Apparatus and methods are described, for use with a contrast agent that is injected into a blood vessel of a subject. The apparatus and methods include 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, one of the image frames is automatically identified as being an image frame in which an angiographic sequence commenced, by determining that, in the image frame, there is a change in a characteristic of the image frame relative to at least one image frame acquired prior to the image frame that is indicative of a presence of contrast agent within the image. An output is generated at least partially in response thereto. Other applications are also described.
