Recently, medical professionals and CT system manufacturers have sought ways to reduce radiation exposure from coronary CT angiography (CCTA) exams. Today, it is possible to perform a CCTA study that delivers a lower radiation dose than the commonly used nuclear stress test. However, education and training are integral to establishing uniform coronary CCTA scanning standards and less dose to patients.
For years, not much attention was paid to the radiation dose associated with CT scanning. Several seminal studies in the early part of the 21st Century linked medical radiation exposure with the possible development of cancer later in life. Since then, there has been a concerted effort to reduce dose as much as diagnostically possible.
Several insurance companies now flag patients who either exceed a radiation dose threshold or have had more than five CT exams to the most vulnerable parts of their anatomy. The FDA suggests that people carry a “radiation dose card” that details all their medical radiation exposure. And CT equipment manufacturers have developed technology that, when used properly, can significantly minimize dose.
Radiation dose is a concern for chest pain patients undergoing CCTA to assess the presence of coronary artery disease. Therefore, it is incumbent upon physicians and technologists that they perform CCTA in accordance with the ALARA principal (As Low As Reasonably Achievable).
Regardless of where the CCTA exam takes place—the emergency department or a physician practice—one of the biggest problems associated with excessive radiation exposure is operator unfamiliarity with dose-reduction protocols. “Whether a patient is imaged electively or in the emergency department, there shouldn’t be a lot of variation in radiation exposure,” says John Lesser, MD, from the Minneapolis Heart Institute. “With CCTA, you have to manipulate parameters to fit the patient. Differences arise because of the patient’s heart rate or size, or operator knowledge.”
Ideally, the CCTA dose should be the same or less than the more commonly used stress nuclear scan. “With earlier models of dual-isotope SPECT scanners, the radiation dose of a cardiac test can be as high as 24 mSv, while a SPECT sestamibi scan can be as high as 12 mSv. We should expect most cardiac CT scans to range between 3 and 12 mSv,” Lesser says.
“The diagnostic results obtained through a CCTA study far outweigh the slight risk of cancer development in later years,” according to Jörg Hausleiter, MD, of the University of Munich, Germany. “However, there are practical means of reducing radiation dose. With modern CT technologies, we will be able to reduce the dose into a range of approximately 5 mSv, or even lower.”
Importance of PROTECTION I
The PROTECTION I trial, of which Hausleiter was lead investigator, was an international, prospective, multicenter study that included 1,965 patients undergoing CCTA between February and December 2007 (JAMA 2009;301:500-507). Researchers found that the median dose-length product (DLP) of patients was 885 mGy cm, which corresponds to an estimated radiation dose of 600 chest x-rays. They also observed a high variability in DLP between study sites (range of median DLPs per site, 331 to 2,146 mGy cm).
The authors wrote that 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. 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.”
Some available methods for reducing CCTA radiation dose include:
- Tube Current Modulation, a helical scan: “You administer a full program dose to certain parts of the cycle, and then you administer a dose reduction of 20 percent for the rest of the cycle,” Lesser says.
- Step-and-Shoot Method, an axial scan. “It exposes without overlapping radiation from adjacent scanner detectors, and exposes a small segment of the cycle. This method also requires a very regular heartbeat and reduces radiation dose the most,” Lesser says.
- Tube Voltage Reduction from the conventional 120 kV to 100 kV, which “can be done very safely without losing diagnostic quality,” Hausleiter says.
- With the new Siemens Definition Flash CT scanner, the “table feed goes very fast, which makes the