X-ray imaging apparatus

Abstract

To provide an X-ray imaging apparatus capable of stabilizing polarization of a photon counting detector and suppressing ineffective exposure of a subject.SOLUTION: An X-ray imaging apparatus comprises an X-ray source that emits an X-ray, a photon counting detector that counts an X-ray photon for each energy, and an image generation unit that generates a medical image of a subject based on a detector output from the photon counting detector. The X-ray imaging apparatus further comprises a control unit that checks, in a case where polarization of the photon counting detector is not stable, the subject not being present in an X-ray irradiation field, which is a range where the X-ray is applied from the X-ray source, and then causes the X-ray source to irradiate the photon counting detector with the X-ray.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an X-ray imaging device equipped with a photon-counting detector, and more particularly to a technique for suppressing the effect of polarization in a photon-counting detector. [Background technology]

[0002] PCCT (Computed Tomography) devices are being developed as X-ray imaging devices equipped with photon-counting detectors that use the photon counting method. Because photon-counting detectors can measure the energy of X-ray photons, PCCT devices can present medical images that contain more information than conventional CT devices, for example medical images that are divided into multiple energy components.

[0003] In photon-counting detectors, which become ready for imaging when a voltage is applied, polarization progresses according to the length of time the voltage is applied, and the peak value of the detector output gradually decreases. Changes in the peak value of the detector output cause errors in the measured energy of X-ray photons, adversely affecting the image quality of medical images that are divided into energy components.

[0004] Patent Document 1 discloses an X-ray imaging device that determines whether the polarization of a photon-counting detector to which a high voltage is applied is stable in order to suppress the effects of polarization of the photon-counting detector, and starts data collection when the polarization is stable. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-194354 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in Patent Document 1, no consideration is given to the progression of polarization due to X-ray irradiation. Polarization of a photon counting detector progresses not only due to application of a high voltage but also due to X-ray irradiation. In order to image a subject after stabilizing polarization due to X-ray irradiation, it is necessary to irradiate the photon counting detector with X-rays just before imaging. In addition, it is necessary to suppress unnecessary exposure of the subject to X-rays irradiated to the photon counting detector.

[0007] Therefore, an object of the present invention is to provide an X-ray imaging apparatus that can stabilize the polarization of a photon counting detector and suppress unnecessary radiation exposure of a subject. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the present invention provides an X-ray imaging device comprising an X-ray source that irradiates X-rays, a photon counting detector that counts X-ray photons for each energy, and an image generating unit that generates a medical image of a subject based on detector output from the photon counting detector, characterized in that the device further comprises a control unit that, when the polarization of the photon counting detector is unstable, causes the photon counting detector to irradiate X-rays from the X-ray source after confirming that the subject is not in an X-ray irradiation field, which is the range where X-rays are irradiated from the X-ray source. Effect of the Invention

[0009] According to the present invention, it is possible to provide an X-ray imaging apparatus capable of stabilizing the polarization of a photon counting detector and suppressing unnecessary radiation exposure of a subject. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing an overall configuration of an X-ray CT apparatus according to a first embodiment. [Diagram 2] FIG. 2 is a diagram showing a configuration of a detector panel according to the first embodiment. [Diagram 3] FIG. 1 is a diagram illustrating the influence of polarization. [Figure 4] FIG. 2 is a diagram illustrating an example of a processing flow according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The X-ray imaging device of the present invention is applied to a device including an X-ray source and a photon counting detector. In the following description, an example in which the X-ray imaging device is an X-ray CT device will be described. EXAMPLES

[0012] 1 shows the overall configuration of the X-ray CT apparatus of this embodiment. The X-ray CT apparatus includes a gantry 1, a computing device 2, a display device 3, and a bed 4, and is an apparatus that counts X-ray photons absorbed by a subject 5 placed on the bed 4 and generates a tomographic image of the subject 5 based on the count number. Each part will be described below.

