Answer to Question #2387 Submitted to "Ask the Experts"

Q

How many head x rays are the limit in one sitting? I had four shots done to make sure I had no metal behind my eyes before I had an MRI (magnetic resonance imaging). Well, they took four and then figured out that they had bad exposure so they took three more. You would think they would check it after the first one. So, seven shots in one day . . . my head should be glowing now or what?

A

I understand your apprehension about the number of x-ray exams that you received, but actually, the skull x-ray exam gives a relatively small dose of radiation to your skull. To quantify the amount of radiation that you received it is necessary to use the term effective dose¹. The effective dose is a measure of the effect that results from a uniform, whole-body exposure taking into account the relative radiosensitivity of the various organs and tissues exposed. The effective dose of one skull x-ray exam is between 0.01 and 0.02 mSv. So the amount of radiation from seven such films is simply seven times the average of 0.015 mSv, or about 0.1 mSv². To put this in perspective, the normal amount of background radiation from the sun, outer space, and radioactive materials that are naturally found in the Earth’s air and soil is 3 mSv per year, which is over 30 times the amount of radiation that you received from the seven skull x-ray exams. To give you an appreciation for the magnitude of radiation in your x rays, from just one CT (computerized tomography) scan of your head, you would have received 2 mSv, or 20 times as much radiation. The MRI that you received does not involve ionizing radiation, and therefore does not add to any long-term risks that you may incur.

What could such risks be? Some interpretation of current research supports the theory that even low doses of ionizing radiation might increase your risk of cancer at some point in your life. The chance that anyone may get a fatal cancer is about 25% even if they receive no additional radiation beyond background levels. For an effective dose of 0.1 mSv, the risk is 0.004% greater than 25% or 25.004%. This increased risk is so slight as to not have any real meaning since natural variations are greater. I hope that this reassures you that although you may have received a slight amount more radiation from the extra x rays, the clinical information that was gained from the precaution far outweighs any risk of radiation that you might have received.

Jamey Lee West, MD
National Institute of Health

1. ICRP Publication 60: 1990 Recommendations of the International Commission on Radiological Protection, Annals of the ICRP Vol. 21/1-3, 1991, Pergamon Press
    
2. BF Wall and D Hart, “Revised radiation doses for typical X-ray examinations. Report on a recent review of doses to patients from medical X-ray examinations in the UK by NRPB. National Radiological Protection Board: British Journal of Radiolology, 70: 437-439 (1997)

Editor’s note: The effective dose of 1 mSv is equivalent to a dose of 100 mrem.