The present invention relates generally to x-ray imaging systems such as backscatter imaging systems, and specifically to controlling the sensitivity and range of such systems as a function of the flux incident during inspection of specified portions of an inspected object.
Current x-ray imaging systems typically make use of a relatively wide angle fan beam exiting an x-ray generator, typically an x-ray tube, formed, in some cases, into a scanning pencil beam by means of a chopper wheel, or otherwise. One such inspection system is depicted in
In typical current practice, the angular coverage of the x-ray beam is determined by the angular extent of the x-ray beam as it exits the x-ray generator, in combination with any subsequent collimating structure. For example, a subsystem that may pick off a pencil-shaped portion of a fan beam produced by the x-ray generator and direct this pencil beam toward the object under inspection, scanning it, point by point, typically in a vertical direction. “Pencil-shaped,” as used herein, refers to a beam having any cross-sectional shape, the extent of each dimension of the cross-section, transverse to the beam propagation direction, being comparable, though not necessarily equal. For example, a tube with a 60 degree wide x-ray beam is able to subtend a maximum angle of 60 degrees. Thus, at a distance of 8 feet away from the tube focal spot, the beam is able to scan an object that is approximately 14 feet high. Moving the object farther away enables taller objects to be completely covered, although the increased distance leads to lower x-ray flux. “Flux,” as used herein and in any appended claims, refers to either the number or total power of x-ray photons crossing a unit cross-sectional area in a unit of time.
Embodiments of the present invention are directed to inspection systems designed for inspecting an object, where the object of inspection may include a person. Preferred embodiments of the invention have a source of penetrating radiation that emits penetrating radiation and at least two primary limiting apertures: a first primary limiting aperture for defining a first field of view of the emitted penetration radiation, and a second primary limiting aperture for defining a second field of view of the emitted penetrating radiation. A spatial modulator forms the emitted penetrating radiation into a beam for irradiating the object with a scanning profile. A mechanism is provided for interposing one of the limiting apertures between the source and the spatial modulator, either on a fixed or contingent mode. In the contingent mode, one primary limiting aperture is used under a first set of conditions and a second primary limiting aperture is used under a second set of conditions.
In further such embodiments, the source of penetrating radiation may be contained within a conveyance that may be a vehicle capable of road travel or may be towed by another vehicle.
In various embodiments, the source of penetrating radiation may be an x-ray tube. Alternatively, the source of radiation may be a radioactive source. The spatial modulator may include one or more rotating chopper wheels.
A description of embodiments of the present invention begins by reference to
Simultaneously with the scanning of exiting x-ray beam 24, either the entire x-ray system, with or without its detectors 20, is moved in a direction transverse to the beam scan direction, or the object under inspection 5 is itself moving past the x-ray system. The combination of the x-ray beam motion and the moving of either the source, or of the object, or both, gives rise to a two dimensional image 6 (represented by an image of a rooster) of the object under inspection.
Preferred embodiments of the present invention provide an operator of the inspection equipment with an option to reduce the field of view at a given object distance. At the same time the x-ray flux is increased within this smaller field of view. Because of the increased flux, detectability of potential objects of interest may be improved in the selected reduced field of view. Such embodiments may be particularly useful when an operator would like to further investigate a potential object of interest that may be poorly defined due to an x-ray flux level which renders a potential object of interest barely visible.
The operator may, while pointing a cursor at the full field-of-view (“FOV”) image, click on the center of the portion he would like to enhance, and re-institute another scan. The new scan has a much smaller angular FOV and an x-ray flux that is higher than the original scan by approximately the same factor by which the angular FOV was reduced. For example, if a first scan subtended an angle of 60 degrees, the second reduced-FOV, or zoomed scan may subtend an angle of 6 degrees centered on a particular portion of the object. The x-ray flux may also be increased by a factor of ten times over this reduced FOV, thereby leading to higher detectability. Reduction of the FOV to less than 10 degrees constitutes a “Zoom Mode” for purposes of the present invention. For an object at a fixed distance from the x-ray source, the pixel size, and thus the perceived resolution, would be unchanged. Alternatively, the chopper wheel aperture size could be changed for improved resolution.
In order to overcome the problem of degraded resolution with increasing range, it is desirable, in certain applications, to employ tighter collimation. Tighter collimation, however, may ultimately require a smaller focal spot size source. In accordance with alternate embodiments of the invention, operation in a mode of reduced field-of-view employs a source of relatively small focal spot size, such as provided, for example, in rotating-anode x-ray tubes.
In another embodiment of the invention, the use of a smaller angular field of view, together with the higher flux inherent from the improved zoom technique, may be used to extend significantly the range from which objects of interest can be detected. But the pencil beam emerging from the moving collimator is continually expanding and the backscatter detectors must be sufficiently large to capture as much backscattered radiation as possible. As a result, system resolution degrades as distance from source to object is increased. Therefore, although the use of the zoom feature will increase range by increasing beam flux on target, resolution will be degraded to some extent. This can be compensated for by reducing the aperture size that defines the incident pencil beam. But this will also reduce flux, and thus there are tradeoffs between the flux gain from the reduced field of view and the flux reduction that would result from narrowing the aperture that defines the pencil beam. However, generally speaking, pencil beam sizes that are of the order of as much as one inch in diameter, or one inch square, will provide adequate resolution for the threats envisioned. This implies that, in almost all circumstances, the flux gain resulting form incorporating the zoom feature will overwhelm any flux reduction required to keep resolution reasonable, at distances up to 100 feet or so.
The scanning technique described above requires two separate scans by the operator, one under “normal” flux and field-of-view conditions, and another under zoom conditions that would require a change in the collimating aperture geometry. In yet another embodiment, a single scan system may always be in zoom mode. In other words, it may be elected to use such a system with a single aperture geometry always set to a particular scan configuration that employs a narrow FOV. Such a system may be designed to be used for either more detailed inspection of smaller objects within the same range as current systems, or, alternatively, as a system capable of longer range scanning, with a range of up to 100 feet. A system such as this, if designed for longer range scanning, may employ an x-ray tube with substantially more power, or higher voltage, or both, than is currently used for backscatter scanning.
It is to be understood that the system described in the foregoing discussion may be fixed in position, or, alternatively, may be carried in a vehicle and may be operated during the course of travel of the vehicle. Operation of an x-ray inspection system under these conditions has been described in patent applications filed by American Science and Engineering, Inc.
Although the invention has been described in detail in terms of various exemplary embodiments, it should be apparent to those skilled in the art that various changes and modifications can be made, such as the application of the above teachings in the context of transmission- or sidescatter-based inspection, all of which modifications will achieve some of the advantages of the invention without departing from the true scope of the invention.
The present application claims priority from U.S. Provisional Application Ser. No. 60/664,278, filed Mar. 21, 2005, which application is incorporated herein by reference.
Number | Date | Country | |
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60664278 | Mar 2005 | US |