[0013] The gantry 1 has a rotating part that rotates and mounts an X-ray tube 6 and a detector panel 7, and a stationary part that supports the rotating part. The X-ray tube 6 is an X-ray source that generates X-rays by irradiating a target with electrons accelerated by a high voltage of about 100 kV. The detector panel 7 is a photon-counting detector that is arranged opposite the X-ray tube 6 across the subject 5 and counts the X-ray photons that have passed through the subject 5 to measure the spatial distribution of the number of X-ray photons. The number of X-ray photons absorbed by the subject 5 is calculated by subtracting the number of X-ray photons that have passed through the subject 5 from the number of X-ray photons when the subject 5 is not present, and this is obtained as projection data. Note that the detector panel 7 is a photon-counting detector that can measure the energy of X-ray photons, so projection data for each energy component is obtained. The detailed structure of the detector panel 7 will be described later with reference to FIG. 2.

[0014] While the X-ray tube 6 and the detector panel 7 rotate around the subject 5, the X-ray tube 6 irradiates the subject 5 with X-rays and the detector panel 7 counts X-ray photons repeatedly, thereby acquiring projection data of the subject 5 from various directions. About 3000 frames of projection data are acquired per second and transmitted to the computing device 2. The bed 4 moves horizontally toward the opening of the gantry 1 to adjust the position of the subject 5 from which projection data is acquired.

[0015] The arithmetic device 2 has the same hardware configuration as a general computer device, and includes a CPU (Central Processing Unit) and memory, and is connected to the display device 3, input device 8, and storage device 9. The arithmetic device 2 generates a tomographic image by image reconstruction using a plurality of pieces of transmitted projection data, and controls each part. For example, the arithmetic device 2 controls the voltage applied to the X-ray tube 6, the rotation speed of the X-ray tube 6 and the detector panel 7, and the like.

[0016] The display device 3 is a liquid crystal display, a touch panel, or the like, and displays the generated tomographic images, etc. The input device 8 is a keyboard, a mouse, or the like, and is used for setting the voltage to be applied to the X-ray tube 6, etc. When the display device 3 is a touch panel, the touch panel functions as the input device 8. The storage device 9 is a hard disk drive (HDD), a solid state drive (SSD), or the like, and stores various data such as programs executed by the CPU, projection data, and tomographic images.

[0017] An example of the detector panel 7 will be described with reference to Fig. 2. The detector panel 7 has an X-ray detector 10 and a collimator 11, and is provided on the rotating part of the gantry 1.

[0018] The X-ray detector 10 is a semiconductor detector that detects X-ray photons that have passed through the subject 5. CdTe or CdZnTe is used for the X-ray detector 10, and a high-voltage electrode 15 is provided on the side where the X-rays are incident, and multiple readout electrodes 13 are provided on the opposite side. A negative high voltage is applied to the high-voltage electrode 15 with respect to the readout electrode 13, which is at ground voltage, and an electric field is formed between the high-voltage electrode 15 and the readout electrode 13. When an X-ray photon is incident on the X-ray detector 10, electrons and holes are generated in a number according to the energy of the X-ray photon. The electrons generated by the incidence of the X-ray photon move to the nearest readout electrode 13 due to the electric field between the electrodes, and are read out as an electrical signal. In other words, the readout electrodes 13 correspond to the detection pixels 12 of the X-ray detector 10.

[0019] The collimator 11 is a metal grid having a plurality of holes and is provided in front of the X-ray detector 10 in order to suppress the incidence of scattered rays generated by the subject 5 or the like into the X-ray detector 10. The collimator 11 is made of a metal having a large specific gravity and atomic number, such as tungsten or molybdenum, and the holes of the collimator 11 are positioned to correspond to the detection pixels 12.

[0020] The effect of polarization of the X-ray detector 10 will be explained using Fig. 3. Fig. 3(a) is an example of an output waveform generated when an X-ray photon is incident on the X-ray detector 10, with the horizontal axis representing time and the vertical axis representing the detector output. Fig. 3(b) is an example of an attenuation curve, which is the change over time in the peak value of the output waveform, with the horizontal axis representing the X-ray irradiation time and the vertical axis representing the peak value of the detector output.

