Abstract: In a CT scanner having a rotating source of radiation and a series of stationary radiation detectors spaced about the axis of rotation of the source, an eclipsing shutter mechanism restricts the width of the diverging fan pattern of radiation to that portion of the patient scan circle which is intermediate the array of stationary detectors and the orbiting source. The eclipsing shutter may include a single elongated slit aperture for flooding the scan circle with radiation or a plurality of smaller apertures defining a collimator for continuously training each one of a plurality of discrete diverging beams in the fan pattern on a single stationary detector during rotation of the source for as long as the separate beam intersects the scan circle. An epicyclic gear train powered by the rotation of the source drives the shutter mechanism in a counter-rotation and causes the collimator to keep each beam aimed at a respective detector.