Medical system and recording medium

Abstract

To provide techniques capable of obtaining optimal delay times.SOLUTION: An embodiment comprises: setting a first delay time TA from when the tube voltage is switched to a high kV to when data collection starts and a second delay time TB from when the tube voltage is switched to a low kV to when data collection starts; determining a high-kV first effective voltage value VH when a high kV is applied to an X-ray tube; determining a low-kV second effective voltage value VL when a low kV is applied to the X-ray tube; and calculating an effective voltage difference representing the difference between the effective voltage value VH and the effective voltage value VL. Furthermore, these steps are executed multiple times to acquire multiple effective voltage differences; and, on the basis of the multiple effective voltage differences, a first optimal delay time TAN and a second optimal delay time TBN are obtained.SELECTED DRAWING: Figure 16

Description

[Technical Field]

[0001] The present invention relates to a medical system capable of switching tube voltages, and a recording medium on which instructions for controlling the medical system are recorded. [Background technology]

[0002] CT systems are known as medical devices that capture images of a subject non-invasively. CT systems are widely used in hospitals and other medical facilities because they can acquire cross-sectional images of the subject in a short scanning time.

[0003] A CT system applies a predetermined voltage to the cathode-anode tube of an X-ray tube to generate X-rays. The generated X-rays pass through the subject and are detected by a detector. The CT system reconstructs a CT image of the subject based on the data detected by the detector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6031618 Summary of the Invention [Problem to be solved by the invention]

[0005] SECT (Single Energy CT) is known as an imaging technique for CT systems. SECT is a method of generating X-rays by applying a predetermined voltage (e.g., 120 kV) to the cathode-anode tube of an X-ray tube, thereby obtaining a CT image of the subject. However, with SECT, different substances can have similar CT values, making it difficult to identify the different substances.

[0006] Therefore, research and development has been conducted on DECT (Dual Energy CT) technology. DECT is a technology that uses X-rays in different energy ranges to distinguish materials, and is becoming widely used because it can obtain images that are useful for diagnosis in clinical settings. DECT technology is known for its kV switching technology, which switches the X-ray tube voltage between low and high.

[0007] With kV switching technology, it is necessary to separate the data acquired by the detector into data corresponding to high tube voltage and data corresponding to low tube voltage. Generally, if the voltage difference between the high and low tube voltages is small, the accuracy of material identification (data identification accuracy) tends to be low. Therefore, with kV switching technology, the tube voltage is set so that the difference between the high and low tube voltages is as large as possible.

[0008] On the other hand, kV switching requires switching between low and high tube voltages in a short time. Ideally, when switching tube voltages, the voltage should change in a rectangular pattern. However, in reality, when switching tube voltages, it takes a certain amount of time for the tube voltage to reach the desired value. For example, when switching from a low tube voltage to a high tube voltage, a certain rise time is required for the tube voltage to reach the desired value. Similarly, when switching from a high tube voltage to a low tube voltage, a certain fall time is required for the tube voltage to reach the desired value. Therefore, to reduce the effects of this tube voltage rise time and fall time, CT systems do not start data acquisition immediately after switching the tube voltage, but instead provide a delay time between switching the tube voltage and starting data acquisition for the corresponding tube voltage.

[0009] However, the rise and fall times of the tube voltage of CT systems vary from one to another, and the delay time set in one CT system is not necessarily the appropriate delay time for another CT system. Furthermore, as CT systems are used over a long period of time, their components deteriorate over time, causing the currently used delay time to deviate from the ideal delay time, resulting in a deterioration of image quality. Therefore, a technique that can determine the optimum delay time is desired. [Means for solving the problem]

