GAMMA-RAY IMAGING

Abstract: The specification describes an instrument and a method for determining information about an object (J), only one side of which is available for examination. The method involves exposing the object to gamma-rays and measuring the position and/or time of arrival of gamma-rays at a detector (D). The instrument includes a source of gamma-rays (S) located so that at least some gamma-rays impact upon the object, and a detector surrounded by a shield (C) having an aperture (A) for facing at the object to be studied. The detector is capable of measuring the position and/or time of arrival at the detector of gamma-rays passing through the aperture.

Description: GAMMA-RAY IMAGING Technical Field The invention relates to the use of gamma-rays to produce an image of an object. In particular, the invention is useful in applications where a 1, 2 or 3-dimensional image is required and there is access to only one side of the object. Background Art Gamma-rays are widely used to produce images of extended objects, for example for medical diagnoses. When there is access to both sides of the object being studied, the conventional approach is to measure the attenuation of a gamma-ray beam passing through the object from a source on one side of the object to a detector on the other. If a wide area beam is used together with a position sensitive detector, a two dimensional map of the object is produced. To produce a 3-dimensional image, multiple two dimensional slices can be combined using computed tomography (CT) techniques. If the object being studied can be injected with a positron emitting nuclide, positron emission tomography (PET) can be used to build up a 3-dimensional image of the object by using the back-to-back 511 keV gamma-rays produced when the positron annihilates. Throughout the specification the term gamma-ray means electromagnetic photons having an energy of about 1 keV or more and includes electromagnetic photons normally known as X-rays which range up to about 100 keV. When there is access to only one side of the object being studied, techniques based on gamma-ray transmission are impossible. Compton scatter imaging (CSI) has been proposed as an alternative method. Gamma-rays from a source pass into the object being studied, undergo a Compton scatter back out of the object and are counted using a suitable detector. Because there is a close relationship between the angle that the gamma-ray scatters through and the energy that it loses, by measuring the energy spectrum of the scattered gamma-rays it is possible to infer the distribution of material within the object of interest . However, unfolding this distribution requires complicated mathematical deconvolution techniques. Alternatively, if a collimated gamma-ray beam is used and the direction of the scattered gamma-rays is determined, direct imaging is possible. However, such systems typically have fairly low efficiencies and scanning is required to build up a full 3-dimensional image. If the object being studied produces gamma-rays itself (examples would include a biological specimen injected with a radiological tracer or a distant astronomical image) , a 2-dimensional image of the radioactive source density can be produced using an Anger camera or a Compton telescope. The former uses a position sensitive gamma-ray detector together with a gamma-ray opaque screen with a small aperture that projects an image of the object being studied onto the detector. Large or multiple apertures can be used to increase the efficiency of the camera, but necessitate the use of mathematical deconvolution techniques to form an image. The Compton telescope makes use of the angle/energy relationship of the Compton scattering process described above to infer the direction of an incident gamma-ray by measuring its interaction with two separate position sensitive detectors. The Compton telescope can be fairly efficient, but again mathematical deconvolution is required to obtain an image. All of these methods suffer from one or more of the following disadvantages: • Access is required to 2 or more sides of the object being studied; • Only 2-dimensional information is obtained; • The object being studied needs to contain radioactive nuclei; • Complex mathematical techniques are required to produce an image of the object; • Scanning of the object and/or source/detector are required to build up an image. Summary of the Invention In a first aspect, the invention provides an instrument for obtaining information about an object, the instrument including: a source of gamma-rays that is so located with respect to an object to be examined that at least some of the gamma-rays impinge on the object; a gamma-ray detector capable of detecting position and/or time of arrival of incident gamma-rays; a gamma-ray shield surrounding the detector having an aperture for facing at the object to be examined; and means for determining information about the object derived from the position and/or time of arrival at the detector of each gamma-ray passing through the aperture wherein the detector is located on the same side of the object as the source of gamma-rays. Preferably the source of gamma-rays is a positron source that is shielded to produce pairs of co-linear and co-incident annihilation gamma-rays. However, the source may be any source of gamma-rays including a source of co- incident gamma-ray pairs. In a second aspect, the invention provides a method including the steps of: generating gamma-rays from a source of gamma-rays causing at least some of the gamma-rays to impact on an object; detecting the position and/or time of arrival of each gamma-ray incident upon a detector; and determining information about the object from the position and/or time of arrival at the detector of the gamma-rays incident upon the detector. In a preferred embodiment of the second aspect of the invention, the source of gamma-rays is positrons and the method includes the step of generating pairs of co- linear and co-incident gamma-rays by shielding a source of positrons with a suitable shield. The invention is designed to form 3-dimensional images of the electron-density of an arbitrary object that can be viewed from one side only. Variations of the invention can be used to produce 1-dimensional (depth) profiles and 2-dimensional transverse density maps. Brief Description of the Drawings Figure 1 is a schematic drawing of a preferred embodiment of the invention; Figure 2 is a schematic drawing of a second embodiment of the invention; Figure 3 is a schematic