The use of scattered X-ray radiation for detection of concealed objects has been taught in many patents. It is well known that X-ray detectors may be placed at any position with respect to the object under scrutiny in order to capture photons scattered through the Compton process by the medium under scrutiny. The various placements of X-ray detectors around the object correspond to forward scatter, backscatter, and side scatter detectors, as the case may be. Since the number of X-ray photons scattered into a given solid angle is small, it is often desirable to capture scattered photons from a large solid angle in order to increase the sensitivity of the detection process. The use of large-area detectors, however, requires the implementation of stratagems in order to determine the precise origin, within the enveloping surface, of the detected photons, i.e., to spatially resolve the shape of the object under scrutiny even if it cannot be observed directly because it is contained within an enveloping surface.
One method of achieving spatial resolution has been to illuminate the object under scrutiny with a "pencil beam" or "flying spot" of X-ray radiation that is scanned with respect to the object in some sort of raster pattern. Obviously, motion of the object, as on a conveyor belt in one direction, is equivalent to a corresponding motion of the X-ray beam. Using this stratagem, only a particular position in a plane transverse to the propagation axis of the X-ray radiation is illuminated at a particular moment, so that an image of the entire scanned object can be reconstructed, but only after the scan is complete.
Using the raster-scanned flying-spot method, each position in the object is only illuminated for a small duration of time, since the entire area of the piece of luggage, for example, must be scanned as the luggage is conveyed on a conveyor belt across the path of the X-ray beam. Under these circumstances, sensitivity may be limited by noise since the system is typically "photon-starved." Spatial resolution is limited to the size of beam, at best, though actual resolution is typically worse due to the necessity of spatial averaging. It is, thus, desirable to gain a multiplex advantage by simultaneously illuminating a larger fraction of the object than would be illuminated by a beam corresponding roughly to the size of a spatial resolution element. To gain this advantage, it is necessary to implement another stratagem for reconstructing an image of the scattering source.
Additionally, implementation of a pencil beam method may entail the weight and mechanical complexity associated with the use of a chopper wheel for moving a heavy lead aperture in order to shape a high-energy x-ray beam.