Answer to Question #11447 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:


When my son was 13 months old, he had a skull fracture. As a result, he had three computed tomography (CT) head scans in total. When we inquired about his radiation dose, we were told he had 12 milligray (mGy), 10 mGy, and 8 mGy, which equals 30 mGy in total. They told us that the radiation was reduced to less than half before the scans. I have two questions:

  1. Is 30 mGy a huge deal and should I be worried?
  2. How many millisieverts (mSv) is 30 mGy? I was told 30 mGy would be 3 mSv, but I don't understand how that can be right after reading many articles about mGy being equal to mSv.

The risks of health effects from radiation doses received during diagnostic-imaging procedures are either too small to be observed or are nonexistent. The benefits from properly performed, clinically indicated, diagnostic-imaging procedures, including CT scans, far outweigh any hypothetical cancer risk. Diagnostic medical-imaging procedures provide a medical benefit even if they do not appear to reveal anything and are of less risk than their alternatives, such as exploratory surgery.

Even if the result of the imaging exam was negative, the physicians were provided information they could use to determine the next course of action. Refusing medical-imaging procedures may result in real and substantial risk by not receiving the clinical benefits of the procedures.

Because the Health Physics Society recommends against quantitative estimates of health risks for radiation doses below 100 mSv, we will not calculate hypothetical risks for diagnostic-imaging procedures. The Society's position statement "Radiation Risk in Perspective" explains in more detail why it is inappropriate to estimate health risks at these doses. Some risk information is available from

As for your question about dose units, CT scanners can provide two dose values: the volume CT dose index (CTDIvol) in mGy and the dose-length product (DLP) in mGy-cm. DLP is the CTDIvol multiplied by the scan length in centimeters (cm) and is the better value for providing a rough estimate of radiation dose in mSv. Assuming a scan length of 10 cm for each of the CT scans, the effective dose would be roughly 2 mSv. For further discussion, I encourage you to read the answer to Question 10540.

Kent Lambert, CHP

Answer posted on 29 March 2016. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.