Apparatus for use with an extraluminal imaging device configured to acquire extraluminal images of a lumen of a body of a subject, the apparatus comprising: an output device; and at least one computer processor configured to: acquire at least one first extraluminal image of the lumen, using the extraluminal imaging device; designate the at least one first extraluminal image as a roadmap image; designate, within the roadmap image, a roadmap pathway; acquire at least one additional extraluminal image of the lumen; generate a score image, by applying a scoring function to at least some pixels within the additional extraluminal image, such that each of the pixels to which the scoring function is applied is designated a score that is indicative of an extent to which the pixel has a given characteristic; based upon the scores of the at least some of the pixels to which the scoring function was applied, determine a correspondence between at least a portion of the roadmap pathway and a curve within the score image; based upon the correspondence between the portion of the roadmap pathway and the curve within the score image, determine a transformation function for mapping between a location within the roadmap image and a location within the additional extraluminal image; and in response thereto, generate an output on the output device. an output device; and at least one computer processor configured to: acquire at least one first extraluminal image of the lumen, using the extraluminal imaging device; designate the at least one first extraluminal image as a roadmap image; designate, within the roadmap image, a roadmap pathway; acquire at least one additional extraluminal image of the lumen; generate a score image, by applying a scoring function to at least some pixels within the additional extraluminal image, such that each of the pixels to which the scoring function is applied is designated a score that is indicative of an extent to which the pixel has a given characteristic; based upon the scores of the at least some of the pixels to which the scoring function was applied, determine a correspondence between at least a portion of the roadmap pathway and a curve within the score image; based upon the correspondence between the portion of the roadmap pathway and the curve within the score image, determine a transformation function for mapping between a location within the roadmap image and a location within the additional extraluminal image; and in response thereto, generate an output on the output device. acquire at least one first extraluminal image of the lumen, using the extraluminal imaging device; designate the at least one first extraluminal image as a roadmap image; designate, within the roadmap image, a roadmap pathway; acquire at least one additional extraluminal image of the lumen; generate a score image, by applying a scoring function to at least some pixels within the additional extraluminal image, such that each of the pixels to which the scoring function is applied is designated a score that is indicative of an extent to which the pixel has a given characteristic; based upon the scores of the at least some of the pixels to which the scoring function was applied, determine a correspondence between at least a portion of the roadmap pathway and a curve within the score image; based upon the correspondence between the portion of the roadmap pathway and the curve within the score image, determine a transformation function for mapping between a location within the roadmap image and a location within the additional extraluminal image; and in response thereto, generate an output on the output device.