WO2016039054A1 - フォトンカウンティングct装置および推定被ばく量算出方法 - Google Patents
フォトンカウンティングct装置および推定被ばく量算出方法 Download PDFInfo
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
- A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
- A61B6/4208—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector
- A61B6/4241—Arrangements for detecting radiation specially adapted for radiation diagnosis characterised by using a particular type of detector using energy resolving detectors, e.g. photon counting
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
- A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
- A61B6/03—Computed tomography [CT]
- A61B6/032—Transmission computed tomography [CT]
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
- A61B6/54—Control of apparatus or devices for radiation diagnosis
- A61B6/542—Control of apparatus or devices for radiation diagnosis involving control of exposure
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- G—PHYSICS
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- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
- G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
- G01T1/2914—Measurement of spatial distribution of radiation
- G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)
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- A61B6/46—Arrangements for interfacing with the operator or the patient
- A61B6/461—Displaying means of special interest
- A61B6/463—Displaying means of special interest characterised by displaying multiple images or images and diagnostic data on one display
Definitions
- the present invention relates to an X-ray CT (Computed Tomography) apparatus (hereinafter referred to as a PCCT apparatus) having a photon counting mode, and more particularly to a technique for managing the exposure amount of a subject in the PCCT apparatus.
- a PCCT apparatus Computed Tomography apparatus
- the X-ray CT apparatus obtains X-ray transmission data of a subject while rotating a pair of an X-ray source and an X-ray detector arranged opposite to each other with the subject interposed therebetween, and calculates a tomographic image (hereinafter referred to as a CT image). And is used as industrial and security inspection devices, medical diagnostic imaging devices, and the like.
- Medical X-ray CT apparatuses include a PCCT apparatus equipped with a photon counting mode.
- a photon counting type detector counts X-ray photons (X-ray photons) transmitted through a subject for each detection element.
- X-ray photons X-ray photons
- the X-ray intensity for each energy value can be obtained by discriminating each counted X-ray photon by the energy value.
- the PCCT apparatus may extract and image only X-rays in a specific energy range and use it for diagnosis. In this case, by reducing X-rays outside the energy range as much as possible, the exposure amount of the patient as the subject can be reduced.
- an X-ray attenuation body (hereinafter referred to as an X-ray filter) capable of changing the thickness is inserted between the X-ray source and the subject. Yes (see, for example, Patent Document 1).
- an X-ray filter capable of changing the thickness
- the exposure dose is calculated by changing the current value when the tube voltage is constant.
- the distribution (spectrum) of the energy value of the irradiated X-rays is changed by the filter (including the bow tie filter), and the exposure amount is also increased. Change. For this reason, an accurate exposure amount cannot be obtained only by changing the current value.
- the present invention has been made in view of the above circumstances.
- the exposure amount of the subject is accurately estimated with a simple configuration regardless of the spectrum shape of the irradiated X-ray.
- an exposure dose by X-rays with a predetermined intensity for each predetermined energy range is obtained and stored as exposure data for each band.
- the number of photons (intensity) of incident X-rays for each energy range is obtained as an X-ray spectrum that is irradiated according to the set imaging conditions and is incident on the detector without a subject.
- the incident X-ray intensity is multiplied by the band-unit exposure data, and the result is added up for the entire energy range. Thereby, the exposure amount by irradiation X-rays irradiated according to the set imaging conditions is estimated.
- the exposure amount of a subject can be estimated accurately with a simple configuration regardless of the spectrum shape of the irradiated X-ray.
- FIG. 1 It is a block diagram of the photon counting CT apparatus of embodiment of this invention.
- (A) And (b) is explanatory drawing for demonstrating the X-ray detector of embodiment of this invention. It is a functional block diagram of the calculating part of embodiment of this invention. It is explanatory drawing for demonstrating the principle of the X-ray photon count of the photon counting CT apparatus.
- (A) is explanatory drawing for demonstrating the band unit exposure amount database of embodiment of this invention
- (b) is explanatory drawing for demonstrating the data stored in the same band unit exposure amount database. is there.
- (A) is explanatory drawing for demonstrating the band unit exposure amount database preparation method of embodiment of this invention
- (b) is for demonstrating the spectrum acquisition method by the spectrum acquisition part of embodiment of this invention.
- a photon counting CT apparatus having a photon counting type detector is used as an X-ray CT apparatus, instead of a conventional integral type (current mode measurement type) detector.
- PCCT apparatus photon counting CT apparatus
- photons (X-ray photons) derived from X-rays transmitted through a subject are counted by a detector.
- X-ray photons have different energies.
- X-ray photons are discriminated and counted for each predetermined energy band. Thereby, the number of X-ray photons for each energy band, that is, the X-ray intensity is obtained.
- FIG. 1 is a schematic configuration diagram of a PCCT apparatus 100 according to the present embodiment.
- the PCCT apparatus 100 of this embodiment includes a UI unit 200, a measurement unit 300, and a calculation unit 400.
- the UI unit 200 receives an input from the user and presents the processing result of the calculation unit 400 to the user. For this reason, the UI unit 200 includes an input device 210 such as a keyboard and a mouse, and an output device 220 such as a display device (monitor) and a printer.
- the display device includes a liquid crystal display, a CRT (Cathode Ray Tube), and the like. Note that the display device may have a touch panel function and be configured to be used as the input device 210.
- the measurement unit 300 irradiates the subject 101 with X-rays and measures X-ray photons transmitted through the subject 101 under the control of the calculation unit 400.
- the measurement unit 300 includes an X-ray irradiation unit 310, an X-ray detection unit 320, a gantry (gantry) 330, a control unit 340, and a table 102 on which the subject 101 is placed.
- a circular opening 331 for placing the subject 101 and the table 102 on which the subject 101 is placed is provided. Inside the gantry 330 are arranged a rotating plate 332 on which an X-ray tube 311 and an X-ray detector 321 described later are mounted, and a driving mechanism for rotating the rotating plate 332.
- the circumferential direction of the opening 331 is defined as the x direction
- the radial direction is defined as the y direction
- the direction orthogonal thereto is defined as the z direction.
- the z direction is the body axis direction of the subject 101.
