WO2004097454A2 - Detector for measuring radioactive fluid - Google Patents

Detector for measuring radioactive fluid Download PDF

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Publication number
WO2004097454A2
WO2004097454A2 PCT/GB2004/001733 GB2004001733W WO2004097454A2 WO 2004097454 A2 WO2004097454 A2 WO 2004097454A2 GB 2004001733 W GB2004001733 W GB 2004001733W WO 2004097454 A2 WO2004097454 A2 WO 2004097454A2
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WO
WIPO (PCT)
Prior art keywords
scintillating member
scintillating
conduit
radioactivity
detecting position
Prior art date
Application number
PCT/GB2004/001733
Other languages
French (fr)
Other versions
WO2004097454A3 (en
Inventor
Michael Leslie Renton
Anthony Michael Westrip
Alexander Ranicar
Original Assignee
Hammersmith Imanet Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Hammersmith Imanet Ltd filed Critical Hammersmith Imanet Ltd
Priority to EP04728851A priority Critical patent/EP1618410A2/en
Publication of WO2004097454A2 publication Critical patent/WO2004097454A2/en
Publication of WO2004097454A3 publication Critical patent/WO2004097454A3/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/003—Scintillation (flow) cells
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T7/00—Details of radiation-measuring instruments

Definitions

  • the present invention relates to the detection of radioactivity in fluids, and in particular relates to the design of scintillators for use with a radiation detection system.
  • the invention particularly, but not exclusively, relates to the detection of radioactivity in radiopharmaceuticals where the radio-nuclides are of the positron or gamma emitting variety.
  • a scintillator usually has a circular cylindrical geometry to conform to a photomultiplier tube detection head.
  • An example of such a scintillator is described in US 5,483,070, which shows a face of the cylinder providing an external surface of the scintillator for optically coupling to a light detection / amplification device such as the photomultiplier tube.
  • the prior art teaches a scintillator in which a small hole is drilled through a diameter of the cylindrical scintillator (e.g. US 5,483,070).
  • a capillary can be threaded through this hole for carrying fluids to be analysed by, for example, positron emission detection.
  • This conventional design has a disadvantage in that a free end of the capillary must be available to thread through the hole of the scintillator.
  • This is particularly disadvantageous since it is often necessary to maintain the ends of the capillary in particular chemical or electrolyte buffer reservoirs, and removal of the capillary from a reservoir to feed it through a scintillator head is inconvenient, time consuming and may result in a necessity to re-prime the fluid delivery system or re-initiate the experiment, and can result in damage to the capillary or detector.
  • a first being a cylindrical scintillator having a diametric slot cut through it
  • a second (described in WO 99/67656) being a cylindrical scintillator having a curved slot cut through it, both of which enable easy insertion and removal of the capillary without needing a free end available.
  • the size of such cylindrical scintillators is constrained by limitations associated with the scintillating material and methods of manufacture, in so far as the scintillator has to be of sufficient size that it does not break during manufacture or use.
  • the scintillator typically has a diameter that is several times greater than the thickness of tube.
  • the capillary extends through a portion of the cylinder's diameter and the larger the scintillator, the greater the length of capillary located within the cylindrical scintillator (and thus the greater the measurement volume). This means that the resolution of measurements relating to steep changes in radioactivity can be poor.
  • a further problem with these known arrangements is that, in order to reduce scattering of scintillated light from the surfaces of the slot into which the capillary is inserted, these surfaces should be kept as clean as possible; clearly, regular ingress and egress of the capillary into the slot will cause particles to be deposited on these surfaces, and increase such unwanted scattering of scintillated light.
  • British patent application GB 2,232,482 describes a scintillator for detecting alpha emissions within a fluid, the scintillator comprising a large number of thin, parallel and spaced-apart sheets of scintillation plastic coupled optically to an end of a photomultiplier tube.
  • the scintillator, or more specifically the sheets of scintillating plastic is inserted into a fluid whose radioactive emissions are to be detected so that the fluid occupies the gaps between the scintillating sheets. Any radioactive particles incident on the sheets cause emission of light within the sheet, and this light is then detected by the photomultiplier in the normal manner.
  • a problem with this arrangement is that the sheets have to be inserted into the radioactive medium, which firstly means that they will require cleaning regularly and secondly that the orientation of the sheets relative to a conduit will be constrained by the internal dimensions of the conduit.
  • the coupling of multiple sheets with the photomultiplier is cumbersome and it is difficult to arrange the sheets in a homogenous manner, both in terms of the relative spacing between the sheets and the angles they make relative to one another. It would be desirable to provide a scintillator that alleviates these problems.
  • radioactivity detection apparatus for detecting radioactivity in a fluid carried in a conduit, the apparatus comprising: a scintillating member responsive to the absorption of radioactivity to generate electromagnetic radiation, the scintillating member having two ends and a side surface, said side surface extending between said two ends; a support for holding the scintillating member in a detecting position with respect to the conduit; and an electromagnetic radiation detection system arranged to provide an electrical signal indicative of radiation released by said scintillating member, wherein at least one end of the scintillating member is arranged to face the electromagnetic radiation detection system, and the apparatus is arranged, when the scintillating member is in said detecting position, such that the scintillating member lies in a plane oriented substantially perpendicular to the conduit and said side surface of the scintillating member faces the conduit.
  • the scintillating member can be arranged beside the conduit whilst being capable of detecting a significant proportion of a type of radioactivity emitted by the fluid in the conduit.
  • the scintillating member can have a relatively uniform cross-section throughout, so that it is less likely to develop cracks during manufacture and/or during use of the apparatus and reduce the working life thereof.
  • the thickness of the scintillating member is not constrained by the need to accommodate a conduit therein, which means that it can have a smaller size. This is beneficial from the point of view of reducing the amount of a type of radiation, such as gamma radiation, which is not of interest, from being absorbed by the scintillating member.
  • the scintillating member comprises an elongate scintillating member, and is arranged so as to form an angular extent about the conduit of at least 30 degrees.
  • the angular extent is 180 degrees or more, and most preferably it is substantially 360 degrees.
  • these latter angular extents can be provided by curving the scintillating member around the conduit; an angular extent of 360 degrees or more conveniently ensures that detection is provided over the entire length of the scintillating member surrounding the conduit.
  • the scintillating member is arranged to lie in a plane substantially perpendicular to the conduit when in the detecting position, thereby optimizing the temporal resolution of the detection volume.
  • the scintillating member is primarily responsive to radioactivity, in particular positrons, having energy between 0.05 MeN and 2 MeN, and more preferably between 0.1 MeN and 1.2 MeN.
  • the thickness of the scintillating member, in a direction extending substantially perpendicular to the conduit is less than 5 mm, and most preferably is less than 2 mm, whilst the width of the scintillating member, in a direction extending along the conduit, is preferably less than 5 mm.
  • the diameter of the scintillating member, in a direction extending along the conduit is preferably less than 5 mm.
  • This preferred configuration thus provides a means of selectively measuring radiation associated with positrons.
  • the scintillating member comprises a plastic scintillator, which has a low density and a low atomic number, and is thus relatively insensitive to gamma radiation.
  • Scintillating plastic is thus an advantageous choice of scintillating material in environments where there is a lot of background gamma radiation, such as is the case in nuclear medicine facilities.
  • plastic scintillating materials are readily formed into shape and are robust, thus being well suited to practical use in nuclear medicine facilities.
  • the electromagnetic radiation detection system comprises a light energy transmission medium and a light energy receiving means, the light energy transmission medium being connectable at one end to said scintillating member and connectable at the other end to the light energy receiving means.
  • the light energy receiving means can be provided by a photomultiplier and the light energy transmission medium by an optical fibre.
  • the support includes a housing comprising a top part and a bottom part, the bottom part including a groove for locating the scintillating member in the detecting position, wherein the housing defines a channel for receiving the conduit, the channel having a depth extending across the thickness of the top and bottom parts and a length extending into the top and bottom parts.
  • the conduit is radially outwardly displaced with respect to an inner surface of the support when the scintillating member is in the detecting position, thereby providing a means of separating the scintillating member from the conduit.
  • Figure 1 is a schematic diagram of a nuclear medicine diagnosis facility in which radioactivity detection apparatus according to an embodiment of the invention can operate;
  • Figure 2a is a schematic perspective diagram of parts of the radioactivity detection apparatus according to an embodiment of the invention.
  • Figures 2b and 2c are sections showing a surface external to the cross section of a scintillating member forming part of the radioactivity detection apparatus according to an embodiment of the invention
  • Figure 2d is a cross-section through a plane passing through both a diameter of the scintillating member of the detector shown in Figure 2a and the longitudinal axis of the conduit, when the detector is in a detecting position;
  • Figures 3a - 3d are cross-sections through line A-A of the schematic diagram of
