X-ray CT device

Abstract

Provided is an X-ray CT device capable of maintaining the image quality of a tomographic image while reducing the irradiation dose of a region that is highly sensitive to radiation. An X-ray CT device is provided with: an X-ray source that irradiates a subject with X-rays; an X-ray detector that detects the X-rays transmitted through the subject; a rotation plate that rotates the X-ray source and the X-ray detector around the subject; an image generation unit that generates a tomographic image of the subject on the basis of a detection signal from the X-ray detector; the X-ray CT device is characterized in that the control part is provided with a modulation pattern setting part, a modulation pattern setting part and a modulation pattern setting part, setting a modulation pattern used in modulation of an irradiation dose, which is an X-ray dose irradiated from an X-ray source during rotation of the rotating plate, such that the irradiation dose to a site having high radiation sensitivity to a subject is reduced than to other sites; and a modulation pattern adjustment unit that adjusts the modulation pattern on the basis of an upper limit speed that is the upper limit of the modulation speed of the irradiation dose.

Description

Technical Field

[0001] The present invention relates to an X-ray CT device for taking cross-sectional images of an object to be examined, and more particularly to a technology for reducing exposure of the object to be examined. Background Art

[0002] The X-ray CT apparatus generates a tomographic image of the subject using projection data from multiple directions obtained by rotating an X-ray source that irradiates the subject with X-rays and an X-ray detector that detects X-rays that pass through the subject around the subject. The generated tomographic image depicts the shape of organs in the subject and is used for image diagnosis.

[0003] As the dose of X-rays irradiating the subject increases, the quality of the tomographic image improves, but the exposure of the subject increases. Since the increase in exposure has an adverse effect on the subject, it is important to suppress the amount of X-rays irradiated, i.e., the irradiation dose, especially to the eyes, breasts, and other parts of the body that are located on the surface and highly sensitive to radiation.

[0004] Patent Document 1 discloses an X-ray source that rotates around a subject to reduce the irradiation dose when the X-ray source is located in front of the subject, which is a part with high radiation sensitivity, than when the X-ray source is located behind the subject.

[0005] Patent Document 1: U.S. Patent No. 9867587

[0006] However, in Patent Document 1, the specific method for controlling the irradiation dose is not sufficiently disclosed, and the irradiation dose may not be controlled according to the preset indicated value. That is, since the modulation speed of the irradiation dose has an upper limit that depends on the performance of the hardware or the upper limit set by the operator, the irradiation dose according to the indicated value may not be obtained. If the irradiation dose according to the indicated value is not obtained, the irradiation dose of the part with high sensitivity to radiation is not reduced sufficiently, or the image quality of the tomographic image is reduced due to insufficient irradiation dose. Summary of the invention

[0007] Therefore, an object of the present invention is to provide an X-ray CT apparatus capable of maintaining the quality of tomographic images while reducing the irradiation dose to a site with high sensitivity to radiation.

[0008] In order to achieve the above-mentioned purpose, the X-ray CT device of the present invention comprises: an X-ray source for irradiating an object with X-rays; an X-ray detector for detecting the X-rays that pass through the object; a rotating plate for rotating the X-ray source and the X-ray detector around the object; an image generating unit for generating a tomographic image of the object based on the detection signal of the X-ray detector; and a control unit for controlling each unit, wherein the X-ray CT device is characterized in that the control unit comprises: a modulation pattern setting unit for setting the modulation pattern used in modulating the amount of X-rays irradiated from the X-ray source during the rotation of the rotating plate, i.e., the irradiation dose, so that the irradiation dose to a part of the object with high radiation sensitivity is less than the irradiation dose to other parts; and a modulation pattern adjusting unit for adjusting the modulation pattern based on an upper limit speed which is an upper limit of the modulation speed of the irradiation dose.

[0009] Effects of the Invention

[0010] According to the present invention, it is possible to provide an X-ray CT apparatus capable of maintaining the quality of tomographic images while reducing the irradiation dose to a site with high sensitivity to radiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a diagram showing an example of the overall configuration of the X-ray CT apparatus according to the first embodiment.

[0012] Figure 2 It is a diagram for explaining modulation of irradiation dose.