- the X-ray irradiation unit 310 generates X-rays and irradiates the subject 101 with the generated X-rays.
- the X-ray irradiation unit 310 includes an X-ray tube 311, an X-ray filter 312, and a bowtie filter 313.
- the X-ray tube 311 irradiates the subject 101 with an X-ray beam by a high voltage supplied in accordance with the control of an irradiation controller 341 described later.
- the irradiated X-ray beam spreads with a fan angle and a cone angle.
- the X-ray beam is applied to the subject 101 as the rotating plate 332 of the gantry 330 described later rotates.
- the X-ray filter 312 adjusts the X-ray dose of X-rays emitted from the X-ray tube 311. That is, the X-ray spectrum is changed.
- the X-ray filter 312 of this embodiment attenuates the X-rays irradiated from the X-ray tube 311 so that the X-rays irradiated from the X-ray tube 311 to the subject 101 have a predetermined energy distribution.
- the X-ray filter 312 is used to optimize the exposure amount of the patient who is the subject 101. For this reason, it is designed so that the dose in the required energy band is increased.
- the bow tie filter 313 suppresses the exposure amount of the peripheral part. Considering that the cross section of the human body that is the subject 101 is an ellipse, it is used to increase the dose near the center and lower the surrounding dose to optimize the exposure dose.
- the X-ray detection unit 320 Each time an X-ray photon enters, the X-ray detection unit 320 outputs a signal capable of measuring the energy value of the X-ray photon.
- the X-ray detection unit 320 includes an X-ray detector 321.
- the X-ray detector 321 of this embodiment includes a plurality of detection elements 322 and a collimator 323 that limits the incident direction to the X-ray detector 321.
- the structure shown in FIG. 2A is repeated in the x direction.
- the X-ray detector 321 has a large number of detection elements 322 arranged in the x direction and the z direction at substantially equal distances from the X-ray generation point of the X-ray tube 311. You may have a structure.
- planar detectors detector modules
- the central portion of the plane is an arc
- pseudo arc shape to obtain an X-ray detector. It is good also as 321.
- Each detection element 322 outputs one pulse of an electrical signal (analog signal) each time an X-ray photon enters.
- the output signal is input to the arithmetic unit 400 described later.
- the detection element 322 an incident X-ray photon is directly converted into an electric signal, for example, a CdTe telluride cadmium telluride semiconductor element is used.
- the detection element 322 may be a scintillator that emits fluorescence upon receiving X-rays and a photodiode that converts fluorescence into electricity.
- the number of detection elements 322 (number of channels) of the X-ray detector 321 is, for example, 1000.
- the size of each detection element in the x direction is, for example, 1 mm.
- the distance between the X-ray generation point of the X-ray tube 311 and the X-ray incident surface of the X-ray detector 321 is 1000 mm.
- the diameter of the opening 331 of the gantry 330 is 700 mm.
- the required time for rotation of the rotating plate 332 depends on the parameters input by the user via the UI unit 200.
- the time required for rotation is 1.0 s / time.
- the number of times of photographing in one rotation by the measurement unit 300 is 900, and one photographing is performed every time the rotating plate 332 rotates 0.4 degrees.
- the control unit 340 controls the X-ray detection in the irradiation controller 341 that controls the irradiation of the X-rays from the X-ray tube 311, the gantry controller 342 that controls the rotational drive of the rotating plate 332, and the X-ray detector 321.
- a controller 343 and a table controller 344 that controls driving of the table 102 are provided. These operate according to control by a measurement control unit 420 of the calculation unit 400 described later.
- the calculation unit 400 controls the overall operation of the PCCT apparatus 100 and performs imaging by processing the data acquired by the measurement unit 300. As shown in FIG. 3, the calculation unit 400 of the present embodiment includes an imaging condition setting unit 410, a measurement control unit 420, a data collection unit 430, an exposure amount estimation unit 440, an image generation unit 450, and a band unit. An exposure dose database (DB) 470.
- DB exposure dose database
- the calculation unit 400 includes a central processing unit (CPU) 401, a memory 402, and an HDD (Hard disk drive) device 403.
- CPU central processing unit
- memory 402 a memory
- HDD Hard disk drive
- each function is realized by causing the central processing unit 401 to load a program stored in the HDD device 403 in advance into the memory 402 and execute it.
- arithmetic unit 400 may be realized by an integrated circuit such as ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array).
- ASIC Application Specific Integrated Circuit
- FPGA Field Programmable Gate Array
- the HDD device 403 stores data used for processing, data generated during processing, data obtained as a result of processing, and the like.
- the processing result is also output to the output device 220 of the UI unit 200.
- the band unit exposure DB 470 is constructed on the HDD device 403, for example.
- the shooting condition setting unit 410 receives and sets shooting conditions from the user. For example, the shooting condition setting unit 410 displays a reception screen for receiving shooting conditions on the display device, and receives the shooting conditions via the reception screen. The user inputs photographing conditions by operating a mouse, a keyboard, and a touch panel, for example, via the reception screen.
- the imaging conditions to be set are, for example, the tube current and tube voltage of the X-ray tube 311, the imaging range of the subject 101, the shape of the X-ray filter 312, the shape of the bow tie filter 313, and the resolution.
- shooting conditions do not necessarily have to be input by the user every time.
- typical shooting conditions may be stored in advance, and read and used.
- the measurement control unit 420 controls the control unit 340 according to the shooting conditions set by the user, and performs measurement.
- the measurement control unit 420 causes the table controller 343 to move the table 102 in a direction perpendicular to the rotary plate 332, and when the shooting position of the rotary plate 332 matches the specified shooting position. To stop moving. Thereby, the arrangement of the subject 101 is completed.
- the measurement control unit 420 instructs the gantry controller 342 to operate the drive motor and start the rotation of the rotating plate 332 at the same timing as the instruction to the table controller 343.
- the measurement control unit 420 instructs the X-ray irradiation timing of the X-ray tube 311 to the irradiation controller 341, and the detection controller 344 instructs the imaging timing of the X-ray detector 321 to 344. Thereby, the measurement control unit 420 starts X-ray irradiation and X-ray photon detection, that is, measurement.
