CN102269826B - X-ray tomography inspection system - Google Patents

Abstract

The present invention relates to X-ray tomography inspection system.A kind of x-ray imaging for checking article checks system, and including x-ray source (10), this x-ray source extends around imaging volume (16) and limits the multiple source points (14) that can therefrom guide X-ray to pass imaging volume.X-ray detector array (12) also extends around imaging volume (16), and is configured to detect the X-ray from source point through imaging volume, and produces the output signal depending on the X-ray detected.Transmitter (20) is configured to transporting articles to pass through imaging volume (16).

Description

X-ray tomography inspection system

The present application is a divisional application entitled "X-ray tomography examination system" with an international application date of 2006, 12/15/2006 and an application number of 200680051488.4.

Technical Field

The present invention relates to X-ray scanning. It has particular application in the security screening of luggage, parcels and other suspicious objects, and it is equally applicable to other suitable applications.

Background

X-ray Computed Tomography (CT) scanners have been used for several years in security screening in airports. Conventional systems include an X-ray tube that rotates about an axis, and an arcuate X-ray detector that also rotates about the same axis at the same speed. The conveyor belt on which the luggage is carried is placed in a suitable hole located near the central axis of rotation and moves along this axis as the tube rotates. A fan-beam of X-radiation passes from the radiation source through the object to be examined to the X-ray detector array.

The X-ray detector array records the intensity of X-rays passing through the object to be examined at several locations along its length. A set of projection data is recorded at each of a number of source angles. From these recorded X-ray intensities, a tomographic (cross-sectional) image can be formed, typically using a filtered back projection (filtered back projection) algorithm. To produce accurate tomographic images of an object, such as a bag or a package, it may be indicated that the X-ray source is required to penetrate through each plane of the object. In the above described configuration, this is achieved by the rotational scanning of the X-ray source and the longitudinal movement of the conveyor on which the object is carried.

In this type of system, the rate at which tomographic scans can be collected depends on the rotational speed of a gantry (gantry) holding the X-ray source and detector array. In modern CT gantries, the entire tube-detector assembly and gantry will complete two to four revolutions per second. This allows up to four or eight tomographic scans to be collected per second, respectively.

With the development of the prior art, single ring X-ray detectors have been replaced by multi-ring detectors. This allows many slices (slices) (typically 8) to be scanned and reconstructed simultaneously using a filtered back-projection method adjusted from a single scanning machine. The radiation source describes a helical scanning movement around the object as the conveyor is continuously moved through the imaging system. This allows the application of more sophisticated cone-beam image reconstruction methods, which may in principle provide a more accurate volumetric image reconstruction.

In another development, swept electron beam scanners (swept electron beam scanners) have been demonstrated in medical applications, whereby the mechanical scanning motion of the X-ray source and detector is avoided, and instead a continuous ring of X-ray detectors surrounding the object under examination, and a moving X-ray source due to the sweeping of the electron beam around an arcuate anode. This allows images to be acquired more quickly than with conventional scanners. However, such swept electron beam scanners are not compatible with a conveyor system, which itself moves close to and parallel to the axis of rotation, because the electron source is located on the axis of rotation.

Disclosure of Invention

The present invention provides an X-ray scanning system for inspecting an article, the system comprising: an X-ray source extending around the scan volume and defining a plurality of source points from which X-rays can be directed through the scan volume; an X-ray detector array also extending around the scan volume and configured to detect X-rays from the source point that have passed through the scan volume and to generate an output signal dependent on the detected X-rays; and a conveyor configured to convey the item through the scan volume.

The present invention also provides a networked inspection system comprising an X-ray scanning system, a workstation, and a connection device configured to connect the scanning system to the workstation, the scanning system comprising: an X-ray source extending around the scan volume and defining a plurality of source points from which X-rays can be directed through the scan volume; an X-ray detector array also extending around the scan volume and configured to detect X-rays from the source point that have passed through the scan volume and to generate an output signal dependent on the detected X-rays; and a conveyor configured to convey the item through the scan volume.

The present invention also provides a sorting system for sorting articles, the system comprising: a tomography scanner configured to scan a plurality of scan areas of respective items to thereby generate a scanner output; an analysis device configured to analyze the scanner output and assign each item to one of a plurality of categories based at least in part on the scanner output; and sorting apparatus configured to sort the items based at least in part on the categories to which they have been allocated.

The present invention also provides an X-ray scanning system comprising: an X-ray source configured to generate X-rays from a plurality of X-ray source locations around a scan region; a first set of detectors configured to detect X-rays that traverse the scan area; a second set of detectors configured to detect X-rays scattered within the scan area; and a processing device configured to process the output from the first set of detectors to produce image data of an image defining a scan area, to analyze the image data to identify objects within the image, to process the output from the second set of detectors to produce scatter data, and to associate portions of the scatter data with the objects.

The present invention also provides a data collection system for collecting data from an X-ray scanner, the system comprising: a memory having a plurality of regions each associated with a respective region of the image; a data input device configured to receive input data from the plurality of X-ray detectors in a predetermined order; a processing device configured to generate X-ray transmission data and X-ray scatter data relating to each of the regions of the image from the input data and store the X-ray transmission data and X-ray scatter data in an appropriate storage area.

The present invention also provides an X-ray scanning system comprising: a scanner configured to scan the object to generate scan data defining a tomographic X-ray image of the object; and a processing device configured to analyze the scan data to extract at least one parameter of the image data and assign the object to one of a plurality of categories based on the at least one parameter.

The present invention also provides an X-ray scanning system comprising: a scanner, the scanner comprising: an X-ray source having a plurality of X-ray source units spaced apart around a scanning volume in a substantially circular configuration, each of the X-ray source units having target regions of two different materials and being configured to generate X-rays of two different energy spectra, and a detector configured to detect X-rays from each of the plurality of target regions to generate two respective scanner outputs, and a processing device configured to: processing signals from the detectors includes combining two respective scanner outputs to produce a plurality of tomographic image data sets, and combining the tomographic image data sets to produce a three-dimensional image of the object.

