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A friend recently had an MRI performed on his back and said he experienced an uncomfortable amount of heat. In his own words, he said: "The MRI tech said the reason I was boiling in the machine was because the machine excites the protons in my body."

I would think if the field strength was really that strong there would be other negative effects on the body. Do MRI machines really induce an extreme heat sensation by exciting protons in the body?

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::sign:: I do know that is the economically silly to run the machines with scientists or engineers, but I just have to say ... arrggghhh! Short answer no. –  dmckee Nov 12 '12 at 20:08
    
It is a concern. They do excite protons in the body, but the main concerns are with avoiding conductive things in the volume, and conductive loops, even those formed between body parts like legs. –  Mike Dunlavey Nov 12 '12 at 21:35
    
@Mike Your own link testifies that it is the RF and inductive currents when inserting or extracting that are a problem. The magnetic field itself and excitation of nuclei or nucleons are non-issues. –  dmckee Nov 13 '12 at 1:36
    
Bart, the deal here is that MRI employs both strong magnetic fields and powerful radio frequency signals. Every conductor is an antenna that picks up the RF and if the currents are able to run for far they will dissipate heat under the impulse of the RF. Enough hear and you burn--real burning. That's a big issue, as is the possibility of loose bits of conductive stuff going ballistic under the force of the field--people have been killed that way. –  dmckee Nov 13 '12 at 1:39
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1 Answer

up vote 13 down vote accepted

MRI scanners can produce heat in rare circumstances.

Guidelines to Prevent Excessive Heating and Burns Associated with MRI Procedures

Magnetic resonance (MR) imaging is considered to be a relatively safe diagnostic modality. However, damaged radiofrequency coils, physiologic monitors, electronically-activated devices, and external accessories or objects made from conductive materials have caused excessive heating, resulting in burn injuries to patients undergoing MR procedures. Heating of implants and similar devices may also occur, but this tends to be problematic primarily for objects made from conductive materials that have elongated shapes or that form loops of a certain diameter. For example, excessive MRI-related heating has been reported for leads, guidewires, certain types of catheters (e.g., catheters with thermistors or other conducting components), and certain external fixation or cervical fixation devices.

skin-to-skin contact points were suspected to be responsible for these injuries, however, the exact mechanism responsible for these incidents is unknown.

Turning Up the Heat on MRI

In 2004, more than 22,000 magnetic resonance imaging (MRI) scanners performed some 65 million studies worldwide, with about half done in the United States. Magnetic resonance imaging has a stellar safety record and is not considered a significant risk. However, it is not hazard free. The worst danger is probably the projectile risk of magnetic objects being sucked into the main magnet, which is dealt with by diligent screening and control of access. Less dramatically, perhaps, is radiofrequency (RF) power deposition and heating in the body due to the MRI excitation field. Power deposition is measured by the specific absorption rate (SAR) in watts per kilogram or by direct thermometry. A third hazard, related to both of these, is the effect of an MRI scanner on devices and leads that are implanted in patients.

my emphasis

Who can have an MRI scan? / Tattoos

(new section)

Some tattoo ink contains traces of metal, but most tattoos are safe in an MRI scanner. Tell the radiographer immediately if you feel any discomfort or heat to your tattoo.

Summary

The essence of these reports seems to be that MRI scans do not normally produce heating in the vast majority of scans performed but that in very rare cases heating can occur either because of conductive implants or due to skin to skin contact or perhaps other circumstances.

Conclusion

Yes, in rare circumstances, MRI scans can produce uncomfortable heating or even burns.

No, "exciting protons in the body" occurs during every MRI scan, "a sensation of extreme heat" does not and is rare.

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Excellent answer -- maybe you can edit it to mention tattoos. Tattoos can heat during MRI. Cold presses or ice packs during the MRI reduce discomfort. Per FDA: "The risks of avoiding an MRI when your doctor has recommended one are likely to be much greater than the risks of complications from an interaction between the MRI and tattoo or permanent makeup. Instead of avoiding an MRI, individuals who have tattoos or permanent makeup should inform the radiologist or technician of this fact in order to take appropriate precautions, avoid complications, and assure the best results." –  MattBagg Nov 13 '12 at 17:42
    
@mb3041023: Updated for tattoos –  RedGrittyBrick Nov 13 '12 at 18:26
    
The "exciting protons in the body" part isn't wrong, the radiofrequency pulses use in NMR and MRI affect the spin of NMR-active nuclei like 1H. And those pulses are also responsible for the heating. –  Fabian Nov 13 '12 at 18:32
    
@Fabian: Yes, I have reworded that part of the answer to try to clarify that for extreme heating to occur, there must be additional factors which MRI procedures are designed to prevent. –  RedGrittyBrick Nov 13 '12 at 19:24
    
your summary states that skin-skin contact is a potential cause for "hot spots," but there is no mention in your excerpts: they mainly deal with "foreign bodies." (too expansive?) –  horatio Nov 13 '12 at 20:15
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