- the measurement control unit 420 measures the entire photographing range by repeating these instructions.
- the data collection unit 430 counts the photons derived from the X-rays detected by the X-ray detector 321 (X-ray photons) for each energy range of the first energy range section, and counts for each energy range. get information.
- the data collection unit 430 of this embodiment includes a data collection system (DAS: Data Acquisition System, hereinafter referred to as DAS), and this DAS counts X-ray photons detected by the measurement unit 300.
- DAS Data Acquisition System
- the DAS acquires the energy value of each X-ray photon detected by the X-ray detector 321 and adds it to the counting result of energy bins (Bin) provided for each energy range according to the energy value.
- the energy bin is a storage area set for each energy range of the first energy range section.
- the first energy range section is obtained by dividing an energy range from 0 keV to the maximum energy of the X-ray tube 311 by a predetermined energy width ⁇ B.
- the energy width ⁇ B is set to 20 keV, for example.
- the maximum energy is 140 keV
- the total energy range 0 keV to 140 keV is changed to B1 (0 to 20 keV), B2 (20 to 40 keV), B3 (40 to 60 keV), B4 (60 to 80 keV), B5 (80 to 100 keV). ), B6 (100 to 120 keV), and B7 (120 to 140 keV).
- the DAS adds to the count result of the energy bin provided in association with the corresponding energy range according to the detected energy value of the X-ray photon.
- the data collection unit 430 counts the number of X-ray photons for each energy range.
- the obtained result shows the distribution of energy values (unit keV) of X-ray photons. Accordingly, the data collection unit 430 thereby obtains the energy distribution (spectrum) of the X-rays detected by the X-ray detector 321.
- the data collection unit 430 outputs the obtained result as counting information.
- the total energy range, the first energy range classification, that is, the number of energy bins, and the energy range corresponding to each energy bin are set in advance according to instructions from the user or the like.
- the image generation unit 450 reconstructs an X-ray CT image from the number of X-ray photons (counting information) stored in each energy bin. For example, the image is reconstructed by performing Log transformation on the number of X-ray photons.
- Various known methods such as the FeldKamp method and the successive approximation method can be used for the reconstruction.
- the image generation unit 450 may perform various correction processes on the count information. Examples of the correction processing performed here include circuit linearity correction, logarithmic conversion processing, offset processing, sensitivity correction, and beam hardening correction.
- projection data stored in all energy bins may not be used for generating an image. Only projection data stored in an energy bin corresponding to a predetermined energy range may be used.
- the exposure dose estimation unit 440 obtains an estimated exposure dose of the subject 101 according to the shooting conditions set by the user.
- the exposure amount band unit exposure amount
- X-rays in each energy range (energy band) of the predetermined second energy range section are irradiated with a predetermined irradiation intensity (unit irradiation intensity).
- the exposure dose estimation unit 440 includes a spectrum (energy distribution) acquisition unit 441 and an estimated exposure dose calculation unit 442.
- a band-unit exposure dose DB 470 created in advance is used.
- the exposure dose estimation unit 440 of the present embodiment presents the calculated estimated exposure dose to the user.
- the presentation is performed, for example, by displaying the estimated exposure dose on a display device.
- the band unit exposure DB 470 holds the exposure amount per unit irradiation intensity in each energy range for each predetermined second energy range section as band unit exposure data.
- the second energy range classification is obtained by dividing the entire energy range of X-ray photons by a predetermined energy width ⁇ E.
- the energy width ⁇ E is, for example, 1 keV.
- the band-unit exposure dose DB 470 divides this total energy range into 140 energy ranges (energy bands) for each ⁇ E (1 keV), The exposure amount per unit irradiation intensity in the energy range is stored.
- the band unit exposure amount DB 470 includes the band unit exposure amounts D (E1), D (E2),... D for each energy range E1, E2,. (En),... D (EN) are stored.
- N is an integer greater than or equal to 1, for example, 140.
- N is an integer from 1 to N.
- the band unit exposure dose is obtained, for example, by irradiating X-rays of known energy from the X-ray tube 311 and actually measuring.
- the actual measurement is performed, for example, with an X-ray measuring instrument 601 inserted at a plurality of positions in the phantom 610 as shown in FIG.
- the phantom 610 is disposed at a position where the subject 101 is disposed.
- a CTDI (Computed Tomography Dose Index) value (unit mSv) is used as the band unit exposure amount.
- the CTDI value obtained by actual measurement is a discrete value at the energy point (E) of irradiated X-rays.
- this discrete value is used as the band unit exposure amount of each energy range.
- the CTDI value D (E) actually measured using the irradiation X-rays of energy E is set as a dose exposure unit in the band in the energy range of ⁇ ⁇ E / 2 with the energy point E as the center. .
- the CTDI value by X-rays of (139 to 140 ⁇ EkeV) is stored in the band unit exposure dose DB 470.
- the CTDI values of the respective energy ranges are ⁇ E / 2, ⁇ E + ⁇ E / 2,... (N ⁇ 1) ⁇ E + ⁇ E / 2, 139 ⁇ E + ⁇ E / 2. It is set as the CTDI value by X-rays of each energy.
- a radioactive ray source that emits monochromatic radiation or radiation of a predetermined energy is used.
- radioactive ray source when a radioactive ray source is used, only X-rays and ⁇ -rays having energy specific to the radioactive substance used can be obtained. For example, when Americium-241 ( 241 Am) is used, 59.5 keV gamma rays are generated. When iodine-125 ( 125 I) is used, 35 keV and 27 keV ⁇ rays are generated. Similarly, other radioactive radiation sources generate only ⁇ rays with a predetermined energy.
- the band unit exposure dose DB 470 interpolates the exposure doses calculated using a plurality of different radioactive sources to obtain the band unit exposure dose of each energy band, Is done.
- the bandwidth unit exposure DB 470 does not necessarily have to be created by actual measurement.
- a physical phenomenon related to the behavior of radiation may be treated as probabilistic, and the unit dose of each energy may be calculated using Monte Carlo simulation that tracks the physical process of radiation (particles) using random numbers. In this case, it is desirable to perform correction by comparing the simulation result with the actually measured value in the energy value that can be actually measured by the radioactive radiation source.
