WO2009125211A1

WO2009125211A1 - Imaging apparatus and method

Info

Publication number: WO2009125211A1
Application number: PCT/GB2009/050269
Authority: WO
Inventors: Max Robinson; Gary Gibson
Original assignee: Durham Scientific Crystals Limited
Priority date: 2008-04-11
Filing date: 2009-03-24
Publication date: 2009-10-15
Also published as: GB0806602D0

Abstract

A method is described for obtaining an image dataset of an object comprising the steps of: providing a source of electromagnetic radiation adapted to emit radiation in a band other than the visible band and a radiation detector system adapted to detect radiation from the source spaced therefrom to define a scanning zone, wherein the detector system is capable of detecting and collecting spectroscopically resolvable information about radiation incident thereon; collecting a dataset of information about the intensity of radiation incident at the detector at least one and preferably a plurality of scanning positions; resolving the dataset spectroscopically to produce a spectroscopically resolved dataset relating incident intensity to frequency across at least a part of the spectrum of the source; mapping the spectroscopically resolved dataset from the spectrum of the source to the visible spectrum to produce a visible dataset by means of a mapping relation that maintains a functional relationship between frequency and intensity in the spectroscopically resolved dataset and frequency and intensity in the visible dataset; preferably generating an image. An apparatus for the same is also described.

Description

IMAGING APPARATUS AND METHOD

The invention relates to a method and apparatus for the imaging of materials. The invention relates both to a method and apparatus for imaging alone and to a method and apparatus that operates by or in conjunction with the generation of other data such as materials characterisation data from an image dataset or otherwise.

The invention in particular relates to an apparatus and method making use of electromagnetic radiation outside the visible range to scan objects where it is desirable to gain visual imaging information and in particular generate a visible light image that better informs a user of the apparatus and/ or method about the internal contents and/ or structure and/ or composition of the objects. This principle is widely employed for example in the security industry, but might also be employed in other areas, for example, without limitation, medical imaging, imaging for quality control purposes or the purposes of determining the integrity of the structure, or the like.

A variety of radiation sources and a variety of principles for the detection of radiation after interaction with an object under test are known. For example, high energy radiation such as x-rays or gamma rays, or other non-visible radiation such as IR, may be used which is capable of interacting with a target material or object. Detectors may be used to measure incident transmitted or back scattered radiation after interaction with a target material or object via a suitable process such as absorption, reflection, diffraction, fluorescence etc.

X-Ray absorption for example has been used as the basis for screening objects to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space. The thicker or more dense an object is then the more it will attenuate an x- ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated.

Typically, to produce a transmission radiograph an x-ray source generates an essentially 2-dimensional beam and detectors of transmitted x-rays are used to build up successive image slices based on transmitted x-rays. A computer is used to generate images based on these transmitted x-rays (and hence differentiating by absorption).

These known apparatus and methods tend to generate single images that show physical structure but allow limited information to be inferred about material content or composition. In essence, at its simplest, all that is being measured is transmissivity of the object to the source radiation. The detector merely collects amplitude information, and does not discriminate transmitted radiation spectroscopically. In most practical systems even this is measured indirectly. At its simplest, a typical linear array x-ray detector comprises in combination a scintillator material responsive to transmitted x-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation.

However, it is known that spectroscopic information from transmitted x- rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically, and that this effect depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately identifying low- and high-energy bands from the full spectrum of x-ray emissions. Such a dual-energy sensor typically comprises a sandwich pair of semiconductor photodiode rays or the like. Filters are used on incoming radiation to shift that radiation into two different energy bands. The differential absorption effect discussed above is exploited by the dual energy detector to differentiate generally between objects having lower and higher atomic number elements predominating.

When exploited as part of a security or like material identification system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. The practical implications of this have led to the use of such detectors in the security industry, and for example in airport x-ray scanners, either to create separate images of metallic items inside luggage (to reveal items hidden metal weapons, such as guns, and knives) or to identify plastic explosives.

A dual energy system confers only limited information about composition. The organic/inorganic division is crude and approximate. Conventional detectors do not give any real spectroscopic information about the spectrum of transmitted x-rays although they detect the presence or otherwise of x-rays within two distinct bands of the spectrum. Ultimately decisions are made based on the attenuation radiograph which is based on the shape of items and their proximity to other objects.

Recent development of detectors that can resolve spectroscopic information about the transmitted X-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of images across these bands to generate multispectral images. For example US5943388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three distinct images. This better exploits the effect of differential spectral absorption by different materials and better approximates transmissivity to composition but is still limited to the information that can be conveyed by a displayed image at a given frequency band or by several such images displayed successively.

It is desirable to make better use of the underlying spectroscopically resolved dataset to supplement the information that can be inferred by an observer of an image compared with that available by plural band imaging alone.

