ARTEFACTS

FIELD OF THE INVENTION

This invention relates, generally, to the detection of particles or substances of interest in objects. Equivalently to obtaining high quality 3D images of the object of interest. More specifically, the invention relates to a detector arrangement and to a detection system. The invention also relates to a method of dynamic positioning a detector arrangement to reduce imaging artefacts.

BACKGROUND OF THE INVENTION

In conventional diamond mining operations, vast amounts of resources such as water and energy are required to process mostly barren rock in order to recover diamonds. Processing of the rock typically includes a damaging sequence of rock crushing and diamond recovery, often with a relatively low yield, for example, approximately 1 carat per ton of rock processed. However, crushing of rock in a conventional fashion may lead to diamond breakage, thereby reducing the profitability of a diamond mine.

Sensor- or detector-based technologies attempt to negate these undesirable effects by enabling early detection of relatively unprocessed diamond bearing rocks which can then be isolated and processed in an environmentally friendly manner that preserves diamond integrity.

One prior art detector-based technology makes use of Positron Emission Tomography (PET) to be able to detect diamonds in rocks. This approach involves irradiating a rock with a gamma ray beam from bremsstrahlung of, for example, 40 MeV electrons, or a different source of photons other than bremsstrahlung, for example, inverse Compton scattering, or other techniques. The rock returns to moderate levels of specific activity within minutes, by which time the PET isotopes represent the dominant residual activity. When the ¹¹ C pet isotope is the dominant activity, after about 30 minutes, the rock is then inspected by way of a detector arrangement to determine whether or not there is a diamond present therein.

Classification data generated when using the above approach is usually associated with photons detected by the detector arrangement. These photons are emitted from the rock as a result of positron annihilation in the rock. The most common outcome of annihilation is the production of nearly co-linear back-to-back 51 1 keV gamma ray photons. Each photon travels through the surrounding material, sometimes changing energy and direction along the way. When the photons reach the detector arrangement, the detector arrangement may output classification data in the form of detector strike/hit event data.

A detector arrangement used in PET diamond imaging may comprise a pair of detector arrays, for instance, respectively located above and below a conveyor belt transporting rock, so as to effectively sandwich the belt and rock travelling thereon. The array may include a top detector plane which consists of a plurality of individual detector elements and a bottom detector plane which consists of a plurality of individual detector elements. The detector elements may typically be in the form of scintillator crystals and photomultiplier tube/pad detectors with suitable electronics.

Usually, two back-to-back photons must be detected substantially at the same time in opposite planes of the detector arrangement to form a line of response (LoR). The LoR corresponds to an imaginary line through the rock connecting strikes on detector elements on opposite sides of the object, with the strikes corresponding to the back-to-back co-linear and co-incident photons emitted by the rock.

Detector arrays are typically discretised into pixels with a fixed geometry aligned with the belt transporting the rock. Each pixel is paired to or associated with one detector element, i.e. to one photomultiplier tube/pad.

In PET reconstruction imaging artefacts may arise due to the discretisation referred to above. The detector elements typically do not have the capability to determine where within a given pixel an event occurred. In fact, in the Inventors’ experience, events are usually remapped as if they can only occur at a given set of discrete positions located at pixel centres. Since a position in a periodic array from the top plane is mapped to a position in a periodic array on the bottom plane, the set of all possible lines of response (LoRs) that join detector elements form regular patterns. As a result, when a PET image is reconstructed, a regular pattern of higher and lower intensity spots may arise. Higher intensity positions are those that lie on direct paths joining the centres of many pairs of detector elements. Lower intensity positions are located between these higher intensity positions and are not on many direct paths.

In light of the above, source points at higher intensity positions may have a greater efficiency in terms of the generation of possible LoRs in a detection system. This may cause a systematic distortion of resulting images.

