GB2329817A

GB2329817A - X-ray detection and imaging of materials

Info

Publication number: GB2329817A
Application number: GB9821037A
Authority: GB
Inventors: Max Robinson; James Paul Owain Evans
Original assignee: Nottingham Trent University
Current assignee: Nottingham Trent University
Priority date: 1997-09-29
Filing date: 1998-09-28
Publication date: 1999-03-31
Anticipated expiration: 2018-09-28
Also published as: GB9720658D0; GB2329817B; GB9821037D0

Abstract

Three-dimensional images or stereo pairs of images are produced using two or more dual energy X-ray transmission sensors and at least one X-ray scatter sensor. Two or more X-ray curtain beams can be provided so as to enhance the stereo effect, and the scatter sensors can be positioned at angles which are characteristic of materials which it is desired to detect. The system may be used to inspect electronic assemblies, or to identify polycrystalline substances such as explosives, drugs or semiconductor materials in items of luggage.

Description

DETECTING, IMPROVING AND CHARACTERISING MATERIAL IN 3-D SPACE This invention relates to arrangements for detecting, imaging and characterising material in 3-D space, using X-rays.

The particular form of apparatus for which this technique is intended is that based on the line-scan principle. This consists of an X-ray source, the beam of which is collimated into a thin curtain, hereinafter called a "curtain beam", and is then detected by a linear array detector. Image information is obtained by having the object of interest move linearly at right angles with respect to the beam and storing successive scans of X-ray transmission information derived from the linear array in a frame store unit. Images formed in the way are related to the X-ray transmission properties of the object.

Our European Patent No. 610084 describes a technique in which a stereoscopic pair of X-ray images are obtained using two diverging curtain beams derived from an X-ray source. Then these are separated into their conjugate slices and a 2-1/2D image built up using software from the resulting slice information.

In that connection a 2-1/2D image is not a true 3D (stereoscopic) image because the image is usually presented on a cathode ray screen which is 2dimensional. Instead the 2-1/2D image is a two dimensional image which has been provided with depth cues such as interposition, linear perspective, shading and shadowing, texture gradient and rotational movement parallax. These clues make the two dimensional image appear some what three dimensional and so the user of the system is provided with an image which has more use than simply a flat shadow graph as is presented by a conventional X-ray image.

That European Patent, however, does provide a user with a final image which can be rotated and looked at from different directions and which can give considerable information as to the relative positioning of different objects within say a suitcase. However it does not give information as to the nature of the items which have been located.

X-Ray systems also exist which can broadly classify materials into inorganic and organic categories by filtering the full spectrum of X-ray emissions into low and high energy bands.

Thus, inorganic materials are impenetrable by the softer X-rays and only hard X-rays will penetrate them. Softer X-rays will pass through organic materials.

Differentiations between organic and inorganic materials generally can be achieved by use of a dual energy sensor. Such a sensor comprises a low energy detector which receives, first of all, the impinging beam of X-rays followed by a filter or attenuator which removes the soft or lower energy X-rays and underneath this will be a high energy detector. With such a dual energy sensor, therefore, one can detect the difference between organic and inorganic materials and software exists which can compare the two signals and enable one to colour images seen on a cathode ray tube according to whether the material found is inorganic or organic.

Even using this technique, however, only flat shadow-graph images are obtained. It is not possible using this method, to locate in object space, objects that are superimposed; nor is it possible to identify their chemical nature.

In particular no information is obtained concerning the crystalline or polycrystalline nature of the object.

These materials scatter X-rays and, because of this, it frequently happens that the resulting X-ray image or shadow graph image which is obtained will hardly detect such polycrystalline material because a very large proportion of the X-rays which have not been absorbed by the material will have been scattered and so not received by the linear array. It is unfortunate that many of the materials which one is seeking, such as explosives, drugs and semiconductor materials, fall within this polycrystalline group and they are, therefore, difficult to detect by using conventional X-ray systems.