[0010] A first aspect of the present invention provides an X-ray tube configured to be able to switch a tube voltage applied to the X-ray tube between a first tube voltage and a second tube voltage; a detector assembly for detecting X-rays emitted from the X-ray tube, the detector assembly including a reference region for detecting X-rays and a filter, the reference region including a first sub-region for detecting X-rays that have passed through the filter and a second sub-region for detecting X-rays that have not passed through the filter; one or more processors, setting a first delay time from when the tube voltage is switched to the first tube voltage to when data acquisition starts, and a second delay time from when the tube voltage is switched to the second tube voltage to when data acquisition starts; determining a first effective voltage value of the first tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the first tube voltage is applied to the X-ray tube; determining a second effective voltage value of the second tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the second tube voltage is applied to the X-ray tube; and Calculating an effective voltage difference representing the difference between the first effective voltage value and the second effective voltage value. and one or more processors executing Including, the one or more processors: a plurality of effective voltage differences are obtained by performing a plurality of steps of setting the first delay time and the second delay time, determining the first effective voltage value, determining the second effective voltage value, and calculating the effective voltage difference, and the first delay time and the second delay time when a maximum effective voltage difference is obtained are obtained based on the plurality of effective voltage differences. This is a medical system that performs the following:

[0011] A second aspect of the present invention provides an X-ray tube configured to be able to switch a tube voltage applied to the X-ray tube between a first tube voltage and a second tube voltage; a detector assembly for detecting X-rays emitted from the X-ray tube, the detector assembly including a reference region for detecting X-rays and a filter, the reference region including a first sub-region for detecting X-rays that have passed through the filter and a second sub-region for detecting X-rays that have not passed through the filter; one or more processors a non-transitory storage medium having stored thereon instructions for execution by the one or more processors of a medical system including: setting a first delay time from when the tube voltage is switched to the first tube voltage to when data acquisition starts, and a second delay time from when the tube voltage is switched to the second tube voltage to when data acquisition starts; determining a first effective voltage value of the first tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the first tube voltage is applied to the X-ray tube; determining a second effective voltage value of the second tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the second tube voltage is applied to the X-ray tube; and Calculating an effective voltage difference representing the difference between the first effective voltage value and the second effective voltage value. Execute The instructions, when executed by the one or more processors, further cause the one or more processors to: a plurality of effective voltage differences are obtained by performing a plurality of steps of setting the first delay time and the second delay time, determining the first effective voltage value, determining the second effective voltage value, and calculating the effective voltage difference, and the first delay time and the second delay time when a maximum effective voltage difference is obtained are obtained based on the plurality of effective voltage differences. It is a storage medium that executes the above. [Effects of the Invention]