According to one aspect of the invention there is provided a method of obtaining an image dataset of an object, and optionally further producing an image of an object, comprising the steps of: providing a source of electromagnetic radiation adapted to emit radiation in a band other than the visible band and a radiation detector system adapted to detect radiation from the source spaced therefrom to define a scanning zone, wherein the detector system is capable of detecting and collecting spectroscopically resolvable information about radiation incident thereon; collecting a dataset of information about the intensity of radiation incident at the detector at least one and preferably a plurality of scanning positions; resolving the collected dataset spectroscopically to produce a spectroscopically resolved dataset relating incident intensity to frequency across at least a part of the spectrum of the source; mapping the spectroscopically resolved dataset from the spectrum of the source to the visible spectrum to produce a visible dataset by means of an appropriate mapping relation that maintains a functional relationship between frequency and intensity in the spectroscopically resolved dataset and frequency and intensity in the visible dataset; optionally displaying as an image the visible dataset.

The detector system is adapted to generate spectroscopic information about radiation incident upon it. That is, the detector exhibits a spectroscopically variable response across at least a substantial part of the spectrum of the radiation source allowing spectroscopic information to be retrieved and the system includes a means to collect and resolve this into a spectroscopically resolved information dataset including at least variations of intensity with frequency rather than just raw intensity alone as might be the case with simple non-spectroscopic detectors. However, instead of merely using this to generate successive images of the object by resolving intensity information successively across a plurality of frequency bands as is the case in the prior art where spectroscopically resolved detectors have been used, in a manner which can give only limited indicative information, an image dataset is generated that inherently includes both spectral and intensity information in a single image.

Thus, the invention exploits the principles of multispectral resolution of radiation outside the visible spectrum incident at the detector more fully and more usefully in generating a single image dataset that relates functionally in both available informational dimensions (intensity and frequency) the spectroscopically resolved dataset to a correspondingly determined intensity and frequency in the image dataset in the visible spectrum. This more closely approximates to how a human observer instinctively expects to view an image, and allows the observer to obtain more information than might be the case from the successive spectrally resolved images known in the prior art. A multispectral variable intensity single image is qualitatively more effective in doing this. The method, the apparatus, and even the image itself, are qualitatively technically more effective as a means for utilising information from the spectroscopically resolved dataset than prior art multiple images.

It will be appreciated from the foregoing that "intensity" in a final image dataset therefore relates in practice to intensity cues perceived by a human user. In those circumstances, whilst absolute luminosity or brightness, or intensity in the narrow physical sense, is likely to be a primary cue, the invention should not be considered to exclude the use of cues in the more general sense such as hue and saturation, which will also cause a given colour or colour slice to be more intensely perceived by a human observer of any resultant image. Brightness, hue and saturation cues may all be used to create perceived image intensity, or brightness alone may be used and hue and saturation reserved to associate with other information derived from the spectroscopically resolved dataset giving even more degrees of information.

Although for convenience the step of mapping the spectroscopically resolved dataset to a visible dataset and the step of displaying as an image the visible dataset are described discretely, this should not be given specific significance as requiring clear and discrete performance. The invention relies upon the generation of a visible light image or a dataset therefor from a spectroscopically resolved intensity dataset outside the visible in which both frequency and intensity information map functionally from the spectroscopically resolved intensity dataset to the image dataset. The language used herein should not be taken to imply the necessity of creation of a discrete visible dataset with existence outside the context of an image, whether transiently or otherwise. The invention encompasses creation of such a dataset but likewise encompasses any method of moving from the spectroscopically resolved dataset to a visible image with spectroscopically resolved intensity variation, whether directly, so that the visible image is in effect itself also the visible dataset, or indirectly via a discrete visible dataset that has either a transient or longer existence and is used to generate an image in suitable imaging means.

In a possible embodiment, the invention comprises resolving radiation outside the visible spectrum incident at the detector across a plurality of discrete frequency bands, for example at least three, and mapping these to discrete colours (which term encompasses both broad and narrow spectral colour bands) in the visible spectrum image. Thus a frequency- specific dataset is collected that comprises at least data about intensity of radiation incident at the detector at a plurality of frequency bands, and for example comprises data representative of the average intensity of radiation incident at the detector across a given frequency band or at least a sufficiently representative part thereof. Thus, in this embodiment spectroscopic resolution of radiation in each of a plurality of bands is represented in a generated image as differentiated colour. A banded mapping is used in that the source spectrum is divided into a plurality of bands, for example between four and eight bands, and different colours are used to represent each such band in the displayed image. Banded mapping to a limited number of colours is found to give particularly effective visual cues to an observer in many cases, often producing clarity in practice that full continuous multispectral resolution would not.

The bandwidth of each band in the source spectrum is not directly pertinent to this embodiment of the invention and useful results can be obtained by any suitable approach to dividing the spectrum, either in whole or in part, into separate bands. For example, the entire spectrum or a substantial part thereof may simply be divided between such a plurality of bandwidths, and frequency-specific intensity information may be derived as a measure representative of intensity across the entire band, and for example an average intensity. Alternatively, a plurality of relatively wide bands, but with discrete gaps therebetween, may be envisaged and analysed on the same basis. Alternatively, "bands" may be narrow even to the point where they essentially approximate to an evaluation of intensity at a single frequency.

In an alternative embodiment, in effect a limiting case of the foregoing as an ever larger plurality of narrower bands is used, the invention comprises resolving radiation incident at the detector effectively continuously across at least a part of the frequency spectrum of the source and mapping this to a correspondingly continuous frequency spectrum within the visible spectrum image

Intensity information may similarly be resolved and presented in the image in a banded manner or by an effectively continuous mapping.

