Multislice CT strikes balance between dose and quality

State-of-the-art CT scanners demonstrate exquisite image quality. The rapid advance of multislice technology has been accompanied, however, by concerns over the increasing medical radiation burden. Radiologists concerned about MSCT dose would be well advised to learn how they can tailor their own scanning protocols to optimize the balance between radiation exposure and image quality, according to Prof. Dr. Willi Kalender, director of the Institute for Medical Physics, University of Erlangen, Germany, speaking at the European Congress of Radiology.

"Dose values need not be a secret. You can be informed," he said. "We now have software that-for whatever scanner you use, whatever range you are scanning-will automatically provide you with effective dose values, with organ dose values, and will let you check the influence of kV."

Dose values for most MSCT examinations range from 1 to 20 mSv. Taking 10 mSv as the typical value, this is four times the annual exposure an individual would receive from natural background radiation in a country such as Germany.

"So we are working in a reasonable range, but as we begin to apply more and more automated exposure controls, I think this will go down further," Kalender said.

Many worries about dose have been driven by the increasing power of CT technology. Since the advent of multislice scanning, the move from four-slice to 64-slice has happened in a matter of years.

The unveiling of dual-source CT technology has attracted yet more attention. This type of scanner comprises two source detector assemblies rotating on a single gantry at 90 degrees to each other. Because one source is slightly smaller than the other, the cone-shaped volume exposed by this second source is also smaller.

Assumptions that "dual source" and "dual power" means "dual dose" are not necessarily true, Kalender said. Radiologists performing a 20-second examination on a standard MSCT scanner should aim to decrease this time to 10 seconds on a dual-source system. Dose will double only if examination times are left unaltered.

"There is no inherent doubling or increase of dose," he said. "It is an increase of x-ray power, thereby allowing us to acquire data faster. In fact, with the design as it is, with the second source exposing a smaller volume, we could actually have a reduction in dose."

The dual-source system should help provide better temporal resolution, he said. Data gathered from a dual-source CT scanner installed at Erlangen have shown that cardiac imaging should be possible without prior administration of beta blockers to slow the heart's rate.

"As a result, we will be able to manage these patients without intervention," Kalender said.

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 in April 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 alter dosage automatically based on attenuation values recorded over the previous 180 degrees arc.

"Although the manufacturers have done a lot on the newer generation scanners to limit the radiation dose, it is still a significant dose for children, and we want to make sure that what we are getting is worth what we are giving," said Dr. John Hauschildt, head of education for the radiology resident program at Children's Hospital and Health Center in San Diego.