Drawings

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal section of a real-time tomography security scan system according to a first embodiment of the invention;

FIG. 1a is a perspective view of an X-ray source of the system of FIG. 1;

FIG. 2 is a plan view of the system of FIG. 1;

FIG. 3 is a schematic side view of the system of FIG. 1;

FIG. 4 is a schematic diagram of a data acquisition system forming part of the system of FIG. 1;

FIG. 5 is a schematic diagram of a threat detection system forming part of the system of FIG. 1;

fig. 6 is a schematic diagram of a baggage sorting system including the scanning system of fig. 1 in accordance with an embodiment of the present invention;

fig. 7 is a schematic diagram of a baggage sorting system according to another embodiment of the present invention;

figures 8a, 8b and 8c are schematic views of baggage sorting systems according to other embodiments of the present invention;

fig. 9 is a schematic diagram of a networked baggage sorting system according to another embodiment of the present invention;

FIG. 10 is a schematic plan view of a free standing scanning system according to another embodiment of the invention;

FIG. 11 is a schematic side view of the system of FIG. 10;

FIG. 12 is a schematic side view of a modular scanning system according to another embodiment of the present invention;

FIG. 13 is a diagram of X-ray scatter events;

FIG. 14 is a longitudinal section of a security scanning system according to another embodiment of the invention;

FIG. 15 is another longitudinal section of the system of FIG. 14 showing how different scattering events are detected;

FIG. 16 is a cross-section of the system of FIG. 14;

FIG. 17 is a schematic diagram of a data acquisition system of the scanning system of FIG. 14;

FIG. 18 is a partial view of a dual energy scanner according to another embodiment of the present invention;

FIG. 19 is another partial view of the scanner of FIG. 18;

FIG. 20 is a schematic view of a dual energy X-ray source according to another embodiment of the present invention;

FIG. 21 is a schematic view of a detector array of a scanner according to another embodiment of the invention;

FIG. 22 is a schematic view of a detector array of a scanner according to another embodiment of the invention;

FIG. 23 is a circuit diagram of the data acquisition circuit of the embodiment of FIG. 21; and

fig. 24 is a circuit diagram of a data acquisition circuit according to another embodiment of the present invention.

Detailed Description

Referring to fig. 1-3, a lobby baggage scanning system 6 includes a scanning unit 8, the scanning unit 8 including a multi-focal length X-ray source 10 and an X-ray detector array 12. The source 10 comprises a large number of source points 14 at respective spaced apart locations on the source and arranged in a full 360 circular array about the axis X-X of the system. It will be appreciated that arrays covering less than a full 360 angle may also be used.

Referring to fig. 1a, in a plane perpendicular to the direction of movement of the conveyor, an X-ray source 10 is composed of a number of source units 11, which source units 11 are spaced apart in a substantially circular configuration around a scanning area 16. Each source unit 11 comprises a conductive metal suppressor 13 having two sides and an emitter element 15 extending along between the two sides of the suppressor. A number of grid elements in the form of grid wires 17 perpendicular to the emitter elements 15 are supported above the suppressor 13. On another plane on the opposite side of the grid wires to the emitter elements, a number of focusing elements in the form of focusing wires 19 are supported. The focusing wires 19 are parallel to the grid wires 17 and are spaced apart from each other at the same intervals as the grid wires, each focusing wire 19 being aligned with a respective one of the grid wires 17.

The focusing wires 19 are supported on two support rails 21 extending parallel to the transmitter elements 15 and are separated from the suppressor 13. The support rail 21 is electrically conductive so that all the focusing wires 19 are electrically connected together. One of the support rails 21 is connected to a connector 23 to provide an electrical connection for the focus wire 19. Each of the grid wires 17 extends under one side of the suppressor 13 and is connected to a respective electrical connector 25 which provides a separate electrical connection for each of the grid wires 17.

An anode 27 is supported above the grid wires 17 and the focus wires 19. The anode 27 is formed as a rod, typically of tungsten or silver plated copper, and extends parallel to the emitter element 15. The grid and focusing wires 17, 19 thus extend between the emitter element 15 and the anode 27. Electrical connector 29 provides an electrical connection to anode 27.

The grid wires 17 are connected to a negative potential, except for the two grid wires 17 which are connected to a positive potential. These positive grid wires extract the electron beam from the area of the emitter element 15 and, by focusing of the focusing wires 19, direct the electron beam onto a point on the anode 27, which forms the X-ray source point of the pair of grid wires. The potentials of the grid wires can thus be switched at any time to select which pair of grid wires is active and thus which point on the anode 27 is the active X-ray source point at any time.

The radiation source 10 can thus be controlled to generate X-rays individually from each of the source points 14 in each of the source units 11, and, referring back to fig. 1, the X-rays from the respective source points 14 are directed inwardly through a scanning area 16 within the circular source 10. The radiation source 10 is controlled by a control unit 18 which controls the potentials applied to the grid wires 17 and thus the X-ray emission from each of the source points 14.

Other suitable X-ray source designs are described in WO 2004/097889. The multi-focal length X-ray source 10 allows the use of electronic control circuitry 18 to select which of a number of individual X-ray source points 14 in the multi-focal length X-ray source is active at any one time. Thus, by electronically scanning the multi-focus X-ray tube, the illusion of X-ray source movement is created without physical movement of the mechanical parts. In this case, the angular velocity of the source rotation may increase to a level that is not at all achievable using conventional rotating X-ray tube assemblies. This rapid rotational scanning translates into an equivalently accelerated data acquisition process and subsequently into rapid image reconstruction.

The detector array 12 is also circular and is disposed about the axis X-X at a position slightly offset from the source 10 along the axis. Radiation source 10 is configured to direct X-rays it generates through a scanning region 16 toward a detector array 12 on the opposite side of the scanning region. The path 18 of the X-ray beam thus traverses the scanning region 16 in a direction substantially or almost perpendicular to the scanner axis X-X so as to intersect near that axis. The volume of the scanned area that is scanned and imaged thus takes the form of a slice perpendicular to the scanner axis. The radiation source is scanned such that each source point emits X-rays for a corresponding period of time, the emission periods being arranged in a predetermined order. As each source point 14 emits X-rays, a signal from the detector 12 is generated which is dependent on the intensity of the X-rays incident on the detector, and the intensity data provided by the signal is recorded in memory. When the radiation source completes its scan, the detector signals can be processed to form an image of the scanned volume.

The conveyor belt 20 passes through the imaging volume from left to right parallel to the axis X-X of the scanner as shown in figure 1. An X-ray scatter shield 22 is positioned around the conveyor 20 upstream and downstream of the main X-ray system to prevent operator radiation dose due to scattered X-rays. The X-ray scattering shields 22 contain leaded rubber strip curtains 24 at their open ends so that the item under inspection 26 is drawn through one curtain as it enters the inspection area and drawn through one curtain as it exits. In the integrated system shown, the main electronic control system 18, the processing system 30, the power supply 32, and the cooling racks 34 are shown mounted below the conveyor 20. The conveyor 20 is configured to operate in a continuous scanning movement, typically at a constant conveyor speed, and typically has a carbon fiber frame within the imaging volume.

