1. A method of imaging the blood flow in a vascular tree of an object ( 3 ) to be examined, which method comprising the steps of: a) forming a series of X-ray projection images (D i ) of the object to be examined ( 3 ) from different projection directions, b) reconstructing a three-dimensional image that contains an image of the vascular tree from the X-ray projection images (D i ), and c) determining a sequence of clusters (C 1, C 2, C 3,... ) from spatially coherent voxels in the three-dimensional image, at least one cluster (C 2 ) that is defined by the direction of the blood flow being determined as from a start cluster (C 1 ) of the vascular tree, said cluster itself acting as the starting point for the determination of at least one next cluster (C 3 ) in the sequence and at least some of its voxels adjoining the voxels of the start cluster and of the next cluster, and for each new cluster of the sequence there being determined the subsequent cluster.