In wave systems, in the event of interference between two different waves, regions of high intensity resulting from constructive interference are referred to as “nodes”, while regions of low intensity that result from destructive interference are called “internodes”. Analogously, the higher intensity positions in a detector arrangement may be referred to as“nodes”, while the lower intensity positions may be referred to as“internodes”.

In the Inventors’ experience, this distortion/effect is not limited to a particular PET image reconstruction technique, but is rather a result of the fixed geometry of detector arrays and/or the remapping of events to pixel centres. The distortion/effect may thus be present at least to some extent regardless of the reconstruction technique employed.

As an example, Figure 1 illustrates a detector array 2 with five top pixels and five bottom pixels extending along an X-axis. These pixels are typically arranged in parallel with a direction of travel (along the X-axis) of a stream of objects (e.g. rock) to be imaged. Figure 1 illustrates all possible LoRs 4 between a top detector plane 6 and a bottom detector plane 8 of the detector array 2. As can be seen in Figure 1 , the most prominent nodes and internodes are located along the midline“M” between the two detector planes 6 and 8 along the Z- axis, where nodes“N” are separated by half of the distance between the centres of directly adjacent detector pixels“P”. The distance between the centres of directly adjacent pixels is hereinafter referred to as the“pixel distance”. Nodes are present at different spacings above and below the midline“M”.

For illustrative purposes, Figure 1 provides a two-dimensional visualisation of the pattern formed by all possible LoRs. It will be understood that a more complex pattern of nodes and internodes would arise if imaging was to be considered in three dimensions. The Inventors have found that when an image is created, the nature and detail of the observed pattern depends on a number of factors, such as the pitch of the pixels, the size of the voxels in the three-dimensional image, and the particular image reconstruction technique used in the imaging process.

The presence of these“nodes” and“internodes” in a reconstructed image may lead to decreased performance, particularly when imaging smaller diamonds. Nodes can lead to bright spots that can be falsely identified as diamonds. On the other hand, if a diamond is positioned at an internode, some of the signal(s) associated with its detection may effectively be distributed to nearby nodes and the diamond could be missed. This effect may also result in inaccurate estimations of diamond size.

There is thus a need to reduce or eliminate imaging artefacts (or the effects thereof) arising as a result of detector configurations employed in PET imaging. Embodiments of the present invention aim to provide an arrangement, system and/or method for addressing or alleviating at least some of the aforementioned issues.

In the context of this specification, the term“object” may be understood to mean a rock particle such as kimberlite, irrespective of the size thereof, or a number of rock particles, or a loose diamond. Thus the terms “object”, “rock”, “particle” and “kimberlite” may be used interchangeably herein. The term“object” may also extend to other objects which are imaged or analysed in a detection method, e.g. a human or animal body or body part of a body.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided a detector arrangement comprising at least one array of detector elements, or pixels, wherein the array includes or is divided into a plurality of sub-units or sub-arrays, each sub-unit including a plurality of detector elements defining a sub-plane, wherein the detector elements are configured to detect photons emitted from an object, located along at least a first axis defined by an object plane, as a result of nuclear reactions within the object wherein one or both of the object and the array of detector elements are operatively displaceable relative to each other, and wherein each sub-plane is individually angled or configured to be individually angled relative to a second axis and/or a third axis, and wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

In accordance with another aspect of the invention there is provided a detector arrangement comprising at least one array of detector elements, or pixels, which define a detector plane, wherein the detector elements are configured to detect photons emitted from an object, located along at least a first axis defined by an object plane, as a result of nuclear reactions within the object, wherein one or both of the object and the array of detector elements are operatively displaceable relative to each other, wherein the detector plane is angled or configured to be angled relative to a second axis and/or a third axis, and wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

In accordance with another aspect of the invention, there is provided a detector arrangement comprising at least one array of detector elements, or pixels, which define a detector plane, wherein the detector elements are configured to detect photons emitted from an object as a result of positron annihilation due to irradiation of the object with photons of a predetermined energy, wherein the object operatively travels relative to the array along a first axis defined by an object plane, and wherein the detector plane is angled or configured to be angled relative to a second axis and/or a third axis, wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

In some embodiments, the detector arrangement may be configured to perform detection on a stream of objects, e.g. rock / rock particles, passing through or travelling past the detector arrangement. The stream may be positioned on a conveying arrangement, e.g. a belt.

