1. A method for automatic detection of a contrast agent inflow in a blood vessel of a patient with a CT system, the method comprising: providing statistical average values of principal components of a previous evaluation of a multiplicity of statistically comparable CT representations by at least two-dimensional position determination of distinctive points of anatomical structures including at least one blood vessel or an ROI for this blood vessel, carrying out one principal component analysis per CT representation and calculating the statistical average values of the principal components; carrying out a multiplicity of chronologically progressive scans in at least one section plane and progressive calculation of CT representations for the respective datum; determining at least two-dimensional automatic position of distinctive points of recognizable anatomical structures, with the exception of the at least one blood vessel or the ROI for this blood vessel, and determining the principal components; determining a deformation vector from the average principal components to the principal components currently ascertained for each CT representation and transformation of at least the position of the at least one blood vessel or the ROI for this blood vessel; determining a change in the image values as a function of time in the region of the at least one blood vessel or the ROI of this blood vessel over a multiplicity of CT representations at the respectively transformed positions of the at least one blood vessel or the transformed ROI for this blood vessel; and at least one of outputting the image values in the region of the at least one blood vessel or the ROI of this blood vessel, and triggering an action if a first threshold of the image values is exceeded.