JP6437345B2 - Photon counting X-ray detector and photon counting X-ray CT apparatus - Google Patents

Description

  Embodiments described herein relate generally to a photon counting X-ray detector and a photon counting X-ray CT apparatus.

  The photon counting X-ray detector can count X-ray photons incident on the detection element. However, when the number of X-ray photons incident within a predetermined period is large, there is a high probability that waveforms generated by individual X-ray photons overlap due to the response time of the detection element. For this reason, the photon counting X-ray detector may count a plurality of X-ray photons as one X-ray photon. This phenomenon is called pulse pileup. When pulse pileup occurs, the photon counting X-ray detector may not be able to accurately count the X-ray photons incident on the detection element.

  Until now, in order to accurately count X-ray photons even when the number of incident X-ray photons is large, measures such as increasing the response speed of the detection element and reducing the number of X-ray photons incident on the detection element have been taken. Has been taken. For example, in a direct conversion photon counting X-ray detector using cadmium telluride (CdTe) or the like, the applied voltage is increased to increase the response speed of the detection element. Alternatively, an indirect conversion type photon counting X-ray detector using a scintillator uses a scintillator with a short fluorescence decay time to increase the response speed of the detection element. In addition, the number of X-ray photons incident on the detection element can be reduced by measures such as increasing the number of detection elements per unit area in the photon counting X-ray detector and reducing the aperture of the detection element. There is also a method of correcting the counting rate for each energy band by regarding a waveform overlapped by pulse pileup as a waveform generated by the incidence of a plurality of low energy X-ray photons. Further, there is a method of providing an integral type DAS (Data Acquisition System) and a photon count type DAS, and using an output of a photon count type when the X-ray photon rate is low and using an integral type output when the X-ray photon rate is high. is there. Note that the above-described waveform means an event in which a state where a voltage or current exceeds a predetermined threshold value continues when an X-ray photon enters the detection element.

JP 2009-78143 A JP 2000-23965 A JP 2000-131440 A JP 2013-227 A

  A problem to be solved by the present invention is a photon counting X-ray detector and a photon counting type capable of appropriately estimating the number of X-ray photons even when the number of incident X-ray photons is large with a simple configuration. An X-ray CT apparatus is provided.

The photon counting X-ray detector according to the embodiment includes a detection element, an observation unit, and an estimation unit. The detection element outputs a response waveform based on the incident X-ray photons. Observation unit observes the number of events that the state in which the response waveform is below a predetermined threshold value continues, or the state in which the response waveform exceeds a predetermined threshold value observing the number of events to continue. Estimating unit, based on the number of observation section has observed, to calculate the incidence within a predetermined period of event observation section has observed, incident within a period of Jo Tokoro to detecting element based on the calculated occurrence rate Estimate the number of X-ray photons.

FIG. 1 is a diagram illustrating a configuration example of a photon counting X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram illustrating an example of a photon counting X-ray detector according to the first embodiment. FIG. 3 is a diagram illustrating an example of a detection circuit provided in a conventional photon counting X-ray CT apparatus. FIG. 4 is a diagram illustrating an example of a response waveform output by the detection element and processed by the waveform shaping unit. FIG. 5 is a diagram illustrating the relationship between the incidence rate and the count rate. FIG. 6 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the first embodiment. FIG. 7 is a diagram illustrating an example of an observation unit included in the detection circuit illustrated in FIG. FIG. 8 is a diagram showing the relationship between the response time of the detection element and the occurrence rate of an event that continues to be below a predetermined threshold in the response waveform. FIG. 9 is a diagram showing the relationship between the incidence rate and the occurrence rate of events that continue to be in a state where the response waveform is below a predetermined threshold. FIG. 10 is a diagram showing the relationship between the threshold for the response waveform and the number of events that continue to be in a state where the response waveform is below the predetermined threshold. FIG. 11 is a diagram illustrating a relationship between the estimation result by the estimation unit or the count result by the counting unit and the reliability. FIG. 12 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the second embodiment. FIG. 13 shows a method of observing the time during which an event in which a state below a predetermined threshold continues in the response waveform continues or the time at which an event in which a state exceeding the predetermined threshold continues in the response waveform continues. It is a figure which shows an example. FIG. 14 shows a threshold for the response waveform and a time for which an event in which the state below the predetermined threshold continues in the response waveform continues or a time for an event in which the state above the predetermined threshold continues in the response waveform It is a figure which shows the relationship. FIG. 15 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the third embodiment.

  Hereinafter, a photon counting X-ray detector and a photon counting X-ray CT apparatus according to embodiments will be described with reference to the drawings. Note that in the following embodiments, description of overlapping contents will be omitted as appropriate.

(First embodiment)
First, the configuration of the photon counting X-ray CT apparatus 1 according to the first embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram illustrating a configuration example of a photon counting X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram illustrating an example of a photon counting X-ray detector according to the first embodiment. As shown in FIG. 1, the photon counting X-ray CT apparatus 1 includes a gantry device 2, a couch device 20, and an image processing device 8. The configuration of the photon counting X-ray CT apparatus 1 is not limited to the following configuration.

  The gantry device 2 irradiates the subject P with X-rays and collects projection data described later. As shown in FIG. 1, the gantry device 2 includes a gantry control unit 3, an X-ray generation device 4, a photon counting X-ray detector 5, a data collection unit 6, and a rotating frame 7.

  The gantry control unit 3 controls the operations of the X-ray generator 4 and the rotating frame 7 under the control of a scan control unit 83 described later. The gantry control unit 3 includes a high voltage generation unit 31, a collimator adjustment unit 32, and a gantry driving unit 33. The high voltage generator 31 supplies a tube voltage to an X-ray tube 41 described later. The collimator adjustment unit 32 adjusts the X-ray irradiation range that the X-ray generator 4 irradiates the subject P by adjusting the aperture and position of the collimator 43. For example, the collimator adjustment unit 32 adjusts the X-ray irradiation range, that is, the X-ray fan angle and cone angle, by adjusting the aperture of the collimator 43. The gantry driving unit 33 rotates the rotary frame 7 to rotate the X-ray generator 4 and the photon counting X-ray detector 5 on a circular orbit around the subject P.

  The X-ray generator 4 generates X-rays that irradiate the subject P. The X-ray generator 4 includes an X-ray tube 41, a wedge 42, and a collimator 43. The X-ray tube 41 generates beam-shaped X-rays that irradiate the subject P with the tube voltage supplied by the high voltage generator 31. The X-ray tube 41 is a vacuum tube that generates beam-like X-rays having a spread along the body axis direction of the subject P. This beam-shaped X-ray is also called a cone beam. The X-ray tube 41 irradiates the subject P with a cone beam as the rotating frame 7 rotates. The wedge 42 is an X-ray filter for adjusting the amount of X-rays irradiated to the subject P. The collimator 43 is a slit for narrowing the X-ray irradiation range in which the amount of X-rays is adjusted by the wedge 42 under the control of the collimator adjustment unit 32.

  As shown in FIG. 2, the photon counting X-ray detector 5 is a multi-row detector in which a plurality of detection elements 51 are regularly arranged in a first direction and a second direction intersecting the first direction. For example, in FIG. 2, the first direction is the channel direction, and the second direction is the slice direction. Here, the channel direction is the circumferential direction of the rotating frame 7, and the slice direction is the body axis direction of the subject P. The detection element 51 outputs a response waveform based on the incident X-ray photons. Specifically, the detection element 51 detects an incident X-ray photon and outputs a response waveform. The response waveform is, for example, time-series data of AD values of voltages or currents output from a detection element 51 described later. Moreover, in the following description, detection means from the time an X-ray photon enters the detection element 51 until the detection element 51 outputs a response waveform.

  The detection element 51 includes, for example, a scintillator and a photodiode. The detection element 51 is connected to a detection circuit described later. The detection element 51 converts each incident X-ray photon into light by a scintillator, and converts this light into electric charge by a photodiode. This electric charge is sent to a detection circuit described later. When a scintillator is provided as the detection element 51, the photon counting X-ray detector 5 is called an indirect conversion type detector.

  The photon counting X-ray detector 5 may be a direct conversion type detector. The direct conversion type detector includes a semiconductor element such as cadmium telluride (CdTe) as the detection element 51. The direct conversion type detector directly converts X-ray photons incident on the detection element 51 into electric charges by a semiconductor element. The charge output from the detection element 51 is such that electrons generated by the incidence of the X-ray photon travel toward the collector electrode having a positive potential and holes generated by the incidence of the X-ray photon have a negative potential. Is output at least one of traveling toward the vehicle. This electric charge is sent to a detection circuit described later.