Referring to FIG. 4, the processing system 30 includes an electronic data acquisition system and a real-time image reconstruction system. The X-ray detector array 12 includes a plurality of rows of single X-ray detectors 50 arranged in a simple linear pattern (e.g., 1X 16). Multiple ring patterns (e.g., 8x16) are also possible. Each detector 50 outputs a signal that depends on the intensity of the X-rays it detects. The multiplexing block 52 multiplexes the output data signals from each of the input X-ray detectors 50, performs data filtering, gain and offset correction, and formats the data into a high-speed serial stream. The selection block 53 takes input from all the multiplexing blocks 52 and selects only a part required for image reconstruction in the entire X-ray data. The selection block 53 also determines unattenuated X-ray beam intensities Io for the appropriate X-ray source points (which will vary for each X-ray source point within the multi-focal-length X-ray tube), by forming a resultant log_e(Ix/Io) the X-ray intensity data Ix from the multiplexing block 52 is processed and then convolved with an appropriate 1-D filter. The resulting projection data is recorded as a sonar chart (sinogram) in which the data is arranged in an array using pixel numbers along one axis (horizontal in this case) and source angles along the other axis (vertical in this case). The data is then passed in parallel from the selection block 53 to a set of backprojection and processor elements 54. The processor element 54 is mapped into hardware using a look-up table with pre-calculated coefficients for selecting the necessary convolved X-ray data and weighting factors for fast backprojection and summation. The formatting block 55 takes data representing a single reconstructed image file from the multiple processor elements 54 and formats the final output image data to fitIn the form of a suitably formatted three-dimensional image generated on a display screen. The output can be generated in real time fast enough for the image to be generated for real time or offline viewing, whereby the system is referred to as a Real Time Tomography (RTT) system.

In this embodiment, the multiplexing block 52 is encoded in software, the selection block 53 and the formatting block 55 are encoded in firmware, and the processor elements are mapped in hardware. However, each of these components may be either hardware or software, depending on the needs of the particular system.

Referring to fig. 5, each final output image for each baggage item is then processed by a threat detection processor 60 within the processing system 30, the threat detection processor 60 being configured to determine whether the imaged baggage item represents a threat. In the threat detection processor 60, the input tomographic image data 62 is passed to a set of low-level parameter extractors 63 (level 1). The parameter extractor 63 identifies image features such as area, texture, and statistical information of constant gray levels. Some extractors work on data of a single 2-dimensional image or slice, some on 3-dimensional images, and some on sonar map data. Where possible, the various extractors act on the same set of input data in parallel, and the various extractors are configured to perform different processing operations and determine different parameters. At the end of the process, the parameters determined by the parameter extractor 63 are passed to a set of decision trees 64 (level 2). Details of the extracted parameters are given below. Decision tree 64 takes a large number (typically all) of the low-level parameters separately and constructs each higher level of information, such as information about the contiguous volume, with relevant statistical information. At the highest level (level 3), the database searcher 65 maps the higher-level parameters generated at level 2 to a "red" probability Pr (threat) indicating the presence of a threat and a "green" probability Pr (security) indicating the security of the item under examination. Processing system 30 uses these probabilities to assign the scanned items to the appropriate security categories and generates automated sort control outputs. The automated sort control output may be a first "green" output indicating that the item is assigned to the open (clear) category, a second "red" output indicating that the item is assigned to the "not open" category, or a third "amber" output indicating that automated sorting with sufficient reliability cannot be implemented to assign the item to either the "open" or "not open" categories. Specifically, if Pr (security) is above a predetermined value (or Pr (threat) is below a predetermined value), an automatic sort output in the form of a first signal will be generated indicating that the article should be assigned to the green channel. If Pr (threat) is above a predetermined value (or Pr (safe) is below a predetermined value), an automatic sort output in the form of a second signal will be generated indicating that the item should be assigned to the red lane. If Pr (threat) (or Pr (security)) is between two predetermined values, an automatic sort output in the form of a third signal will be generated indicating that the article cannot be assigned to either the red channel or the green channel. The probabilities may also be output as other output signals.

The parameters to be determined by the parameter extractor 63 are typically related to a statistical analysis of the pixels within individual regions of the 2-dimensional or 3-dimensional image. To identify individual regions in an image, a statistical edge detection method is used. The method starts on a certain pixel and then checks if neighboring pixels are part of the same region, moving outward as the region grows. In each step, the average intensity of a region is determined by calculating the average intensity of the pixels within the region, and the intensity of the next pixel adjacent to the region is compared to the average to determine if its intensity is sufficiently close to the average for the pixel to be added to the region. In this case, the standard deviation of the pixel intensities within the region is determined and if the intensity of the new pixel is within the standard deviation, it is added to the region. Otherwise, it is not added to the region, and this defines the edge of the region as the boundary between pixels in the region and pixels that have been examined and not added to the region.

Once the image has been divided into a plurality of regions, parameters of the regions may be measured. One such parameter is a measure of the variance of the pixel intensities within a region. If it is high this may indicate a lump material (lumpy material) which may be found for example in homemade bombs, whereas if the variance is low this may indicate a homogeneous material such as a liquid.

Another parameter that is measured is the skew (skewedness) of the distribution of pixel values within a region, which is determined by measuring the skew of the histogram of pixel values. A gaussian distribution (i.e., a distribution without bias) indicates that the material within the region is uniform, while a distribution with high bias indicates non-uniformity within the region.

As described above, these low-level parameters are passed up to the decision trees 64, where higher-level information is constructed with the determined higher-level parameters in the decision trees 64. One such higher level parameter is the surface area to volume ratio of the identified region. The other is a measure of similarity, which in this case is the cross-correlation between the shape of the region and the shape of the template stored in the system. The template shape is configured to correspond to the shape of an item having a security threat, such as a firearm or detonator. These high-level parameters are used as explained above to determine the threat level posed by the imaged object.