- the band unit exposure dose DB 470 is created in advance at a predetermined timing before photographing, such as when the apparatus is manufactured or installed. At this time, the estimated exposure dose can be obtained more accurately by reducing the energy range width ⁇ E.
- the exposure amount in each energy range may not be due to X-rays having a predetermined unit intensity. X-rays with different intensities may be used.
- the X-ray intensity used when calculating the exposure dose is also stored in the band unit exposure dose DB 470.
- the estimated exposure dose is calculated by taking into account the X-ray intensity at the time of data acquisition.
- the spectrum acquisition unit 441 uses the count information for each energy range section collected by the data collection unit 430 and the energy distribution (spectrum) of X-rays emitted from the X-ray tube 311 according to the imaging conditions set by the imaging condition setting unit 410. Get. At this time, the spectrum is obtained without arranging the subject 101 as shown in FIG.
- the spectrum acquisition unit 441 acquires a spectrum in a state where these filters used for actual photographing are installed.
- the spectrum acquisition unit 441 instructs the measurement control unit 420 to irradiate X-rays according to the imaging conditions without the subject 101, and acquires a spectrum.
- the spectrum acquired by the spectrum acquisition unit 441 is a discrete spectrum of the X-ray intensity incident on the X-ray detector 321 for each energy range of the first energy range section.
- each energy range width ⁇ B according to the first energy range section of the PCCT apparatus 100 is set to each of the second energy range sections that are intervals used in the band unit exposure amount DB 470 described later.
- the description will be made assuming that the energy range is the same as the width ⁇ E, and that each energy range is the same.
- the estimated exposure amount calculation unit 442 includes band unit exposure amount data that is exposure amount data per unit intensity for each energy range (energy band) of a predetermined second energy range section of X-rays, and a spectrum acquisition unit.
- the estimated exposure dose is calculated using the spectrum acquired by 441. That is, using the value of the band unit exposure DB 470 and the spectrum acquired by the spectrum acquisition unit 441, the exposure amount (estimated exposure amount) of the subject 101 when shooting is performed under the set shooting conditions.
- the estimated exposure dose EsD (E) at this energy value E is expressed by the following equation (1).
- EsD (E) D (E) ⁇ S (E) (1)
- the band-unit exposure dose D held in the band-unit exposure DB 470 and the values that can be taken by the spectrum S acquired by the spectrum acquisition unit 441 are both discrete values with an interval ⁇ E. Accordingly, the estimated dose calculation unit 442 actually calculates the estimated dose EsD all according to the following equation (3).
- S (i ⁇ E) is the spectrum of the energy band between (i-1) ⁇ ⁇ E and i ⁇ ⁇ E
- D (i ⁇ E) is the energy between (i-1) ⁇ ⁇ E and i ⁇ ⁇ E. This is the exposure amount of the band in band units.
- the total range is set from 1 to infinity for convenience.
- the upper limit of the generated photon energy is determined by the voltage value set by the X-ray source, and therefore it is sufficient to add within the range.
- the above equation (3) is expressed by the following equation (4).
- the estimated exposure amount calculation unit 442 of the present embodiment multiplies the band unit exposure amount of the energy range section and the X-ray intensity of the energy range section for each first energy range section. To calculate the estimated exposure dose of the energy range category. And the estimated exposure amount of the whole energy range is obtained by adding the estimated exposure amount of each energy range division.
- FIG. 7 is a processing flow of the photographing process of the present embodiment.
- the band unit exposure DB 470 is created in advance.
- the shooting condition setting unit 410 receives and sets shooting conditions from the user via the UI unit 200 (step S1101) (step S1102).
- Imaging conditions for accepting input include tube voltage, tube current, thickness and shape of the X-ray filter 312, and shape of the bow tie filter 313.
- the exposure dose estimation unit 440 calculates the estimated exposure dose under the accepted imaging conditions (step S1103). Then, the exposure dose estimation unit 440 presents the calculation result to the user (step S1104) and accepts an input indicating whether or not it is possible (step S1105). At this time, the exposure dose estimation unit 440 may be configured to further display not only the estimated exposure dose but also the spectrum as a calculation result.
- step S1105 upon receiving a possible instruction from the user, the measurement control unit 420 performs measurement in accordance with the shooting conditions set in step S1102 (step S1106), and the data collection unit 430 collects data.
- the image generation unit 450 generates an image from the data collected by the data collection unit 430 (step S1107), and ends the process.
- step S1105 if a non-permitted instruction is received from the user, the process returns to step S1101, and the shooting condition setting unit 410 receives a new shooting condition.
- the shooting condition setting unit 410 may automatically change the shooting condition without receiving an input of a new shooting condition from the user. In this case, the process returns to step S1102, the changed shooting condition is set, and the process is repeated.
- the impossibility instruction is usually given when the estimated exposure dose is large. Therefore, for example, the tube voltage may be automatically reduced. Alternatively, only an instruction from the user to increase or decrease the exposure amount may be received, and the tube voltage may be changed by a predetermined voltage accordingly.
- step S1105 the estimated exposure dose is presented to the user, and an acceptability instruction is accepted.
- the present invention is not limited to this.
- the shooting condition setting unit 410 automatically determines according to the estimated exposure dose calculated in step S1103, and changes the shooting condition as necessary. You may comprise.
- a threshold value for determining whether or not it is possible is held in advance.
- a parameter to be changed when it is determined as impossible and a change amount thereof are also held.
- the imaging condition setting unit 410 allows the process to proceed to step S1106 and execute measurement. On the other hand, if it is equal to or greater than the threshold, the imaging condition setting unit 410 subtracts the tube voltage from the current value by ⁇ V, and repeats the processing from step S1102.
- the exposure amount estimation unit 440 instructs the measurement control unit 420 and the data collection unit 430 to count X-rays detected by irradiating X-rays without the subject 101 under the imaging conditions at that time. Is obtained (step S1201).
- the spectrum acquisition unit 441 acquires a spectrum (energy value (X-ray intensity) of each energy range) based on the count information (step S1202).
- the estimated dose calculation unit 442 calculates the estimated dose for the entire energy range.
