Practical x-ray scatter measurements for volume CT detector design

To help design a volume CT scanner, we measured x-ray scatter through large irradiated volumes, with and without detector collimator. An x-ray tube (125 to 150 kV) with an adjustable diaphragm irradiates volumes 25 to 200 mm thick. The scattering objects are water cylinders (approximate diameters 200, 300, and 500 mm). Complementary apertures (between the object and the detector collimator, along a line from the source) select "scatter" or "direct" detector signals. A direct-defining hole in a lead plate mounts over a pilot hole in a thin plastic sheet. With the lead plate removed, a scatter-defining plug fits into the pilot hole to block the same solid angle. We tested three styles of collimator: (1) Blades are two thick steel bars; the length changes and the spacing equals the detector aperture diameter. (2) In aluminum-interspaced lead grids, the spacing is smaller than the detector aperture. (3) Stacks are equally-spaced thin metal sheets, with gaps comparable to the detector aperture, and achieve low scatter at large aspect ratio. We show a useful design approximation: along the central beam line, for a given voltage and collimator, the scatter-to-direct ratio depends only on the irradiated volume.