  A detection circuit described later is connected to the detection element 51. The detection circuit includes a calculation unit, a counting unit, an observation unit, and an estimation unit. The calculation unit calculates the energy of the X-ray photon that has generated an event in which a state in which the response waveform exceeds a predetermined threshold continues. The counting unit counts the number of X-ray photons (count value) that has generated an event in which the state of exceeding a predetermined threshold in the response waveform continues. Alternatively, the counting unit may transmit a predetermined period to the detection element for each one or a plurality of energy bands set on the energy distribution of the X-rays irradiated by the X-ray tube 41 based on the result calculated by the calculation unit. The number (count value) of X-ray photons incident on the inside is counted. Here, for example, a count rate indicating the number per unit time is used as the count value. The observation unit observes an event in which a state where the response waveform is below a predetermined threshold continues. Alternatively, the observation unit observes an event in which a state exceeding a predetermined threshold in the response waveform continues. The estimation unit calculates an occurrence rate of the event observed by the observation unit within a predetermined period, and estimates the number (estimated value) of X-ray photons incident on the detection element within the predetermined period based on the occurrence rate. Here, as the estimated value, for example, an incidence rate indicating the number per unit time is used. The predetermined period is, for example, a period during which each projection data for generating a CT image is collected. Details of the detection circuit and projection data will be described later. In addition, there are two types of events: an event in which a state below a predetermined threshold in the response waveform continues and an event in which a state above a predetermined threshold in the response waveform continues.

  As illustrated in FIG. 1, the data collection unit 6 includes a count data collection unit 61 and an estimated data collection unit 62. The counting data collection unit 61 collects data output from the counting unit and generates projection data based on the data. The estimation data collection unit 62 collects data output from the estimation unit, and generates projection data based on this data. The projection data generated by the count data collection unit 61 and the projection data generated by the estimation data collection unit 62 are sent to a preprocessing unit 84 described later.

  The rotating frame 7 is an annular frame that supports the X-ray generator 4 and the photon counting X-ray detector 5 so as to face each other with the subject P interposed therebetween. The rotating frame 7 is driven by the gantry driving unit 33 and rotates on a circular orbit around the subject P at a high speed.

  The couch device 20 includes a couch driving device 21 and a top plate 22. The couch driving device 21 moves the subject P within the rotating frame 7 by moving the top plate 22 on which the subject P is placed in the body axis direction under the control of the scan control unit 83 described later. . Note that the gantry device 2 performs a helical scan that scans the subject P in a spiral shape by rotating the rotating frame 7 while moving the top plate 22, for example. Alternatively, the gantry device 2 performs a conventional scan in which the subject P is scanned by rotating the rotating frame 7 while the position of the subject P is fixed after the top plate 22 is moved. Alternatively, the gantry device 2 executes a step-and-shoot method in which a conventional scan is performed in a plurality of scan areas by moving the position of the top plate 22 at regular intervals.

  The image processing apparatus 8 receives an operation of the photon counting X-ray CT apparatus 1 by the user. Further, the image processing device 8 performs processing such as reconstruction on the projection data collected by the gantry device 2. The image processing apparatus 8 includes an input unit 81, a display unit 82, a scan control unit 83, a preprocessing unit 84, a data storage unit 85, an image generation unit 86, an image storage unit 87, and a control unit 88. Is provided.

  The input unit 81 is a mouse, a keyboard, or the like used by the user of the photon counting X-ray CT apparatus 1 for inputting various instructions and various settings. The input unit 81 transfers the instruction and setting information received from the user to the control unit 88. The display unit 82 is a monitor that is referred to by the user. The display unit 82 displays a GUI (Graphical User Interface) or the like for receiving various settings from the user via the input unit 81 as a result of various image processing.

  The scan control unit 83 controls operations of the gantry control unit 3, the data collection unit 6, and the bed driving device 21 under the control of the control unit 88. Specifically, the scan control unit 83 controls the gantry control unit 3 to rotate the rotating frame 7 and irradiate X-rays from the X-ray tube 41 when performing photon counting CT imaging. The opening degree and position of 43 are adjusted. The scan control unit 83 controls the data collection unit 6 under the control of the control unit 88. In addition, the scan control unit 83 moves the top 22 by controlling the bed driving device 21 when performing photon counting CT imaging under the control of the control unit 88.

  The preprocessing unit 84 performs logarithmic conversion, offset correction, sensitivity on the projection data generated by the data collection unit 6, that is, the projection data generated by the count data collection unit 61 and the projection data generated by the estimation data collection unit 62. Correction processing such as correction, beam hardening correction, and scattered ray correction is performed and stored in the data storage unit 85. Note that the projection data subjected to the correction processing by the preprocessing unit 84 is also referred to as raw data (Raw Data). The data storage unit 85 stores raw data, that is, projection data that has been subjected to correction processing by the preprocessing unit 84.

  The image generation unit 86 reconstructs the projection data stored in the data storage unit 85 and generates a CT image. That is, the image generation unit 86 generates a CT image based on at least one of the projection data generated by the count data collection unit 61 and the projection data generated by the estimation data collection unit 62. As the reconstruction method, there are various methods, for example, back projection processing. In addition, as the back projection processing, for example, an FBP (Filtered Back Projection) method can be cited. Note that the image generation unit 86 may perform reconstruction processing by, for example, a successive approximation method. Further, the image generation unit 86 can also generate a CT image for each substance discriminated by substance discrimination based on the projection data generated by the counting data collection unit 61. The image storage unit 87 stores the CT image generated by the image generation unit 86.

  The control unit 88 controls the photon counting X-ray CT apparatus 1 by controlling the operations of the gantry device 2, the couch device 20, and the image processing device 8. The control unit 88 controls the scan control unit 83 to execute scanning, and collects projection data from the gantry device 2. The control unit 88 controls the preprocessing unit 84 to perform the above-described correction processing on the projection data. The control unit 88 controls the display unit 82 to display the projection data stored in the data storage unit 85 and the image data stored in the image storage unit 87.

  The data storage unit 85 and the image storage unit 87 described above can be realized by a RAM (Random Access Memory), a semiconductor memory element such as a flash memory, a hard disk, an optical disk, or the like. In addition, the scan control unit 83, the preprocessing unit 84, the image generation unit 86, and the control unit 88 described above are an integrated circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA) or a central processing unit (CPU). And an electronic circuit such as an MPU (Micro Processing Unit).

  Next, the detection circuit 520 provided in the conventional photon counting X-ray CT apparatus will be described with reference to FIG. FIG. 3 is a diagram illustrating an example of a detection circuit provided in a conventional photon counting X-ray CT apparatus.

  As illustrated in FIG. 3, the detection circuit 520 includes a waveform shaping unit 5210, an output unit 5220, a removal unit 5230, a calculation unit 5240, and a counting unit 5250. Here, the waveform shaping unit 5210 is, for example, a waveform shaping amplifier. The output unit 5220 is, for example, a comparator.

  First, the configuration of the detection circuit 520 will be described. An input terminal of the waveform shaping unit 5210 is connected to the detection element 51. The output terminal of the waveform shaping unit 5210 is connected to one of the input terminals of the output unit 5220. The removal unit 5230 is connected to one of the input terminals of the output unit 5220. The output terminal of the output unit 5220 is connected to the input terminal of the calculation unit 5240. The output terminal of the calculation unit 5240 is connected to the input terminal of the counting unit 5250. The output terminal of the counting unit 5250 is connected to the counting data collecting unit 61. The counting unit 5250 includes one or more counters that count only X-ray photons having energy belonging to each energy band set on the energy distribution of X-rays irradiated by the X-ray tube 41. When there is one counter, one energy band to be counted is set, and all X-ray photons having energy belonging to the set energy band are counted. One energy band to be set may be a total energy of 0 or more, or may be a specific energy range corresponding to a purpose such as noise removal.

  Next, the operation of the detection circuit 520 will be described. The waveform shaping unit 5210 amplifies the response waveform output from the detection element 51 to a necessary level while suppressing an increase in noise. The removing unit 5230 supplies a predetermined voltage to the output unit 5220 and sets a threshold value for removing noise included in the response waveform output from the detection element 51. The output unit 5220 outputs only the response waveform exceeding the threshold set by the removal unit 5230 among the response waveforms generated by the X-ray photons incident on the detection element 51 to the calculation unit 5240. The calculation unit 5240 calculates the wave height, waveform area, and the like of the response waveform output from the output unit 5220. The counting unit 5250 regards the wave height, waveform area, and the like calculated by the calculating unit 5240 as the energy of the X-ray photon, and X-ray photons for each energy band set on the X-ray energy distribution irradiated by the X-ray tube 41. Count. The counting unit 5250 outputs the counted result to the counting data collecting unit 61.

  Next, the behavior of the detection circuit 520 with respect to the number of X-ray photons incident on the detection element 51 will be described with reference to FIGS. 4 and 5.

  FIG. 4 is a diagram illustrating an example of a response waveform output by the detection element and processed by the waveform shaping unit. In each of FIGS. 4A, 4B, and 4C, the vertical axis represents voltage or current, and the horizontal axis represents time. In FIG. 4, the response waveform output by the output unit 5220 is indicated by a solid line, and the response waveform based on the incident X-ray photon that overlaps with another response waveform is indicated by a dotted line. . The threshold for the response waveform is indicated by a straight line Th. In the following description, the vertical axis of the response waveform is a voltage.

