JP7102190B2 - X-ray CT device and calibration method - Google Patents

Description

Embodiments of the present invention relate to X-ray CT devices, calibration methods, and phantoms.

In an X-ray CT (Computed Tomography) apparatus, a technique called Photon Counting CT, which performs imaging by counting the number of X-ray photons (photons), is known. In the photon counting CT, a photon counting type (photon counting type) X-ray detector is used. The photon counting type X-ray detector measures the intensity of X-rays by counting the number of incident X-ray photons. Further, the photon counting type X-ray detector utilizes the fact that when converting an X-ray photon into an electric charge, an amount of electric charge corresponding to the energy of the X-ray photon is generated, and each X-ray photon is used. Measure energy. Therefore, in the photon counting CT, it is possible to obtain an energy spectrum showing the distribution of the number of photons according to the energy.

In photon counting CT, it is possible to discriminate and reconstruct the basic substance components contained in the imaging region (substance discrimination reconstruction). The material discrimination reconstruction utilizes the fact that the amount of attenuation of the energy spectrum is different for each substance. In the substance discrimination reconstruction, it is possible to identify a substance existing in the X-ray pathway and calculate the concentration and permeation length of the substance.

Correction (calibration) data is generated in advance in order to calibrate (calibrate) the concentration and transmission length of the substance obtained by photon counting CT. In order to generate this correction data, it is necessary to collect projection data (energy spectrum) under various conditions in which the composition (chemical composition), concentration, and transmission length of the substance are known, which is troublesome. Further, in the photon counting type detector, the energy spectrum changes according to the difference in the X-ray dose incident on the detector. In order to calibrate in consideration of this change, it is necessary to collect the projection data by changing the X-ray dose conditions, which is more time-consuming.

Japanese Unexamined Patent Publication No. 6-94648 Japanese Unexamined Patent Publication No. 2000-298105

The problem to be solved by the present invention is X-ray C, which can easily generate correction data.
To provide a T-device, a calibration method, and a phantom.

The X-ray CT apparatus according to the embodiment includes an acquisition unit and a generation unit. The acquisition unit is a cylindrical phantom having a plurality of concentric layers, and a phantom having a different composition of a substance and a concentration of the substance contained in each of the plurality of layers is provided at a plurality of rotation angles. By scanning with, the first energy spectrum at each rotation angle is obtained. The generation unit corrects the third energy spectrum obtained from the subject based on the first energy spectrum and the second energy spectrum which is the theoretical energy spectrum obtained from the phantom under preset conditions. Generate correction data for.

FIG. 1 is a block diagram showing a configuration example of the X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram for explaining a configuration example of the cylindrical phantom 100 according to the first embodiment. FIG. 3 is a diagram for explaining an example of the internal structure of the cylindrical phantom 100 according to the first embodiment. FIG. 4 is a diagram for explaining the relationship between the rotation angle and the transmission length in the cylindrical phantom 100 according to the first embodiment. FIG. 5 is a diagram for explaining the transmission length of the cylindrical phantom 100 according to the first embodiment. FIG. 6 is a diagram for explaining a correction data generation process using the cylindrical phantom 100 according to the first embodiment. FIG. 7 is a diagram for explaining an imaging process by the X-ray CT apparatus 1 according to the first embodiment. FIG. 8 is a diagram for explaining a comparative example. FIG. 9 is a diagram showing a configuration example of the cylindrical phantom 200 according to the second embodiment.

Hereinafter, the X-ray CT (Computed Tomography) apparatus, the calibration method, and the phantom according to the embodiment will be described with reference to the drawings. The embodiments described below are merely examples, and are not limited to the contents described in the embodiments. Further, in principle, the contents described in one embodiment are similarly applied to other embodiments.

The X-ray CT apparatus described in the following embodiment is an apparatus capable of performing photon counting CT. That is, the X-ray CT apparatus described in the following embodiment is an apparatus capable of reconstructing X-ray CT image data by counting the X-rays that have passed through the subject using a photon counting type detector. .. The configuration of the X-ray CT apparatus according to the following embodiment is particularly effective in photon counting CT, but when a conventional integral type (current mode measurement method) detector or dual energy CT is performed. It is also applicable to.

(First Embodiment)
FIG. 1 is a block diagram showing a configuration example of the X-ray CT apparatus according to the first embodiment. As shown in FIG. 1, the X-ray CT device 1 according to the first embodiment includes a pedestal device 10, a bed device 30, and a console device 40. In FIG. 1, for convenience of illustration, the gantry device 10 is depicted at two locations, but typically, one X-ray CT device 1 is provided with one gantry device 10.

In the present embodiment, the rotation axis of the rotation frame 13 in the non-tilt state or the longitudinal direction of the top plate 33 of the bed device 30 is defined as the Z-axis direction. Further, the axial direction orthogonal to the Z-axis direction and horizontal to the floor surface is defined as the X-axis direction. Further, the axial direction orthogonal to the Z-axis direction and perpendicular to the floor surface is defined as the Y-axis direction.

The gantry device 10 includes an X-ray tube 11, an X-ray detector 12, a rotating frame 13, an X-ray high voltage device 14, a control device 15, a wedge 16, a collimator 17, and a DAS (Data Acquisition System). Has 18 and.

The X-ray tube 11 is a vacuum tube that generates X-rays by irradiating thermoelectrons from the cathode (filament) toward the anode (target) by applying a high voltage from the X-ray high voltage device 13. For example, the X-ray tube 11 includes a rotating anode type X-ray tube that generates X-rays by irradiating a rotating anode with thermoelectrons.