Preferably, the method comprises use of a mapping function that maps between frequency in the spectroscopically resolved dataset and frequency in the visible dataset and eventual image in a serial manner. That is to say, where a banded approach is used successive frequency bands in the spectroscopically resolved dataset are mapped to successive frequencies in the visible spectrum and where a continuous approach is used the bandwidth of the spectroscopically resolved dataset is mapped continuously and monochromatically to a bandwidth within the visible spectrum such that adjacent frequencies in the spectroscopically resolved dataset remain adjacent in the visible dataset and eventual image. Preferably, higher frequency radiation in the spectroscopically resolved dataset is mapped to higher frequency radiation in the visible spectrum in the visible dataset.

A successive and for example continuous mapping function need not be linear. A bandwidth in the spectroscopically resolved dataset may be mapped to a bandwidth in the visible dataset by any order of function, in particular the better to present the information as a visible image giving technically useful data to an observer about an object under image.

Appropriate and where applicable frequency-variable numerical weighting factors may be applied to data in a frequency-specific dataset prior to or as part of the process of image generation to produce a suitably modified/ meaningful visible dataset or image without departing from the principle of the invention. Such weighting factors might for example correct for intensity variations in a given source spectrum, for noise of any kind, or for any other factor that it might be desirable to account for to improve the technically representative character of the visible dataset or image.

Preferably the method comprises use of a mapping function that maps between intensity in the spectroscopically resolved dataset and intensity in the visible dataset and eventual image in a serial manner. That is to say, where a banded approach is used successive intensity levels in the spectroscopically resolved dataset are mapped to successive intensity levels in the image and where a continuous approach is used the intensity of the spectroscopically resolved dataset is mapped continuously to a functionally corresponding intensity at the appropriate frequency within the visible spectrum. Preferably, the mapping function is a reciprocal function at least in the general sense that higher intensity in the spectroscopically resolved dataset is mapped to lower intensity in the visible spectrum image.

This alternative is particularly suited to examples of the invention where incident radiation at the detector is based on transmitted intensity, as is the case for example with x-ray or gamma-ray transmission radiography. In these cases, the more meaningful material information is generated primarily by the radiation absorption behaviour of an object or material. Transmitted radiation could indicate absence or mere transparency at a given frequency, whereas absorption at a given frequency is more directly characteristic of the material. Thus, a multispectral variable intensity image in accordance with the invention confers greater technical information if reduced intensity at a given frequency in the spectroscopically resolved dataset (indicating presence of a material) is mapped as increased intensity in a visible image (the more effectively to indicate presence of the material).

The principles of the invention can be applied to any spectrum of source radiation outside the visible spectrum which interacts with an object in a scanning zone in such a way as to generate useful information, which in the context of the invention means that it interacts with an object in a spectroscopically variable manner depending upon some characteristic of the object, and in particular depending upon its material composition. This spectroscopically variable intensity response outside the visible range is then mapped directly and functionally to a visible image exhibiting corresponding variation both spectroscopically and in relation to intensity. Known examples of methods exploiting such interactions and to which the present invention could be applied include transmission imaging, back scattered imaging, fluorescence imaging and combinations thereof.

As indicated above, in a particularly preferred embodiment, the method involves at least the creation of a transmission dataset and for example a transmission image based on the intensity of radiation transmitted through an object. Transmission radiography will generally give more information about bulk objects, which is particularly significant in relation to security scanning systems where a bulk imaging technique is likely to be especially useful. However, the principles of the invention are also applicable to images created via surface scanning techniques.

The radiation source preferably comprises a source to deliver high-energy radiation such as ionizing radiation, for example high energy electromagnetic radiation such as x-rays and/ or gamma rays, or subatomic particle radiation, and the detection system is adapted correspondingly to detect radiation in this spectrum. The radiation source for example is a broadband x-ray or gamma-ray source capable of producing broad spectrum emission over a wide range of x-ray or gamma- ray energies. In a preferred embodiment, the method involves the use of high-energy electromagnetic radiation such as x-rays or gamma-rays, the source is an x-ray or a gamma-ray source, and the detector system is an x-ray or gamma-ray detector system.

Many materials exhibit a spectroscopically varying response to x-rays or gamma-rays, and accordingly the resolution of the incident radiation spectroscopically across the spectrum of the source will allow the information about this variable response to be processed and mapped to a visible image in accordance with the invention. In a particularly preferred embodiment, the method comprises a method of collecting transmission information from high-energy radiation such as x- rays or gamma-rays, and comprises providing a source of such high- energy radiation and a radiation detector system spaced therefrom to define a scanning zone therebetween, in such manner that measuring the intensity of radiation incident upon the detector system in use constitutes measuring transmissivity of an object in the scanning zone.

The invention is consequently discussed hereinafter in the context of examples relating to the generation of transmission radiographs. However, the more general applicability of the invention as set out above will be understood.

It will be understood that although reference is made herein for convenience to the imaging of an object this should not be considered to limit the application of the invention to the imaging of single homogenous objects. Indeed, for many envisaged applications, an "object" is likely to consist of multiple heterogeneous materials and/or to be a container or other agglomeration of multiple articles, so that any radiation path is likely to pass through multiple different materials having varied properties. One of the particular advantages of the invention is that it can facilitate resolution of such varied materials.

