1. A method for determining collateral information describing blood flow in collaterals of a blood vessel system in a target region of a patient from a four-dimensional vascular data set recorded by a medical imaging device describing, in the four-dimensional vascular data values, temporal flow of a contrast medium, marked blood constituents, or the temporal flow of the contrast medium and the marked blood constituents through the target region, the method comprising: segmenting the blood vessel system in the four-dimensional vascular data set; determining collaterals among the segmented blood vessels by a collateral classifier; determining, for all determined collaterals, a diameter of the collaterals taking into account the segmentation, a filling parameter describing a filling of the collaterals, and a time parameter describing a time response relative to a reference point in the blood vessel system from a temporal course of image values in a monitored portion of the collaterals; and determining the collateral information for, or as a function of, the filling parameter and the time parameter.