Apparatus, including a probe having a distal end insertable into a nasal sinus of a human patient, and a location sensor positioned within the distal end. A sinuplasty balloon is positioned on the distal end at a selected opening of the nasal sinus. A processor receives first signals from the location sensor while the distal end is inserted into the nasal sinus and prior to positioning of the balloon at the selected opening, and generates a first map of the sinus. The processor inflates the balloon when it is at the selected opening, so as to enlarge the selected opening, and subsequently deflates the balloon. The processor then receives second signals from the location sensor and generates therefrom a second map of the sinus. The processor registers the first map with the second map and generates from the registered maps a numerical increase in size of the selected opening.