Answer to Question #13908 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:


I had a computed tomography (CT) scan today. The technologist told me my dose was 372.42 milligray (mGy), which I believe is 37 Rem. What am I missing here? Does that number sound right to you?


The dose metrics for CT scans can be very confusing. The bottom line is that your dose was neither 372 mGy nor 37 rem.

For CT scans, the machine indicates the CT Dose Index to a volume (CTDIvol) in mGy and the Dose Length Product (DLP) in mGy-cm. Neither of those are patient dose—they are measures of the radiation that was output by the CT scanner. The CTDIvol is a dose value that was measured at a specific location in a phantom (an acrylic cylinder usedfor CT quality assurance testing) and the DLP is the CTDIvol multiplied by the length of the scan. The CTDIvol is very useful for comparing the radiation output during different types of scans and between different CT scanners. 

The value the technologist provided to you may have been the CTDIvol. If so, the dose at a point in an acrylic or plastic cylinder used for quality control testing of CT scanners would be 372.42 mGy under the parameters used in your scan. It is actually more likely that the DLP of your scan was 372.42 mGy-cm because it is within the expected range for CT scan DLP values.

The DLP in mGy-cm is more useful for calculating effective dose. However, the effective dose is not the actual dose to any specific patient; it is a useful measure to compare risks from radiation exposures to only part of the body based on averages across the population. The effective dose depends on the radiation sensitivity of the organs and tissues that were exposed to the radiation as well as the dose absorbed in the tissue. If you received a CT scan of your chest with a DLP of 372.42 mGy-cm, the effective dose is approximately 5.2 mSv. A head CT with the same DLP value would have an effective dose of approximately 0.8 mSv and an abdomen/pelvis CT scan would have an effective dose of approximately 5.6 mSv. 

The risk of health effects from doses below 100 mSv are either too low to measure or do not exist. While there is a very low, theoretical risk from the radiation in a CT scan, there is a real benefit from the CT scan. The imaging allows your physician to make an accurate diagnosis and provide the appropriate treatment.

Deirdre H. Elder, MS, CHP, CMLSO

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.
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