1. A method, comprising: inserting a tool into a blood vessel; while the tool is within the blood vessel, using an extraluminal image-acquisition device acquiring an extraluminal image of the blood vessel; using a processor, in the extraluminal image of the blood vessel, automatically detecting a location of a portion of the tool with respect to the blood vessel; using the processor, using the detected location of the portion of the tool, automatically designating a target longitudinal section of the blood vessel, based upon a presence of the portion of the tool in a vicinity of the target longitudinal section; using the processor, automatically performing quantitative analysis on the target longitudinal section of the blood vessel within the extraluminal image; and generating an output on a display, based upon the quantitative analysis.