A system enables a user to mark a ROI of a vessel portion having detected boundaries and extend the detection of vessel boundaries into a region identified by the user. An anatomical blood vessel detection and display system includes a user interface enabling a user, via a display of an image presenting anatomical vessels, to, mark a region of interest in a vessel identified by detected boundaries in the image and extend the marked region of interest of the vessel. A detector automatically detects a boundary of the vessel in the extended marked region of interest by detecting an edge representing a change in luminance in data representing the image. An image data processor identifies a minimum vessel diameter in the extended marked region of interest between the detected boundaries and initiates identification of the location of the minimum vessel diameter in the image.