1. A method for use with a contrast agent that is injected into a blood vessel of a subject, comprising: acquiring a plurality of image frames of the subject's blood vessel, acquisition of the image frames commencing prior to injection of the contrast agent into the blood vessel; using at least one processor, identifying a first one of the image frames as being an image frame in which an angiographic sequence commenced within the acquired image frames, by: calculating a vesselness descriptor for each one of the image frames, by determining an extent to which pixels within the image frame are likely to correspond to the blood vessel, by, for each of the image frames, counting a number of possible centerline points of a vessel that are located near to possible edge lines of the vessel; and determining that, in the first image frame, there was an increase in the vesselness descriptor relative to at least one image frame acquired prior to the first image frame; and using the at least one processor, generating an output at least in response to identifying the image frame in which the angiographic sequence commenced.