A computer-implemented method of identifying vascular territories in an imaging volume representing an anatomical region, including: obtaining, for each volume element of a plurality of volume elements into which the imaging volume is divided, arrival time data representing an estimated arrival time of a contrast agent at the volume element; defining one or more seed regions, each seed region associated with one of a plurality of vascular territories and containing seed volume elements for which the estimated arrival time does not exceed a threshold time; identifying, on the basis of the arrival time data, unassigned elements of the plurality of volume elements which are connected with a seed volume element; and assigning said unassigned elements of the plurality of volume elements to one of said vascular territories.