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Can MRIs have any deleterious effect on one's health? Or would it be safe to have say 5 MRIs per year?

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One of the greatest benefits of MRI scans is their safety. Unlike PET, X-ray, CT and most other scans, MRIs use the properties of body tissues in magnetic fields to produce an image. The MRI machine produces a powerful magnetic field which interacts with body tissues to produce radio waves, which are in turn interpreted by a computer to determine the location of the tissues.1

This does not, however, mean MRIs are completely without risk. First and foremost, an MRI machine is basically a giant magnet. If one has metal in his or her body, from a medical implant, car accident, or even an improperly done tattoo, the MRI machine can move it, potentially violently. In the case of medical implants, an MRI machine can easily destroy any electrical components.2 There are a number of different reasons metal might be present in the body.3 This is why MRI technologists are trained to ask, repeatedly, about any metal that may be present.

In addition to metal, some MRIs involve contrast media, chemicals injected or consumed that increase visibility of certain tissues in MRI scans. These chemicals are normally removed by the body rather quickly, and except in individuals with reduced kidney function, are for the most part safe. However, there have been cases of allergic reactions and side effects.4 Also, recently, questionable studies have presented possible links between gadolinium-based contrast agents and nephrogenic systemic fibrosis, though the American FDA has stated that this link is only significant in patients with kidney disease, as "NSF has not been reported in patients with normal kidney function".5

In Short

In healthy individuals, with properly-functioning kidneys, no medical implants, no improperly-done tattoos, and no metal embedded in the body, MRIs are completely safe, once or even quite often.

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    Good answer, thanks. Explicitly annotated links would be helpful, so people can see what they are about to click. – Faheem Mitha Mar 31 '15 at 21:56
  • @FaheemMitha Bottom left corner of the screen in most browsers, and I'm working on Meta now to implement a better reference system – TheEnvironmentalist Mar 31 '15 at 22:07
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    Yes, I know I can see the link. :-) I meant, it is helpful if the link is annotated in the question itself, with a title or other suitable material. If you don't want to interrupt the flow of text in the answer, perhaps you could put it at the bottom, like a footnote? – Faheem Mitha Mar 31 '15 at 22:10
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    @Tim While I certainly appreciate the effort, it's been nagging me that the meta post now seems like self-promotion. My intent was to present an improvement for the community, not to brag about my answers. My position remains the same. For that reason, as well as improvements in flow and consistency with the medical standard, I have rolled back both. I felt you had every right to know, given the time you put into both. – TheEnvironmentalist Apr 16 '15 at 4:05
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    @dev101 Noted, and source replaced. I also added some details from the alternative source (FDA) for additional information. – TheEnvironmentalist Jan 8 at 20:52
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While they're sometimes confused by the public, X-Ray CT (computed tomography) and MRI (magnetic resonance imaging) work by entirely different priniciples. X-Ray radiation is harmful, and any unnecessary exposure should be limited. MRI works by putting the subject into a very strong magnetic field and using radiowaves to excite specific nuclei.

The potential dangers of MRI are equivalent to the dangers of strong magnetic fields and radiowaves (and potentially any contrast medium that is used, which can cause allergic reactions).

According to the FDA:

There are no known harmful side-effects associated with temporary exposure to the strong magnetic field used by MRI scanners.

Due to the strong magnetic field you need to avoid bringing any metal objects near the MRI, they will be attracted by the magnet and become dangerous projectiles.

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