  • Figure 2a showing different angular extents defined by the configuration of the scintillating member of the detector shown in Figure 2a relative to the conduit, when the detector is in a detecting position;
  • Figure 4 is a schematic perspective diagram showing in more detail the radiation detection system and the scintillating member forming part of the detector shown in Figure 2a;
  • Figure 5a is a graph showing the light output response for scintillating plastic BC- 412
  • Figure 5b is a graph showing spectral response of light energy receiving means forming part of the radiation detection system shown in Figure 4;
  • Figure 6 shows cross section and end views of a ferrule and a nut assembly for use in joining parts of the radiation detection system shown in Figure 4;
  • Figure 7a is a plan view of a lid part forming part of a housing for the radioactivity detection apparatus according to an embodiment of the invention;
  • Figure 7b shows cross section and plan views of a bottom part forming part of the housing for the radioactivity detection apparatus according to an embodiment of the invention
  • Figures 8a and 8b show cross section and plan views of an alternative arrangement of the bottom part forming part of the housing.
  • Figure 9 shows schematic plan and side views of a carrier for the radioactivity detection apparatus according to an embodiment of the invention.
  • Radioactive isotopes within a body.
  • the radioisotopes are attracted to specific organs, bones or tissues, and the emissions produced by the radioisotopes are used provide information about a particular type of disease.
  • Examples of radioisotopes commonly used in nuclear medicine include carbon-11, oxygen-15, fluorine-18 and bromine-75.
  • Positron emission tomography is an example of a nuclear medicine diagnostic technique whereby images of physiological function of organs are acquired by imaging the decay of radio-isotopes bound to molecules having known biological properties.
  • Suitable radio-isotopes are synthesized into a carrier (also called a tracer) that enables the radio- isotope to be delivered to the organ under examination.
  • a carrier also called a tracer
  • Such a carrier is commonly referred to as a radiopharmaceutical.
  • detector 100 An embodiment of the radioactivity detection apparatus, hereinafter referred to as detector 100, will be described in the context of a radiopharmaceutical generation system (herein referred to as a water generation system).
  • a radiopharmaceutical generation system hereinafter referred to as a water generation system.
  • One commonly used radio-isotope is Oxygen-15 ( 15 O), which is produced by deuteron bombardment of natural nitrogen through the 14 N(d,n) 15 O nuclear reaction, and a radiopharmaceutical associated with the Oxygen-15 radio-isotope is O-labelled water.
  • FIG. 1 is a schematic diagram of such a facility 110, showing a delivery path 101 associated with a radiopharmaceutical, together with the detector 100, which is positioned in the vicinity of the scanner 107.
  • water generator 105 receives, as input, saline 109; radioactive Oxygen-15; and hydrogen, and generates radioactive 5 O-labelled water under control of processing unit 111.
  • This 15 O-labelled water is pumped along delivery tube 101, passing through the detector 100 en route for the subject 103.
  • Embodiments of the invention are concerned with a detector whose construction is simpler and less sensitive to manufacturing stresses than known detectors, and whose shape is not controlled by material and manufacturing constraints in such a way as to significantly limit the resolution and ability to discriminate between radiation particles.
  • An embodiment of the invention will now be described with reference to Figure 2a, which is a perspective schematic diagram showing part of the conduit (or delivery tube) 101 having first and second sides 222a, 222b, through which the radiopharmaceutical is delivered, together with an embodiment of the detector 100.
  • the detector 100 comprises a scintillating member 201 having a side surface 204 and two ends (only one shown, end 206), the side surface being external of its cross-section.
  • the detector 100 also includes a support (not shown in Figure 2a; described in detail below) for holding the scintillating member 201 in a detecting position relative to the tube 101; and an electromagnetic radiation detection system 210 including light energy receiving means 211, which preferably comprises a photomultiplier (described in more detail below) and a light energy transmission medium 221 connectable at one end 223 to said scintillating member 201 and at connectable at the other end 225 to the light energy receiving means 211.
  • the light energy transmission medium 221 comprises an optical fibre, and its configuration and attachment at ends 223, 225 is described in more detail below.
  • Figures 2b, 2c and 2d show a scintillating member 201 having a circular cross- section, wherein the side surface 204 is the outer surface of the scintillating member 201;
  • Figure 2c shows scintillating member 201 having a rectangular cross-section, wherein the side surface 204 is either side surface 281a or side surface 281b.
  • Figure 2d is a cross-section through a scintillating member 201 having a circular cross-section.
  • the scintillating member 201 has an inner side 205a and an outer side 205b and it can be seen that, in the detecting position, the side surface 204 of the scintillating member 201 is arranged to face the tube 101.
  • the scintillating member 201 is preferably elongated and positioned in a plane that is substantially perpendicular to the axis 231 of the conduit. As can be seen from Figures 3a - 3d, the scintillating member 201 forms an angular extent 301 about the axis 231 of the tube 101, the magnitude of which is dependent on the configuration of the scintillating member 201 relative to the conduit 101: in Figure 3a the angular extent 301 is approximately 45 degrees, whilst in Figure 3b the angular extent 301 is around 100 degrees, in Figure 3c the angular extent is around 210 degrees, and in Figure 3d the angular extent 301 is around 350 degrees.
  • the angular extent 301 is at least 360 degrees, such that there is no line of sight to the tube 101. In this latter configuration, the angular extent 301 ensures that circumferential geometry is provided in the plane of the axis of the scintillating member 201.
  • An advantage of having an angular extent 301 in excess of 360 degrees is that the radiation detection apparatus can tolerate movement of the conduit 101, relative to the scintillating member 201, while the scintillating member 201 still provides detection around the entire circumference of the conduit 101.
  • the cross-section of the scintillating member 201 (given by the thickness and width thereof, where thickness means distance in a direction extending substantially perpendicular to the axis 231 of the conduit and width means distance in a direction extending along the axis 231 of the conduit) is selected so as to optimise the efficiency of detecting positron ( ⁇ ) particles by optimizing interactions of positron particles (prior to annihilation) with the scintillating material while minimizing interactions of gamma ( ⁇ ) radiation.
  • the light energy generated in the scintillating member 201 can be considered to be primarily indicative of positron energy reaching the scintillating member 201 and thus of that flowing through the conduit 101.
  • the preferred geometry of the cross-section of the scintillating member 201 is dependent on the material of the member 201 and the radioisotope characterising the radiopharmaceutical.
  • the material of the scintillating member 201 is scintillating plastic, such as Bicron BC-412TM, as supplied by Bicron, Newbury, Ohio, USA. Scintillating plastic is a preferred material because of its low density and atomic number (Z- number), meaning that it is a relatively poor detector of gamma radiation; in addition, plastic scintillators have a relatively large light output and a short decay time, making them suitable for measurements that require fast response times.
  • the atomic composition of Bicron BC-412 is 5.23 Hydrogen atoms per cc (x 10 22 ); 4.74 Carbon atoms per cc (x 10 22 ); and 3.37 electrons per cc (x 10 23 ).
  • Bicron 412 is a preferred type of plastic scintillator, since it is capable of detecting radiation having energy ranging between 100 KeN and 5 MeN; positron energies associated with the radioisotopes commonly used in PET - carbon- 11, oxygen-15, fluorine- 18 and bromine-75 - fall within this range.
  • Examples of most preferred geometries associated with a scintillating member 201 manufactured from Bicron 412 are isotope-dependent and as follows: for positron emissions from 15 O-labelled water, for which the average positron energy is 721 KeN and the maximum energy is 1.75 MeN, the optimum diameter, with respect to average energy, is between 1.75 and 2.25 mm; for positron emissions from a radiopharmaceutical including carbon-11, for which the average positron energy is 395 KeN and the maximum energy is 0.97 MeN, the optimum diameter, with respect to average energy, is between 1.15 and 1.35 mm; and for positron emissions from a radiopharmaceutical including fluorine- 18, for which the average positron energy is 245 MeN and the maximum energy is 0.64 MeN, the optimum diameter, with respect to average energy, is between 0.50 and 0.70 mm.
  • the scintillating member 201 In order to resolve abrupt changes in radioactivity, such as may be expected when the 15 O-labelled water first comes into view of the scintillating member 201 and when it stops passing the scintillating member 201, the scintillating member 201 should present as small a measurement area as possible tt> the conduit 101.
  • the width of the scintillating member 201 In order to capture the initial magnitude of the radioactivity as the 15 O-labelled water comes into view of the scintillating member 201, and minimize the extent to which it is averaged out by values before (background) or after (some value less than the initial magnitude), the width of the scintillating member 201 should be as small as possible (as defined above, the width is the size of the scintillating member in a direction extending along the conduit).
  • a scintillating member 201 having a diameter of 1 mm is preferred, since it represents a compromise between ease of shaping on the one hand and resolving steep changes in radioactivity and absorbing positron particles whilst avoiding absorption of gamma radiation on the other.
  • scintillating members 201 having diameters less than 5 mm, and more preferably less than 2 mm can be used.
  • the scintillating member 201 preferably has a circular cross-section and can be formed from a rod of scintillating plastic having an appropriate size diameter.
  • a circular cross-section rod is preferred to rods of other cross-sections since it is easier to form curved shapes from rods having a circular cross-section than it is with rods having a rectangular cross-section. Moreover, in the process of forming such curved rods, those having circular cross-section are less likely to develop cracks and areas of localized stress than those of rectangular cross-section.
