The disclosure is directed to an apparatus and method for generating a two-dimensional back-projected image of a slice of an object, and also to an optical filtering apparatus and method which, inter alia, is useful therein. Successive beams of light are generated, each having a narrow elongated cross-section with an optical intensity that is a function of the density characteristic of the slice of the object as measured at a particular relative rotational angle. The beams are optically filtered and spread in a direction transverse to their length. Optical filtering with a filter function having both positive and negative components is preferably performed using a single-channel time modulated technique. A television camera receives the spread filtered beams. Relative rotation is effected between the beams and the camera, the rotation being related to the relative rotational angles of the object associated with the generation of the successive beams of light. A digital frame store accumulates the intensities sensed at elemental areas of the image sensing means. The accumulated intensities comprise a two-dimensional back-projected image of the slice of the object which can then be displayed on a television display. In a further form of the disclosure, back-projected images are obtained using an optical "loop" processor which employs a single-channel time modulated filtering technique.