[0013] Figure 3 This is a diagram showing an example of a functional block of Example 1.

[0014] Figure 4 This is a diagram showing an example of the flow of processing for calculating the upper limit rate of the irradiation dose.

[0015] Figure 5 This is a diagram showing an example of the result of measuring the temporal change of the tube current.

[0016] Figure 6 This is a diagram showing an example of the process flow of Example 1.

[0017] Figure 7 This is a diagram showing an example of the flow of processing for setting the modulation pattern of the irradiation dose.

[0018] Figure 8 This is a diagram showing an example of the flow of processing for adjusting a set modulation pattern.

[0019] Fig. 9 It is a diagram for explaining adjustment of the modulation pattern.

[0020] Explanation of symbols

[0021] 10-subject, 100-scanning gantry unit, 101-X-ray source, 102-rotating plate, 103-collimator, 104-opening unit, 105-workbench, 106-X-ray detector, 107-data collection unit, 108-rotating plate control unit, 109-workbench control unit, 110-X-ray control unit, 111-high voltage generating unit, 120-operating unit, 121-input unit, 122-image generating unit, 123-storage unit, 124-system control unit, 125-display unit, 301-modulation pattern setting unit, 302-modulation pattern adjusting unit. DETAILED DESCRIPTION

[0022] Hereinafter, embodiments of an X-ray CT (Computed Tomography) apparatus according to the present invention will be described with reference to the accompanying drawings. The X-ray CT apparatus is an apparatus that generates tomographic images as medical images used for diagnosis of a subject, etc.

[0023] [Example 1]

[0024] use Figure 1 , the overall structure of the X-ray CT apparatus of Example 1 is described. The X-ray CT apparatus includes a scanning gantry 100 and an operating unit 120. The scanning gantry 100 is disposed in an imaging room surrounded by a shielding material for shielding X-rays, and the operating unit 120 is disposed in an operating room located outside the imaging room.

[0025] The scanning gantry unit 100 includes an X-ray source 101, a rotating plate 102, a collimator 103, an X-ray detector 106, a data collection unit 107, a table 105, a rotating plate control unit 108, a table control unit 109, an X-ray control unit 110, and a high voltage generator 111. The X-ray source 101 is a device for irradiating an object 10 placed on the table 105 with X-rays, such as an X-ray tube device. The collimator 103 is a device for limiting the irradiation range of X-rays. The rotating plate 102 includes an opening 104 for allowing the object 10 placed on the table 105 to enter, and carries the X-ray source 101 and the X-ray detector 106, so that the X-ray source 101 and the X-ray detector 106 rotate around the object 10.

[0026] The X-ray detector 106 is disposed opposite to the X-ray source 101 and is a device that acquires the spatial distribution of X-rays by having a plurality of detection elements for detecting X-rays. The detection elements of the X-ray detector 106 are arranged two-dimensionally in the rotation direction and the rotation axis direction of the rotating plate 102. The data collection unit 107 is a device that collects the spatial distribution of X-rays acquired by the X-ray detector 106 as digital data.

[0027] The rotating plate control unit 108 is a device for controlling the rotation and tilt of the rotating plate 102. The table control unit 109 is a device for controlling the up-down, forward-backward, left-right, and right-left movements of the table 105. The high voltage generator 111 is a power source for generating a tube voltage, which is a voltage applied to the X-ray source 101, and a tube current, which is a current supplied to the X-ray source 101. The X-ray control unit 110 is a device for controlling the output of the high voltage generator 111. The rotating plate control unit 108, the table control unit 109, and the X-ray control unit 110 are, for example, an MPU (Micro-Processing Unit).

[0028] The operation unit 120 includes an input unit 121, an image generation unit 122, a display unit 125, a storage unit 123, and a system control unit 124. The input unit 121 is a device for inputting examination data such as the name of the subject 10, the examination date and time, and the imaging conditions, and is, for example, a keyboard, a pointing device, a touch panel, etc. The image generation unit 122 is a device for generating a tomographic image using the digital data collected by the data collection unit 107, and is, for example, an MPU or a GPU (Graphics Processing Unit). The display unit 125 is a device for displaying the tomographic image generated by the image generation unit 122, and is, for example, a liquid crystal display and a touch panel. The storage unit 123 is a device for storing the digital data collected by the data collection unit 107, the tomographic image generated by the image generation unit 122, the program executed by the system control unit 124, the data used by the program, and is, for example, an HDD (Hard Disk Drive) or an SSD (Solid State Drive). The system control unit 124 is a device that controls each unit such as the rotating plate control unit 108 , the stage control unit 109 , and the X-ray control unit 110 , and is, for example, a CPU (Central Processing Unit).

