MRI scans take longer to perform than an average CT scan. In the context of an unstable patient this may produce difficulties in monitoring and continuing care. Furthermore, most critically ill patients have a number of devices attached any of which can be affected or even damaged by the magnetic field.
MRI scans can be noisy. The repeated banging sound that occurs during the scanning process is accounted for by fluctuations in electrical current within the intense magnetic field. This noise further adds to the anxiety experienced by some patients within the device.
Older machines are constructed around a long narrow cylindrical magnet which some have found claustrophobic. Newer devices have shorter magnets, allowing less enclosure of the body.
Clinicians are recommended to be selective in the ordering of regional scans to minimise the time patients spend within the machine.