JP4737104B2 - Method for calculating position information of photon detector and positron CT apparatus using the same - Google Patents

Description

  The present invention relates to a photon detector position information calculation method for obtaining position information by detecting photons such as X-rays and γ-rays, and a positron CT apparatus using the same.

  X-rays and γ-rays are photons with high energy, and thus are likely to cause scattering, particularly Compton scattering, as an interaction with a substance (see, for example, Non-Patent Document 1). The probability of occurrence of this Compton scattering depends on the substance, but in a general photon detection element substance, it becomes more prominent as the energy of the photon becomes higher at several MeV or less. Therefore, in a medical diagnostic apparatus, a positron tomography apparatus (positron CT apparatus or PET apparatus) (Also referred to as Patent Document 1).

  A nuclear medicine diagnostic apparatus such as a positron CT apparatus has a function of calculating position information of a light source (radiation source) from position information of a detector and generating a medical image from the position information. Therefore, the quality of medical images is improved by obtaining detailed detector position information. As an example, a detector is subdivided to obtain position information in the plane direction, and a multilayered DOI (Depth Of Interaction) detector is used to obtain position information in the depth direction (for example, Non-Patent Document 2).

In addition, when Compton scattering occurs in the detector, output may be obtained from a plurality of detection elements. In order to detect such Compton scattering, a detector capable of obtaining detailed position information (for example, a multi-anode photomultiplier tube) is used (for example, see Non-Patent Document 3). .
Nuclear Encyclopedia “Interaction between γ-rays and substances” http://sta-atm.jst.go.jp:8080/03060306_1.html Japanese Patent Laid-Open No. 7-113873 "DOI measurement device" http://www.nirs.go.jp/usr/medical-imaging/ja/study/jPET_D4_2006/p87_90.pdf `` Analysis of the interaction in the detector '' http://www.nirs.go.jp/usr/medical-imaging/ja/study/jPET_D4_2006/p87_90.pdf

However, the conventional example having such a configuration has the following problems.
That is, the conventional apparatus has a problem that the position information of the detection element deteriorates when Compton scattering in which output is generated from a plurality of detection elements occurs in the detector.

  The present invention has been made in view of such circumstances, and by calculating position information with high accuracy, position information calculation of a photon detector that can improve image quality by reducing deterioration of position information. It is an object of the present invention to provide a method and a positron CT apparatus using the method.

In order to achieve such an object, the present invention has the following configuration.
That is, the invention according to claim 1 is a photon detector position information calculation method configured by combining a plurality of different types of photon detection elements, and is generated in a photon detection element among scattering events having different scattering angles. A process of discriminating a scattered event as a generated scattering event based on an output from a photon detector, and a process of obtaining position information where a photon has interacted by performing a centroid calculation based on an output corresponding to the generated scattering event. , Are provided.

  [Operation / Effect] According to the first aspect of the present invention, when the photon is incident on the photon detector, the type of the photon detection element is different. Therefore, any scattering of different scattering events based on the output of the photon detector. Whether it is an event can be determined. Then, the photon detection element that caused the interaction is specified by the scattering angle of the generated scattering event, and the center of gravity calculation is performed based on the output of the photon detector corresponding to the generated scattering event, so that the influence of scattering can be suppressed. it can. Therefore, by calculating the position information with high accuracy, it is possible to reduce the deterioration of the position information and prevent the image quality from being improved.