  • the scintillating member 201 has a curved shape, as can be expected for arrangements where the angular extent 301 exceeds 100 degrees (or thereabouts), the rod is first heated, then bent in a plane of the longitudinal axis of the rod (e.g. using a mould), and subsequently cooled down slowly so as to minimise the formation of cracks in the curved portion of the rod.
  • the whole of the surface external of the cross section of the scintillating member 201 (in this embodiment side surface 204) is preferably coated with a silvered reflective coating to ensure that scintillated light is reflected back into the body of the scintillating member 201.
  • a layer of acrylic black paint is then applied to this reflective layer to reduce background light reaching the scintillating member 201.
  • the light transmission medium 221 is preferably a length of optical fibre having a reflective outer sheath and a layer of cladding such as black PNC, and end 223 has a recess for locating the end 206 of the scintillating member 201.
  • optical cement can be applied to the region surrounding the recess, thereby fixedly joining the scintillating member 201 to the optical fibre 221.
  • the optical cement is then preferably painted with black acrylic paint, or similar, to reduce the amount of external light reaching the optical fibre 221.
  • the light receiving means 211 is preferably a photomultiplier module, whose radiant sensitivity is matched to the wavelength of maximum emission associated with the scintillating member 201.
  • Figure 5a which is a graph showing the emission spectra associated with scintillating plastic Bicron 412, the wavelength of maximum emission is 434 nm (reference 501); a suitable choice of photomultiplier is Hamamatsu module, model numbers H5773 / H5783 / H5784, whose radiant sensitivity peaks around 420 nm (reference 503, figure 5b).
  • the H5784 module has a low noise amplifier with a cable output 401, which has an input and output connection (not shown), the input being connected to a power supply, rated at between 11.5 and 15.5 N and the output being connected to an amplifier and suitable analyzer, such as OrtecTM 590A Amplifier and Timing Single-Channel Analyzer, available from Advanced Measurement Technology Berkshire, UK, which includes both a low-noise shaping amplifier and a timing single- channel analyzer.
  • an amplifier and suitable analyzer such as OrtecTM 590A Amplifier and Timing Single-Channel Analyzer, available from Advanced Measurement Technology Berkshire, UK, which includes both a low-noise shaping amplifier and a timing single- channel analyzer.
  • the photomultiplier 211 has a circular window 403 of around 8 mm diameter, which, for an optical fibre having a diameter of approximately 3 mm, means that some shielding is required between end 225 of the optical fibre 221 and the window 403.
  • the end 225 of the optical fibre 221 is coupled to the photomultiplier 211 by means of a ferrule 601 and nut 611 assembly;
  • Figure 6 shows end views 600a, 600b and section A- A 600c of the ferrule 601, together with a longitudinal cross section 600d of nut 611.
  • the ferrule 601 includes a shoulder 602 that is adapted to engage in a recess in the photomultiplier tube outer casing (within which window 403 is located); a through hole 604, of diameter suitable to accommodate the optical fibre 221; and slits 606 along a portion 607 of the ferrule 601, which provide a means of varying the diameter of the ferrule 601 in the region of that portion 607.
  • the outer surface of the ferrule 601 includes a threaded portion 608, which engages with a corresponding threaded portion 613 on the inner surface of the nut 611.
  • the threaded portion 613 leads into a tapered portion 615, which, when the threaded portion 613 of the nut 611 engages with the threaded portion 608 of the ferrule 601, applies a force in the radial direction on the slits 606 of the ferrule, thereby effectively clamping the ferrule onto the optical fibre 221.
  • the ferrule 601 includes attachment holes 620, for attaching the ferrule 601 to the photomultiplier tube outer casing.
  • the ferrule 601 is fabricated from PNC, most preferably black PNC, so as to reduce the amount of light entering the window 403, and the nut is fabricated from brass.
  • the support for holding the scintillating member 201 in a position that is suitable for measuring radioactivity flowing through the conduit 101 will now be described, with reference to Figures 7a and 7b.
  • the scintillating member 201 is preferably located within a housing comprising a bottom part 701 (Figure 7b) and a lid part 703 ( Figure 7a), which are interconnected by fixing means 720 (e.g. screws) insertable into holes 722 extending through the lid part 703 into the bottom part 701.
  • the bottom part 701 has a rectangular shape, and is provided with a bottom face 702, a front wall 704a and a rear wall 704b, which are perpendicular to the bottom face 702, and two side walls 704c, which are also perpendicular to the bottom face 702.
  • the front wall 704a, the rear wall 704b, and the side walls 704c all have substantially the same height.
  • the bottom part 701 is provided with a top face 706, which has a groove, or recess, 707 therein for locating the scintillating member 201, and the front wall 704a has a hole 708 therethrough for accommodating the optical fibre 211.
  • the bottom part 701 and top part 703 define a channel 711 into which the conduit
  • the channel 711 having a length that extends from the front wall 704a towards the rear wall 704b, a depth that extends through the full depth of both the top and bottom parts 701, 703, and a width, in the plane of the top face 706, that is at least twice the diameter of the conduit 101.
  • the channel 711 has an end portion 713, which is preferably curved and tapered, so as to provide a means of loosely holding the conduit 101 in place, and the end portion 713 is preferably displaced laterally from the front wall 704a of the bottom part 701 by a distance that is at least several times the width of the channel 711.
  • the conduit 101 is radially outwardly displaced with respect to the extents of the channel 711, the extents effectively defining an inner surface of the housing (701, 703).
  • the scintillating member 201 thus is thus physically separated from the conduit 101, which helps minimize the amount of dirt collecting on the scintillating member 201 and means that the scintillating member 201 does not have to bear the strain of repeated insertions and removals of the conduit 101 associated with a busy measurement environment.
  • the housing is made out of lead, so as to minimize the amount of gamma radiation that can reach the scintillating member 201, and the housing 701, 703 is light tight and radiation absorbent thereby minimizing noise arising from background light and background radiation.
  • the housing alternatively comprises an insert 801 having a top face 803 in which the groove 707 is located, while the bottom part 701 comprises a recess 805 for accommodating the insert 801.
  • the insert 801 is adapted to be inserted in the recess 805 such that the top face 706 of the bottom part lies flush with the top face 803 of the insert, and the top part, bottom part and insert 701, 703, 801 collectively define the channel 711.
  • the insert 801 can be fabricated from brass, preferably hard brass, or from lead.
  • the insert 801 is clamped in place by tightening the fixing means 720 in holes 722.
  • Figure 9 is a schematic diagram showing the housing 701, 703 and detector 100, with the lid part 703 removed from the housing, mounted on carrier 900.
  • the light energy receiving means 211 is preferably housed within a box 902, which is mountable on the carrier 900, is light tight and radiation absorbent, and has a hole (not shown) in a front face 904 thereof for receiving the ferrule 601 (shown schematically in Figure 9).
  • the carrier 900 additionally includes holes or similar (not shown) for hanging or fixing the carrier 900 on a hook or peg or in a threaded hole during measurements.
  • the detector 100 should be calibrated using a well defined radioactive source such as radioactive germanium ( 68 Ge) or radioactive water (H 2 15 O).
  • a well defined radioactive source such as radioactive germanium ( 68 Ge) or radioactive water (H 2 15 O).
  • the bottom part 701 and insert 801 can include a through hole 730, in which a germanium crystal is placed, and measurements taken by the detector. These measurements can subsequently be used to calibrate those in relation to a fluid passing through the conduit 101.
  • scintillating plastic has a low density and a low atomic number, which means that it is relatively insensitive to gamma radiation (provided the thickness of the scintillating member is less than around 5 mm).
  • plastic scintillating materials are inexpensive, readily formed into different shapes, robust (thus well suited to busy scanning environments), easy to light-proof and provide an excellent signal to noise ratio.
  • Scintillating plastic is thus a preferred choice of scintillating material in this, and other environments where there is a lot of background gamma radiation.
  • other scintillating materials could be used, such as sodium iodide (Nal(TI); chemical formula Nal); bismuth germanium (BGO, chemical formula Bi 4 (GeO 4 ) 3 ); Yttrium aluminium perovskit activated by Cerium (YAP:Ce); Yttrium aluminium garnet activated by Cerium (YAG:Ce, chemical formula Y 3 Al 5 O 7 ); or Lutetium Aluminum Garnet activated by Cerium (LUG:Ce, chemical formula Lu 3 Al 5 O 7 ).
  • Table 1 The physical and luminescence properties of these scintillators are set out in Table 1 below:
  • the scintillating member 201 When the scintillating member 201 is fabricated from one of these crystalline materials, approximately 50 - 60 mm of lead shielding will be required around the scintillating member 201 to minimize the amount of background gamma radiation that reaches the scintillating member 201.
  • the scintillating member 201 is described as being connected to a photomultiplier at end 206. Thus, in order to reach the photomultiplier, any light that is generated by the scintillating member 201 has to be reflected, by the inner walls of the side surface 204, along its length.
  • the detector could additionally include a further electromagnetic radiation detection system connected to the other end of the scintillating member 201, which is arranged to detect radiation generated in the region of the other end.
  • the radiation detection system 210 is described as comprising an optical fibre and photomultiplier arrangement, the light energy receiving means 211 could alternatively comprise an avalanche photo diode (APD) It will be understood that the present disclosure is for the purpose of illustration only and the invention extends to modifications, variations and improvements thereto.
  • APD avalanche photo diode

