Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example.
To obtain additional information about the contents of the luggage and other objects, detectors may be provided to detect scattered radiation, as described in U.S. Pat. No. 5,642,394, for example. Systems may combine detection of scattered radiation with the detection of transmitted radiation.
Another technique to enhance the information that may be derived about the composition of the contents of an object is to scan the object with radiation beams having two different energy distributions. A ratio of the attenuation detected at two energy levels is indicative of the atomic numbers of the material through which the radiation beam passes. Dual energy systems enable better detection of plastic materials and illegal drugs, for example.
One disadvantage of radiographic imaging is that all items within the object in the path of the radiation beam are superimposed on the image. If there are many items in the object, it may be difficult to distinguish among them. The identification of dangerous items is thereby hampered. In addition, the orientation and shape of the items within the object could affect whether they can be identified on a radiograph. Thin sheets of explosive materials may also be difficult to identify on a radiograph, particularly if they are oriented perpendicular to the scanning beam.
Computed tomography (“CT”) enables the reconstruction of the cross-sectional images of the contents of an object, facilitating the identification of the items in the luggage. CT images also provide higher resolution, greater image contrast and greater sensitivity to characteristics of the object being scanned, than radiographs. However, reconstruction of CT images of an object requires a large number of scans of the object at a plurality of angles. Conducting a sufficient number of scans for CT reconstruction is time consuming. Depending on the system used, CT imaging of an entire piece of luggage may be too slow for practical use in screening luggage in airports, for example.
In U.S. Pat. No. 5,567,552 (“the '552 patent”), a source of X-ray radiation is provided on one side of an inner surface of a rotating module and a detector array is provided on the opposite side. Luggage is moved through the module incrementally. The module rotates to scan the luggage at a plurality of angles, at each incremental position. The inspection speed may be increased by pre-screening with a line-scan. Then, only suspicious regions identified by the prescreening step are subjected to CT imaging. U.S. Pat. No. 6,078,642 (“the '642 patent”) discloses a CT scanning system for luggage similar to the '552 patent, where data processing techniques are used to speed the inspection rate to scan the entire piece of luggage, without requiring pre-scanning. The module rotates as a piece of luggage is continuously moved through the module, providing helical volumetric CT scanning.
U.S. Pat. No. 5,410,156 discloses an explosives detection system for scanning luggage in airports including a neutron radiation source on one side of an object and a two dimensional detector array on the opposite side of the object. The object is supported on a rotatable platform. Rotation of the platform during scanning enables optional tomographic imaging of an object on the platform, to create three dimensional distributions of hydrogen, carbon, nitrogen and oxygen per a cubic volume through the sample. The ratios of these elements are determined for small volume increments of the sample. Neural net methods are used to determine whether a volume increment contains an explosive.
While the smuggling of contraband, such as guns and explosives, onto planes in carry-on bags and in luggage has been a well known, ongoing concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers. Only a small proportion of the cargo containers brought to the United States by boat are inspected, for example. “Checkpoint terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting sidewalls, may be of comparable sizes as cargo containers, at least when supporting objects. The term “cargo conveyance” is used to refer to all types of cargo containers and comparably sized pallets (and other such platforms) supporting objects.
In contrast to the size ranges of cargo conveyances, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40×0.60 meters. Scanning systems for checked luggage have travel openings that are only slightly larger. Since only bags that fit through the tunnel may be inspected, such systems cannot be used to inspect cargo containers. The low energies used in typical X-ray luggage and bag scanners, described above, are also too low to penetrate through the much larger cargo containers. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.
It is known to inspect cargo containers supported by vehicles, such as trucks, by mobile systems. For example, in U.S. Pat. No. 5,638,420, large containers are inspected by a system on a movable frame. A source of a fan beam, a cone beam or a pencil beam of X-ray radiation, such as a linear accelerator with an accelerating potential in the MeV range, is mounted on one side of the frame. A detector array is mounted on an opposing side of the frame. The frame may be self-propelled and advances across the length of the container. Radiographic images are generated for analysis by an operator. Other mobile systems are disclosed in U.S. Pat. Nos. 6,292,553 and 5,917,883, for example.
The high resolution, improved image contrast and the ability to distinguish small differences in characteristics of items within an object that are provided by CT scanning would be advantageous in the inspection of cargo conveyances. The CT scanning units used in airports for luggage and the like discussed above are not readily scaleable to the large sizes required to scan cargo containers. For example, to accommodate most cargo conveyances, the rotating modules of the '552 patent or the '642 patent would need to be greatly enlarged. Such large rotating units, carrying both the sources and the detectors, would be very expensive and would be difficult to operate and maintain.
U.S. Pat. No. 6,628,745 B1 discloses a CT system for cargo containers comprising a rotating and vertically displaceable platform to support the container. An electron beam is magnetically deflected along a linear target, as the container is rotated, to generate a radiation beam that moves along the target to scan a slice of the container. CT images may be generated from the detected radiation. An elevator then moves the platform vertically and the container is scanned again to form CT images of other slices.
FIG. 1 is a front view of an example of a detector module 1 known in the art. In this example, the detector module 1 comprises a housing 2, a detector element section 3 within the housing and an electronics section within the housing. The housing 2 defines an open window in front of the detector element section 3 for radiation to pass through. Behind the window are detector elements 4. The electronics sections 5, 6, 7 comprise digital drivers in sections 5, 6 and analog readouts in section 7. In some detector modules, the digital drivers may only be provided on one side of the module, in section 5, for example. In use, shielding material (not shown) is placed in front of the electronics sections 5, 6, 7 to protect the electronics from damage by radiation. However, the shielding in front of the electronics sections 5, 6, 7 may not be sufficient to protect the electronics at the high levels of radiation, such as at 6 MeV and above, necessary to penetrate through larger cargo conveyances.
Despite the various designs for the inspection of large objects such as cargo containers disclosed in the patents discussed above and in other references, much of the inspection of cargo conveyances is done manually, if at all. “Checkpoint terror”, U.S. News and World Report, Feb. 11, 2002, p. 52. Practical, efficient, non-intrusive CT radiation scanners for the inspection of large objects, such as cargo conveyances, are still needed. Improved radiation scanners for the inspection of smaller objects, such as luggage, including improved CT imaging of smaller objects, are also needed.