  In the present invention, the scattering events having the different scattering angles include the first scattering event in which the scattering angle θ is in the range of θ1 to θ2 (forward scattering) and the second scattering in the range of θ3 to θ4 (backward scattering). And when the photon detection element is stacked in the order of the first photon detection element and the second photon detection element in the direction of photon emission, the generated scattering event is the first scattering event. In some cases, the first photon detection element is specified as the position where the interaction has occurred, and when the generated scattering event is the second scattering event, the second photon detection element is determined as the position where the interaction has occurred. It is preferable to specify (Claim 2). The accuracy of the center of gravity calculation due to scattering is low because the scattering angle θ is in the range of θ1 to θ2 (forward scattering) and the second scattering event is in the range of θ3 to θ4 (backscattering). Therefore, by limiting the scattering events to these and carrying out the method of the present invention, the positional information accuracy can be improved as compared with the prior art. In particular, when the photon detection element is a photon detector in which the first photon detection element and the second photon detection element are stacked in this order from the photon emission direction, the generated scattering event is the first scattering event. The first photon detection element is specified as the position where the interaction has occurred, and if the generated scattering event is the second scattering event, the second photon detection element is specified as the position where the interaction has occurred By doing so, position information accuracy can be improved.

  In the present invention, when determining the scattering event, it is preferable that the output of the photon detector is divided into at least two time windows and specified based on the output ratio in each time window. Scattering events of different scattering angles can be distinguished by setting a time window in which the output of the photon detector is divided by a specific time range, and the output ratio in each time window.

  The invention according to claim 4 is a positron CT apparatus that detects photons from a subject, and is configured by combining a plurality of different types of photon detection elements, and is emitted from a radionuclide administered to the subject. A photon detector for detecting a photon, a scattering event discriminating unit for discriminating a scattering event occurring in a photon detection element as a generated scattering event based on an output from the photon detector among scattering events of different scattering angles, and a photon A position information calculation unit that identifies a photon detection element that has caused an interaction, and performs position calculation based on an output corresponding to the generated scattering event to obtain position information where the photon has caused an interaction. It is characterized by that.

  [Operation / Effect] According to the invention described in claim 4, when the photon is incident on the photon detector, the type of the photon detection element is different. Therefore, the scattering event discriminating unit differs based on the output of the photon detector Determine which scatter event is a scatter event. And since a positional information calculation part performs a gravity center calculation based on the output of the photon detector according to the generated scattering event, it can suppress the influence of scattering. Therefore, by calculating the position information with high accuracy, it is possible to prevent the position information from being reduced and to improve the image quality.

  In the present invention, the scattering events having different scattering angles include a first scattering event in which the scattering angle θ is in the range of θ1 to θ2 (forward scattering) and a second scattering event in the range of θ3 to θ4 (backscattering). If the photon detection element is stacked in the order of the first photon detection element and the second photon detection element from the photon emission direction, the position information calculation unit generates When the scattering event is the first scattering event, the first photon detection element is specified as the position where the interaction has occurred, and when the generated scattering event is the second scattering event, the interaction occurs. Preferably, the second photon detection element is identified as the position (Claim 5), and when the scattering event discriminating unit discriminates the scattering event, the output of the photon detector is divided into at least two time windows. , Based on the output ratio in each time window It is preferable to identify can (claim 6).

  According to the photon detector position information calculation method according to the present invention, when the photon is incident on the photon detector, the type of the photon detection element is different. Whether it is an event can be determined. Since the center of gravity is calculated based on the output of the photon detector corresponding to the generated scattering event, the influence of scattering can be suppressed. Therefore, by calculating the position information with high accuracy, it is possible to prevent the position information from being reduced and improve the image quality.

An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram illustrating a schematic configuration of a positron CT apparatus according to an embodiment, and FIG. 2 is a longitudinal sectional view illustrating a schematic configuration of a photon detector and a schematic diagram illustrating an example of a scattering event.

  The positron CT apparatus includes a gantry unit 1, a data collection system 3, and a data processing unit 5. The gantry unit 1 includes a bed 7 and a gantry 9 on which a subject M to which a positron emitting nuclide is administered is placed. The bed 7 includes a base 11 that can be raised and lowered, and a top plate 13 that is movable in the horizontal direction (Z direction) above the base 11.

  The gantry 9 has an opening 15 in the center that can accommodate the subject M together with the top plate 13. A plurality of photon detectors 17 (described later in detail) for detecting positrons (photons) emitted from the subject M are embedded around the opening 15. Outputs from the plurality of photon detectors 17 are provided to the data collection system 3.

