Cardiac CT with Somatom Definition Image source: Siemens Healthcare

Prospectively gated step and shoot and retrospectively gated helical techniques delivered comparable image quality using a 256-slice CT scanner for cardiac CT angiography (CCTA) exams, with the prospectively gated step and shoot technique associated with a 62 percent reduction in radiation dose, according to a study published in the January issue of Academic Radiology.

Although CCTA provides a highly accurate mechanism for the detection of coronary artery disease, radiation dose remains a challenge as dose of retrospectively gated helical (RGH) CTA ranges from 12 to 28 mSv. Previous studies with 64-, 128-slice and dual source CT have suggested that the prospective gated step and shoot (PGSS) technique results in dose reduction between 52 to 85 percent. The dose range for conventional coronary angiography hovers between 5 and 12 mSv.

Taiwanese researchers retrospectively compared radiation dose and image quality for CCTA on a 256-slice system using PGSS and RGH techniques between January 2009 and April 2009. One hundred twenty-six patients received RGH CTA, and 126 received PGSS CCTA.

Two independent cardiovascular radiologists, each with more than five years experience and blinded to the scan types, analyzed and graded the images according to a four-point scale. A score of three or lower was considered acceptable for routine clinical diagnostic purposes, wrote lead author Wei-Yip Law, MD of Shin Kong Wu Ho-Su Memorial Hospital in Taipei, Taiwan.

Image quality scores for RGH and PGSS groups were 1.522 +/- 0.317 and 1.500 +/- 0.374, respectively, reported Law and colleagues. “Overall, 50.6 percent and 54.2 percent of segments received a score of 1 (best image quality) for PGSS and RGH techniques, respectively, and notably, 0 percent and 0.1 percent of coronary segments were non-evaluative (score = 4) in RGH and PGSS groups, respectively,” according to the researchers.

They also reported a 62 percent reduction in effective radiation dose in the PGSS cohort.

Law and colleagues pointed out that 256-slice CT offers faster gantry rotation times, larger z-coverage and increased x-ray tube power, which may allow physicians to scan patients with higher heart rates and heart rate variability. In fact, they found weak correlation for heart rate in the RGH group and no correlation between the motion score and heart rate variability in either technique.

The researchers acknowledged the dose advantages of the PGSS technique and pointed out that the prospective helical acquisition technique mode may provide additional dose reduction. However, Law and colleagues cited advantages of the RGH mode, particularly the ability to evaluate cardiac function, which may help physicians better determine prognosis or therapy for some patients. In addition, 320-slice scanning may deliver further dose reduction by avoiding overscanning, they continued.

The researchers defined several limitations to the study. It did not compare PGSS and RGH with conventional angiography, nor did they scan patients with heart rates exceeding 75 beats/minute with PGSS, which may have biased image quality in favor of PGSS. Law and colleagues recommended further study to confirm the diagnostic accuracy compared to coronary artery angiography.