[0012] In the present invention, multiple effective voltage differences are obtained by performing the steps of setting the first delay time and the second delay time, determining the first effective voltage value, determining the second effective voltage value, and calculating the effective voltage difference multiple times. Therefore, it is possible to determine a delay time suitable for setting the effective voltage difference to a large value based on the multiple effective voltage differences. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a block diagram of a CT system 100 according to the present embodiment. [Figure 2] FIG. 10 is an explanatory diagram for switching the tube voltage. [Figure 3] FIG. 1 is an explanatory diagram of a high kV period and a low kV period of a tube voltage waveform. [Figure 4] 10 is a diagram showing an example of an effective voltage value VH corresponding to the tube voltage in a period 21 and an effective voltage value VL corresponding to the tube voltage in a period 22. FIG. [Figure 5] FIG. 10 is an explanatory diagram of a delay time. [Figure 6] FIG. 1 shows the effective voltage difference obtained for a rectangular waveform 30. [Figure 7] FIG. 2 is an explanatory diagram of a detector assembly 108. [Figure 8] FIG. 1 is an enlarged view of a reference area 109 of a detector assembly 108. [Figure 9] FIG. 1 is a flow diagram of a method for creating a baseline. [Figure 10] FIG. 10 is a diagram illustrating a method for calculating an index value M. [Figure 11] FIG. 10 is an explanatory diagram of a baseline creation method. [Figure 12] FIG. 1 shows a baseline 60. [Figure 13] FIG. 1 is a flow chart for determining the optimum delay time during installation of a CT system. [Figure 14] FIG. 10 is an explanatory diagram of a reference delay time. [Figure 15] FIG. 1 is an explanatory diagram of a method for calculating an index value M (=MHr) corresponding to a high kV and an index value M (=MLr) corresponding to a low kV. [Figure 16] FIG. 10 is an explanatory diagram of a method for determining an effective voltage value. [Figure 17] 10 is a diagram specifically illustrating a method for determining effective voltage values ​​VHr and VLr using a baseline 60. FIG. [Figure 18] FIG. 10 is a diagram illustrating an example in which the delay time is set to a time longer than the reference delay time. [Figure 19] FIG. 1 is an explanatory diagram of a method for calculating an index value M (=MHL) corresponding to a high kV and an index value M (=MLL) corresponding to a low kV. [Figure 20] FIG. 10 is an explanatory diagram of a method for determining an effective voltage value. [Figure 21] FIG. 10 is a diagram illustrating an example in which the delay time is set to a time shorter than the reference delay time. [Figure 22] FIG. 1 is an explanatory diagram of a method for calculating an index value M (=MHS) corresponding to a high kV and an index value M (=MLS) corresponding to a low kV. [Figure 23]FIG. 1 shows data sets Q1, Q2, and Q3. [Figure 24] FIG. 10 is a flow diagram of a method for determining an optimal delay time when a user periodically performs calibration of the CT system. [Figure 25] FIG. 10 is an explanatory diagram of the latest registered delay time value. [Figure 26] FIG. 10 is a diagram showing a delay time set to a time different from the registered delay time (TAN, TBN). [Figure 27] FIG. 10 is an explanatory diagram of a method for calculating a fifth effective voltage difference D5. [Figure 28] FIG. 1 shows data sets Q4 and Q5. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, a description will be given of an embodiment of the invention, but the present invention is not limited to the following embodiment.

[0015] FIG. 1 is a block diagram of a CT system 100 according to this embodiment. The CT system 100 includes a gantry 102 and a table 116 . The gantry 102 has a bore 107 into which a subject 112 is transferred and scanned.

[0016] An X-ray tube 104, a filter section 103, a pre-collimator 105, a detector assembly 108, and the like are attached to the gantry 102.

[0017] The X-ray tube 104 generates X-rays when a predetermined voltage is applied to the cathode-anode tube. The X-ray tube 104 is configured to be able to rotate on a path centered on a rotation axis 206 within the XY plane. Here, the Z direction represents the body axis direction, the Y direction represents the vertical direction (the height direction of the table 116), and the X direction represents the direction perpendicular to the Z direction and the Y direction. In this embodiment, the X-ray tube 104 is an X-ray tube compatible with a kV switching system that can switch the tube voltage applied to the X-ray tube 104 between a first tube voltage and a second tube voltage. Note that although the CT system 100 includes one X-ray tube 104 in this embodiment, it may also include two X-ray tubes 104.

[0018] The filter section 103 includes, for example, a flat plate filter and / or a bowtie filter. The pre-collimator 105 is a member for narrowing down the irradiation range of X-rays so that unnecessary areas are not irradiated with X-rays.

[0019] The detector assembly 108 includes a plurality of detector elements 202. The plurality of detector elements 202 detects x-rays 106 emitted from the x-ray tube 104 and passing through an object 112, such as a patient. Thus, the detector assembly 108 can acquire projection data for each view.

[0020] Projection data detected by the detector assembly 108 is collected by the DAS 214. The DAS 214 performs predetermined processing on the collected projection data, including sampling and digital conversion. The processed projection data is sent to the computer 216. The computer 216 stores the data from the DAS 214 in a storage device 218. The storage device 218 includes one or more storage media for storing programs, instructions to be executed by a processor, and the like. The storage media may be, for example, one or more non-transitory computer-readable storage media. The storage device 218 may include, for example, a hard disk drive, a floppy disk drive, a compact disk read / write (CD-R / W) drive, a digital versatile disk (DVD) drive, a flash drive, and / or a solid-state recording drive.

