Apparatus (40) is provided, including a capsule (50), adapted to be swallowed by a subject (54), the capsule (50) including (a) at least one radiation source (60), adapted to emit radiation having anenergy of at least 10 keV, and (b) at least one photon detector (62), adapted to detect photons generated responsively to the emitted radiation, the photons having an energy of at least 10 keV. The apparatus (40) additionally includes a control unit (64), adapted to analyze data regarding the photons in order to generate information useful for identifying a clinically-relevant feature of a gastrointestinal (GI) tract (72) of the subject (54).