due to missile effect.

3. Intra-ocular foreign body: potential for injury resulting in impaired vision. Plain radiographs or CT scan with the patient looking up and down may be required to exclude an intra-ocular foreign body before a scan.

4. Cochlear implants: these devices are programmed by a magnet, and so subjecting the device to a large magnetic field will interfere with function and also risk dislodging mechanical parts; the potential exists for a heating effect through the induction of a current in the cable.

5. Pregnancy: the effect of MRI on early pregnancy is unknown. Scans are therefore not performed during the first trimester.

6. Prosthetic heart valves: some older models of prosthetic valves are contraindicated, individual cases should be checked.