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19 July 2008

Answer to Question #4671 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:

When instructing students in clinic I made the familiar statement that the majority of scatter produced by the patient—for example, a portable chest x ray—will be in the direction of backscatter, coming directly back in the direction from which it originated. My understanding is that the least amount of scatter radiation exits the patient at an angle 90 degrees from the direction of the primary beam. However, when I researched several texts, I can find little referencing the angle of scatter. I would like to make accurate suggestions to technicians/students who find they cannot leave the room nor gain six feet in distance when imaging a patient in the hospital room. Please suggest references with your response.

The definitive reference has been a 1972 article in Radiology by E.D. Trout and J.P. Kelley titled "Scattered Radiation from a Tissue-Equivalent Phantom for X Rays from 50 to 300 kVp." This can be a little hard to get a hold of, though.

In 2004 another article came out in the British Journal of Radiology by G. McVey and H. Weatherburn titled "A Study of Scatter in Diagnostic X-Ray Rooms." In it are several scenarios and the amount of scatter generated by each.

Their results, which are similar to those by Trout, did show that the greatest scatter was at scatter angles between about 135 degrees and 150 degrees for kVps of 49 and 121 (just as a frame of reference, if the photons go straight ahead onto the film, that would be 0 degrees and if they came straight back at the x-ray tube, that would be 180 degrees). So, what they say confirms what you have been teaching—that the greatest operator exposure actually comes when you're halfway between the patient and the x-ray tube. They have a graph showing that about 10% of all the scatter is occurring at 150 degrees (and about 3% at 135 degrees).

The actual exposure someone gets is highly dependent on patient size, the size of the x-ray field, kVp, and other such parameters. In this article, it states that increasing the kVp from 49 to 121 will increase the percent of scatter in the 135-degree direction by 64%. It also shows that going from a field size of 10 x 10 to one of 20 x 20 increases the percent of scatter from 7% to 25%.

I, too, looked in several reference texts. I couldn't find anything as good as this article.

Hope this helps.

Kelly Classic
Certified Medical Health Physicist

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