The angle or angles of the detector plane relative to the second axis and/or the third axis may be selected such that a point in the object plane, when travelling through or past the detector arrangement, traverses at least one pixel distance, and preferably several pixel distances, of a lateral dimension of the array, along the second axis and/or along the third axis.

The longitudinal direction of the conveying arrangement may define the first axis (“the X-axis”) and a transverse direction of the conveying arrangement may define the second axis (“the Y-axis”). The third axis (“Z-axis”) may extend perpendicularly to the first axis and the second axis, from the conveying arrangement to the detector plane. The detector plane defined by the array may thus be angled or tilted about the Y-axis or the Z-axis defined by the conveying arrangement.

The object plane may be defined by a conveying surface of the conveying arrangement. The first axis may be a direction of travel of the stream of objects through the detector arrangement, e.g. along the length of the belt.

The at least one array of detectors may include a pair of spaced apart detector arrays with their detector planes oriented generally parallel to each other. The detectors may be spaced apart on opposite sides of the conveying arrangement, e.g. above and below the belt. Each detector array may include, or be divided into, a plurality of sub-units or sub-arrays. Each sub-unit or sub-array may include a plurality of detector elements, or pixels, defining a sub-plane. Each sub-plane may be individually angled or titled relative to the second axis and/or the third axis, substantially in the manner described above.

The photons emitted from the object may be as a result of positron annihilation due to irradiation of the object with photons of a predetermined energy and/or insertion of radioisotopes into the object by any means. The case of the latter is particularly relevant in the applications of the present invention in medical PET imaging wherein radioisotopes are ingested/injected into human/animal bodies.

In some example embodiments for medical applications of the technology disclosed herein, coherent Brehmsstrahlung sources may be used for electron radiation free efficient gamma only activation of PET isotopes.

The photons may be gamma ray photons. The photons may undergo a nuclear interaction with carbon, creating radioactive isotopes and thereby enabling the detector arrangement to be used for the detection of diamond. The predetermined energy may be the energy at which a giant dipole resonance (GDR) occurs in the nuclear reaction between the photons and carbon, allowing for efficient activation of carbon. The detector arrangement may be configured to detect photons having an energy level of approximately 51 1 keV and rejecting photons not having the energy level of approximately 51 1 keV.

In some example embodiments of the invention, the predetermined energy may be outside the energy at which GDR occurs (either higher or lower). GDR is associated with the highest cross-section, but it will be appreciated that there is non zero cross-section in other energy areas, so in some applications one may be able get activation from photons without using GDR (either higher or lower energy).

It will be understood by those skilled in the art, that the isotopes introduced or produced may vary depending on the application of the disclosure herein. For example, in medical applications, activation of 15-Oxygen (which decays with a 2 minute half-life) would result in detection of photons by the arrangement described herein with the human/animal body returning to a “normal” condition after a few minutes.

It will be noted that outputs from the detector arrangement such as detector data associated with and/or indicative of and/or corresponding to photons detected by the detector arrangement may be used to generate images of the object/s.

