X-ray imaging device

Abstract

The invention provides an X-ray imaging device which stabilizes polarization of a photon counting detector and can suppress invalid radiation of a subject. This X-ray imaging device is provided with: an X-ray source that irradiates X-rays; a photon counting detector that counts X-ray photons by energy; the X-ray imaging apparatus includes a photon counting type detector, and an image generation unit that generates a medical image of a subject on the basis of a detector output from the photon counting type detector, the X-ray imaging apparatus further including a control unit that controls the photon counting type detector to generate the medical image of the subject when the polarization of the photon counting type detector is unstable. The X-ray is irradiated from the X-ray source to the photon counting detector after confirming that the subject is not present in an X-ray irradiation field, which is a range in which the X-ray is irradiated from the X-ray source.

Description

Technical Field

[0001] The present invention relates to an X-ray imaging device including a photon counting detector, and more particularly to a technology for suppressing the influence of polarization of the photon counting detector. Background Art

[0002] As an X-ray imaging device equipped with a photon counting detector, the development of PCCT (Photon Counting Computed Tomography) devices is being promoted. Since the photon counting detector can measure the energy of X-ray photons, the PCT device can present medical images containing more information than the existing CT device, such as medical images divided into multiple energy components.

[0003] The photon counting detector, which is enabled to take images by applying voltage, is polarized according to the length of time the voltage is applied, and the peak value of the detector output gradually decreases. The change in the peak value of the detector output causes an error in the energy of the measured X-ray photons, which has a negative impact on the image quality of the medical images 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 influence of the polarization of the photon counting detector, and starts data collection when the polarization is stable.

[0005] Patent Document 1: Japanese Patent Application Publication No. 2021-194354

[0006] However, the progress of polarization caused by X-ray irradiation is not considered in Patent Document 1. Polarization of the photon counting detector is performed not only by applying a high voltage but also by irradiation of X-rays. In order to photograph the subject after stabilizing the polarization based on the irradiation of X-rays, it is necessary to perform irradiation of X-rays to the photon counting detector before photographing. In addition, it is necessary to suppress ineffective radiation of the subject caused by the X-rays irradiated to the photon counting detector. Summary of the invention

[0007] Therefore, an object of the present invention is to provide an X-ray imaging apparatus capable of stabilizing the polarization of a photon counting detector and suppressing ineffective radiation from a subject.

[0008] In order to achieve the above-mentioned purpose, the present invention provides an X-ray imaging device, which comprises: an X-ray source, which irradiates X-rays; a photon counting detector, which counts X-ray photons according to energy; and an image generating unit, which generates a medical image of a subject based on the detector output from the photon counting detector. The X-ray imaging device is characterized in that it also comprises a control unit, which, when the polarization of the photon counting detector is unstable, causes the X-rays to be irradiated from the X-ray source to the photon counting detector after confirming that the subject does not exist within the range of the X-rays irradiated from the X-ray source, that is, in the X-ray irradiation field.

[0009] Effects of the Invention

[0010] 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 ineffective radiation from a subject. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a diagram showing the overall structure of the X-ray CT apparatus according to the first embodiment.

[0012] Figure 2 It is a diagram showing the structure of the detector panel of Example 1.

[0013] Figure 3 This is a diagram for explaining the influence of polarization.

[0014] Figure 4 This is a diagram showing an example of the processing flow of Example 1.

[0015] Explanation of symbols

[0016] 1-gantry, 2-computing device, 3-display device, 4-bed, 5-subject, 6-X-ray tube, 7-detector panel, 8-input device, 9-storage device, 10-X-ray detector, 11-collimator, 12-detection pixel, 13-readout electrode, 15-high voltage electrode, 301-pulse output. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The X-ray imaging device of the present invention is applicable to a device having 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.

[0018] [Example 1]

[0019] Figure 12 shows the overall structure of the X-ray CT apparatus of this embodiment. The X-ray CT apparatus is a device that includes a gantry 1, a computing device 2, a display device 3, and a bed 4, 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 value. Each part is described below.

[0020] The gantry 1 has a rotating portion that carries an X-ray tube 6 and a detector panel 7 for rotation, and a stationary portion that supports the rotating portion. The X-ray tube 6 is an X-ray source that generates X-rays by causing electrons accelerated at a high voltage of about 100 kV to hit a target. The detector panel 7 is a photon counting detector that is arranged opposite to the X-ray tube 6 with the subject 5 in between, counts X-ray photons that have passed through the subject 5, and measures 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 is acquired as projection data. In addition, 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 can be acquired. Use Figure 2 The detailed structure of the detector panel 7 will be described later.

[0021] While the X-ray tube 6 and the detector panel 7 are rotating around the subject 5, the projection data of the subject 5 from various directions are acquired by repeating the X-ray irradiation of the subject 5 by the X-ray tube 6 and the counting of X-ray photons by the detector panel 7. About 3,000 projection data are acquired per second and sent 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 for which the projection data is acquired.

