Background: To assess the diagnostic performance and enhancement of coronary CT with reduced contrast medium using the spiral flow tube compared with the T-shaped tube. Methods We retrospectively included 444 patients who underwent catheter examination within 2 months after coronary CT. The first 222 patients received 21.0 mgI/kg/s of contrast medium using the T-shaped tube and the injected contrast medium was reduced by 6% (19.8 mgI/kg/s) in the last 222 patients using the spiral flow tube. We compared the per vessel diagnostic performance for detecting obstructive stenosis (≥50%) by catheter examination and the enhancement of proximal coronary arteries. Results The sensitivity, specificity, positive predictive value, negative predictive value and accuracy was 96%, 84%, 72%, 98% and 87% using the spiral flow tube and 93%, 83%, 68%, 97% and 86% using the T-shaped tube, respectively, without significance. The enhancement of left main (spiral flow vs T-shape, 407 ± 53 vs 407 ± 62 HU, p = 0.95) and right coronary (419 ± 58 vs 415 ± 61 HU, p = 0.49) arteries did not show significant difference. Conclusions The spiral flow tube would maintain the diagnostic performance and coronary enhancement with reduced contrast medium compared with the T-shaped tube.