[0021] The computer 216 includes one or more processors 217. The computer 216 uses the one or more processors 217 to output commands and parameters to the DAS 214, the X-ray controller 210, and / or the gantry motor controller 212 to control system operations such as data acquisition and / or processing. The computer 216 also uses the one or more processors to perform various processes such as signal processing, data processing, and image processing in each step of the flow described below. Although the one or more processors 217 are shown in FIG. 1 as being included in the computer 216, the one or more processors 217 may be distributed between the computer 216 and other components (e.g., the X-ray controller 210, the gantry motor controller 212, the table motor controller 118, etc.).

[0022] An operator console 220 is coupled to the computer 216. An operator can operate the operator console 220 to enter predetermined operator inputs related to the operation of the CT system 100 into the computer 216. The computer 216 receives operator inputs, including commands and / or scan parameters, via the operator console 220 and controls system operation based on the operator inputs. The operator console 220 can include a keyboard (not shown) or a touch screen for the operator to enter commands and / or scan parameters.

[0023] The X-ray controller 210 controls the X-ray tube 104 based on instructions from the computer 216. The gantry motor controller 212 also controls the gantry motor based on instructions from the computer 216 so that components such as the X-ray tube 104 and the detector assembly 108 rotate.

[0024] Although FIG. 1 shows only one operator console 220 , more than one operator console may be coupled to computer 216 .

[0025] CT system 100 may also be coupled to multiple remotely located displays, printers, workstations, and / or similar devices, for example, via wired and / or wireless networks.

[0026] In one embodiment, for example, CT system 100 may include or be coupled to a picture archiving and communication system (PACS) 224. In an exemplary implementation, PACS 224 may be coupled to a remote system, such as a radiology department information system, a hospital information system, and / or an internal or external network (not shown).

[0027] The computer 216 supplies instructions to the table motor controller 118 for controlling the table 116. Based on the received instructions, the table motor controller 118 can control the table motor so as to move the table 116. For example, the table motor controller 118 can move the table 116 so that the subject 112 is positioned appropriately for imaging.

[0028] As described above, DAS 214 samples and digitally converts projection data acquired by detector elements 202. Image reconstructor 230 then reconstructs an image using the sampled and digitally converted data. Image reconstructor 230 includes one or more processors that may perform the image reconstruction processing. While image reconstructor 230 is shown in FIG. 1 as a separate component from computer 216, image reconstructor 230 may form part of computer 216. Computer 216 may also perform one or more functions of image reconstructor 230. Furthermore, image reconstructor 230 may be located remotely from CT system 100 and operatively connected to CT system 100 using a wired or wireless network.

[0029] Image reconstructor 230 can store the reconstructed image in storage device 218. Image reconstructor 230 may also transmit the reconstructed image to computer 216. Computer 216 can transmit the reconstructed image and / or patient information to a display device 232 communicatively coupled to computer 216 and / or image reconstructor 230.

[0030] The various methods and processes described herein may be stored as executable instructions on a non-transitory storage medium within the CT system 100. The executable instructions may be stored on a single storage medium or may be distributed across multiple storage media. One or more processors included in the CT system 100 perform the various methods, steps, and processes described herein in accordance with the instructions stored on the storage medium.

[0031] The CT system 100 is configured as described above. In this embodiment, the CT system is configured to be able to image a subject using a kV switching technique that switches the tube voltage of the X-ray tube between a high tube voltage (hereinafter referred to as "high kV") and a low tube voltage (hereinafter referred to as "low kV").

[0032] With kV switching technology, it is necessary to separate the data acquired by the detector into data corresponding to high kV and data corresponding to low kV. Generally, if the voltage difference between high kV and low kV is small, the accuracy of material identification (data identification accuracy) tends to deteriorate. Therefore, with kV switching technology, it is necessary to switch the tube voltage so that the difference between high kV and low kV is as large as possible.

