Answer to Question #2424 Submitted to "Ask the Experts"

Q

Thank you for your service. I am confused on doses for CT scans. In Question 743 of your Ask the Expert feature it states that a CT scan can be two to three times an annual dose of background radiation. Question 1695 states that a CT scan dose of 2 rads is 20 years of background radiation. Question 636 lists 111 mrem for a head cat and Question 736 lists 5 rad as a possible dose. Although I don't understand all the different terms and how to convert between them, these seem to be quite a contradiction between 2 years and 20 years for a CT scan. Another radiology source listed 3 years for an abdominal CT scan. How many years of background radiation is 1 rad and also 1 rem? Thank you for your explanations and helping me to understand this. I have a sister and a son who have had head CT scans and I have had several abdominal studies and would like to know the risk we have been exposed to. I read in one of your answers that 1 rem is a 4 out of 1,000,000 chance of cancer and I would like to know how much 1 rad would increase risk and if it is similar. Thank you.

A

You are correct when you say that this is all confusing. The individuals who answered these questions tried to provide answers that could be understood by the individual submitting the question. Taken individually, each answer is good. However, when comparing all the answers, it might seem, as you correctly pointed out, that the answers are different. And, they may very well be depending on the area of the body subjected to CT scanning. A CT of the head will yield a different dose to the head than what a CT of the abdomen will yield to the abdomen. It is not any different from a plain x-ray image of the head vs. a plain x-ray image of the abdomen. The radiation dose required for the head x-ray image is different from the radiation dose required to produce an image of the abdomen image.

The confusion over rad and rem or mrad and mrem is easy to solve. When talking about x-ray exposures, 1 rad is equal to 1 rem and 1 mrad is equal to 1 mrem. Similarly, 1,000 mrad or mrem equals 1 rad or rem, respectively.

A further point of confusion is that some writers provide doses in mrad, mrem, rad, or rem to the part of the body, e.g., head or abdomen, being imaged. Others average the dose over the entire body to try to give an equivalent whole-body dose that would represent the same risk as the higher dose to the specific portion of the body. The calculated dose that averages the radiation dose over the entire body is called the “effective dose equivalent.” For example, a dose of 2 rad or 2,000 mrad to the head would be equivalent, in risk, to a dose to the whole body of perhaps 110 mrad effective dose equivalent.

While doses to the part of the body being imaged in a CT procedure will be typically in the range of 2 to 5 rad, the calculated effective dose equivalent, i.e., the calculated whole-body dose that would present the same risk as a dose to the portion of the body, will be less and in the range of 0.1 to 1 rad.

It should be noted that the dose that we receive from natural background each year is approximately 300 mrem. A 2 rem dose to the head that calculates out to have a 300 mrad or 300 mrem effective dose equivalent will present the same risk as one year’s worth of natural background radiation.

The exact risk from doses of 1 to 5 rad or rem to the total body is uncertain and quite low. It is inappropriate to try to even calculate an exact risk for such low doses.

Kenneth L. Miller, CHP, CMHP
Penn State Hershey Medical Center