Background The transradial approach is regarded as a useful vascular site for coronary procedures. The aim of this study was to test whether 4Fr catheters assisted by ACIST variable rate injector system can produce comparable angiographic quality and reduce the risk of radial artery injury compared to hand manifold 6 Fr catheters.Methods A total of 1816 patients were studied consecutively, among whom 856 patients received coronary angiography by 4 Fr catheters (4Fr group) and 960 patients by 6 Fr catheters (6Fr group). Angiographic and procedural characteristics were observed and recorded. The luminal inner radial arterial diameter before and after the procedure were collected.Results The baseline clinical characteristics were similar in both groups. There were no significant differences in procedure time, radiation dose and quality scores in both groups (P>0.05), but more contrast media was delivered in the 6Fr group (P