Referring to fig. 6, an in-line real-time tomographic baggage sorting system includes the scanning system 6 of fig. 1, and a conveyor 20 passes through the scanning system 6. Downstream of the scanning system 6, a sorting apparatus 40 is configured to receive individual pieces of baggage from the conveyor 20 and move them to an open or "green" lane conveyor 42 or an uninhibited or "red" lane conveyor 44. Sorting apparatus 40 is controlled by automated sorting output signals from processing system 30 via control line 46, which represent decisions by processing system 30 as to whether an item is open, and also by signals from workstation 48 to which sorting apparatus 40 is connected via line 45. The images from the scanning system 6 and signals from the processing system 30 indicative of the red and green probabilities and nominal decisions of the processing system 30 are also fed to the workstation 48. The workstation is configured to display images on the screen 47 so that they are visible to the operator and provide a display indicating green and red probabilities and nominal automated sorting decisions. A user at the workstation may review the images and probabilities and the automated sort output and decide whether to accept or override the decision if the scanning system decides to assign an item to the red or green category or whether to enter the decision if the scanning system decides to assign an item to the "amber" category. The workstation 48 has a user input 49 that enables a user to send a signal to the sorting apparatus 40 that can be recognized by the sorting apparatus as overriding the decision of the scanning system. If the override signal is received by the sorting apparatus, the sorting apparatus does override the decision of the scanning system. If no override signal is received, or indeed if an acknowledgement signal is received from the workstation confirming the decision of the scanning system, the sorting apparatus sorts the item based on the decision of the scanning system. If the sortation system receives an "amber" signal from the scanning system relating to an item, it initially assigns the item to a "red" category to be placed in the red channel. However, if it receives an input signal from the workstation indicating that the item should be in the "green" category before it sorts the item, it sorts the item into the green channel.

In a modification of the system of fig. 6, sorting may be fully automated such that the processing system gives only one of two sort outputs "open" and "not open" to assign items to either the green or red channel. It is also possible for the processing system to determine only one probability Pr (threat) with a threshold and assign the item to one of the two categories depending on whether the probability is above or below the threshold. In this case, the allocation is still temporary and the operator still has the option of overriding the automatic sorting. In another modification, an automatic category assignment of the scanning system is used as the final assignment without user input at all. This provides a fully automated sorting system.

In the system of fig. 6, the scanning speed is matched to the conveyor speed so that baggage may be moved at a constant speed from the loading zone (where it is loaded onto conveyor 20) through scanning system 6 and onto sorting device 40. The conveyor 20 extends a distance L between the exit of the scanning system 6 and the sorting device 40. During the travel of the baggage item over the distance L on the conveyor 20, the operator may observe the image data of the inspected item and the initial category assignment determined by the scanning system and confirm or reject the automatic decision of the RTT system. Typically, the baggage is then either accepted onto the open aisle and forwarded on to prepare for transport or rejected onto the non-open aisle for further investigation.

In this RTT multi-focal-length system, the RTT scanning unit 8 can operate at the highest baggage belt speed, thus eliminating the need for baggage queuing or other steering mechanisms for optimal system operation. In an integrated system such as this, the limited throughput capability of conventional rotating source systems is a significant constraint. This often means placing multiple conventional CT machines in parallel and using a complex baggage handling system to switch the item to be inspected to the next available machine. This complexity can be avoided by the arrangement of fig. 6.

Referring to figure 7, a second embodiment of the invention comprises a redundant system in which two RTT scanning systems 70, 72 are placed in series on the same conveyor 74 so that if one system is taken out of service, the other can continue to scan for baggage. In either case, the conveyor belt 74 may continue to pass through both scanning systems 70, 72 at a standard operating belt speed.

Referring to fig. 8a, in a third embodiment a more complex redundant system is provided which operates two RTT systems 82, 84 in parallel. The first main entry conveyor 86 brings all the items to be sorted to a first sorting device 88, which first sorting device 88 may transfer the items onto either of two further conveyors 90, 92. Each of the two conveyors 90, 92 passes through a respective one of the scanning systems 82, 84 that will scan the item and enable a decision to be made as to whether to release the item. On each of the two conveyors 90, 92, a further sorting device 94, 96 is provided, which sorting device 94, 96 is configured to sort the baggage onto a common 'green channel' conveyor 98 for onward transport or, in the case of the item not being released, onto a 'red channel' conveyor 100, on which conveyor 100 it can be subjected to further investigation. In this configuration, the input transmitter 86 and the 'green channel' transmitter may be run at a speed higher than the RTT transmitter speed, typically up to twice that speed. For example, in this case, the main entrance conveyor 86 and the common 'green channel' conveyor move at a speed of 1m/s, while the scanning conveyors 82, 84 travel at half that speed, i.e., 0.5 m/s. Of course, the system may be extended with more parallel RTT systems such that the ratio of the speed of the main entry conveyor to the speed of the scanner conveyor is equal or substantially equal to the number of scanners in parallel, although the sorting apparatus may become unreliable at main conveyor speeds greater than about 1 m/s.

Referring to fig. 8b, in another embodiment, the baggage sorting system comprises a number of RTT scanners 81b, 82b, 83b, typically up to about 60 in one system, each associated with a respective check-in station. Sorting devices 84b, 85b, 86b are associated with each RTT scanner, and baggage is transported on a conveyor from each RTT scanner to its associated sorting device. Each sorting device 84b, 85b, 86b sorts baggage in response to signals from its scanner to either a common open aisle conveyor 88b or a common reject aisle conveyor 87 b. A further backup RTT scanner 89b is provided on the reject lane conveyor 87b, which backup RTT scanner 89b has associated sorting means 90b that can leave a baggage on the reject lane conveyor 87b or transfer it to the release lane conveyor 88 b.

Under normal operation, each of the primary scanners 81b, 82b, 83b sorts baggage and the backup or redundant scanner 89b provides further inspection of the items sorted into the reject lane. If the scanner determines that the baggage item represents no or sufficiently low threat, it passes the baggage item to an open aisle. If one of the one-time scanners is inoperative or malfunctioning, its associated sorting apparatus is configured to sort all baggage from that scanner to a reject lane. The backup scanner 89b then scans all of these bags and controls their sorting between the open aisle and the reject aisle. This enables all of the check-in stations to continue to function while the faulty scanner is being repaired or replaced.

Referring to fig. 8c, in another embodiment, baggage from each of the check-in stations is transferred via a plurality of individual conveyors onto a central loop or endless conveyor belt 81c, on which conveyor belt 81c the baggage continuously circulates. A number of sorting devices 82c, 83c, 84c are each configured to transfer an item of baggage from the loop 81c to a respective conveyor leading to a respective RTT scanner 85c, 86c, 87 c. The sorting devices 82c, 83c, 84c are controlled by the scanners to control the rate at which baggage items are fed to each of the scanners. From the scanner, the conveyor passes all baggage items to a common exit conveyor 88c leading to another sorting device 89 c. It is controlled by all scanners to sort each of the baggage items between the open aisle 90c and the reject aisle 91 c.

To track the movement of each item of baggage, each item is assigned a 6 digit ID and its position on the conveyor belt as recorded when the item first entered the system. The scanner can thus identify which item of baggage is being scanned at any one time and associate the scan results with the appropriate item. The sorting device can thus also identify individual baggage items and sort them on the basis of their scanning results.

The number of scanners and the speed of the conveyor in the present system are configured so that if one of the scanners is inoperative, the remaining scanner can process all baggage being fed onto loop 81c from the check-in station.