[0021] In a photon counting detector, a pulse output 301 is generated every time an X-ray photon is incident during a dead time τ, and the peak value of the pulse output 301 is proportional to the photon energy. Fig. 3(a) shows an example of a pulse output 301L when the photon energy of the incident X-ray photon is relatively low, and a pulse output 301H when the photon energy is relatively high. That is, the detector output is divided by threshold values ​​Th1, Th2, Th3, etc., to discriminate the X-ray photons according to their photon energy.

[0022] However, when X-rays are continuously irradiated onto the X-ray detector 10, electrons and holes are trapped inside, and polarization progresses as the X-ray irradiation time passes. Since polarization retards the movement of electrons and holes, some of the electrons generated between the electrodes cannot reach the readout electrode 13 within the dead time τ, and as the irradiation time increases, the pulse height value decreases as the irradiation time increases, as shown in FIG. 3(b). That is, even if X-ray photons of the same photon energy are incident, they will have different pulse height values, so an error occurs in the energy of the measured X-ray photons. The change in the pulse height value is largest immediately after the X-rays are irradiated, for example, during period A in FIG. 3(b), and decreases as time progresses to periods B and C. That is, the polarization of the X-ray detector 10 stabilizes as the X-ray irradiation time passes.

[0023] Therefore, in the first embodiment, the polarization of the X-ray detector 10 is stabilized by irradiating the X-rays immediately before imaging the subject. If the polarization progresses too much, the characteristics of the X-ray detector 10 deteriorate, causing an increase in noise and a decrease in the count rate, so it is desirable to stop the irradiation of the X-rays during a period in which the peak value is equal to or greater than a predetermined threshold, for example, during period B instead of period C in FIG. 3(b).

[0024] The process flow of the first embodiment will be described with reference to FIG.

[0025] (S401) The arithmetic unit 2 determines whether or not the polarization of the X-ray detector 10 is stable. If the polarization is not stable, the process proceeds to S402, and if the polarization is stable, the process proceeds to S405.

[0026] Whether the polarization of the X-ray detector 10 is stable or not is determined based on, for example, the time elapsed since the previous imaging. That is, if the time elapsed since the previous imaging is shorter than a predetermined time, it is determined that the polarization is stable. Based on the time elapsed since the previous imaging, whether the polarization is stable or not can be determined by a simple process. The predetermined time is set in advance based on the result of the advance measurement, and is set to, for example, 10 minutes. Note that the greater the irradiation dose, the more the polarization progresses, so the predetermined time is set according to the irradiation dose. By setting the predetermined time according to the irradiation dose, whether the polarization is stable or not can be determined more accurately.

[0027] (S402) The arithmetic unit 2 judges whether or not the subject 5 is in the X-ray irradiation field. The X-ray irradiation field is a range in which X-rays are irradiated from the X-ray tube 6. If the subject 5 is not in the X-ray irradiation field, the process proceeds to S404, and if the subject 5 is in the X-ray irradiation field, the process returns to S402 via S403.

[0028] Whether or not the subject 5 is in the X-ray radiation field is determined based on, for example, the position of the bed 4. That is, if the bed 4 is in the X-ray radiation field, it is determined that the subject 5 is in the X-ray radiation field. By basing the determination on the position of the bed 4, the presence or absence of the subject 5 can be determined without adding new hardware.

[0029] Also, whether or not the subject 5 is in the X-ray irradiation field may be determined based on an image of a camera that captures the X-ray irradiation field. That is, if the subject 5 is included in the image of the camera, it is determined that the subject 5 is in the X-ray irradiation field. By basing the image of the camera, the presence or absence of the subject 5 can be determined even if the subject 5 protrudes from the bed 4 and is in the X-ray irradiation field.