The method of the invention is not limited in its application to the mobile scanning of objects. Information pertinent to intensity variation in the radiation incident at the detector for a given scanning event, and hence the material composition of an object or objects in a scanning zone and for example in a transmission path, can be obtained by a single scanning event, for example of a stationary object being scanned by a single beam of appropriate geometry, for example a pencil beam or conical beam. In such circumstance the method merely includes placing the object in a scanning zone to obtain such a single scan and single dataset of data of information about radiation incident at the detector.

However, in a preferred embodiment information is collected regarding the object in a scanning zone, and in particular the transmissivity of an object under test in the scanning zone in a plurality of scanning positions between which the object is translated and/ or rotated. In accordance with this embodiment of the method, the method comprises the additional step of causing an object to move relative to and for example through the scanning zone as a plurality of successive datasets of information about radiation incident at the detector are collected.

For clarification it should be understood that where used herein a reference to the generation of image encompasses the creation of information dataset or "image dataset", for example in the form of a suitable stored and manipulatable data file, from which a visual representation of the underlying structure of the object under investigation could be produced without undue further effort on suitable apparatus, and references to displaying this image are references to presenting an image generated from such a dataset in a visually accessible form, for example on a suitable display means.

The method of the invention makes use of a detector system enabled to generate spectroscopic information about the transmitted radiation, and for example comprising an array one or more detectors that can generate spectroscopic information about the transmitted radiation. That is, the detector exhibits a spectroscopically variable response across at least a substantial part of the radiation spectrum of the source allowing spectroscopic information to be retrieved.

By analogy, in accordance with a further aspect of the invention there is provided an apparatus for obtaining and preferably further for displaying an image dataset of an object comprising: a source of electromagnetic radiation adapted to emit radiation in a band other than the visible band; a radiation detector system adapted to detect radiation from the source band spaced therefrom to define a scanning zone, wherein the detector system is capable of detecting and collecting spectroscopically resolvable information about radiation incident thereon; a resolution module to collect a dataset of information about the intensity of radiation incident at the detector and resolve the dataset spectroscopically to produce a spectroscopically resolved dataset relating incident intensity to frequency across at least a part of the spectrum of the source; a mapping module to map the spectroscopically resolved dataset from the spectrum of the source to the visible spectrum to produce an appropriate visible dataset by means of a mapping relation that maintains a functional relationship between frequency and intensity in the spectroscopically resolved dataset and frequency and intensity in the visible dataset; optionally, a display means to display as an image the visible dataset.

In a preferred embodiment, the apparatus involves the use of high-energy electromagnetic radiation such as x-rays or gamma-rays, the source is an x-ray or a gamma-ray source, and the detector system is an x-ray or gamma-ray detector system. In a particularly preferred embodiment, the apparatus is enabled to collect transmission information from high-energy radiation such as x-rays or gamma-rays, and comprises a source of such high-energy radiation and a radiation detector system spaced therefrom to define a scanning zone therebetween, in such manner that measuring the intensity of radiation incident upon the detector system in use constitutes measuring transmissivity of an object in the scanning zone.

Optionally, the apparatus is adapted to collect in use transmission intensity data with an object in a single scanning position and for example includes a means to retain an object in a scanning position such as a receptacle into which an object can be placed. Additionally or alternatively it may include a conveyor to convey an object into and out of such scanning position.

Optionally, the apparatus is adapted to collect in use transmission intensity data with an object in a plurality of scanning positions as the object moves through the scanning zone, and preferably to collect in use data for an image of an object in the scanning zone, and preferably a succession of images as the object moves through the scanning zone, in that it further comprises an object handler to cause an object to move relative to and through the scanning zone in use.

Preferably, the apparatus further includes an image generation apparatus to generate an image dataset from the visible dataset; and preferably further an image display adapted to display an image.

The display means is conveniently a simple two dimensional display screen, for example a conventional video display screen (which term is intended to encompass any direct display or projection system exploiting any cathode ray tube, plasma display, liquid crystal display, liquid crystal on silicon display, light emitting diode display or like technology). It is a particular advantage that the method can be envisaged for use with, and the apparatus for the invention incorporated into, the standard display screens of comparable existing systems for example in the security or medical imaging fields.

The radiation source should produce a distribution of energies across a suitable spectral range for resolution by the detector and resolution module, and is typically an x-ray source. Tungsten is the most appropriate target, but others could be used.

It is necessary that the detector system is enabled to detect radiation in a manner which is spectroscopically resolvable. Preferably, a detector system, or some or all discrete detector elements making up a multielement system, may be inherently adapted to produce spectroscopic resolution in that a system or element exhibits a direct spectroscopic response. In particular a system or element is fabricated from a material selected to exhibit inherently as a direct material property a direct variable electrical and for example photoelectric response to different parts of the source spectrum. For example, the detector system or element comprises a semiconductor material or materials preferably formed as a bulk crystal, and for example as a bulk single crystal (where bulk crystal in this context indicates a thickness of at least 500 μm, and preferably of at least 1 mm). For example a system or element is a direct bandgap material. The materials making up the semiconductor are preferably selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide, thorium bromide. Group M-VI semiconductors, and especially those listed, are particularly preferred in this regard. Combination of these and any other such materials may be considered which give spectroscopic x-ray detection rather than merely detecting amplitude of transmitted x-rays and thus enable resolution of radiation incident at the detection system into intensity data which is determined across the spectrum, and thus allow mapping to a visible image including information varying in both information dimensions, to illustrate variations in intensity with frequency.