  FIG. 5 is a diagram illustrating the relationship between the incidence rate and the count rate. In FIG. 5, the vertical axis represents the counting rate counted by the counting unit 5250, and the horizontal axis represents the incidence rate. The dotted line D in FIG. 5 represents the relationship between the two when the counting rate counted by the counting unit 5250 is proportional to the incidence rate. A solid line S in FIG. 5 represents the behavior of the counting rate counted by the counting unit 5250 with respect to the incidence rate.

  FIG. 4A shows a response waveform Wa output by the detection element 51 and processed by the waveform shaping unit 5210 when the number of X-ray photons (for example, incidence rate) incident within a predetermined period is small. ing. This corresponds to, for example, the case indicated by the straight line A in FIG. In this case, there is a high probability that the time interval at which the X-ray photons are incident on the detection element 51 is longer than the response time of the detection element 51. For this reason, the probability that a pulse pileup will occur becomes low.

  The response waveform Wa includes five waveforms, that is, a waveform Pa1, a waveform Pa2, a waveform Pa3, a waveform Pa4, and a waveform Pa5. Each of these response waveforms exceeds the threshold for the response waveform indicated by the straight line Th and then falls below this threshold. That is, these waveforms do not overlap each other. Therefore, the counting unit 5250 can count the number of waveforms included in the response waveform Wa, that is, the number of X-ray photons incident on the detection element 51 (for example, incidence rate) almost accurately. This corresponds to the fact that the difference Da between the count rate counted by the counter 5250 and the count rate on the straight line A shown in FIG. 5 is close to zero.

  FIG. 4B shows the response waveform output by the detection element 51 and processed by the waveform shaping unit 5210 when the number of X-ray photons incident within a predetermined period is large (for example, when the incidence rate is large). Wb is shown. This corresponds to, for example, the case indicated by the straight line B in FIG. In this case, there is a high probability that the time interval at which the X-ray photons are incident on the detection element 51 is shorter than the response time of the detection element 51. For this reason, the probability that a pulse pileup will occur becomes high.

  The response waveform Wb includes nine waveforms, that is, a waveform Pb1, a waveform Pb2, a waveform Pb3,..., A waveform Pb8, and a waveform Pb9. The waveform Pb1, the waveform Pb2, the waveform Pb3, the waveform Pb6, and the waveform Pb7 all exceed the threshold for the response waveform indicated by the straight line Th and then fall below the threshold. That is, these response waveforms do not overlap each other. For this reason, the counting unit 5250 can count these waveforms individually.

  However, the waveform Pb4 and the waveform Pb5 overlap each other and form a single waveform P1 that falls below this threshold after exceeding the threshold for the response waveform indicated by the straight line Th. For this reason, the counting unit 5250 counts the waveform Pb4 and the waveform Pb5 as one waveform. In addition, the waveform Pb8 and the waveform Pb9 overlap each other, and form a single waveform P2 that falls below the threshold value after exceeding the threshold value for the response waveform indicated by the straight line Th. For this reason, the counting unit 5250 counts the waveform Pb8 and the waveform Pb9 as one waveform. Therefore, the counting unit 5250 accurately determines the number of X-ray photons (count rate) corresponding to the number of waveforms included in the response waveform Wb, that is, the number of X-ray photons incident on the detection element 51 (for example, the incidence rate). Cannot count. This corresponds to a large difference Db between the counting rate counted by the counting unit 5250 and the counting rate on the assumption that the counting rate is proportional to the incidence rate on the straight line B shown in FIG. The response waveform Wb shown in FIG. 4B includes nine waveforms. However, the counting unit 5250 counts the number of waveforms as seven because of pulse pileup.

  FIG. 4C shows a waveform shaping unit that is output by the detection element 51 when the number of X-ray photons incident within a predetermined period (for example, the incidence rate) is larger than that shown in FIG. The response waveform Wc processed by 5210 is shown. This corresponds to, for example, the case indicated by the straight line C in FIG. In this case, the probability that the time interval at which the X-ray photons are incident on the detection element 51 is shorter than the response time of the detection element 51 is higher than that shown in FIG. For this reason, the probability that a pulse pileup will occur is higher than in the case shown in FIG.

  The response waveform Wc includes 14 waveforms, that is, a waveform Pc1, a waveform Pc2, a waveform Pc3,..., A waveform Pc13, and a waveform Pc14. The waveform Pc1, the waveform Pc2, and the waveform Pc3 overlap each other and form a single waveform P10 that falls below this threshold value after exceeding the threshold value for the response waveform indicated by the straight line Th. For this reason, the counting unit 5250 counts the waveform Pc1, the waveform Pc2, and the waveform Pc3 as one waveform. Similarly, the waveform Pc4, the waveform Pc5, and the waveform Pc6 also overlap each other and form one waveform P20 that falls below the threshold value after exceeding the threshold value for the response waveform indicated by the straight line Th. For this reason, the counting unit 5250 counts the waveform Pc4, the waveform Pc5, and the waveform Pc6 as one waveform. Further, the waveform Pc7, the waveform Pc8,..., The waveform Pc13, and the waveform Pc14 also overlap with adjacent waveforms, and after forming a single waveform P30 that falls below this threshold after exceeding the threshold for the response waveform indicated by the straight line Th. ing. Therefore, the counting unit 5250 counts the waveform Pc7, the waveform Pc8,..., The waveform Pc13, and the waveform Pc14 as one waveform.

  Therefore, the counting unit 5250 cannot accurately count the number of waveforms included in the response waveform Wc, that is, the number of X-ray photons incident on the detection element 51 (for example, the incidence rate). This corresponds to the fact that the difference Dc between the count rate counted by the counting unit 5250 and the count rate on the straight line C shown in FIG. 5 is larger than the above-described difference Db. The response waveform Wc shown in FIG. 4C includes 14 waveforms. However, the counting unit 5250 counts the number of waveforms as three because of pulse pileup.

  The phenomenon described with reference to FIG. 4 and FIG. 5 is established for both the count rate for each energy band by the counting unit 5250 and the value obtained by adding these over all energy bands or a plurality of energy bands.

  From the above description, the counting rate for each energy band by the counting unit 5250 and the value obtained by adding them over all energy bands or a plurality of energy bands are increased as the number of X-ray photons incident on the detection element 51 increases. It turns out that it leaves | separates from the value on the dotted line D. FIG. Therefore, when the number of X-ray photons incident on the detection element 51 is large, the counting unit 5250 may not be able to accurately count the X-ray photons.

  Next, a detection circuit included in the photon counting X-ray CT apparatus 1 according to the first embodiment will be described with reference to FIGS. FIG. 6 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the first embodiment. FIG. 7 is a diagram illustrating an example of an observation unit included in the detection circuit illustrated in FIG. FIG. 8 is a diagram showing the relationship between the response time of the detection element and the occurrence rate of an event that continues to be below a predetermined threshold in the response waveform. FIG. 9 is a diagram showing the relationship between the incidence rate and the occurrence rate of events that continue to be in a state where the response waveform is below a predetermined threshold. FIG. 10 is a diagram showing the relationship between the threshold for the response waveform and the number of events that continue to be in a state where the response waveform is below the predetermined threshold. FIG. 11 is a diagram illustrating a relationship between the estimation result by the estimation unit or the count result by the counting unit and the reliability.

  As shown in FIG. 6, the detection circuit 52a includes a waveform shaping unit 521, an output unit 522, a removal unit 523, a calculation unit 524, a counting unit 525, a processing unit 526a, and an estimation unit 527a. Here, the waveform shaping unit 521 is, for example, a waveform shaping amplifier. The output unit 522 is, for example, a comparator.

  First, the configuration of the detection circuit 52a will be described. An input terminal of the waveform shaping unit 521 is connected to the detection element 51. The output terminal of the waveform shaping unit 521 is connected to one of the input terminals of the output unit 522. The removal unit 523 is connected to one of the input terminals of the output unit 522. The output terminal of the output unit 522 is connected to the input terminal of the calculation unit 524 and the input terminal of the processing unit 526a. The output terminal of the calculation unit 524 is connected to the input terminal of the counting unit 525. The output terminal of the counting unit 525 is connected to the counting data collecting unit 61. The counting unit 525 includes one or more counters that count only X-ray photons having energy belonging to each energy band set on the energy distribution of the X-rays irradiated by the X-ray tube 41. When there is one counter, one energy band to be counted is set, and all X-ray photons having energy belonging to the set energy band are counted. One energy band to be set may be a total energy of 0 or more, or may be a specific energy range corresponding to a purpose such as noise removal. The output terminal of the processing unit 526a is connected to the input terminal of the estimation unit 527a. The output terminal of the estimation unit 527a is connected to the estimation data collection unit 62.

  Here, the processing unit 526a includes, for example, an observation unit 5261a and a setting unit 5262a as illustrated in FIG. In this case, the input terminal of the observation unit 5261a corresponds to the input terminal of the processing unit 526a described above, and the output terminal of the observation unit 5261a corresponds to the output terminal of the processing unit 526a described above.

