Answer to Question #11710 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:

Q

I went in for a head computed tomography (CT) scan about a month ago. It was done with and without contrast. I did my research ahead of time and found that the average dose for this procedure is 4 millisieverts (mSv). When I asked the radiology department what my dose was, they said I had a CT dose index (vol) (CTDIvol) of 69 milligrays (mGy) for each scan and a dose length product (DLP) of 1,308 for each scan (so for both scans, total CTDIvol of 138 mGy and total DLP of 2,616). They told me this was around 5.5 mSv. The dose in mSv seems very low, but my question is how can I have a dose with such a high mGy total but such a low mSv?

If I'm looking at this incorrectly, please correct me, but wouldn't 138 mGy be equal to many, many years of background radiation, all going to the head within minutes? I understand that the yearly background radiation is around 3 mGy. Does my CTDIvol seem excessive? I'm only 25 and I really hope I did not make a mistake for my future health.

A

The head CT scan you received is not a threat to your future health. The CTDIvol is well within the normal range, and based on the DLP, I estimate the same effective dose in mSv as the hospital provided you.

You have asked a question, however, that requires a little physics to explain. When we are done, you will be eligible for your atomic merit badge. The best way, I think, is to explain each of the terms.

CTDIvol is the absorbed dose (described below) measured in an acrylic phantom. These measurements are made for each CT scan protocol used, and the data are entered into the CT scanner's computer. It is not a dose to any person.

DLP is the dose length product. It is the CTDIvol multiplied by the length of the scan. The units are mGy centimeters (mGy cm). The DLP can be used to calculate a rough estimate of the effective dose. From it I calculated an effective dose (described below) of 5.5 mSv for you. Remember this is a rough estimate; it could be off by 20% either way.

Absorbed dose in mGy is the amount of energy (joules [J]) deposited in a material per mass of the material (kilogram [kg]) from ionizing radiation such as x rays. A joule per kilogram (J kg-1) is called a gray (Gy). Since a Gy is a large amount, milligray (mGy) is more often used.

For this example, let's assume a person received 50 mGy to the brain from a CT scan. The rest of the body, however, received very little radiation, so the absorbed dose to the rest of the body is approximately 0 mGy. If, on the other hand, a whole-body scan was performed without varying the machine parameters, then the absorbed dose to each organ/tissue of the whole body would be 50 mGy.

But clearly a partial-body exposure to radiation would carry less risk than the same absorbed dose delivered to the whole body. Since background radiation exposes the whole body, comparing background in terms of absorbed dose in mGy to the dose to the head in mGy from a CT scan is not valid.

Effective dose is derived by taking the absorbed dose to an organ or tissue, multiplying it by the risk factor for adverse health effects from radiation absorbed by that particular organ, and then summing up each of these for all exposed organs/tissues. So the effective dose would be determined by taking the absorbed dose to the brain times the risk factor for the brain, plus the absorbed dose for the thyroid times the risk factor for the thyroid, plus the absorbed dose to the lungs times the risk factor for the lungs, etc. Effective dose is measured in sieverts (Sv). Again a Sv is a large quantity so millisieverts (mSv) are more commonly used.

Effective dose, then, can be used to compare partial-body exposures because it takes into account the chances of detrimental consequences from exposure of each organ or tissue. So we can compare background radiation to a head CT scan in terms of effective dose. Here, you received approximately 5.5 mSv and background radiation (on average in the United States) is about 3 mSv per year. At low doses such as these, the time it takes to deliver the dose has no effect.

If you take a group of people that received no radiation except background radiation and a second group of people that received 100 mSv in addition to background radiation, there would be no difference in the two groups in terms of health effects.

Because the brain and nervous system are not very sensitive to radiation, the risk factor is low compared to other organs. And since the brain receives most of the radiation dose from a head CT scan, the absorbed dose can be high and the effective dose can be low.

One other thing to keep in mind: the very real medical benefit from the CT scan is much greater than the hypothetical risk from the scan.

Kent Lambert, CHP

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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