Jan. 23, 2001 A new Children's Hospital Medical Center of Cincinnati study shows that some children who get computed tomography (CT) scans receive doses of radiation at strengths at least five times greater than necessary. Radiologists can reduce doses of radiation without compromising information in the images obtained, according to Lane F. Donnelly, M.D., a staff radiologist at Cincinnati Children's and the study's lead author.
CT is the primary imaging technique for children with abdominal pain, suspected appendicitis, abdominal trauma, a suspected mass, or complicated pneumonias. Like general radiography, CT uses ionizing radiation, or x-rays. The radiation dose associated with CT, however, is much larger than the dose associated with other imaging procedures. Radiation dose is particularly important in children because of the relatively increased lifetime cancer risk of children compared to adults.
"CT is often a necessary part of the evaluation of children with suspected illness," says Dr. Donnelly, whose study is published in the February edition of the American Journal of Roentgenology. "The benefit from the information obtained by the CT greatly outweighs the radiation risk. However, although the risks of radiation associated with a CT examination are extremely low, they are not zero. It's not only imperative to make an accurate diagnosis, but also to strive to make the risks as minimal as possible."
CT accounts for about 4 percent of medical x-ray examinations. But it contributes to an estimated 40 percent of the total radiation dose to the population. Two technical factors -- tube current and pitch -- "can be adjusted easily to reduce radiation exposure in the pediatric population and have a profound affect," says Dr. Donnelly. Tube current refers to the amount of electrons used to generate the x-ray source. Pitch refers to the ratio between the rate at which the table the patient lies on moves through the scanner and the rate at which the scanner spins.
"The major disadvantage of decreasing the tube current is an increase in noise, but this potential is counterbalanced in younger patients by their smaller size," says Dr. Donnelly. "In addition, pitch can be increased by about half to reduce radiation dose by about a third, without any loss of image quality."
CT is able to depict anatomy at different levels within the body. This ability, known as cross-sectional imaging, is possible because the x-ray source rotates around the patient during a CT scan, encircling the patient and capturing anatomical detail from many angles. Each rotation of the x-ray beam produces a single cross-sectional "slice" of anatomy, like the slices in a loaf of bread. A computer then creates an image by stacking the individual image slices. Using this technology, physicians can view the inside of anatomic structures, a feat not possible with general radiography.
Unfortunately, most studies of CT have been done on adults, and the same dose of radiation typically has been used on children.
In addition, many CT units are equipped with software that automatically chooses tube current based on optimal image quality calculated for adults. "Efforts must be made to override these automatic parameters when imaging children," says Dr. Donnelly.
The radiologists in Cincinnati Children's section of body imaging, in the department of Radiology, began examining lower doses of radiation about two years ago, when Kathleen Emery, M.D., a staff radiologist, realized that some other studies had begun to examine reduced dosages of radiation for CT of the chest. "It began to bother us that we used parameters based on adults studies," she says. "Unlike general radiography, there is no penalty for using a higher dose of radiation, whereas in regular x-rays, too high a dose would result in a black picture."
In the two years the radiologists have adjusted tube current and pitch, they are unaware of any cases in which a diagnosis was undetected on a reduced dose CT but became evident later. In addition, they have not had to repeat studies at an increased tube current because of poor technical quality.
Collaborating on the study were Dr. Emery, Alan S. Brody, M.D., Tal Laor, M.D., Victoria M. Gylys-Morin, M.D., and Christopher G. Anton, M.D., all of the department of Radiology at Cincinnati Children's; Stephen R. Thomas, Ph.D., University of Cincinnati College of Medicine; and Donald P. Frush, M.D., department of Radiology, Duke University.
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