The wedge 16 is a filter for adjusting the X-ray dose emitted from the X-ray tube 11. Specifically, the wedge 16 transmits and attenuates the X-rays emitted from the X-ray tube 11 so that the X-rays emitted from the X-ray tube 11 to the subject P have a predetermined distribution. It is a filter to do. For example, the wedge 16 is a filter made of aluminum so as to have a predetermined target angle and a predetermined thickness. The wedge 16 is sometimes called a wedge filter or a bow-tie filter.

The collimator 17 is a lead plate or the like for narrowing the irradiation range of X-rays transmitted through the wedge 16, and a slit is formed by a combination of a plurality of lead plates or the like. The collimator 17 may be called an X-ray diaphragm.

The X-ray detector 12 detects the X-rays emitted from the X-ray generator 11 and passed through the subject P, and outputs an electric signal corresponding to the X-ray dose to the DAS 18. The X-ray detector 12 has, for example, a plurality of X-ray detection element trains in which a plurality of X-ray detection elements are arranged in the channel direction along one arc centered on the focal point of the X-ray tube. The X-ray detector 12 has, for example, a structure in which a plurality of X-ray detection element sequences in which a plurality of X-ray detection elements are arranged in the channel direction are arranged in a slice direction (column direction, low direction).

Further, the X-ray detector 12 is an indirect conversion type detector having, for example, a grid, a scintillator array, and an optical sensor array. The scintillator lake array has a plurality of scintillators, and the scintillator has a scintillator crystal that outputs a photon amount of light according to an incident X dose. The grid is arranged on the surface of the scintillator array on the X-ray incident side, and has an X-ray shielding plate having a function of absorbing scattered X-rays. The grid may also be called a collimator (one-dimensional collimator or two-dimensional collimator). The optical sensor array has a function of converting into an electric signal according to the amount of light from the scintillator, and has, for example, an optical sensor such as a photomultiplier tube (PMT). The X-ray detector 12 may be a direct conversion type detector having a semiconductor element that converts incident X-rays into an electric signal. The X-ray detector 12 is an example of an X-ray detector.

Here, the X-ray detector 12 according to the first embodiment is a photon counting type detector capable of performing photon counting CT. In the photon counting type detector, each X-ray detection element outputs one pulse of an electric signal each time an X-ray photon is incident. In the photon counting CT, the number of X-ray photons incident on the detection element can be counted by discriminating the individual pulses output by the X-ray detection element. Further, in the photon counting CT, the energy value of the counted X-ray photon can be measured by performing an arithmetic process based on the pulse intensity.

The X-ray high-voltage device 14 has an electric circuit such as a transformer and a rectifier, and has a function of generating a high voltage applied to the X-ray tube 11. It has an X-ray control device that controls an output voltage according to the X-ray. The high voltage generator may be a transformer type or an inverter type. The X-ray high-voltage device 14 may be provided on the rotating frame 13 described later, or may be provided on the fixed frame (not shown) side of the gantry device 10. The fixed frame is a frame that rotatably supports the rotating frame 13. The X-ray high-voltage device 14 is an example of an X-ray high-voltage unit.

The DAS 18 has an amplifier that amplifies the electric signal output from each X-ray detection element of the X-ray detector 12, and an A / D converter that converts the electric signal into a digital signal, and has detection data. To generate. The detection data generated by the DAS 18 is transferred to the console device 40. DAS18 is an example of a data collection unit.

Further, in the photon counting CT, the DAS 18 counts the number of X-ray photons incident on each X-ray detection element using the signal output from the X-ray detector 12. For example, the DAS 18 calculates the energy value from the peak value of each pulse output from each X-ray detection element and the system-specific response function. Alternatively, the DAS 18 calculates the energy value by integrating the pulse intensities. Then, the DAS 18 distributes the calculated energy value to a plurality of energy discrimination regions (energy bins) using a comparator. Then, the DAS 18 counts the number of pulses distributed to each energy discrimination region as the number of X-ray photons. The count values distributed to the plurality of energy discrimination regions are energy spectra showing the distribution of the number of photons according to the energy. That is, the detection data generated by the DAS 18 at a certain tube position (rotation angle) is surface data including an energy spectrum corresponding to the position of each X-ray detection element. The DAS 18 generates detection data for the entire circumference (360 degrees) when X-rays are continuously exposed from the X-ray tube 11 during the rotation of the rotating frame 13.

The rotating frame 13 is an annular frame that supports the X-ray tube 11 and the X-ray detector 12 so as to face each other and rotates the X-ray tube 11 and the X-ray detector 12 by a control device 15 described later. The rotating frame 13 is further provided with an X-ray high voltage device 14 and a DAS 18 in addition to the X-ray tube 11 and the X-ray detector 12 to support the rotating frame 13. The detection data generated by DAS18 is a non-rotating portion (for example, a fixed frame. Not shown in FIG. 1) of the gantry device by optical communication from a transmitter having a light emitting diode (LED) provided in the rotating frame. It is transmitted to a receiver having a photodiode provided in.) And transferred to the console device 40. The method of transmitting the detection data from the rotating frame to the non-rotating portion of the gantry device is not limited to the above-mentioned optical communication, and any method may be adopted as long as it is a non-contact type data transmission. The rotating frame 13 is an example of a rotating unit.

The control device 15 has a processing circuit having a CPU and the like, and a drive mechanism such as a motor and an actuator. The control device 15 has a function of receiving an input signal from an input interface 43, which will be described later, attached to the console device 40 or the gantry device 10 to control the operation of the gantry device 10 and the bed device 30. For example, the control device 15 controls to rotate the rotating frame 13 in response to an input signal, controls to tilt the gantry device 10, and controls to operate the bed device 30 and the top plate 33. The control for tilting the gantry device 10 is based on the tilt angle (tilt angle) information input by the input interface attached to the gantry device 10, and the control device 15 rotates around an axis parallel to the X-axis direction. It is realized by rotating. The control device 15 may be provided in the gantry device 10 or in the console device 40. The control device 15 is an example of a control unit.

