AU2001276493A1

AU2001276493A1 - Portable x-ray imaging apparatus

Info

Publication number: AU2001276493A1
Application number: AU2001276493A
Authority: AU
Inventors: David Burrows; Anthony Carter; Geraint Dermody; Ian Jupp; Ian Pleasants
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2000-08-09
Filing date: 2001-08-02
Publication date: 2002-02-18
Also published as: GB0019452D0; GB0301800D0; EP1308033A1; CA2417708A1; WO2002013517A1; GB2382760A

Description

PORTABLE X-RAY IMAGING APPARATUS

The invention relates to portable imaging apparatus and particularly to apparatus for imaging objects on the remote side of a barrier.

It is well known to carry out security screening using x-ray apparatus. For example, at airports, passenger hand luggage is screened for explosives articles. Such screening tends to be carried out using shadow techniques. By means of a movable x-ray beam from a continuously driven x-ray source, the luggage may be line scanned from one side. To the other side is a large array of x-ray detectors. Any x-ray impermeable objects within the luggage cast a shadow on to the array. The shadows are converted to screen images which are scrutinised by an operator.

Security personnel have a requirement for a non-invasive system of determining the contents of an item without having to relocate it. For example, unclaimed luggage left on the platform of an underground train station requires explosion threat assessment, but moving the luggage in order to determine its contents is of itself risky. Hence, there is a need to be able to determine the contents in situ.

Coded aperture x-ray cameras are well lαiown. These may conventionally have a suitable x-ray detector and a mask situated between the detector and an object under assessment. The mask has x-ray transparent areas arranged in a pre-determined, coded, pattern. X-ray flux from each point source on the object is masked so as to project on to the detector a pattern of flux corresponding to the pattern of the mask. The position of the projected flux pattern is determined by the originating direction of the flux. An image of the object may be reconstructed by decoding the accumulated flux patterns.

The coded aperture camera has advantages over its forerunner, the single pinhole camera.

To maintain high resolution, a single pinhole needs to have as small a diameter as possible. To ensure sufficient sensitivity, the pinhole needs to be as large as possible. Thus, the choice of aperture diameter in a single pinhole camera involves a degree of compromise. However, the coded aperture camera, because it in effect has a pattern of multiple pinholes, achieves both high resolution and sufficient sensitivity.

Coded aperture imaging is used in astronomy.

X-rays impinging upon an object may undergo Compton scattering. This is the result of interaction between the x-ray photons and the electrons of the object. The x-rays may be scattered at various angles and flux intensities, according to the composition of the object. Some will be back-scattered to the same side of the object as the source.

Known back-scatter x-ray screening apparatus which derives an image from x-rays back- scattered from an object is not easily portable, not least because it typically tends to use a mechanically scanned x-ray beam, necessitating equipment of a size and weight which does not lend itself to portability.

Conventional thinldng is that back-scattered x-rays produced intermittently by a portable pulsed source offer insufficient flux for imaging.

The invention provides apparatus for imaging an object on the remote side of a barrier comprising source means for illuminating the object through the barrier with radiation, detector means for detecting radiation back-scattered from a plurality of points on the object, mask means having a plurality of radiation transparent areas arranged in a predetermined pattern wherein the back-scattered radiation from each point is masked thereby to project on to the detector means at a position determined by the position of the point a pattern of radiation corresponding to the pattern of the mask, image generation means for generating an image of the object from an analysis of the accumulated plurality of patterns of radiation. Using a mask having a plurality of radiation transparent areas enables the apparatus according to the invention to perform imaging with good angular resolution and maximises the radiation reaching the detector.

The apparatus may be portable, that is to say, it may be of a size and weight which lends itself to portability. Individual elements of the apparatus may be portable or the apparatus may be portable as a whole. Portability enables the apparatus to be taken to the location of an object requiring assessment which, as aforementioned, is advantageous in the case of objects requiring elimination as security threats.