  Next, the operation of the detection circuit 52a will be described. The waveform shaping unit 521 amplifies the response waveform output from the detection element 51 to a necessary level while suppressing an increase in noise. The removing unit 523 supplies a predetermined voltage to the output unit 522, and sets a threshold value for removing noise included in the response waveform output from the detection element 51. The output unit 522 outputs, to the calculation unit 524 and the processing unit 526a, only the waveform exceeding the threshold set by the removal unit 523 among the waveforms generated by the X-ray photons incident on the detection element 51. The calculation unit 524 calculates the wave height, the waveform area, and the like of the waveform output from the output unit 522. The counting unit 525 considers the wave height, waveform area, and the like calculated by the calculation unit 524 as the energy of the X-ray photon, and the X-ray photon for each energy band set on the energy distribution of the X-ray irradiated by the X-ray tube 41 Count. The counting unit 525 outputs the counted result to the counting data collecting unit 61.

  The observation unit 5261a observes an event in which a state where the response waveform is below a predetermined threshold continues. For example, the observation unit 5261a observes this phenomenon by observing a phenomenon in which the response waveform is below a predetermined threshold and a phenomenon in which the response waveform is above a predetermined threshold. In this case, the observation unit 5261a observes the number of events that continue to be in a state of being below a predetermined threshold in the response waveform, for example, by the following method.

  The observation unit 5261a compares the voltage of the response waveform received from the output unit 522 with the voltage supplied from the setting unit 5262a in order to set a threshold value. The observation unit 5261a outputs a negative voltage to the estimation unit 527a during a period in which the voltage of the response waveform received from the output unit 522 is larger than the voltage supplied from the setting unit 5262a to set the threshold value. The observation unit 5261a outputs a positive voltage to the estimation unit 527a during a period in which the voltage of the response waveform received from the output unit 522 is smaller than the voltage supplied from the setting unit 5262a to set the threshold value.

  Here, the phenomenon in which the voltage output to the estimation unit 527a switches from negative to positive corresponds to a phenomenon in which the response waveform falls below a predetermined threshold. The phenomenon in which the voltage output to the estimation unit 527a switches from positive to negative corresponds to a phenomenon that exceeds a predetermined threshold in the response waveform. Therefore, the observation unit 5261a observes a phenomenon in which the voltage output to the estimation unit 527a is switched from negative to positive and subsequently switched from positive to negative. Therefore, the observation unit 5261a can observe the number of events in which the state where the response waveform is below a predetermined threshold continues by observing the number of the phenomena.

  Alternatively, the observation unit 5261a confirms that one event in which the state below the predetermined threshold in the response waveform continues occurs between two events in which the state above the predetermined threshold continues in the response waveform. By utilizing this, an event in which a state where the response waveform is below a predetermined threshold value continues may be observed. In this case, the observation unit 5261a observes the number of events in which the state of being below a predetermined threshold in the response waveform continues by calculating the number of times of outputting a positive voltage from the number of times of outputting a negative voltage. Can do. For example, the observation unit 5261a outputs a negative voltage five times, outputs a positive voltage four times between these negative voltages, and outputs a positive voltage before the first negative voltage and after the last negative voltage. , The observation unit 5261a can calculate the number “6” of outputting the positive voltage by adding “1” to the number of times “5” of outputting the negative voltage. In addition, the observation unit 5261a simply counts the number of times the positive voltage is output, thereby observing the number of events that continue to be below a predetermined threshold in the response waveform, and transmits the result to the estimation unit 527a. can do.

  In the above description, the case where the observation unit 5261a observes an event in which a state where the response waveform is below a predetermined threshold continues is taken as an example. However, the observation unit 5261a may observe an event in which a state where the response waveform exceeds a predetermined threshold continues. For example, the observation unit 5261a observes this phenomenon by observing a phenomenon in which the response waveform exceeds a predetermined threshold and a phenomenon in which the response waveform falls below a predetermined threshold. In this case, the observation unit 5261a observes the number of events in which a state in which the response waveform exceeds a predetermined threshold continues, for example, by the following method.

  As in the first embodiment, the observation unit 5261a outputs a positive voltage or a negative voltage to the estimation unit 527a. The observation unit 5261a observes a phenomenon in which the voltage output to the estimation unit 527a is switched from positive to negative and subsequently switched from negative to positive. Therefore, the observation unit 5261a can observe the number of events in which the state of exceeding the predetermined threshold in the response waveform continues by observing the number of this phenomenon.

  Alternatively, the observation unit 5261a confirms that one event in which the state exceeding the predetermined threshold in the response waveform continues occurs between two events in which the state below the predetermined threshold in the response waveform continues. By utilizing this, an event in which a state exceeding a predetermined threshold in the response waveform continues may be observed. In this case, the observation unit 5261a observes the number of events in which the state exceeding the predetermined threshold in the response waveform continues by calculating the number of times of outputting the negative voltage from the number of times of outputting the positive voltage. Can do. For example, the observation unit 5261a outputs a positive voltage five times, outputs a negative voltage four times between these positive voltages, and outputs a negative voltage before the first positive voltage and after the last positive voltage. , The observation unit 5261a can calculate the number “6” of outputting the negative voltage by adding “1” to the number of times “5” of outputting the positive voltage. The observation unit 5261a simply counts the number of times the negative voltage is output, thereby observing the number of events in which the response waveform continues to exceed a predetermined threshold, and transmits the result to the estimation unit 527a. can do.

  The estimation unit 527a calculates the occurrence rate of events observed by the observation unit 5261a within a predetermined period. That is, the estimation unit 527a calculates the occurrence rate of observed events within a predetermined period based on the number observed by the observation unit 5261a. Specifically, the estimation unit 527a calculates the number of events that continue to be in a state where the response waveform is below a predetermined threshold in the response waveform in a predetermined period, and the occurrence rate of events in which the state below the predetermined threshold in the response waveform continues. Calculate as Alternatively, the estimation unit 527a calculates the number of events in which the state exceeding the predetermined threshold in the response waveform in the predetermined period continues as the occurrence rate of events in which the state exceeding the predetermined threshold in the response waveform continues. . Note that these occurrence rates may be normalized. In addition, the occurrence rate calculated by the observation unit 5261a is the number of events in which the number of events observed by the observation unit 5261a continues to be below a predetermined threshold in the response waveform observed by the observation unit 5261a, and the observation unit 5261a This is the ratio of the observed response waveform to the sum of the number of events that continue to exceed the predetermined threshold.

  Also, the sum of the occurrence rate of events that continue to be below a predetermined threshold in the response waveform and the occurrence rate of events that continue to be above the predetermined threshold in the response waveform in a predetermined period is 100. %. For this reason, the estimation unit 527a detects an event in which the state in which the state below the predetermined threshold in the response waveform continues even if the observation unit 5261a observes an event in which the state above the predetermined threshold continues in the response waveform. The incidence can be calculated.

  Note that the frequency of occurrence of an event that continues to be lower than a predetermined threshold in the response waveform is lower than the frequency of occurrence of an event that continues to be higher than the predetermined threshold in the response waveform. For this reason, it is preferable that the observation unit 5261a observes an event in which a state where the response waveform is below the predetermined threshold continues rather than an event in which the state where the response waveform exceeds the predetermined threshold continues. Accordingly, the estimation unit 527a can calculate the occurrence rate with a small load.

  Furthermore, the estimation unit 527a estimates the number of X-ray photons incident on the detection element 51 within a predetermined period based on the calculated event occurrence rate. Specifically, the estimation unit 527a estimates the number of X-ray photons (for example, incidence rate) incident on the detection element 51 within a predetermined period by a method described below. In the following description, a case where monochromatic X-rays enter the detection element 51 will be described as an example.

  The phenomenon in which X-ray photons enter the detection element 51 follows a Poisson process. The time interval in which the phenomenon according to the Poisson process occurs follows an exponential distribution. Therefore, the time interval at which the X-ray photons are incident on the detection element 51 follows an exponential distribution. That is, the probability density function f (t; λ) of the time interval t at which the X-ray photons are incident on the detection element 51 is expressed by the following equation (1). Here, λ is the incidence rate.

Here, the response time of the detection element 51 is t ₀ . The response time t ₀ of the detection element 51 is the time from the time when one waveform exceeds the threshold for the response waveform to the time when the waveform falls below the threshold. When the time interval t at which the X-ray photons are incident on the detection element 51 is longer than the response time t ₀ of the detection element 51, adjacent waveforms on the response waveform do not overlap. That is, when the time interval t at which X-ray photons are incident on the detection element 51 is greater than the response time t ₀ of the detection element 51, an event occurs in which the response waveform continues to be below a predetermined threshold. Therefore, the occurrence rate F (t> t ₀ ; λ) of the event in which the state of being below the predetermined threshold in the response waveform continues is obtained by integrating equation (1) from t ₀ to infinity. This is expressed by the following equation (2). Equation (2) is also called a cumulative distribution function.

FIG. 8 is a graph created based on an equation that takes the natural logarithm of both sides of Equation (2). Here, the unit of the response time t ₀ is nsec, and the unit of the count rate is Mcps (10 ⁶ count per second). As shown in FIG. 8, the natural logarithm of the occurrence rate F (t> t ₀ ; λ) of the event in which the state of being below the predetermined threshold in the response waveform continues is proportional to the response time t ₀ of the detection element 51. . In this case, the proportionality constant is −λ. Therefore, as shown in FIG. 8, the incidence rate F (t> t ₀ ; λ) decreases as the incidence rate λ increases.