drawing of a third embodiment of the invention; and Figure 4 is a schematic drawing of another embodiment of the invention. Detailed Description of the Drawings Figure 1 shows: (i) a gamma-ray detector (D) which is instrumented to provide the position and time of an incident gamma-ray; (ii) a collimator (C) made of lead or another suitable gamma-ray shielding material containing an aperture (A) in its front face and (iii) a positron source (S) surrounded by sufficient shielding material that positrons emitted by the source are brought to rest and annihilate in the vicinity of the source. The operation of the embodiment is as follows. A positron from the source (S) comes to rest in the shielding surrounding the source and annihilates, producing two 511 keV gamma-rays travelling back-to-back. One of the gamma- rays (1) is detected in detector (D) and the time and position of its arrival noted. The other gamma-ray (2) enters the object being examined (J) and scatters at some point (P) within the object. The scattered gamma-ray is then detected in detector (D) and its position and time of arrival noted. The positions of the two gamma-rays in detector (D) and the time between their arrival suffices to calculate the scattering position (P) . By measuring gamma- rays from a large number of positron annihilation events, a profile of the probability of scattering and hence the electron-density inside the object (J) can be determined. The electron density in turn can be approximately related to the physical density of matter inside the object. Figure 2 depicts a gamma-ray source (S) producing 2 or more coincident gamma-rays, gamma-ray detectors (D and D') , and a collimator (C) containing an aperture (A) . Gamma-ray (1) is detected in (D') travelling directly from the source and gamma-ray (2) is detected in (D) after scattering at point (P) in the object being studied (J) . Gamma-ray detector (D') can be omitted, with both gamma- rays being detected in detector (D) . Figure 3 depicts a gamma-ray source (S) , gamma- ray detector (D) , and a collimator (C) containing an aperture (A) . Gamma-rays are detected in (D) after scattering at point (P) in the object being studied (J) . Figure 4 depicts a gamma-ray or positron source (S) producing 2 or more coincident gamma-rays, gamma-ray detectors (D and D') , and a collimator (C) containing an aperture (A) . One gamma-ray is detected directly in detector (D) or (D') if used; the other gamma-ray is detected in (D) after scattering at point (P) in the object being studied (J) . Gamma-ray detector (D') can be omitted, with both gamma-rays being detected in detector (D) . Modes for Carrying out the Invention The physical dimensions and construction of the embodiments depend on the spatial resolution that is required when mapping the density of object (J) and the field of view required. Detector (D) may comprise one or more slabs of a scintillator material having a fast light decay time. The slab(s) are read out by a multiplicity of light detectors such as photomultiplier tubes or semiconductor diodes. Timing and possibly amplitude information from these detectors may be used to determine the position and arrival time of an incident gamma-ray. It will be appreciated that this description represents only one possible realisation of detector (D) and other detectors designs could be used without affecting the underlying nature of the invention. The collimator (C) should be sufficiently opaque to gamma-rays to shield the detector (D) from gamma-rays scattered from the object (J) , other than those gamma-rays passing through aperture (A) . The size and form of aperture (A) should be chosen to optimise the spatial resolution and efficiency of the invention. The following variations on the first embodiment are also included: 1. A gamma-ray imaging device as per the first embodiment, with the positron source (S) replaced by a gamma-ray source which produces at least two coincident gamma-rays per decay. One gamma-ray is detected in detector (D) or in a small detector (D') immediately surrounding the source (S) and its time of arrival noted. The other gamma ray scatters from the object (J) and its time and arrival in detector (D) noted. Aperture (A) is made small enough that scattered gamma-rays project an image of object (J) onto detector (D) . From the position of the scattered gamma-ray in detector (D) and the time between its arrival and the arrival of the directly detected gamma-ray, the scattering position (P) and hence the density profile of the object (J) can be determined. By selecting a source producing gamma-rays of suitable energy, the penetration of the imaging device into object (J) can be controlled. 2. A gamma-ray imaging device as per the second embodiment, with the source (S) replaced by a gamma-ray source where only one gamma-ray per decay is used. No timing information is measured or used. Such a device would permit a 2-dimensional map of the density of object (J) to be determined, with the density profile over the third coordinate (radial distance from the source (S) ) being averaged. 3. A gamma-ray profiling device as per the first embodiment, with the arrival position of the two gamma-rays in detector (D) not being measured or used. The difference between the arrival times of the two gamma-rays is used to determine the density profile of object (J) in 1-dimension (radial distance from the source (S) ) . The source (S) can either comprise a positron emitting source as in the main invention, or a source producing two coincident gamma-rays as per variation 1 above; in this instance, one of the gamma-rays may be detected in a small detector (D') surrounding the source. Collimator (C) and aperture (A) can be adjusted to control the transverse size of the region of object (J) that is examined. Figures 2, 3 and 4 illustrate these variations. Other minor variations, within the spirit of the main invention and the variations described above, are also included within the scope of the invention. Industrial Applicability The invention has utility in the following applications : 1. Detection of buried landmines, relying on the fact that mines have a different density from the surrounding soil. 2. Detection of other buried objects having dimensions a few cm or larger. 3. Non-invasive measurement of refractory linings inside burners or furnaces . 4. Non-invasive measurement of the build up of deposits/scale inside pipelines. 5. Non-invasive measurement of density of materials flowing inside pipelines. Other uses of the invention are also conceivable.

