For the sake of fanfiction, I am trying to figure out if it would be possible for Tony Stark to have either a CAT scan or an MRI. I am thinking not, since I would bet that either would interfere with the electromagnetic function of the arc reactor.
Wise and knowledgeable dopers, educate me! 
As of the end of Iron Man 3:
the arc reactor was removed from his body
so I don’t think it will be an issue.
I doubt the arc reactor is MRI safe, so the scanner’s magnetic field would rip the thing right out of him. People with metal implants as a general rule, need to be aware of what’s in them so they can list them on the pre-scan questionnaire. For example, my artificial spinal disk is MRI safe to 3 Tesla.
Ignoring that, the original point of the arc reactor was to power an electromagnet to keep a piece of shrapnel from destroying his heart. That shrapnel would be a disqualified for an MRI on its own.
As for CT scans, as long as it’s not disrupted by X-rays, it should be OK, but it would make one heck of an artifact in the images. I’ve got some images of a sinus CT scan I had, and the amalgam fillings in my teeth made wild starburst distortions.
That sounds to me like a software problem. If the machine is expecting to see just flesh, then a very strong but small absorber like a filling would in fact show up as something like that… But the machine should be programmed to consider the possibility of fillings, which would result in showing them exactly as they are.
And I had figured that an MRI would be bad, too: Even if (as seems likely) the device is made of low-magnetization materials, it appears to be something resembling a miniature Tokomak, in which case a strong external field would disrupt the inner workings. On the other hand, though, that would also render him very vulnerable to magnetic-based weapons, which someone is going to throw at him eventually, so it’s likely that he’s designed it to be well-shielded against external fields.
MRI is right out. It will either rip it out of his chest, or there will be horrible artifact.
On CT there would be a massive amount of artifact. It is a software issue in part, but not one that has been resolved by any means.
CT works by
(1) rotating an x-ray tube around the patient and repeatedly shooting x-rays through the patient, with paired x-ray detectors on the other side.
(2) determining the amount of radiation that makes it through the patient, then
(3) The computer then uses the data from each detector to back calculate the electron density of each voxel within the slice, kind of like a sudoko, but using the Beer-Lambert law (Il=Ioe^ul), because the x-ray photons drop off exponentially. A CT image is actually a map of the electron densites of the tissues.
As a result, each calculated electron density is kind of dependent on the electron densities of the adjacent tissues. A sudden drop off from metal to flesh tends to cause streaky high and low density artifacts in the surrounding tissues. CT companies spend lots of time an effort minimizing this, but it is a daily problem. Dental amalgam is brutal, mostly because there’s usually so much of it, spread all all over the mouth.
Oh, yeah, either technology will definitely give inaccurate results for him, unless you had a very smart operator basically re-designing the machine just for him.
Perhaps a sonogram?
Thanks for the info, everyone. Even if I am writing fanfiction about comic book characters, I do like to try to keep things as realistic as possible.
The sonogram hadn’t occurred to me, and is a good idea. Thanks!
I’ve seen how Wolverine failed to fare against Magneto.
Seems to me Magneto would turn Stark into a huge cable-less wrecking ball.
Same Universe (Marvel); Have they ever met?
—G?
This may be of use to you in your quest for answers: http://marvel.com/news/tv/2012/3/20/18347/magneto_attacks_iron_man_armored_adventures