In accordance with another aspect of the invention, there is provided a detector arrangement comprising at least one array of detector elements, or pixels, wherein the array includes or is divided into a plurality of sub-units or sub-arrays, each sub-unit including a plurality of detector elements defining a sub-plane, wherein the detector elements are configured to detect photons emitted from an object as a result of positron annihilation due to irradiation of the object with photons of a predetermined energy, wherein the object operatively travels relative to the array along a first axis defined by an object plane, and wherein each sub-plane is individually angled or configured to be individually angled relative to a second axis and/or a third axis, wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

In accordance with another aspect of the invention, there is provided a detection system which includes:

a detector arrangement comprising at least one array of detector elements, or pixels, which define a detector plane, wherein the detector elements are configured to detect photons emitted from an object as a result of positron annihilation due to irradiation of the object with photons of a predetermined energy; and

a conveying arrangement configured to allow the object to travel relative to the array along a first axis defined by an object plane,

wherein the detector plane is angled or configured to be angled relative to a second axis and/or a third axis, wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane. In accordance with another aspect of the invention, there is provided a detection system which includes:

a detector arrangement comprising at least one array of detector elements, or pixels, wherein the array includes or is divided into a plurality of sub-units or sub arrays, each sub-unit including a plurality of detector elements defining a sub-plane, wherein the detector elements are configured to detect photons emitted from an object as a result of positron annihilation due to irradiation of the object with photons of a predetermined energy; and

a conveying arrangement configured to allow the object to travel relative to the array along a first axis defined by an object plane,

wherein each sub-plane is individually angled or configured to be individually angled relative to a second axis and/or a third axis, wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

In accordance with another aspect of the invention, there is provided a diamond mine processing system which includes a detector arrangement and/or a detection system substantially as described above.

In accordance with another aspect of the invention, there is provided a method of positioning a detector arrangement to reduce imaging artefacts, the method including:

providing a detector arrangement substantially as described above;

providing a conveying arrangement which allows an object to travel relative to the detector arrangement along a first axis defined by an object plane; and

positioning the detector arrangement such that its detector plane, or, if present, each sub-plane, extends at an angle relative to a second axis and/or a third axis, wherein the second axis extends transversely across the object plane relative to the first axis, and the third axis extends perpendicularly to the first axis away from the object plane.

Positioning the detector arrangement may include angling or tilting the detector arrangement relative to the conveying arrangement such that a point in the object plane, when travelling through or past the detector arrangement, traverses at least one pixel distance, and preferably several pixel distances, of a lateral dimension of the array, along the second axis and/or along the third axis.

It will be appreciated that the method may be a dynamic method of positioning the detector arrangement.

The techniques developed in order to image diamonds using PET can be transferred back to a medical setting using detector outputs. Patients who have been activated, for example by ingesting radioisotopes, can pass through detectors, instead of the traditional static setting, where the patient remains in one position surrounded by detectors. This offers similar benefits to the mining setting, in terms of increasing throughput (in this case patients imaged per hour), and helping with uniformity of PET response for image reconstruction.

Those skilled in the art would appreciate that outputs from the detector arrangement described herein may be used to generate higher quality three- dimensional images.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a conceptual side view of a detector arrangement, in which all possible LORs between a top detector plane and a bottom detector plane are illustrated;

Figure 2 shows a schematic diagram of an example of a diamond mine processing system in which embodiments of the invention may be implemented;

Figure 3 shows an illustration of kimberlite rock on a moving belt between detector planes of a detector arrangement;

Figure 4 shows a conceptual top view of a first example of a detector arrangement according to an embodiment of the invention; Figure 5 shows a conceptual side view of a second example of a detector arrangement according to an embodiment of the invention; and

Figure 6 shows a conceptual side view of a third example of a detector arrangement according to an embodiment of the invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible, and may even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

It will be appreciated that the phrase“for example,”“such as”, and variants thereof describe non-limiting embodiments of the presently disclosed subject matter. Reference in the specification to “one example embodiment”, “another example embodiment”, “some example embodiment”, or variants thereof means that a particular feature, structure or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the presently disclosed subject matter. Thus, the use of the phrase“one example embodiment”,“another example embodiment”,“some example embodiments”, or variants thereof does not necessarily refer to the same embodiment(s).

Unless otherwise stated, some features of the subject matter described herein, which are, described in the context of separate embodiments for purposes of clarity, may also be provided in combination in a single embodiment. Similarly, various features of the subject matter disclosed herein which are described in the context of a single embodiment may also be provided separately or in any suitable sub combination.

