Abstract: A linear detector selectively detects successive line images from different angles while the linear detector and a radiation source are rotated about a patient. Radiation-transmission images are generated and detected and converted to computer-grabbed images by a computer which acquires, processes, stores and reconstructs the images. A visual monitor display reconstructs video images including a planar image and tomographic images representing sections orthogonal to the planar image. Selectively-positionable field-delineating lines corresponding to intersectional outlines between a simulation beam and the planar image are overlaid on the planar image. Beam-delineating lines are updated by the computer to correspond to intersectional outlines as the field-delineating lines are positioned.