The bed device 30 is a device for placing and moving the subject P to be scanned, and includes a base 31, a bed drive device 32, a top plate 33, and a support frame 34. The base 31 is a housing that supports the support frame 34 so as to be movable in the vertical direction. The bed driving device 32 is a motor or an actuator that moves the top plate 33 on which the subject P is placed in the long axis direction of the top plate 33. The top plate 33 provided on the upper surface of the support frame 34 is a plate on which the subject P is placed. In addition to the top plate 33, the bed drive device 32 may move the support frame 34 in the long axis direction of the top plate 33.

The console device 40 includes a memory 41, a display 42, an input interface 43, and a processing circuit 44. Although the console device 40 will be described as a separate body from the gantry device 10, the gantry device 10 may include a part of each component of the console device 40 or the console device 40.

The memory 41 is realized by, for example, a RAM (Random Access Memory), a semiconductor memory element such as a flash memory, a hard disk, an optical disk, or the like. The memory 41 stores, for example, projection data and reconstructed image data. The memory 41 is an example of a storage unit.

The display 42 displays various information. For example, the display 42 outputs a medical image (CT image) generated by the processing circuit 44, a GUI (Graphical User Interface) for receiving various operations from the operator, and the like. For example, the display 42 is a liquid crystal display or a CRT (Cathode Ray Tube) display. Further, the display 42 may be provided on the gantry device 10. Further, the display 42 may be a desktop type, or may be composed of a tablet terminal or the like capable of wireless communication with the console device 40 main body. The display 42 is an example of a display unit.

The input interface 43 receives various input operations from the operator, converts the received input operations into electric signals, and outputs the received input operations to the processing circuit 44. For example, the input interface 43 receives from the operator collection conditions for collecting projection data, reconstruction conditions for reconstructing a CT image, image processing conditions for generating a post-processed image from a CT image, and the like. .. For example, the input interface 43 includes a mouse, a keyboard, a trackball, a switch, and a button. It is realized by a joystick or the like. Further, the input interface 43 may be provided in the gantry device 10. Further, the input interface 43 may be composed of a tablet terminal or the like capable of wireless communication with the console device 40 main body. The input interface 43 is an example of an input unit.

The processing circuit 44 controls the operation of the entire X-ray CT device 1. For example, the processing circuit 44 executes the system control function 441, the preprocessing function 442, the reconstruction processing function 443, the image processing function 444, the acquisition function 445, and the generation function 446. The processing circuit 44 is an example of a processing unit. The acquisition function 445 and the generation function 446 will be described in detail later.

The system control function 441 controls various functions of the processing circuit 44 based on the input operation received from the operator via the input interface 43. For example, the system control function 441 controls the data collection process in the gantry device 10 by controlling the operation of the gantry device 10. Further, the system control function 441 controls the operation of the gantry device 10 so that the data collection process is executed under the imaging conditions specified by the operator. The system control function 441 is an example of a control unit.

The preprocessing function 442 generates data that has undergone preprocessing such as logarithmic conversion processing, offset correction processing, sensitivity correction processing between channels, and beam hardening correction on the detection data output from the data acquisition circuit 14. The data before preprocessing (detection data) and the data after preprocessing may be collectively referred to as projection data. The preprocessing function 442 is an example of a preprocessing unit.

The reconstruction processing function 443 generates CT image data by performing reconstruction processing using a filter correction back projection method, a successive approximation reconstruction method, or the like on the projection data generated by the preprocessing function 442. Further, the reconstruction processing function 443 is an example of the reconstruction unit.

The detection data obtained by photon counting CT includes information on the energy spectrum of X-rays attenuated by passing through the subject P. Therefore, the reconstruction processing function 443 can, for example, reconstruct the X-ray CT image data (substance discrimination image data) that images the energy components of each substance (substance discrimination reconstruction). In addition, the reconstruction processing function 443 can also reconstruct monochromatic X-ray image data, density image data, effective atomic number image data, and the like.

The image processing function 444 uses a known method to obtain CT image data of an arbitrary cross section or three-dimensional image data generated by the reconstruction processing function 443 based on an input operation received from the operator via the input interface 43. Convert to image data. The reconstruction processing function 443 may directly generate the three-dimensional image data. The image processing function 444 is an example of an image processing unit.

The overall configuration of the X-ray CT device 1 according to the first embodiment has been described above. Under such a configuration, the X-ray CT apparatus 1 according to the first embodiment executes each processing function described below in order to easily generate correction (calibration) data.

That is, in the X-ray CT apparatus 1, the acquisition function 445 is a cylindrical phantom having a plurality of concentric layers, and at least one of the composition of the substance and the concentration of the substance contained in each of the plurality of layers is different. By scanning the phantom at each of a plurality of rotation angles, the first energy spectrum at each rotation angle is acquired. Then, the generation function 446 corrects the third energy spectrum obtained from the subject based on the first energy spectrum and the second energy spectrum which is the theoretical energy spectrum obtained from the phantom under preset conditions. Generate correction data for this purpose. The acquisition function 445 is an example of an acquisition unit. The generation function 446 is an example of a generation unit.

First, a configuration example of the cylindrical phantom 100 according to the first embodiment will be described with reference to FIG. FIG. 2 is a diagram for explaining a configuration example of the cylindrical phantom 100 according to the first embodiment. FIG. 2 shows an example of the appearance of the cylindrical phantom 100.

As shown in FIG. 2, the cylindrical phantom 100 is a phantom having a cylindrical shape. For example, the cylindrical phantom 100 is placed on the top plate 33 so that the central axis C of the cylindrical phantom 100 coincides with the rotational central axis of the rotating frame 13. The cylindrical phantom 100 can be placed on the top plate 33 by using an arbitrary jig or support member.