  The data collection system 3 includes a data collection unit 19, a scattering event determination unit 21, and a position information calculation unit 23. The data collection unit 19 includes an amplifier, an ADC (analog / digital converter), a TDC (time / digital converter), a delay circuit, a double event determination unit, and the like, and relates to photons incident on the pair of photon detectors 17 at the same time. Collect and digitize the signal. As will be described later, the scattering event determination unit 21 determines a scattering event that has occurred inside the photon detector 17 as a generated scattering event. The position information calculation unit 23 calculates the center of gravity based on the output corresponding to the generated scattering event, and obtains position information where the photon has interacted.

  The data processing unit 5 includes a reconstruction unit 25, a control unit 27, a display device 29, an instruction unit 31, and a drive unit 33. The reconstruction unit 25 reconstructs the RI distribution image of the subject M based on the position information from the position information calculation unit 23, the collection signal from the data collection unit 19, and transmission data separately collected. The control unit 27 advances or retracts the top panel 13 via the drive unit 33 based on processing for displaying the RI distribution image on the display device 29 based on the data of the reconstruction unit 25 and instructions via the instruction unit 31 such as a mouse. Processing to be driven, collection start instruction to the data collection system 3, and the like are performed.

The photon detector 17 is configured, for example, as shown in FIG.
That is, the photon detector 17 is a so-called two-layer DOI detector configured by combining a plurality of different photon detection elements and further stacking them. Specifically, a first photon detection element 35 and a second photon detection element 37 are provided in order from the direction in which the photons are emitted. A light guide 39 is disposed on the second photon detection element 37 side, and a plurality of photomultiplier tubes 41 are disposed on the ride guide 39. The photomultiplier tube 41 is preferably a so-called monoanode type. Further, each of the first photon detection element 35 and the second photon detection element 37 is constituted by a plurality of blocks. The first photon detection element 35 is made of, for example, LSO (ruthium silicon oxide), and, for example, the second photon detection element 37 is made of GSO (silicate / gatrinium).

  In a photon detection element such as BGO (bismuth germanate) generally used in a positron CT apparatus, an annihilation photon having an energy of 511 [keV] within the element causes a photoelectric effect and Compton scattering. The probability of waking up is comparable. Thus, Table 1 shows the results of calculating the Compton scattering and photoelectric absorption probabilities of 511 [keV] photons when BGO is employed as the photon detection element.

  Also, as the photon energy decreases, the scattering rate decreases and the photoelectric absorption rate increases dramatically. Therefore, in the following explanation, after one occurrence of Compton scattering, the phenomenon that caused the photoelectric effect is described. An example will be described.

  FIG. 2 illustrates a scattering event that occurs in the photon detector 17. Here, a case where the scattering angle θ is in the range of 30 ° <θ <60 ° is a first scattering event SE1, and a case where the scattering angle θ is in the range of 120 ° <θ <150 ° is a second scattering event SE2. When the scattering angle θ is outside the above range, the degree of influence on the accuracy of the position information is low, and is not considered in this example.

  First, a method for discriminating which scattering event occurs in the photon detector 17 will be described with reference to FIG. FIG. 3 is a graph showing the output for each scattering event.

  The two output waveforms shown in FIG. 3 are output waveforms when the energy of 511 [keV] is all applied to the first photon detection element 35 and the second photon detection element 37. Depends on the material. Therefore, the output ratios measured by dividing the output waveforms of the first photon detection element 35 and the second photon detection element 37 into a plurality of time windows TW, for example, two time windows TW = T1 and T2, respectively. It is defined as 2.

  However, these output ratios are values when the photon detection element is NaI and the output is 100. Therefore, when the photon detection element 35 is LSO and the photon detection element 37 is GSO, the energy E (35) of the first photon detection element 35 = 40 to 76, and the second photon detection element 37 Energy E (37) = 20 is given. However, in the following description, as described in Table 2, the energy E (35) = 70 of the first photon detection element 35 and the energy E (37) = 20 of the second photon detection element 37 are set as follows. Used.