If one just adds a scatter detector, one part of the problem can be addressed, i.e. the presence or not of crystalline material, since substances in both these classes of materials have well documented molecular structure and X-ray scatter angles. However, the location of the polycrystalline material in 3-D space is still unknown.

Whilst the line-scan X-ray technique is widely used in security applications where the detection of explosives and narcotics is an important feature, knowledge of the existence of such substances, combined with their exact location in three dimensional space, would be a considerable improvement on present techniques.

Therefore, the present invention in one aspect aims to provide a composite detector arrangement which can provide both 3D (stereoscopic) and 2-1/2D X-ray images of objects and also identify the structures within such objects by their X-ray scattering properties, in particular their coherent scattering (diffraction) signatures. In another aspect the invention aims to increase the sensitivity of scatter detection to enable one to identify and also locate in space polycrystalline materials.

The technique can also be used in the inspection of electronic assemblies to map the location of wires, connectors, plastic encapsulants and also semiconductor material itself. The invention therefore provides the ability to inspect enclosed or covered electronic assemblies as during manufacture.

Therefore, according to one aspect of the invention there is provided a method of obtaining a stereoscopic pair of images of an object with identification of polycrystalline material in that object, in which the object is moved through at least two diverging beams of X-rays, and the output of at least two parallel dual energy linear detectors associated with the beams provides a stereoscopic pair of images, and in which a scatter detector is positioned half way in between the or each pair of Xray curtain beams so that the scatter detector receives an input of scattered X-rays from each beam, to provide an increased amount of scatter radiation to detect polycrystalline material and assist in the sensitivity of the overall system.

Also according to this aspect of the invention there is provided apparatus for creating a stereoscopic pair of images of an object with identification of polycrystalline material in that object, comprising an X-ray source with two or more diverging curtain beams of X-rays, each beam being associated with one of two or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images, and a scatter detector positioned half way in between the or each pair of X-ray curtain beams so that the scatter detector receives an input of scattered X-rays from each beam, to provide an increased amount of scatter radiation to detect polycrystalline material and assist in the sensitivity of the overall system.

Thus, when you have a scatter detector the amount of energy it receives is relatively low and so it is often quite difficult to detect sufficient signal for accurate usage. By following the invention, however, double the amount of scatter radiation is detected and this assists in the sensitivity of the overall system.

Because each X-ray image obtained is detected by a dual energy sensor and by a scatter X-ray sensor, as an object being examined is moved through the X-rays, the dual energy sensor provides information about the nature of the object found and one can build up, if desired, separate 3D (stereoscopic) or, 2-1/2D pictures of the organic and inorganic objects in the item and superimpose them. This will help to solve correspondence problems for 3D (stereoscopic) images.

In addition the scatter image can be used to detect and identify the polycrystalline material of a particular object, which may be only relatively faintly observed in the direct X-ray images, using software to indicate this on a video screen once the 2-l/2D image has been formed.

According to another aspect of the invention there is provided a method of obtaining a stereoscopic pair of images of an object with identification of polycrystalline material in that object, in which the object is moved through at least two diverging beams of X-rays, and the output of parallel dual energy linear detectors associated with the beams provides a stereoscopic pair of images, and in which as many scatter detectors as dual energy detectors are positioned so that a stereoscopic pair can be derived from each of the high energy and low energy detectors in the dual energy pair and also a stereoscopic pair can be derived from the scatter detectors.

Also according to this aspect of the invention there is provided apparatus for producing a stereoscopic pair of images of an object with identification of polycrystalline material in that object, comprising an X-ray source with two or more diverging curtain beams of X-rays, each beam being associated with one of two or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images, and as many scatter detectors as dual energy detectors positioned so that a stereoscopic pair can be derived from each of the high energy and low energy detectors in the dual energy pair and also a stereoscopic pair can be derived from the scatter detectors.

