WO1996009560A1

WO1996009560A1 - Gamma ray imaging

Info

Publication number: WO1996009560A1
Application number: PCT/GB1995/002101
Authority: WO
Inventors: Paul Michael Willson; Anthony John Dean; Anthony John Bird
Original assignee: Kennedy & Donkin Systems Control Limited; University Of Southampton
Priority date: 1994-09-21
Filing date: 1995-09-07
Publication date: 1996-03-28
Also published as: GB2293742A; GB9419040D0

Abstract

A gamma ray imager is described having an imager head assembly (2) comprising a coded aperture mask (4), a gamma ray shield (6) and an array (8) of detector elements. The imager head assembly is rotated during exposure to provide an extended field of view over the instantaneous field of view provided by the imager head assembly (2). The rotation of the imager head assembly (2) takes place about a vertical axis (22) through the coded aperture mask (4) and is driven by a stepper motor.

Description

GAMMA RAY IMAGING

This invention relates to the field of gamma ray imaging. More particularly, this invention relates to gamma ray imaging using coded aperture masks.

The use of coded aperture masks and arrays of gamma ray detectors is known in the field of gamma ray astronomy. This technology has been successfully applied in balloon-borne and satellite-borne instruments for investigation of high energy cosmic events such as black holes, super novae and active galactic nuclei. These applications require high angular resolution and extreme sensitivity despite the presence of interfering background radiation. n a coded mask imaging technique, a mask with opaque and transparent regions (the coded aperture) is placed in front of a position sensitive photon detector (see Figure 1 of the accompanying drawings). A beam of photons incident on the mask will be absorbed where it strikes opaque areas of the mask, thus casting a shadow onto the detector. For a point source of photons the registration of this shadow on the detector may be used to determine the direction of the source, provided that the mask pattern is suitably chosen. A collection of point sources casts overlapping shadows and with an appropriately chosen mask pattern the directions of the point sources may be determined by correlation techniques. A complex field can be imaged by determining the contribution of each pixel in the source to the combined "shadowgram".

Many possibilities exist for the choice of the mask pattern, the simplest form being the pinhole camera, which can be regarded as a coded mask imager in which the mask is opaque except for a single small region - the pinhole. In this case there is a one-to-one correspondence between intensities at positions in the position- sensitive detector and image brightness. However, where the flux is low, as in gamma ray astronomy or environmental monitoring, it is advantageous to use a mask with a larger open fraction - it can be shown that in the background limited case the best signal to noise ratio is obtained with a mask that is half transparent and half opaque.

There are known mathematical methods for generating optimum mask patterns with a larger open area than a single pinhole and for which image reconstruction techniques can produce an unbiased estimate for each resolution element in the field of view. All of the patterns can be considered to consist of an array of holes of the same size repeated on a regular grid. The formally optimum patterns are cyclic and, if the recorded shadow contains a complete number of cycles of the pattern, then the imaging properties are the same as for the single pinhole, i.e. the point source response function is triangular with a width given by the ratio of the hole size to detector-mask separation. However, the signal to noise ratio is much better. In the background limited case with 50% open mask, the statistical significance of the determination of the flux from the source direction is given by N./(N_tot)*, where N_β and N_tot are respectively the number of detected source counts and the total number of counts from all sources and the background. It is of note that coded-mask systems produce "statistical'' images, i.e. no direction is assigned to an individual photon, but rather it is the ensemble of all detected photons which yield a probabilistic estimate of the flux in each direction. Most image reconstruction techniques are based on correlation of the observed shadowgram with those which would be expected from sources situated in each pixel.

Coded-mask imaging relies on simple geometry and ray-optics and the performance of real systems is found to be essentially the same as predicted from the properties of the detector and the mask. Because the recorded shadowgram is analogous to a hologram in that information about a particular part of the field of view is spread over the entire detector, the technique is tolerant of detector (or mask) defects, e.g. the existence of obscured or unusable regions. The simple geometric nature of the imaging process means that sources can be accurately positioned within the field of view with few problems of alignment or calibration.

