Technology, protocols aid CCTA dose reduction

 
 
 

PHOENIX–The utilization of CT in imaging has exploded over the past several years as technology has evolved its capabilities to deliver even greater diagnostic information for clinicians; however, the boom in deployment of the modality has raised questions about acceptable levels of radiation for patients undergoing procedures, according to a presentation this week at the 2008 North American Society for Cardiovascular Imaging (NASCI) annual meeting.

“In the past couple years there have been a number of articles, both in the professional literature and in the lay press, that have brought the issue of radiation dose to the forefront,” noted James P. Earls, MD, director of the cardiac CT lab at the Inova Heart and Vascular Center in Falls Church, Va.

Earls, speaking on managing radiation issues in coronary CT angiography (CCTA), cited Ludwig Mies van der Rohe’s famous comment, “less is more,” as the guiding principle for clinicians performing the exam.

A recent prospective study presented by Meyer J. Hausleiter, MD, and colleagues at the 2008 American College of Cardiology conference in Chicago looked at CTA dose rates in 50 study sites. The researchers reviewed 1,965 consecutive CCTA exams and reported a mean estimated dose of 15.4 mSv.

More troubling was that the dose rate was highly variable: both between sites, 5.7 mSv to 36.5 mSv; and between 64-slice CT systems 11.8 mSv to 25.2 mSv. However, Earls reported that three methods used to reduce radiation dose in CCTA were highly effective.

“ECG-modulation was able to achieve a 20 percent reduction in dose, 110kV tube current saw a 50 percent reduction in dose, and prospective step-and-shoot techniques brought a 68 percent reduction, according to the study,” he said.

Practices investigating the addition of CCTA to their service line need to be aware that there is little correlation between a standard chest CT and a CCTA exam when it comes to radiation dose, Earls noted.

 “When we do cardiac CT, traditionally with retrospective gating, we have to use a relatively low pitch value. This lower pitch causes multiple overlap in areas of exposure, which then causes increase in radiation to the patient,” he said.

 Earls also noted that tube current is left on throughout the acquisition, a shorter temporal resolution requirement results in a higher flux of photons and thinner detectors also require a higher dose. A combination of these elements leads to an increased radiation burden by the patient in a CCTA.

“Carefully adjusting your mA to the weight and body mass index of the patient, limiting the Z-axis coverage to small fields of view, and using cardiac-specific filters can all contribute to a lower dose to the patient while maintaining adequate image quality,” he added.

Another technique, prospective triggering, or prospectively gated axial (PGA) acquisition, uses a combined step-and-shoot axial data acquisition and an incrementally moving table with prospective adaptive ECG triggering. According to Earls, this method takes advantage of the large 40 mm (64 x 0.625mm) volume coverage available with a 64-row CT scanner that enables complete coverage of the heart in 2 to 3 steps.

“The result is very little overlap between the scans, significant 50 to 80 percent reduction in radiation dose, and more robust and adaptive ECG-gating,” he said. However, because of the heart rate limitations inherent with the PGA technique in its current form, good heart rate control is essential for its use.

Earls and his colleagues at Inova use oral beta blockers or calcium channel blockers when beta blockers are contraindicated, if a patient’s resting heart rate is greater than 60 beats per minute (bpm). In addition, a small oral dose of metoprolol is given to minimize heart rate variability in patients with a heart rate of 55-59 bpm in order to maximize image quality during the exam.

He noted that the PGA technique can be used in up to 90 percent of patients and that it lowers the effective dose by approximately 80 percent.

“CT parameters can be optimized to lower dose, so you don’t have to be a radiation physics heavyweight in order to get good diagnostic image quality,” he said.