[0022] The computing device 2 has the same hardware structure as a general computer device, and is equipped with a CPU (Central Processing Unit) or a memory, etc., and is connected to the display device 3, the input device 8, and the storage device 9. The computing device 2 generates a tomographic image by image reconstruction using a plurality of transmitted projection data, and controls each part. For example, the computing 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, etc.

[0023] The display device 3 is a liquid crystal display, a touch panel, etc., and displays the generated tomographic image, etc. The input device 8 is a keyboard, a mouse, etc., and is used to set the voltage applied to the X-ray tube 6, etc. In addition, when the display device 3 is a touch panel, the touch panel functions as the input device 8. The storage device 9 is a HDD (Hard Disk Drive) or SSD (Solid State Drive), etc., and stores various data such as programs executed by the CPU, projection data, and tomographic images.

[0024] use Figure 2 An example of the detector panel 7 will be described. The detector panel 7 includes an X-ray detector 10 and a collimator 11 , and is provided in a rotating portion of the gantry 1 .

[0025] 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 in the X-ray detector 10, and a high voltage electrode 15 is provided on the side where the X-rays are incident, and a plurality of readout electrodes 13 are provided on the opposite side. A negative high voltage is applied to the high voltage electrode 15 relative to the readout electrode 13, which is a 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 corresponding 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 through the electric field between the electrodes and are read out as an electrical signal. That is, the readout electrode 13 corresponds to the detection pixel 12 of the X-ray detector 10.

[0026] The collimator 11 is a metal grid having a plurality of holes and is provided at the front stage of the X-ray detector 10 to suppress the scattered rays generated on the subject 5 and the like from being incident on the X-ray detector 10. A metal having a large specific gravity and atomic number, such as tungsten or molybdenum, is used for the collimator 11, and the holes of the collimator 11 are aligned in a manner corresponding to the detection pixels 12.

[0027] use Figure 3 The influence of polarization of X-ray detector 10 will be described. Figure 3 (a) is an example of an output waveform generated when an X-ray photon enters the X-ray detector 10, with the horizontal axis representing time and the vertical axis representing the detector output. Figure 3 (b) is an example of a decay curve showing the temporal change of the peak value of the output waveform, in which the horizontal axis represents the X-ray irradiation time and the vertical axis represents the peak value of the detector output.

[0028] In a photon counting detector, a pulse output 301 is generated every time an X-ray photon is incident within the insensitive time τ, and the wave height value of the pulse output 301 is proportional to the photon energy. Figure 3(a) shows pulse output 301L when the photon energy of the incident X-ray photon is low and pulse output 301H when it is high. That is, by distinguishing the detector output by thresholds Th1, Th2, Th3, etc., X-ray photons are identified by photon energy.

[0029] However, if X-rays continue to irradiate the X-ray detector 10, electrons or holes are captured inside and polarized as the X-ray irradiation time passes. Since polarization delays the movement of electrons or holes, part of the electrons generated between the electrodes cannot reach the readout electrode 13 within the insensitive time τ, as shown in FIG. Figure 3 As shown in (b), the wave height value decreases as the irradiation time increases. That is, even if X-ray photons with the same photon energy are incident, they will have different wave height values, so the energy of the measured X-ray photons will have errors. In addition, the change in the wave height value is different immediately after the X-ray irradiation (for example, Figure 3 The polarization of (b) is the largest during period A), and decreases with the passage of time during periods B and C. That is, the polarization of the X-ray detector 10 becomes stable as the irradiation time of the X-rays passes.

[0030] Therefore, in the first embodiment, the polarization of the X-ray detector 10 is stabilized by irradiating X-rays before imaging the subject. If the polarization is excessive, the characteristics of the X-ray detector 10 deteriorate, resulting in an increase in noise and a decrease in the count rate. Therefore, it is preferable to stabilize the polarization during the period when the peak value reaches a predetermined threshold or higher (for example, not during the period when the peak value reaches a predetermined threshold or higher). Figure 3 (b) during C, but during B) the X-ray irradiation is stopped.

[0031] use Figure 4 The processing flow of Example 1 is described.

[0032] (S401)

[0033] The calculation device 2 determines whether the polarization of the X-ray detector 10 is stable. If the polarization is unstable, the process of S402 is performed, and if the polarization is stable, the process of S405 is performed.

[0034] Whether the polarization of the X-ray detector 10 is stable is determined, for example, based on the time that has passed since the last shot. That is, if the time that has passed since the last shot is shorter than the specified time, the polarization is determined to be stable. Based on the time that has passed since the last shot, it is possible to determine whether the polarization is stable through simple processing. The specified time is pre-set based on a pre-measurement result, for example, 10 minutes. In addition, the greater the irradiation dose, the more polarization is performed, so the specified time is set according to the irradiation dose. By setting the specified time according to the irradiation dose, it is possible to more accurately determine whether the polarization is stable.