[0029] The high voltage generator 111 generates a tube voltage and a tube current according to the imaging conditions set via the input unit 121, so that the X-rays corresponding to the imaging conditions are irradiated from the X-ray source 101 to the subject 10. The X-ray detector 106 detects the X-rays irradiated from the X-ray source 101 and transmitted through the subject 10 using a plurality of detection elements, and acquires the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate controller 108, and rotates based on the imaging conditions input from the input unit 121, especially based on the rotation speed, etc. The table 105 is controlled by the table controller 109, and moves relative to the rotating plate 102, so that the imaging position designated for the subject 10 is moved to the imaging field of view, which is the range for detecting the transmitted X-rays.

[0030] By repeating X-ray irradiation based on the X-ray source 101 and X-ray detection based on the X-ray detector 106 as the rotating plate 102 rotates, projection data as an X-ray projection image of the subject 10 is measured at various projection angles. The projection data is associated with the view (View) representing each projection angle, the channel (ch) number as the detection element number of the X-ray detector 106, and the column number. The measured projection data is sent to the image generation unit 122. The image generation unit 122 generates a tomographic image by back-projecting a plurality of projection data. The generated tomographic image is displayed on the display unit 125 as a medical image, or stored in the storage unit 123.

[0031] In an X-ray CT apparatus, as the amount of X-rays irradiated to the subject 10, i.e., the irradiation dose, increases, the quality of the generated tomographic image improves, but the exposure dose of the subject 10 increases. Since the increase in exposure dose has an adverse effect on the subject 10, it is necessary to suppress the irradiation dose. In particular, in order to reduce the exposure dose of parts such as eyes and breasts that are located on the surface of the body and have high radiation sensitivity, the irradiation dose to the subject 10 is sometimes modulated during the rotation of the rotating plate 102.

[0032] use Figure 2 , the irradiation dose modulated during the rotation of the rotating plate 102 is described. Figure 2 The upper part of shows an X-ray source 101 rotating around the head of the subject 10. When the rotation angle θ of the X-ray source 101 is in the range of θ1 to θ2, the X-rays irradiate the eyes, which are parts with high sensitivity to radiation, and therefore, in order to reduce the exposure amount of the eyes, the irradiation dose is modulated according to the rotation angle θ.

[0033] exist Figure 2 In the lower part of FIG. 1 , a solid line is used to show an example of a modulation pattern of the irradiation dose. In addition, the vertical axis is the irradiation dose, and the horizontal axis is the rotation angle θ. For comparison, the irradiation dose before modulation is shown by a dotted line. In addition, since the X-ray source 101 rotates at a constant speed, the rotation angle θ is proportional to time. Figure 2 In the modulation pattern illustrated in , D_L is set to be less than the irradiation dose D_0 before modulation under θ12<θ<θ21 to reduce the exposure amount under θ1<θ<θ2. In addition, in order to compensate for the reduction in irradiation dose under θ12<θ<θ21, D_H is set to be more than the irradiation dose D_0 before modulation under θ<θ11 and θ22<θ. Furthermore, the irradiation dose is changed from D_H to D_L under θ11<θ<θ12, and from D_L to D_H under θ21<θ<θ22.

[0034] Since the modulation speed of the irradiation dose has an upper limit that depends on the hardware performance, the change of the irradiation dose from D_H to D_L or from D_L to D_H is not timely, which sometimes leads to insufficient reduction of exposure or degradation of the image quality of the tomographic image. Therefore, in Example 1, according to the upper limit speed which is the upper limit of the modulation speed of the irradiation dose, the modulation pattern is adjusted to set the irradiation dose of the part with high sensitivity to radiation to be lower than the irradiation dose of other parts. By adjusting the modulation pattern according to the upper limit speed, it is possible to reduce the irradiation dose of the part with high sensitivity to radiation while maintaining the image quality of the tomographic image.

