MRI is one of the least invasive imaging technologies available today, and has been in use for over thirty years. In that time, it has proven to be very safe. This safety record is due to appropriate knowledge and remediation of potential hazards.
Magnetic Field - One source of concern is the main magnetic field. This field is on the order of a million times stronger than the ambient magnetic field at the surface of the earth and exerts tremendous translational and rotational force on ferromagnetic objects.
Cryogens - Most magnet systems are cooled with liquid helium. The purpose of the cooling is to permit the coil that generates the magnetic field to become super-conducting, which occurs within a few degrees of absolute zero. As helium boils at 4.2K, a failure in the cooling system can result in a rapid boil-off. Such an event is rare, and MRI systems are designed to direct the resulting gas safely out of human-occupied areas. However, the failure of the this venting system may result in helium gas displacing oxygen, creating a risk of asphyxiation.
Radiofrequency Energy (RF) - Another component of MRI is an applied RF field to perturb, or flip, the spins of protons of the tissue being imaged. Clinical magnets are carefully designed to prevent too much RF from being delivered to the body.
Gradient Magnetic Fields - The third component of MRI is a gradient magnetic field. This field allow localization of the signal from tissue. Some types of scans require rapidly changing gradient fields, and conductors in a changing magnetic field will host electrical currents. One effect of these currents that has been noted is peripheral nerve stimulation.
Sound - Another effect of the gradient fields is the production of a great deal of noise. Some sequences can exceed 110 dB within the bore of the magnet. For this reason, hearing protection is mandated.