A system for detecting clinically-relevant features of a gastrointestinal (GI) tract of a subject. The system comprises an external magnetic field generator adapted to be affixed to a patient for generating at predetermined time intervals a magnetic field with respect to the subject. The magnetic field has a predetermined polarity and flux density, thereby establishing a patient coordinate system based upon the magnetic field. The system includes a capsule adapted to be swallowed by a subject. The system further includes at least one radiation source emitting X-ray or gamma radiation and at least one radiation detector. The radiation detector is configured to detect in a first energy window collimated X-ray fluorescence radiation from the X-ray contrast agent composition excited by the emitted radiation, and to detect a second energy window Compton-backscattered radiation from the X-ray contrast agent and the wall of the GI tract produced in response to the emitted radiation. The system includes a magnetic field detector configured to generate a first output responsive to a detected polarity and detected levels of flux density during exposure to the externally generated magnetic field. A processor generates a second output based upon relative changes in the first output, the first output varying as a function of relative changes in physical location or angular orientation of the capsule associated with movement of the capsule within the GI tract. A control unit configured to selectively enable or disable X-ray or gamma ray emissions from the capsule produced by the at least one radiation source based upon the second output, such that the GI tract is selectively irradiated upon detected movement of the capsule.
