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Category: Medical and Dental Patient Issues

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

Q

I recently had a head computed tomography (CT) scan. I was concerned about the level of radiation produced by the CT. I was informed the dose-length product (DLP) was 734.73 mGy-cm. The DLP seems like a very large amount. How can this translate to mSv? I thought mSv = mGY.

A

The unit millisievert (mSv) can be used for several different radiation doses of which the two primary ones are effective dose and equivalent dose. The effective dose in mSv is the risk-based metric you are interested in knowing.

The unit milligray (mGy) is used for other types of radiation doses, but for this discussion the only one we need to know is absorbed dose. For x rays, gamma rays, and beta radiation, the conversion factor between absorbed dose in mGy and equivalent dose in mSv is one (1). So, in this case, we can say mGy equals mSv. But the equivalent dose is not equal to the effective dose except when the entire body is uniformly exposed to radiation. This is almost never the case in medicine, so we need to take into consideration that only part of the body was exposed and also take into consideration how sensitive to radiation those exposed body parts are. Effective dose does this.

Effective dose can be estimated based on the DLP using the method described in the American Association of Physicists in Medicine (AAPM) Report No. 96, The Measurement, Reporting, and Management of Radiation Dose in CT, January 2008. In your case the effective dose from the head CT is around 1.5 mSv.

Kent Lambert, CHP, FHPS

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 18 April 2019. 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.