Apparatus and methods are described including acquiring a plurality of image frames of a blood vessel of a subject, acquisition of the image frames commencing prior to injection of a contrast agent into the blood vessel. A first image frame is identified as being the image frame in which an angiographic sequence commenced 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, and by 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. An output is generated at least partially in response to identifying the first image frame. Other applications are also described.