The invention generally relates to medical imaging systems that instantly and/or automatically detect borders. Embodiments of the invention provide an imaging system that automatically detects a border at a location within a vessel in response only to navigational input moving the image to that location. In some embodiments, systems and methods of the invention operate such that when a doctor moves an imaging catheter to a new location with in tissue, the system essentially instantly finds, and optionally displays, the border(s), calculates an occlusion, or both.