The internal imaging chamber was a sensor device capable of reading an individual's internal physiology. Following the Dominion War, Starfleet established a standard protocol by which all personnel were required to pass through such a chamber before reporting for duty, to verify that they had not been replaced by a Changeling.
In 2401, the USS Titan crew discovered that a Changeling agent had fully mimicked internal organs of their target Sidney La Forge, confirming that such infiltrators could now fool internal imaging chambers. (PIC: "Imposters")