While MRI scanning may not be as safe as radiologists claim, most injuries and reactions are rare. However, the same cannot be said for the dyes that are injected before the screening is carried out.
Although it was originally believed that the high quality of the 'pictures' afforded by MRI would eliminate the need for injectible dyes, this has not proved to be so. Contrast agents are still needed to detect brain tumours, for example. Unlike the contrast materials used for CT (computed tomography) scanning, which contain iodine, those used for MRI are magnetically active substances.
Currently, the only MRI contrast materials approved by the US Food and Drug Administration (FDA) are chelates, typically containing the rare earth element gadolinium. When injected into a patient's veins, this works similarly to iodine-based contrast agents, but is supposed to be far safer, with severe reactions only affecting about one in 350,000 patients.
Of all the reports made to the UK Adverse Reaction Reporting Scheme from 1977 to 1983, nearly half concerned methylene diphosphonate, used for imaging bone, and a third involved colloids, used for liver scans. The majority of complaints were due to patients' hypersensitivity to the dyes.
The most conservative estimate is that one in a thousand people will have such a reaction - a figure far higher than was originally believed (Br J Radiol, 1984; 57: 1091-6).