Abstract

Embodiments of the present invention relate to the detection of radioactivity in fluids, and in particular to the design of scintillators for use with a radiation detection system. The invention provides radioactivity detection apparatus for detecting radioactivity in a fluid carried in a conduit (101), the apparatus comprising: a scintillating member (204) responsive to the absorption of radioactivity to generate electromagnetic radiation, the scintillating member having two ends (201, 206) and a side surface, said side surface extending between said two ends; a support for holding the scintillating member in a detecting position with respect to the conduit; and an electromagnetic radiation detection system (211) arranged to provide an electrical signal indicative of radiation released by said scintillating member, wherein at least one end of the scintillating member is arranged to face the detection system, and the apparatus is arranged, when said scintillating member is in said detecting position, such that the scintillating member lies in a plane oriented substantially perpendicular to the conduit and said side surface of the scintillating member faces the conduit.

Description

Radioactivity detector
The present invention relates to the detection of radioactivity in fluids, and in particular relates to the design of scintillators for use with a radiation detection system. The invention particularly, but not exclusively, relates to the detection of radioactivity in radiopharmaceuticals where the radio-nuclides are of the positron or gamma emitting variety.
Background In conventional detection systems, a scintillator usually has a circular cylindrical geometry to conform to a photomultiplier tube detection head. An example of such a scintillator is described in US 5,483,070, which shows a face of the cylinder providing an external surface of the scintillator for optically coupling to a light detection / amplification device such as the photomultiplier tube. Where detection of radiation from fluids in capillary feed systems is required, in one arrangement, the prior art teaches a scintillator in which a small hole is drilled through a diameter of the cylindrical scintillator (e.g. US 5,483,070). A capillary can be threaded through this hole for carrying fluids to be analysed by, for example, positron emission detection. This conventional design has a disadvantage in that a free end of the capillary must be available to thread through the hole of the scintillator. This is particularly disadvantageous since it is often necessary to maintain the ends of the capillary in particular chemical or electrolyte buffer reservoirs, and removal of the capillary from a reservoir to feed it through a scintillator head is inconvenient, time consuming and may result in a necessity to re-prime the fluid delivery system or re-initiate the experiment, and can result in damage to the capillary or detector. Furthermore, where hazardous substances, particularly those including radio-nuclides of high activity, are being passed through the capillary, it is most desirable to minimise occasions on which the system must be "opened", since such an operation introduces a high risk of spillage of radioactive material and contamination of personnel and equipment.
Two solutions to this problem have been proposed, a first being a cylindrical scintillator having a diametric slot cut through it, and a second (described in WO 99/67656) being a cylindrical scintillator having a curved slot cut through it, both of which enable easy insertion and removal of the capillary without needing a free end available. The size of such cylindrical scintillators is constrained by limitations associated with the scintillating material and methods of manufacture, in so far as the scintillator has to be of sufficient size that it does not break during manufacture or use. Thus, the scintillator typically has a diameter that is several times greater than the thickness of tube. Since the conversion of gamma radiation to light is dependent on the thickness of the scintillator (the thicker the scintillator material, the more likely it is that gamma radiation will be converted into light), it is difficult to suppress conversion of gamma radiation to light, and thus to distinguish between measurements of positron particles and gamma radiation.
In these prior art arrangements, the capillary extends through a portion of the cylinder's diameter and the larger the scintillator, the greater the length of capillary located within the cylindrical scintillator (and thus the greater the measurement volume). This means that the resolution of measurements relating to steep changes in radioactivity can be poor.
A further problem with these known arrangements is that, in order to reduce scattering of scintillated light from the surfaces of the slot into which the capillary is inserted, these surfaces should be kept as clean as possible; clearly, regular ingress and egress of the capillary into the slot will cause particles to be deposited on these surfaces, and increase such unwanted scattering of scintillated light.
British patent application GB 2,232,482 describes a scintillator for detecting alpha emissions within a fluid, the scintillator comprising a large number of thin, parallel and spaced-apart sheets of scintillation plastic coupled optically to an end of a photomultiplier tube. The scintillator, or more specifically the sheets of scintillating plastic, is inserted into a fluid whose radioactive emissions are to be detected so that the fluid occupies the gaps between the scintillating sheets. Any radioactive particles incident on the sheets cause emission of light within the sheet, and this light is then detected by the photomultiplier in the normal manner. A problem with this arrangement is that the sheets have to be inserted into the radioactive medium, which firstly means that they will require cleaning regularly and secondly that the orientation of the sheets relative to a conduit will be constrained by the internal dimensions of the conduit. In addition, the coupling of multiple sheets with the photomultiplier is cumbersome and it is difficult to arrange the sheets in a homogenous manner, both in terms of the relative spacing between the sheets and the angles they make relative to one another. It would be desirable to provide a scintillator that alleviates these problems.
Summary of the Invention
According to a first aspect of the present invention there is provided radioactivity detection apparatus for detecting radioactivity in a fluid carried in a conduit, the apparatus comprising: a scintillating member responsive to the absorption of radioactivity to generate electromagnetic radiation, the scintillating member having two ends and a side surface, said side surface extending between said two ends; a support for holding the scintillating member in a detecting position with respect to the conduit; and an electromagnetic radiation detection system arranged to provide an electrical signal indicative of radiation released by said scintillating member, wherein at least one end of the scintillating member is arranged to face the electromagnetic radiation detection system, and the apparatus is arranged, when the scintillating member is in said detecting position, such that the scintillating member lies in a plane oriented substantially perpendicular to the conduit and said side surface of the scintillating member faces the conduit.
Since the external surface of the scintillating member is arranged to face the conduit, the scintillating member can be arranged beside the conduit whilst being capable of detecting a significant proportion of a type of radioactivity emitted by the fluid in the conduit. The scintillating member can have a relatively uniform cross-section throughout, so that it is less likely to develop cracks during manufacture and/or during use of the apparatus and reduce the working life thereof. Moreover, the thickness of the scintillating member is not constrained by the need to accommodate a conduit therein, which means that it can have a smaller size. This is beneficial from the point of view of reducing the amount of a type of radiation, such as gamma radiation, which is not of interest, from being absorbed by the scintillating member.
Advantageously the scintillating member comprises an elongate scintillating member, and is arranged so as to form an angular extent about the conduit of at least 30 degrees. Preferably the angular extent is 180 degrees or more, and most preferably it is substantially 360 degrees. In one arrangement, these latter angular extents can be provided by curving the scintillating member around the conduit; an angular extent of 360 degrees or more conveniently ensures that detection is provided over the entire length of the scintillating member surrounding the conduit. The scintillating member is arranged to lie in a plane substantially perpendicular to the conduit when in the detecting position, thereby optimizing the temporal resolution of the detection volume.