Here, the response time t ₀ of the detection element 51 depends on the performance of the detection element such as, for example, the light attenuation characteristic of the scintillator, and in addition to the response waveform set by the voltage supplied from the setting unit 5262a to the observation unit 5261a. Depends on the threshold. The latter is clear from the definition of the response time t ₀ of the detection element 51 described above. Therefore, FIG. 8 shows that the behavior of the incidence rate λ of the incidence rate λ at a certain response time t _{0 with} respect to the natural logarithm (t> t ₀ ; λ) is as shown in FIG. FIG. 9 shows a straight line ThS, a straight line ThM, and a straight line ThL. The straight line ThS represents the relationship between the incidence rate λ and the incidence rate F (t> t ₀ ; λ) when the threshold is small. The straight line ThL represents the relationship between the incidence rate λ and the incidence rate F (t> t ₀ ; λ) when the threshold is large. The straight line ThM represents the relationship between the incidence rate λ and the incidence rate F (t> t ₀ ; λ) when the threshold is larger than that of the straight line ThS and smaller than that of the straight line ThL.

As described above, the estimation unit 527a calculates, as an occurrence rate F (t> t ₀ ; λ), the number of events in which the response waveform in a predetermined period continues to be below a predetermined threshold. Therefore, the estimation unit 527a can estimate the incidence rate of the X-rays incident on the detection element 51 using FIG.

A case where the setting unit 5262a supplies a voltage corresponding to the straight line ThM to the observation unit 5261a will be described as an example. Here, it is assumed that the threshold value corresponding to the straight line ThM is a threshold value at which the response time t ₀ of the detection element 51 is 60 nsec. When the incidence rate F (t> t ₀ ; λ) is 0.1, the incidence rate λ is estimated to be about 40 Mcps from the straight line ThM in FIG. When the incidence rate F (t> t ₀ ; λ) is 0.01, the incidence rate λ is estimated to be about 80 Mcps from the straight line ThM in FIG. Note that the estimation unit 527a can also estimate the incidence rate of the X-rays incident on the detection element 51 using the straight line ThS or the straight line ThL.

  When the observation unit 5261a observes an event in which the state where the response waveform is below the predetermined threshold continues, the threshold continues to be below the predetermined threshold in the response waveform observed by the observation unit 5261a. It is preferable that the number of events is set to be a predetermined number or more. When the observation unit 5261a observes an event in which the state in which the response waveform exceeds a predetermined threshold continues, or the threshold continues in a state in which the response waveform observed by the observation unit 5261a exceeds the predetermined threshold It is preferable that the number of events is set to be a predetermined number or more. In these cases, the setting unit 5262a sets the threshold based on an instruction input by the user using the input unit 81, for example. The setting unit 5262a sets a threshold value based on the estimated incidence rate range. The estimated incidence rate range is also called a dynamic range.

  As shown in FIG. 10, when the threshold value represented by the straight line Th1 is set for the response waveform W, two events E11 and E12 in which the response waveform continues to be below a predetermined threshold value occur. When the threshold value decreases, the number of events in which the state of being below the predetermined threshold continues in the response waveform observed by the observation unit 5261a decreases, and thus the state of the estimation unit 527a being below the predetermined threshold in the response waveform continues. The reliability of the number of events that continue to be below a predetermined threshold in the response waveform in a predetermined period calculated as the occurrence rate of the event to be reduced decreases. For this reason, the reliability of the number of X-ray photons incident within the predetermined period estimated by the estimation unit 527a is lowered.

  However, when the threshold value represented by the straight line Th2 is set for the response waveform W, the six events E21, E22, E23, E24, E25, and E26 in which the response waveform continues to be below the predetermined threshold value are displayed. Occur. As the threshold value increases, the number of events that continue to be below a predetermined threshold value in the response waveform observed by the observation unit 5261a increases. For this reason, the number of events in which the state that is below the predetermined threshold in the response waveform in the predetermined period calculated by the estimation unit 527a as the occurrence rate of the event in which the state below the predetermined threshold in the response waveform continues Reliability is improved. For this reason, the reliability of the number of X-ray photons incident within the predetermined period estimated by the estimation unit 527a is improved.

  Note that the content having the same meaning as the content described with reference to FIG. 10 also holds for an event in which a state in which a response waveform exceeds a predetermined threshold continues.

  The photon counting X-ray detector 5 preferably outputs both the result estimated by the estimating unit 527a and the result counted by the counting unit 525. That is, the estimation of the number of X-ray photons (for example, the incidence rate) by the estimation unit 527a and the counting (for example, the count rate) of the X-ray photons by the counting unit 525 are preferably performed in parallel. This is because the result estimated by the estimation unit 527a is more suitable for generating a CT image depending on the position of the detection element 51 and the positions of the X-ray tube 41 and the photon counting X-ray detector 5 being scanned. This is because the result counted by the counting unit 525 may change more frequently when it is more suitable for generating a CT image.

  Alternatively, the photon counting X-ray detector 5 outputs the result estimated by the estimating unit 527a for each position of the detection element 51 and the positions of the X-ray tube 41 and the photon counting X-ray detector 5 being scanned. It is also possible to switch between the case and the case in which the result counted by the counting unit 525 is output. In this case, either the result estimated by the estimation unit 527a or the result counted by the counting unit 525 is output for each position of the detection element 51 and each position of the X-ray tube 41 and the photon counting X-ray detector 5 being scanned. It is preferable to record whether or not.

  As a method for switching between the case where the estimation unit 527a outputs the estimation result and the case where the counting unit 525 outputs the counting result, for example, as shown in FIG. Accordingly, the reliability of the result of each method may be obtained in advance, and switching may be performed at an incidence rate or count rate at which the reliability is switched (incidence rate or count rate corresponding to the intersection Ct in FIG. 12). That is, when the estimation result or counting result is smaller than the incidence rate or counting rate corresponding to the intersection, the counting unit 525 outputs the counting result, and when it is larger, the estimation unit 527a outputs the estimation result. Here, the reliability is an index indicating a divergence between the incidence rate or count rate of actually incident X-ray photons and the estimation result of the estimation unit 527a or the count result of the counting unit 525, and is small if the divergence is small. The higher the value, the lower the value. The reliability is, for example, a difference between the incidence rate or count rate of actually incident X-ray photons and the estimation result of the estimation unit 527a or the count result of the counting unit 525.

  In addition, the observation unit 5261a is configured so that the total value of the results counted by the counting unit 525 is the number of X-ray photons incident on the detection element 51 when X-rays irradiated by the X-ray tube 41 are incident on the detection element 51. It is only necessary to have a data capacity necessary for observing an event in which a state where the response waveform is below a predetermined threshold value continues when a deviation from a predetermined range including a value assumed to be proportional to . This is because, when the sum of the count rates is within a predetermined range including the number of X-ray photons incident on the detection element 51 (for example, the incidence rate), the X-ray photons incident within a predetermined period by the estimation unit 527a. This is because it is not necessary to estimate the number of incidents (for example, incidence rate).

  As described above, since the photon counting X-ray detector 5 according to the first embodiment includes the observation unit 5261a, the setting unit 5262a, and the estimation unit 527a, the detection element 51 even if a pulse pileup occurs. The number of X-ray photons incident on can be estimated. That is, since the photon counting X-ray CT apparatus 1 according to the first embodiment includes the observation unit 5261a, the setting unit 5262a, and the estimation unit 527a, the number of X-ray photons can be calculated even if a pulse pileup occurs. Can be estimated appropriately.

  The photon counting X-ray detector 5 according to the first embodiment includes an observation unit 5261a, a setting unit 5262a, and an estimation unit 527a. Therefore, when the X-ray is incident on the detection element 51, the image generation unit 86 assumes that the total of the results counted by the counting unit 525 is proportional to the number of X-ray photons incident on the detection element 51. When deviating from the predetermined range including the value, a CT image can be generated based on the result estimated by the estimation unit 527a.

  The observation unit 5261a, the setting unit 5262a, and the estimation unit 527a can be realized with a small-scale circuit. For this reason, the detection circuit 52a is highly integrated. Therefore, the photon counting X-ray detector 5 according to the first embodiment can suppress an increase in product cost. Moreover, the photon counting X-ray detector 5 according to the first embodiment can further make the photon counting X-ray CT apparatus 1 compact.

(Second Embodiment)
A detection circuit included in the photon counting X-ray CT apparatus according to the second embodiment will be described with reference to FIGS. FIG. 12 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the second embodiment. FIG. 13 shows a method of observing the time during which an event in which a state below a predetermined threshold continues in the response waveform continues or the time at which an event in which a state exceeding the predetermined threshold continues in the response waveform continues. It is a figure which shows an example. FIG. 14 shows a threshold for the response waveform and a time for which an event in which the state below the predetermined threshold continues in the response waveform continues or a time for an event in which the state above the predetermined threshold continues in the response waveform It is a figure which shows the relationship.

