The invention relates to X-ray scanning and in particular to X-ray scanning wh ich uses mu ltiple plane images to generate a three-dimensional representation of an object. The invention may be used in particular in the field of baggage scanning.

Typical baggage scanning of bags at airports uses a primary X-ray scanner located on a baggage transport system that draws baggage through the X-ray scanner continuously. Baggage is scanned by X-ray, essentially scanning a 2 dimensional slice through the baggage. The operator identifies and separates a small percentage of baggage for further secondary scanning and allows the rest to pass through. Secondary scanning can take place using a number of different technologies. However, typical ly secondary scann ing takes place on single items of baggage that are not on a conveyor, but which may be loaded into a machine and are stationary during secondary inspection. This allows the use of more advanced and accurate inspection techniques at the expense of speed - it is not practicable to check more than a small fraction of baggage in this way.

One method of analysis is a method known as CT scanning (computed tomography), wh ich is used in particular for med ical appl ications. This involves taking a number of two-dimensional sections through the sample through a full 360° from which a three-dimensional representation is calculated. This is typically achieved by rotating an X-ray source and detector around the sample to be imaged while moving the sample linearly along the axis of rotation. The combined motion is therefore helical. Accurate recordal of the position of the sample, source and detector is necessary to ensure accurate calculation of the image.

Such CT scanners are large, bulky and extremely expensive. They are as a result generally not preferred for use in baggage scanning. Where they are used, they are used for secondary inspection in view of the low throughput and the need for accurate alignnnent of the sample and detectors.

One patent, US 7,319,737, suggests the use of a laminographic or tomosynthesis method for baggage scanning. Unlike CT scanning, in which images are ta ken from essential ly al l ang les, the method known as tomosynthesis takes images from a limited range of angles and calculates a three-dimensional representation from the scans. A problem not addressed by this patent is however the al ignment of the images. Tomosynthesis requires very accurate alignment of the images taken at different angles. If it is intended to take images of a sample at different times, then the motion of the sample needs to be exactly controlled so that the exact motion of the sample between images is known in order to be able to accurately combine the images to build up a three-dimensional representation of the sample.

This means that the system proposed in US 7,31 9,737 does not appear suitable for use with conventional baggage handling conveyor systems as already installed in airports and similar locations, since the motion of baggage along such conveyor systems is not sufficiently controlled to accurately build up a three-dimensional representation. Instead the system would require a much more accurate conveyor of the type conventionally used in CT scans Summary of Invention

According to an aspect of the invention there is provided a system according to claim 1 . By combining the visual images and the X-ray images, the visual images can be used to locate exactly the position and orientation of the object to be imaged as it travels along the conveyor. This allows a three-dimensional representation of the X-ray image to be calculated. According to another aspect of the invention, there is provided a method according to claim 10.

Brief Description of Drawings

Embodiments of the invention will now be described , purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 illustrates a top view of objects moving along the conveyor; and Figure 2 shows a detail view of baggage moving around a corner of the conveyor.

The drawings are schematic and not to scale.

Detailed Description

A baggage conveyor 2 of the type used to carry baggage 8 in an airport or other facility takes baggage along a predetermined path. Such paths are almost never exactly straight and pre-existing baggage conveyors thus typically include several bends or corners 4.

A baggage inspection system 10 is mounted at a corner 4 along the conveyor. A 90° corner is selected in the embodiment described since such corners frequently occur. Thus, it is general ly possible to locate the baggage inspection system 10 around a pre-existing corner in baggage conveyors. However, it is not essential to use a 90° corner - all that is required is that the motion of the baggage around the corner allows a sufficient variety of images to be captured to carry out the method. A 180° corner is particularly suitable, though it occurs less often. The geometric centre 6 of the corner is indicated.

The baggage inspection system 10 includes a wide angle X-ray source 12, i.e. without a collimator, which emits X-rays over a wide angle, preferably at least 40°, or further preferably at least 55°. The source is arranged away from the centre 6 of the bend in the baggage conveyor but on the same side of the conveyor as the centre 6. A plurality of line X-ray detectors 14 are arranged on the opposite side of the conveyor to the X-ray source such that as baggage moves along the conveyor round the corner plane images defined by the X- ray source and the line of each detector are taken as the baggage moves.

