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
  • G01T1/16—Measuring radiation intensity
  • G01T1/161—Applications in the field of nuclear medicine, e.g. in vivo counting
  • G01T1/164—Scintigraphy
  • G01T1/1641—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras
  • G01T1/1648—Ancillary equipment for scintillation cameras, e.g. reference markers, devices for removing motion artifacts, calibration devices
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/161—Applications in the field of nuclear medicine, e.g. in vivo counting
  • G01T1/164—Scintigraphy
  • G01T1/1641—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras
  • G01T1/1642—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
  • A61B6/4208—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector
  • A61B6/4258—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector for detecting non x-ray radiation, e.g. gamma radiation

Definitions

• the present invention relates to a detection head and a collimator for a gamma-camera and more particularly for a so-called "pixel" gamma-camera.
• gamma-pixel camera is understood to mean a camera sensitive to gamma radiation and the detection head of which comprises a plurality of juxtaposed individual elementary detectors.
• the invention finds applications in medical imaging, such as, for example, in scintigraphy and in SPECT emission tomography (Single Photo-Emission Computed Tomography).
• the gamma-cameras conventionally used in medical imaging are cameras of the Anger type. Reference may be made to this subject in document (1), the reference of which is indicated at the end of this description. Gamma cameras are used in particular to visualize the distribution in the body, or in an organ, of molecules marked by a radioactive isotope previously injected into the patient.
• FIG. 1 shows more precisely a detection head 10 of a gamma-camera of the Anger type, arranged opposite a member 12.
• the detection head 10 includes a collimator 20, a scintillator crystal 24, a light guide 22 and a plurality of photomultiplier tubes 26 juxtaposed so as to cover one face of the light guide 22, opposite the scintillator crystal 24.
• the scintillator is, for example, a Nal (Ti) crystal.
• the collimator is in the form of a lead disc crossed by a plurality of channels 21 for the passage of gamma radiation, substantially identical and parallel to each other. The disc is placed against the scintillator 24 so that the channels 21 are perpendicular to the surface of this crystal.
• a divergent or convergent collimator can be used.
• the function of the collimator 20 is to select from all the gamma rays 30 emitted by the member 12 those which reach the detection head substantially under normal incidence.
• the selective nature of the collimator makes it possible to increase the resolution and the sharpness of the image produced.
• the increase in resolution comes at the expense of sensitivity.
• the opening and the length of the channels 21 are determined as a function of the energy of the examination and of the spatial resolution-derived sensitivity compromise.
• the spacing of the channels is chosen the greater the greater the energy of the received radiation.
• the known collimator channels have a hexagonal (or round for high energies) section.
• This shape is dictated not only by imperatives of uniformity of detection but also by manufacturing constraints of the collimator.
• the doses of radioactive product injected into the patient must necessarily be limited.
• the intensity of the emitted radiation is relatively low.
• the extent and thickness of the intermediate walls separating the channels of the collimator should be reduced in order to limit excessive losses of "useful" radiation.
• collimators are produced, the channels of which have a hexagonal section. This form also has the advantage of facilitating the production of collimators.
• hexagonal shape is retained insofar as it is relatively close to the circular shape and allows a substantially uniform detection.
• the thickness of the walls which delimit the channels is generally chosen in a range going from 0.2 to 2 mm.
• the characteristic size of the opening of the channels, that is to say the distance between dishes of the hexagonal section is of the order of 1.5 to 4.5 mm.
• the depth of the channels is generally chosen from 30 to 50 mm.
• Known collimators are generally manufactured by a technique of assembling lead sheets shaped to constitute the channels. According to another known technique, the collimators can also be obtained by molding in a needle mold.
• each interaction is designated by "event" detected from a gamma photon with the material of the detector . for example with the scintillator crystal.
• the photomultipliers 26 are designed to emit, during each event, an electrical pulse proportional to the number of light photons received from the scintillator 24.
• the photomultipliers 26 are not directly attached to the scintillator crystal 24 but are separated from the latter by the light guide 22.
• Photomultipliers emit a signal whose amplitude is proportional to the total quantity of light produced in the scintillator by gamma radiation, that is to say proportional to its energy.
• the individual signal of each photomultiplier also depends on the distance which separates it from the point of interaction 30 of the gamma radiation with the material of the scintillator. Indeed, each photomultiplier delivers a current pulse proportional to the light flux it has received.