A collimator is preferably provided to produce an emitted beam of suitable geometry from the source. The geometry of the emitted beam will determine the most useful geometry of the detector system.

For example, a beam may be collimated to have a spread in one or two dimensions, in particular to co-operate respectively with one or more linear detectors or with an area detector. Conveniently, linear and/or area detectors comprise linear and/or area arrays of a plurality of individual detector elements as above described.

The invention in one possible embodiment relates to an apparatus and method operating on the line-scan principle, in which three dimensional objects are caused to move through a scanning zone and imaging information collected by a linear detector.

Imaging apparatus which employs the line-scan principle is well known. Typically, such apparatus will consist of a radiation source, the beam of which may be collimated into a curtain, usually referred to as a "curtain beam", and is then detected by a linear detector for example comprising a linear photodiode array. Image information is obtained by having the object of interest move linearly for example at right angles with respect to the beam and storing successive scans of radiation transmission information derived from the linear array from which a complete image frame can be compiled.

Accordingly, in this line-scan embodiment, the method comprises: providing a source and a detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one and preferably a plurality of linear detectors capable of generating spectroscopically resolvable information about radiation; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in the manner above described.

Accordingly, in this line-scan embodiment, the apparatus comprises: a radiation source and a detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one and preferably a plurality of linear detectors capable of generating spectroscopically resolvable information about incident radiation.

Preferably, the source is an x-ray or gamma-ray source.

In accordance with this embodiment the radiation source is preferably collimated to produce a curtain beam and is thus, for example a curtain beam x-ray source as will be familiar from conventional line scan apparatus.

Preferably the detector system comprises a plurality of linear detectors linearly or angularly spaced apart in generally parallel conformance in serial array. Each linear detector may comprise a linear array of detector elements. The x-ray source may comprise a single primary source adapted to generate a beam such as a curtain beam aligned to be incident upon each linear detector in the spaced serial array at a suitable angular separation, from example by a suitable beam splitting apparatus. A single beam may be generated. Alternatively, multiple beams may be generated from a single source. Alternatively, multiple sources may be provided each generating a beam such as a curtain beam incident upon a linear detector in the serial array. The x-ray source may comprise a source combining any or all of the foregoing principles.

The provision of a plurality of linear detectors in accordance with this preferred embodiment of the invention offers an additional functionality. Data can be collected for an equivalent plurality of transmission paths as an object passes through a scanning zone. The provision of such a plurality of transmission paths between a source and differently positioned linear detectors or detector arrays gives the collected information at least some of the characteristics of the information collected by a conventional CT scanning apparatus, and allows the data to be processed additionally in a manner known from that technology.

For example, multiple transmission path data may be used to generate multiple images and thus improve the information content of the imaging aspect of operation in a familiar manner. Additionally or alternatively, multiple transmission paths through a given part of an object will lead to a varying of the effective through thickness, which can be employed to draw inferences about material content, again in a manner to some extent at least analogous to that known from CT scanning, and reinforce or further inform the inferences drawn by the derivation of data indicative of the mass attenuation constant in accordance with the basic principles of the invention. It will be appreciated however that although there are some aspects that can be seen as to some extent at least analogous to CT scanning, the invention is not a CT scanning technique and important differences remain.

With the latter application in mind in particular, it will generally be preferable if a single source is provided and used for the multiple ray paths created by having a multiple array of linear detectors. This guarantees that the incident spectrum for each ray path is essentially the same, and eliminates one possible uncertainty. However, the same principles could be applied to systems using multiple sources of reasonable spectral reliability.

The invention in an alternative possible embodiment relates to an apparatus and method operating on a two dimensional area scan principle, in which three dimensional objects are caused to move through a scanning zone and imaging information collected by an area array detector.

Accordingly, in this embodiment, the method comprises: providing a source and a detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one area array detector capable of generating spectroscopically resolvable information about incident radiation; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in accordance with any preceding claim.

The apparatus is modified accordingly. The invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is general schematic of a possible apparatus to implement the invention; Figure 2 illustrates a typical radiation source spectrum, and illustrates how it might be partitioned and mapped to the visible to implement the invention;

Figure 3 illustrates a typical method of applying differentiating colours to a frequency sliced area dataset generated from a spectrum partitioned in accordance with figure 2;

Figure 4 is an illustration of operation of a possible mode of operation of the invention.

In the illustrated embodiment, the apparatus employs a line scan principle to generate an x-ray image. In airline security applications, the principle is encountered in particular in relation to hand baggage scanners. X-ray imaging might also be used in principle as a supplementary system for hold baggage but this is less common. However it will be understood that the principles of the invention are equally applicable to imaging of stationary objects and for imaging for other purposes such as medical or product quality control purposes where different considerations may apply (for example, in particular, where there might be prior knowledge of an "expected" material profile against which results and generated images could be compared).