In the following description, the direction parallel to the central axis C is defined as the z-axis direction. Further, the axial direction perpendicular to the z-axis direction and horizontal to the floor surface is defined as the x-axis direction. Further, the axial direction orthogonal to the z-axis direction and perpendicular to the floor surface is defined as the y-axis direction.

An example of the internal structure of the cylindrical phantom 100 according to the first embodiment will be described with reference to FIG. FIG. 3 is a diagram for explaining an example of the internal structure of the cylindrical phantom 100 according to the first embodiment. The left view of FIG. 3 is a cross-sectional view of the cylindrical phantom 100 in the xy plane. The right view of FIG. 3 is a cross-sectional view of the cylindrical phantom 100 in the yz plane.

As shown in FIG. 3, the cylindrical phantom 100 has plate members 101, 102, 103, 104, 105. The plate members 103, 104, and 105 are cylindrical plate members having substantially the same axial length and different radii from each other. The plate members 103, 104, 105 are arranged concentrically so that the central axis of each plate member 103, 104, 105 coincides with the central axis C. The plate members 101 and 102 are circular plate members having a radius substantially the same as the radius of the plate member 103. The plate members 101 and 102 are arranged at both ends of the plate members 103, 104 and 105 in the axial direction so that the central axis C passes through the center of each plate member 101 and 102.

That is, the cylindrical phantom 100 has a layer structure formed by three layers 100A, 100B, and 100C. Specifically, the layer 100A corresponds to a cylindrical space formed between the plate member 103 and the plate member 104. Further, the layer 100B corresponds to a cylindrical space formed between the plate member 104 and the plate member 105. Further, the layer 100C corresponds to a cylindrical space formed inside the plate member 105. In other words, each layer of the cylindrical phantom 100 is configured as a cylindrical or cylindrical container.

Further, the three layers 100A, 100B and 100C contain substances having different X-ray absorption amounts. As an example, the three layers 100A, 100B, and 100C are filled with iodine contrast agent solutions having different concentrations. The iodine contrast agent solution is a liquid in which the iodine contrast agent is dissolved in water or an arbitrary solvent at an arbitrary concentration.

Here, among the three layers 100A, 100B, and 100C, the layer closer to the center contains a substance having a smaller amount of X-ray absorption, and the layer farther from the center contains a substance having a larger amount of X-ray absorption. For example, when an iodine contrast medium solution having three different line absorption coefficients μ1, μ2, μ3 (μ1 <μ2 <μ3) is prepared, an iodine contrast medium solution having a line absorption coefficient μ1 (the solution having the lowest concentration) is prepared. Layer 100C is filled. Further, the layer 100B is filled with an iodine contrast agent solution having a line absorption coefficient of μ2. Further, the layer 100A is filled with an iodine contrast medium solution having a linear absorption coefficient of μ3. The line absorption coefficient is proportional to the concentration of the substance. The hatching shades of the layers 100A, 100B, and 100C shown in FIG. 3 correspond to the magnitude of the linear absorption coefficient (concentration) of the substance contained in each layer.

The content described in FIG. 3 is merely an example, and the embodiment is not limited to this. For example, in FIG. 3, the case where the cylindrical phantom 100 has a three-layer structure has been described, but the number of layers can be set arbitrarily.

Further, in FIG. 3, the case where the composition (chemical composition) of the substance contained in each layer 100A, 100B, 100C is an iodine contrast medium has been described, but the present invention is not limited to this, and X such as a calcium compound used as a pseudo-human bone can be used. Any substance capable of absorbing the line can be applied.

Further, in FIG. 3, the case where the composition of the substances contained in each of the layers 100A, 100B, and 100C is uniform has been described, but the present invention is not limited to this, and a plurality of types of compositions may be used. In this case, for example, layer 100A and layer 100B contain an iodine contrast agent, and layer 100C contains a calcium compound. Also in this case, it is preferable that the layer closer to the center contains a substance having a smaller amount of X-ray absorption and the layer farther from the center contains a substance having a larger amount of X-ray absorption. be. The amount of X-rays absorbed is, for example, the amount of absorption per unit volume.

Further, the plate members 101, 102, 103, 104, 105 are formed of, for example, acrylic resin, but the present invention is not limited to this, and any material that can be used as a phantom of the X-ray CT apparatus 1 can be applied. However, as the material to be applied, a material having a small amount of X-ray absorption and transparency is preferable.

The relationship between the rotation angle and the transmission length in the cylindrical phantom 100 according to the first embodiment will be described with reference to FIG. FIG. 4 is a diagram for explaining the relationship between the rotation angle and the transmission length in the cylindrical phantom 100 according to the first embodiment. FIG. 4 illustrates the positional relationship between the X-ray tube 11, the X-ray detector 12, and the cylindrical phantom 100. Specifically, the upper part of FIG. 4 shows the positional relationship when the rotation angle is 0 degrees (when the X-ray tube 11 is located at the apex of the rotation frame 13), and the lower part of FIG. 4 shows the rotation angle of 90 degrees. The positional relationship when it is a degree is shown.

As shown in FIG. 4, the X-ray path P1 incident on the detection element D1 changes to the path P2 due to the rotation of the rotation frame 13. However, the cylindrical phantom 100 has a structure in which the central axis C is arranged so as to coincide with the center of the rotating frame 13 and is laminated concentrically. Therefore, even if the positions of the X-ray tube 11 and the X-ray detector 12 change due to the rotation of the rotating frame 13, the apparent positional relationship between the X-ray tube 11, the X-ray detector 12, and the cylindrical phantom 100 changes. do not do. As a result, the permeation length of each layer in the path P1 and the permeation length of each layer in the path P2 do not change.