[0030] Furthermore, whether or not the subject 5 is in the X-ray irradiation field may be determined based on an image obtained by low-dose imaging. Note that low-dose imaging is imaging using an X-ray dose lower than the X-ray dose irradiated from the X-ray tube 6 when capturing a medical image of the subject 5. In other words, if the subject 5 is included in the image obtained by low-dose imaging, it is determined that the subject 5 is in the X-ray irradiation field. By using the image obtained by low-dose imaging, the presence or absence of the subject 5 can be determined without adding new hardware, even if the subject 5 protrudes from the bed 4 and is in the X-ray irradiation field.

[0031] (S403) The arithmetic device 2 causes the display device 3 to display a message warning that the subject 5 is in the X-ray irradiation field. In response to the message displayed on the display device 3, S402 and S403 are repeated until the operator operates the bed 4 to remove the subject 5 from the X-ray irradiation field. Note that when the number of times S402 and S403 are repeated reaches a predetermined threshold, the flow of the process may be terminated. Also, in S403, the arithmetic device 2 may control the bed 4 to remove the subject 5 from the X-ray irradiation field.

[0032] (S404) The computing device 2 controls the X-ray tube 6 to irradiate X-rays to the X-ray detector 10. The X-ray irradiation time is a predetermined length, and may be set according to the irradiation dose.

[0033] (S405) The computing device 2 controls the gantry 1 and the bed 4 to image the subject 5. That is, the bed 4 is moved into the gantry 1, and the X-ray tube 6 and the detector panel 7 are rotated around the subject 5 while repeating X-ray irradiation and counting of X-ray photons, thereby obtaining projection data from various directions. Then, a tomographic image is generated as a medical image by image reconstruction using the multiple projection data.

[0034] 4, it is possible to stabilize the polarization of the photon counting detector and suppress unnecessary exposure of the subject 5. That is, after it is confirmed that the subject 5 is not in the X-ray irradiation field, X-rays are irradiated so as to stabilize the polarization of the photon counting detector, thereby suppressing unnecessary exposure of the subject 5. In addition, since the polarization of the photon counting detector is stabilized by irradiation with X-rays before the subject 5 is imaged, there is no adverse effect on the image quality of the medical image.

[0035] The X-ray imaging apparatus of the present invention is not limited to the above-mentioned embodiment, and the components can be modified without departing from the gist of the invention. In addition, the components disclosed in the above-mentioned embodiment can be appropriately combined. Furthermore, some components can be deleted from all the components shown in the above-mentioned embodiment. [Explanation of symbols]

[0036] 1: gantry, 2: computing device, 3: display device, 4: bed, 5: subject, 6: X-ray tube, 7: detector panel, 8: input device, 9: memory device, 10: X-ray detector, 11: collimator, 12: detection pixel, 13: readout electrode, 15: high voltage electrode, 301: pulse output.

Claims

1. An X-ray source that emits X-rays; a photon counting detector that counts X-ray photons for each energy; An X-ray imaging apparatus comprising an image generating unit that generates a medical image of a subject based on a detector output from the photon counting detector, An X-ray imaging device further comprising a control unit that, when the polarization of the photon counting detector is not stable, causes the photon counting detector to irradiate X-rays from the X-ray source after confirming that the subject is not in an X-ray irradiation field, which is the range where X-rays are irradiated from the X-ray source.

2. 2. The X-ray imaging device according to claim 1, The control unit is configured to confirm that the subject is not in the X-ray irradiation field based on a position of a bed on which the subject is placed.

3. 2. The X-ray imaging device according to claim 1, Further comprising a camera for photographing the X-ray radiation field, The control unit is configured to confirm that the subject is not in the X-ray irradiation field based on the image captured by the camera.

4. 2. The X-ray imaging device according to claim 1, The control unit is characterized in that it confirms that the subject is not in the X-ray irradiation field based on an image generated by irradiating a lower X-ray dose than the X-ray dose used to capture the medical image.

5. 2. The X-ray imaging device according to claim 1, The control unit determines that the polarization of the photon counting detector is stable when the time elapsed since the previous imaging is shorter than a predetermined time.

6. 6. The X-ray imaging device according to claim 5, The X-ray imaging apparatus according to claim 1, wherein the predetermined time is set in accordance with an amount of radiation.

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