In a modification of the present embodiment, the sorting devices 82c, 83c, 84c which select which articles are conveyed to the respective scanners are not controlled by the scanners, but are each configured to select articles from the loop 81c to feed them to the respective scanner at a predetermined rate.

Referring to FIG. 9, a networked system in accordance with another embodiment includes three scanning systems 108 similar to the scanning system of FIG. 6 and four operator workstations 148. The video image outputs from the three RTT scanning systems 108 are connected through respective high bandwidth point-to-point video links to a real-time disk array 109 that provides a redundant video switch 110 with transient storage for the raw image data. The disk array 109 is in turn connected to each of the workstations 148. The video switch 110 is thus able to transfer the raw video images output from each of the scanning systems 108 from its temporary storage to any of the workstations 148 where the raw video images can be utilized to generate 3-dimensional video images that can be viewed offline. The outputs from the scanning system for the red/green probability signal and the automated sorting assignment signal are connected to a redundant conventional ethernet switch 112, which ethernet switch 112 is also connected to each of the workstations. The ethernet switch is configured to switch each of the probability signal and the sort distribution signal to the same workstation 148 as the associated video signal. This allows image data from multiple machines to be connected, along with automatic assignments and probabilities assigned to the assignments, to a bank of operator workstations 148 where an operator may monitor the performance of the baggage inspection system and determine the destination of baggage assigned an amber threat level.

Alternatively, a networked system includes a single scanning system 108 and workstation 148 connected to a server. The video image output from the scanning system 108 is connected to a real-time disk array 109 that provides instantaneous storage for the raw image data. The disk array 109 is in turn connected to a workstation 148. The probability signal and assignment signal outputs are sent to the workstation 148 along with the associated video image outputs to be monitored by the operator. The networked single scanning system may be part of a networked system having multiple scanning systems.

Referring to fig. 10 and 11, in another embodiment, an in-line scanner has a conveyor belt 160 that is just as long as a main scatter shield 162. In this stand-alone system configuration, an item for inspection is placed onto the conveyor belt 160 and the item is loaded into the system. The article is then scanned by the scanner machine 164 and an image is generated. In conventional systems, items are often pre-screened with a simple delivery X-ray system to identify potential threat zones prior to computed tomography screening of selected planes in the object. This application is only for real-time multi-focal-length systems. Here, no pre-screening is used and a true three-dimensional image of the entire article will be obtained.

In some embodiments, the trajectory of the source points in a multi-focal-length X-ray source will extend in an arc over an angular range of only 180 degrees plus the fan beam angle (typically in the range of 40-90 degrees). Advantageously, the number of discrete source points is chosen to satisfy the Nyquist sampling theorem. In some embodiments, as in the embodiment of FIG. 1, a full 360 degree ring of source points is used. In this case, the dwell time per source point is increased for a given scan rate, beyond the 180+ fan beam configuration, and this is advantageous in improving the reconstructed image signal-to-noise ratio.

The scanner system of fig. 1 is an integrated scanner system in that the control, processing, power and cooling units 18, 30, 32, 34 are housed in a unit with the scanning system 8 and the shield 22. Referring to fig. 12, in another embodiment a modular system is provided in which some or all of the control, processing, power and cooling racks 218, 230, 232, 234 are located remotely from the scanning unit 208 containing the multi-focal length X-ray source and sensor array. It is advantageous to use a modular design for ease of installation, which is particularly advantageous in a baggage handling hall environment where the system may be suspended from a ceiling or in an area with limited access. Alternatively, the entire system may be configured as an integrated unit having a plurality of sub-assembly units co-located within a single housing.

In some embodiments, including the embodiment of fig. 1, a single ring of X-ray detectors is used. It is cheaper to construct and provide sufficient signal-to-noise performance with a simple fan-beam image reconstruction algorithm even at higher image scan rates. In other embodiments (particularly for larger image reconstruction circle diameters), it is preferable to use a multi-ring sensor array having a plurality of circular or partially circular sensor groups arranged adjacent to each other spaced along an off-source axis of the system. This enables the use of more complex cone-beam image reconstruction algorithms in the processing system. The use of multiple ring sensors increases the dwell time of each source point, resulting in a larger integrated signal size and thus an improvement in the signal-to-noise ratio of the reconstructed image.

The center of the design of the above described embodiment using a multi-focal length X-ray source based computed tomography system is the relationship between the angular velocity of rotation of the radiation source and the velocity of the conveyor system through the scanner. In the extreme case of the conveyor being stationary, the thickness of the reconstructed image slice is determined entirely by the size of the X-ray focal distance and the area of the elements of the X-ray detector array. As the conveyor speed increases from zero, the object under examination will pass through the imaging slice during the rotation of the X-ray beam and will introduce additional blurring in the reconstructed image in the direction of the slice thickness. Ideally, the X-ray source will rotate faster than the conveyor speed so that blurring in the slice thickness direction will be minimized.

Computed tomography systems based on multi-focal-length X-ray sources for baggage inspection provide good ratios of rotational angular velocity of the radiation source to linear velocity of the conveyor for the purpose of high probability detection of threat materials and objects in the inspected item. By way of example, in the embodiment of FIG. 1, the conveyor speed is 0.5m/s, as is common in airport systems. The radiation source can achieve 240 source rotations per second around the conveyor, so that the object under examination will move a distance of 2.08mm across the imaging slice during scanning. In a conventional system with a source rotation of 4 revolutions per second, the object under examination will move a distance of 62.5mm across the imaging slice during scanning at the same belt speed.

The primary purpose of an inspection system for detecting threat materials is to accurately detect the presence of the threat material and to pass all other materials when not suspect. The greater the blur in the slice direction due to the movement of the conveyor during the scan, the greater the partial volume artifact (artifact) in the reconstructed image pixels and the less accurate the reconstructed image density. The less accurate the reconstructed image density, the more likely the system will be to alert non-threat materials and not true threat materials. Thus, real-time tomography (RTT) systems based on multi-focal-length X-ray source technology may provide significantly enhanced threat detection capabilities at higher conveyor speeds compared to conventional mechanically rotating X-ray systems.

Because an extended arcuate anode is used in a multi-focal-length X-ray source, the electron source can be switched so that it jumps over the full length of the anode, rather than scanning sequentially to emulate the mechanical rotation observed in conventional computed tomography systems. Advantageously, in order to minimize the instantaneous thermal load on the anode, the X-ray focal length will be switched to maximize the distance from the current anode radiation position to all previous radiation positions. This temporal spreading of the X-ray emission points is advantageous in minimizing partial volume effects due to the conveyor movement, thereby further improving the pixel accuracy of the reconstruction.