- the estimated exposure amount calculation unit 442 calculates an estimated exposure amount EsD (i ⁇ E) in the i-th energy range, that is, the energy range (band) between (i ⁇ 1) ⁇ ⁇ E and i ⁇ ⁇ E (Ste S1204). As described above, the calculation is performed by multiplying the band unit exposure amount D (i ⁇ E) held in the band unit exposure amount DB 470 of the energy range (band) by the spectrum S (i ⁇ E) of the energy range. .
- the estimated radiation amount calculation unit 442 the estimated exposure of ESD of the calculated i-th energy range (band) to (AiderutaE), is added to the estimated amount of exposure ESD all the entire energy range (step S1205).
- the estimated dose calculation unit 442 repeats the above processing until the counter i becomes larger than the total number N (steps S1206 and S1207). Then, the estimated exposure amount EsD all at the time when the counter i reaches N + 1 is set as the estimated exposure amount, and the process is terminated.
- the estimated exposure dose EsD (i ⁇ E) calculated for each energy range is added to the EsD all calculated so far to obtain the estimated exposure dose of the entire energy range.
- the estimated exposure doses may be summed up.
- the exposure dose estimation unit 440 may further include a correction unit 443 as shown in FIG.
- the correction unit 443 corrects the influence of scattered radiation on the estimated exposure amount calculated by the estimated exposure amount calculation unit 442.
- the correction unit 443 performs correction by subtracting the scattered dose from the energy value of each energy range of the first energy range section acquired by the spectrum acquisition unit 441.
- the scattered dose is estimated by, for example, calculating the scattered dose for each energy range segment incident on each detection element 322 by Monte Carlo simulation including the collimator 323 and the substrate on the back of the detection element 322.
- the correction unit 443 uses the Monte Carlo simulation to detect each detection element 322.
- the scattered dose incident on is calculated for each energy range.
- amendment part 443 each subtracts a scattered dose from a measurement X dose for every energy range, and obtains the dose after correction
- the estimated dose calculation unit 442 estimates the dose using the corrected dose. That is, the corrected dose EsD all is calculated using the corrected dose as S (i ⁇ E) in the above equation (3).
- an estimated exposure amount can be calculated more accurately.
- the PCCT apparatus 100 includes the X-ray irradiation unit 310 that irradiates X-rays, the photon counting X-ray detector 321 that detects the X-rays, and the X-ray detector 321.
- the X-ray photons derived from the X-rays detected in step 1 are counted for each energy range of the predetermined first energy range section, and the data collection unit 430 for obtaining count information for each energy range, and the imaging set by the user
- An exposure dose estimation unit 440 that obtains an estimated exposure dose of the subject 101 according to conditions, and the exposure dose estimation unit 440 distributes the energy distribution of X-rays emitted from the X-ray irradiation unit 310 according to the imaging conditions.
- a spectrum acquisition unit 441 that obtains the spectrum from the count information of each energy range of the first energy range section, and a predetermined number of X-rays
- An estimated exposure amount calculation unit 442 for calculating the estimated exposure amount using the band unit exposure amount data which is exposure amount data per unit intensity for each energy range of the energy range section and the spectrum. .
- the band unit exposure amount database 470 may be created by interpolating the exposure dose calculated using a plurality of radioactive sources having different energies.
- the said exposure amount estimation part 440 may further be provided with the correction
- the display device may further display the X-ray spectrum.
- the PCCT apparatus 100 can accurately estimate the exposure dose with a simple configuration regardless of the spectrum shape of the irradiated X-rays. Therefore, even if the shape of the irradiation spectrum is changed using a filter or the like, the exposure dose can be estimated with high accuracy according to the imaging conditions, and the accuracy of exposure dose management of the subject 101 is increased. Accordingly, the inspection can be executed efficiently.
- each energy range (energy range by 1st energy range division) of energy bin set to PCCT apparatus 100, and each energy range (energy by 2nd energy range division) of band unit exposure amount DB470. (Range) is described as matching.
- the data collection unit 430 sets energy bins for each unit band ⁇ E and counts X-ray photons, and the spectrum acquisition unit 441 matches each energy range of the band unit exposure DB 470. For each energy range, an X-ray intensity is obtained and a spectrum is obtained.
- the energy bin bandwidth (energy range width) ⁇ B is different from each energy range width ⁇ E of the band unit exposure dose DB 470.
- a processing method in such a case will be described.
- the energy range width of the band unit exposure dose DB 470 is 1 keV.
- the bandwidth of the energy bin of the PCCT apparatus 100 is 1 keV, the amount of data becomes enormous.
- the maximum energy of the X-ray tube 311 is 120 keV, 120 energy bins are required, and the spectrum acquisition unit 441 discriminates X-ray photons into 120 energy bands. The same applies to the subsequent measurement processing.
- the energy bandwidth ⁇ B is set larger than the energy range width ⁇ E of the band unit exposure amount DB 470 ( ⁇ B > ⁇ E).
- FIG. 9 illustrates a case where ⁇ B is 10 times ⁇ E.
- the estimated dose calculation unit 442 performs multiplication after matching the widths of the two.
- the estimated exposure amount calculation unit 442 converts either one of the count information and the band unit exposure amount data into a value acquired in the other energy range section, and calculates the estimated exposure amount. .
- a conversion method As a conversion method, a method (first method) of converting each band unit exposure amount of the band unit exposure amount DB 470 into a value of each energy range of the energy bin, and an energy range of the energy bin acquired by the spectrum acquisition unit 441 There is a method (second method) in which the X-ray intensity for each is converted into the value of each energy range of the band unit exposure dose DB 470.
- the average value of the band unit exposure amount for each energy range of the first energy range section in the band unit exposure amount DB 470 is calculated, and the band unit exposure of each energy range of the first energy range section is calculated. Amount.
- the band unit exposure amount DB 470 includes 0 to 1 keV, 1 to 2 keV, 2 to 3 keV,..., 9 to 10 keV, 10 to 11 keV, The dose unit D of the band unit in each energy range is held. Further, when the energy range width ⁇ B of the PCCT apparatus 100 is 10 keV and the maximum tube voltage is 120 keV, the spectrum acquisition unit 441 has each energy range of 0 to 10 keV, 10 to 20 keV, 20 to 30 keV,..., 110 to 120 keV. X-ray intensity is acquired.
