Apparatus and methods are described including inserting a tool into a blood vessel, and, while the tool is within the blood vessel, acquiring an extraluminal image of the blood vessel. In the extraluminal image of the blood vessel, a location of a portion of the tool with respect to the blood vessel is detected automatically. In response to detecting the location of the portion of the tool, a target portion of the blood vessel that is in a vicinity of the portion of the tool is designated automatically. Using the extraluminal image, quantitative vessel analysis is performed on the target portion of the blood vessel. Other embodiments are also described.