[0035] (S402)

[0036] The computing device 2 determines whether the subject 5 is present in the X-ray irradiation field. The X-ray irradiation field is a range where X-rays are irradiated from the X-ray tube 6. If the subject 5 is not present in the X-ray irradiation field, the processing of S404 is performed. If the subject 5 is present in the X-ray irradiation field, the processing returns to S402 via S403.

[0037] Whether the subject 5 is present in the X-ray irradiation field is determined, for example, based on the position of the bed 4. That is, if the bed 4 is present in the X-ray irradiation field, it is determined that the subject 5 is present in the X-ray irradiation field. By determining the presence or absence of the subject 5 based on the position of the bed 4 without adding new hardware.

[0038] Furthermore, it is also possible to determine whether the subject 5 is present in the X-ray irradiation field based on the image of the camera that captures the X-ray irradiation field. That is, if the subject 5 is included in the camera image, it is determined that the subject 5 is present in the X-ray irradiation field. By using the image based on the camera, even if the subject 5 extending from the bed 4 is present in the X-ray irradiation field, it is possible to determine whether the subject 5 is present.

[0039] It is also possible to determine whether the subject 5 is present in the X-ray irradiation field based on the image obtained by low-dose imaging. In addition, low-dose imaging is imaging using an X-ray dose lower than the X-ray dose irradiated from the X-ray tube 6 when taking a medical image of the subject 5. That is, if the subject 5 is included in the image obtained by low-dose imaging, it is determined that the subject 5 is present in the X-ray irradiation field. By using the image based on low-dose imaging, even if the subject 5 extending from the bed 4 is present in the X-ray irradiation field, it is possible to determine whether the subject 5 is present or not without adding new hardware.

[0040] (S403)

[0041] The computing device 2 displays a message on the display device 3 to warn that the subject 5 is in the X-ray irradiation field. S402 and S403 are repeated according to the message displayed on the display device 3 until the operator operates the bed 4 to make the subject 5 retreat from the X-ray irradiation field. In addition, if the number of repetitions of S402 and S403 reaches a predetermined threshold, the processing flow can be terminated. In addition, in S403, the computing device 2 can also control the bed 4 to make the subject 5 retreat from the X-ray irradiation field.

[0042] (S404)

[0043] The calculation device 2 controls the X-ray tube 6 to irradiate X-rays toward the X-ray detector 10. The irradiation time of X-rays may be a predetermined length, or may be set according to the irradiation dose.

[0044] (S405)

[0045] The computing device 2 controls the gantry 1 and the bed 4 to image the subject 5. That is, the bed 4 is moved to 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 acquiring projection data from various directions. Then, a tomographic image is generated as a medical image by image reconstruction using a plurality of projection data.

[0046] By using Figure 4 The processing flow described above stabilizes the polarization of the photon counting detector and can suppress ineffective radiation from the subject 5. That is, after confirming that the subject 5 does not exist in the X-ray irradiation field, X-rays are irradiated to stabilize the polarization of the photon counting detector, thereby suppressing ineffective radiation from the subject 5. Furthermore, since the subject 5 is imaged after the polarization of the photon counting detector is stabilized by the irradiation of X-rays, there is no adverse effect on the image quality of the medical image.

[0047] In addition, the X-ray imaging device of the present invention is not limited to the above-mentioned embodiments, and the constituent elements can be deformed and concretized within the scope of the invention. In addition, the multiple constituent elements disclosed in the above-mentioned embodiments can be appropriately combined. In addition, some constituent elements can be deleted from all the constituent elements shown in the above-mentioned embodiments.

Claims

1. An X-ray imaging device comprising: An X-ray source, irradiating X-rays; Photon counting detectors, which count X-ray photons by energy; and an image generating unit that generates a medical image of the subject based on the detector output from the photon counting detector, The X-ray imaging device is characterized in that It also includes a control unit that, when polarization of the photon counting detector is unstable, causes the X-ray source to irradiate the photon counting detector with X-rays after confirming that the subject is not within the range of the X-rays irradiated from the X-ray source, that is, in the X-ray irradiation field.

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

3. The X-ray imaging device according to claim 1, characterized in that: The X-ray imaging device further includes a camera for photographing the X-ray irradiation field. The control unit confirms that the subject is not present in the X-ray irradiation field based on the image captured by the camera.

4. The X-ray imaging device according to claim 1, wherein: The control unit confirms that the subject is not present in the X-ray irradiation field based on an image generated by irradiating an X-ray dose lower than the X-ray dose when the medical image is captured.

5. The X-ray imaging device according to claim 1, characterized in that: The control unit determines that the polarization of the photon counting detector is stable when the elapsed time from the last imaging is shorter than a predetermined time set in advance.

6. The X-ray imaging device according to claim 5, characterized in that: The predetermined time is set according to the irradiation dose.

Images