The successful use of coded aperture imaging demands a high performance position sensitive gamma ray detection system. An array of conventional gamma ray detectors, scintillator crystals each coupled to a photo-multiplier tube, present severe limitations of resolution, stability and cost. Advances in solid state detectors have made possible the construction of arrays of small (typically 1 centimetre square) scintillator crystals coupled to PIN photodiodes and extremely low noise electronics which offer substantially better resolution and stability. Gamma ray imagers generally include many detector elements to provide an adequate resolution over the fixed field of view. The large number of these sophisticated elements, generally in excess of 200, and often over 1000, represents a major part of the cost and complexity of such imagers, severely restricting their application.

Viewed from one aspect the invention provides apparatus for gamma ray imaging, said apparatus comprising: an imager head assembly having an array of gamma ray detector elements disposed in a detector plane, a gamma ray shield enclosing said array, said gamma ray shield having an aperture opening therein and a coded aperture mask disposed over said aperture opening for creating a shadowgram in said detector plane, said imager head having an associated instantaneous field of view; an imager head actuator for moving said imager head assembly whilst said array detects gamma rays; and image constructing means responsive to signals representing gamma rays detected by said array and signals representing associated imager head assembly positions for constructing a gamma ray image over an extended field of view larger than said instantaneous field of view.

The invention provides a less complex and less expensive gamma ray imager. The invention enables a reduction in the size of the array and number of gamma ray detector elements for a given field of view, resolution and sensitivity by moving the image head assembly to effectively scan the array through a larger virtual shadowgram. In addition to the direct saving through the reduction in the number of detector elements - required, the processing electronics is also simplified: fewer data channels are needed to handle the data from the detectors. The incorporation of scanning in the construction of a larger image also provides for flexibility in the creation of a seamless and concurrent image of any chosen field of view larger than the instantaneous one. Despite the addition of shielding to the detector array, the reduced size of the imager head assembly means that the weight of the imager is not significantly increased above that of a static imager of comparable field of view, resolution and sensitivity and the imager head is more compact making it more readily transportable.

As previously discussed, coded aperture imaging is a statistical process where the gamma ray events over an extended exposure period are collected and then a two dimensional correlation analysis performed to yield the image representation. In the context of such extended exposure operation, the moving of the imager head assembly during gamma ray detection is opposite to what would normally be considered to be desirable. Indeed, in order to provide finer imaging detail, it would be considered desirable to take measures to hold the imager head assembly as still as possible. In practice the controlled movement of the imager head assembly during the exposure period can be dealt with by supplying the image constructing means with signals indicative of the imager head assembly position so that this factor can be taken into account for each gamma photon detected. The statistical nature of the imager using a coded aperture mask enables this approach. By contrast, consider a normal optical camera in which movement of the camera during the exposure period produces a blurred image and is extremely undesirable.

The imager head assembly can be moved in one or more directions as required by the particular application. The most generally applicable degree of freedom that may be provided is one in which said imager head actuator rotates said imager head assembly about a vertical axis.

When rotating about such an axis it is strongly desirable that the axis should lie in the plane of the coded aperture mask. This eliminates parallax errors that would otherwise be introduced into the system and require correction.

Whilst the difference in size between the instantaneous field of view and the extended field of view could be of any magnitude within reason, an advantageous balance between the necessary exposure times, the size, weight, cost and complexity of the equipment and the imaging performance is achieved when the solid angle of said extended field of view is between three and seven times larger than said instantaneous field of view. Whilst any level of shielding could, within practical limits, be selected, an advantageous balance between the benefit of improved sensitivity and increased cost and weight of the shielding is achieved when said gamma ray shield has a shielding factor in the range of /(ø/*4π) to (ø/4π), where Φ is the instantaneous field of view in Steradians.

Once a gamma ray image has been generated it is advantageous to the analysis of the image if it may be overlaid on top of an image from a different spectral range (e.g. visible, ultraviolet or infrared radiation) . In the applications where the imager of the invention is intended to be used (e.g. industrial radiation monitoring) , the ability to identify gamma ray hotspots is particularly useful. In order to facilitate this, preferred embodiments of the invention comprise a camera fixed to said imager head assembly to capture an image of a scene under analysis to be overlaid with a gamma ray image obtained with said imager head assembly.