  When the energy of the pair annihilation photon is Eγ = 511 [keV], the energy Eγ ′ of the Compton scattered photon and the energy Ee ′ imparted by Compton scattering are as follows.

Eγ ′ = 5111 / (2-cos θ) [keV]
Ee ′ = 511−Eγ ′ [keV]

  In this case, the energy output E (T, SE) obtained in the two types of scattering events SE1, SE2 is given as shown in Table 3 based on the outputs in the respective time windows TW.

The ratio P of the two energy outputs obtained here is
P = E (T1, SE) / E (T2, SE)
Which depends on the scattering angle θ. In addition, said code | symbol SE represents 1st scattering event SE1 or 2nd scattering event SE2.

  FIG. 4 shows the distribution of the energy output ratio P divided into the first scattering event SE1 and the second scattering event SE2, respectively. The energy output ratio P is biased to θ = 30 ° and θ = 150 ° because the Compton scattering angle distribution has peaks in the front (θ = 0 °) and backward (θ = 180 °). As can be seen from the distribution of this energy output ratio P, each scattering event SE is a distribution having upper and lower limits, and they are separated. Therefore, the scattering event SE can be determined by setting an appropriate threshold value. Specifically, if the threshold value is 2.0 to 3.2, the first scattering event SE1 can be determined, and if the threshold value is 3.3 to 3.6, the second scattering event SE2 can be determined. This determination method is executed by the scattering event determination unit 21 described above.

  Since the scattering event SE can be determined from the distribution of the energy output P obtained by the above-described method, a method for calculating the position information IP (x, y, z) will be described next. This calculation method is executed by the position information calculation unit 23.

  A flow from the distribution of the energy output ratio P obtained as described above to obtaining the position information IP (x, y, z) where the photon has interacted will be described with reference to FIG. FIG. 5 is a flowchart relating to calculation of position information.

  In FIG. 5, the arrow from the waveform / scatter event discrimination (21) toward the center of gravity calculation means that the information on the scattering event SE is reflected in the center of gravity calculation as a generated scattering event. An example of the reflection method is shown below. In the following description, a case where the position information (x, y, z) is calculated by performing the centroid calculation based on the outputs of the plurality of photomultiplier tubes 41 provided in the photon detector 17 will be exemplified.

  As shown in FIG. 2, when the generated scattering event is determined as the first scattering event SE1, the position information IP (x, y) is to be corrected to the position of the first photon detection element 35. As shown in the figure, the position information IP (x, y, z) is obtained by performing the centroid calculation using only the energy output E (T1) that has a large contribution from the first photon detection element 35 that falls quickly. On the other hand, when the generated scattering event is determined as the second scattering event SE2, it is desired to correct the position information IP (x, y) to the position of the second photon detection element 37, and as shown in FIG. The position information IP (x, y, z) is obtained by performing the centroid calculation using only the energy output E (T2) to which the contribution of the second photon detection element 37, which is slow down, is large. As described above, by performing the centroid calculation using the energy output P including much contribution of the original position information IP, more accurate position information IP (x, y, z) can be obtained.

  In the embodiment, when the scattering angle θ is 0 ° <θ = 30 ° and 150 ° ≦ θ ≦ 180 °, the correction effect obtained is small, so that the centroid calculation is energy output E (T1) + E (T2). Is going. Further, when the scattering angle θ is 60 ° ≦ θ ≦ 120 °, the influence of scattering on both the first photon detection element 35 and the second photon detection element 37 is the same, so that the centroid calculation is similarly performed. Is performed by energy output E (T1) + E (T2).

  As described above, when the photon is incident on the photon detector 17, since the types of the first photon detection element 35 and the second photon detection element 37 are different, the scattering event determination unit 21 outputs the output of the photon detector 17. To determine which scattering event is a different scattering event. Then, the position information calculation unit 23 specifies the photon detection elements 35 and 37 that have caused the interaction based on the scattering angle of the generated scattering event, and calculates the center of gravity based on the output of the photon detector 17 corresponding to the generated scattering event. As a result, the influence of scattering can be suppressed. Therefore, by calculating the position information IP with high accuracy, it is possible to reduce the deterioration of the position information IP and improve the image quality.

