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
  • G01T1/1644—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using an array of optically separate scintillation elements permitting direct location of scintillations
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
  • G01T1/2914—Measurement of spatial distribution of radiation
  • G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)
• • 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
  • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
  • A61B6/03—Computed tomography [CT]
  • A61B6/037—Emission tomography

Definitions

• the present invention relates to the art of nuclear medicine and diagnostic imaging. It finds particular application in reducing errant scintillations produced by back scattered rays from reaching an associated scintillation crystal during positron emission tomography (“PET”)- It is to be appreciated that the present invention may also be used in conjunction with single photon emission computed tomography (“SPECT”), whole body nuclear scans, transmission imaging, other diagnostic modes and/or other like applications.
• PET positron emission tomography
• SPECT single photon emission computed tomography
• scintillation crystals are widely used for radiation detection in many industries.
• large area crystals e.g., crystal plates about 20 mm thick
• thallium-doped sodium iodide Na(Tl)
• the crystals are formed on glass plates about 20 mm thick.
• the plates are typically a blend of silicon, sodium, boron, calcium, and aluminum oxides selected for their transparency and index of refraction.
• the other face of the crystal plate was commonly encased with an aluminum sheet to form a hermetic seal.
• the glass plate functions as a light guide between the scintillation crystal and photomultiplier tubes or other photo detectors.
• the light guide optically spreads the scintillation light pulse to an optimal size.
• the glass material is chosen by considering its physical properties. The index of refraction is selected to conform with the scintillation crystal. The optical absorption or transparency at the wavelength of the emitted scintillations is minimized. Other properties of the glass are also of interest, such as the strength and thermal expansion properties along with the cost of manufacturing.
• the present inventor has found that at 511 keV, the about 20 mm sodium iodide crystal only absorbs and converts about 12% of the radiation into light. The remaining radiation passes through the crystal into the glass plate with a portion being Compton scattered in the crystal. Much of the radiation that reaches the glass plate is Compton scattered in the plate. Radiation that is back scattered, i.e. scattered back into the scintillation crystal, typically gives up a little over half of its energy to the scattering interaction, depending on the scattering angle. The radiation which is back scattered into the crystal along various angles has an energy of about 170 keV to about 300 keV. In this energy range, particularly at oblique angles, the back scattered radiation is substantially completely converted into light. The scintillations from the back scattered radiation are noise or errors which degrade the resultant image.
• the present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others .
• a scintillation camera includes a scintillation crystal mounted on a first side of a light guide.
• Light detectors are mounted on a second side of the light guide.
• the light guide includes a chemical element having an atomic number of greater than or equal to 40.
• a method of detecting gamma radiation includes receiving gamma rays with a scintillation crystal. A portion of the gamma rays are photoelectrically absorbed in the scintillation crystal and emit light. A portion of the gamma rays are passed through the scintillation crystal. The emitted light is passed through a light guide, which includes a chemical element having an atomic number of greater than or equal to 40, to photo detectors. Photoelectric absorption of the portion of the gamma rays that pass through the scintillation crystal in the light guide is maximized. Compton scattering of the gamma rays in the light guide is minimized.
• a primary advantage of the present invention is that it reduces or eliminates stray radiation events from back scattered radiation.
• Another advantage of the present invention is that the count rates are increased.
• Another advantage of the present invention is that if more than one energy window is used, scattered gamma rays contaminating the lower energy window from the higher energy window are reduced. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.
• FIGURE 1 illustrates a diagrammatic illustration of a nuclear camera system according to the present invention
• FIGURE 2 is a diagrammatic illustration of gamma ray interactions within the scintillation crystal and the light guide of FIGURE 1;
• FIGURE 3 illustrates a histogram of an energy spectrum of Nal for detected coincidence events achieved in a computer simulation using standard low-Z glass;
• FIGURE 4 illustrates a histogram of an energy spectrum of Nal for detected coincidence events achieved in a computer simulation using high-Z glasses.
• a nuclear camera system 10 includes a plurality of detectors heads (“detectors”) 12 mounted for movement around a subject 14 in an examination region 16.
• Each of the detectors 12 includes a light guide 18 and a scintillation crystal 20 that converts a radiation event into a flash of light energy or scintillation.
• An array of sensors 22, e.g. 59 sensors, is arranged to receive the light flashes from the scintillation crystal 20.
• the sensors include photomultiplier tubes. However, other sensors are also contemplated.
• Each of the sensors 22 generates a respective analog sensor output pulse (e.g., tube output pulse) in response to the received light flash. Furthermore, each of the sensors 22 is electrically connected to analog-to-digital converters 24. The analog-to-digital converters 24 convert the analog sensor output pulses to a series of digital sensor output values. A processor 26 determines coordinates in two dimensions of the location and the energy of the scintillation event that occurred in the crystal.
• a coincidence detector 28 determines when scintillations are detected concurrently in both scintillation crystals.
• the spatial coordinates of the events which define a connecting ray, are communicated to a reconstruction processor 30.
• An angular orientation of the detector heads 12 around the subject is determined by an angular position encoder 32.