[0035] use Figure 3 , the functional blocks of the first embodiment are described. In addition, these functional blocks may be constituted by dedicated hardware using an ASIC (Application Specific Integrated Circuit) or an FPGA (Field-Programmable Gate Array), or may be constituted by software running on the system control unit 124. In the following description, the case where the functional blocks of the first embodiment are constituted by software is described.

[0036] In the first embodiment, a modulation pattern setting unit 301 and a modulation pattern adjusting unit 302 are provided. Each unit will be described below. In addition, an upper limit speed which is an upper limit of the modulation speed of the irradiation dose is stored in advance in the storage unit 123 .

[0037] The modulation pattern setting unit 301 sets a modulation pattern used in modulating the amount of X-rays irradiated from the X-ray source 101 to the subject 10 during the rotation of the rotating plate 102, that is, the irradiation dose. In the modulation pattern set by the modulation pattern setting unit 301, Figure 2 As shown in the example, the irradiation dose to the parts with high radiation sensitivity, such as eyes or breasts, is reduced compared to the irradiation dose to other parts. In addition, the modulation pattern can also be set to make the amount of transmitted X-rays at each rotation angle uniform. By making the amount of transmitted X-rays uniform, the balance between the image quality of the tomographic image and the exposure amount of the subject is maintained. In addition, the modulation pattern set by the modulation pattern setting unit 301 can also be displayed on the display unit 125.

[0038] The modulation pattern adjustment unit 302 reads the upper limit speed, which is the upper limit of the modulation speed of the irradiation dose, from the storage unit 123, and adjusts the modulation pattern set by the modulation pattern setting unit 301 according to the upper limit speed. The upper limit speed is calculated based on the result of measuring the temporal change of the irradiation dose in advance and stored in the storage unit 123.

[0039] use Figure 4, an example of the process of calculating the upper limit rate of the irradiation dose is described step by step.

[0040] (S401)

[0041] The system control unit 124 measures the temporal change of the tube current, which is the current supplied to the X-ray source 101. Since the irradiation dose is proportional to the tube current, the temporal change of the tube current is equivalent to the temporal change of the irradiation dose. In addition, since the modulation speed is different when the tube current rises and falls, the temporal change is measured separately when it rises and falls.

[0042] Figure 5 This is an example of the result of measuring the time variation of the tube current. The solid line indicates the rise, and the dotted line indicates the fall. The time variation during the rise is measured by setting the indicated value of the tube current to the maximum value when the tube current is zero, and the time variation during the fall is measured by setting the indicated value of the tube current to zero when the tube current is the maximum value.

[0043] (S402)

[0044] The system control unit 124 calculates the upper limit speed using the change over time measured in S401 . Figure 5 The slope at each point of the temporal change shown in exemplifies the upper limit of the modulation speed of the tube current, so the upper limit speed is calculated for each tube current value during the rising and falling periods. The upper limit speeds calculated for each tube current value during the rising and falling periods are stored in the storage unit 123.

[0045] (S403)

[0046] The system control unit 124 generates an approximate function for the temporal change measured in S401. As the approximate function, for example, a quadratic function such as the following equation is used.

[0047] K_up(t)=A_up·t^2+B_up·t+C_up……(Formula 1)

[0048] Here, t is time, K_up(t) is the temporal change of the tube current during the rise, and A_up, B_up, and C_up are coefficients.

[0049] By generating the approximate function illustrated in (Equation 1), it is sufficient to store coefficients instead of the upper limit speed for each tube current value, thereby reducing the capacity of the storage unit 123. In order to improve the accuracy of the approximate function, the logarithm or square root of the tube current may be combined.

[0050] By using Figure 4The processing flow described above calculates the upper limit speed as the upper limit of the modulation speed of the irradiation dose and stores it in the storage unit 123. The upper limit speed stored in the storage unit 123 is used to adjust the modulation pattern used for modulating the irradiation dose while the rotating plate 102 rotates.

[0051] use Figure 6 , an example of the processing flow of Example 1 is described step by step.

