Answer to Question #12423 Submitted to "Ask the Experts"

I have had a computer tomography (CT) scan of the brain without contrast, a magnetic resonance imaging (MRI) scan of the brain with and without contrast, an MRI of the carotid arteries, and a magnetic resonance angiography (MRA) scan of the brain in 2008. I also had an MRI of the abdomen and pelvis with contrast in 2011. I had a thyroid scan with a 10 MBq capsule of ¹³¹I in September 2017 and an exercise nuclear stress test in November 2017 with 400 MBq of ^99mTc sestamibi at rest followed by 31.85 mci ^99mTc sestamibi. There was another exercise nuclear stress test in 2012. I had an MRI of my abdomen with and without contrast in October 2014 with Gadovist 10 ml. I had an MRI of lumbo-sacral spine with and without contrast two times on different occasions. How much radiation have I received from all these procedures? I am 54 years of age.

The short answer is that you got just the right amount of radiation that was necessary to avoid serious adverse effects on your health, including thyroid gland problems, heart attacks, and strokes.

You're probably concerned that the ionizing radiation used in CT scans and nuclear medicine tests may result in you getting cancer at some time later in your life. You're not alone. It is clear that ionizing radiation can cause cancer in humans at high doses.

Whether or not there is a significant chance of getting cancer from the very low doses from diagnostic tests is less clear. For that reason, radiologists, imaging equipment designers, and medical physicists strive to ensure that CT scans and nuclear medicine exams are carried out so that every patient gets as little radiation as possible but still gets valuable information from their exam.

Although we don't know the cancer risk due to very low doses with certainty, it is possible to calculate a "worst case" estimate of the chances that men of your age who underwent all the same tests you had would get cancer. For 100,000 50-year-old men who got those tests, about 200 of them might get radiation-induced leukemia or cancer at some point later in life. This is a tiny additional risk, and there may be no additional risk at all.  

The chance of one of our 100,000 hypothetical patients being diagnosed with cancer at some time in his life due to the natural history of cancer is very high. About half of them would get cancer that was not caused by their exposure to radiation. Put another way, the negligible risk from radiation is far outweighed by the benefits of the medical tests. Considering that strokes and heart attacks affect 875,000 people per year in the United States, your decision to have the tests done to protect your health and quality of life was a good one.

You also had a number of MRI and MRA studies done. The magnetic fields and radiofrequency radiation used in an MRI produce nonionizing radiation, which does not affect cells and tissues in the same way that the radiation from CT scans and nuclear tests does. No long-term radiation-related health effects are expected from your MRI or MRA studies.

Robert E. Reiman, MSPH, MD
Associate Professor of Radiology
Faculty, Medical Physics Graduate Program
Radiation Safety Division