Referring to Figure 2 of the drawings, an example of a diamond mine processing system is generally indicated by reference numeral 12. Diamond detection will be used as an example implementation of embodiments of the invention in this description. However, it should be appreciated that alternative embodiments extend to other types of detection and to imaging applications in high volume and high rate environments, e.g. PET imaging/detection applied to the body of a human or animal (such as total-body medical PET and PET video).

The system 12 typically includes a classification or detection unit 10 which is usually a computerised unit or system configured to perform imaging and to detect diamonds as individual, separate objects, as embedded in host objects or as objects included in a mass of other objects.

The diamond mine processing system 12 may be located at or adjacent a diamond mine and may comprise suitable conventional mining equipment such as a crusher 14 to coarsely crush mined rock to sizes of approximately 160 mm diameter, or less. The system 12 further comprises a suitable irradiator 16 to irradiate the crushed rock with photons. The photons which irradiate the rock may be from gamma ray beams from bremsstrahlung of approximately 40 MeV electrons. Instead, or in addition, these photons may be from inverse Compton scattering, plasma wakefield device, or the like. The photons are preferably at an energy at which giant dipole resonance (GDR) occurs due to a nuclear reaction between the photons and carbon in the rock. GDR refers to one particular manner in which a photon interacts with a nucleus, and which happens at a particular photon energy. It has been found that the most efficient activation may be obtained if the photon energy is tuned to the GDR. However, it will be understood that photons still interact with carbon nuclei at other energies.

The system 12 comprises a hopper arrangement 18, 19 to hold the rock while it is being irradiated and to hold irradiated rock for a predetermined period of time. The irradiated rock returns to moderate levels of specific activity within minutes, by which time PET isotopes represent the dominant residual activity. In this regard, the hopper arrangement 18, 19 is configured to hold the irradiated rock for a hold-time of between twenty and thirty minutes at which time the ¹¹ C PET isotope is the dominant activity. The hopper arrangement 18, 19 may then automatically release the rock after the hold time.

The hopper arrangement 18, 19 may include an irradiation hopper 18 and a hold hopper 19 (as shown in the example of Figure 2). In such a case, the irradiation hopper 18 is configured to hold the rock while it is being irradiated by the irradiator 16 and the hold hopper 19 is configured to hold activated material for the predetermined time, e.g., about 30 minutes. The irradiation hopper 18 may take various forms, depending on the implementation, e.g., it may include a vertical chute or three horizontal conveyor belts.

The system 12 comprises a conveying arrangement 20 comprising suitable conveyor belts which are non-attenuating to PET photons to transport rock in the system 12 in an automated fashion. The conveyor arrangement 20 may be configured to transport rock in a rock stream at a constant predetermined speed in the system 12, for example, 1 m per second.

The system also comprises a detector arrangement 22 which is located downstream from the hopper arrangement 18, 19 and adjacent the conveyor arrangement 20, particularly the belt thereof, so as to detect PET photons emitted therefrom. The direction of travel of rock through the detector arrangement 22 is indicated by the arrow marked“A” in Figure 2. This direction of travel, which is along the length of the belt 20, can defined as a first axis or X-axis, as will be described in greater detail below.

In one example embodiment, and as shown in Figures 2 and 3, the detector arrangement 22 may comprise a pair of detector arrays 22.1 and 22.2 which are located above and below the belt, respectively, so as to effectively sandwich the belt and rock travelling thereon. The array 22.1 may consist of a plurality of individual detector elements/pixels, defining a top detector plane, and the array 22.2 may consist of a plurality of individual detector elements/pixels, defining a bottom detector plane. In one example embodiment, the detector arrangement 22 comprises detectors suitable for detecting photons. In this regard the detectors of the arrangement 22 may be in the form of scintillator crystals and photomultiplier tube (PMT) or pad detectors with suitable electronics.

