Aug. 8, 2005 Two minor changes in technique could make a major difference in 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 a survey 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 at The University of Texas, M.D. Anderson Cancer Center in Houston, TX. The radiation dose depends on X-ray tube kilovoltage (kVp), X-ray tube current and X-ray tube positioning, she said. Dr. Cody and her colleagues studied 21 CT scanners, representing three different vendors and 11 different models. They found that "the lowest radiation exposure was achieved using 80 kVp, minimum X-ray current and a 180-degree tube position. If these settings can be used for survey CT scans, the associated radiation exposure could be reduced to that of less than one chest X-ray," Dr. Cody said.
When the X-ray tube is put in the 180-degree position it is underneath the patient so the radiation beam strikes the table the patient is lying on first, "allowing the table to absorb the lowest-energy X-rays," she said. This reduces the exposure to the breast, in particular and does not have any effect on the quality of the CT survey image, Dr. Cody said.
On the other hand, "when the X-ray tube voltage or current is altered, the image quality will be affected," she said. "We encourage all radiologists to think about how the survey scan is used. If, for example, the survey CT scan is used only to plan for the examination, high-quality survey CT images may not be needed. In these cases we suggest that sites consider decreasing their X-ray tube voltage and current so that the relative radiation exposure from the survey scan is as low as possible, while maintaining a good enough image," she said.
"Default settings on CT scanners vary, and even though the survey scan makes up a small fraction of the total radiation dose in a CT scan, it is important for all institutions to check those default settings to make certain they are 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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