The source means may illuminate the object with x-ray radiation, that is, radiation in the electromagnetic spectrum of wavelengths less than about lA.

The source means may comprise a pulsed x-ray source. Pulsed x-ray sources, because they require only sufficient components to produce a pulse of x-rays, tend to be compact and portable.

The mask pattern is preferably selected, according to a mathematical function, so as to code the radiation projected on to the detector means in a manner which can be subsequently decoded. The mask may be used in a so-called pyramidal configuration, in which it occupies a smaller area than the detector means, a reverse pyramidal configuration, in which it occupies a larger area than the detector means, or may be substantially the same size as the detector means.

The detector means preferably has sufficient spatial resolution in order to make use of a high resolution projected pattern. Also preferably, if an x-ray source is used, the detector means needs to be able to stop x-ray photons of energies up to 200 KeN. The detector means is further preferably capable of processing a high rate of incident radiation. For instance if a portable, pulsed, x-ray source is used, the detector needs to have the capability to process the maximum amount of projected radiation in a given time. An energy resolving capability is also preferable so that the detector means can be tuned to a particular energy window thereby contributing to the optimisation of image quality.

If an x-ray source is used, the detector means may comprise a scintillation detector having an electron bombardment intensifier tube including a converter screen, a photocathode, an accelerator stage and a charge coupled device. Such detector means has an energy resolution capability. Alternatively, the detector means may comprise a solid state detector, for instance, amorphous silicon based, having a relatively large collection area, a high spatial resolution and may be coupled to a converter screen for the detection of higher energy x-rays .

The image generation means may comprise a system driven by software to decode, by mathematical analysis, the accumulated patterns of radiation projected on to the detector means thereby to reconstruct an image of the object. The mathematical analysis may comprise filtered deconvolution, matrix inversion, maximum likelihood/maximum entropy reconstruction or cross-correlation.

The invention will now be described, by way of example, with reference to the following drawings, in which:

Figure 1 is a schematic diagram of apparatus according to the invention;

Figure 2 is a diagram of a typical coded pattern for a mask used in the apparatus shown in figure 1 ;

Figure 3 is a schematic cross-sectional diagram of an example of masked flux patterns of the type which may be seen in the apparatus according to the invention; and Figures 4(a)-(c) are diagrams of an object for assessment, flux patterns projected on to detector means and an image generated in apparatus according to the invention respectively.

With reference to figure 1, an object 4 under assessment is situated to one side of a barrier 6. The object is represented schematically in the figure as a three dimensional body but could be a gun, a knife, an explosives detonator etc. The barrier 6 could be the skin of a brief case, suit case etc.

Portable imaging apparatus, indicated generally at 1A^S located on the other side of the barrier 6, typically one metre away. The apparatus 1 comprises a source 2, a detector 8 and a mask 10.

The source 2 is a hand held, in the sense of hand transportable, pulsed x-ray source, nominally 500 x 200 x 200 mm, weighing approximately 10 Kg. The source is self- powered by a 18N rechargeable battery pack and includes a spiral capacitor which creates the EMF required to accelerate electrons at a target and thereby produce x-rays. The source 2 emanates successive 60ns pulses of x-rays, typically of energies up to in the region of 300 KeN, which are collimated. X-ray pulses are produced every 50ms for up to in the region of 1000 cycles.

The separately portable detector 8 is a known electron bombardment image intensifier tube, nominally 350 x 200 x 150 mm, weighing approximately 10 Kg. The detector 8 essentially comprises (none of the following shown) a converter screen (columnar grown Csl (TI) scintillator) which converts x-ray photons into visible light photons, a photocathode which emits photoelectrons in response to photon bombardments, a single accelerator stage for accelerating the photoelectrons and a high resolution charge coupled device on to which the photoelectrons are focused. Such a detector provides an energy resolution capability so that the energy of the x-ray photons may be determined. The mask 10 is a hexagonal coded aperture mask, approximately 200mm across, having x-ray transparent areas in a pre-determined pattern. The pattern is chosen according to an autocorrelation function of an appropriate form, that is, a delta function. In effect, the mask can be considered as an array of, typically 1000, elements, and selected elements, typically 500, are x-ray transparent. Figure 2 shows typical coded aperture mask patterning.