The motion applied to the imager head assembly by the imager head actuator could be a stop/start motion to advance the imager head assembly along its scanning path. However, in preferred embodiments said imager head actuator continuously rotates said imager head assembly throughout a scanning operation.

Continuously rotating the imager head assembly provides for a smoother and more reliable operation. The continuous motion may be compensated for by the image constructing means that is supplied with signals representing the imager head assembly position.

In order to deal efficiently with the detected events from this moving system, gamma rays detected by said array are allocated by said image constructing means to pixels within a virtual shadowgram over said extended field of view in dependence upon said signals representing imager head assembly positions.

The allocation of detected gamma events to the virtual shadowgram may be exploited by embodiments of the invention in which said array of gamma ray detector elements undersamples said shadowgram produced by said coded aperture mask, said array of gamma ray detector elements having fewer elements than said coded aperture mask.

In this way a saving may be made in the total number of the relatively expensive detector elements that are required, with a trade off being made in the sensitivity of the imager.

An advantage of the statistical nature of the imaging operation and the movement of the imager head assembly is that the pixels in the virtual shadowgram may be made smaller than would normally be dictated by the finite size of the detector elements. Thus, whilst a detector element might have a one square centimetre cross section, it can be moved via the imager head actuator through small distances (e.g. 0.1 centimetres) and the difference in gamma ray count at these different positions used to resolve to a level smaller than that of the detector element itself.

An indication of the distance of objects in the gamma ray image from the imager can be made by providing that said image constructing means constructs a plurality of gamma ray images assuming differing magnifications of said coded aperture mask on said array.

The different magnifications correspond to different source to imager separations and the one which yields the best image definition will correspond to the distance of the actual source from the imager. This analysis of best definition may be automated. Such analysis of the distance of the sources is made more precise by the improved resolution of the images. This improved precision enables better estimates of the absolute strength of the sources to be made permitting reduced work in radiation areas and better assessment of hazards. In order to facilitate ready transport of the imager said imager head assembly is demountably fitted to a support stand.

A particularly convenient way to fit the imager head assembly to the support stand is by a spigot joint.

In order to provide positive location of the imager head assembly it is desirable that said spigot joint is keyed against rotation when said imager head assembly is in an operational position.

A particular ability of an imaging system of this type is to provide at least some spectroscopic analysis of the received gamma rays. In this way, not only can the gamma ray source be located, its nature may be determined, i.e. the energy of the gamma rays is characteristic of their source nuclide.

It is preferred to use a programmed general purpose computer (such as a personal computer (PC)) as the image constructing means. Such a general purpose computer is also able to coordinate data collection, scanning control and optical picture capture as well as post exposure image construction and presentation. Such a programmed computer provides the benefits of simplicity, flexibility, economy, easy data transfer and portability.

Viewed from another aspect, this invention provides a method of gamma ray imaging using an imager head assembly having an array of gamma ray detector elements disposed in a detector plane, a gamma ray shield enclosing said array, said gamma ray shield having an aperture opening therein and a coded aperture mask disposed over said aperture opening for creating a shadowgram in said detector plane, said imager head having an associated instantaneous field of view, said method comprising the steps of: moving said imager head assembly with an imager head actuator whilst said array detects gamma rays; and in response to signals representing gamma rays detected by said array and signals representing associated imager head assembly positions, constructing a gamma ray image over an extended field of view larger than said instantaneous field of view.

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

Figure 1 illustrates the general principle of coded aperture mask imaging; Figure 2 illustrates example coded aperture masks;

Figure 3 schematically illustrates the operation of a mask;

Figure 4 illustrates an imager head assembly;

Figure 5 illustrates an imager;

Figure 6 illustrates the mounting used for the imager head assembly;

Figure 7 illustrates scanning through a virtual shadowgram; and

Figure 8 illustrates the electronic arrangement of the imager.