• the reconstruction processor 30 reconstructs a volumetric image representation from the rays passed by the coincidence detector.
• the volumetric image representation is stored in a volume image memory 34.
• a video processor 36 converts operator selected portions of the volumetric image presentation into an appropriate format for display on a monitor 38, such as a video monitor, CCD monitor, active matrix monitor, high resolution printer, or the like.
• Typical human readable displays include slice images, volume or surface renderings, projection images, and the like.
• the light guide 18 includes a glass material having a chemical element with an atomic number ⁇ ⁇ 40.
• the light guide 18 includes oxides of barium, lanthanum, and/or lead. The light guide optically spreads light pulses from the scintillation crystal 20 to a predetermined size.
• the guide 18 includes a transparent plastic material in which atoms with an atomic number ⁇ ⁇ 40 are bonded into the polymer chain.
• the chemical composition of a glass light guide having an atomic number ⁇ ⁇ 40 causes the detection of scattered high energy photons to be minimized.
• the use of high-Z glass is beneficial when considering the properties of sodium iodide crystal and commercially available glasses.
• the two (2) dominant processes through which photons in the energy range of nuclear medicine e.g., between ⁇ 70 keV and ⁇ 600 keV
• photoelectric absorption and Compton scattering are photoelectric absorption and Compton scattering.
• a gamma ray 60 strikes the nucleus 62 in the crystal and is completely absorbed such that its energy transferred to one or more electrons (s) , with the occasional emission of an x-ray.
• the electrons change quantum levels emitting light 64 of a characteristic wavelength.
• the light guide passes the light, with an intensity vs. spatial location represented by curve 66.
• a gamma ray 70 strikes a nucleus 72 in the crystal or the light guide and its energy is partially transferred, resulting in an energetic electron and a scattered gamma ray 74 of lower energy, which may itself be photoelectrically absorbed or Compton scattered.
• the scattered gamma ray still has sufficient energy, e.g. about 200 keV to undergo photo electric absorption when it strikes a nucleus 76 in crystal causing the emission of light 78.
• a resultant intensity vs. spatial location curve 80 is spatially shifted relative to the true entry point of the gamma ray 70 into the crystal, causing inaccuracies in the resultant image.
• the desired scenario for nuclear imaging is complete absorption of gamma rays within the scintillation crystal itself. Such complete absorption triggers a primary scintillation event.
• a majority of the gamma rays are absorbed because the Nal (TI) crystal has a very high stopping power at these energies.
• the energy of the scattered gamma rays is lower, typically between ⁇ 170 keV and ⁇ 300 keV. Even at this reduced energy, there is a high probability that the scattered gamma rays will reach the scintillation crystal and trigger electronics for contributing to the dead time, and limiting the count rate or generating false data. There are other ways for undesired scattered photons to reach the scintillation crystal. X-rays or bremsstrahlung x-rays also may be produced in the components behind the crystal, and these may also be directed towards and detected in the crystal.
• Typical glasses contain mostly silicon dioxide (Si0 2 ) and other oxides of low-Z elements (e.g., aluminum, sodium, boron, and calcium). Glass of this type is used in gamma cameras. Heavier glasses exist, such as, for example, flint glass (containing lead oxide (PbO) ) , some crown glasses (containing barium oxide (BaO) , and rare earth glasses (containing lanthanum oxide La 2 0 3 ) . Usually, the optical properties of heavier glasses are not as desirable for gamma cameras relative to the lighter glasses.
• the potential benefit is that the maximum count rate is increased, thereby improving performance for coincidence imaging and other studies having very high event rates (e.g, first-pass cardiac imaging) .
• higher-Z glass light guides reduce the amount of contamination from the scatter behind the scintillation crystal and improve the quality of the image. For example, if dual isotope imaging is performed with 18 F-FDG (511 keV) and 99m Tc-mibi (140 keV) , scatter from the 511 keV gamma rays in the 140 keV energy window is substantial. It is to be understood that using the higher-Z glass is also beneficial for such dual isotope imaging.
• FIGURE 3 illustrates a histogram of an energy spectrum of Nal for detected coincidence events achieved in a computer simulation using standard low-Z glass.
• a typical gamma camera detector was modeled as a 10 mm slab of Nal crystal, a 15 mm slab of glass as the light guide, a 10 mm slab of copper to simulate the other elements in the detector (magnetic shielding, photomultiplier tube electrodes, electronics, mounting plates, etc.), and a 12.5 mm slab of lead to simulate the detector bucket.
• the simulation investigated the case of a dual-head camera system in PET mode, which detects coincident pairs of gamma rays at 511 keV emitted from a scattering medium.
• FIGURE 4 illustrates histograms of an energy spectrum of Nal for detected coincidence events achieved in a computer simulation using standard glass (indicated as the graph with a backscatter peak 110) along with typical compositions of barium crown glass (indicated as the graph with a backscatter peak 112) and light flint glass (indicated as the graph with a backscatter peak 114) .
• the barium crown glass and light flint glass both contain lead and high-Z glasses.
• the intensity of the backscatter peaks 112, 114 are significantly less for the high-Z barium crown glass and light flint glass, respectively, relative to the peak 110 for the standard, low-Z glass.
• a reduced backscatter peak indicates fewer scattered events are detected, minimizing the contribution of the undesired events to the dead time of the camera.
• the number of scattered events at -140 keV is much lower, which is helpful for 18 F/" ra Tc dual isotope imaging.
• the photomultiplier tubes be manufactured with high-Z glass instead of, or in addition to, the light guide including the high-Z glass.

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• Engineering & Computer Science (AREA)
• Measurement Of Radiation (AREA)
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• 2001
  • 2001-09-21 IL IL14974501A patent/IL149745A0/xx unknown
  • 2001-09-21 JP JP2002529256A patent/JP2004510139A/ja active Pending
  • 2001-09-21 WO PCT/US2001/029699 patent/WO2002025310A2/en not_active Ceased
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