Magnetic Resonance Imaging came about during the 1970s, providing a much clearer and more accurate picture of the internal environment of a living human body. Basically, an extremely strong magnetic field is used to interact with the nuclear magnetization of atoms in the body, providing RF fields with excess photons to construct the imaging component.
The field strength of MRIs has consistently increased, and there is some concern over occupational exposure. There have also been some anecdotal reports of symptoms by people who have worked with MRIs or other high-powered magnets.
On the other hand, there has been no evidence that strong static magnetic fields have any effect on human health, although nothing should be taken for granted. The jury is definitely still out on electromagnetic fields, and that may have bearing on the MRIs overall level of safety, but for the time being you can probably rest assured that whatever is bringing you to the MRI chamber is almost certainly a bigger risk than anything the machine could pose!
And when compared to more obvious risk factors associated with other modern, accepted imaging techniques, MRI and associated field risks appear in a better light. For example, x-rays and any other form of radiation used for scans or treatments are known to carry risks associated with bombardment of the body with sub-atomic particles. One can easily understand those risks – think tiny particles or energy penetrating the body.
Because MRI uses the measurement of changes to existing nuclear magnetic fields, there really is no analogy to other forms of radiation. Of course, in time we may learn of other risk factors, but until that time the benefits clearly outweigh the known risks.