Figure 1 schematically illustrates the operation of coded aperture mask imaging. The source creates a shadowgram by virtue of the photons passing through transparent regions in the coded aperture mask. The shadowgram is then subject to computer analysis to recover an image of the source.

On a theoretical level, if the image in the source plane is denoted by S(x,y) and the aperture transmission function is denoted by A(x,y) , then the spatial distribution of the detected flux in the shadowgram is given by: D(x,y) = A(x,y) * S(x,y) ♦ B(x,y)

where * is the convolution operator and B represents the source independent background. An image S' of the object may be obtained by filtering the detected shadowgram with a suitable function G(x,y), in a process known as a cross-correlation de-convolution in which:

S' (x.y) = G(x.y) * A(x,y) • S(x.y) + G(x,y) * B(x.y)

Choosing G to be the inverse of A would, theoretically, yield a perfect reconstruction of the source distribution in the absence of noise. In practice, this procedure may cause a large amplification of the noise term, and both A and G are selected to minimise the noise component introduced into the reconstructed image.

Another way of viewing the above is to consider that the cross- correlation method compares the detected photon distribution with the expected photon distribution for a source at each point in the field of view. The strength of that correlation determines the source intensity which is then ascribed to that -point in the image plane. This is a simple method which works well when the coded aperture has been selected to have an auto-correlation function with flat side lobes, as is .the case for the mask described herein.

Cross-correlation de-convolution is particularly suitable when the number of pixels involved is relatively small. This is the case in a gamma ray imager described herein. The computational tasks involved in such cross-correlation de-convolution are of a level where a commercially available personal computer provides sufficient processing power to achieve the de-convolution reasonably quickly. Figure 2 illustrates masks suitable for use with a 15 x *l array of detector elements. A 1:1 correspondence between the mask and detector pixel sizes is chosen. The basic pattern illustrated in Figure 2 is a 59-element mask arranged in a 1. x *t array (minus one pixel). Th-Ts mask is also suitable for 15 x . , 15 x 2 and 15 x 1 arrays of detector elements utilising suitable undersampling correction as the virtual shadowgram is constructed during a scanning operation. The left hand pattern in Figure 2 shows the basic mask, with the right hand pattern showing the full mask used to achieve the desired imager head field of view. With detector pixels and mask pixels measuring 1cm x 1cm and the mask placed 20cm from the array of detectors, the following performance parameters are achieved:

1. Angular resolution = 2.9 degrees;

2. Imager head field of view = 39 degrees x 9 degrees;

3. Nominal image field of view 39 degrees x 45 degrees; and

4. Point source location approximately 0.3 degrees (lOσ); 2cm at 4m

The mask thickness is determined by a number of factors. The primary requirement is to achieve greater than 7 % attenuation throughout the selected operational gamma ray energy band, i.e. approximately 200 to 700keV in this case. A secondary requirement is that the mask material should not produce fluorescence photons within the energy band detector. Finally, the mask should be as thin as possible, not only to minimise the mass, but also to reduce the effect of vignetting by the finite mask thickness. This effect will be particularly apparent near the edges of the field of view, and will be particularly apparent when the mask thickness is more than 1.5cm.

The two materials that are traditionally used in this application are lead and tungsten. In practice, tungsten provides considerably higher attenuation for a given thickness making it preferable in this application. The basic mask is constructed of 1cm thick tungsten elements using a fundamental 1cm x 1cm mask element size. A strong and compact array is achieved by sandwiching and gluing the tungsten elements in the pattern illustrated in Figure 2 between two thin (approximately 0.5mm) aluminium sheets with suitable attachment points provided to fix the mask to a support structure. A number of masks of different patterns for various uses (e.g. point sources, extended sources, strong sources etc.) may be provided as interchangeable "optics'¹ for the imager.

Coded aperture imaging has to date been used in astrophysics applications where the source of the gamma rays can be regarded as being at infinity. However, when the source distance becomes finite (and in particular for applications where the source distance may not be well known in advance) the operation and analysis of the coded aperture system becomes more complex. As can be seen in Figure 3. the projected mask pixel size in the detector plane will not be the same for sources at different distances. A coded aperture imager can effectively "focus" at a given distance by applying a suitable demagnification factor to the mask pattern when before it is used in the de-convolution process. Higher magnifications correspond to the source being closer.

