Given: Tattoo ink containing ferromagnetic compounds (most stereotypically, dark inks which contain iron or iron oxide) will potentially experience induced currents inside an MRI, get hot, and burn the skin.
Given: There’s no way to “block” magnetic field lines (divergence of the magnetic field is zero, thank you Maxwell), but you can give them a more attractive path; hence, the use of Permalloy and mu-metal and similar to protect magnetism-sensitive materials from strong magnetic fields.
Therefore: It might be possible to use a mu-metal or Permalloy contraption to selectively redirect magnetic field lines around a tattoo which would otherwise heat up. Is that reasonable, or would you need to surround the affected region in an anatomically implausible fashion?
Maybe. But at least according to Mythbusters, it’s not actually a concern. Link to video from that episode. Summary, even with homemade, iron oxide heavy ink, there was no effect.
I’d think that the biggest question would be the locations of the tattoo and of the body part to be imaged. An MRI of the head with a tattoo on the ankle would be a lot easier to deal with than an MRI of the lungs and a tattoo on the chest.
Reading through that article it seems, right off that bat, like Chronos mentions, that tattoos not in the immediate area being studied aren’t an issue. I can state that the black tattoo I have on my left forearm didn’t burn or tingle when they scanned my right shoulder.
With that said, I’m not sure what good directing the magnetic waves around that area would do.
That same article does go on to mention that what they do right now is inform the patients, let them know to hit the button if they feel anything and if the tech feels it’s necessary, they wrap the tattooed area in wet towels.
Some years ago I had radiation treatment for cancer. Prior to that I had Both and MRI and a cat scan to determine exactly where the radiation beam should be aimed. After they did this, they tattooed three very small dots on my abdomen to mark the targets.
I asked the MRI technician if I should henceforth answer “yes” to the pre-MRI question do you have any tattoos. Not only couldn’t they answer the question, they’d never been asked it before. It’s my impression they really don’t know about MRIs and tattoos.
How do you know the ink is ferromagnetic? The presence of iron doesn’t guarantee this in itself; low-spin iron(II) compounds can be diamagnetic (e.g. oxygenated hemoglobin)
I’d have thought a metallic device that interacts with the magnetic fields inside an MRI is surely going to do one or more of the following:
[ul]
[li]Experience mechanical forces[/li][li]Experience induced currents, possibly resulting in heating[/li][li]Screw up the imaging[/li][/ul]
The heating is caused by induced currents caused by the radiofrequency (RF) and by the magnetic gradient fields applied to the region being scanned (not by the extremely powerful static magnetic field). If the tattoo is near the region to be imaged, you cannot “block” these without affecting the image. Conversely, if the tattoo is far from the region being imaged, it is unlikely to be significantly affected.
Most tattoos don’t actually heat up that much. Also the skin is sensitive to heat, and an alert patient can communicate the fact before significant burns occur, in general estimation. This is more of a problem for debilitated or unconscious patients (who are still unlikely to be burned, but cannot call out if they are).
Also, they can cause local artifacts, so even with the minimal risk of burning, it’s good to ask about them.
While in the absence of the static magnetic field, mu metal and Permalloy might shield the RF and gradient fields as Derleth proposes, there are a least two problems. First, the enormous static field is vastly higher then the saturation field for high mu materials, thus the high permeability necessary for the shielding effect would not be present. Second, any distortion of the static field caused be the material is likely to cause severe anomalies in and near the region of the “shields.”
OK, the general tone seems to be that nobody’s interested in solving the problems associated with the shields/redirectors because a damp towel solves the most pressing problem. Fair enough.
It was still an interesting little exploration of a physics concept.
…because how do you shield or redirect the mag waves around the area being studied?
You’ll even note that several times it’s been mentioned that the tattoo has shown up on the image. MRIs aren’t like ‘full body x-rays’, they generally show a very small area, maybe less than 6 inches across. So between tattoos in areas not being looked at not getting burned and not being able to ‘shield’ (in any way I can think of) tattoos that are between the imager and the injury, I’m not sure what the other options are.
Also, with how many people have tattoos (and how popular they’ve become in the last 10 years) if there is a real problem beyond the extremely small handful of cases, I’m sure a better way will be found to deal with them. Whether it’s adjusting the scan, using other diagnostic methods first to see if they can rule out what they’re looking for without using an MRI or maybe find something better then ‘here ya go, put a towel on it’.
But as of right now, when you have what, less than 10 reported cases out of millions of scans, I’m not sure GE etc is going to put too much research into this.