The higher temporal resolution of RTT systems allows for a higher level of accuracy in automatic threat detection. With this higher level of accuracy, it is possible to operate the RTT system in an unattended mode, resulting in a simple two-state output indication, one state corresponding to a green or open allocation and the other corresponding to a red or non-open allocation. The green bag is released for onward transmission. The red bag represents a higher threat level and should coordinate with the passenger and prohibit the passenger from traveling.

Further embodiments of the invention will now be described in which data relating to scattering of X-rays and data relating to transmitted X-rays are recorded and used to analyse scanned items of baggage.

Referring to fig. 13, as a beam 300 of X-rays passes through an object 302, some of the X-rays pass directly through it and exit the object, which travels in the same direction as the X-rays entered the object. Some of the X-rays are scattered at a scatter angle θ, which is the difference between the direction they enter the object and the direction they leave the object. It is well known that two types of scattering occur: coherent or bragg scattering centered around a scattering angle of 5 °, typically 4-6 °, and incoherent or compton scattering of X-rays scattered at larger angles. Bragg scattering increases linearly with the atomic number of the object and obeys the following equation:

nλ＝2dsinθ

wherein,

n is an integer

λ is the wavelength of the X-rays

d is the interatomic distance in the object.

Thus, the amount of bragg scattering gives information about the atomic structure of the object. However, it does not vary smoothly with atomic number.

The amount of compton scattering depends on and varies smoothly with the electron density of the object, so that the amount of scattering at larger scattering angles gives information about the electron density of the object and thus about its atomic number.

Referring to fig. 14, a security scanning system according to another embodiment of the invention includes the same multi-focal-length X-ray source 410 as in fig. 1, and a circular detector array 412 and transmitter 420 also as in fig. 1. However, in this embodiment, the system includes another cylindrical detector array 422, the cylindrical detector array 422 extending around the transmitter at the same radius as the circular detector array 412, but axially on the other side of the radiation source 410. While the circular detector array is configured to detect X-rays transmitted through the object 426, the cylindrical detector array 422 is configured to detect X-rays scattered in the object. Scatter detector array 422 is made up of a large number of circular arrays or rings 422a, 422b of detectors, with the detectors in each ring being equally spaced around the transmitter so that they are arranged in a number of straight rows extending along the axis of the scanner.

The detectors in scatter detector array 422 are energy-resolving detectors such that each X-ray interaction with a respective detector produces a detector output indicative of the energy of the X-ray. These detectors can be fabricated from wide bandgap III-V or II-IV semiconductor materials such as GaAs, HgI, CdZnTe or CdTe, narrow bandgap semiconductors such as Ge, or composite scintillation detectors such as nai (ti) with photomultiplier readout.

Referring to fig. 15, a collimator 428 is disposed in front of the scatter detector 422. The collimator 428 provides a barrier that prevents X-rays from reaching the various detectors unless the X-rays are from a particular receive direction. As can be seen in fig. 16, for each detector in the array 422, the receive direction passes through the central longitudinal axis X-X of the scanner. However, as can be seen from fig. 15, the receiving direction is not perpendicular to the axis X-X, but is inclined at an angle of about 5 ° to the plane of the detector rings 422a, 422b in a direction towards the radiation source 410.

Referring to fig. 15, it will be appreciated that X-rays incident on any one of the detectors of the array 422 must be scattered out of a corresponding small sub-volume within the relatively thin imaging volume lying on a line in the path of the X-ray beam and the direction of reception from the detector 422. For any coherently scattered X-rays, the axial position of the detector that detects it will be determined by the distance to the active X-ray source point where the scatter occurred. The detector axially closest to radiation source 410 will detect the X-rays scattered furthest from the active X-ray source point. For example, X-rays scattered from a point closest to active X-ray source point 410a will be detected by a detector that is farther from radiation source 410 than X-rays scattered from point z (which is farther from the active X-ray source point). Thus, at any one time, when an active X-ray source point can be identified, the axial position of the detector that detected the scattered X-rays can be used to determine the scatter position along the X-ray beam direction.

It will also be appreciated from figure 15 that for the system to work it is important that the X-ray beam be narrowly focused along the axis of the scanner. The spread of the beam in the transverse direction (i.e. using fan beam spreading in the transverse direction) will still allow such localization of coherent scatter events.

Referring to fig. 16, since the collimator 428 is aligned with the axis of the scanner, X-rays from the active source point 410a that are subject to coherent scatter will only be detected by the detector row 422a located on the opposite side of the scanner axis from the active source point, and, depending on how narrow the collimator is focused, may be detected by one or more of the rows that are close to it on either side. If the X-rays are defined as a straight narrow "pencil" beam, then any X-rays that are incoherently scattered at large angles will not be detected at all because they will be cut off by the collimator 428. An example of such an X-ray is shown by arrow 'a' in fig. 16. However, if a fan beam of X-rays is generated from active source point 410a that is spread through the imaging volume slice in a direction perpendicular to the scanner axis, X-rays directed further away from the scanner axis may undergo incoherent scatter and reach the detectors on either side of row 422a opposite the active source point. Arrows b and c show examples of these X-rays. It should be noted that in order to reach any detector 422b, a scatter event must occur in a plane passing through the scanner axis and that detector 422 b. This means that for a given active source point and a particular detector, the location of the detected X-ray scatter event can be identified as lying on a plane passing through the scanner axis and the detector. If the exact location of the scattering event is to be determined, then additional information is required. For example, if information about the location of an object within the imaging volume is available from, for example, tomographic imaging data, then, as explained in more detail below, scatter may be correlated with the most likely object.

From the bragg scatter data, for each detected scatter event, a combination of X-ray energy and scatter angle can be used to determine the interatomic distance d of the material in which the scatter event occurred. In practice, it can be assumed that the scattering angle is constant and energy is used to distinguish between different materials. For compton scattering, the level of scattering from each of the scattering volumes gives an indication of the density of the material in that volume. The ratio of compton scattering to coherent scattering can also be determined and used as another parameter for characterizing the material of the imaged object.

Due to the short dwell time for each X-ray source point, the number of detected scattered X-rays for each source point is always very small, typically less than five. In order to form a reasonable coherent scatter signal, scatter data must be collected for all source points within the tomographic scan, and then the results of the various sub-volumes of the imaging volume are accumulated. For a scanner with 500 source points and one average of coherent diffraction scatter results per scan of each subvolume, then after accumulating a set of data, each subvolume will have 500 results associated with it corresponding to the 500 scatter events within that subvolume. A typical sub-volume occupies an area of a few square centimeters in the imaging plane and has a volume thickness of a few millimeters.