[0052] (S601)

[0053] The modulation pattern setting unit 301 sets a modulation pattern of the irradiation dose so that the irradiation dose to a site with high sensitivity to radiation is reduced compared to the irradiation dose to other sites.

[0054] use Figure 7 , an example of the processing flow of S601 is described step by step.

[0055] (S701)

[0056] The system control unit 124 acquires a transmission X-ray image of the subject 10. The transmission X-ray image acquired in S701 is an image called a scanogram image or a scout image, and is used to set the range of CT imaging.

[0057] (S702)

[0058] The system control unit 124 uses the transmission X-ray image acquired in S701 to create a cross-sectional model of the subject 10, and calculates a modulation pattern of the irradiation dose based on the cross-sectional model. The cross-sectional model is a model in which the cross-sectional shape of the subject 10 is approximated as, for example, an ellipse. By creating the cross-sectional model, the X-ray transmission length through the rotation center of the rotating plate 102 can be calculated for each rotation angle. Furthermore, the modulation pattern of the irradiation dose is calculated so that the amount of transmitted X-rays for the X-ray transmission length calculated for each rotation angle is uniform. In addition, in the case where the cross-sectional shape of the subject 10 can be regarded as a perfect circle, the modulation pattern of the irradiation dose is as follows: Figure 2 As shown by the dotted line in the lower section of , it is constant regardless of the rotation angle θ. The process of S702 is not essential.

[0059] (S703)

[0060] The modulation pattern setting unit 301 obtains a range in which the irradiation dose is reduced, i.e., a low dose range. The low dose range may be set by an operator using the input unit 121 for the transmission X-ray image obtained in S701, or by the system control unit 124 using an image recognition function. The modulation pattern setting unit 301 obtains the set low dose range.

[0061] (S704)

[0062] The modulation pattern setting unit 301 sets the modulation pattern of the irradiation dose according to the low dose range acquired in S703. The modulation pattern set by the modulation pattern setting unit 301 is, for example, Figure 2 The lower section of the modulation pattern is represented by a solid line. In addition, it is preferable to set the modulation pattern in such a way that the total amount of irradiation dose during one rotation of the rotating plate 102 is equal before and after the modulation. In the case where it is difficult to maintain the total amount of irradiation dose before and after the modulation, the modulation pattern may be set in such a way that the difference in the total amount of irradiation dose before and after the modulation is less than a predetermined threshold value. Furthermore, the amount of irradiation dose reduction under θ11<θ<θ22 may be compensated by increasing the irradiation dose under θ11+π<θ<022+π. Return to Figure 6 Description.

[0063] (S602)

[0064] The modulation pattern adjustment unit 302 adjusts the modulation pattern set in S601 according to the upper limit speed which is the upper limit of the modulation speed of the irradiation dose.

[0065] use Figure 8 , an example of the processing flow of S602 is described step by step.

[0066] (S801)

[0067] The modulation pattern adjustment unit 302 determines whether there is a period of modulation speed exceeding the upper limit speed in the modulation pattern set in S601. The determination of whether there is a period of modulation speed exceeding the upper limit speed, i.e., an excess period, is performed when the irradiation dose is decreased and when the irradiation dose is increased.

[0068] If there is an excess period in the modulation pattern, the process proceeds to S802, and if there is no excess period, the process flow ends.

[0069] (S802)

[0070] The modulation pattern adjustment unit 302 changes the modulation speed during the period determined to exceed the upper limit speed in S801 to the upper limit speed. By changing the modulation speed during the period to the upper limit speed, it is possible to prevent insufficient reduction of the irradiation dose to a site with high radiation sensitivity.

[0071] In addition, if the modulation speed during the overrun period is changed to the upper limit speed, the total amount of irradiation dose during one rotation of the rotating plate 102 decreases, so the irradiation dose to the parts other than the parts with high radiation sensitivity can be increased to maintain the total amount of irradiation dose. By maintaining the total amount of irradiation dose during one rotation of the rotating plate 102, it is possible to suppress the degradation of the image quality of the tomographic image.

