Purpose: To evaluate the relationship between the phase window width and image quality in prospectively ECG-gated 320-detector row coronary CTA, and to evaluate the relationship between heart rate and the number of cardiac phases with diagnostic quality images.
Methods: Thirty-six phases (60-95% R-R, 1% increments) were reconstructed in 41 consecutive prospectively gated single R-R 320 x 0.5 mm detector row coronary CTA patients. For each phase, two cardiovascular imagers retrospectively documented the phases considered diagnostic for the left anterior descending (LAD), left circumflex (LCx), and right coronary artery (RCA). The smallest phase window width including at least one diagnostic phase for 95% of coronary arteries was determined, and after accounting for sampling variation, the same smallest window width was estimated for the general population. Inter-rater agreement was determined. A linear regression model evaluated the relationship between heart rate and width of diagnostic phase windows.
Results: Widening the phase window width increases the proportion of coronary arteries with at least one diagnostic phase. Among the 41 patients, 95% of vessels had a diagnostic phase in the 72-77% phase window. Accounting for sampling variation, the 72-81% phase window has a 0.95 probability of including a diagnostic phase for 95% of coronary arteries in the general population. Interobserver agreement was 0.959 with 0.95 confidence interval [0.908, 0.987]. Patients with a lower heart rate had significantly more diagnostic phases.
Conclusions: For prospectively ECG-gated single heart beat coronary CTA, a phase window width of 10% will reduce patient radiation and yield diagnostic images in >90% of patients. Heart rate control is an important component of 320-detector row prospectively gated CT dose reduction.