Given the presently increasing threats of violence, the inspection of vehicles, including, but not limited to, luggage and cargo at transit points, has become almost universally mandatory. The screening of small and large objects is required to detect the presence of contraband such as explosives, weapons, narcotics and dangerous chemicals. Non-invasive inspection is typically achieved using X-ray scanning systems. Transmission based X-ray systems are traditionally used to inspect trucks and cargo containers, in particular when these may contain high-density materials and/or nuclear materials.
In backscatter-based systems, X-rays are used for irradiating a vehicle or object being inspected, and X-rays that are scattered back by the object are processed to provide images which help identify the presence of contraband. Typically, a backscatter imaging system utilizes a flying spot which is derived from a rotating collimator located close to a wide angle source of X-rays, such as a constant potential X-ray tube. As the collimator rotates, the resulting pencil beam of X-rays sweeps over the surface of the object to be inspected and interacts with the surface. Some of the X-rays backscatter away from the surface of the object and in the direction from which the X-rays originated. Some of the X-rays will penetrate, and pass through, the object. Those X-rays that backscatter away from the object may be captured by X-ray detectors which are located at some distance from the surface of the object. Using trigonometry, it is possible to form a one-dimensional image of the backscattered signal intensity, assuming that the surface of the object is flat. If motion is introduced between the X-ray system and the object under inspection, a two-dimensional image may then be generated
In an alternative configuration, a fan beam of X-rays may be used to irradiate a whole line across the object, and a combination of collimators and detectors can be used to capture the backscatter image.
In another alternative configuration, a multi-focus X-ray source whose individual source points are arranged in a linear array may be placed behind a parallel collimator array such that each source point is constrained to irradiate only a small portion of the object under inspection. By sequencing the emission from each X-ray source point in turn, the effect of sweeping the X-ray beam across the object is generated and the resulting X-ray backscatter signal may be recorded in X-ray detectors which are located at some distance from the surface of the object. In each configuration, a constant potential X-ray source is utilized with the imaging signal being purely the magnitude of the backscattered X-ray signal at the X-ray detector.
All the above mentioned configurations of known X-ray backscatter imaging systems are based on the assumption that the objects being screened are planar or straight sided. Also, the backscatter signal has a dependence on the distance, r, which scales approximately as 1/r4. Therefore, in order for an X-ray backscatter system to produce a distortion-free image, the photon detector must be equidistant from all portions of the object being scanned. Thus, such systems are well-adapted for producing images of trucks or railcars, which generally have vertical sidewalls.
They are not, however, as well suited for scanning aircrafts or even cars which have oval or elliptical cross-sections because, as a collimated beam scans an oval or cylindrical surface, some portions of the scanned surface are located closer to the photon detector(s) than other portions. The variations in distance from the detector(s) produce distortions in the backscattered image. For the same reason, producing accurate and distortion-free images when screening luggage (such as on a conveyer system) and even when screening people, remains a challenge for backscatter X-ray imaging systems.
Therefore, what is needed is an inspection system that produces distortion free images for accurately determining the presence of concealed illegal materials in different types of objects, such as luggage, cargo and vehicles. Such a system should also be capable of providing both depth and material type information for an object being scanned. There is also a need for the inspection system to be mobile and non-intrusive, with an ability to work in various orientations, scanning ranges, and fields of view to suit different kinds of inspection applications.