• small graphs A, B, C show that photomultipliers 26a, 26b and 26c located at different distances from an interaction point 30 deliver signals with different amplitudes.
• the position of the interaction point 30 and the energy of a gamma photon is calculated in the gamma camera from the signals coming from the set of photomultipliers by carrying out a barycentric weighting of the contributions of each photomultiplier.
• Anger-type gamma cameras have a drawback, however, due to the fact that the number of light photons created at each event in the scintillating crystal obeys a Poisson statistic.
• the number of photoelectrons torn from the photocathode of photomultipliers also obeys Poisson statistics.
• the calculations of position and energy are marred by an inaccuracy linked to the fish fluctuations of the number of light photons and the number of photoelectrons for each event. The greater the standard deviation of the fluctuations, the smaller the number of photons or photoelectrons.
• the intrinsic spatial resolution of the gamma-camera is characterized by the width at mid-height of the distribution of the positions calculated for the same collimated point source placed on the crystal.
• the resolution is of the order of 3 to 4 mm at 140 keV. Furthermore, the energy of the gamma photon is calculated by summing the contributions of all the photomultipliers that have received light. It is also marred by a statistical fluctuation. The energy resolution is characterized by the ratio of the width at half-height of the distribution of the energies calculated over the average value of the distribution, for the same source. It is around 9 to 11% at 140 keV.
• Detection heads for gamma cameras are also known, in which the scintillator crystal and the photomultipliers are replaced by solid detectors arranged in the form of an array of individual detectors. In such a case the spatial resolution of the gamma-camera depends on the size of the. individual detectors.
• FIG. 2 shows for information and very schematically a detection head with solid detectors.
• the detection head 40 includes a plurality of individual elementary semiconductor detectors 42. These are, for example, CdTe or CdZnTe type detectors.
• the individual detectors are substantially identical to each other and are juxtaposed in the form of a matrix network to form a detection plane 44.
• the detectors 42 are also transferred to a printed circuit board 46 and are connected to preamplifiers (not shown) on this card.
• the card 46 makes it possible to collect the detection signals from the various individual detectors, to format them and to direct them to a unit for computing and processing information 48. This unit makes it possible to calculate the position and the energy of the events .
• a detection head in accordance with FIG. 2 has, compared to the detection head in FIG. 1, the advantage of a significant improvement in the energy resolution because the number of charges created in the semiconductor is ten times the number of light photons created in the scintillator crystal.
• a gamma camera equipped with a detection head in accordance with FIG. 2, can also be equipped with a collimator for selecting the radiations substantially perpendicular to the detection head.
• FIG. 3 shows, in partial top view, a gamma camera with solid detectors equipped with a conventional collimator.
• a collimator 20 similar to that of FIG. 1. It comprises a plurality of channels 21 of hexagonal section, juxtaposed and with a main axis substantially perpendicular to the detection plane 44. Each channel is delimited by a lead sheet folded in hexagonal shape. The lead sheets, thus formed, are juxtaposed and joined to form the collimator 20.
• Such a structure called “honeycomb”, is particularly practical to produce and is well known for the manufacture of collimators such as used on "Anger" type cameras.
• the collimator can also be molded in a mold having needles of hexagonal section.
• FIGS. 4A, 4B, 4C and 4D This problem is highlighted and illustrated by FIGS. 4A, 4B, 4C and 4D.
• FIGS. 4A to 4D show, in top view, different relative positions of an individual detector 42 of the detection head of FIG. 3 with respect to the channels 21 of the collimator 20. For reasons of simplification, a single individual detector and a only part of the collimator are shown. Furthermore, the scale of FIGS. 4A to 4D is slightly larger than that of FIG. 3.
• an object of the present invention is to propose a detection head which does not have the limitations and drawbacks mentioned above.
• Another object of the invention is to provide a detection head with individual semiconductor detectors and with a collimator of regular shape, and which has a uniform sensitivity.
• An aim is also to propose a collimator for a detection head with juxtaposed individual detectors, allowing uniform detection.
• the invention more specifically relates to a gamma-camera detection head comprising:
• the term "length and width of the repeat pattern of the channels” means the length and width of the channels, including the thickness of the walls of material which delimit the channels.
• the length and width of the elementary detectors means the length and the width of their sensitive part including the thickness of non-sensitive "dead" zones which possibly surround the sensitive parts.
• the length and the width of the elementary detectors are understood to include the thickness of these walls.
