Coronary CT angiography study rules out coronary artery disease in a 58-year-old asymptomatic man with multiple cardiovascular risk factors.  Image source: U. Joseph Schoepf, MD, Medical University of South Carolina, Charleston.

Lowering the tube voltage to 100 kVp from the standard 120 kVp delivers diagnostic quality images, as well as one-third less radiation exposure, according to results from the PROTECTION II trial published in the November issue of JACC: Cardiovascular Imaging.

Jörg Hausleiter, MD, from the University of Munich, and colleagues randomly assigned 400 non-obese patients from eight European study sites to undergo CCTA with either the 100 kVP or 120 kVp protocol. The primary endpoint was to demonstrate noninferiority in image quality with the 100 kVp protocol on a scale of 1 (nondiagnostic) to 4 (excellent). Image quality was adjudicated by two experienced readers at a central location.

"The experience in reading CCTAs should not influence the current study results," Hausleiter said in an interview. "Many studies including the large study by Gilbert Raff et al (JAMA 2009), demonstrate that CCTA scan protocols applying 100 kVp for dose reduction can be applied in daily practice without impacting the clinical usefullness of CCTA."

Secondary endpoints included radiation dose and need for additional diagnostic tests during follow-up.

Researchers found no significant differences between the 100 kVp and 120 kVp protocols in terms of image quality (3.3 vs. 3.28, respectively). There were no differences in image quality between the retrospectively ECG-gated acquisitions (93.2 percent) and prospectively-gated acquisitions (6.8 percent) or in image quality among the different body mass index groups.

"The majority of study scanners were not equipped with prospective scanning techniques at the time of study," Hausleiter said. "However, in the subsequent PROTECTION III study, which has been presented during ACC 10, we compared prospective with retrospective scanning, applying a comparable study design as in PROTECTION II and found no difference."

The investigators also found no differences in image quality among the various CT scanners: GE Healthcare's LightSpeed VCT (one site) and LightSpeed VCT XT (one site); Siemens Healthcare's Somatom Sensation 64 (one site) and Somatom Definition Dual Source (two sites); and Toshiba Medical Systems' Aquilion 64 (three sites).

The mean radiation exposure in the current study was significantly reduced from 12.2 mSv to 8.4 mSv with the 100 kVp protocol, a 31 percent reduction in dose and in line with catheter angiography. "This needs to be compared with radiation doses of approximately 9 mSv and of as high as 41 mSv for sestamibi and thallium myocardial rest-stress nuclear scans, respectively," the authors wrote.

Motion artifacts and low contrast were the main reasons for a nondiagnostic image quality score
in 97 and 3 percent of nondiagnostic coronary arteries, respectively. Extensive coronary calcifications and increased image noise were not identified as reasons for nondiagnostic image quality, according to the study.

Researchers did not separate differences between single-source and dual-source scanners. However, they are currently enrolling patients in a PROTECTION IV study, "which compares the Siemens high-pitch scan mode (Flash mode) with conventional scan modes for coronary CT angiographies. Again, we want to demonstrate the high pitch scan mode is non-inferior to conventional modes, while radiation dose is significantly reduced," Hausleiter said.

The investigators found no significant differences in image quality score between patients with and without need for additional diagnostic testing.

During follow-up, 27 patients of the 100 kVp group underwent additional testing for suspected obstructive coronary artery disease (20 patients with invasive coronary angiography, four patients with stress nuclear cardiac perfusion imaging and three patients with stress cardiac MR).

In the 120 kVp group, 38 patients underwent additional tests (32 patients with invasive coronary angiography, four patients with stress nuclear cardiac perfusion imaging and two patients with stress cardiac MR).

In an accompanying JACC editorial, Pat Zanzonico, PhD, from Memorial Sloan-Kettering Cancer Center in New York City, said, "The foregoing data make a compelling case that, despite a significant degradation of image aesthetics, diagnostic information content is maintained using a 100 kVp rather than a 120 kVp scan protocol for CCTA in non-obese patients while reducing dose by 30 percent. The 100 kVp protocol should therefore be considered for coronary CTA in all such patients."