A capsule is coupled to a tether that is manipulated to position the capsule and a scanner included within the capsule at a desired location within a lumen in a patient's body. Images produced by the scanner can be used to detect Barrett's Esophagus (BE) and early (asymptomatic) esophageal cancer after the capsule is swallowed and positioned with the tether to enable the scanner in the capsule to scan a region of the esophagus above the stomach to detect a characteristic dark pink color indicative of BE. The scanner moves in a desired pattern to illuminate a portion of the inner surface. Light from the inner surface is then received by detectors in the capsule, or conveyed externally through a waveguide to external detectors. Electrical signals are applied to energize an actuator that moves the scanner. The capsule can also be used for diagnostic and/or therapeutic purposes in other lumens.