  In addition, since the photomultiplier tube 41 is a monoanode type, the amount of data does not increase and the calculation time does not increase. Furthermore, since the waveform discrimination method used for discrimination of the scattering event is used, an increase in processing time of hardware can be suppressed.

  The present invention is not limited to the above-described embodiment, and can be modified as follows.

  (1) In the above-described embodiment, one photon detector 17 includes a plurality of monoanode type photomultiplier tubes 41. However, the photomultiplier tube may be a multi-anode type. Thereby, more detailed detector position information can be obtained.

  (2) In the embodiment described above, the photon detector 17 is of the two-layer DOI type in which the photon detection elements are stacked in the depth direction, but the present invention is limited to the configuration of the photon detector 17. For example, the configuration shown in FIG. 6 may be adopted. FIG. 6 is a longitudinal sectional view showing another configuration of the photon detector.

  That is, the photon detector 17A has a configuration in which the first photon detection element 35 and the second photon detection element 37 are not stacked in the depth direction but these elements are stacked alternately in the plane direction. In this case, although the accuracy of the scattering angle and the position information IP that can be corrected are different, it is possible to similarly suppress the deterioration of the position information due to the influence of scattering.

  (3) In the above-described embodiment, in the case of a scattering event where the scattering angle θ is in the range of 60 ° <θ <120 °, the center of gravity is calculated using the energy output E, but it is discarded without being used for the center of gravity calculation. Although the sensitivity is lowered, the accuracy of the position information IP can be improved.

  (4) In the embodiment described above, there are two time windows TW, but three or more time windows may be set.

  (5) In the embodiment described above, crystal determination (DOI determination) using waveform discrimination is performed, but a method of performing crystal determination (DOI determination) using a position map may be used.

  (6) In the embodiment described above, the photon detection element uses a scintillator, but a photon detection element such as a semiconductor or ceramic may be used.

  In the above-described embodiment, the description has been made using the graph showing the energy output ratio for each scattering event in FIG. 4, but a supplementary explanation will be given here with reference to FIG. 7. FIG. 7 is a supplementary explanatory diagram of FIG. FIG. 4 described above is drawn by extracting only the scattering event of the correction target angle, but in FIG. 7, the distribution obtained for all the events is drawn. In addition, the relationship shown in Tables 3-5 mentioned above is used for the relationship between the scattering angle θ and energy.

  An explanation will be given starting from the smaller energy ratio. First, a peak due to the scattering angle θ = 0 ° of the scattering event SE1 (an event in which energy is reduced by the second photon detection element 37) appears at 12/8 = 1.5 from the energy ratio. Then, the tail due to the forward scattering event (SE1) having a small scattering angle so as to lead to the peak has an energy ratio value of 4.0 ° = 4.0. The reason for pulling this tail is as follows.

  First, forward scattering has an occurrence probability peak at θ = 0 °. Secondly, the detector block is arranged so that the incident angle of radiation has a distribution centered on 0 °, and the incident light is incident at an incident angle = 0 ° that is incident from the side of the detector. Is unlikely. For these reasons, the radiation that forms the first scattering event SE1 has a large incidence with a shallow incident angle and a shallow scattering angle, and the other minor components form the tail.

  Next, a tail is pulled from 3.2 where the distribution by the second scattering event SE2, which is backscattering with a large scattering angle, is 180 ° scattering to 4.0 which is 90 ° scattering. This is because backscattering has a peak of occurrence probability at θ = 180 °. Then, a peak due to an event (SE3) in which all energy is reduced by the first photon detection element 35 is generated at 60/10 = 6.0.

