Cutting CT dose stands out as necessary but difficult challenge

The typical CT exam exposes patients to the equivalent of between 100 and 250 chest x-rays. This fact escapes most physicians, including radiologists, according to Dianna D. Cody, Ph.D., chief of radiologic physics at the University of Texas M.D. Anderson Cancer Center.

Cutting that dose is not easy, but it has to be done, she said at the Society for Imaging Informatics in Medicine annual meeting opening April 27 in Austin.

"CT really is a big monster," Cody said.

Although CT accounts for only about a third of the exams that involve ionizing radiation, it imposes two-thirds of the overall dose applied to patients. One in about four people underwent a CT exam in 2002, she said. Eleven percent of those exams were performed on children.

"This translates into more than seven million pediatric CT exams," she said.

Altering scan time, table pitch, collimation, and kilovoltage changes the effective dose, defined as the amount that goes to body organs. None has as great an effect as downsizing the kV. Unfortunately, dialing back the power can also have a dramatic effect on image quality, in some cases rendering the images diagnostically worthless, as noise and artifacts obscure disease processes.

Some compromise is needed, however, especially when imaging children.

Cody suggests settling on an acceptable level of noise in images and then running the CT to deliver images of that quality. The ideal would be establishment of some generally acceptable standards or guidelines for dose and image quality, but radiology so far has done very little in this regard.

"You would think that since CT is a big deal, somebody would be coming up with standards," Cody said. "But the only thing out there is a CT accreditation program from the American College of Radiology, which has established some reference values."

These values address acceptable doses for the head, adult abdomen, and pediatric abdomen of a five-year-old.

Until more detailed guidelines are provided, radiologists can turn to efforts by vendors to curb dosage, using algorithms that vary dosage according to body parts. Such body-dependent radiation typically applies lower doses to areas more easily penetrated and higher doses to difficult ones.

"The shoulders need lots of photons, but fewer are needed through the chest," Cody said.

The most sophisticated programs, such as one implemented by Siemens Medical Solutions, alter dosage on the fly based on attenuation values recorded over the previous 180° arc.