Dose-saving strategies play catch up to greater CT use

Efforts by researchers and radiation safety advocates in the U.S. and abroad have led to the development of several techniques and strategies to reduce radiation dose from CT scanning.

The use of CT scanning has steadily increased in much of the world during the last decade. While CT represents 12% of diagnostic imaging procedures that require ionizing radiation, it accounts for about half of the overall estimated effective radiation dose afforded to patients.

The relationship between medical radiation and lifetime chance of developing cancer remains controversial. What is clear, however, is that dose has increased and most physicians remain oblivious to the magnitude of this increase, according to Dr. Fred A. Mettler Jr., chief of radiology and nuclear medicine at the New Mexico Federal Regional Medical Center. Mettler spoke at the 2007 National Council on Radiation Protection meeting.

The first step to reducing dose is to determine whether a proper clinical indication exists and which diagnostic studies, including CT, are really necessary, said Dr. Andrew Einstein, an assistant professor of cardiology, radiology, and medicine at Columbia University.

"For a number of settings, there are appropriateness criteria or guidelines by various national organizations as to where CT should and should not be used," he said. "Once it is determined that CT is appropriate, a number of methods can be employed to minimize dose to the patient."

Coronary artery disease remains the main killer of men and women in the U.S. and a significant burden to the healthcare system, with related annual costs of nearly $150 billion. Imaging experts say that 64-slice coronary CT angiography may become the diagnostic gold standard for this condition. What might be the lifetime cancer risk associated with the exam?

Einstein and colleagues estimated this risk by analyzing the Biological Effects of Ionizing Radiation (BEIR) VII Phase 2 report and the most current data available on radiation bioeffects using a computerized simulation model. The BEIR VII is a National Academy of Sciences-sponsored effort to study the risks of ionizing radiation exposure at doses below 100 mSv.

The researchers found that 64-slice coronary CTA scans put young women at a greater risk of developing cancer later in life than any other group of patients. They suggest that alternative diagnostic tests should be considered for these patients (JAMA 2007;298[3]:317-323).

ADJUSTING CURRENT AND FILTERING NOISE

Tube current modulation (mAs), considered one of the more popular dose reduction strategies, refers to dose attenuation or adjustment in the x, y, and z imaging planes to fit a patient's body size and mass. Researchers have also explored the feasibility of manipulating tube voltage settings (kVp) in combination with modulating tube current to maximize image quality while reducing noise.

Tube current modulation has been shown to reduce dose by about 50% in some cases while keeping image noise levels steady. The clinical literature notes the convenience of these schemes for dose reduction in small patients, in particular children. Recently published studies have also made a case for their application in overweight and obese patients, who pose an increasing challenge for U.S. healthcare in general and for imaging in particular (AJR 2007;188[2]:553-562).

Devices in the latest generation of CT scanners feature automated exposure controls that take care of the xyz modulation. Most recently, ECG-gated tube current modulation, also referred to as "step-and-shoot," has been introduced into clinical cardiovascular practice.

A number of papers presented at the 2007 Society of Cardiovascular Computed Tomography meeting showed that a reduction of about 50% in dose was possible for single- and dual-source 64-slice CT scanners using the step-and-shoot technique. One in particular, by Dr. Sabha Bhatti from the University of Wisconsin Hospital and Clinics in Madison, showed the technique provides dose levels that could at least match those of coronary catheter angiography.

Noise reduction filter technology segments raw data in a computer workstation, processes its components separately to correct for noise and other irregularities while preserving CT "numbers," then recombines the processed data. The technique allows for reducing dose while maintaining image quality. The literature notes multiple examples of the technique's clinical application in gastrointestinal (Radiology 2004;232[2]:611-620), chest (Eur Radiol 2003;13:717-723), and cardiovascular imaging (Eur Radiol 2005;15[4]:721-726).

OTHER TECHNIQUES

A few healthcare providers and insurance companies have recently begun implementing programs that monitor either the number of CT exams a patient has or the cumulative radiation dose exposure. Dr. Steven Birnbaum, a radiologist based in Nashua, NH, championed a pioneering system, adopted by the New Hampshire Blues, that alerts physicians when an individual patient has had more than five CT exams to the neck, chest, abdomen, or pelvis. At least one insurance carrier, NIA/Magellan Health Services, offers its customers a similar plan that interrupts the preauthorization process if a patient exceeds 50 mSv of exposure within 12 months.

"Dr. Birnbaum has gone beyond just simply looking at when was the most recent CT or making sure you are not repeating one unnecessarily. His program is probably on the cutting edge of dose management," said Dr. James Brink, chair of diagnostic radiology at Yale University.

Different types of shielding techniques are gaining attention in the radiology community. The idea behind them is to place a cover or shield on top of radiation-sensitive areas, such as gonads or breasts, that will limit the radiation scatter exposure when imaging areas in close proximity. The technique could be beneficial mostly to pediatric patients and young women.

An international multicenter study proposed using bismuth shields for female pediatric patients to minimize dose from multislice scanning. Dr. Bradley L. Fricke and colleagues in Ohio, North Carolina, Virginia, and Spain found that bismuth shields effectively reduced dose without affecting image quality (AJR 2003;180:407-411).

"Using these shields reduces dose by about 30% or so. This could help reduce breast dose to women as well as children," said Kalpana M. Kanal, Ph.D., an assistant professor of radiology at the University of Washington in Seattle.

A technique that is becoming popular, particularly among cardiovascular imagers, proposes narrowing the field-of-view around the organ system under evaluation. Cardiologists at the Wisconsin Heart Hospital, for instance, do not perform a full chest CT when they do heart exams.

"Certainly, when we have bypass grafts or look for aortic disease, we need to extend the scan. But we only radiate the parts of the body that we really need to see, and in our case it is usually just the heart," said Dr. Samuel Wann, an interventional cardiologist. "We shouldn't extend the field-of-view to include other parts of the body that aren't needed for a primary diagnosis."

PROS AND CONS

The flurry of studies and statements regarding radiation dose with CT is pushing radiologists and other physicians to redefine the "as low as reasonably achievable," or ALARA, concept. On the one hand, there is a legitimate concern to reduce dose to minimize the risk of cancer, especially in young patients. On the other, it is important not to lose the potentially life-saving diagnostic value of imaging tests. In many cases, overdoing dose reduction may lead to missed disease. Brink recalled one case in which his chief of CT had to bring a patient back to the scanner because of a suspicious lesion that had been identified first as a likely renal cyst. The level of noise was so high that the diagnosis was not accurate enough. After a second scan with a higher dose, they could identify the lesion as malignant, which was confirmed after surgery (Figure 2).

"Always cutting the dose for the sake of dose reduction without paying attention to what you are losing from a diagnostic standpoint is not appropriate," Brink said.

Cardiologists agree.

"I am reluctant to use these dose reduction techniques when it's important that I get the right diagnosis," Wann said. "The patients I take care of are far more likely to die of coronary disease than of hypothetical radiation damage. So I like to be reasonable about it, not fanatical."

Mr. Abella is associate editor of Diagnostic Imaging.