In FIG. 4, for convenience of illustration, only 0 degrees and 90 degrees have been illustrated and described, but the apparent positional relationship does not change regardless of the rotation angle. That is, in the path of X-rays incident on each detection element, the transmission length of each layer is constant at any rotation angle. Therefore, the projection data collected by using the cylindrical phantom 100 includes the influence of mechanical blurring and vibration accompanying the rotation of the rotating frame 13.

The transmission length of the cylindrical phantom 100 according to the first embodiment will be described with reference to FIG. FIG. 5 is a diagram for explaining the transmission length of the cylindrical phantom 100 according to the first embodiment. FIG. 5 illustrates a case where the rotation angle is 0 degrees.

In FIG. 5, assuming that the non-transmitted X-ray intensity (X-ray dose) emitted from the X-ray tube 11 is I0 (E, mA), the X-ray intensity I (E, θ, mA) incident on each detection element. Is expressed by the following equation (1).

In formula (1), E represents the energy of X-rays. Further, mA indicates the tube current of the X-ray tube 11. θ indicates the angle formed by the X-ray path incident on each detection element with respect to the vertical direction. μ1 (E) indicates the line absorption coefficient of the substance contained in the layer 100C at the energy E. μ2 (E) indicates the line absorption coefficient of the substance contained in the layer 100B at the energy E. μ3 (E) indicates the line absorption coefficient of the substance contained in the layer 100A at the energy E. L1 indicates the transmission length of the layer 100C. L2 indicates the transmission length of the layer 100B. L3 indicates the transmission length of the layer 100A. Although the line absorption coefficient μ (E) has energy dependence, the description of “(E)” may be omitted.

Here, the transmission lengths L1, L2, and L3 are represented as follows according to the angle θ. In the following, d indicates the distance between the focal point S and the central axis C of the X-ray tube 11. r1 indicates the radius of the layer 100C. r2 indicates the radius of layer 100B. r3 indicates the radius of layer 100A. θ1 is a value that satisfies the following equation (2). θ2 is a value that satisfies the following equation (3). θ3 is a value that satisfies the following equation (4).

First, when θ = 0, the transmission lengths L1, L2, and L3 are represented by the following equations (5) to (7).

Next, when 0 <θ <θ1, the transmission lengths L1, L2, and L3 are represented by the following equations (8) to (10). Note that a indicates the distance of the perpendicular line drawn from the center (central axis C) with respect to the straight line indicating the path of the angle θ, and is a value satisfying “a = d × sin (θ)”.

When θ1 ≦ θ <θ2, the transmission lengths L2 and L3 are represented by the following equations (11) and (12). In this case, the transmission length L1 is 0.

When θ2 ≦ θ <θ3, the transmission length L3 is represented by the following equation (13). In this case, the transmission lengths L1 and L2 are 0.

In this way, the cylindrical phantom 100 can treat the transmission length in each detection element as known. Thereby, the operator can collect the detection data of the substance concentration, the permeation length, and the X-ray condition (X-ray dose) only by collecting the detection data from one cylindrical phantom 100.

Next, the process of generating correction data using the cylindrical phantom 100 according to the first embodiment will be described with reference to FIG. FIG. 6 is a diagram for explaining a correction data generation process using the cylindrical phantom 100 according to the first embodiment. The process shown in FIG. 6 is started, for example, when the input interface 43 receives an instruction from the operator to start the correction data generation process.

As shown in FIG. 6, in step S101, the acquisition function 445 projects X-rays onto the cylindrical phantom 100 at a plurality of rotation angles. For example, the acquisition function 445 continuously irradiates X-rays from the X-ray tube 11 by supplying a constant tube current to the X-ray tube 11 while rotating the rotating frame 13.

In step S102, the acquisition function 445 acquires the first energy spectrum for each rotation angle. For example, the acquisition function 445 acquires the energy spectrum of the X-ray photon incident on each detection element of the X-ray detector 12 as the first energy spectrum by controlling the DAS 18. As a result, the acquisition function 445 acquires the first energy spectrum for the entire circumference (360 degrees) for each detection element. Here, the acquisition function 445 performs scanning in a state where the central axis C of the cylindrical phantom 100 and the rotation center of the X-ray tube 11 coincide with each other, so that the first energy spectrum having a constant transmission length in each detection element is performed. To get. As a result, the first energy spectrum includes the influence of mechanical blurring and vibration accompanying the rotation of the X-ray tube 11.

In step S103, the generation function 446 reads the theoretical second energy spectrum from the memory 41. Here, the second energy spectrum is a theoretical energy spectrum obtained from the cylindrical phantom 100 under preset conditions. The second energy spectrum is associated with each of the tube current, the composition of the substance, the concentration of the substance, and the transmission length, and is stored in the memory 41 in advance. That is, the memory 41 stores the second energy spectrum in association with at least one of the tube current supplied to the X-ray tube, the composition of the substance, the concentration of the substance, and the transmission length of the substance.

The second energy spectrum is not limited to the case where it is stored as a value according to a preset condition, and may be stored as a function according to a tube current, a substance concentration, a transmission length, and the like. That is, the memory 41 stores the second energy spectrum as a function corresponding to at least one of the tube current supplied to the X-ray tube, the composition of the substance, the concentration of the substance, and the transmission length of the substance. When the second energy spectrum is stored as a function, the generation function 446 can calculate a value according to a preset tube current, a substance concentration, a transmission length, or the like each time.

In step S104, the generation function 446 generates correction data based on the first energy spectrum and the second energy spectrum. For example, the generation function 446 compares the first energy spectrum with the second energy spectrum. Then, the generation function 446 calculates a value for eliminating the difference as correction data based on the difference between the first energy spectrum and the second energy spectrum.