Thus, one has as many scatter detectors as dual energy detectors are positioned so that a stereoscopic pair can be derived from each of the high energy and low energy detectors in the dual energy pair and also a stereoscopic pair can be derived from the scatter detectors. This greatly assists in the location of objects and the solution of the correspondence problem normally associated with a weak signal from which one is deriving a stereoscopic pair and also enables one to identify, for each of the 2-1/2D images which can be produced, the nature of the material seen. Then an operator has additional options available to him. He can, for example, simply look at a stereoscopic image or a 2-1/2D image produced by the scatter detectors.

This will, therefore, simply show up polycrystalline material like drugs and explosives without showing up the other organic and inorganic materials and so this may well provide the operator with additional help in trying to assess an image.

According to a further aspect of the invention two or more scatter detectors may be associated with one or each of the X-ray curtain beams but positioned at different predetermined angles.

Also according to this further aspect of the invention there is provided apparatus for obtaining an X-ray image of an object, comprising at least one Xray curtain beam through which the object is passed, at least one linear detector associated with the or each beam for detecting the image, and two or more scatter detectors associated with one or each of the X-ray curtain beams but positioned at different predetermined angles, so as to look for the characteristic scatter signature for a polycrystalline material.

Thus, each polycrystalline material like the explosive known as semtex has a particular very well known scatter spectrum. By positioning the scatter detectors at known angles of peak scatter one can, therefore, look for the characteristic scatter signature for such a material.

According to yet another aspect of the invention one may provide three or more diverging curtain beams of X-rays with three of more associated linear detectors, preferably of the dual energy type.

Also according to this aspect of the invention there is provided apparatus for creating a two dimensional solid model representation of an object with psychological cues as to depth, known as a 2iD solid mould or of preparing a stereoscopic pair of images, comprising an X-ray source with three or more diverging curtain beams of X-rays, each beam being associated with one of three or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images.

By following such an arrangement one can gain additional information and obtain an exaggeration of the depth of a 3D (stereoscopic) image or a 2-1/2D image if required to assist in identifying an object.

Thus using, for example, a larger diverging angle between two curtain beams of X-rays than has normally been used in the past one can separate the various slices explained in our above noted European Patent by a larger amount and so present these with enhanced depth on the final 3D (stereoscopic) and 2-1/2D image.

Also by providing three X-ray curtain beams one has a combination of three different stereoscopic pairs. An operator can, therefore, switch from one pair to another pair for direct stereoscopic viewing or for forming the 2-1/2D image, both providing fuller information about a particular object which he is trying to examine. Sometimes it will be better to use a stereoscopic pair which have a relatively narrow angle between them and on other occasions to have a stereoscopic pair with the largest possible angle between them.

One or more of these aspects of the invention can be combined so as to provide enhanced information for an operator and several of the specific embodiments discussed below show such combinations.

In embodiments of the invention two or more divergent primary curtain beams, preferably from the same X-ray source, are provided such that the angle between them is consistent with producing a stereoscopic image or images, and in addition the scattered or diffracted secondary beams are chosen so as to be either detected additively or detected in such a way as to provide their own stereoscopic data from the scattering (polycrystalline) regions of the object.

The first of these options is particularly advantageous where the scattered signal has a low value and so by providing two or more primary curtain beams, an additive diffraction signal can be obtained.

The second option would provide not only stereoscopic images of both low and high energy X-ray transmission signals, but also a stereoscopic view of the polycrystalline regions of the object.

By having a composite detector consisting of effectively three parts, i.e. both high and low energy transmission signals, plus a diffraction signal, the process of building solid image (2-1/2D) models of the object is greatly improved. In that connection the outputs of the detectors from each detector will provide a stereoscopic pair of images which can be converted to a 2-1/2D image in the manner described, for example, in our European Patent No. 6100804 to which reference is made for an explanation of this.

The invention will now be described by way of example, with reference to the accompanying drawings which are diagrams of various embodiments of the invention.