As the baggage 8 travels along the conveyor round the corner 4 it will rotate, and so the motion of the baggage is not perfectly defined by the geometry of the conveyor to a level of accuracy sufficient for tomosynthesis.

The baggage inspection system 10 includes a camera 18 mounted above the conveyor to record the motion of baggage along the conveyor.

A controller 20 is connected to the X-ray source 12, X-ray detectors 14 and camera 18. The controller 20 includes an image recognition and analysis module 22 which in combination with camera 1 8 forms a visual recordal system which determines both the position and orientation of the baggage 8 moving along the conveyor.

The position, i .e. location and orientation , of the baggage is recorded continuously together with the image signal from the camera. An image reconstruction mod u l e 24 th en con stru cts a th ree-dimensional X-ray representation of the baggage.

In the embodiment described, the controller 20 is a computer, with memory and a processor, in combination with code forming the image recognition and analysis module which is arranged to cause the computer to determine the position and orientation of the baggage 8 moving along the controller as well as code form ing the reconstruction module 24 to construct the three- dimensional X-ray representation of the baggage.

However, the image recognition and analysis module 22 and the image reconstruction module 24 may both be implemented in hardware or software.

In the embodiment described a sum and add algorithm is used to construct the three-dimensional representation. However, alternative algorithms may be used such as a simulated algebraic reconstruction technique. See for example Dobbins I II, J .T., and D.J . Godfrey. "Digital x-ray tomosynthesis: current state of the art and clinical potential." Physics in Medicine and Biology, 2003: R65-R106, and Van de Sompel , D. , and M . Brady. "Systematic Performance Analysis of SART as Applied to Digital Breast Tomosynthesis." In Digital Mammography, 561 -569. Berlin / Heidelberg: Springer, 2008

As the baggage 8 passes through the baggage inspection system 10 images of the baggage are taken at different orientations, with respect to the baggage of at least 20°, preferably at least 40°, as typical baggage moves through the apparatus as illustrated in more detail in Figure 2. Note that the baggage rotates somewhat as it moves, but because of the off-centre location of the source 12 a variety of images are taken at different angles with respect to the baggage a l l owi n g effect ive co n stru ctio n of th e th ree-dimensional representation to take place.

By providing a visual baggage location system the need to use a specially accurate conveyor is avoided so the system according to the invention can be mounted on existing conveyor systems. This greatly reduces the cost of installing the system.

In a specific implementation, two PaxScan detectors 14 were used as the detectors and a tungsten target X-ray source 12 used as source 12. PaxScan detectors are full two-dimensional detectors. In an alternative embodiment, multiple cameras 18 are provided.

In some arrangements, it may be possible to use multiple relatively low cost cameras 18, for example web-cameras, and the combined images will allow the image recognition and analysis module 22 to identify the positions of the baggage, especially the edges of the baggage, at least as effectively and in many cases more effectively than using a single higher cost camera.

In particular, multi-station convergent photogrammetry may be used to combine the images from multiple cameras 18 and locate the edges of objects on the conveyor. This can establish real-time tracking of objects to detect differential motion and also register the position and orientation of the objects to accurately define their topography. Even using low-cost cameras, mm scale resolution may be achieved.

Further, the number of detectors that are required may be varied. Since the baggage moves with respect to the detectors, in some cases only one detector may be required, with multiple images captured at different times as the baggage moves through the detectors. Alternatively, multiple detectors may give a greater variety of images and hence improved tomosynthesis reconstruction. The detectors may be line detectors, with a plurality of pixels arranged along a line, the image being built up over time. However, to capture more data for easier image reconstruction, full two-dimensional detectors with a two-dimensional array of pixels are preferred.

Although the embodiments described above relates to an airport baggage conveyor, the invention is not limited to baggage as the object to be detected, but may use any objects that may be placed on a conveyor and imaged. The system may therefore be useful also in industrial applications.