If you’re on social media, or anywhere on the internet for that matter, you’ve likely seen warnings about cat-eye nail polish being “unsafe” to wear during an MRI scan. There are numerous social media posts, videos, and articles from purported experts warning of the potential for heating and patient burns.
Provider-level responses to this perceived new risk are often to refuse patient access to MRI for those with metallic nail polish or delay an exam while the material is -- at least in one case -- ground off from the patient’s fingernails.
What is cat-eye nail polish?
Cat-eye nail polish is a gel polish containing fine particles of iron oxide. While the polish is wet, these particles can be manipulated into various holographic patterns by holding a small magnetic tool, often in the cap of the polish bottle, close to the nail. After the desired pattern has been achieved, a clear topcoat is applied and is allowed to dry or cure under UV light.
MR safety experts Kristin Seitz (left) and Tobias Gilk (right)
Why the hubbub? Metallic particles are not unique to nail polish. Iron oxide is a common ingredient in many cosmetic products -- mascara, bronzer, tinted sunscreen, etc. Yet we don’t see TikTok videos warning patients to wash their faces or arrive for their MRI without makeup. It’s likely the fact that the materials are promoted as “magnetic” that has increased the level of MRI safety anxiety over this particular cosmetic.
Risks/potential risks
Heating
The radiofrequencies (RF) used during MR imaging impart a significant amount of energy into patients and, when that energy is focused or concentrated, it can cause heating and burns. Notice the word "can," not "will."
To date, there have been no FDA MAUDE reported incidences of RF heating of metallic nail polish -- or any other topical cosmetic product. ASTM/US FDA standards indicate that objects with an electrically conductive length of 2 cm (approximately 0.75 inch) or less are not at risk of clinically significant levels of RF heating (which is to say, heating significantly more than the surrounding tissues) in MR imaging at 3.0 T or lower field strengths, even when the object is within the volume of RF energy deposition.
Why is this true? Let’s take what we know about resonant circuitry. For an object to heat via resonant circuitry when exposed to RF, the electrically conductive length of the object needs to closely match ½ wavelength of the transmitted RF. For a 1.5-T MRI scanner, which uses an RF transmit frequency of 64 MHz, this length would be about 25–30 cm (approximately 12–13 inches) inside human tissue, and much longer in air. At 3.0 T, which uses an RF transmit frequency of 128 MHz, the length of greatest concern would be approximately 12–15 cm (roughly 6–7 inches) in human tissue, and much longer in air. This is because resonant circuit heating risk inversely correlates electrical length with field strength of exposure: higher field strength = risk of shorter conductive object heating. The physics of the different field strengths and transmit RF frequencies makes the ASTM/US FDA published 2-cm length of concern rather conservative, particularly at lower field strengths.
Artifacts
The risks of image distortion or artifacts are real. The presence of metallic and magnetic objects/substances inside the bore alters the main magnetic field, which in turn alters the precessional frequency of hydrogen protons, which can cause image artifacts. The reach of these artifacts or distortions will depend on several factors, including field strength, the magnetizability of the materials involved, the pulse sequences being run, and more. But at their worst, these artifacts will generally only extend only a few inches from the object. Thus, with respect to imaging, this risk should only really apply to imaging of the hand, wrist, or anatomy that is very close to the placement of the hand during imaging.
Heating potential/risk management
Heating effects are a result of interactions of materials with the transmitted RF during MR exams. For the polish to interact with transmitted RF, the hands need to be within the volume of the RF transmit coil. While MRI manufacturers do all provide spatial information about the static magnetic field and time-varying gradients throughout the bore, curiously they do not routinely provide end user information on the physical extents of the integrated RF body coil. For many systems the volume of RF deposition extends approximately 30 cm (1 ft) both superior and inferior from isocenter, for a roughly 60-cm (2-ft) superior-to-inferior volume of the bore.
Fortunately, the polish is on fingers, which are on hands, which can often be moved to more superior positions for imaging between the knee and heart/T-spine, or kept fully extended, well inferior of isocenter for C-spine or brain imaging, moving them out of the volume of RF deposition for that study. Movement of the hands out of the volume of RF deposition has the added benefit of having the hands well away from the region of interest, diminishing any potential artifact risks as well.
Mitigation
If imaging anatomy other than hands and hands can’t be moved >30 cm superior or inferior of image center, such as while imaging the elbow, it is recommended to remove the nail polish. As an alternative approach, one could move the MRI patient’s hands toward midline. RF energy in the ~60-cm body coil volume is transmitted from behind the bore walls/ceiling, and anatomy further from the walls (i.e., closer to midline of the MR bore) will receive significantly less incident RF and will be less likely to heat. Heating risks may also be mitigated through the use of cold compresses.
Artifact
When imaging other body parts (not hands), simply make sure that the hands with the nail polish aren’t close to the region of interest.
If imaging hands or wrist, it would be advisable to remove the metallic nail polish due to possible artifacts and degradation of image quality.
Conclusion
With the exception of imaging of the hand or other anatomy close to fingers with metallic nail polish, there are many risk-mitigating approaches individual MRI providers can take apart from removing the polish. In fact, for most patients/exams, indications to remove the polish as a prerequisite for MRI imaging are made irrelevant by simply having the patient move their hands out from the volume of RF deposition. The presence of metallic nail polish products should not be an automatic MRI safety exclusion criteria for patients seeking MRI exams.
Kris Seitz has been an MRI technologist for over 30 years and is currently an instructor in the Radiography program at The Ohio State University. Kris is certified in radiography, computed tomography, and magnetic resonance Imaging by the ARRT and as an MR Safety Officer (MRSO) by the International Board of Magnetic Resonance Safety (IBMRS). She also serves on the Board of Directors of the IBMRS and the Ohio Society of Radiologic Technologists. Kris has authored articles on MR and MR safety for ASRT Scanner and Radiologic Technology.
Tobias "Toby" Gilk is the founder of Gilk Radiology Consulting and co-host of The Invisible Force podcast on the AuntMinnie Podcast Network. An architect by training, he has spent over 20 years focusing on MRI safety, initially through the architecture and planning of MRI facilities, but growing into the technology, clinical practice, regulation, and economics of MRI safety. Gilk holds both MR Safety Officer (MRSO) and MR Safety Expert (MRSE) certifications from the American Board of Magnetic Resonance Safety (ABMRS). An evaluator of serious reportable events (SRE), he is also a volunteer member of the Technical Expert Panel (TEP) of the National Quality Forum, and co-author of "The Technologist MRI Safety Handbook."
The comments and observations expressed are those of the author and do not necessarily reflect the opinions of AuntMinnie.com.