  As illustrated in FIG. 12, the detection circuit 52b includes a waveform shaping unit 521, an output unit 522, a removal unit 523, a calculation unit 524, a counting unit 525, a processing unit 526b, an estimation unit 527b, and a supply unit 528b. Further, as illustrated in FIG. 12, the processing unit 526b includes an observation unit 5261b and a setting unit 5262b.

  The configuration of the detection circuit 52b will be described. The detection circuit 52b includes a processing unit 526b, an estimation unit 527b, and a supply unit 528b instead of the processing unit 526a and the estimation unit 527a included in the detection circuit 52a according to the first embodiment. Therefore, the processing unit 526b, the estimation unit 527b, and the supply unit 528b will be described.

  The input terminal of the processing unit 526b is connected to the output terminal of the output unit 522. The output terminal of the processing unit 526b is connected to the input terminal of the estimation unit 527b. The output terminal of the estimation unit 527b is connected by the estimation data collection unit 62. The supply unit 528b is connected to the processing unit 526b.

  Next, the operation of the detection circuit 52b will be described. The operations of the waveform shaping unit 521, the output unit 522, the removal unit 523, the calculation unit 524, and the counting unit 525 are the same as those in the first embodiment, and thus detailed description thereof is omitted.

  The observation unit 5261b observes an event in which a state where the response waveform is below a predetermined threshold continues. The observation unit 5261b observes an event by, for example, observing a phenomenon in which the response waveform is below a predetermined threshold and a phenomenon in which the response waveform is above a predetermined threshold.

  The observation unit 5261b observes, for example, the time during which an event in which a state where the response waveform is below a predetermined threshold continues continues by the following method. Specifically, the observation unit 5261b counts a clock at a time when an event in which a state of being lower than a predetermined threshold in the response waveform continues continues, so that the predetermined threshold in the response waveform generated in the predetermined period is obtained. Observe the time during which the state of continuing below is continuing.

  First, the supply unit 528b supplies a clock to the observation unit 5261b. For example, the supply unit 528b supplies the clock CR to the observation unit 5261b. As shown in FIG. 13A, the clock CR includes a clock Cd and a clock Cs. In the clock Cd, when the voltage of the response waveform Wa exceeds the threshold value represented by the straight line Th in the clock CR, that is, the event in which the state in which the response waveform Wa exceeds the predetermined threshold value continues. Clock. In the clock Cs, in the clock CR, when the voltage of the response waveform Wa is below the threshold value represented by the straight line Th, that is, when the event in which the state of being below the predetermined threshold value continues in the response waveform Wa continues. Clock. Note that the clock Cd and the clock Cs are the same signal.

  The observation unit 5261b compares the voltage of the response waveform Wa received from the output unit 522 with the voltage supplied from the setting unit 5262b in order to set a threshold value. The observation unit 5261b outputs a negative voltage to the estimation unit 527b when the voltage of the response waveform Wa received from the output unit 522 is larger than the voltage supplied from the setting unit 5262b to set the threshold value. The observation unit 5261b outputs a positive voltage to the estimation unit 527b when the voltage of the response waveform Wa received from the output unit 522 is smaller than the voltage supplied from the setting unit 5262b to set the threshold value.

  Further, the phenomenon that the voltage output to the estimation unit 527b switches from negative to positive corresponds to a phenomenon that the response waveform Wa falls below a predetermined threshold. The phenomenon in which the voltage output to the estimation unit 527b is switched from positive to negative corresponds to a phenomenon that exceeds a predetermined threshold in the response waveform Wa. Therefore, the observation unit 5261b can discriminate between the clock Cd and the clock Cs described above by observing these phenomena.

  The observation unit 5261b observes the time when the positive voltage is output by counting the clock Cs. As a result, the observation unit 5261b can observe the time during which an event in which the response waveform Wa is below the predetermined threshold continues and transmits the result to the estimation unit 527b.

  In addition, the observation unit 5261b generates one event in which the state of being lower than the predetermined threshold in the response waveform Wa continues between two events in which the state of being higher than the predetermined threshold is continued in the response waveform Wa. By utilizing this fact, an event may be observed in which the state in which the response waveform Wa is below a predetermined threshold continues.

  In this case, the observation unit 5261b observes the time when the negative voltage is output by counting the clock Cd. Then, the observation unit 5261b subtracts the observed time from the predetermined period. As a result, the observation unit 5261b can calculate the time during which an event in which the response waveform Wa is below the predetermined threshold continues and transmits the result to the estimation unit 527b.

  In the above description, the case where the observation unit 5261b observes an event in which a state where the response waveform Wa is below a predetermined threshold continues is taken as an example. Alternatively, the observation unit 5261b may observe an event in which a state where the response waveform Wa exceeds a predetermined threshold continues. The observation unit 5261b observes an event by, for example, observing a phenomenon in which the response waveform Wa is below a predetermined threshold and a phenomenon in which the response waveform is above a predetermined threshold.

  For example, the observation unit 5261b observes a time during which an event in which a state exceeding a predetermined threshold in the response waveform continues continues by the following method. Specifically, the observation unit 5261b counts a clock at a time when an event in which a state exceeding a predetermined threshold value continues in the response waveform Wa continues, so that the response waveform Wa generated in a predetermined period has a predetermined value. Observe the time during which an event that continues to exceed the threshold is continued.

  In this case, contrary to the case described above, the observation unit 5261b observes the time when the negative voltage is output by counting the clock Cd. As a result, the observation unit 5261b can observe the time during which an event in which the response waveform Wa exceeds a predetermined threshold continues and transmits the result to the estimation unit 527b.

  Alternatively, the observation unit 5261b generates one event in which the state of exceeding the predetermined threshold in the response waveform Wa continues between two events in which the state of falling below the predetermined threshold in the response waveform Wa continues. By utilizing this, an event in which a state exceeding a predetermined threshold in the response waveform Wa continues may be observed.

  In this case, the observation unit 5261b observes the time when the positive voltage is output by counting the clock Cs. Then, the observation unit 5261b subtracts the observed time from the predetermined period. As a result, the observation unit 5261b can calculate the time during which an event in which the response waveform Wa exceeds a predetermined threshold continues and transmits the result to the estimation unit 527b.

  Based on the time observed by the observation unit 5261b, the estimation unit 527b calculates an occurrence rate within a predetermined period of an event in which a state where the response waveform is below a predetermined threshold continues. In this case, the occurrence rate calculated by the estimation unit 527b is, for example, the ratio of the observed time to the predetermined period. The predetermined period is, for example, a view. The view is the position of the X-ray tube 41 on a circular orbit centered on a point in the subject P.

  For example, the estimating unit 527b may be configured to detect an event in which a state in which a state below a predetermined threshold continues in a response waveform generated in a predetermined period continues from a time in which the state continues below a predetermined threshold in the response waveform. Calculate the incidence. Specifically, the estimation unit 527b calculates, as an event occurrence rate, a ratio of time during which an event in which a state of being below a predetermined threshold continues in a response waveform for a predetermined period continues. In this case, the estimation unit 527b estimates the incidence rate of X-rays incident on the detection element 51 using FIG. 9 described above.

  Alternatively, the estimation unit 527b may be configured to detect an event in which a state in which a state exceeding a predetermined threshold continues in a response waveform generated in a predetermined period continues from a time in which the state in which the state exceeds a predetermined threshold continues. Calculate the incidence. Specifically, the estimation unit 527b calculates, as an event occurrence rate, a ratio of a time during which an event in which a state exceeding a predetermined threshold continues in a response waveform for a predetermined period continues. In this case, the estimation unit 527b calculates the sum of the occurrence rate of the event in which the state below the predetermined threshold in the response waveform continues and the occurrence rate of the event in which the state above the predetermined threshold in the response waveform continues. Utilizing the fact that it becomes 100%, the occurrence rate of an event in which the state of being below a predetermined threshold in the response waveform continues is calculated. Then, the estimation unit 527b estimates the incidence rate of the X-rays incident on the detection element 51 using FIG. 9 described above.

  As shown in FIGS. 13B and 13C, when the number of X-ray photons that enter the detection element 51 within a predetermined period (for example, the incidence rate) increases, a predetermined threshold value is set in the response waveform. The time during which an event that continues to fall below is continued decreases. Then, the reliability of the occurrence rate of the event that the state where the response waveform calculated by the estimation unit 527b is below the predetermined threshold continues is lowered. In order to suppress this, the threshold is set so that the time during which an event in which the state of being below the predetermined threshold continues in the response waveform observed by the observation unit 5261b generated in the predetermined period continues is equal to or longer than the predetermined time. It is preferably set. This will be described with reference to FIG.

  As shown in FIG. 14, the clock CR includes a clock Cd, a clock Ca, and a clock Cs. The clock Cs is a clock when the voltage of the response waveform W is below the threshold value represented by the straight line Th1 in the clock CR. The clock Ca is a clock when the voltage of the response waveform W in the clock CR is above the threshold value represented by the straight line Th1 and below the threshold value represented by the straight line Th2. The clock Cd is a clock when the voltage of the response waveform W exceeds the threshold value represented by the straight line Th2 in the clock CR. The clock Cd, the clock Ca, and the clock Cs are the same signal.