In the embodiments of the invention shown in Figures 1 to 3 two or more curtain beams of X-rays from the same source are provided. By appropriate choice of primary beam angles and also appropriate choice of scattered beam angles a composite detector can be constructed which will produce stereoscopic images of high energy and low energy transmission Xrays. In addition, positioning an appropriate scatter detector between or around the primary beams provides the correct angle for collection of the scattered (diffracted) beams. In this way, the scattered signal can be intensified by the summation of the signals collected, due to each primary beam.

Figure 1 shows an arrangement, using two beams 1 and 2 with a scatter detector positioned equally between the two linear parallel dual energy detectors.

In this way the signal received by the scatter detector can be increased.

Two possible arrangements using three primary curtain beams are as shown in Figures 2 and 3. The arrangement of Figure 2 has two important properties.

One is that three stereoscopic views are obtained by combining images from beams 1 and 2, 2 and 3 and 1 and 3. The other is that two scatter detectors can be used to either obtain four times the signal in the arrangement shown in Figure 2 or alternatively each scatter detector can be arranged to detect a diffracted signal from a different angle which as explained above can provide further materials identification.

Figure 3 shows an arrangement where three primary curtain beams are used in an asymmetric geometry.

Similar advantages exist as for the arrangements of Figure 3 as for Figure 2, but now there will be a range of stereoscopic viewing angles, increasing from beams 1 and 2, through 2 and 3 and then 1 and 3 to give the advantages noted above. Again the scatter detectors can be used in both modes described above in connection with Figure 2.

In the embodiment of the invention shown in Figures 4 to 6, each primary curtain beam 1 and 2, or 1, 2 and 3, has one complete composite detector associated with it. This consists of the dual energy transmission intensity feature and a scatter (diffraction) detector.

Arrangements using two and three beams are shown respectively in Figures 4 and 5. An advantage of this technique is that a composite stereoscopic image can now be produced in which the spatial positions of organic, inorganic and specified crystalline materials can be identified and 3D stereoscopic image obtained as described above. The visual information can be presented using standard stereoscopic display techniques. In addition the production of solid image models (i.e. 2-1/2D images) is greatly improved by providing this extra materials identification. There is also an improvement of the solid image modelling process, since the solution of the stereoscopic correspondence problem is simplified.

Figure 6 shows an arrangement with three curtain beams and so provides the similar options to Figure 3.

However, the scatter detectors are shown at the same angle relative their associated curtain beam and so their output can also be used to provide 3D (stereoscopic) or 2-1/2D image models from three different pairs of scatter beams.

The X-ray source must contain a distribution of energies. Tungsten is the most appropriate target, but others could be used.

The scattered radiation detector should have both position sensitivity as well as energy resolution and must be collimated to ensure that only the defined scattering directions are observed. The detector could be in the form of a linear array or an area array. My European Patent Application No 87398517.9 (Serial No 261 984) describes examples of linear array detectors for detecting the X-ray transmission images for creating a stereoscopic pair reference is made to that Application for full details and the contents thereof are to be deemed incorporated herein. The scatter detector could be made from a variety of materials such as HPGe of CdZuTe but is not limited to these.

The dual energy X-ray transmission part of the composite detector could be either based on scintillator materials and photodiodes or on scintillators in conjunction with optical fibres. In each case the emitted X-ray spectrum is filtered into high and low energy regions which broadly signify inorganic material.

Systems using more than three primary beams are of course possible.