Rather than being a disadvantage to the operation of the system, this effect can be turned to advantage by using it to yield some information as to the distance of a source from, the detector. This is achieved by de-convolving the detected shadowgram using a range of demagnification factors and determining for each object within the gamma ray image which demagnification factor gives the best gamma ray image definition for that object. Determining which demagnification factor gives the best definition results in an indication of the distance of that object from the detector.

Figure 4 illustrates a vertical cross-section through an imager head assembly having a 15 x 4 array of detector elements. The imager head assembly 2 comprises a coded aperture mask 4, a lead shield 6 and an array 8 of detector elements. Each detector element is formed of a lcm x 1cm x 2cm block of high-Z CsI(Tl) scintillator material 10 fixed to a photodiode 12 with a fast pre-amplifier 14 responsive to the photodiode 12. The individual gamma ray detectors are supported within a honeycomb of thin (less than 0.5mm) aluminium rectangular tubes 15- The honeycomb is mechanically and optically closed at either end by means of a thin (less than 1mm) aluminium plates which are glued at the Csl end and mechanically detachable at the preamplifier end. Such an arrangement provides a compact, robust and electrically and optically isolated system for which the individual detection elements are easily accessible. In order to reduce the weight of the shield 6, the array 8 is mounted such that the direction of view for incident gamma rays is through the pre-amplifiers to the Csl crystals. Negligible attenuation occurs for the gamma rays, which typically have an energy in excess of 200keV, when passing through the electronics. The probability of direct gamma ray interaction in the photodiode 12 is also small at these energies. An integral unit of this type is advantageously manageable for pre-assembly testing and later for examination and repair as necessary. A multi-way connector is provided for the electronic interface to the associated personal computer that functions as the means of image reconstruction. The shield 6 is fabricated from lead and is the heaviest part of the imager. Accordingly, the shield 6 is associated with the overall support structure. The shield comprises a bottom and four sides which are held together by means of a framework of mild steel or aluminium. The framework also provides attachment points for the mask 4, an associated optical camera, a support structure and any alignment mechanisms. The array of detection elements 8 is attached to the bottom (back) element of the shield 6.

Figure 5 illustrates the imager head assembly 2 mounted on a support structure in the form of a tripod 16. A rotating mount 18 is provided between the tripod 16 and the imager head assembly 2. An imager head actuator, in the form of the rotating mount 18, is most conveniently arranged between the imager head assembly 2 and the tripod 16. The scanning movement in this case is made within the horizontal plane so that simple rotation will suffice. The imager head assembly 2 will have a typical mass of about 25kg, which can be supported by this type of mounting. The drive for the actuator is provided by a geared stepping motor controlled by the personal computer. Standard commercially available systems are available that are capable of a rotational accuracy of 1/100 degrees and so an open loop control system is quite adequate. This reduces the system cost since no shaft encoder is required for position feedback. The signals representing the imager head assembly position are essentially generated within the personal computer which has the information as to where the imager head assembly 2 has been directed. An optical camera 20 is mounted on the imager head assembly 2 and provides direction sighting and an optical image that may be overlaid upon the gamma ray image. The imager head assembly 2 rotates about a vertical axis 22 that passes through the coded aperture mask 4 and the centre of the objective lens of the optical camera 20. Control to the imager head actuator 18 from the remote personal computer is provided via a cable 24. Cable 25 carries the signals from the array 8 representing detected gamma ray events to a remote personal computer. The imager head actuator 18 can be controlled to vary the extent of the angle of scan through which the imager head assembly 2 is moved (i.e. to vary the extended field of view). The duration of the scan can also be varied depending upon the gamma ray source strength to reach an appropriate exposure period. In addition, the positions at which the optical camera 20 takes one or more video images to cover the extended field of view can also be varied.