- the estimated exposure dose calculation unit 442 extracts 10 band unit exposure doses of 0 to 1 keV, 1 to 2 keV, 2 to 3 keV,..., 9 to 10 keV from the band unit exposure dose DB 470, and averages these values. Is calculated as the band unit exposure amount in the energy range of 0 to 10 keV. For other energy ranges, the same calculation is performed to obtain the band unit exposure amount of each energy range by the first energy range section.
- the estimated dose calculation unit 442 interpolates the X-ray intensity of a finer energy range from the X-ray intensity of each energy range acquired by the spectrum acquisition unit 441 by the first energy range section by interpolation. obtain. First, an intermediate energy value of each energy range of the first energy range section is determined as an energy range value by the second energy range section. And using these, the value of the energy range of another 2nd energy range division is calculated by interpolation.
- the energy range width ⁇ B by the first energy range section is 10 keV
- the energy range width ⁇ E by the second energy range section is 1 keV.
- the estimated dose calculation unit 442 converts the X-ray intensity of each energy range width acquired by the spectrum acquisition unit 441 into an X-ray intensity of 1/10 energy range width.
- the X-ray intensity in the energy range of 1 keV in the vicinity of the intermediate value of 5 keV is set to 1/10 of the initial value.
- the X-ray intensity in the energy range of 1 keV in the vicinity of 15 keV is set to 1/10 of the initial value
- the X-ray intensity in the energy range of 1 keV in the vicinity of 25 keV, 1/10 of the initial value is obtained from the obtained X-ray intensities of 5 keV, 15 keV, 25 keV,....
- the X-ray intensity of each energy range in 1 keV increments of the entire range is obtained by interpolation.
- the X-ray intensity by X-ray photons in that state is 0.
- the maximum tube voltage for example, 120 kV
- X-rays exceeding this tube voltage are not generated. Therefore, the X-ray intensity of the X-ray photon at the maximum tube voltage is also set to zero.
- the X-ray intensity of each energy range is obtained by interpolation from these boundary values and each X-ray intensity of 5 keV, 15 keV,.
- the interpolation is performed by, for example, linear interpolation or interpolation using a spline function.
- the exposure amount can be estimated with high accuracy. Even in the case where the PCCT apparatus 100 cannot count in units of energy ranges equivalent to the band unit exposure amount, the exposure amount can be estimated with high accuracy regardless of the spectrum shape.
- the transfer data amount can be reduced when obtaining the spectrum by setting it wide.
- it is configured to obtain a spectrum that matches the energy range of the band unit exposure DB 470 by changing the energy range to be measured for each measurement, instead of converting after acquiring the spectrum. May be.
- the number of energy bins is 12.
- the first measurement measurement is made in the energy range of 0 to 12 keV, and in each energy bin, the energy band of 0 to 1 keV, 1 to 2 keV, ... 11 to 12 keV, respectively.
- X-ray photons are counted.
- the second measurement measurement is made in the energy range of 12 to 24 keV, and X-ray photons in the energy band of 12 to 13 keV, 13 to 14 keV,..., 23 to 24 keV are counted in each energy bin, respectively.
- measurement in the energy range of 0 to 120 keV is realized.
- the measurement control unit 420 controls the measurement in this way, so that a finer energy bandwidth can be measured with the same number of energy bins. Therefore, even with a PCCT apparatus having a small number of energy bins, it is possible to obtain an X-ray intensity for each energy range equivalent to the band-unit exposure dose DB 470, and the estimated exposure dose calculation unit 442 calculates a highly accurate estimated exposure dose. It can be calculated.
- the exposure dose is confirmed using only the estimated exposure dose.
- an image acquired under the shooting conditions may be presented to the user as reference data and asked to make a determination.
- the calculation unit 400 further includes an image database (image DB) 490 that holds images acquired in association with shooting conditions, as shown in FIG.
- image DB 490 is constructed in the HDD device 403.
- the image DB 490 is created by storing image data acquired in association with the shooting conditions for acquiring the image every time the image is acquired.
- the image DB 490 stores image data that can specify the image quality in association with the shooting conditions that affect the image quality among the shooting conditions.
- imaging conditions that affect image quality include, for example, tube voltage, tube current, the shape of the X-ray filter 312, the shape of the bow tie filter 313, and the like.
- the image data held in the image DB 490 may be further associated with the physique information of the subject 101.
- the physique information of the subject 101 includes, for example, height, weight, waist circumference, chest circumference, and the like. Further, when an image is already held under the same shooting condition at the time of storage, the image may be updated to the latest one.
- the exposure dose estimation unit 440 presents the image held in the image database according to the set shooting condition to the user together with the estimated exposure dose in step S1104.
- the presented image data is image data held in the image DB 490 in association with the shooting conditions at that time.
- FIG. 10B shows a screen example 710 displayed. As shown in this figure, in this case, an estimated exposure dose 711 and image data 712 are presented to the user. As described above, a spectrum may be further displayed.
- step S1105 the user instructs to perform photographing as possible in step S1105, and otherwise instructs that it is not possible. In this case, the process returns to step S1101 to change the shooting conditions.
- the estimated exposure dose and the image quality image are simultaneously presented to the user. Therefore, the user can grasp these simultaneously. Therefore, the user can grasp the diagnostic ability of the obtained image and can prevent invalid exposure due to insufficient dose.
- the calculation unit 400 may further include an estimated dose database (estimated dose DB) 480 that holds estimated doses calculated in association with imaging conditions.
- the exposure dose estimation unit 440 refers to the estimated exposure dose DB 480 prior to acquisition of the X-ray spectrum, and if the estimated exposure dose is held in association with the set imaging conditions, the exposure dose is stored. The estimated exposure dose is obtained as the estimated exposure dose of the subject.
- the exposure dose estimation unit 440 does not calculate the estimated exposure dose but estimates the estimated exposure dose. Acquired from the quantity DB 480.
- this estimated dose DB 480 stores the estimated dose in association with the imaging conditions.