The system 12 also comprises a suitable sorter 24 which may be an electronically controlled mechanical sorter 24 configured to sort potentially diamondiferous or in other words diamond containing rocks or loose diamonds from potentially barren rocks or in other words rocks without diamonds therein.

The detection unit 10 is communicatively coupled to the detector arrangement 22 and to the sorter 24 so as to receive classification data from the detector arrangement 22 and to generate suitable control signals to control the sorter 24 to sort diamondiferous rocks from barren rocks. In this regard, it is important for the unit 10 to process the classification data with sufficient speed in order to be able to send the activation signal (data) to the sorter 24 in time.

The sorter 24 may be configured to sort diamonds or diamondiferous rocks into one or more categories according to one or more specific properties of the diamond/diamondiferous rock detected, as opposed to simply sorting the same from barren rocks.

The unit 10 may be coupled to the detector arrangement 22 and/or the sorter 24 in a hardwired fashion, or in a wireless fashion. In one example embodiment, the unit 10 is communicatively coupled to the arrangement 22 via a communications network which may comprise one or more different types of communication networks. In this regard, the communication network may be one or more of the Internet, a local area network (LAN), a wide area network (WAN), a metropolitan area network (MAN), various types of telephone networks (e.g., Public Switch Telephone Networks (PSTN) with Digital Subscriber Line (DSL) technology) or mobile networks (e.g., Global System Mobile (GSM) communication, General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), and other suitable mobile telecommunication network technologies), or any combination thereof. It therefore follows that though it may not necessarily be practical, it is envisaged that in some example embodiments, the unit 10 need not be at the site of the mine but may be remote therefrom. In some embodiments, the arrangement 22 may form part of the unit 10. In some embodiments, the arrangement 22 and the belt 20 may together form a detection system while the unit 10 is defined as a classification system.

The unit 10 is configured/programmed to receive classification data and to determine whether or not the object is potentially a diamond or diamondiferous by processing the received classification data. The classification data is typically associated with photons detected by the arrangement 22 which are emitted from the object as a result of positron annihilation in the irradiated object received from the hopper 18, 19. In particular, referring to Figure 3 of the drawings, when a PET isotope in the kimberlite rock releases a positron particle through beta-decay, the positron annihilates with a nearby electron after following a short path that can involve multiple scattering events. The most common outcome of this annihilation is the production of nearly co-linear back-to-back 51 1 keV gamma ray photons. Each photon then travels through the surrounding material, sometimes changing energy and direction along the way. When the photons reach the detector arrangement arrays 22.1 , 22.2 at S1 , S2, the arrangement 22 outputs classification data in the form of detector strike/hit event data, for example, which comprises data indicative of the location of the hit on the arrays 22.1 , 22.2, the energy of the photon, and a time stamp. This allows the position of a diamond (see“D” in Figure 3) to be determined or estimated.

As explained in the“Background” section above, detector arrays such as the arrays 22.1 and 22.2 are usually discretised into pixels with a fixed geometry aligned with the belt 20 transporting the rock. Each pixel is paired to or associated with one detector element. The detectors typically do not have the capability to determine where within a given pixel an event occurred, and events are usually remapped as if they can only occur at a given set of discrete positions located at pixel centres. Since a position in a periodic array from the top plane is mapped to a position in a periodic array on the bottom plane, the set of all possible lines of response (LoRs) that join detector elements form regular patterns (this was described above with reference to Figure 1 ). As a result, when a PET image is reconstructed, a regular pattern of higher and lower intensity spots (“nodes” and“internodes”) may arise, as described above. This may lead to imaging artefacts.

The Inventors have found that certain principles of sampling theory may be applied to reduce imaging artefacts in PET detection, particularly when dealing with the detection of diamonds. In sampling theory in the time domain, the discretisation of sampling imposes a maximum frequency cut-off. If the signal in question is bandwidth limited, and the sampling rate is twice the bandwidth limit, then the sampling does not introduce artefacts and the sampling rate is termed the“Nyquist rate".