Each pulse of x-rays is directed from the source 2 towards the barrier 6 with the intention of illuminating the object 4. Some of the x-ray photons which penetrate the barrier 6 impinge upon the object 4. Due to Compton scattering, a proportion of the photons impinging upon the object 4 will be back-scattered towards detector 8. Each back- scattering point source on the object 4 will produce an x-ray flux. The mask 10 interrupts any flux whose path is towards the detector 8 and a pattern of x-ray flux corresponding to the coded pattern of the mask is projected on to the detector 8. The position at which the pattern is projected is determined by the originating direction of the flux. Figure 3 shows how flux originating from two different, spaced apart, points on the object, producing fluxes 1 and 2 respectively, projects a flux pattern corresponding to the coded mask pattern at different positions on the detector. The position of the flux pattern on the detector is shown by the lines corresponding to the direction of the flux making the pattern, and in certain areas the two patterns overlap.

The source 2 is run for between 100 and 1000 pulses, according to the distance to the object 4, thickness of the barrier 6, by which time a sufficient number of patterns have been projected on to the detector 8 to enable the analysis and reconstruction of an image. This is achieved using a PC (not shown) which mathematically decodes the accumulated patterns and using the decoded information generates a screen image of the object under assessment. Figure 4 sequentially shows (a) an object under assessment, (b) the accumulated patterns of back-scattered flux projected from the object on to the detector 8 and (c) an image of the object reconstructed from the accumulated patterns.

Claims

1. Apparatus for imaging an object on the remote side of a barrier comprising source means for illuminating the object through the barrier with radiation, detector means for detecting radiation back-scattered from a plurality of points on the object, mask means having a plurality of radiation transparent areas arranged in a pre-determined pattern wherein the back-scattered radiation from each point is masked thereby to project on to the detector means at a position determined by the position of the point a pattern of radiation corresponding to the pattern of the mask means, image generation means for generating an image of the object from an analysis of the accumulated plurality of patterns of radiation.

2. Apparatus according to claim 1 which is portable.

3. Apparatus according to claim 1 or claim 2 wherein the source means illuminates the object with x-ray radiation.

4. Apparatus according to claim 3 wherein the source means is a pulsed x-ray source.

5. Apparatus according to any of claims 1 to 4 wherein the mask pattern is selected, according to mathematical function, thereby to code the radiation projected on to the detector means in a manner which can be subsequently decoded.

6. Apparatus according to claim 3 or claim 4 wherein the detector means is able to stop x-ray photons of energies up to 200 KeV.

7. Apparatus according to any preceding claim wherein the detector means has an energy resolving capability.

8. Apparatus according to claim 3 or claim 4 wherein the detector means comprises a scintillation detector having an electron bombardment intensifier tube including a converter screen, a photocathode, an accelerator stage and a charge coupled device.

9. Apparatus according to claim 3 or claim 4 wherein the detector means comprises a solid state detector.

10. Apparatus according to claim 9 wherein the solid state detector is coupled to a converter screen for the detection of higher energy x-rays.

11. Apparatus according to any preceding claim wherein the image generation means comprises a system driven by software to decode, by mathematical analysis, the accumulated patterns of radiation projected on to the detector means thereby to reconstruct an image of the obj ect.

12. Apparatus according to claim 11 wherein the mathematical analysis comprises filtered deconvolution, matrix inversion, maximum likelihood/maximum entropy reconstruction or cross-correlation.

13. Apparatus for imaging an object on the remote side of a barrier substantially as herein described with reference with to figures 1 to 3 of the drawings.