Figure 1 illustrates a suitable apparatus for this example mode of operation. The illustrated embodiment uses a single x-ray source collimated to produce a curtain beam incident upon the three linear detectors 3A to 3C (which in the embodiment each comprise a linear array of detector elements). Thus, a plurality of ray paths 5A to 5C are generated in the scanning zone by means of a plurality of curtain beams incident upon a linearly or angularly spaced array of such linear detectors. Incident ray paths 5A to 5C are shown through the scanning zone between the x-ray source 1 and, respectively, the detectors 3A to 3C.

In the embodiment, the linear array detectors 3A to 3C comprise material capable of spectroscopic resolution of incident x-rays, and in the specific example comprise cadmium telluride although the skilled person will appreciate that other material selections may be appropriate. To exploit this spectral resolution, the x-ray source emits x-ray across a broad energy spectrum. In the example a tungsten source is used, although the skilled person would appreciate that other materials might be appropriate.

An endless belt conveyor 7 causes an object to be scanned 9 to move in a direction d so as to intercept the ray paths 5A to 5C in the scanning zone. The envisaged application of this embodiment of the invention is as a security scanner, and object 9 can be considered typically to be a container that is expected to contain a variety of distinct objects which it would be useful and desirable to characterise compositionally and to view effectively in a third dimension (for example, an item of airline hold baggage). However, the skilled person would readily appreciate that the same principles can be applied for example to the scanning of objects for internal examination purposes, to medical scanning, and to similar applications.

Datasets of transmitted intensity information are generated by building up transmitted information from each of the three detectors 3A to 3C. The processing of a dataset of information by resolving, at least to some extent, a relationship between incident energy/ wavelength and transmitted intensity in accordance with the principles of the invention is carried out as below with reference to figure 2 in a resolution module.11 , mapped to a visible dataset by a mapping module 13, and displayed on the screen 15. Further exploitation of the multiple datasets which can be created by the apparatus of figure 1 in a preferred mode of operation is discussed with reference to figure 3.

The source (1 ) is preferably tungsten source, which gives a characteristic plot of x-ray intensity (I) versus wavelength (λ) as is illustrated in Figure 2. Figure 2 illustrates how this spectrum might be divided to operate a system in accordance with the principles of the invention. The overall spectrum is divided into successive relatively broad bands (t1 to t5) that are used to build up an energy-differentiated dataset of transmitted intensity. In this regard, the apparatus follows the same basic principles as conventional energy-differentiated apparatus. It differs in the way the resolved dataset is mapped to an image dataset (figure 2b).

In the illustrated example, essentially the entire intensity spectrum of the source is resolved into five wavelength bands (t1 to t5). It will be appreciated that other banding approaches, for example involving narrower and/ or discrete bands, could be employed. Moreover, although the embodiment shows a banded approach, it will be understood that a continuous mapping from the transmitted intensity spectrum to the visible spectrum could also be applied.

The detector system measures the transmitted intensity across the spectrum of the source, and a resolution module 11 resolves this into a spectroscopically resolved intensity dataset. In the example, this is done by calculation of an average intensity, respectively It1 , It2, It3, It4 and It5 for each frequency band (t1 to t5). This x-ray spectrum is mapped to the visible spectrum as represented in Figure 2b by a mapping module 13 applying a mapping function which relates each frequency (t1 to t5) in the transmitted spectrum to a corresponding frequency v1 to v5 (that is, to a corresponding colour) in the visible dataset shown in Figure 2b.

The intensity to be applied and generated in any resultant image displayed on the display screen 15, at each visible wavelength or wavelength band in the visible dataset 2b is related functionally to the average intensity across the relevant transmission band in the transmitted dataset.

In the preferred embodiment, as illustrated, consecutive frequency bands in the source spectrum are directly mapped to consecutive discrete colours in the visible spectrum. This is a usual preferred mode of operation, but for specific applications an alternative arrangement in which particular colours in the visible image are applied via another protocol to particular bands in the transmitted dataset might be appropriate.

An image is conveniently displayed on suitable display means 15. In this case, the apparatus operates to generate a transmission radiograph of an object.

It is known that reduced transmitted intensity (It) at a given part of the spectrum corresponds to increased absorption by an object in the transmission path in the scanning zone. It is absorption that is particularly characteristic of material properties. Accordingly, it is absorption that it is desirable to illustrate in the visible dataset, and ultimately in a visible image generated therefrom. It follows that the transmitted intensity (It) in the transmitted dataset at a given wavelength and the visible intensity (Iv) at a given wavelength in the visible dataset are more effectively functionally related via a function which has reciprocal character at least to the extent that a higher intensity (It) in the spectroscopically resolved dataset is mapped to a lower intensity (Iv) in the visible dataset. This functional relationship need not be a reciprocal or inverse function in the narrow mathematical sense provided it meets this requirement.

In one possible mapping methodology, the transmitted intensity (It) at each wavelength, and in the case of the embodiment at each wavelength band, is compared with the source intensity (I₀) to generate a measure of the reduction in intensity at that frequency or frequency band. This reduction in intensity is consequent upon a proportional absorption of the source intensity by material in a transmission ray path, and consequently on the material characteristics of material in a transmission ray path. The corresponding intensity at the corresponding functionally related visible band is an expression of this reduction. Greater reduction, corresponding to greater absorbance in the primary dataset, maps to greater intensity in the image dataset. In a preferred embodiment of this methodology, the calculated reduction in intensity at a given frequency is normalised relative to the source intensity. Such a methodology is of course not limited to the example embodiment of x-ray source or to transmission radiography, but is generally applicable across the range of the invention.

