1. Field of the Invention
The embodiments described herein relate generally to inspection of containers and, more particularly, to producing images of at least one object of interest in a container to facilitate detecting objects within the container.
2. Description of Prior/Related Art
At least some known inspection systems construct an image of a container and analyze the image to detect explosives, drugs, weapons, and/or other contraband objects within the container. Some known inspection systems, including some known carry-on and checked baggage inspection systems, use a computed tomography (CT) scanner to produce images of the interior of the container. In some known CT systems, an X-ray fan beam source and a detector array are disposed opposite each other in a gantry. The gantry is rotated around the container such that the angle at which the X-ray fan beam intersects the container constantly changes. The detector array acquires a “view,” or group of X-ray attenuation measurements, at each of multiple gantry angles. A “scan” of the object includes a set of views made at different gantry angles, or view angles, during one revolution of the X-ray source and detector about the container.
At least some known CT systems use three-dimensional (3-D) volumetric scanners to acquire CT scan data representing an entire target volume. One example of a 3-D volumetric CT scanner is a helical CT scanner, in which the container is continually moved substantially parallel to the axis of gantry rotation while the X-ray attenuation data is being acquired, such that the path of the X-ray source defines a helix with respect to the container. Three-dimensional representations of the entire volume scanned by a 3-D volumetric scanner can be reconstructed using well known tomographic reconstruction algorithms, for example, direct Fourier or filtered back-projection methods.
At least some known inspection systems also include a prescanner to produce a two-dimensional scan projection (SP) image of the container for presentation to an operator for inspection. Some known prescanners operate by moving the container under a fan beam of X-rays from a stationary X-ray source. X-ray intensities, after being attenuated by the container and the objects within it, are measured by a stationary array of detectors. Such prescanners may use a separate stationary X-ray source and detector array, or they may employ a primary 3-D volumetric CT scanner in a stationary prescanner mode to produce two-dimensional SP images before conducting a helical scan. Inspection system operators typically receive extensive training, and/or have accumulated extensive experience, in recognizing certain types of objects in a two-dimensional SP image. As a result, for at least some inspection system operators, inspection of an SP image, rather than of a full three-dimensional representation from the CT scan, increases a speed and accuracy of the identification of certain types of objects.
The use of a dedicated prescanner also may provide other benefits. For example, at least some known prescanners provide dual energy scanning of the container. Collecting data for a low-energy scan and a high-energy scan allows such inspection systems to reconstruct, for example, a density image and/or atomic number image of the contents of the container to facilitate identification of objects and materials in the container.
Unfortunately, the use of a prescanner also may increase a cost and complexity of the inspection system, for example by requiring a separate stationary X-ray source and detector array, or by requiring additional hardware and software in the 3-D volumetric CT scanner to support a stationary prescanner mode. The time required to conduct the prescan also adds to the overall time required for scanning a container. Some known inspection systems avoid these potential drawbacks by forgoing a prescanner, instead re-projecting the 3-D volumetric CT scan data into two-dimensional images similar to SP images from a prescanner. However, in at least some such systems, a resolution of the re-projected two-dimensional SP-type images is less than a resolution of the SP images produced by a prescanner, and/or less than a resolution of the original three-dimensional CT representation. This reduced resolution may cause objects of interest identifiable from the three-dimensional CT scan data, such as thin wires, not to be visible in the re-projected two-dimensional image.
For example, FIG. 3 is a right perspective view of certain representative objects of interest that may lie within a container 18. In FIG. 3, container 18 and any non-numbered objects within container 18 are rendered transparent for ease of viewing. Container 18 might include objects of interest such as, for example but not by way of limitation, a thin wire 60 between two otherwise non-descript objects 61 and 62, a plastic explosive material 64, and/or overlapping objects 66 and 68.
FIG. 4 shows a prior art two-dimensional SP-type image that might be obtained using a suitable method for re-projecting an SP-type image from data obtained from a 3-D volumetric scan of container 18 containing representative objects of interest 60, 64, 66 and 68, as shown in FIG. 3. Alternatively, an image such as that illustrated in FIG. 4 might be obtained using a suitable method for re-projecting an SP-type image from a three-dimensional representation of container 18 that was in turn generated from 3-D volumetric scan data from a scan of container 18.
With reference to FIGS. 3 and 4, otherwise non-descript objects 61 and 62 appear in the SP-type image as disconnected areas 71 and 72, with no indication of thin wire 60, although thin wire 60 would have been detected in the inspection of the three-dimensional image representation. Similarly, plastic explosive 64 appears as a non-descript region 74 in the SP-type image, although in certain embodiments a multiple-energy scan by inspection system 10 would have detected a material comprising plastic explosive 64 as a material of interest. In addition, overlapping objects 66 and 68 appear as a single unified region 76 in the SP-type image, although the presence of separate objects 66 and 68 would have been detected in the inspection of the three-dimensional image representation.