A 38-year-old intravenous drug abuser presented with weakness in both legs and loss of bowel and bladder control. An epidural abscess causing cauda equina compression was suspected and hence MRI was the investigation of choice. However, she said she had lost a needle in her groin 3 months ago. She was declined surgical intervention to remove the needle, given the risks involved. The needle was localised with a pelvic x-ray and was found to be lying on the left femoral vessels (fig 1). The duty neuroradiologist was reluctant to perform an MRI scan considering the possibility of the metal heating up and causing damage to the femoral vessels. Instead CT scanning was carried out to plan urgent neurosurgical intervention. The management was thought to be suboptimal in the absence of an MRI scan. CT myelography was not an option because of the infected epidural space. The vascular surgeons were requested to remove the needle so that an MRI scan could be carried out safely. However, the risks of surgery were considered to be greater than the risks of the needle causing harm in a magnetic field. In the absence of intervention, the patient could be left unable to walk and doubly incontinent for the rest of her life. The risks and benefits were carefully analysed. A vascular radiologist further confirmed that the risk of harm was minimal with such a small metallic foreign body. We went ahead with 1.5 Tesla MRI scan of the thoracolumbar spine uneventfully.

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.