[0072] use Fig. 9 , the adjustment of the modulation pattern is explained. Fig. 9 In FIG. 1 , the modulation pattern before adjustment is indicated by a dotted line, and the modulation pattern after adjustment is indicated by a solid line. Figure 2 The modulation speed is equivalent to the slope of the irradiation dose with respect to the rotation angle.

[0073] In the modulation pattern before adjustment, the modulation speed during the period of θ11<θ<θ12 exceeds the upper limit speed during the descent, and the modulation speed during the period of θ21<θ<θ22 exceeds the upper limit speed during the rise, so the modulation speed during each exceeding period is changed to the upper limit speed. In addition, the period of θ12<θ<θ21 is not changed so that the irradiation dose of the part with high sensitivity to radiation is maintained at D_L, and the period of irradiation dose change becomes θ13<θ<θ12 and θ21<θ<θ23. And, in order to compensate for the reduction in irradiation dose caused by the change in the modulation speed during the exceeding period, the irradiation dose under θ<θ13 and θ23<θ is increased from D_H to D_HH.

[0074] In addition, when the irradiation dose D_HH to parts other than parts with high radiation sensitivity exceeds the maximum output of the high voltage generating unit 111 or the threshold value set by the operator, the irradiation dose D_L to parts with high radiation sensitivity may be increased. By increasing the irradiation dose D_L to parts with high radiation sensitivity, the image quality of the tomographic image can be maintained even when the irradiation dose to parts other than parts with high radiation sensitivity is insufficient. Furthermore, when the irradiation dose D_L to parts with high radiation sensitivity is lower than the minimum output of the high voltage generating unit 111 or the threshold value set by the operator, the irradiation dose to parts other than parts with high radiation sensitivity may be reduced. By reducing the irradiation dose to parts other than parts with high radiation sensitivity, the exposure amount of the subject 10 can be reduced. Return to Figure 6 Description.

[0075] (S603)

[0076] The system control unit 124 images the subject 10 using the modulation pattern adjusted in S602 .

[0077] By using Figure 6The processing flow described above images the subject 10 using a modulation pattern adjusted according to the upper limit rate of the modulation rate of the irradiation dose, and thus can reduce the irradiation dose to a site with high radiation sensitivity while maintaining the image quality of the tomographic image.

[0078] The above is a description of the embodiments of the present invention. The present invention is not limited to the above embodiments, and the constituent elements can be deformed and concretized within the scope of the main purpose of the invention. In addition, multiple constituent elements disclosed in the above embodiments can be appropriately combined. In addition, some constituent elements can be deleted from all the constituent elements shown in the above embodiments.

Claims

1. An X-ray CT device, comprising: An X-ray source irradiates the subject with X-rays; An X-ray detector for detecting X-rays transmitted through the subject; A rotating plate to rotate the X-ray source and the X-ray detector around the subject; an image generating unit that generates a tomographic image of the subject based on the detection signal of the X-ray detector; and Control department, control each department, The X-ray CT device is characterized in that: The control unit has: a modulation pattern setting unit that sets a modulation pattern used in modulating the amount of X-rays irradiated from the X-ray source during the rotation of the rotating plate, i.e., an irradiation dose, so that the irradiation dose to a portion of the subject having high radiation sensitivity is reduced compared to the irradiation dose to other portions; and The modulation pattern adjustment unit adjusts the modulation pattern according to an upper limit rate that is an upper limit of a modulation rate of the irradiation dose.

2. The X-ray CT apparatus according to claim 1, characterized in that: The modulation pattern adjustment unit changes a modulation speed exceeding the upper limit speed in the modulation pattern set by the modulation pattern setting unit to the upper limit speed.

3. The X-ray CT apparatus according to claim 2, characterized in that: The modulation pattern adjustment unit increases the irradiation dose to the other parts to compensate for the total amount of irradiation dose reduced by the change in the modulation speed.

4. The X-ray CT apparatus according to claim 3, characterized in that: The modulation pattern adjustment unit increases the irradiation dose to the site with high radiation sensitivity when the irradiation dose to the other site exceeds an upper limit value.

5. The X-ray CT apparatus according to claim 3, characterized in that: The modulation pattern adjustment unit reduces the irradiation dose to the other parts when the irradiation dose to the part with high sensitivity to radiation is lower than a lower limit value.

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