Claims: Claims : 1. An instrument for obtaining information about an object, the instrument including: a source of gamma-rays that is so located with respect to an object to be examined that at least some of the gamma-rays impinge on the object; a gamma-ray detector capable of detecting position and/or time of arrival of incident gamma-rays; a gamma-ray shield surrounding the detector having an aperture for facing at the object to be examined; and means for determining information about the object derived from the position and/or time of arrival at the detector of each gamma-ray passing through the aperture wherein the detector is located on the same side of the object as the source of gamma-rays. 2. An instrument according to claim 1, wherein the source of gamma-rays is a positron source that is shielded to produce pairs of co-linear and co-incident annihilation gamma-rays. 3. An instrument according to claim 1, wherein the source of gamma-rays is a source of co-incident gamma-ray pairs . 4. An instrument according to claim 2, wherein one of each pair of gamma-rays is detected directly and the other after scattering from the object, the co-linear nature of positron decay gamma-ray pairs being used to infer the scattering position of the gamma-ray interacting in the object from the detected position of the directly detected gamma-ray. 5. An instrument according to claim 4, wherein the difference in arrival times between the two gamma-rays of each pair is used to additionally infer the distance of penetration of the scattering gamma-ray into the object. 6. An instrument according to claims 1 or 3, wherein the detector detects the position of gamma-rays scattered by an object and the aperture in the gamma-ray shield is sufficiently small that it projects an image of the object onto the gamma-ray detector, enabling the scattering position of the gamma-ray inside the object to be inferred. 7. An instrument according to claim 3 wherein the difference in arrival times between the two gamma-rays is used to infer the distance of penetration of the scattering gamma-ray into the object. 8. An instrument according to claims 3 or 6 wherein the difference in arrival times between the two gamma-rays is used to additionally infer the distance of penetration of the scattering gamma-ray into the object. 9. An instrument according to any one of claims 1 to 8, wherein the distribution of positions of gamma-ray scatters in the object is used to infer the density distribution of the object. 10. An instrument according to claim 9, wherein the detector comprises one or more slabs of a scintillator material having a fast decay time and the slabs are read out by a multiplicity of light detectors. 11. An instrument according to claim 10, wherein the light detectors are photomultiplier tubes or semi-conductor diodes . 12. A method for determining information about an object, the method including the steps of: generating gamma-rays from a source of gamma- rays; causing at least some of the gamma-rays to impact on an object; detecting the position and/or time of arrival of each gamma-ray incident upon a detector; and determining information about the object from the position and/or time of arrival at the detector of the gamma-rays incident upon the detector. 13. A method according to claim 12, wherein the source of gamma-rays is a source of co-incident gamma-ray pairs. 14. A method according to claim 12, wherein the source of gamma-rays is positrons and the method includes the step of generating pairs of co-linear and co-incident gamma-rays by shielding a source of positrons with a suitable shield. 15. A method according to claim 13 wherein the difference in arrival times between gamma-rays of the coincident pair are used to infer information about the depth of scattering in the object. 16. A method according to claim 14 wherein the co- linear nature of gamma-rays from positron decay is used to infer information about the gamma-ray scattering position in the object. 17. A method according to claim 16 wherein the difference in arrival times between the direct and scattered gamma-rays from the positron decay are used to infer additional information about the depth of scattering in the object. 18. An method according to any one of claims 12 to 17, wherein the distribution of positions of gamma-ray scatters in the object is used to infer the density distribution of the object.

Inventors: TICKNER JAMES RICHARD, TICKNER, JAMES, RICHARD
Applicants: COMMW SCIENT IND RES ORG, TICKNER JAMES RICHARD, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION, TICKNER, JAMES, RICHARD
Application year: 2000
Main classification: G01T1
IPC codes: G01N23, G01T1