Do you have tattoos? – A discussion around tattoos and MRI scans
By Martin J. Stuart (Cardiff Healthcare Innovation Ltd.)
If you have ever had an MRI (magnetic resonance imaging) scan, you would have filled out what is known as a screening form. The form asks questions to determine whether or not it is safe for you to be scanned in an environment that combines a high magnetic field with radio frequency pulse sequences. Many of the questions address immediate problems, such as implantable devices like pacemakers. If you have a pacemaker it is likely that you would be ruled out for suitability for an MRI scanning procedure, as the risk of disruption to the device would be too high and the results of device failure potentially catastrophic. Although medical device manufacturers are working to develop more MRI compatible equipment, and new products are appearing on the market that allow for MRI scanning of patients with previously contraindicated devices, it is probably still generally recognisedthat most patients with pacemakers would be unsuitable for an MRI scan. One question that is also likely to appear on an MRI screening form would ask if the patient has tattoos. For many years I worked in a brain imaging centre that used MRI to scan research participants and this question often caused the most confusion for people participating in an MRI study. The confusion didn’t arise from the existence of a tattoo (which is fairly obvious), but from the reason as to why a tattoo might be a problem in an MRI scanner. To add to this confusion, MRI operators generally had no satisfactory answer to this quite reasonable question. The standard line was that there is a danger that tattoos could contain metal particles that could cause a heating effect in the tissue surrounding the tattoo and that this was particularly true of tattoos received in some eastern European countries or in prisons. Having made this explanation myself, though unsure of what evidence there was for it, I proceeded to allow the tattooed participant into the scanner. I’m unsure how many people I scanned over a decade, but it must have been a few hundred. I never turned anyone away from a scan because of tattoos, only advising that if they felt any discomfort then they should alert me to the fact, and I would stop the scan. No one ever complained of localised heating effects around tattoos and I never stopped a scan because of it (claustrophobia is a different story). Many years ago, the television programme, MythBusters, decided to test the myth of exploding tattoos, and tattooed some ink that was heavily doped with iron oxide onto some pig skin. The result showed some image distortion from the ferrous content of the ink, but the pig skin failed to explode in the scanner. The MythBusters test was obviously a made for television bit of entertainment, but it’s interesting to check how much actual science has gone into the investigation of tattoo heating in MRI. In a paper from 1997, Kreidstein, Giguere and Freiberg (1)reported a case study where a patient was having an MRI scan that resulted in a burning pain from the site of a tattoo. This pain manifested on entering the MRI room, but before a scanning sequence was performed, suggesting that the 1.5 Tesla static magnetic field had caused the pain. In order for an MRI scan to be carried out the tattoo was surgically excised, and the patient was successfully scanned following the tattoo removal. Some tests were then carried out on the excised tattoo using a standard (presumably ferrite) horseshoe magnet, that caused “pronounced displacement of the tattooed skin” (Kreidstein et al, 1997, p.1717). This case seems to suggest that the pain was caused by deflection of the iron oxide particles in the tattoo, due to the scanner’s strong magnetic field. In another case study from 2011, by Ross and Matava (2), a patient received a burn localised to an area at the extremity of a tattoo. The suspected cause of the burn was again the presence of iron oxide in the tattoo ink, although on this occasion, the patient had been subjected to radio frequency (RF) pulses used in MRI. It is known that electrically conductive cables left resting on patients during MRI scanning procedures can cause serious burns. The reason for this is that a trailing cable can act as an antenna for the RF pulses generated during MRI scans. The RF can induce a heating current in the cable itself, which can in turn cause burns to the patient. From both of the above cases the primary culprit for the tattoo pain arising from MRI seems to be iron oxide. If the pain is caused by the deflection of ferrous material due to a high magnetic field, then iron oxide could be a likely candidate. If the pain is caused by some kind of electrical current induced heating effect, it would seem less likely that iron oxide was to blame, because it isn’t a great conductor (check out rust). The problem is identifying the source of the pain effect (magnetic field, radio frequency pulses or a combination of the two) and what component is in the ink that would manifest the pain in the area of the tattoo. Interestingly (and worryingly), although the equipment used for tattooing is tightly controlled, there seems to be little or no regulation about what goes into tattoo inks (although this is currently on the agenda for the European standards committees). If one takes the view that the likelihood of a tattoo related injury in MRI is related to the electrical conductivity of the ink used, the shape of the tattoo (straight lines, loops and spirals) and the intensity and locality (in relation to the tattoo) of the RF field, then there might be another contender in the ink composition that could account for any burns that might be reported. Although iron oxide is frequently used for ink colouring, the basis for many inks appears to be carbon powder. This can be in a form such as crushed graphite from a pencil (as demonstrated by tattooing carried out in prisons where pencil lead is the most readily available ingredient for making tattoo ink) or as a cheap carbon powder that can be used to make the bulk of the ink. Carbon powder is the main ingredient in black inks, and it would be worth investigating the prevalence of MRI related burns in tattoos with black inks compared to those with reds, greens, blues, etc. It may well be that tattoo injuries in MRI are related to a variety of conditions that arise, from ink composition to radio frequency pulse sequences and magnetic field strengths. To me, there is great scope for further investigation, because as long as the question “do you have tattoos?” continues to be asked of patients in MRI, there is a need to provide an adequate scientific answer as to why it might be a problem.
- Kreidstein, M.L., Giguere, D.B., & Freiberg, A. (1997). MRI interaction with tattoo pigments: case report, pathophysiology, and management. Plastic and reconstructive surgery, 99 6, 1717-20.
- Ross, J.R., & Matava, M.J. (2011). Tattoo-Induced Skin “Burn” During Magnetic Resonance Imaging in a Professional Football Player. Sports health.