Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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G11MA-RAY INAGING
TeChnical Field
The inventioa 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
Ccaqpton scatter back out of the object and are counted
using a suitable detector. Because there is a close
relationship between the angle that the qamma-ray scatters
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throuQh 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 gamaa-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 Conpton 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;
a Only 2-dimensional information is obtained;
+ The object being studied needs to contain radioactive
nuclei;
0 Complex mathematical techniques are required to produce
an image of the object;
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= 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 2-dimensional information about
the distribution of mass of an object, the instrument
including;
a source of positrons, shielded so that the
positrons annihilate and produce co-linear gamma-rays
pairs in the immediate vicinity of the source, said source
being located with respect to an object to be examined
that at least some of the gamma-rays inpinge on the
object;
a gamma-ray shield surrounding a gamma-ray
detector having an aperture for facing at the object to be
examined, said gamma-ray detector being located on the
same side of the object as the source of positrons, and
said gamma-ray detector being capable of detecting the
arrival position of gamma-rays of said gamma-ray pairs
travelling directly from the source and the arrival
position of gamma-rays of said gamma-ray pairs after
scattering from the object and passing through said
aperture; and
means for determining 2-dimensional information
about the object from the direction of flight of the
directly detected gamma-ray and the arrival position of
the scattered gamma-ray of each gamma-ray pair.
In a second aspect, the invention provides an
instrument for obtaining 2-dimensional information about
the distribution of mass of an object, the instrument
including:
a source of gamma-rays 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 shield surrounding a gamma-ray
detector having an aperture for facing at the object to be
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examined said aperture being sufficiently small to project
an image of the object onto aaid detector, said gamma-ray
detector being located on the same side of the object as
the source of Qamma-raya and being capable of detecting
the arrival position of gamma-rays scattered from the
object and passing through the aperture; and
means for determining 2-dimensional information
about the object derived from the detected positions of
scattered gamma-rays.
In a third aspect, the invention provides an
instrument for obtaining 1-dimensional information about
the distribution of mass of an object, the instrument
including:
a source of Qamma-rays, said source being 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 shield surrounding one or more
detectors having an aperture for facing at the object to
be examined;
a detector located on the same aide of the object
as said source, said detector being capable of determining
the arrival time of gamma-raya having scattered from the
- object and inferring the departure time of said gamma-rays
from said source; and
means for determining 1-dimensional information
about the object from the arrival times of said scattered
gamma-rays and said inferred departure times of said
gamma-rays.
In a fourth aspect, the invention provides a
method for obtaining 2-dimensional information about the
distribution of mass of an object, the method including:
generating co-linear gamma-ray pairs using a
position source;
causing at least some of the gamma-rays to impact
on an object;
detecting the position of arrival of each gamma-
ray pair incident upon a detector located on the same side
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of the object as said source; and
determining 2-dimensional information about the
object from the direction of flight of the directly
detected Qamma-ray and the arrival position of the
scattered gamma-ray of each gamma-ray pair.
In a fifth aspect, the invention provides a
method for obtaining 2-dimensional information about the
distribution of a mass of an object, the method including:
generating Qamma-raya using a Qamma-ray source;
causing at least some of the gamma-rays to impact
on an object;
shielding a detector located on the same side of
the object as said gamma-ray source with a shield having a
aperture having a size sufficiently small enough to
project an image of the object onto said detector;
detecting the position of each gamma-ray
scattered from the object incident upon said detector; and
determining 2-dimensional information about the
object from the detected position of the scattered Qamma-
rays.
In a sixth aspect, the invention provides a
method for obtaining 1-dimensional information about the
distribution of mass of an object, the method including:
generating gamma-rays using a gamma-ray source;
causing at least some of the gamma-rays to impact
on an object;
determining the arrival times at said detector of
gamma-rays having scattered from said object;
inferring departure times of said to be scattered
gamma-rays from said source; and
determining 1-dimensional information about the
object from the arrival times of said scattered gamma-rays
and the inferred departure times of said to be scattered
Qamma-rays.
Brief Description of the Drawings
Figure 1 is a schematic drawing of a preferred
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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 (5) 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
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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
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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
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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 gamvaa-rays
as per variation 1 above; in this instance, one of the
ganrma-rays may be detected in a amall 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.
