JP2009229336A - Method and device for improving sensitivity of positron image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a signal causing false positional information (LOR) and noise is generated due to scattering simultaneous counting and pileup effect in positron image measurement. <P>SOLUTION: The resolution and sensitivity of the obtained positron image are deteriorated. In order improve the sensitivity of the positron image by reducing the signal providing the false positional information and the signal causing the noise, in two annihilation gamma-rays simultaneously measured by two facing gamma-ray detectors, a window is set by using the energy sum and the energy difference of the signals in addition to an energy window by a conventional method. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for improving the sensitivity of a positron image by increasing the signal-to-noise ratio of a positron image obtained by simultaneously measuring two gamma rays centered on 511 keV emitted along with positron annihilation.

  Medical imaging diagnostic equipment using positrons, such as positron emission tomography (PET), administers a drug labeled with a positron emitting nuclide into a living body, and emits gamma rays that are emitted when the positron and the electrons in the body disappear. Detect and determine the biodistribution of the radiopharmaceutical.

  The acquisition of the positron image is based on the simultaneous measurement of two annihilation gamma rays centered on energy 511 keV emitted in the opposite direction of about 180 degrees with the annihilation of the positron-electron pair. When two annihilation gamma rays are simultaneously measured by two opposing detectors, it is recognized as a true coincidence, and it is assumed that positrons disappear on a line of response (LOR) connecting the detectors. Since a large number of detector pairs are arranged around the subject, a large number of LORs are obtained. A positron image is obtained from the position information of the detector defining the LOR by a reconstruction method.

  The conventional method uses two detectors facing each other to measure simultaneously when an annihilation gamma ray pair discriminated by an energy window centered on 511 keV enters a certain time range (time window). .

  However, not only the energy resolution of the detector is finite, but in order to obtain data in a short time and to reduce the exposure dose of the subject, the energy window is expanded and the clock value is earned for measurement. is doing.

  Therefore, the measured coincidence value includes photons other than gamma rays due to annihilation of true positron pairs, such as coincidence coincidence and scattering coincidence. These cause deterioration of sensitivity (signal-to-noise ratio (S / N ratio)) during positron image reconstruction, and greatly affect the quantitative deterioration of measurement.

  So far, the resolution can be improved by a method of geometrically collimating gamma rays (see Patent Document 1) or a method using a semiconductor detector with high resolution but low detection efficiency (see Non-Patent Document 1). Has been tried.

However, in these methods, since the number of detected gamma rays decreases, the statistical fluctuation of data increases and the sensitivity deteriorates. When smoothing processing or the like is performed in order to improve sensitivity, there has been a problem that the resolution is deteriorated.
Japanese Patent No. 3555276 Nuclear Instruments and Methods in Physics Research, A576, 435-440 (2007)

  In the conventional positron image acquisition technology, annihilation gamma ray pairs, which are energy discriminated by an energy window centered on 511 keV using two opposing detectors, are signaled within a certain time range (time window) by a coincidence counting circuit. When it is detected, the coincidence is counted.

  However, since an event caused by the following phenomenon is recognized as a true event even if it is not caused by positron pair annihilation, the LOR defined by two detectors that detect two gamma rays is true. Deviates from the annihilation coordinates. Therefore, the resolution and sensitivity of the obtained positron image are deteriorated.

  In some cases, annihilation gamma rays change the traveling direction by Compton scattering inside the subject and are recognized as coincidence counts. This is called scattering coincidence counting. Scattering coincidence is a cause of noise because it measures gamma rays that are not originally in that position.

  Furthermore, an event caused by the following phenomenon causes noise. Therefore, the resolution and sensitivity of the obtained positron image are deteriorated.

  When a gamma ray detector measures many gamma rays in a short time, a pulse is generated as if a single gamma ray equal to the sum of the energy of these gamma rays was detected. Such a phenomenon is called a pile-up effect. The pile-up effect involving gamma rays whose direction and energy have changed due to scattering produces false LOR information. Furthermore, background radiation such as other radiation and natural radiation generated in the medical field also causes noise.

