This study aimed to compare the effects of the automated and manual methods for determining the optimal phase of image reconstruction with the use of 64-slice coronary CT angiography (CCTA). In 63 patients, the optimal trigger delay was determined using an automated technique (PhaseXact) and by manual phase selection from the images reconstructed at a 10 ms and 1% increment of the R-R interval. The image quality was evaluated using four-step grading. The mean heart rate in all the patients was 70 +/- 10.1 bpm (range: 52-98 bpm). The frequency of obtaining diagnostic quality images was highest with the manual absolute timing method (97.6% of 753 segments), while it was 94.7% with manual relative timing and 91.9% with an automated method. The mean image quality score (1.3 +/- 0.2) based on the manual absolute timing was significantly better (P = 0.002, one-way ANOVA; P < 0.05, Scheffe's test) than the scores determined with the use of the other two techniques (manual relative timing, 1.4 +/- 0.3; automated method, 1.5 +/- 0.4). The best quality coronary CT images were obtained using image reconstruction with manual absolute timing for the ECG-trigger delay, while an automated method and manual relative timing provided diagnostic quality images in most of the segments.