CT radiation dose cut; image quality still high
In some instances, it can be reduced by 50%
A new study shows that the radiation dose used in some chest CT examinations can be reduced by as much as 50% without jeopardizing the radiologist’s ability to make a diagnosis. And while current standard dosages do not pose a major health threat, the lead author asserts, it’s in the patient’s best interest to get the doses as low as possible while still maintaining a quality image.
"Why seek to lower the radiation doses? Because currently, the risks are not zero," asserts James G. Ravenel, MD, assistant professor of radiology at the Medical University of South Carolina in Charleston. "The guiding principle in radiation always has been to get the dose as low as reasonably achievable."
Ravenel goes on to explain that there seemed to be a "hole" in the body of research dealing with CT scan radiation. "While we look at CT scans generally from an image quality perspective, there have been several studies that looked at the appropriate dose, but they did not address the question of how the image quality and dose interrelate," he says. "We wanted to find out: Can we lower the dose without lowering the image quality?"
As the study notes, "The predominant risk to patients undergoing chest CT is the induction of cancer."1 How high is the risk? "The risk from a typical chest CT scan can be compared with other everyday risks," the authors explain. "An effective does of 6 mSv is comparable to the risk of dying from lung cancer after smoking approximately 100 packs of cigarettes, or the risk of dying in an automobile when driving a distance of approximately 5,000 miles."1
The figure of 6 mSv, a dosage measure, correlates directly to the mAs factor, which represents the actual tube current. So, for example, an mAs of 280, the standard that had been used at Ravenel’s facility, equates to 6 mSv, and an attendant fatality risk of three per 10,000 patients. (For additional data on fatality risk, see chart, below left.)
| Effective Dose and Radiation Risks for Chest CT Examinations | ||
| mAs | mSv | Cancer Fatality Estimate (per 10,000 scans) |
| 280 | 6.0 | 3.0 |
| 220 | 4.7 | 2.4 |
| 160 | 3.4 | 1.7 |
| 120 | 2.6 | 1.3 |
| 80 | 1.7 | 0.9 |
| 40 | 0.9 | 0.4 |
| Source: American Journal of Roentgenology, August 2001. | ||
One of the challenges in dealing with CT scans is that, unlike with X-rays, quality is not sacrificed when the dosage is increased. "In a normal chest X-ray, the more dosage you give, the blacker the image becomes," Ravenel explains. So he and his team sought to achieve a delicate balance. "With a certain lower dosage, you will start to see a decrease in quality, essentially in noisy, or grainy images," he says. "We wanted to know how we could go down [and not sacrifice quality]."
Finding the right balance
In search of the minimum radiation dose that maintained good quality, the researchers seemed to determine that 160 mAs may be the magic number. "The constant error rate at > 160 mAs implies that observers cannot differentiate between images generated at 160 mAs, 220 mAs, and 280 mAs. On the other hand, these data show that below 160 mAs, a monotonic reduction of the error rate is seen, implying that observers can differentiate between these images,"1 they wrote. At 160 mAs, the cancer fatality estimate drops to 1.7 patients per 10,000, or nearly half the rate when the dosage is 280.
The authors cautioned that radiation is not a one-size-fits-all process. "Radiologists and other imaging professionals who are responsible for developing scanning protocols need to address the trade-off between patient dose and image quality for each diagnostic imaging study," they noted. "This balancing of dose and image quality should be performed explicitly to ensure that patient doses are kept as low as reasonably achievable. . . . Low-dose CT is being promoted for general screening applications, whereas high-dose CT may be appropriate for the detection of some subtle diseases."1
From the perspective of a quality manager, says Ravenel, "The major two factors are dose and image quality. In essence, what probably should be done is to ask their radiologists what the mAs is, and whether they believe they can reduce that dosage without decreasing their confidence in making interpretations."
Part of the answer may lie with the specific scanner you institution has, he says. "Ours is a [General Electric]; quality managers may have to get a feel for other companies’ machines," he advises. We have moved on to work with others and have found similar dose reductions are possible."
Do his findings have implications for other areas of radiology? "Our study was specific for chest CT scans," he explains. "The same could probably be done with abdominal or head CT scans, but whether the magnitude would be the same remains to be seen."
Reference
1. Ravenel JG, Scalzetti EM, Huda W, Garrisi W. Radia-tion exposure and image quality in chest CT examinations. AJR 2001; 177:279-284.
Need more information?
For more information, contact: James G. Ravenel, MD, Assistant Professor, Thoracic Imaging Dept. Radiology, Box 250322, Medical University of South Carolina, 169 Ashley Ave., Charleston, SC 29425. Telephone: (843) 792-4932. E-mail: [email protected].
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