An x-ray source emits a cone beam a rapidly rotating, x-ray-opaque disc with four narrow radial slots. The slots break the cone beam into fan beams that are emitted to an x-ray-opaque plate with a narrow slit. As each fan beam moves across the plate, the slit produces a scanning x-ray pencil beam. The backscatter detector is mounted adjacent to the plate and has a slightly larger slit that is aligned with the plate slit. The pencil beam enters the object space through the detector slit. The pencil beam moves rapidly in a line across the object space, 20 cm in 0.1 second. Simultaneously, the assemblage of x-ray source, disc, plate, and detector moves slowly in the x direction at 1 mm in 0.05 second. Thus, the raster scan of the 20 cm×20 cm region is accomplished in 10 seconds.