- the estimated exposure dose DB 480 holds the calculated estimated exposure dose in association with the imaging conditions set when the estimated exposure dose is calculated. By creating.
- the estimated dose DB 480 is built in the HDD device 403.
- the exposure amount estimation unit 440 first determines whether or not the imaging condition that matches the set imaging condition is stored in the estimated exposure DB 480 before acquiring the spectrum. . And when it memorize
- the calculation unit 400 has been described as being included in the PCCT apparatus 100, but is not limited to this configuration.
- the PCCT apparatus 100 may be constructed on an information processing apparatus that can transmit and receive data and is independent of the PCCT apparatus 100.
- the UI unit 200 may have an independent configuration capable of transmitting / receiving information to / from the PCCT apparatus 100.
- the UI unit 200 and the calculation unit 400 may be realized by a single information processing apparatus.
- FFS Fluorescence focal spot imaging
- the focal position moving method of the X-ray tube 311 is determined according to the resolution of the subject 101 and set as imaging conditions.
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Abstract
Description
本実施形態では、X線CT装置として、従来の積分型(電流モード計測方式)の検出器ではなく、フォトンカウンティング方式の検出器を有するフォトンカウンティングCT装置(PCCT装置)を用いる。PCCT装置では、被写体を透過したX線に由来する光子(X線フォトン)を、検出器で計数する。
計測部300は、演算部400による制御に従って、被写体101にX線を照射し、被写体101を透過したX線フォトンを計測する。計測部300は、X線照射部310と、X線検出部320と、ガントリ(Gantry:構台)330と、制御部340と、被写体101を載置するテーブル102と、を備える。
ガントリ330の中央には、被写体101と、被写体101を載置するテーブル102とを配置するための円形の開口部331が設けられる。ガントリ330の内部には、後述するX線管311およびX線検出器321を搭載する回転板332と回転板332を回転させるための駆動機構が配置される。
X線照射部310は、X線を発生し、発生したX線を被写体101に照射する。X線照射部310は、X線管311と、X線フィルタ312と、ボウタイ(bowtie)フィルタ313と、を備える。
X線検出部320は、X線フォトンが入射する毎に、当該X線フォトンのエネルギー値を計測可能な信号を出力する。X線検出部320は、X線検出器321を備える。
制御部340は、X線管311からのX線の照射を制御する照射制御器341、回転板332の回転駆動を制御するガントリ制御器342、X線検出器321におけるX線検出を制御する検出制御器343、テーブル102の駆動を制御するテーブル制御器344、を備える。これらは、後述する演算部400の計測制御部420による制御に従って動作する。
演算部400は、PCCT装置100の動作全体を制御し、計測部300で取得したデータを処理することにより、撮影を行う。本実施形態の演算部400は、図3に示すように、撮影条件設定部410と、計測制御部420と、データ収集部430と、被ばく量推定部440と、画像生成部450と、帯域単位被ばく量データベース(DB)470と、を備える。
撮影条件設定部410は、ユーザから撮影条件を受け付けて設定する。例えば、撮影条件設定部410は、撮影条件を受け付ける受付画面を表示装置に表示し、受付画面を介して撮影条件を受け付ける。ユーザは、受付画面を介して、例えば、マウス、キーボード、タッチパネルを操作することにより、撮影条件を入力する。
計測制御部420は、ユーザが設定した撮影条件に従って、制御部340を制御し、計測を実行する。
データ収集部430は、X線検出器321が検出したX線に由来するフォトン(X線フォトン)を、予め定めた第一のエネルギー範囲区分のエネルギー範囲毎に計数し、当該エネルギー範囲毎の計数情報を得る。本実施形態のデータ収集部430は、データ収集システム(DAS:Data Acquisition System、以下DASと表記)を備え、このDASが、計測部300が検出したX線フォトンの計数を行う。
画像生成部450は、各エネルギービンに保存されたX線フォトン数(計数情報)から、X線CT画像を再構成する。画像は、例えば、X線フォトン数に対し、Log変換を行い、再構成する。再構成には、FeldKamp法、逐次近似法など、各種の公知の手法を用いることができる。
被ばく量推定部440は、ユーザが設定した撮影条件に応じて、被写体101の推定被ばく量を得る。本実施形態では、予め定めた第二のエネルギー範囲区分の各エネルギー範囲(エネルギー帯域)のX線を、予め定めた照射強度(単位照射強度)で照射した際の、被ばく量(帯域単位被ばく量)を用い、撮影条件で設定された照射X線による、被写体101の被ばく量(推定被ばく量)を推定する。
帯域単位被ばく量DB470は、予め定めた第二のエネルギー範囲区分毎の各エネルギー範囲の、単位照射強度あたりの被ばく量を帯域単位被ばく量データとして保持する。
スペクトル取得部441は、データ収集部430が収集したエネルギー範囲区分毎の計数情報から、撮影条件設定部410が設定した撮影条件に従って、X線管311から照射されたX線のエネルギー分布(スペクトル)を得る。このとき、スペクトルは、図6(b)に示すように、被写体101を配置せずに得る。