Advantageously the scintillating member is primarily responsive to radioactivity, in particular positrons, having energy between 0.05 MeN and 2 MeN, and more preferably between 0.1 MeN and 1.2 MeN.
Preferably the thickness of the scintillating member, in a direction extending substantially perpendicular to the conduit, is less than 5 mm, and most preferably is less than 2 mm, whilst the width of the scintillating member, in a direction extending along the conduit, is preferably less than 5 mm. Thus for a scintillating member having a circular cross section, its diameter is preferably less than 5 mm. This has the benefit of being sufficiently thick and wide for arriving positrons to be converted into light energy, whilst not annihilating, and being too thin for arriving gamma radiation to be converted into light energy. This preferred configuration thus provides a means of selectively measuring radiation associated with positrons.
Preferably the scintillating member comprises a plastic scintillator, which has a low density and a low atomic number, and is thus relatively insensitive to gamma radiation. Scintillating plastic is thus an advantageous choice of scintillating material in environments where there is a lot of background gamma radiation, such as is the case in nuclear medicine facilities. Moreover, plastic scintillating materials are readily formed into shape and are robust, thus being well suited to practical use in nuclear medicine facilities.
Preferably the electromagnetic radiation detection system comprises a light energy transmission medium and a light energy receiving means, the light energy transmission medium being connectable at one end to said scintillating member and connectable at the other end to the light energy receiving means. The light energy receiving means can be provided by a photomultiplier and the light energy transmission medium by an optical fibre.
Preferably the support includes a housing comprising a top part and a bottom part, the bottom part including a groove for locating the scintillating member in the detecting position, wherein the housing defines a channel for receiving the conduit, the channel having a depth extending across the thickness of the top and bottom parts and a length extending into the top and bottom parts. Advantageously the conduit is radially outwardly displaced with respect to an inner surface of the support when the scintillating member is in the detecting position, thereby providing a means of separating the scintillating member from the conduit. Further features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, made with reference to the accompanying drawings.
Brief Description of Drawings Figure 1 is a schematic diagram of a nuclear medicine diagnosis facility in which radioactivity detection apparatus according to an embodiment of the invention can operate;
Figure 2a is a schematic perspective diagram of parts of the radioactivity detection apparatus according to an embodiment of the invention;
Figures 2b and 2c are sections showing a surface external to the cross section of a scintillating member forming part of the radioactivity detection apparatus according to an embodiment of the invention;
Figure 2d is a cross-section through a plane passing through both a diameter of the scintillating member of the detector shown in Figure 2a and the longitudinal axis of the conduit, when the detector is in a detecting position; Figures 3a - 3d are cross-sections through line A-A of the schematic diagram of
Figure 2a, showing different angular extents defined by the configuration of the scintillating member of the detector shown in Figure 2a relative to the conduit, when the detector is in a detecting position;
Figure 4 is a schematic perspective diagram showing in more detail the radiation detection system and the scintillating member forming part of the detector shown in Figure 2a;
Figure 5a is a graph showing the light output response for scintillating plastic BC- 412;
Figure 5b is a graph showing spectral response of light energy receiving means forming part of the radiation detection system shown in Figure 4;
Figure 6 shows cross section and end views of a ferrule and a nut assembly for use in joining parts of the radiation detection system shown in Figure 4; Figure 7a is a plan view of a lid part forming part of a housing for the radioactivity detection apparatus according to an embodiment of the invention;
Figure 7b shows cross section and plan views of a bottom part forming part of the housing for the radioactivity detection apparatus according to an embodiment of the invention;
Figures 8a and 8b show cross section and plan views of an alternative arrangement of the bottom part forming part of the housing; and
Figure 9 shows schematic plan and side views of a carrier for the radioactivity detection apparatus according to an embodiment of the invention.
Detailed description of the drawings
Nuclear medicine involves the use of radioactive isotopes (radioisotopes) within a body. The radioisotopes are attracted to specific organs, bones or tissues, and the emissions produced by the radioisotopes are used provide information about a particular type of disease. Examples of radioisotopes commonly used in nuclear medicine include carbon-11, oxygen-15, fluorine-18 and bromine-75.
Positron emission tomography (PET) is an example of a nuclear medicine diagnostic technique whereby images of physiological function of organs are acquired by imaging the decay of radio-isotopes bound to molecules having known biological properties. Suitable radio-isotopes are synthesized into a carrier (also called a tracer) that enables the radio- isotope to be delivered to the organ under examination. Such a carrier is commonly referred to as a radiopharmaceutical.
An embodiment of the radioactivity detection apparatus, hereinafter referred to as detector 100, will be described in the context of a radiopharmaceutical generation system (herein referred to as a water generation system). One commonly used radio-isotope is Oxygen-15 (15O), which is produced by deuteron bombardment of natural nitrogen through the 14N(d,n)15O nuclear reaction, and a radiopharmaceutical associated with the Oxygen-15 radio-isotope is O-labelled water. Typically, O-labelled water is produced on-line in a radiopharmaceutical generator, details of which can be found in an article entitled "Technical performance and operating procedure of a bedside [15O] water infuser", authored by H Touchon-Danguy et al and published in the Journal of Label. Cmpds. Radiopharm., volume 37 pages 662 - 664, in 1995. Figure 1 is a schematic diagram of such a facility 110, showing a delivery path 101 associated with a radiopharmaceutical, together with the detector 100, which is positioned in the vicinity of the scanner 107. In overview, water generator 105 receives, as input, saline 109; radioactive Oxygen-15; and hydrogen, and generates radioactive 5O-labelled water under control of processing unit 111. This 15O-labelled water is pumped along delivery tube 101, passing through the detector 100 en route for the subject 103.
Embodiments of the invention are concerned with a detector whose construction is simpler and less sensitive to manufacturing stresses than known detectors, and whose shape is not controlled by material and manufacturing constraints in such a way as to significantly limit the resolution and ability to discriminate between radiation particles. An embodiment of the invention will now be described with reference to Figure 2a, which is a perspective schematic diagram showing part of the conduit (or delivery tube) 101 having first and second sides 222a, 222b, through which the radiopharmaceutical is delivered, together with an embodiment of the detector 100. In this embodiment the detector 100 comprises a scintillating member 201 having a side surface 204 and two ends (only one shown, end 206), the side surface being external of its cross-section. The detector 100 also includes a support (not shown in Figure 2a; described in detail below) for holding the scintillating member 201 in a detecting position relative to the tube 101; and an electromagnetic radiation detection system 210 including light energy receiving means 211, which preferably comprises a photomultiplier (described in more detail below) and a light energy transmission medium 221 connectable at one end 223 to said scintillating member 201 and at connectable at the other end 225 to the light energy receiving means 211. Preferably the light energy transmission medium 221 comprises an optical fibre, and its configuration and attachment at ends 223, 225 is described in more detail below. Firstly aspects of the scintillating member 201 will be described, with reference to