Referring now to FIG. 17, a data acquisition system configured to accumulate data from the scatter detector array 422 of the scanner of FIGS. 14-16 includes a multi-channel analyzer (MCA)500 associated with each of the detectors 422. Each MCA 500 is configured to receive an output signal from the detector and to assign each detected X-ray to one of a number of X-ray energy ranges or channels and to output a signal indicative of the energy range within which the detected X-ray falls. The multiplexer 502 is configured to receive an output from each of the MCAs 500. A look-up table 504 is also provided having a plurality of entries therein that identify, for a given source point and detector, a sub-volume within the imaging volume in which X-rays are scattered. The system also includes an image memory 506 comprising a plurality of memory areas 508, each of the memory areas 508 associated with a respective sub-volume within the scanner imaging plane.

Data is automatically loaded into the various memory regions 508 by the multiplexer 502 under the direction of the look-up table 504. The look-up table is loaded with coefficients that map respective combinations of the detectors 422 and MCA 500 to corresponding image locations 508, one look-up table entry for each X-ray source location, prior to scanning. Those pixels (i.e., detector 422) that are in the forward direction (i.e., substantially in the direction in which photons travel from the radiation source before any interaction) are assumed to record coherent scattered photons at a beamlet angle of about 4-6 degrees. Those pixels 422 that are not in the forward direction are assumed to record incoherent scattered photons due to the compton scattering effect. Thus, the image memory 506 is effectively "three-dimensional" -two dimensions represent positions in the image, while the third dimension maintains the scatter spectrum for coherent scatter (low 8 bits) and incoherent scatter (high 8 bits). The lookup table 504 will also instruct the multiplexer 502 as to the type of data collected for the respective MCA 500 at each projection so that the appropriate memory space is filled.

Once scatter data has been collected for a given scan, the data is transmitted to the master RTT data acquisition system 512 described above with reference to fig. 4, and is synchronized with the master RTT data acquisition system 512 by the projection sequencer 510. Thus, the reconstructed image data and scatter data are simultaneously passed to a threat detection system, which may use it to determine appropriate analysis parameters.

For each scan, tomographic image data from the transmission detector 412 will yield data related to the X-ray attenuation of the individual pixels of the image, which in turn correspond to the respective sub-volume of the tomographic imaging volume. This data is obtained as described above with reference to fig. 4. The data from the scatter detectors 422 provides data relating to the amount of coherent scatter within each sub-volume and data relating to the amount of incoherent scatter within each sub-volume as described above. The data may be analyzed in a threat detection processor similar to that of fig. 5. In this case, the parameters of the extracted data may relate to image data or scatter data or a combination of two or more types of data. Examples of parameters extracted from the data are the ratio of coherent scatter to incoherent scatter, the type of material determined from coherent scatter data, the density of material determined from incoherent scatter data, the correlation of CT image pixel values to scatter data. Also, scattering data parameters corresponding to the parameters described above for the transmission data may also be determined.

Referring to FIG. 18, in another embodiment of the present invention, a transmission detector 512 for producing tomographic image data is configured to measure X-ray transmission over different energy ranges. This is achieved by having two sets of detectors 512a, 512b each forming a ring around the transmitter. The two groups are located at different axial positions in the direction of travel of the conveyor, in this case axially adjacent to each other. The first group 512a has no filter in front of it, but the second group 512b has a metal filter 513 between it and the X-ray source 510. The first set of detectors 512a thus detects transmitted X-rays over a wide energy range, while the second set 512b detects X-rays only in a narrow portion of the range at the high energy end.

As the item to be scanned moves along the conveyor, its respective thin volume or slice may be scanned once using the first set of detectors 512a and then scanned again using the second set 512 b. In the illustrated embodiment, two adjacent volumes are scanned simultaneously using the same radiation source 510, such that data for each of the two volumes is collected by a respective one of the detector sets 512a, 512 b. After a volume of the article has passed through the two sets of detectors and scanned twice, two sets of image data may be formed using two different X-ray energy ranges, each image containing transmission data (and thus attenuation data) for a respective pixel of the image. The two sets of image data may be combined by subtracting the image data of the second detector set 512a from the image data of the first detector set 512b to obtain corresponding image data for the low energy X-ray components.

The difference between the X-ray transmission data for each single energy range and the data for two different ranges, such as high energy and low energy, may be recorded for each pixel of the image. This data can then be used to improve the accuracy of the CT image. It may also be used as another parameter in a threat detection algorithm.

It will be appreciated that other methods may be used to obtain transmission data for different ranges of X-ray energies. In a modification of the system of fig. 18 and 19, balanced cancellation filters may be used on both detector groups. The filters are chosen such that there is a narrow window of energy that is traversed by both of them. The image data of the two sets of detectors can then be combined to obtain transmission data for this narrow energy window. This enables chemically specific imaging to be obtained. For example, bone specific images can be generated by using a filter that balances out around the calcium K edge energy. It is clear that this chemistry specific data can be effectively used in threat detection algorithms.

In another embodiment, instead of using separate filters, two sets of detectors are used that are sensitive to different energies of X-rays. In this case, stacked detectors are used, including a thin front detector sensitive to low-energy X-rays while allowing higher-energy X-rays to pass through, and a thick rear detector sensitive to high-energy X-rays passing through the front detector. Also, attenuation data of different energy ranges may be used to provide energy specific image data.

In another embodiment, two scans of each slice of the object are performed with two different X-ray beam energies achieved by using different tube voltages in the X-ray source, e.g. 160kV and 100 kV. The different energies result in X-ray energy spectra that are shifted with respect to each other. Since the energy spectrum is relatively flat over a portion of the energy range, the energy spectrum is similar over many portions of the range. However, a portion of the energy spectrum will vary significantly. Thus, a comparison image of two tube voltages can be used to identify portions of the object that have significantly varying attenuation between the two images. This therefore identifies image regions with high attenuation in the narrow spectral portion that varies between images. This is therefore an alternative way of obtaining energy specific attenuation data for each of the sub-volumes within the scanned volume.

Referring to fig. 20, in another embodiment of the present invention, two different X-ray energy spectra are generated by providing an anode 600 in an X-ray tube having target areas 602, 604 of two different materials. In this case, for example, the anode comprises a copper base 606 having a tungsten target region 602 and a uranium target region 604. The electron source 610 has a large number of source points 612 that can be individually activated. On opposite sides of the path of the electron beam 616 there is provided a pair of electrodes 612, 614, which electrodes 612, 614 can be controlled to switch the electric field on and off to control the path of the electron beam such that it impinges on one or the other of the target areas 602, 604. The energy spectrum of the X-rays generated at the anode will vary depending on which of the target areas the electron beam 616 impinges.