As an example, the generation function 446 utilizes the fact that the transmission length of each detection element is constant (known) at any rotation angle, and X-rays absorbed in each layer from the first energy spectrum of the entire circumference are absorbed. Calculate the absorption amount of. Then, the generation function 446 calculates the correction data by comparing the calculated X-ray absorption amount of each layer with the theoretical value of the absorption amount absorbed by the substance contained in each layer.

As the correction data, for example, a difference value between the first energy spectrum and the second energy spectrum may be used, or a value obtained by an arbitrary calculation method may be used. Further, as the correction data, the average value with the difference value at the adjacent rotation angles may be used, or the average value of the first energy spectrum acquired by X-ray projection of two or more laps may be used for calculation. good. The process of calculating the correction data is not limited to the method described above, and can be executed by any method.

In step S105, the generation function 446 stores the generated correction data in the memory 41. For example, the generation function 446 stores the correction data of each detection element at each rotation angle in the memory 41 in association with each of the tube current, the composition of the substance, the concentration of the substance, and the transmission length. Not limited to this, the generation function 446 can also store correction data in any unit.

Note that FIG. 6 has described the case where scanning is performed with a single tube current, but the present invention is not limited to this. For example, the tube current supplied to the X-ray tube 11 may be changed to repeatedly execute the correction data generation process of FIG. In this case, the acquisition function 445 acquires a plurality of first energy spectra having different X-ray doses by performing a plurality of scans in which the tube current supplied to the X-ray tube 11 is changed. Then, the generation function 446 generates a plurality of correction data having different X-ray doses per unit time based on a plurality of first energy spectra having different X-ray doses.

The imaging process by the X-ray CT apparatus 1 according to the first embodiment will be described with reference to FIG. 7. FIG. 7 is a diagram for explaining an imaging process by the X-ray CT apparatus 1 according to the first embodiment. The process shown in FIG. 7 is started, for example, when the input interface 43 receives an instruction from the operator to start the imaging process.

As shown in FIG. 7, in step S201, the system control function 441 projects X-rays onto the subject at a plurality of rotation angles. For example, the system control function 441 continuously irradiates X-rays from the X-ray tube 11 while the rotating frame 13 is rotating.

In step S202, the system control function 441 acquires a third energy spectrum for each rotation angle. For example, the system control function 441 acquires the energy spectrum of the X-ray photon incident on each detection element of the X-ray detector 12 as the third energy spectrum by controlling the DAS 18. As a result, the system control function 441 acquires a third energy spectrum for the entire circumference (360 degrees) for each detection element. The third energy spectrum is appropriately pretreated by the pretreatment function 442.

In step S203, the reconstruction processing function 443 reads the correction data from the memory 41. Here, the correction data is generated by the generation process according to FIG. 6 and is stored in the memory 41 in advance.

In step S204, the reconstruction processing function 443 corrects the third energy spectrum by using the correction data. For example, the reconstruction processing function 443 corrects the count value for each energy discrimination region included in the third energy spectrum based on the correction data.

In step S205, the reconstruction processing function 443 executes the substance discrimination reconstruction based on the corrected third energy spectrum. As a result, the reconstruction processing function 443 generates, for example, substance discrimination image data.

In step S206, the processing circuit 44 displays the image data. For example, the processing circuit 44 causes the display 42 to display the substance discrimination image data generated by the reconstruction processing function 443.

As described above, the X-ray CT apparatus 1 according to the first embodiment is a cylindrical phantom having a plurality of concentric layers, and the composition of the substance and the concentration of the substance contained in each of the plurality of layers. By imaging a phantom in which at least one of them is different at each of a plurality of rotation angles, a first energy spectrum at each rotation angle is acquired. Then, the X-ray CT apparatus 1 corrects the energy spectrum obtained from the subject based on the first energy spectrum and the second energy spectrum which is a theoretical energy spectrum obtained from the phantom under preset conditions. Generate correction data for this purpose. According to this, the X-ray CT apparatus 1 can easily generate correction data.

A comparative example will be described with reference to FIG. FIG. 8 is a diagram for explaining a comparative example. As shown in FIG. 8, as a comparative example, plate-shaped phantom 90s having the same composition but different concentrations may be used. In this case, if the rotating frame 13 is rotated as in a normal scan, the transmission length of each detection element changes. Therefore, when the plate-shaped phantom 90 is used, the energy spectrum is collected without rotating the rotating frame 13. However, in this case, the influence caused by the rotation of the rotating frame 13 cannot be calibrated. For example, it is known that scattered rays differ depending on the rotation angle, and that mechanical blurring and vibration occur with rotation, but the plate-shaped phantom 90 cannot be corrected by taking these effects into consideration. ..

On the other hand, the cylindrical phantom 100 according to the present embodiment has a structure in which the central axis C is arranged so as to coincide with the center of the rotating frame 13 and is laminated concentrically. As a result, in the cylindrical phantom 100, the path of X-rays incident on each detection element can be made constant at any rotation angle. Therefore, the projection data collected by using the cylindrical phantom 100 includes the influence of scattered rays for each rotation angle and the influence of mechanical blurring and vibration accompanying the rotation of the rotation frame 13.

Further, as another comparative example, a cylindrical phantom having a uniform concentration may be used. In this case, the longer the transmission length, the larger the amount of X-ray absorption (attenuation). Therefore, there is an extreme difference in the amount of X-ray absorption between the detection element near the center of the X-ray detector and the detection element near the outer edge of the X-ray detector. Therefore, it is difficult to perform accurate detection with both detection elements, and a statistical error occurs in either detection element. Further, in order to generate correction data with different densities, a large number of phantoms having different densities are used, and it is necessary to generate correction data while exchanging the phantoms each time, which is troublesome.