Figures 2b, 2c and 2d. Figure 2b shows a scintillating member 201 having a circular cross- section, wherein the side surface 204 is the outer surface of the scintillating member 201; Figure 2c shows scintillating member 201 having a rectangular cross-section, wherein the side surface 204 is either side surface 281a or side surface 281b. Figure 2d is a cross-section through a scintillating member 201 having a circular cross-section. The scintillating member 201 has an inner side 205a and an outer side 205b and it can be seen that, in the detecting position, the side surface 204 of the scintillating member 201 is arranged to face the tube 101. The scintillating member 201 is preferably elongated and positioned in a plane that is substantially perpendicular to the axis 231 of the conduit. As can be seen from Figures 3a - 3d, the scintillating member 201 forms an angular extent 301 about the axis 231 of the tube 101, the magnitude of which is dependent on the configuration of the scintillating member 201 relative to the conduit 101: in Figure 3a the angular extent 301 is approximately 45 degrees, whilst in Figure 3b the angular extent 301 is around 100 degrees, in Figure 3c the angular extent is around 210 degrees, and in Figure 3d the angular extent 301 is around 350 degrees. In a preferred arrangement, the angular extent 301 is at least 360 degrees, such that there is no line of sight to the tube 101. In this latter configuration, the angular extent 301 ensures that circumferential geometry is provided in the plane of the axis of the scintillating member 201. An advantage of having an angular extent 301 in excess of 360 degrees is that the radiation detection apparatus can tolerate movement of the conduit 101, relative to the scintillating member 201, while the scintillating member 201 still provides detection around the entire circumference of the conduit 101.
The cross-section of the scintillating member 201 (given by the thickness and width thereof, where thickness means distance in a direction extending substantially perpendicular to the axis 231 of the conduit and width means distance in a direction extending along the axis 231 of the conduit) is selected so as to optimise the efficiency of detecting positron (β) particles by optimizing interactions of positron particles (prior to annihilation) with the scintillating material while minimizing interactions of gamma (γ) radiation. In this way the light energy generated in the scintillating member 201 can be considered to be primarily indicative of positron energy reaching the scintillating member 201 and thus of that flowing through the conduit 101. The preferred geometry of the cross-section of the scintillating member 201 is dependent on the material of the member 201 and the radioisotope characterising the radiopharmaceutical. In one embodiment the material of the scintillating member 201 is scintillating plastic, such as Bicron BC-412™, as supplied by Bicron, Newbury, Ohio, USA. Scintillating plastic is a preferred material because of its low density and atomic number (Z- number), meaning that it is a relatively poor detector of gamma radiation; in addition, plastic scintillators have a relatively large light output and a short decay time, making them suitable for measurements that require fast response times. Bicron BC-412 is the product name for a plastic based on polyvinyltolulene, having chemical formula 2-CH3C6H CH:=CH . The atomic composition of Bicron BC-412 is 5.23 Hydrogen atoms per cc (x 1022); 4.74 Carbon atoms per cc (x 1022); and 3.37 electrons per cc (x 1023). Bicron 412 is a preferred type of plastic scintillator, since it is capable of detecting radiation having energy ranging between 100 KeN and 5 MeN; positron energies associated with the radioisotopes commonly used in PET - carbon- 11, oxygen-15, fluorine- 18 and bromine-75 - fall within this range. Examples of most preferred geometries associated with a scintillating member 201 manufactured from Bicron 412 are isotope-dependent and as follows: for positron emissions from 15O-labelled water, for which the average positron energy is 721 KeN and the maximum energy is 1.75 MeN, the optimum diameter, with respect to average energy, is between 1.75 and 2.25 mm; for positron emissions from a radiopharmaceutical including carbon-11, for which the average positron energy is 395 KeN and the maximum energy is 0.97 MeN, the optimum diameter, with respect to average energy, is between 1.15 and 1.35 mm; and for positron emissions from a radiopharmaceutical including fluorine- 18, for which the average positron energy is 245 MeN and the maximum energy is 0.64 MeN, the optimum diameter, with respect to average energy, is between 0.50 and 0.70 mm. In order to resolve abrupt changes in radioactivity, such as may be expected when the 15O-labelled water first comes into view of the scintillating member 201 and when it stops passing the scintillating member 201, the scintillating member 201 should present as small a measurement area as possible tt> the conduit 101. For example, in order to capture the initial magnitude of the radioactivity as the 15O-labelled water comes into view of the scintillating member 201, and minimize the extent to which it is averaged out by values before (background) or after (some value less than the initial magnitude), the width of the scintillating member 201 should be as small as possible (as defined above, the width is the size of the scintillating member in a direction extending along the conduit).
Thus a scintillating member 201 having a diameter of 1 mm is preferred, since it represents a compromise between ease of shaping on the one hand and resolving steep changes in radioactivity and absorbing positron particles whilst avoiding absorption of gamma radiation on the other. However, scintillating members 201 having diameters less than 5 mm, and more preferably less than 2 mm can be used. As stated above, the scintillating member 201 preferably has a circular cross-section and can be formed from a rod of scintillating plastic having an appropriate size diameter. A circular cross-section rod is preferred to rods of other cross-sections since it is easier to form curved shapes from rods having a circular cross-section than it is with rods having a rectangular cross-section. Moreover, in the process of forming such curved rods, those having circular cross-section are less likely to develop cracks and areas of localized stress than those of rectangular cross-section. In the event that the scintillating member 201 has a curved shape, as can be expected for arrangements where the angular extent 301 exceeds 100 degrees (or thereabouts), the rod is first heated, then bent in a plane of the longitudinal axis of the rod (e.g. using a mould), and subsequently cooled down slowly so as to minimise the formation of cracks in the curved portion of the rod.
The whole of the surface external of the cross section of the scintillating member 201 (in this embodiment side surface 204) is preferably coated with a silvered reflective coating to ensure that scintillated light is reflected back into the body of the scintillating member 201. A layer of acrylic black paint is then applied to this reflective layer to reduce background light reaching the scintillating member 201.
Aspects of the light receiving means 211 and light transmission medium 221 will now be described, with reference to Figures 4, 5, 6. Turning firstly to Figure 4, the light transmission medium 221 is preferably a length of optical fibre having a reflective outer sheath and a layer of cladding such as black PNC, and end 223 has a recess for locating the end 206 of the scintillating member 201. Once the end 206 has engaged into the recess, optical cement can be applied to the region surrounding the recess, thereby fixedly joining the scintillating member 201 to the optical fibre 221. The optical cement is then preferably painted with black acrylic paint, or similar, to reduce the amount of external light reaching the optical fibre 221.
The light receiving means 211 is preferably a photomultiplier module, whose radiant sensitivity is matched to the wavelength of maximum emission associated with the scintillating member 201. As can be seen from Figure 5a, which is a graph showing the emission spectra associated with scintillating plastic Bicron 412, the wavelength of maximum emission is 434 nm (reference 501); a suitable choice of photomultiplier is Hamamatsu module, model numbers H5773 / H5783 / H5784, whose radiant sensitivity peaks around 420 nm (reference 503, figure 5b). The H5784 module has a low noise amplifier with a cable output 401, which has an input and output connection (not shown), the input being connected to a power supply, rated at between 11.5 and 15.5 N and the output being connected to an amplifier and suitable analyzer, such as Ortec™ 590A Amplifier and Timing Single-Channel Analyzer, available from Advanced Measurement Technology Berkshire, UK, which includes both a low-noise shaping amplifier and a timing single- channel analyzer.