This normalised expression of the reduction in intensity attributable to absorption can be expressed mathematically, for any given frequency/ wavelength or frequency/ wavelength band, by the relation Iv = (I₀ - It)/ Io where Io is the source intensity and It the intensity incident at the detector (for example where a banded system is used the average intensity across the band in each case). When calculated in this way, a significant reduction of intensity in the transmitted spectrum, corresponding to a significant material absorption, is reflected as relative brightness rather than darkness in the visible dataset and the resulting visible image. This is significantly more effective in conveying specific compositional information, since it relates brightness in the image directly to absorption at a given wavelength. Thus, an image generated in accordance of the apparatus and method of the invention conveys significantly more information in practice to a user.

The principle of the invention is applicable to various detector geometries, and for example for both line scan and area array detectors. Area array detectors might be preferred for many imaging applications, but a line scan principle is equally applicable.

An example of the power of the invention in generating datasets of image information with greater utility is made with reference to the frequency sliced area dataset shown in figure 3.

An area array detector is used to collect information resolved in x, y directions in familiar manner. This dataset is spectroscopically resolved in accordance with the invention by slicing into a plurality of energy bands in the manner described with reference to figure 2 as represented in the figure. A colour may be assigned to each band. The specific colour palette may be selected as appropriate. For example, colours may be assigned in consecutive direct or inverse relationship to the transmitted frequency, or assigned in accordance with alternative specific protocols to identify particular bands in a particular manner, either on a false colour basis or in an approximation to some real colour representation. Each energy band (and hence each colour band in the visible dataset) varies in presented intensity in accordance with a mapping relation that maintains a functional relationship between intensity in the spectroscopically resolved dataset and intensity in the visible slice presenting as a perceptible intensity in a resultant visible image. While absolute physical intensity or brightness is likely to be a primary cue in this regard, variations in visible cues in the more general sense such as hue and saturation, which will also cause a given colour slice to be more intensely perceived by a human observer of any resultant image, might be exploited.

In a further refinement, it would be possible to generate additional utility from resolved energy and for example energy sliced information by processing the data and volume rendering it.

Although the invention requires only a single ray path, the embodiment of figure 1 presents plural ray paths through an object. Figure 4 illustrates an additional effect that can be created by images generated by means of the multiple ray paths provided by the embodiment of figure 1 which can further enhance the information provided.

As an object 9 passes through incident ray paths 5A to 5C (see figure 4a) three images are generated in which the object is oriented differently relative to the x-ray source 1. Successive display of these images will cause the object to appear to rotate as is illustrated in figure 4b.

This ability in effect to get a view of the object which is in effect rotatable in a third dimension can be seen in some respects as analogous to CT scanning. In a conventional CT scanner, relative rotational movement between scanner and scanned object (usually, by orbital movement of the scanner) allows a rotatable image to be collected. The multiple image generated in this example offers some of these features as a result of the multiple ray paths provided by the apparatus, but with a less complex geometry, and for example on a simple linear conveyor such as is typically used in security scanning systems. This offers additional image functionality. It will be appreciated however that although there are some aspects of this embodiment, and of the information collected thereby, that can be seen as to some extent at least analogous to CT scanning, the embodiment of the invention is not a CT scanning technique and important differences remain.

In this way, in accordance with the invention, an apparatus and method is described which can offer specific material characterisation based on resolved energy detection and data processing to identify materials by the absence or reduction of characteristically scattered band. All this information is obtained from the primary transmitted beam by spectroscopic resolution and processing of the primary collected dataset and without the need for specific detection of characteristically scattered signals.

Claims

1. A method of obtaining an image dataset of an object comprising the steps of: providing a source of electromagnetic radiation adapted to emit radiation in a band other than the visible band and a radiation detector system adapted to detect radiation from the source spaced therefrom to define a scanning zone, wherein the detector system is capable of detecting and collecting spectroscopically resolvable information about radiation incident thereon; collecting a dataset of information about the intensity of radiation incident at the detector at least one and preferably a plurality of scanning positions; resolving the dataset spectroscopically to produce a spectroscopically resolved dataset relating incident intensity to frequency across at least a part of the spectrum of the source; mapping the spectroscopically resolved dataset from the spectrum of the source to the visible spectrum to produce a visible dataset by means of a mapping relation that maintains a functional relationship between frequency and intensity in the spectroscopically resolved dataset and frequency and intensity in the visible dataset.

2. The method of claim 1 wherein the step of mapping the dataset comprises use of a mapping function that maps between frequency in the spectroscopically resolved dataset and frequency in the visible dataset in a serial manner.

3. The method of claim 2 wherein higher frequency radiation in the spectroscopically resolved dataset is mapped to higher frequency radiation in the visible spectrum in the visible dataset.

4. A method in accordance with any preceding claim wherein frequency-variable numerical weighting factors are applied to data in a frequency-specific dataset prior to or as part of the process of image generation to produce a suitably modified visible dataset.