  In order to improve the sensitivity of the positron image by removing the spurious coincidence value and noise caused by the scattering coincidence and the pile-up effect as described above, it was measured simultaneously by two opposing gamma ray detectors. For two annihilation gamma rays, a window is set using the energy sum and energy difference of signals in addition to the energy window of the conventional method.

  The effects of noise caused by scattering coincidence, pile-up and background radiation from the positron image are reduced, and the sensitivity of the positron image can be improved.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  First, a conventional method will be described with reference to FIG. In the conventional positron image acquisition technology, annihilation gamma ray pairs that are energy-discriminated by energy windows centered at 511 keV using two opposing detectors are within a certain time range (time window) by a coincidence counting circuit. If a signal is detected at, it is regarded as a true clock value. The position of the detector pair at that time is recorded, and a positron image is obtained by reconstructing LOR information obtained from a number of detector pairs.

A method for improving the signal-to-noise ratio (S / N ratio) will be described with reference to FIG. Let X and Y be the energy of gamma rays measured by two detectors facing each other. Assume that the energy resolution ΔE can be represented by a circle indicated by a dotted line in FIG. In the conventional method, a box window with one side ΔE indicated by a broken line in FIG. 2 is set based on the resolution of single measurement, and the coincidence count within the range is counted. However, in this case, the noise in the portion (A in FIG. 2) that protrudes from the resolution is also counted. Therefore, in addition to X and Y, X + Y and XY are obtained, and a box window with a side length of √ (2) ΔE is obtained.
If you set, it will look like the square shown by the solid line. Therefore, the coincidence count of the portion (octagon) surrounded by both the broken box window and the solid box window is taken. In this way, the portion A included in the first window is removed, and only the count inside the almost circular shape (although there is a slight deviation between the octagon and the circle) can be taken out. As a result, it is possible to reduce scattering coincidence and pile-up effects due to scattered gamma rays whose energy has been reduced due to internal scattering or the like. Furthermore, other radiation and natural radiation generated at the medical site can be reduced. Therefore, the S / N ratio can be improved. This principle is common for all detectors that measure.

  Using the measurement system shown in FIG. 3, the energy of two gamma rays simultaneously measured by two opposing gamma ray detectors was measured simultaneously.

  Aluminum single crystal was used as the scatterer. 20 microcurie positron emission source sodium 22 was sandwiched between two aluminum single crystal samples. Since 20 microcurie is much weaker than the source intensity used in normal positron image acquisition, the effect of scattering coincidence and pileup should be small. Therefore, it can be said that the conditions for performance evaluation are stricter than the conditions used in PET and the like.

  The gamma ray detector includes a BGO (bismuth / germanium / oxide) scintillator and a photomultiplier tube. Two annihilation gamma rays are detected by two detectors X and Y, amplified by an amplifier (Amp), and then measured by a coincidence circuit when two gamma rays enter within a time window of 300 ns. The energy spectrum of the detectors X and Y is displayed two-dimensionally by an analog / digital conversion circuit (ADC) built-in type two-dimensional multi-channel analyzer (MCA). FIG. 4 shows an example of a two-dimensional image obtained by using an aluminum single crystal as a scatterer.

  In the two-dimensional spectrum of FIG. 4, it can be seen that the peaks due to the two 511 keV gamma rays are concentric. In the conventional box window indicated by the wavy line, noise enters. In addition to the conventional box window, an energy window close to a perfect circle can be set by setting a box window that is set using the energy sum and energy difference of the gamma ray signal indicated by the solid line. Can be reduced.

A similar experiment was performed for a gamma ray detector comprising a BaF ₂ (barium fluoride) scintillator and a photomultiplier tube. FIG. 5 shows an example of a two-dimensional image obtained using an aluminum single crystal as a scatterer.

  In the two-dimensional spectrum of FIG. 5, it can be seen that the peaks due to the two 511 keV gamma rays are concentric. In the conventional box window indicated by the wavy line, noise enters. In addition to the conventional box window, an energy window close to a perfect circle can be set by setting a box window that is set using the energy sum and energy difference of the gamma ray signal indicated by the solid line. Can be reduced.