On the other hand, the X-ray CT apparatus 1 according to the present embodiment uses the cylindrical phantom 100 to generate correction data. Since the cylindrical phantom 100 has a plurality of layers containing substances having different concentrations, it is possible to generate a plurality of density and transmission length correction data by one X-ray projection. Further, by changing the tube current of the X-ray tube for each scan, it is possible to generate correction data having different incident X-ray doses. Therefore, the X-ray CT apparatus 1 can easily generate correction data in a short time.

Further, in the cylindrical phantom 100, the layer closer to the center contains a substance having a smaller amount of X-ray absorption, and the layer farther from the center contains a substance having a larger amount of X-ray absorption. Therefore, the difference in the amount of X-ray absorption between the detection element near the center of the X-ray detector 12 and the detection element near the outer edge of the X-ray detector 12 is reduced, and thus both detection elements. It is possible to reduce the statistical error caused by.

(Second embodiment)
In the first embodiment, the case where the cylindrical phantom 100 having a plurality of layers is integrally formed has been described, but the embodiment is not limited to this. For example, the cylindrical phantom may have a configuration in which each layer can be exchanged.

A configuration example of the cylindrical phantom 200 according to the second embodiment will be described with reference to FIG. 9. FIG. 9 is a diagram showing a configuration example of the cylindrical phantom 200 according to the second embodiment.

As shown in FIG. 9, the cylindrical phantom 200 has a cylindrical phantom 210 and cylindrical phantoms 220 and 230. The cylindrical phantom 210 and the cylindrical phantoms 220 and 230 have substantially the same axial length.

Further, the length of the outer diameter of the cylindrical phantom 210 is substantially the same as the length of the inner diameter of the cylindrical phantom 220. Therefore, the cylindrical phantom 210 can be accommodated inside the cylindrical phantom 220.

Further, the length of the outer diameter of the cylindrical phantom 220 is substantially the same as the length of the inner diameter of the cylindrical phantom 230. Therefore, the cylindrical phantom 220 can be accommodated inside the cylindrical phantom 230.

In this way, the cylindrical phantom 200 forms a layered structure composed of three layers by combining the cylindrical phantom 210 and the cylindrical phantoms 220 and 230. As a result, the cylindrical phantom 200 includes a plurality of phantoms corresponding to each of the plurality of layers.

That is, in the cylindrical phantom 200, the phantoms (cylindrical phantom 210, cylindrical phantom 220, 230) constituting each layer are configured to be separable from each other. Therefore, the operator can arrange any substance in each layer by appropriately exchanging the phantoms constituting each layer.

For example, a plurality of cylindrical phantoms 210 containing substances having different X-ray absorption amounts, a plurality of cylindrical phantoms 220 containing substances having different X-ray absorption amounts, and substances having different X-ray absorption amounts. A plurality of cylindrical phantoms 230 containing the same are prepared in advance. As a result, the operator can use any combination of phantoms.

The content described in FIG. 9 is merely an example, and the embodiment is not limited to this. For example, the cylindrical phantom 200 is not limited to a three-layer structure, and may have a layer structure composed of an arbitrary number of layers. The cylindrical phantom 200 has the same configuration as the cylindrical phantom 100, except that the phantoms constituting each layer are separable.

(Other embodiments)
In addition to the above-described embodiments, various different embodiments may be implemented.

(Application to other than photon counting CT)
For example, in the above-described embodiment, the case where the correction data generation method according to the present embodiment is applied to the photon counting CT has been described, but the present invention is not limited to this. For example, the correction data generation method according to the present embodiment can be applied even when a conventional integral type detector or dual energy CT is performed. In order to obtain the effect of the present application that correction data can be easily generated according to various conditions such as the composition, concentration, and permeation length of the substance, the method for generating correction data according to the present embodiment. Is preferably applied to photon counting CT or multi-energy CT. The multi-energy CT is an imaging method that processes data of a plurality of types (two or more types) of energy, and any imaging method such as Kv switching, dual source, and stacked detector method can be applied.

(3rd generation CT / 4th generation CT)
The X-ray CT device 1 includes a Rotate / Rotate-Type (3rd generation CT) in which an X-ray tube 11 and an X-ray detector 12 rotate around a subject as a unit, and a large number of X-rays arranged in a ring shape. There are various types such as Stationary / Rotate-Type (4th generation CT) in which the line detection element is fixed and only the X-ray tube rotates around the subject, and any type can be applied to the present embodiment.

(Multi-console)
Although the console device 40 has been described as executing a plurality of functions on a single console, a plurality of functions may be executed by different consoles. For example, the functions of the processing circuit 44 such as the preprocessing function 442 and the reconstruction processing function 443 may be distributed.

(Integrated server)
The processing circuit 44 is not limited to the case where it is included in the console device 40, and may be included in an integrated server that collectively performs processing on detection data acquired by a plurality of medical diagnostic imaging devices.

(Processing subject of post-processing)
The post-processing described in the embodiments may be performed on either the console 40 or an external workstation. Further, both the console 40 and the workstation may process at the same time.

(Full scan / Half scan)
In order to reconstruct the CT image, the projection data of 360 ° around the subject is required, and the projection data of 180 ° + fan angle is required even in the half scan method. Any reconstruction method can be applied to the present embodiment.

(Single tube / multi-tube)
In the present embodiment, a so-called multi-tube type X-ray CT in which a plurality of pairs of an X-ray tube 11 and an X-ray detector 12 are mounted on a rotating ring also in a single-tube type X-ray CT device 1. It can also be applied to the device 1.

