MRI is a technology that uses a strong magnetic force, radiofrequency waves and a computer to create images of the body. A typical MRI uses a magnetic field of 1.5 Telsa which is a field strength about 30 000 times more powerful than the earth’s magnetic field.1
An important safety concern with MRI is the machine’s ability to attract metal objects.2
MRI scanning in patients with metallic foreign bodies can lead to five dangerous interactions: projectile effect, twisting, burning, artefacts, and device malfunction (interference with a pacemaker).3
All patients should be thoroughly screened for foreign bodies before undergoing an MRI study.4
However, having a metallic foreign body is not an absolute contraindication but can cause anxiety among patients and physicians. Patients need to be assessed on an individual basis. In doubtful cases useful advice can be obtained from the Institute of Magnetic Resonance Safety, Education and Research (IMRSER). A reference manual for magnetic resonance safety, implants and devices by Professor Frank G Shellock is a comprehensive guide which relates the experience of ferromagnetic testing and is available for reference in most MRI suits. Furthermore, in doubtful cases ferromagnetic testing can be carried out in MRI suits in a controlled fashion.
- Having a metallic foreign body is a relative contraindication for having an MRI scan.
- Ferromagnetic foreign bodies in the eyes and aneurysm clips in the brain may migrate in a magnetic field with disastrous results, and hence an MRI scan is absolutely contraindicated.
- In doubtful cases it is best to defer the scan until specialist advice is available.
- A metallic implant may distort the MRI images.