  In the above-described embodiment, the scattering angle θ is corrected up to 60 °. In this case, about half of the second scattering event SE2 with the scattering angle θ = 180 ° is determined as the first scattering event SE1 with the scattering angle θ = 60 °, and this is calculated particularly in the present invention. There is no problem. That is, the position information IP where the interaction has occurred is calculated using the second scattering event SE2 with the scattering angle θ = 180 ° as the first scattering event SE1, and the position where the interaction has occurred as the second scattering event SE2. This is because, when the information IP is calculated, although the position information IP is different, the accuracy of the position information IP is improved and the LOR (Line Of Response) is not changed.

It is a block diagram which shows schematic structure of the positron CT apparatus which concerns on an Example. It is the longitudinal cross-sectional view which shows schematic structure of a photon detector, and the schematic diagram which shows the example of a scattering event. It is a graph which shows the output for every scattering event. It is a graph which shows the output ratio for every scattering event. It is a flowchart concerning calculation of position information. It is a longitudinal cross-sectional view which shows the other structure of a photon detector. FIG. 5 is a supplementary explanatory diagram of FIG. 4.

Explanation of symbols

M ... Subject 1 ... Gantry part 3 ... Data collection system 5 ... Data processing part 17 ... Photon detector 19 ... Data collection part 21 ... Scattering event determination part 23 ... Position information calculation part 25 ... Reconstruction part 35 ... First Photon detection element 37 ... Second photon detection element 39 ... Light guide 41 ... Photomultiplier tube SE1 ... First scattering event SE2 ... Second scattering event

Claims (6)

  In the photon detector position information calculation method configured by combining multiple photon detection elements of different types, among the scattering events with different scattering angles, the scattering event that occurred in the photon detection element is used as the output from the photon detector. A photon characterized in that it comprises: a process for discriminating as a generated scattering event on the basis of it; and a process for obtaining position information where the photon has interacted by performing a centroid calculation based on an output corresponding to the generated scattering event. Detector position information calculation method.   2. The method of calculating positional information of a photon detector according to claim 1, wherein the scattering events having different scattering angles are the first scattering event having a scattering angle θ in the range of θ1 to θ2 (forward scattering), and θ3 to θ4. A second scattering event in the range of (backscattering), and the photon detection element is stacked in the order of the first photon detection element and the second photon detection element in the photon emission direction. If the generated scatter event is the first scatter event, the first photon detection element is identified as the position where the interaction occurred, and if the generated scatter event is the second scatter event, A method for calculating position information of a photon detector, wherein the second photon detection element is specified as a position where an action has occurred.   3. The method of calculating position information of a photon detector according to claim 1 or 2, wherein when determining a scattering event, the output of the photon detector is divided into at least two time windows, and based on an output ratio in each time window. A method for calculating position information of a photon detector, characterized by specifying.   In a positron CT apparatus that detects photons from a subject, the scattering is different from a photon detector that is configured by combining a plurality of different types of photon detection elements and detects photons emitted from radionuclides administered to the subject. Among the scattering events in the corner, the scattering event discriminating unit that discriminates the scattering event generated in the photon detection element as the generated scattering event based on the output from the photon detector, and the photon detection element in which the photon has interacted are specified. And a position information calculation unit that obtains position information on which photons interact with each other based on an output corresponding to the generated scattering event, and a positron CT apparatus.   5. The positron CT apparatus according to claim 4, wherein the scattering events having different scattering angles include a first scattering event having a scattering angle θ in a range of θ1 to θ2 (forward scattering), and θ3 to θ4 (backscattering). If the second scattering event is in the range and the photon detection element is stacked in the order of the first photon detection element and the second photon detection element from the photon emission direction, the position information When the generated scattering event is the first scattering event, the calculation unit specifies the first photon detection element as the position where the interaction has occurred, and when the generated scattering event is the second scattering event, A positron CT apparatus, wherein the second photon detection element is specified as the position where the interaction occurs.   The positron CT apparatus according to claim 4 or 5, wherein when the scattering event discriminating unit discriminates the scattering event, the output of the photon detector is divided into at least two time windows, and the output ratio in each time window is obtained. A positron CT apparatus characterized by being specified based on.

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