If it is not possible to sample at least at the Nyquist rate, then the statistical technique of “oversampling” may be employed. This technique involves combining multiple sets of samples (K) which are each below the Nyquist Rate. The samples in the oversampling set must be uncorrelated. In practice, the samples will be time stepped at an interval above the Nyquist Rate and will therefore be independent. It can be shown that combining them (an integral) leads to a Vtf improvement in the signal to noise rate.

This technique of oversampling in the time domain, which involves displacing independent samples within the main sample time interval, may be applied to PET reconstruction in the space domain. If a measurement frame is locked on the moving stream of rock to be imaged, with belt motion stroboscopically frozen by means of correction for belt velocity, the efficiency of a single LoR can be considered. For example, a perpendicular LoR extending along a Z-axis (the axis perpendicular to the direction of travel of the belt and extending between the detector planes), connecting the centres of two pixels, can be considered, along with a slightly offset, but also perpendicular, LoR. For the reasons set out above, these two LoRs may map into the same two pixels, usually so long as the offset d is less than half the pixel period, D /2. If K sets of measurements are taken, stepping detector position K times within the dimension of the pixel, Dx, a different distribution of efficiencies would be produced for a given LoR source point, as the exact cross-over point at which it became mapped into the next pixel would depend on its position at the resolution of Dx/K. In this way, the Inventors have found that it is possible to gain in resolution and substantially eliminate the inhomogeneous efficiency of LoR source points by the technique of oversampling. In practical terms, this approach can be applied in PET imaging by adjusting the orientation/angle of detector planes relative to an object plane defined by a conveying surface of the belt. Examples are provided below.

For the purposes of the examples in Figures 4 to 6, a coordinate system is defined where x is the direction that a conveyor belt 30 travels in, y is the direction transverse along the belt 30, and z is the direction perpendicular to the belt from a top detector plane 32/40 to a bottom detector plane 34. The conveying surface carrying the rock between the planes 32/40 and 34 (i.e. on which objects to be imaged operatively travel relative to the detectors) is referred to as the“object plane” (see“O” in Figure 5).

Therefore, the x direction, extending along the length of the belt 30, defines a first axis (“the X-axis”), the width of the belt 30, i.e. the / direction transverse across the belt 30, defines a second axis (“the Y-axis”), and a third axis (“the Z-axis”) extends perpendicularly to the X-axis and the Y-axis away from the belt 30, i.e. between the detector planes 32/40 and 34.

When analysed along the X-axis, the motion of the belt 30 serves to smooth out all“nodes” and“internodes” that could have been formed. This can effectively be seen as“automatic” oversampling as a result of the conventional geometry and motion employed in the PET process. At any given point in time, a point on the belt 30 could be along the line (LoR) joining two detector elements, or between two different lines. As the belt 30 moves through the pattern of nodes and internodes, every position on the belt spends substantially as much time at nodes as at internodes, and no position has a significant positive or negative bias. Equivalently, every LoR of a particular angle will, in time, penetrate each pixel in a near continuum of positions, so that it crosses pixel boundaries with a precision greater than the pixel level discretisation. The detector record of LoR end points, when mapped back by velocity correction, will essentially have been subjected to the oversampling process. In the integral case referred to above, this provides a resulting distribution which has become decoupled (at least to some extent) from the original discretisation.

Along the Y-axis, artefacts may be observed if the detector planes 32 and 34 are exactly aligned with the belt motion. In other words, this would be the case if the two axes defined by the planes 32, 34 are aligned with and parallel to the X-axis and Y-axis defined by the belt 30. For example, if a given belt position along the Y-axis is between two detector positions on the Y-axis, it will stay between those detector positions throughput its journey along the object plane O between the detector planes 32, 34. In order to achieve a similar smoothing out of nodes and internodes as is observed in the x direction, a rotation/tilt/angling of the detector arrangement relative to the belt 30 can be introduced, as shown in Figure 4.