• rectangle is understood to designate the shape of a quadrilateral whose four angles are straight.
• rectangular also designates a square shape which is only a particular case of rectangular shape.
• the sensitive detector surface facing with channels of the collimator is substantially identical for each elementary detector. This makes it possible to obtain an excellent uniformity of response from the detection head thus equipped.
• the shape of the elementary detectors and / or that of the repeating pattern is square.
• the invention also relates to a collimator for a gamma camera.
• the collimator comprises a plurality of channels for the passage of gamma radiation, substantially identical and parallel to each other, the channels having a square cross section.
• the invention relates to a gamma camera comprising a collimator or a detection head as described above.
• - Figure 1 is a diagram showing in a simplified manner the operation of an Anger type camera equipped with a collimator.
• - Figure 2 shows schematically a semiconductor detection head for gamma-camera, with a plurality of individual detectors.
• FIG. 3 is a partial top view of a detector according to Figure 2 equipped with a collimator with channels of hexagonal section.
• FIG. 5 is a partial schematic view from above of a detection head according to the invention.
• FIG. 6 is a partial schematic view from above of a detection head according to the invention according to an alternative embodiment.
• FIG. 5 is a partial top view of a detection head 140 according to the invention.
• the detection head comprises a plurality of elementary detectors 142 juxtaposed in the form of a matrix array to form a detection plane 144.
• the elementary detectors 142 are CdTe or CdZnTe type semiconductor detectors and have in the detection plane 144 a square surface of 3 to 5 mm on a side.
• each detector in the detection plane 144 has a central part 150 sensitive to gamma radiation and a non-sensitive peripheral edge 152.
• the central sensitive part also has a square surface of 3 to 5 mm side. This rating is indicated on the figure with the reference L.
• the peripheral edge 152 has a thickness E of the order of 3 mm.
• the detection head further includes preamplifier circuits for collecting the signals from the detectors 142 and directing them to a processing unit.
• the detection head 140 also includes a collimator 120 disposed in front of the detection plane 144.
• the collimator 120 can be disposed directly in contact with the detection plane 144.
• the collimator has a plurality of channels 121 for the passage of gamma radiation arranged perpendicular to the detection plane. It can be noted that the channels are not necessarily perpendicular to the detection plane but can form, in a particular application, a divergent or convergent beam. Channels 121, juxtaposed, are arranged according to a repetition pattern of individual channels each having a square section according to the detection plane.
• each square section is a submultiple of the side of the square area of the individual detectors.
• the length and the width of the repeating pattern correspond to a third of the length and the width of the elementary detectors in the detection plane.
• 9 channels including their walls, correspond to the surface of each detector.
• each square channel 121 has an opening with a side l ⁇ of of the order of 1.33 mm and is delimited by a wall 123 with a thickness ei of the order of 0.1 mm.
• Figure 6 shows an alternative embodiment of the detector of the invention.
• the side that is to say the width and the length of the repeating pattern are equal to half the side of the surface of each elementary detector in the detection plane.
• FIG. 6 The elements represented in FIG. 6 are, with the exception of their dimensions, similar to those of FIG. 5. They are designated by the same references and one can refer to their subject in the description above.
• each channel has an opening with a side £ 2 of 1.7 mm and is surrounded by a wall 123 with a thickness e 2 of 0.3 mm.
• collimators 120 as shown in FIGS. 5 and 6 can be produced by electro-erosion from a solid block of absorbent material such as a solid block of lead.
• EDM needles having a shape corresponding to that of the channels are advanced in the block to form the channels. This method facilitates the realization of channels with a square section and makes it possible to obtain sharp angles.
• the collimators according to the invention can also manufacture the collimators according to the invention by molding.
• the channels are defined by square section.
• These needles are preferably slightly pyramidal to facilitate demolding of the collimators.

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• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
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• High Energy & Nuclear Physics (AREA)
• Molecular Biology (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Measurement Of Radiation (AREA)
• Nuclear Medicine (AREA)

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• 1996
  • 1996-12-30 FR FR9616199A patent/FR2757954B1/fr not_active Expired - Fee Related
• 1997
  • 1997-12-29 EP EP97953961A patent/EP0950198A1/de not_active Ceased
  • 1997-12-29 WO PCT/FR1997/002438 patent/WO1998029764A1/fr not_active Application Discontinuation
  • 1997-12-29 JP JP52970498A patent/JP2001507453A/ja not_active Ceased
  • 1997-12-29 US US09/331,582 patent/US6365900B1/en not_active Expired - Fee Related

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