5. A method in accordance with any preceding claim wherein the step of mapping the dataset comprises use of a mapping function that maps between intensity in the spectroscopically resolved dataset and intensity in the visible dataset in a serial manner.

6. A method in accordance with claim 5 wherein the mapping function is such that higher intensity in the spectroscopically resolved dataset is mapped to lower intensity in the visible dataset.

7. A method in accordance with any preceding claim comprising the use of high-energy electromagnetic radiation such as x-rays or gamma-rays, wherein the source is an x-ray or a gamma-ray source, and the detector system is an x-ray or gamma-ray detector system.

8. A method in accordance with any preceding claim wherein the source and the detector system are so arranged as to measure transmissivity of an object in the scanning zone, and wherein the method involves generation of a visible dataset suitable for generation of a transmission radiograph of the object.

9. A method in accordance with any preceding claim wherein the step of resolving the dataset comprises resolving radiation outside the visible spectrum incident at the detector across a plurality of discrete frequency bands and mapping these to discrete colours in the visible dataset.

10. A method in accordance with any preceding claim wherein the step of resolving the dataset comprises resolving radiation effectively continuously across at least a part of the frequency spectrum of the source and mapping this to a correspondingly continuous frequency spectrum within the visible spectrum image

11. A method in accordance with any preceding claim operating on the line-scan principle, comprising: providing a source and a detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one linear detector capable of generating spectroscopically resolvable information about incident radiation; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in accordance with any preceding claim.

12. A method in accordance with claim 11 wherein the radiation source is used to generate a curtain beam.

13. A method in accordance with claim 11 or 12 wherein the detector system comprises a plurality of linear detectors in a laterally spaced serial array at a suitable angular separation, and where data is collected from the resultant multiple ray paths between source and array of linear detectors.

14. A method in accordance with any preceding claim operating on a two dimensional area-scan principle, comprising: providing a source and a detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one area array detector capable of generating spectroscopically resolvable information about incident radiation; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in accordance with any preceding claim.

15. A method of presenting an image of an object comprising generating a visible dataset in accordance with any preceding claim and displaying the visible dataset as a viewable image.

16. An apparatus for obtaining and preferably further for displaying an image dataset of an object comprising: a source of electromagnetic radiation adapted to emit radiation in a band other than the visible band; a radiation detector system adapted to detect radiation from the source band spaced therefrom to define a scanning zone, wherein the detector system is capable of detecting and collecting spectroscopically resolvable information about radiation incident thereon; a resolution module to collect a dataset of information about the intensity of radiation incident at the detector and resolve the dataset spectroscopically to produce a spectroscopically resolved dataset relating incident intensity to frequency across at least a part of the spectrum of the source; a mapping module to map the spectroscopically resolved dataset from the spectrum of the source to the visible spectrum to produce a visible dataset by means of a mapping relation that maintains a functional relationship between frequency and intensity in the spectroscopically resolved dataset and frequency and intensity in the visible dataset

17. An apparatus in accordance with claim 16 wherein the source comprises an x-ray or a gamma-ray source, and the detector system comprises an x-ray or gamma-ray detector system.

18. An apparatus in accordance with claim 16 or 17 wherein the apparatus is enabled to collect transmission information from an object in a scanning zone and comprises a source of radiation and a radiation detector system spaced therefrom to define a scanning zone therebetween, in such manner that measuring the intensity of radiation incident upon the detector system in use constitutes measuring transmissivity of an object in the scanning zone.

19. An apparatus in accordance with one of claims 16 to 18 further including an image generation apparatus to generate an image dataset from the visible dataset.

20. An apparatus in accordance with claim 19 further including an image display adapted to display a viewable image from the image dataset.

21. An apparatus in accordance with one of claims 16 to 20 wherein the detector system is inherently adapted to produce spectroscopic resolution in that it exhibits a direct spectroscopic response.

22. An apparatus in accordance with claim 21 wherein the detector system comprises a material selected to exhibit inherently as a direct material property a direct variable electrical and for example photoelectric response to different parts of the source spectrum.

23. An apparatus in accordance with claim 22 wherein the detector system comprises a direct bandgap semiconductor material.

24. An apparatus in accordance with one of claims 21 to 23 wherein the detector system comprises a semiconductor material formed as a bulk crystal, and for example as a bulk single crystal.

25. An apparatus in accordance with one of claims 21 to 24 wherein the detector system comprises a semiconductor material selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide, thorium bromide.

26. An apparatus in accordance with one of claims 21 to 25 wherein the detector system comprises a semiconductor material selected from Group M-VI semiconductors.

27. An apparatus in accordance with one of claims 16 to 26 operating on the line-scan principle, comprising: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one linear detector capable of generating spectroscopically resolvable information about incident x-radiation.

28. An apparatus in accordance with claim 27 wherein the radiation source is a collimated to produce a curtain beam.

29. An apparatus in accordance with claim 27 or 28 wherein the detector system comprises a plurality of linear detectors in a laterally spaced serial array at a suitable angular separation such that intensity data may be collected in use from the resultant multiple ray paths between source and array of linear detectors.

30. An apparatus in accordance with one of claims 16 to 26 operating on an area imaging principle, comprising: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one two dimensional area detector capable of generating spectroscopically resolvable information about incident x-radiation in two dimensions.