Consider a point on the belt 30 with the detector plane 32 angled relative to the object plane O / belt 30 as shown in Figure 4. At position Έ”, it is nearly halfway between two detector pixels. At position“F”, it is in the middle of a pixel. At position “G”, it is halfway between two pixels. As a result of the angle between the detector planes 32, 34 and the belt 30, each point on the belt 30 may therefore pass smoothly between nodes and internodes, as is the case“automatically” (without rotation being employed) for the x direction.

The angle of the detector planes 32, 34 with respect to the belt 30 may be chosen based on the pixel distance in the relevant arrays and the overall length of the detector arrangement, to ensure that each point in the object plane O, when travelling through the planes 32, 34, is able to traverse several pixel distances of a lateral dimension“L” of the planes 32, 34 (or the array), and/or several pixel distances along the Y-axis, along its path. It will be understood that, for illustrative purposes, the pixel sizes in Figures 4 to 6 have been exaggerated and that pixels would be substantially smaller in practice, leading to the crossing of many pixel distances with respect to the dimension“L”.

A similar effect may be achieved along the Z-axis by rotating/angling the detector planes 32, 34 relative to the belt around the Y-axis (as opposed to the angling relative to the Z-axis described with reference to Figure 4), as shown in Figure 5. In this case, each point on the belt 30 will traverse several pixel distances along the Z- axis, thereby passing smoothly between nodes and internodes.

As is evident from the examples in Figures 4 and 5, when the detector arrangement, or individual detector planes, is/are tiled/angled relative to the belt, at a front and back of the detector, the belt becomes offset from the middle of the detector planes. As a result, regions that are relatively far from the centre of the belt may not have as large a detector coverage as is desirable, and therefore may have a lower detector response. To address this potential issue, the angling/tilting may be applied to detector sub-units or sub-arrays, as shown in Figure 6.

In the example of Figure 6, each detector plane (only the top plane 40 is shown) is divided into 12 sub-units 42. The detector elements or pixels of these sub-units define“sub-planes” (see“S” in Figure 6). In this example, the individual sub-units 42 are rotated with respect to the belt 30, around the Z-axis (it will be understood that they may alternatively or additionally be rotated about the Y-axis). Additionally, the sub-units 42 are offset relative to each other, so that pixels do not line up where the sub-units 42 meet. This allows a point on the object plane O of the belt 30 to traverse between different relative pixel positions, without introducing a large overall rotation to the detector plane 40, such that the belt 30 remains near the centre of the detector plane 40.

The detector arrangements described with reference to Figures 4 to 6 may be applied in a processing and/or detection system such as the system 12 described with reference to Figure 2.

A detector arrangement, a detection system and a method of positioning a detector arrangement are therefore provided herein. By angling detector planes relative to the object plane on which objects to be imaged are carried, the presence of higher and lower intensity spots on the resulting image may be reduced or eliminated. The techniques outlined above make it possible to reduce imaging artefacts by leveraging the movement of the conveying arrangement (e.g. belt), without needing to introduce any other motion, e.g. vibration of the belt. In this way, the number of false diamonds identifications (false positives) and/or the number of missed diamonds may be reduced, possibly leading to more profitable operations. It is envisaged that at least some of the techniques and configurations described herein may find application outside of diamond (or other precious particle) detection. For instance, at least some of the techniques and configurations described herein may be applied in PET imaging/detection applied to the body or a body part of a human or animal. In the medical application of the disclosure herein, it will be appreciated that the conveyor arrangement may be in the form of a displaceable bed or platform or displaceable detector system which is configured to spatially displace a human/animal body, or part thereof, in the object plane relative to the detector arrangement. In this regard, the spatial displacement may include travel in the first direction and/or multiple direction including the first direction. For example, in one embodiment, the bed or platform may move the object back and forth along the first direction within the object plane.