Claims

CLAIMS 1. A method of obtaining a stereoscopic pair of images of an object with identification of polycrystalline material in that object, in which the object is moved through at least two diverging beams of X-rays, and the output of at least two parallel dual energy linear detectors associated with the beams provides a stereoscopic pair of images, and in which a scatter detector is positioned half way in between the or each pair of X-ray curtain beams so that the scatter detector receives an input of scattered X-rays from each beam, to provide an increased amount of scatter radiation to detect polycrystalline material and assist in the sensitivity of the overall system.
2. A method as claimed in Claim 1 in which the stereoscopic pair are used to build up separate 3D (stereoscopic) or 2-1/2D pictures of the organic and inorganic objects in the item and superimpose them.
3. A method as claimed in either preceding claim in which a stereoscopic pair so obtained is resolved in a manner as claimed in any claim of European Patent No. 610 084.
4. A method of obtaining a stereoscopic pair of images of an object with identification of polycrystalline material in that object, in which the object is moved through at least two diverging beams of X-rays, and the output of parallel dual energy linear detectors associated with the beams provides a stereoscopic pair of images, and in which as many scatter detectors as dual energy detectors are positioned so that a stereoscopic pair can be derived from each of the high energy and low energy detectors in the dual energy pair and also a stereoscopic pair can be derived from the scatter detectors.
5. A method as claimed in Claim 4 when applied to a method as claimed in Claim 1 or Claim 2.
6. A method of obtaining an X-ray image of an object in which the object is passed through at least one Xray curtain beam and an image is detected by a sensor associated with the or each curtain beam, and in which two or more scatter detectors are associated with one or each of the X-ray curtain beams but positioned at different predetermined angles, so as to look for the characteristic scatter signature for a polycrystalline material.
7. A method as claimed in Claim 5 when applied to a method as claimed in any of claims 1 to 5.
8. A method of creating a two dimensional solid model representation of an object with psychological cues as to depth, known as a 2MD solid model or of preparing a stereoscopic pair of images comprising providing passing the object through three or more diverging curtain beams of X-rays with three or more associated parallel linear detectors.
9. A method as claimed in Claim 8 in which the linear detectors are of the dual energy type.
10. A method as claimed in Claim 8 or Claim 9 when applied to a method as claimed in any of claims 1 to 7.
11. A method of obtaining an X-ray image, substantially as herein described with reference to any of the Figures of the accompanying drawings.
12. Apparatus for creating a stereoscopic pair of images of an object with identification of polycrystalline material in that object, comprising an X-ray source with two or more diverging curtain beams of X-rays, each beam being associated with one of two or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images, and a scatter detector positioned half way in between the or each pair of Xray curtain beams so that the scatter detector receives an input of scattered X-rays from each beam, to provide an increased amount of scatter radiation to detect polycrystalline material and assist in the sensitivity of the overall system.
13. Apparatus as claimed in Claim 12 in which each linear detector is of the dual energy type.
14. Apparatus for producing a stereoscopic pair of images of an object with identification of polycrystalline material in that object, comprising an X-ray source with two or more diverging curtain beams of X-rays, each beam being associated with one of two or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images, and as many scatter detectors as dual energy detectors positioned so that a stereoscopic pair can be derived from each of the high energy and low energy detectors in the dual energy pair and also a stereoscopic pair can be derived from the scatter detectors.
15. Apparatus as claimed in Claim 14 as applied to an apparatus as claimed in either Claim 12 or Claim 13.
16. Apparatus for obtaining an X-ray image of an object, comprising at least one X-ray curtain beam through which the object is passed, at least one linear detector associated with the or each beam for detecting the image, and two or more scatter detectors associated with one or each of the X-ray curtain beams but positioned at different predetermined angles, so as to look for the characteristic scatter signature for a polycrystalline material.
17. Apparatus as claimed in Claim 16 as applied to an apparatus as claimed in any of claims 12 to 15.
18. Apparatus for creating a two dimensional solid model representation of an object with psychological cues as to depth, known as a 2iD solid mould or of preparing a stereoscopic pair of images, comprising an X-ray source with three or more diverging curtain beams of X-rays, each beam being associated with one of three or more parallel linear detectors, whereby the output of the linear detectors for any pair of beams provides a stereoscopic pair of images.
19. Apparatus as claimed in Claim 18 as applied to an apparatus as claimed in any of claims 12 to 17.
20. Apparatus for obtaining an X-ray image, substantially as herein described with reference to any of the Figures of the accompanying drawings.