The photomultiplier 211 has a circular window 403 of around 8 mm diameter, which, for an optical fibre having a diameter of approximately 3 mm, means that some shielding is required between end 225 of the optical fibre 221 and the window 403. Turning also to Figure 6, the end 225 of the optical fibre 221 is coupled to the photomultiplier 211 by means of a ferrule 601 and nut 611 assembly; Figure 6 shows end views 600a, 600b and section A- A 600c of the ferrule 601, together with a longitudinal cross section 600d of nut 611. The ferrule 601 includes a shoulder 602 that is adapted to engage in a recess in the photomultiplier tube outer casing (within which window 403 is located); a through hole 604, of diameter suitable to accommodate the optical fibre 221; and slits 606 along a portion 607 of the ferrule 601, which provide a means of varying the diameter of the ferrule 601 in the region of that portion 607. The outer surface of the ferrule 601 includes a threaded portion 608, which engages with a corresponding threaded portion 613 on the inner surface of the nut 611. The threaded portion 613 leads into a tapered portion 615, which, when the threaded portion 613 of the nut 611 engages with the threaded portion 608 of the ferrule 601, applies a force in the radial direction on the slits 606 of the ferrule, thereby effectively clamping the ferrule onto the optical fibre 221. The ferrule 601 includes attachment holes 620, for attaching the ferrule 601 to the photomultiplier tube outer casing. Preferably the ferrule 601 is fabricated from PNC, most preferably black PNC, so as to reduce the amount of light entering the window 403, and the nut is fabricated from brass. When the optical fibre 221 is attached to the photomultiplier 211, (clamping in place by means of the ferrule 601 and nut 611 assembly), optical grease is applied to join the end of the optical fibre 221 thereto.
The support for holding the scintillating member 201 in a position that is suitable for measuring radioactivity flowing through the conduit 101 will now be described, with reference to Figures 7a and 7b. The scintillating member 201 is preferably located within a housing comprising a bottom part 701 (Figure 7b) and a lid part 703 (Figure 7a), which are interconnected by fixing means 720 (e.g. screws) insertable into holes 722 extending through the lid part 703 into the bottom part 701. Preferably the bottom part 701 has a rectangular shape, and is provided with a bottom face 702, a front wall 704a and a rear wall 704b, which are perpendicular to the bottom face 702, and two side walls 704c, which are also perpendicular to the bottom face 702. The front wall 704a, the rear wall 704b, and the side walls 704c all have substantially the same height. The bottom part 701 is provided with a top face 706, which has a groove, or recess, 707 therein for locating the scintillating member 201, and the front wall 704a has a hole 708 therethrough for accommodating the optical fibre 211. The bottom part 701 and top part 703 define a channel 711 into which the conduit
101 may be inserted, the channel 711 having a length that extends from the front wall 704a towards the rear wall 704b, a depth that extends through the full depth of both the top and bottom parts 701, 703, and a width, in the plane of the top face 706, that is at least twice the diameter of the conduit 101. The channel 711 has an end portion 713, which is preferably curved and tapered, so as to provide a means of loosely holding the conduit 101 in place, and the end portion 713 is preferably displaced laterally from the front wall 704a of the bottom part 701 by a distance that is at least several times the width of the channel 711. Thus when the scintillating member 201 is in the detecting position, the conduit 101 is radially outwardly displaced with respect to the extents of the channel 711, the extents effectively defining an inner surface of the housing (701, 703). In use, the scintillating member 201 thus is thus physically separated from the conduit 101, which helps minimize the amount of dirt collecting on the scintillating member 201 and means that the scintillating member 201 does not have to bear the strain of repeated insertions and removals of the conduit 101 associated with a busy measurement environment. Preferably the housing is made out of lead, so as to minimize the amount of gamma radiation that can reach the scintillating member 201, and the housing 701, 703 is light tight and radiation absorbent thereby minimizing noise arising from background light and background radiation.
Referring to Figures 8a and 8b, the housing alternatively comprises an insert 801 having a top face 803 in which the groove 707 is located, while the bottom part 701 comprises a recess 805 for accommodating the insert 801. The insert 801 is adapted to be inserted in the recess 805 such that the top face 706 of the bottom part lies flush with the top face 803 of the insert, and the top part, bottom part and insert 701, 703, 801 collectively define the channel 711. In this arrangement the insert 801 can be fabricated from brass, preferably hard brass, or from lead. The insert 801 is clamped in place by tightening the fixing means 720 in holes 722. Figure 9 is a schematic diagram showing the housing 701, 703 and detector 100, with the lid part 703 removed from the housing, mounted on carrier 900. The light energy receiving means 211 is preferably housed within a box 902, which is mountable on the carrier 900, is light tight and radiation absorbent, and has a hole (not shown) in a front face 904 thereof for receiving the ferrule 601 (shown schematically in Figure 9). The carrier 900 additionally includes holes or similar (not shown) for hanging or fixing the carrier 900 on a hook or peg or in a threaded hole during measurements.
In order to produce reliable quantitative, as well as qualitative measurements, the detector 100 should be calibrated using a well defined radioactive source such as radioactive germanium (68Ge) or radioactive water (H2 15O). Referring back to Figures 7b and 8b, the bottom part 701 and insert 801 can include a through hole 730, in which a germanium crystal is placed, and measurements taken by the detector. These measurements can subsequently be used to calibrate those in relation to a fluid passing through the conduit 101.
When the detector 100 is used to take measurements close to a patient in a PET facility, such as the one described above with reference to Figure 1, all of the infused positron radioactivity passing the detector 100 enters the patient, becomes annihilated and is emitted as gamma radiation. There is therefore a considerable amount of background radiation in the vicinity of the detector 100. As stated above, scintillating plastic has a low density and a low atomic number, which means that it is relatively insensitive to gamma radiation (provided the thickness of the scintillating member is less than around 5 mm). Moreover, plastic scintillating materials are inexpensive, readily formed into different shapes, robust (thus well suited to busy scanning environments), easy to light-proof and provide an excellent signal to noise ratio. Scintillating plastic is thus a preferred choice of scintillating material in this, and other environments where there is a lot of background gamma radiation. However, other scintillating materials could be used, such as sodium iodide (Nal(TI); chemical formula Nal); bismuth germanium (BGO, chemical formula Bi4(GeO4)3); Yttrium aluminium perovskit activated by Cerium (YAP:Ce); Yttrium aluminium garnet activated by Cerium (YAG:Ce, chemical formula Y3Al5O7); or Lutetium Aluminum Garnet activated by Cerium (LUG:Ce, chemical formula Lu3Al5O7). The physical and luminescence properties of these scintillators are set out in Table 1 below:
Table 1: Comparison of physical and luminescence parameters of scintillators
Figure imgf000016_0001
When the scintillating member 201 is fabricated from one of these crystalline materials, approximately 50 - 60 mm of lead shielding will be required around the scintillating member 201 to minimize the amount of background gamma radiation that reaches the scintillating member 201. In the embodiment above, the scintillating member 201 is described as being connected to a photomultiplier at end 206. Thus, in order to reach the photomultiplier, any light that is generated by the scintillating member 201 has to be reflected, by the inner walls of the side surface 204, along its length. As is known, reflection of light incurs losses, so that, for long and curved scintillating members, a proportion of the light will be lost before reaching the electromagnetic radiation detection system 210. Accordingly, the detector could additionally include a further electromagnetic radiation detection system connected to the other end of the scintillating member 201, which is arranged to detect radiation generated in the region of the other end.
Whilst in the foregoing embodiments the radiation detection system 210 is described as comprising an optical fibre and photomultiplier arrangement, the light energy receiving means 211 could alternatively comprise an avalanche photo diode (APD) It will be understood that the present disclosure is for the purpose of illustration only and the invention extends to modifications, variations and improvements thereto.

