Study cautions cardiac CT labs to be cautious. Image Source: NYU Internal Medical Blog

Use of cardiac CT angiography (CCTA) has the potential to expose patients to high doses of radiation, and methods available to reduce radiation dose are not frequently used, according to a study in today’s issue of the Journal of the American Medical Association.

“With the constantly increasing number of CCTA-capable scanners worldwide, the volume of CCTA scans performed is likely to show substantial further increase,” the authors wrote. They added that the clinical usefulness of CCTA for the assessment of coronary artery disease has to be weighed against the radiation exposure of CCTA and the small but potential risk of cancer. Many clinicians may still be unfamiliar with the magnitude of radiation exposure that is received during CCTA in daily practice and with the factors that contribute to radiation dose, according to the researchers.

Jörg Hausleiter, MD, of the Klinik an der Technischen Universität München in Munich, Germany, and colleagues investigated the magnitude of radiation dose of CCTA in daily practice, factors contributing to radiation dose and the use of currently available strategies to reduce radiation dose.

The PROTECTION I trial (Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice I), an international, multi-center study (21 university hospitals and 29 community hospitals) includes 1,965 patients undergoing CCTA between February and December 2007. They identified independent predictors associated with radiation dose, which was measured as dose-length product (DLP), which best mirrors the radiation a patient is exposed to by the entire CT scan, according to the authors. 

Hausleiter and colleagues found that the median DLP of the patients in the study was 885 mGy cm, which corresponds to an estimated radiation dose of 600 chest x-rays. They observed a high variability in DLP between study sites (range of median DLPs per site, 331-2,146 mGy cm).

The researchers found that the independent factors associated with radiation dose were:

• Patient weight (relative effect on DLP, 5 percent);
• Absence of stable sinus rhythm (10 percent effect);
• Scan length (a 1 cm increase in the scan length was associated with a 5 percent increase in DLP);
• Use of electrocardiographically controlled tube current modulation (resulting in a reduction of DLP of 25 percent, applied in 73 percent of patients);
• 100-kV tube voltage (46 percent reduction of DLP, applied in 5 percent of patients);
• Sequential scanning (78 percent reduction; applied in 6 percent of patients);
• Experience in cardiac CT (1 percent reduction); number of CCTAs per month; and
• Type of 64-slice CT system (for highest vs. lowest dose system, 97 percent effect).

“The study demonstrates that radiation exposure can be reduced substantially by uniformly applying the currently available strategies for dose reduction, but these strategies are used infrequently,” the authors wrote. “An improved education of physicians and technologists performing CCTA on these dose-saving strategies might be considered to keep the radiation dose ‘as low as reasonably achievable’ in every patient undergoing CCTA.”

“As CCTA is being used more frequently worldwide for diagnosing coronary artery disease, all strategies for reducing radiation exposure will finally reduce the patient’s lifetime cancer risks,” the investigators wrote. “Although the associated risk is small (estimated lifetime attributable risk of death from cancer after an abdominal CT scan is 0.02 percent) relative to the diagnostic information for most CT studies, this risk needs to be realized especially when repeated CT scans are being performed.”

In an accompanying editorial, Andrew J. Einstein, MD, PhD, of the Columbia University College of Physicians and Surgeons in New York City, wrote that there are a number of implications from this study for patient care.

Einstein said that the study indicated several factors:

• First, the results reinforce that CCTA is still a potentially high-dose procedure, and like all procedures involving the use of ionizing radiation, a patient-specific risk-benefit analysis should always be performed to justify the exam.
• Second, the findings suggest that dose-reduction methods can be used in the majority of patients, which should serve as “a wake-up call to cardiac CT laboratories that do not routinely use the methods.”
• Third, the study reveals a degree of variability in radiation dose between sites that had not been previously appreciated, but which offers the potential to decrease radiation burden from CCTA, while maintaining diagnostic image quality by instituting quality improvement programs to close the gap.
• Fourth, the lack of clinically significant association between procedure volume and dose suggests that despite the association between case volumes and quality of care, even high-volume centers can benefit from such quality improvement programs.

“The international system of radiological protection stands on three principles: justification, optimization and diagnostic reference levels. PROTECTION I provides valuable information pertaining to each of these in the context of CCTA, and as such makes an important addition to the evidence base,” Einstein wrote.