A scanning flexible endoscope includes a tether attached to a capsule. The tether controls a disposition of the capsule within a body lumen such as the esophagus. A scanning device in the capsule optically scans the adjacent tissue on the inside surface of the body lumen as the capsule is moved axially through the body lumen. A non-contact sensor responds to indicia on the tether to measure a relative position of the tether and the capsule in the body lumen. The indicia can be analog or digital in form and the sensor can be either magnetic, or optical. A wiper is optionally provided to remove bodily fluids from the tether when pulled from the body lumen past the sensor. A pulse of fluid can be delivered to the distal end of the tether to cause the body lumen to distend, facilitating free movement of the capsule in the lumen.
