Is Magnetic Resonance Imaging Safe?

In Chapter 18 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss safety issues in magnetic resonance imaging.

Safety issues in MRI include forces on magnetic objects in and around the patient such as aneurysm clips, hairpins, pacemakers, wheel chairs, and gas cylinders (Kanal et al. 2007), absorbed radio-frequency energy (Problem 21), and induced currents from rapidly-changing magnetic field gradients. The rapid changes of magnetic field can stimulate nerves and muscles, cause heating in electrical leads and certain tattoos, and possibly induce ventricular fibrillation. Induced fields are reviewed by Schaefer et al. (2000). Cardiac pacemakers are being designed to be immune to the strong—and rapidly varying—magnetic and rf fields (Santiniet al. 2013).

Recently, two more safety issues have emerged. The first is the possibility of genetic damage caused by MRI. This question is examined in the article “Will an MRI Examination Damage Your Genes?” by Kenneth Foster, John Moulder, and Thomas Budinger (Radiation Research, Volume 187, Pages 1-6, 2017). Foster and Moulder are cited extensively in Chapter 9 of IPMB, when we discuss the risks of low-frequency electric and magnetic fields. They are two of my heroes in the fight against pseudoscience. Budinger has studied MRI safety for years. They write

We conclude that while a few studies raise the possibility that MRI exams can damage a patient’s DNA, they are not sufficient to establish such effects, let alone any health risk to patients. Based on the failure of decades of research on biological effects of static and RF fields to establish genotoxic effects of such fields at levels comparable to those used in clinical MRI, we consider that genotoxic effects of MRI are highly unlikely. The likely increase in risk, if it were present at all, from a one-off MRI exam would surely be very small and possibly nil, but could not be proven to be zero.

In my opinion, the phrase “highly unlikely” is generous.

A second, more serious, safety issue is risks associated with the MRI contrast agent gadolinium. In IPMB, Russ and I explain

Differences in relaxation time are easily detected in an [MRI] image. Different tissues have different relaxation times. A contrast agent containing gadolinium (Gd³⁺), which is strongly paramagnetic, is often used in magnetic resonance imaging. It is combined with many of the same pharmaceuticals used with ^99mTc [an isotope used extensively in nuclear medicine], and it reduces the relaxation time of nearby nuclei. Gadolinium has been used to assess ischemic myocardium.

MRI using gadolinium was recently discussed in a point/counterpoint article (“The Use of Gadolinium-Based Contrast Agents Should be Discontinued Until Proven Safe,” Medical Physics, Volume 44, Pages 3371–3374, 2017). Moderator Colin Orton writes

Gadolinium-based contrast agents (GBCAs) are widely used in MRI to increase the visibility of tissues. Some believe, however, that due to their documented toxicity, clinical use of these agents should be discontinued until proven safe. This is the premise debated in this month’s Point/Counterpoint. Arguing for the Proposition is Stacy Matthews Branch, Ph.D. Dr. Branch is a biomedical consultant, medical writer, and veterinary medical doctor…. Arguing against the Proposition is Michael F. Tweedle, Ph.D. Dr. Tweedle is the Stefanie Spielman Professor of Cancer Imaging and Professor of Radiology at The Ohio State University.

I am a big fan of point/counterpoint articles, and we discuss one every Friday in my Medical Physics class. This debate has more substance than the genetic damage controversy, but I tend to agree with Tweedle when he concludes

But is dissociated Gd a risk factor beyond NSF [Nephrogenic Systemic Fibrosis, a disease shown to be associated with some Gadolinium-based contrast agents]? At what level and for what? Research to better understand the risks of GBCAs should certainly continue. But discontinuation of all GBCAs would result in complete loss of their benefit, probably in loss of human life due to inaccurate or imprecise diagnosis, while we search for an hypothesized chronic toxicity of unknown seriousness that we, at this point, have no reason in evidence to anticipate. The reasonable response to the new findings is further research into chronic tolerance and more discriminating use of the available GBCAs.

A recent article featured in medicalphysicsweb highlighted new MRI contrast agents based on manganese instead of gadolinium, that may be safer.

Manganese-based contrast could allow safer MRI. A team at Massachusetts General Hospital has developed a potential alternative to gadolinium-based MRI contrast agents, which carry significant health risks for some patients and cannot be used in patients with poor renal function. In tests on baboons, the researchers demonstrated that the manganese-based agent Mn-PyC3A produced equivalent contrast enhancement of blood vessels to that of a gadolinium-based agent (Radiology doi:10.1148/radiol.2017170977).

A discussion of significant MRI safety issues can be found here.

So is magnetic resonance imaging safe? For the vast majority of MRIs that do not use any contrast agent, I would say overwhelmingly yes. When gadolinium is used, there is a small risk that in most cases will be far less significant than the benefit of obtaining the image.