X-ray imaging with coded masks.

How can one image objects from which high-energy X rays emanate Actually the imaging is made possible by a technique that is similar to the one applied in making medical X-ray photographs. To make such photographs a film with an X-ray sensitive emulsion is placed directly behind a body part that is irradiated from the front by a small point source of X rays. The film in effect records the shadow cast by those components of the part that absorb X rays. A somewhat old-fashioned word for the technique is skiagraphy, from the Greek skia, meaning shadow. Now suppose the X rays come not from a point source such as a medical X-ray tube but from an extended source of unknown shape-perhaps a huge cloud of hot intergalactic plasma. Because the extended source can be envisioned as consisting of numerous point sources, the shadow of any intervening object is blurred. The reason is that each component point source causes a slightly different shadow to be cast on the film. Yet if one knows the shape of the intervening object precisely, one can easily predict the form of the shadow it would cast when illuminated by a single point source ofmore » X rays. By comparsing the shadows produced by all possible combinations of point sources with the actual recorded shadow, it might then be possible to reconstruct the shape of the extended source. That, in fact, is the operating principle of coded-mask X-ray imaging. The known object between the X-ray source and the detector is a coded mask: an X-ray-opaque plate that has a pattern of holes. The key to this type of imaging lies in the selection of a pattern that enables one to reconstruct an image of the X-ray source from the form of the mask's shadow.« less