A method for automatic detection of a contrast agent inflow in a blood vessel of a patient with a CT system, and CT system for carrying out this method, are disclosed. At least one embodiment of the invention relates to a method which determines the position of at least one blood vessel in section image representations in a CT examination without external intervention with the aid of an active shape or active appearance model, measures the inflow of contrast agent in this region in a targeted way and automatically initiates at least one action in the event of inflowing contrast agent.