The imager head actuator 18 is illustrated in more detail in Figure 6. The mounting is detachable. This is provided by the use of a spigot joint having a male portion 26 and a female portion 28. Keying between the two parts of the spigot joint against unwanted rotation is provided by engagement balls 30. The male portion 26 of the spigot joint is rotatably fixed to the tripod 16 via an upper sealed roller bearing 32 and a lower sealed ball bearing 34. A stepper motor and gearbox 3 drives the male portion 26 via a keyed shaft 8. An electrical connector 40 supplies power and control signals from the personal computer to the stepper motor and gearbox 36.

The rotation of the imager head actuator 18 extends the field of view of the system to be greater than the instantaneous field of view. Figure 7 illustrates the relationship between a source, the mask

4 and the detector array 8. The detector array 8 effectively traverses a static shadowgram of the mask created by the various sources within the scene under study. The first step in image construction by the personal computer is to allocate each detected gamma ray event from a detector element to a pixel within the reconstructed static shadowgram. The pixels in the recreated static shadowgram are much narrower than the detector pixels, typically being 1mm wide and 10mm high. Each detected gamma ray event would be allocated to the virtual pixel which lay behind the centre point of the detector pixel at the moment of detection. This correspondence can be determined by reference to the imager head actuator position as instructed (or as encoded if a head position encoder is provided) .

Once a scan has been completed, the data in the recreated virtual shadowgram is de-convolved by the personal computer to produce a gamma ray image of the scene under study. The cross-correlation algorithm previously discussed may be used and yields good results. The deconvolution process is either applied to all events detected or is applied selectively according to the magnitude of each event detected and digitised. Selection by magnitude enables an image of emissions by specific nuclides, each having particular gamma ray emission lines (with corresponding event magnitudes) , to be made to characterize the distribution and nature of the sources in the extended field of view. As previously discussed, the virtual shadowgram is de-convolved in this way for different magnifications of the mask corresponding to a series of different distances of source from the imager. Comparison of the images generated for different distances show the sources at their highest definition at their actual distance from the imager. This comparison may be made visually by the user or may be implemented automatically with conventional image processing techniques within the personal computer to create a three-dimensional image of the sources. The resolution of the image in depth will be significantly lower than that for the other dimensions.

The three-dimensional image may be used to convert the brightness of the sources in the field of view into estimates of their absolute intensity, which is indicative of the hazard they represent. Displaying this data permits the imager to be used to eliminate other source assessment tasks and for the planning of activity in an area with potentially dangerous gamma ray sources.

Figure 8 illustrates the electrical connections within the system. The array of detectors are coupled to high-gain, low-noise pre-amplifiers that are in turn multiplexed together by a multiplexer 42 onto an analogue to digital converter 46 (ADC) which digitizes the sensed gamma ray events for transfer into a personal computer 44. Events large enough to trigger the discriminator 50 cause the logic 5 to select the signal with the multiplexer 42 for digitisation by the ADC 46. The personal computer 44 drives a stepper motor controller 48 that supplies the rotating mount 18 associated with the stepper motor and gearbox 36. The optical camera 22 is linked to a frame grabber 5 that records optical images in front of the imager head assembly 2 when instructed by the personal computer 44. The reconstructed gamma ray image is overlaid on the optical image and displayed on a display unit 54.

Claims

1. Apparatus for gamma ray imaging, said apparatus comprising: an imager head assembly having an array of gamma ray detector elements disposed in a detector plane, a gamma ray shield enclosing said array, said gamma ray shield having an aperture opening therein and a coded aperture mask disposed over said aperture opening for creating a shadowgram in said detector plane, said imager head having an associated instantaneous field of view; an imager head actuator for moving said imager head assembly whilst said array detects gamma rays; and image constructing means responsive to signals representing gamma rays detected by said array and signals representing associated imager head assembly positions for constructing a gamma ray image over an extended field of view larger than said instantaneous field of view.

2. Apparatus as claimed in claim 1, wherein said imager head actuator rotates said imager head assembly about a vertical axis.

3- Apparatus as claimed in claim 2, wherein said vertical axis lies in the plane of said coded aperture mask.

4. Apparatus as claimed in any one of the preceding claims, wherein the solid angle of said extended field of view is between three and seven times larger than said instantaneous field of view.

