Radiation Safety & Testing in CT

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Keeping radiation dosages in check is a priority for CT scans of adults and especially pediatric patients who run more risks of radiation exposure because they are still growing. Check out the variety of techniques, notably on souped-up multislice scanners, to measure and control dose while producing excellent images.

The use of computed tomography is on the rise. In February 2003, the United States Food and Drug Administration (FDA) reported that the use of computed tomography (CT) for diagnostic studies, cancer staging, treatment planning and real-time visualization during intervention had risen dramatically based on data collected for the Nationwide Evaluation of X-ray Trends (NEXT) Survey. Twenty-nine percent of all CT units in the U.S. are equipped to perform multislice spiral scanning, according to the survey.

David J. Brenner, PhD., DSc, professor of radiation oncology and public health at Columbia University in New York compares the statistic of close to 58 million CTs completed in 2000 with the four million CT scans accomplished in 1980. Brenner continues that although the NEXT report did not provide an exact number of CT studies in pediatrics, it's on the order of 10 percent, or roughly six million scans.

BALANCING SAFETY WITH IMAGE QUALITY

From the perspective of radiation dose safety, several concerns arise.

The broad issue involves the fact that CT examinations deliver a higher radiation dose to the patient than a conventional radiograph because, by its design, CT produces several radiographs from different directions that are then combined to produce the image. While radiation dose to adults is of major significance, the implications for pediatric patients assume critical proportions.

If the CT scanner is set to deliver the normal adult milliamp per second dose (mAs) to a child, the resulting dose will be effectively higher for the child because an adult body provides more shielding, explains Brenner. "For the same mAs, the organ dose can be three times higher, depending on the size of the child."

Brenner describes two converging factors that make increased dose more dangerous to children from a carcinogenic risk perspective. Radiation renders its greatest deleterious effect on dividing cells. Children and neonates, as part of the normal growth process, produce large numbers of rapidly dividing cells. Therefore, children are more sensitive to radiation than adults. Given the fact that children are thinner than adults, often the quality of the image can be maintained even with reduced radiation dose.

Brenner notes that one way to reduce dose dramatically for children is to use other imaging modalities in place of CT. He says research studies estimate that approximately one third of the CT scans performed can be replaced by other imaging exams or alternative diagnostic strategies. For example, in the diagnosis of appendicitis, ultrasound scans or just observing the child over several hours can will provide the diagnostic information required. Many children who present with marked abdominal tenderness become automatic candidates for a CT scan. While these studies do provide excellent diagnostic information, the benefit may not outweigh the risk from increased radiation exposure.

The major CT vendors have recognized the risks involved, and have developed methods to address these concerns for both adults and children.

Philips Medical Systems approached these issues from several directions. Intellibeam technology places two titanium filters and a wedge-shaped filter into the port of the x-ray tube of the Brilliance scanner to eliminate low energy x-ray skin dose that does not contribute to the quality of the image, according to Hugh Morgan, PhD, research scientist in the CT engineering department at Philips. "Typically, our system will deliver about 40 percent less skin dose than the competition, but will not reduce image quality," says Morgan.

Jim Green, Philips' senior vice president and general manager of CT business, says that their multislice scanners help to reduce dose through increased coverage.

Philips has just introduced the Brilliance 40, a 40-slice version that can provide 25 mm of coverage at 0.625 slice thickness, and 44 mm of coverage if the slice thickness is set at 1.2 mm. The system is currently being used for coronary artery analysis and stroke assessment as well as other studies. They anticipate that it will significantly reduce dose for pediatric patients because since the scan is accomplished