Two minor changes in technique could make a major differencein the radiation dose used in survey CT scans, a new study shows.Survey scans are those that are performed before a regular CT scan,usually to plan for the examination.
The radiation dose in asurvey scan can be as high as that of four chest X-rays, said Dianna D.Cody, PhD, associate professor in the department of imaging physics atThe University of Texas, M.D. Anderson Cancer Center in Houston, TX.The radiation dose depends on X-ray tube kilovoltage (kVp), X-ray tubecurrent and X-ray tube positioning, she said. Dr. Cody and hercolleagues studied 21 CT scanners, representing three different vendorsand 11 different models. They found that "the lowest radiation exposurewas achieved using 80 kVp, minimum X-ray current and a 180-degree tubeposition. If these settings can be used for survey CT scans, theassociated radiation exposure could be reduced to that of less than onechest X-ray," Dr. Cody said.
When the X-ray tube is put in the180-degree position it is underneath the patient so the radiation beamstrikes the table the patient is lying on first, "allowing the table toabsorb the lowest-energy X-rays," she said. This reduces the exposureto the breast, in particular and does not have any effect on thequality of the CT survey image, Dr. Cody said.
On the other hand,"when the X-ray tube voltage or current is altered, the image qualitywill be affected," she said. "We encourage all radiologists to thinkabout how the survey scan is used. If, for example, the survey CT scanis used only to plan for the examination, high-quality survey CT imagesmay not be needed. In these cases we suggest that sites considerdecreasing their X-ray tube voltage and current so that the relativeradiation exposure from the survey scan is as low as possible, whilemaintaining a good enough image," she said.
"Default settings onCT scanners vary, and even though the survey scan makes up a smallfraction of the total radiation dose in a CT scan, it is important forall institutions to check those default settings to make certain theyare using as low a technique as possible," Dr. Cody said.
The study appears in the August 2005 issue of the American Journal of Roentgenology.
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