  When the threshold value represented by the straight line Th1 is set for the response waveform W, only the clock Cs is the clock when the event that the state of being lower than the predetermined threshold in the response waveform continues among the clock CR continues. It is. When the threshold value decreases, the number of clocks when the voltage of the response waveform W is below the threshold value decreases. Therefore, the estimation unit 527b calculates the occurrence rate of the event in which the response waveform continues to be below the predetermined threshold value. In the response waveform for the predetermined period, the reliability of the proportion of the time during which the state where the state of being lower than the predetermined threshold continues continues is lowered. For this reason, the reliability of the number of X-ray photons incident within the predetermined period estimated by the estimation unit 527b is lowered.

  However, when the threshold value represented by the straight line Th2 is set with respect to the response waveform W, the clock when the state in which the state below the predetermined threshold value continues in the response waveform of the clock CR continues is the clock Cs and clock Ca. As the threshold value increases, the number of clocks increases when the voltage of the response waveform W is below the threshold value, so that the estimation unit 527b calculates the occurrence rate of an event in which the response waveform continues to be below the predetermined threshold value. In the response waveform for the predetermined period, the reliability of the ratio of the time during which the state where the state of being below the predetermined threshold continues continues is improved. For this reason, the reliability of the number of X-ray photons incident within the predetermined period estimated by the estimation unit 527b is improved.

  Alternatively, the threshold value is set so that a time during which an event in which a state exceeding the predetermined threshold value continues in the response waveform observed by the observation unit generated in a predetermined period continues is a predetermined time or more. It is preferable.

  In this case, contrary to the case described above, the threshold is set so that the number of clocks increases when an event in which the response waveform W continues to be below the predetermined threshold continues. Thereby, the observation unit 5261b continues the event in which the state exceeding the predetermined threshold in the response waveform for the predetermined period calculated as the occurrence rate of the event in which the state exceeding the predetermined threshold in the response waveform continues. The reliability of the percentage of time that is being improved. For this reason, the reliability of the number of X-ray photons incident within the predetermined period estimated by the estimation unit 527b is improved.

  As described above, since the photon counting X-ray detector according to the second embodiment includes the observation unit 5261b, the setting unit 5262b, and the estimation unit 527b, even if a pulse pileup occurs, the detection element 51 The number of incident X-ray photons can be estimated. That is, since the photon counting X-ray CT apparatus according to the second embodiment includes the observation unit 5261b, the setting unit 5262b, and the estimation unit 527b, the number of X-ray photons is appropriately set even when pulse pileup occurs. Can be estimated.

  The photon counting X-ray detector according to the second embodiment includes an observation unit 5261b, a setting unit 5262b, and an estimation unit 527b. Therefore, when the X-ray is incident on the detection element 51, the image generation unit 86 assumes that the total of the results counted by the counting unit 525 is proportional to the number of X-ray photons incident on the detection element 51. When deviating from a predetermined range including a value, a CT image can be generated based on the result estimated by the estimation unit 527b.

  The observation unit 5261b, the setting unit 5262b, and the estimation unit 527b can be realized with a small-scale circuit. For this reason, the detection circuit 52b is highly integrated. Therefore, the photon counting X-ray detector according to the second embodiment can suppress an increase in product cost. The photon counting X-ray detector according to the second embodiment can further make the photon counting X-ray CT apparatus compact.

  Furthermore, in the photon counting X-ray detector according to the second embodiment, the observation unit 5261b observes a time during which an event in which the response waveform generated in a predetermined period continues below a predetermined threshold continues. Alternatively, the time during which an event in which the response waveform generated in a predetermined period continues exceeding the predetermined threshold is continued is observed. In addition, the estimation unit 527b is configured to detect an event in which a state in which a state below a predetermined threshold is continued in a response waveform generated in a predetermined period continues from a time in which a state in which the state is below a predetermined threshold continues. The rate of occurrence is calculated or the response waveform that occurs in a predetermined period of time continues to be in a state where the state exceeding the predetermined threshold continues, and the event in which the state exceeding the predetermined threshold in the response waveform continues Calculate the incidence. For this reason, in the photon counting X-ray detector according to the second embodiment, there is an event in which a state where the generated response waveform is below a predetermined threshold continues, or a state where the response waveform is above the predetermined threshold. Even if the number of events that continue is small, the number of X-ray photons that have entered the detection element 51 can be accurately estimated if the duration of these events continues to some extent.

(Third embodiment)
As shown in FIG. 9 above, when the occurrence rate of the observed event is low or when the observed event lasts for a short time, the occurrence rate of the observed event changes slightly, and the count rate varies greatly. Resulting in. Therefore, the photon counting X-ray detector according to the third embodiment estimates the number of X-ray photons based on the observation results for each of a plurality of threshold values.

  A detection circuit included in the photon counting X-ray CT apparatus according to the third embodiment will be described with reference to FIG. FIG. 15 is a diagram illustrating an example of a detection circuit included in the photon counting X-ray CT apparatus according to the third embodiment.

  As illustrated in FIG. 15, the detection circuit 52c includes a waveform shaping unit 521, an output unit 522, a removal unit 523, a calculation unit 524, a counting unit 525, a processing unit 526c, and an estimation unit 527c.

  The configuration of the detection circuit 52c will be described. The detection circuit 52c includes a processing unit 526c and an estimation unit 527c instead of the processing unit 526a and the estimation unit 527a included in the detection circuit 52a according to the first embodiment. Therefore, the processing unit 526c and the estimation unit 527c will be described.

  The processing unit 526c includes an observation unit 5261c and a setting unit 5262c. The observation unit 5261c includes a response waveform observation unit 5261x, a response waveform observation unit 5261y, and a response waveform observation unit 5261z. The setting unit 5262c includes a voltage supply unit 5262x, a voltage supply unit 5262y, and a voltage supply unit 5262z.

  The input terminal of the response waveform observation unit 5261x is connected to the output terminal of the output unit 522. The output terminal of the response waveform observation unit 5261x is connected to the input terminal of the estimation unit 527c. The voltage supply unit 5262x is connected to the input terminal of the response waveform observation unit 5261x. The input terminal of the response waveform observation unit 5261y is connected to the output terminal of the output unit 522. The output terminal of the response waveform observation unit 5261y is connected to the input terminal of the estimation unit 527c. The voltage supply unit 5262y is connected to the input terminal of the response waveform observation unit 5261y. The input terminal of the response waveform observation unit 5261z is connected to the output terminal of the output unit 522. The output terminal of the response waveform observation unit 5261z is connected to the input terminal of the estimation unit 527c. The voltage supply unit 5262z is connected to the input terminal of the response waveform observation unit 5261z. An output terminal of the estimation unit 527c is connected to the estimation data collection unit 62.

  Next, the operation of the detection circuit 52c will be described. The setting unit 5262c sets a plurality of threshold values. That is, the voltage supply unit 5262x supplies the first voltage to the response waveform observation unit 5261x. The first voltage corresponds to the first threshold value. The voltage supply unit 5262y supplies the second voltage to the response waveform observation unit 5261y. The second voltage corresponds to the second threshold value. The voltage supply unit 5262z supplies the third voltage to the response waveform observation unit 5261z. The third voltage corresponds to the third threshold value.

  The observation unit 5261c observes an event in which a state where the response waveform is below a predetermined threshold is continued for each of the plurality of thresholds. That is, the response waveform observation unit 5261x observes an event in which a state where the response waveform is below the threshold continues with respect to the threshold set by the first voltage supplied by the voltage supply unit 5262x. The response waveform observation unit 5261y observes an event in which a state where the response waveform is below the threshold continues with respect to the threshold set by the second voltage supplied by the voltage supply unit 5262y. The response waveform observation unit 5261z observes an event in which a state where the response waveform is below the threshold continues with respect to the threshold set by the third voltage supplied by the voltage supply unit 5262z.

  For example, the response waveform observation unit 5261x, the response waveform observation unit 5261y, and the response waveform observation unit 5261z observe an event by observing a phenomenon that is lower than a threshold value set in the response waveform and a phenomenon that is higher than the threshold value set in the response waveform. To do. Alternatively, the response waveform observation unit 5261x, the response waveform observation unit 5261y, and the response waveform observation unit 5261z set the predetermined threshold value in the response waveform during two events in which the response waveform continues to exceed the predetermined threshold value. Utilizing the occurrence of one event that continues to be in a state of being lower, an event in which the state of being lower than a predetermined threshold is continued in the response waveform is observed. These specific methods are as described above.

  The observation unit 5261c observes an event in which a state where the response waveform exceeds a predetermined threshold for each of the plurality of thresholds continues. That is, the response waveform observation unit 5261x observes an event in which the response waveform continues to exceed the threshold with respect to the threshold set by the first voltage supplied by the voltage supply unit 5262x. The response waveform observation unit 5261y observes an event in which the state in which the response waveform exceeds the threshold is continued with respect to the threshold set by the second voltage supplied by the voltage supply unit 5262y. The response waveform observation unit 5261z observes an event in which the response waveform continues to exceed the threshold with respect to the threshold set by the third voltage supplied by the voltage supply unit 5262z.

