Generally, X-ray inspection systems radiate X-rays from one direction and take a planar (2D) X-ray transmission image. The same principle applies to medical X-ray machines and airport baggage screening. On the other hand, CT systems use computers to analyze X-ray images taken from multiple directions to generate three-dimensional (3D) X-ray images.
In a medical CT scanner, the X-ray tube and detector rotate around the human body to continuously generate slice images (tomographic images). In industrial CT systems, the inspected object is rotated to produce a tomographic image (CT image).　Cone-beam CT, which can produce multilayered tomographic images with a single rotation, is mainly used.