(X-ray imaging device)
Further, the above embodiment is applicable not only to the X-ray CT device but also to other imaging devices using X-rays. For example, the above embodiment is effective for an imaging method in which an X-ray source and an X-ray detector are rotated to perform imaging.

(Cloud storage of correction data)
The correction data generated by the above embodiment is not limited to the case where the correction data is stored in the memory 41 of the console device 40, and may be stored in the cloud, for example. For example, a cloud server that can connect to the X-ray CT device 1 via a communication network such as the Internet stores correction data in response to a storage request from the X-ray CT device 1.

(Application to dental CT)
The cylindrical phantom 100 described in the above embodiment can also be applied to dental CT. That is, the dental CT can perform calibration (generation of correction data) using the cylindrical phantom 100.

Further, for example, in the above embodiment, it has been described that each processing function is realized by a single processing circuit, but a processing circuit is configured by combining a plurality of independent processors, and each processor executes a program. The function may be realized by executing it.

The term "processor" used in the above description refers to, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an application specific integrated circuit (ASIC), or a programmable logic device (for example, an application specific integrated circuit). It means a circuit such as a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA). The processor realizes the function by reading and executing the program stored in the storage circuit 110. Instead of storing the program in the storage circuit 110, the program may be directly incorporated in the circuit of the processor. In this case, the processor realizes the function by reading and executing the program embedded in the circuit. It should be noted that each processor of the present embodiment is not limited to the case where each processor is configured as a single circuit, and a plurality of independent circuits may be combined to form one processor to realize its function. good. Further, a plurality of components in each figure may be integrated into one processor to realize the function.

Further, for example, each component of each of the illustrated devices is a functional concept, and does not necessarily have to be physically configured as shown in the figure. That is, the specific form of distribution / integration of each device is not limited to the one shown in the figure, and all or part of the device is functionally or physically dispersed / physically distributed in arbitrary units according to various loads and usage conditions. Can be integrated and configured. Further, each processing function performed by each device may be realized by a CPU and a program analyzed and executed by the CPU, or may be realized as hardware by wired logic.

Further, among the processes described in the above-described embodiment, all or a part of the processes described as being automatically performed can be manually performed, or the processes described as being manually performed. It is also possible to automatically perform all or part of the above by a known method. In addition, the processing procedure, control procedure, specific name, and information including various data and parameters shown in the above document and drawings can be arbitrarily changed unless otherwise specified.

Further, the correction data generation method described in the above-described embodiment can be realized by executing a correction data generation program prepared in advance on a computer such as a personal computer or a workstation. This correction data generation method can be distributed via a network such as the Internet. Further, this correction data generation method is executed by recording on a computer-readable recording medium such as a hard disk, flexible disk (FD), CD-ROM, MO, or DVD, and reading from the recording medium by the computer. You can also do it.

According to at least one embodiment described above, correction data can be easily generated.

Although some embodiments of the present invention have been described, these embodiments are presented as examples 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 gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, as well as in the scope of the invention described in the claims and the equivalent scope thereof.

1 X-ray CT device 100 Cylindrical phantom 44 Processing circuit 445 Acquisition function 446 Generation function

Claims (10)

A cylindrical phantom having a plurality of concentric layers, in which at least one of the composition of the substance contained in each of the plurality of layers and the concentration of the substance is different, is scanned at a plurality of rotation angles. And the acquisition unit that acquires the first energy spectrum at each rotation angle,
For correction for correcting the third energy spectrum obtained from the subject based on the first energy spectrum and the second energy spectrum which is a theoretical energy spectrum obtained from the phantom under preset conditions. An X-ray CT apparatus including a generator for generating data. The acquisition unit acquires the first energy spectrum having a constant transmission length in each detection element by performing the scan in a state where the central axis of the phantom and the rotation center of the X-ray tube coincide with each other.
The X-ray CT apparatus according to claim 1. The first energy spectrum includes the effects of mechanical blurring and vibration associated with the rotation of the X-ray tube.
The X-ray CT apparatus according to claim 2. The acquisition unit acquires a plurality of the first energy spectra having different X-ray doses by performing a plurality of scans in which the tube current supplied to the X-ray tube is changed.
The generation unit generates a plurality of the correction data having different X-ray doses per unit time based on the plurality of first energy spectra having different X-ray doses.
The X-ray CT apparatus according to any one of claims 1 to 3. Among the plurality of layers of the phantom, the layer closer to the center contains a substance having a smaller amount of X-ray absorption, and the layer farther from the center contains a substance having a larger amount of X-ray absorption.
The X-ray CT apparatus according to any one of claims 1 to 3. A storage unit for storing the second energy spectrum is further provided in association with at least one of the tube current, the composition of the substance, the concentration of the substance, and the transmission length.
The X-ray CT apparatus according to any one of claims 1 to 5. A storage unit for storing the second energy spectrum is further provided as a function corresponding to at least one of the tube current, the composition of the substance, the concentration of the substance, and the transmission length.
The X-ray CT apparatus according to any one of claims 1 to 5. A reconstruction unit that corrects and reconstructs the third energy spectrum collected by scanning the subject using the correction data is further provided.
The X-ray CT apparatus according to any one of claims 1 to 7. The phantom includes a plurality of phantoms corresponding to each of the plurality of layers.
The X-ray CT apparatus according to any one of claims 1 to 8. A cylindrical phantom having a plurality of concentric layers, in which at least one of the composition of the substance contained in each of the plurality of layers and the concentration of the substance is different, is scanned at a plurality of rotation angles. To obtain the first energy spectrum at each rotation angle,
Correction data for correcting the energy spectrum obtained from the subject based on the first energy spectrum and the second energy spectrum which is a theoretical energy spectrum obtained from the phantom under preset conditions. Generate,
Calibration methods, including that.

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