推定被ばく量算出部442は、X線の、予め定めた第二のエネルギー範囲区分のエネルギー範囲(エネルギー帯域)毎の、単位強度あたりの被ばく量データである帯域単位被ばく量データと、スペクトル取得部441が取得したスペクトルとを用いて、推定被ばく量を算出する。すなわち、帯域単位被ばく量DB470の値と、スペクトル取得部441が取得したスペクトルとを用いて、設定された撮影条件で撮影した場合の、被写体101の被ばく量(推定被ばく量)を算出する。
EsD(E)=D(E)×S(E) ・・・(1)
推定被ばく量算出部442が算出する推定被ばく量EsDallは、EsD(E)を、全エネルギー範囲について積算したものである。従って、以下の式(2)で表される。
次に、演算部400による本実施形態の撮影処理の流れについて説明する。図7は、本実施形態の撮影処理の処理フローである。なお、帯域単位被ばく量DB470は、予め作成されているものとする。
次に、ステップS1103の推定被ばく量算出処理の流れを図8に従って説明する。ここでは、エネルギー範囲の幅をΔE、エネルギー範囲数(区分数)をNとする。
なお、被ばく量推定部440は、図3に示すように、さらに、補正部443を備えてもよい。この補正部443は、推定被ばく量算出部442が算出した推定被ばく量における散乱線の影響を補正する。本実施形態では、補正部443は、スペクトル取得部441が取得した、第一のエネルギー範囲区分の各エネルギー範囲のエネルギー値から、散乱線量を減算することにより補正する。
そして、前記帯域単位被ばく量データベース470は、複数の異なるエネルギーの放射性線源を用いて算出した被ばく量を補間することにより作成されてもよい。
また、前記被ばく量推定部440は、前記算出した推定被ばく量における散乱線の影響を補正する補正部443をさらに備えてもよい。
算出した前記推定被ばく量を表示する表示装置をさらに備えてもよい。そして、前記表示装置には、前記X線のスペクトルがさらに表示されてもよい。
上記実施形態では、PCCT装置100に設定されるエネルギービンの各エネルギー範囲(第一のエネルギー範囲区分によるエネルギー範囲)と、帯域単位被ばく量DB470の、各エネルギー範囲(第二のエネルギー範囲区分によるエネルギー範囲)とは一致しているものとして説明している。
上記実施形態では、被ばく量の確認は、推定被ばく量のみを用いて行う。しかしながら、これに限定されない。例えば、その撮影条件で取得される画像も、参考データとしてユーザに合わせて提示し、判断を仰ぐよう構成してもよい。
画像DB490は、画像を取得する毎に、当該画像を取得した撮影条件に対応づけて取得した画像データを格納することにより作成される。本変形例では、図10(a)に例示するように、画像DB490には、撮影条件のうち、画質に影響のある撮影条件に対応づけて、画質を特定可能な画像データが保持される。
また、演算部400は、撮影条件に対応づけて算出した推定被ばく量を保持する推定被ばく量データベース(推定被ばく量DB)480をさらに備えてもよい。この場合、被ばく量推定部440は、X線のスペクトルの取得に先立ち、推定被ばく量DB480を参照し、設定された撮影条件に対応づけて推定被ばく量が保持されている場合、当該保持されている推定被ばく量を、被写体の推定被ばく量として得る。
Claims (10)
- X線を照射するX線照射部と、
前記X線を検出するフォトンカウンティング方式のX線検出器と、
前記X線検出器で検出したX線に由来するX線フォトンを、予め定めた第一のエネルギー範囲区分のエネルギー範囲毎に計数し、当該エネルギー範囲毎の計数情報を得るデータ収集部と、
ユーザが設定した撮影条件に応じて、被写体の推定被ばく量を得る被ばく量推定部と、を備え、
前記被ばく量推定部は、
前記撮影条件に従って前記X線照射部から照射されたX線のエネルギー分布であるスペクトルを、前記第一のエネルギー範囲区分の各エネルギー範囲の計数情報から得るスペクトル取得部と、
X線の、予め定めた第二のエネルギー範囲区分のエネルギー範囲毎の、単位強度あたりの被ばく量データである帯域単位被ばく量データと、前記スペクトルとを用いて、前記推定被ばく量を算出する推定被ばく量算出部と、を備えること
を特徴とするフォトンカウンティングCT装置。 - 請求項1記載のフォトンカウンティングCT装置であって、
前記第二のエネルギー範囲区分のエネルギー範囲毎の、前記帯域単位被ばく量データを保持する帯域単位被ばく量データベースを備えること
を特徴とするフォトンカウンティングCT装置。 - 請求項1記載のフォトンカウンティングCT装置であって、
前記被ばく量推定部は、前記算出した推定被ばく量における散乱線の影響を補正する補正部をさらに備えること
を特徴とするフォトンカウンティングCT装置。 - 請求項2記載のフォトンカウンティングCT装置であって、
前記帯域単位被ばく量データベースは、複数の異なるエネルギーの放射性線源を用いて算出した被ばく量を補間することにより作成されること
を特徴とするフォトンカウンティングCT装置。 - 請求項1記載のフォトンカウンティングCT装置であって、
算出した前記推定被ばく量を撮影条件に対応づけて保持する推定被ばく量データベースをさらに備え、
前記被ばく量推定部は、前記スペクトルの取得に先立ち、前記推定被ばく量データベースを参照し、設定された前記撮影条件に対応づけて前記推定被ばく量が保持されている場合、当該保持されている推定被ばく量を前記被写体の推定被ばく量として得ること
を特徴とするフォトンカウンティングCT装置。 - 請求項1記載のフォトンカウンティングCT装置であって、
前記推定被ばく量算出部は、前記計数情報および前記帯域単位被ばく量データのいずれか一方を他方のエネルギー範囲区分で取得した値に換算し、前記推定被ばく量を算出すること
を特徴とするフォトンカウンティングCT装置。 - 請求項1記載のフォトンカウンティングCT装置であって、
算出した前記推定被ばく量を表示する表示装置をさらに備えること
を特徴とするフォトンカウンティングCT装置。 - 請求項7記載のフォトンカウンティングCT装置であって、
前記表示装置には、前記スペクトルがさらに表示されること
を特徴とするフォトンカウンティングCT装置。 - 請求項7記載のフォトンカウンティングCT装置であって、
撮影条件毎に取得した画像を保持する画像データベースをさらに備え、
前記表示装置には、設定された前記撮影条件に応じて前記画像データベースに保持される画像をさらに表示すること
を特徴とするフォトンカウンティングCT装置。 - ユーザが設定した撮影条件に従って照射されるX線を検出し、
前記検出したX線に由来するX線フォトンを、予め定めた第一のエネルギー範囲区分のエネルギー範囲毎に計数し、当該エネルギー範囲毎の計数情報を得、
前記計数情報から前記X線のエネルギー分布であるスペクトルを得、
X線の、予め定めた第二のエネルギー範囲区分のエネルギー範囲毎の、単位強度あたりの被ばく量データである帯域単位被ばく量データと、前記スペクトルとを用いて、前記撮影条件における推定被ばく量を算出すること
を特徴とするフォトンカウンティングCT装置における推定被ばく量算出方法。
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