1. A method for segmenting blood vessels in a perfusion related map from perfusion MR images comprising: clustering pixels from the perfusion related map into at least an arterial pixel class and a venous pixel class by performing a multi-class cluster analysis performed by an electronic processor applying one or more perfusion curve parameters representing the degree to which the tissue entirely consists of blood and one or more curve parameters representing different measures of contrast arrival at the tissue obtainable from perfusion curves for pixels in the perfusion MR images, wherein arterial pixels belong to cluster(s): whose pixels have the earliest contrast arrival; and whose pixels consist of blood to the highest degree; venous pixels belong to cluster(s): whose pixels have the latest contrast arrival; and whose pixels consist of blood to the highest degree; and generating a vessel mask based on arterial pixels and venous pixels.