Feature article

How Safe is an MRI?

What is an MRI?

Magnetic resonance imaging (MRI) is a widely utilized imaging system that obtains detailed images of organs and tissues through the body. MRI is unique from many other imaging techniques, such as X-rays and CT scans, because it does not use ionizing radiation. Instead, an MRI scanner uses a powerful magnetic field and radio waves to activate the hydrogen atoms within your body. Once energized, the hydrogen atoms emit energy that is detected by the MRI machine, and interpreted by a computer into an image.

Modern MRI technology is capable of creating a 3-D, cross sectional image of the interior of the body. Radiologists can interpret the image and easily differentiate between healthy and unhealthy tissues, determining if any abnormalities exist in different parts of the body. MRI is the preferred tool when physicians are attempting to establish a diagnosis, view potential conditions, and assess progress to treatments. The brain, spine, and joints are most often examined with MRI; however, this imaging system can be used on any part of the body.

Many studies have concluded that MRI is one of the safest technologies for imaging the body. The examination causes no pain, and the magnetic field produces no known tissue damage of any kind.1 When proper safety guidelines are followed, people of all ages can undergo an exam. Furthermore, if advised by one’s physician, it is acceptable to have multiple scans within a short time period, as the scan itself presents no negative side effects. These images provide physicians with valuable diagnostic and prognostic information, allowing them to confidently assess a patient’s condition.

Safety Precautions Required Before MRI

Most concerns about MRI involve people who have metal embedded in their body. The powerful magnetic field of the MRI system is very strong and will attract any iron containing objects.1 When in use, an MRI can slightly shift or heat up embedded metal, potentially harming a patient. Metal objects can be drawn into the magnetic field, and the activity of medical devices may be disrupted.

When a patient prepares for an MRI exam, they will be asked to fill out a screening form to indicate whether or not they have any foreign objects within their body. Possible hazardous objects include:

  • Certain cardiac pacemakers or implanted cardioverter defibrillators
  • Certain vascular clips placed to prevent intracranial aneurysm bleeding
  • Some medication pumps
  • Certain cochlear implants
  • A bullet, shrapnel, or other metallic fragments

It is important to note that some of these medical devices, such as certain cardiac pacemakers, are acceptable for MRI. Patient’s must inform their radiologist about the exact type of device, to ensure their safety.1 In addition, before entering the MRI system room, patient’s will be instructed to remove all metallic objects from pockets and hair, such as jewelry, wallets, cell phones, and hearing aids.

Dr. Max Wintermark, Chief of Neuroradiology at Stanford University, understands the importance of taking extreme precautions during an MRI exam. If a patient has implants or embedded metal, indicating that they cannot safely have an MRI, then they will use a different scanning technology instead.2

Furthermore, during an MRI exam, patients will receive a gown to wear. This is because some clothes may contain metal in unexpected places, such as underwear and socks. These metal particles may heat up in the exam, causing some pain and discomfort to the patient.

Is Gadolinium Contrast Safe?

While the MRI scan itself is considered very safe, in recent years there has been a growing concern over the safety of gadolinium-based contrast agents used in conjunction with MR. Gadolinium dye is a chemical substance that is injected in a the body and used to enhance the quality of the image, allowing the radiologist to more accurately identify normal and abnormal tissue. In late 2014, a study was released showing the contrast agent is deposited and retained in the brain. This study, combined with a small percentage of patients who claimed their health was affected following gadolinium exams, has caused some radiologists and patients to question the safety of the dye.3 Studies have shown that the dye may remain in various parts of the body.4 Patients who have kidney problems may not be able to have an MRI with gadolinium. In rare cases, the dye may cause an allergic reaction.

Dr. Wintermark has stated that the reports from patients who attribute health issues to gadolinium contrast agents certainly have the attention of radiologists. These reports have sparked additional research into gadolinium dye to determine if the contrast agent does pose a threat to patient’s health. “We don’t know what the clinical significance is, which is why we continue to do clinical research on this,” said Sheela Agarwal, M.D., U.S medical affairs, Bayer Healthcare, at a RSNA session.

However, physicians are reluctant to link gadolinium agents to illness because there is very little clinical evidence showing a direct correlation.3 In fact, clinical evidence shows that in using the dye in hundreds of thousands of patients over the past few decades, it is safe in most patients. Results from about 50 studies in the last few years do not indicate that gadolinium retention leads to chronic disease, and as Sheela Agarwal explains, “It’s very difficult for us as clinicians to call something a disease if we cannot definitely connect causality."3 The Food and Drug Administration has stated that there is no clinical evidence directly linking gadolinium retention to adverse health effects in patients, and has ultimately concluded that the benefit of all approved gadolinium-based contrast agents continues to outweigh any potential risks.

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Is MRI Safe for Pregnant Women? 

As the usefulness and prevalence of MRI continues to increase, it is inevitable that exam requests for women who are pregnant will occur. After years of studying the effect of MRI on pregnant women, there is no known evidence suggesting that MRI is harmful to a pregnant woman or her fetus. However, according to a large cohort study from Ontario, Canada gadolinium dye should be avoided at all times during pregnancy, as it is associated with increased risks to the fetus.6 In general, MRI exams for pregnant patients should be reviewed on a case-to-case basis, and the risk-benefit ratio needs to be determined by their physician and performed only if absolutely essential.5

References

1. "Magnetic Resonance Imaging (MRI) Safety". RadiologyInfo.org. Web. 30 July 2018. <https://www.radiologyinfo.org/en/info.cfm?pg=safety-mr>.

2. Karen Weintraub. "Do M.R.I. Scans Cause Any Harm?" The New York TImes. 23 June 2017. Web. 30 July 2018. <https://www.nytimes.com/2017/06/23/well/live/do-mri-scans-cause-any-harm.html>

3. Dave Fornell "The Debate Over Gadolinium MRI Contrast Toxicity". Imaging Technology News. 16 February 2018. Web. 30 July 2018 <https://www.itnonline.com/article/debate-over-gadolinium-mri-contrast-toxicity>.

4. "FDA Drug Safety Communication: FDA warns that gadolinium-based contrast agents (GBCAs) are retained in the body; requires new class warnings." FDA.gov. 16 May 2018. Web. 12 April 2019. <https://www.fda.gov/Drugs/DrugSafety/ucm589213.htm>.

5. Fergus Coakley et al. "CT and MR Pregnancy Guidelines". UCSF Department of Radiology and Biomedical Imaging. Web. 30 July 2018. <https://radiology.ucsf.edu/patient-care/patient-safety/ct-mri-pregnancy#pregnancy-mri-patients>.

6. Joel G. Ray, et al. "Association Between MRI Exposure During Pregnancy and Fetal and Childhood Outcomes." JAMA. September 2016; 316(9):952-961. Web. 30 July 2018. doi: 10.1001/jama.2016.12126