1. A method, comprising: inserting a tool into a blood vessel; while the tool is within the blood vessel, acquiring an extraluminal image of the blood vessel; in the extraluminal image of the blood vessel, automatically detecting a location of a portion of the tool with respect to the blood vessel; in response to detecting the location of the portion of the tool, automatically designating a target portion of the blood vessel that is in a vicinity of the portion of the tool; and using the extraluminal image, performing quantitative vessel analysis on the target portion of the blood vessel.