Claims

Claims
1. Radioactivity detection apparatus for detecting radioactivity in a fluid carried in a conduit, the apparatus comprising: a scintillating member responsive to the absorption of radioactivity to generate electromagnetic radiation, the scintillating member having two ends and a side surface, said side surface extending between said two ends; a support for holding the scintillating member in a detecting position with respect to the conduit; and an electromagnetic radiation detection system arranged to provide an electrical signal indicative of radiation released by said scintillating member, wherein at least one end of the scintillating member is arranged to face the detection system, characterised in that the apparatus is arranged, when said scintillating member is in said detecting position, such that the scintillating member lies in a plane oriented substantially perpendicular to the conduit and said side surface of the scintillating member faces the conduit.
2. Apparatus according to claim 1, wherein the scintillating member comprises an elongate scintillating member.
3. Apparatus according to claim 1 or claim 2, wherein the scintillating member is radially outwardly displaced with respect to an inner surface of the support when the scintillating member is in the detecting position.
4. Apparatus according to any one of the preceding claims, wherein the scintillating member is arranged so as to form an angular extent about the conduit of at least 30 degrees.
5. Apparatus according to claim 4, wherein the angular extent is at least 180 degrees.
6. Apparatus according to claim 4, wherein the angular extent is substantially 360 degrees.
7. Apparatus according to any one of the preceding claims, wherein the thickness of the scintillating member, in a direction extending substantially perpendicular to the conduit, is less than 5 mm.
8. Apparatus according to claim 7, wherein the thickness is less than 2 mm.
9. Apparatus according to any one of the preceding claims, wherein the width of the scintillating member, in a direction extending along the conduit, is less than 5 mm.
10. Apparatus according to any one of the preceding claims, wherein the scintillating member has a substantially circular cross-section.
11. Apparatus according to any one of the preceding claims, the electromagnetic radiation detection system comprising a light energy transmission medium and a light energy receiving means, the light energy transmission medium being connectable at one end to said scintillating member and connectable at the other end to the light energy receiving means.
12. Apparatus according to any one of the preceding claims, wherein the scintillating member is primarily responsive to radioactivity having energy between 0.05 MeN and 2 MeN, and more preferably between 0.1 MeV and 1.2 MeN.
13. Apparatus according to any one of the preceding claims, wherein the scintillating member is responsive to the absorption of positron particles.
14. Apparatus according to any one of the preceding claims, wherein the scintillating member comprises a plastic scintillator.
15. Apparatus according to any one of the preceding claims, wherein the support includes a housing comprising a top part and a bottom part, the bottom part including a groove for locating the scintillating member in the detecting position, wherein the housing defines a channel for receiving the conduit, the channel having a depth extending across the thickness of the top and bottom parts and a length extending into the top and bottom parts.
16. Apparatus according to claim 15, wherein the channel extends into the top and bottom parts to form an end portion therein, the end portion being curved so as to locate the conduit.
17. A nuclear medicine facility comprising apparatus according to any preceding claim.
18. A positron emission tomography scanning facility according to claim 17.
19. A support for radioactivity detection apparatus for detecting radioactivity in a fluid carried in a conduit, wherein the apparatus comprises a scintillating member responsive to the absorption of radioactivity to generate electromagnetic radiation, the scintillating member having two ends and a side surface; and an electromagnetic radiation detection system arranged to provide an electrical signal indicative of radiation released by said scintillating member, wherein the support is arranged to hold the scintillating member in a detecting position with respect to the conduit such that said side surface of the scintillating member faces the conduit, the support comprising: a housing having a top part and a bottom part, the bottom part including a groove for locating the scintillating member in the detecting position, wherein the housing defines a channel for receiving the conduit, the channel having a depth extending across the thickness of the top and bottom parts and a length extending into the top and bottom parts.
PCT/GB2004/001733 2003-04-30 2004-04-22 Detector for measuring radioactive fluid WO2004097454A2 (en)

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Citations (3)

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WO1995012825A1 (en) * 1993-11-03 1995-05-11 Packard Instrument Company, Inc. Flow cell for use in a flow scintillation analyzer
WO1999053340A1 (en) * 1998-04-08 1999-10-21 Imperial College Innovations Limited Scintillation head
WO1999067656A1 (en) * 1998-06-23 1999-12-29 Imperial College Innovations Ltd. Scintillation head

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JPS5713381A (en) * 1980-06-30 1982-01-23 Aloka Co Ltd Radiation detector

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Publication number Priority date Publication date Assignee Title
WO1995012825A1 (en) * 1993-11-03 1995-05-11 Packard Instrument Company, Inc. Flow cell for use in a flow scintillation analyzer
WO1999053340A1 (en) * 1998-04-08 1999-10-21 Imperial College Innovations Limited Scintillation head
WO1999067656A1 (en) * 1998-06-23 1999-12-29 Imperial College Innovations Ltd. Scintillation head

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Title
PATENT ABSTRACTS OF JAPAN vol. 0060, no. 75 (P-114), 12 May 1982 (1982-05-12) & JP 57 013381 A (ALOKA CO LTD), 23 January 1982 (1982-01-23) *

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