Abstract: A system and method are disclosed for sensing and controlling the operation of a transaxial reconstruction tomographic apparatus having a movable radiation source for directing radiation through a subject and a radiation detector movable synchronously with the source in an orbital path about the subject to detect radiation emerging from the subject.The system includes circuitry and apparatus including an optical incremental encoder for producing index signals indicating when the detector passes predetermined locations in its orbital path, a frequency multiplier is responsive to the encoder for producing other more frequent signals indicating by interpolation the relative location of the detector with respect to the locations delineated by the index signals. Processing circuitry produces signals defining the boundaries of segments of the orbital scanning path, according to a scan segment magnitude and orientation indicated by the computational apparatus.