Answer to Question #5128 Submitted to "Ask the Experts"

Several days ago I had a pelvic/abdomen CT scan. I was informed by the clinic that my "radiation dose for CT ABO/Pelvis was 285.28 mGy." This has confused me, as I have been led to believe that the usual effective dose for this type of CT scan is 10 to 16 mGy.

It was a contrast CT, with pictures taken pre- and post-contrast. I was given a drink 45 minutes before the procedure. Once in the CT machine, two scans were initially taken with me holding my breath and my arms above my head. After these were taken, I was given iodine intravenously and another scan was completed again with my arms above my head and holding my breath. That is three scans in total. Could 285.28 mGy be the correct dose that was administered and, if it was, is that my effective dose?

There are two different measurements in your question. The value of 285 mGy is probably the output of the CT scanner. The effective dose in mSv is computed by (1) determining which organs were irradiated, (2) multiplying the dose to that organ based on the machine output in mGy by a tissue weighting factor that is associated with the risk of developing cancer or another adverse effect, and (3) then summing up all of the tissues of interest.

As listed on our Web page Doses from Medical Radiation Sources, the pelvic or abdomen CT scan delivers an effective dose of approximately 10 mSv, which will depend on the type of unit and areas scanned. As you had two detailed scans—with and without contrast—the total dose is about 20 mSv. The first scan was what they call a scout film, which is at a lower dose than the high-resolution scans.

I would also direct you to our Web page Risk/Benefit of Medical Radiation Exposures, which will give you more information about your risks from this procedure.

John Jacobus, MS
Certified Health Physicist