This embodiment uses an X-ray source similar to that of figure 1a, with the different target areas formed as parallel strips extending along the anode 27. For each active electron source point, two different X-ray spectra can be generated, depending on which target material is used. The radiation source may be configured to switch between the two target regions of each electron source point when it is active. Alternatively, the scan along anode 27 may be performed twice, once for one target material and once for the other. In either case, additional electron beam focusing wires may be required to ensure that the electron beam only irradiates one or the other target material at a time.

Depending on the angle at which the X-ray beam is extracted from the anode, the beams from the two target regions 602, 604 may in some cases be configured to pass through the same imaging volume and be detected by a common detector array. Alternatively, they may be configured to traverse adjacent slices of the imaging volume and be detected by individual detector arrays. In this case, various portions of the imaged article may be scanned twice as the article passes along with the conveyor in a similar manner to the arrangement of fig. 18.

Referring to fig. 21, in yet another embodiment, two detector arrays axially adjacent to each other are provided in a single scanner, one detector array 710 corresponding to the detector array of fig. 1 and configured to form an RTT image, and the other detector array 712 having a higher resolution and configured to produce a high resolution projection image of the scanned object. In the present embodiment, the high resolution detector array 712 includes two parallel linear arrays 714, 716, each of the two linear arrays 714, 716 configured to detect X-rays of different energies, such that dual energy projection images may be generated. In the embodiment of FIG. 22, the high resolution array 812 includes two stacked arrays, i.e., a thin array located above that is configured to detect lower energy X-rays and transparent to higher energy X-rays, and a thicker array located below that is configured to detect higher energy X-rays. In both cases, both detector arrays are configured to be axially close enough to be able to detect X-rays from a single linear array of source points.

To provide a projection image, data needs to be captured from all detectors in the high resolution arrays 712, 812 when only one source point is active. Referring to fig. 23, to achieve this, each detector 718, 818 in the high resolution array is connected to an integrator 750. The integrator includes an amplifier 752 in parallel with a capacitor 754. An input switch 756 is provided between the detector 718 and the amplifier 752, a reset switch 758 is provided across the input terminals of the amplifier, a further reset switch 759 is connected across the capacitor 754, and a multiplexing switch 760 is provided between the integrator and the analog-to-digital converter ADC.

In operation, when detector 718 is not required to be active, all switches except multiplexing switch 760 are closed. This ensures that the capacitor 754 is not charged and remains intact. Then, at the beginning of the period in which the detector is required to collect data, both reset switches 758, 759 are closed so that any X-rays detected by detector 718 will cause an increase in the charge on capacitor 754, which will result in an integration of the signal from detector 718. When the period for data acquisition ends, the input switch 756 is opened so that the capacitor will remain charged. Then, to read the integrated signal from the integrator, the output switch 760 is closed to connect the integrator to the ADC. This provides an analog signal to the ADC that is determined by the level of charge on capacitor 754 and thus indicates the number of X-rays detected by detector 718 during the period of connection to the integrator. The ADC then converts the analog signal to a digital signal for input to a data acquisition system. To generate a single projection image, all high resolution detectors are used to collect data simultaneously while one of the X-ray source points is active.

Referring to fig. 24, in another embodiment, each detector 718 is connected to two integrators 750a, 750b in parallel, each of the two integrators 750a, 750b being the same as the integrator of fig. 23. The outputs from the two integrators are connected to the ADC via their output switches 760a, 760 b. This enables each integrator to be configured to integrate the signal from detector 718 at different points in the scan of the X-ray source and thus collect data for separate images from different angles through different X-ray source points. For example, it may be used to generate a plurality of projection images from orthogonal directions, which may be used to construct a high resolution 3-dimensional image from which the location of features in the imaged package may be determined in three dimensions.

Since a high resolution image may help identify items such as filaments that require higher resolution, it may be useful when it is combined with an RTT image.

Claims (14)

1. An X-ray scanning system, the X-ray scanning system comprising:

(a) a scanner, the scanner comprising:

(i) an X-ray source comprising a plurality of X-ray source units spaced apart around a scanning volume in a substantially circular configuration, each of the X-ray source units comprising an anode and a plurality of electron source points that can be individually activated, wherein two target areas of different materials are formed on the anode and are configured to generate X-rays of two different energy spectra, wherein each electron source point is configured to switch between the two target areas when active, and

(ii) a plurality of detectors configured to detect X-rays from each of the target regions, wherein each detector is connected to two integrators, and each integrator is configured to integrate a signal from the detector at different points in a scan of the X-ray source, thereby producing two respective scanner outputs, an

(b) A processing device configured to:

(i) processing signals from the detectors, including combining two corresponding scanner outputs to produce a plurality of tomographic image data sets, an

(ii) The tomographic image data sets are combined to produce a three-dimensional image of the object.

2. The X-ray scanning system of claim 1 wherein said detector comprises a common detector array configured to detect X-rays from each of said target zones.

3. The X-ray scanning system of claim 1 wherein said detector comprises two detector arrays each configured to detect X-rays from one of said target zones.

4. The X-ray scanning system of claim 2 wherein said detector array is a circular array.

5. The X-ray scanning system of claim 3 wherein each detector array is a circular array.

6. The X-ray scanning system of any of claims 1-5 wherein the target area is formed as parallel strips extending along the anode.

7. The X-ray scanning system of claim 1 wherein the electron source point is configured to generate an electron beam, and the X-ray source further comprises a pair of electrodes disposed on opposite sides of a path of the electron beam, the pair of electrodes being controllable to turn on and off an electric field to control the path of the electron beam such that the electron beam strikes one or the other of the target regions.

8. The X-ray scanning system of any of claims 1-5 wherein the detectors extend around a scan volume and the X-ray source is configured to direct X-rays through the scan volume.

9. The X-ray scanning system of any of claims 1-5 further comprising a conveyor configured to transport an item through the scan volume.

10. An X-ray scanning system according to any of claims 1 to 5 wherein the processing means is configured to generate attenuation data indicative of the level of X-ray attenuation in a plurality of portions of the scanned volume from the signal from the detector.

11. The X-ray scanning system of claim 10 wherein said attenuation data is in the form of image data suitable for producing an image of a scanned item.

12. The X-ray scanning system of claim 11 wherein said attenuation data is adapted to produce a three dimensional tomographic image.

13. The X-ray scanning system of any of claims 1-5 further comprising a display device configured to display images derived from the image data set.

14. An X-ray scanning system according to claim 11 comprising analysis means configured to analyse the attenuation data and generate an output signal dependent on a parameter of the attenuation data.