  In a conventional PET apparatus, a pair of annihilation gamma rays are simultaneously measured in a certain time window using a plurality of detectors, and the presence of a drug is assumed with equal probability on a line of response (LOR) connecting both detectors. is doing. In contrast, TOF-PET measures the detection time difference between a pair of gamma rays to obtain position information on the straight line of the drug, and is expected to improve image quality. Further, in the DOI-PET apparatus, position information on the depth position of the scintillator where gamma rays are incident can be identified by a three-dimensional position detector. If the depth position information is known, resolution degradation at the periphery can be expected.

  The present invention includes a conventional human PET apparatus, a small animal PET apparatus, a next-generation human DOI-PET apparatus, a small animal DOI-PET apparatus, a human TOF-PET apparatus, and a small animal TOF-PET apparatus. It can be applied to all positron imaging apparatuses, and the S / N ratio can be improved. The present invention can be used in scintillators such as LSO, GSO, BGO, and PWO, positron imaging devices using CdTe semiconductor detectors, Ge semiconductor detectors, and other similar devices.

Conventional positron image acquisition system. A true clock when a pair of annihilation gamma rays discriminated by an energy window centered at 511 keV using two opposing detectors detects a signal within a certain time range (time window) by a coincidence circuit. Recognized as a number. Let X and Y be the energy of gamma rays measured by two detectors facing each other. Assume that the energy resolution ΔE can be represented by a circle indicated by a dotted line. In the conventional method, a box window with one side ΔE indicated by a broken line is set based on the resolution of single measurement, and coincidence counts within that range are counted. However, in this case, the noise of the portion (A) that protrudes from the resolution is also counted. Therefore, in addition to X and Y, X + Y and XY are obtained, and for these, a box window having a side length of √ (2) ΔE If you set, it will look like the square shown by the solid line. Therefore, the coincidence count of the portion (octagon) surrounded by both the broken box window and the solid box window is taken. In this way, the portion A included in the first window is removed, and only the count inside the almost circular shape (although there is a slight deviation between the octagon and the circle) can be taken out. As a result, it is possible to reduce scattering coincidence and pile-up effects due to scattered gamma rays whose energy has been reduced due to internal scattering or the like. Furthermore, other radiation and natural radiation generated at the medical site can be reduced. Therefore, the S / N ratio can be improved.
Measuring system. This system measures two annihilation gamma rays simultaneously with two gamma ray detectors. The gamma ray detector (PMT) is composed of a BGO scintillator (or BaF ₂ scintillator) and a photomultiplier tube. When two annihilation gamma rays are detected by two detectors X and Y, amplified by an amplifier (Amp), and then two gamma rays enter within a time window of 300 ns by an A / D converter built-in coincidence circuit It is measured. The energy spectrum of the detectors X and Y is displayed two-dimensionally by the two-dimensional MCA with a built-in A / D converter. A two-dimensional image obtained using an aluminum single crystal as a scatterer. Corresponding to the gamma ray detectors X and Y, they are two-dimensionally displayed as the X and Y axes. BGO is used as the scintillator. The center coordinates of the box correspond to (511 keV, 511 keV). A two-dimensional image obtained using an aluminum single crystal as a scatterer. Corresponding to the gamma ray detectors X and Y, they are two-dimensionally displayed as the X and Y axes. BaF ₂ is used as the scintillator. The center coordinates of the box correspond to (511 keV, 511 keV).

Claims (4)

  In the positron imaging method, for two signals having a center energy of 511 keV detected by opposing detectors, the energy of the two signals is in a predetermined energy range, and the sum and difference of the energy of the two signals are A method for forming a positron image, wherein a signal having a predetermined energy range is determined as a positron annihilation gamma ray signal.   2. The positron image forming method according to claim 1, wherein the signal is a gamma ray signal emitted when the positron and the electron in the living body disappear.   In a positron imaging apparatus, for two signals with a center energy of 511 keV detected by opposing detectors, a window in which the energy of the two signals is in a predetermined energy range and the sum and difference of the energy of the two signals, respectively. Forming a window having a predetermined energy range, and determining a signal passing through the two windows as a positron annihilation gamma ray signal.   4. The positron image forming apparatus according to claim 3, wherein the signal is a gamma ray signal that is emitted when the positron and the electron in the living body disappear.