5. Apparatus as claimed in any one of the preceding claims, wherein said gamma ray shield has a shielding factor in the range of /(Φ/4π) to (Φ/4π), where Φ is the instantaneous field of view in Steradians.

6. Apparatus as claimed in any one of the preceding claims, comprising a camera fixed to said imager head assembly to capture an image of a scene under analysis from a different energy range in the electromagnetic spectrum to be overlaid with a gamma ray image obtained with said imager head assembly.

7. Apparatus as claimed in any one of the preceding claims, wherein said imager head actuator continuously rotates said imager head assembly throughout a scanning operation.

8. Apparatus as claimed in any one of the preceding claims, wherein gamma rays detected by said array are allocated by said image constructing means to pixels within a virtual shadowgram over said extended field of view in dependence upon said signals representing imager head assembly positions.

9- Apparatus as claimed in claim 8, wherein said array of gamma ray detector elements undersamples said shadowgram produced by said coded aperture mask, said array of gamma ray detector elements having fewer elements than said coded aperture mask.

10. Apparatus as claimed in any one of claims 8 and 9- wherein said pixels in said virtual shadowgram are smaller than said elements of said array.

11. Apparatus as claimed in any one of the preceding claims, wherein said image constructing means constructs a plurality of gamma ray images assuming differing magnifications of said coded aperture mask on said array.

12. Apparatus as claimed in any one of the preceding claims, wherein said elements of said array generate signals indicative of gamma ray energy to enable said image constructing means to perform spectroscopic analysis to present images according to selected energies characteristic of emitting nuclides.

13. Apparatus as claimed in claims 11 and 12, wherein said image constructing means produces images indicative of source intensity, position, distance and composition.

14. Apparatus as claimed in any one of the preceding claims, wherein said imager head assembly is demountably fitted to a support stand.

15. Apparatus as claimed in claim 14, wherein said imager head assembly is demountably fitted to said support stand by a spigot joint.

16. Apparatus as claimed in claim 15. wherein said spigot joint is keyed against rotation when said imager head assembly is in an operational position.

17. Apparatus as claimed in any one of the preceding claims, wherein each of said elements within said array comprises an inorganic scintillator coupled to a photodiode supplying input to a high gain amplifier.

18. Apparatus as claimed in any one of the preceding claims, wherein said imager head actuator comprises a stepper motor.

19. Apparatus as claimed in any one of the preceding claims, wherein said image constructing means is remotely located from said imager head assembly.

20. Apparatus as claimed in claim 19. wherein said image constructing means comprises a programmed general purpose computer.

21. Apparatus as claimed in claim 20, wherein said programmed general purpose computer coordinates gamma event detection, control of imager head movements and gamma ray image data presentation.

22. Apparatus as claimed in claims 6 and 20, wherein said programmed general purpose computer coordinates operation of said camera.

23. A method of gamma ray imaging using an imager head assembly having-an array of gamma ray detector elements disposed in a detector plane, a gamma ray shield enclosing said array, said gamma ray shield having an aperture opening therein and a coded aperture mask disposed over said aperture opening for creating a shadowgram in said detector plane, said imager head having an associated instantaneous field of view, said method comprising the steps of: moving said imager head assembly with an imager head actuator whilst said array detects gamma rays; and in response to signal^'s representing gamma rays detected by said array and signals representing associated imager head assembly positions, constructing a gamma ray image over an extended field of view larger than said instantaneous field of view.

24. A method of planning work operations within a radioactive environment comprising constructing a gamma ray image with an apparatus for gamma ray imaging as claimed in any one of claims 1 to 21.

25. A method controlling removal of radioactive material from an area comprising constructing a gamma ray image of said area using an apparatus for gamma ray imaging as claimed in any one of claims 1 to 21 and characterising imaged radioactive material in dependence upon said gamma ray image to control removal operations.

26. Apparatus for gamma ray imaging substantially as hereinbefore described with reference to the accompanying drawings.

27. A method of gamma ray imaging substantially as hereinbefore described with reference to the accompanying drawings.