  For example, the response waveform observation unit 5261x, the response waveform observation unit 5261y, and the response waveform observation unit 5261z observe an event by observing a phenomenon that is lower than a threshold value set in the response waveform and a phenomenon that is higher than the threshold value set in the response waveform. To do. Alternatively, the response waveform observing unit 5261x, the response waveform observing unit 5261y, and the response waveform observing unit 5261z exceed the predetermined threshold in the response waveform during two events in which the response waveform continues to be below the predetermined threshold. Utilizing the occurrence of one event in which the current state continues, the event in which the state exceeding the predetermined threshold in the response waveform continues is observed. These specific methods are as described above.

  The estimation unit 527c estimates the number of X-ray photons that have entered the detection element 51 within a predetermined period based on the observation result of the observation unit 5261c for each of the plurality of threshold values. Specifically, first, the estimation unit 527c is a result observed by the response waveform observation unit 5261x, the response waveform observation unit 5261y, and the response waveform observation unit 5261z by the same method as in the first embodiment or the second embodiment. For each, the number of X-ray photons (for example, incidence rate) incident on the detection element 51 within a predetermined period is estimated. Then, the estimation unit 527c outputs statistical values such as the estimated average value and median value of the number of X-ray photons to the estimated data collection unit 62 as a final result.

  In the third embodiment, the case where the observation unit 5261c includes the response waveform observation unit 5261x, the response waveform observation unit 5261y, and the response waveform observation unit 5261z is described as an example, but the present invention is not limited to this. The number of response waveform observation units provided in the detection circuit 52c is not particularly limited. In addition, the estimation unit 527c may output not the statistical value as a final result but a result obtained by using a table created in advance to the estimation data collection unit 62. Alternatively, the estimation unit 527c may output a result obtained by using a previously created calculation formula instead of the statistical value as the final result to the estimation data collection unit 62. For example, the estimation unit 527c is based on a result estimated by the response waveform observation unit 5261x, a result estimated by the response waveform observation unit 5261y, and a result observed by the response waveform observation unit 5261z. A result obtained by multiplying the estimated result by weighting and taking the sum may be output as a final result.

  As described above, since the photon counting X-ray detector according to the third embodiment includes the observation unit 5261c, the setting unit 5262c, and the estimation unit 527c, even if a pulse pileup occurs, the detection element 51 The number of incident X-ray photons can be estimated. That is, since the photon counting X-ray CT apparatus according to the third embodiment includes the observation unit 5261c, the setting unit 5262c, and the estimation unit 527c, a CT image can be generated even if a pulse pileup occurs. it can.

  The observation unit 5261c, the setting unit 5262c, and the estimation unit 527c can be realized with a small-scale circuit. For this reason, the detection circuit 52c is highly integrated. Therefore, the photon counting X-ray detector according to the third embodiment can suppress an increase in product cost. Moreover, the photon counting X-ray detector according to the third embodiment can further reduce the size of the photon counting X-ray CT apparatus.

  Furthermore, in the photon counting X-ray detector according to the third embodiment, the setting unit 5262c sets a plurality of thresholds, and the observation unit 5261c is below a predetermined threshold in the response waveform for each of the plurality of thresholds. Based on the result of observation by the observing unit 5261c for each of the plurality of thresholds by observing the event in which the state continues exceeding the predetermined threshold in the response waveform, or the estimation unit 527c To estimate the number of X-ray photons incident within a predetermined period. For this reason, the photon counting X-ray detector according to the third embodiment has a low incidence of these events when the number of these events is small or when the duration of these events is short. Even in this case, the number of X-ray photons incident on the detection element 51 within a predetermined period can be accurately estimated.

  In the above-described embodiment, the case where the detection circuit processes the response waveform output from the detection element 51 has been described as an example. However, the present invention is not limited to this. For example, the response waveform output from the detection element 51 may be stored, and the above-described processing may be performed on the stored response waveform.

  The image processing method described in the above-described embodiment may be performed by an image processing apparatus installed independently of the photon counting X-ray CT apparatus. For example, the image processing apparatus having the same function as the control unit 88 shown in FIG. 1 uses the response waveform acquired from the photon counting X-ray CT apparatus 1 or the PACS database or the electronic medical record system database as described above. The image processing method described above may be performed.

  Each component described above is functionally conceptual and does not necessarily have to be physically configured as illustrated. In other words, the specific form of distribution or integration of each component is not limited to the illustrated one, and all or a part thereof may be functionally or physically in an arbitrary unit depending on various loads or usage conditions. It can be configured to be distributed or integrated. Furthermore, each processing function of each component is realized by a CPU and a program executed on the CPU, all or any part thereof. Alternatively, all or any part of each processing function of each component is realized as hardware by wired logic.

  The method described in the above embodiment can be realized by executing a program prepared in advance on a computer such as a personal computer or a workstation. This program can be distributed via a network such as the Internet. The program can also be executed by being recorded on a computer-readable recording medium such as a hard disk, a flexible disk (FD), a CD-ROM, an MO, and a DVD, and being read from the recording medium by the computer.

  According to at least one of the embodiments described above, it is possible to accurately count X-ray photons even when the number of X-ray photons incident within a predetermined period is large while realizing high integration of the detection circuit. .

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

51 Detection Element 5261a Observation Unit 527a Estimation Unit

Claims (10)

A detection element that outputs a response waveform based on the incident X-ray photons;
An observation unit for observing the number of events that continue to be below a predetermined threshold in the response waveform , or to observe the number of events that continue to be above a predetermined threshold in the response waveform;
Based on the number observed by the observation unit, the occurrence rate of the event observed by the observation unit within a predetermined period is calculated, and X incident on the detection element within the predetermined period based on the occurrence rate An estimator for estimating the number of line photons;
A photon counting X-ray detector. A setting unit for setting the threshold value;
The setting unit sets a plurality of the threshold values,
The observation unit observes an event in which a state of being below a predetermined threshold in the response waveform for each of the plurality of thresholds continues, or sets a predetermined threshold in the response waveform for each of the plurality of thresholds. Observe the event that continues to exceed,
The estimating unit, based on the result of the observation portion is observed for each of the plurality of the threshold values, to estimate the number of incident X-ray photons within the predetermined period of time to the detection device, according to claim 1 Photon counting X-ray detector. 3. The photon counting X-ray detector according to claim 1, further comprising a counting unit that counts the number of X-ray photons that have generated an event in which a state of exceeding a predetermined threshold continues in the response waveform. A calculation unit that calculates energy of an X-ray photon that has generated an event in which a state of exceeding a predetermined threshold in the response waveform continues;
Based on the result calculated by the calculation unit, the counting unit sends the predetermined value to the detection element for each of one or a plurality of energy bands set on the energy distribution of the X-rays irradiated by the X-ray tube. 4. The photon counting X-ray detector according to claim 3 , which counts the number of X-ray photons incident during the period. The observation unit assumes that the total of the results counted by the counting unit is proportional to the number of X-ray photons incident on the detection element when X-rays irradiated by an X-ray tube are incident on the detection element. If that deviates from a predetermined range including the value when, having data capacity required in terms of the state of the response waveform is below a predetermined threshold is observed events to continue, according to claim 3 or claim 4 2. A photon counting X-ray detector according to 1. The result estimating unit has estimated and the counting section is output both as a result of the counting, photon counting X-ray detector according to any one of the preceding claims 3. Wherein the predetermined period is a period in which each projection data is collected for generating a CT image, photon counting X-ray detector according to any one of claims 1 to 6. A detection element that outputs a response waveform based on an X-ray photon incident within a predetermined period, and a number of events that are provided for each of the detection elements and that continue to be below a predetermined threshold in the response waveform are observed. Or an observation unit for observing the number of events that continue to exceed a predetermined threshold in the response waveform, and based on the number observed by the observation unit, the occurrence rate of events observed by the observation unit A photon counting X-ray detector having an estimation unit that calculates and estimates the number of X-ray photons incident on the detection element within the predetermined period based on the calculated incidence rate;
An image generation unit that generates a CT image based on a result estimated by the estimation unit;
A photon counting X-ray CT apparatus. The photon counting X-ray detector includes a calculation unit that calculates energy of an X-ray photon that has generated an event in which a state in which the response waveform exceeds a predetermined threshold continues, and a result calculated by the calculation unit And a counter for counting the number of X-ray photons incident on the detection element within the predetermined period for each of one or a plurality of energy bands set on the energy distribution of the X-rays. A photon counting X-ray CT apparatus according to claim 8 . When the X-ray photon is incident on the detection element, the image generation unit assumes that the sum of the results counted by the counting unit is proportional to the number of X-ray photons incident on the detection element. The photon-counting X-ray CT apparatus according to claim 9 , wherein a CT image is generated based on a result estimated by the estimation unit when deviating from a predetermined range including.