[0033] Fig. 2 is an explanatory diagram of switching the tube voltage, showing a waveform 10 of the tube voltage. Assume that a low kV is applied to the X-ray tube immediately before time t1. At time t1, the tube voltage applied to the X-ray tube 104 switches from low kV to high kV. Ideally, the tube voltage switches in a rectangular pattern, but in reality, due to a transient phenomenon in the tube voltage, it is not possible to instantly change from low kV to high kV at time t1, and it takes a certain amount of time to reach a predetermined voltage value.

[0034] Then, at time t2, a predetermined time after time t1, the tube voltage of the X-ray tube switches from high kV to low kV. However, due to a transient phenomenon of the tube voltage, it is not possible to instantly change from high kV to low kV at time t2, and it takes a certain amount of time to reach the predetermined voltage value.

[0035] Next, at time t3, a predetermined time after time t2, the tube voltage of the X-ray tube switches from low kV to high kV. However, due to a transient phenomenon in the tube voltage, the voltage cannot instantaneously change from low kV to high kV at time t3; it takes a certain amount of time to reach the predetermined voltage value.

[0036] Thereafter, switching between high kV and low kV is similarly performed. Therefore, the waveform of the tube voltage includes periods of high kV and periods of low kV.

[0037] FIG. 3 is an explanatory diagram of the high kV period and the low kV period of the tube voltage waveform. The upper part of Fig. 3 shows the tube voltage waveform 10 shown in Fig. 2, and the lower part shows a rectangular waveform 20. The rectangular waveform 20 is a waveform that represents a period 21 corresponding to a high kV and a period 22 corresponding to a low kV. Period 21 represents the period from time t1 when the tube voltage starts to switch from a low kV to a high kV, to time t2 when the tube voltage starts to switch from a high kV to a low kV.

[0038] On the other hand, period 22 represents the period from time t2 when the tube voltage starts to switch from high kV to low kV to time t3 when the tube voltage starts to switch from low kV to high kV.

[0039] In the above example, period 21 represents a high-kV period, and period 22 represents a low-kV period. However, as shown in FIG. 3, the tube voltage does not change in a rectangular pattern but rather changes according to a constant rise time and fall time. Therefore, period 21 includes not only voltages sufficiently close to high kV but also voltages immediately after switching from low kV to high kV. Therefore, when considering the tube voltage during period 21, it is necessary to consider the effective voltage value rather than the ideal value of the tube voltage. Similarly, period 22 includes not only voltages sufficiently close to low kV but also voltages immediately after switching from high kV to low kV. Therefore, when considering the tube voltage during period 22, it is necessary to consider the effective voltage value rather than the ideal value of the tube voltage. FIG. 4 shows an example of the effective voltage value VH corresponding to the tube voltage during period 21 and the effective voltage value VL corresponding to the tube voltage during period 22.

[0040] In kV switching technology, the data acquired by the detector must be separated into data corresponding to high kV and data corresponding to low kV. Generally, a small effective voltage difference D between the effective voltage values ​​VH and VL tends to reduce the accuracy of material identification (data identification accuracy), so the effective voltage difference D must be as large as possible. However, in the example of Figure 4, period 21 includes not only voltages sufficiently close to high kV but also the voltage immediately after switching from low kV to high kV. Therefore, the effective voltage value VH cannot be made sufficiently large due to the influence of the voltage immediately after switching from low kV to high kV. Similarly, period 22 includes not only voltages sufficiently close to low kV but also the voltage immediately after switching from high kV to low kV. Therefore, the effective voltage value VL cannot be made sufficiently small due to the influence of the voltage immediately after switching from high kV to low kV. Therefore, the effective voltage difference D cannot be made sufficiently large. To address this problem, a certain delay time is provided between when the tube voltage is changed and when the acquisition of projection data begins. Next, this delay time will be explained.

[0041] FIG. 5 is an explanatory diagram of the delay time. 5 shows a tube voltage waveform 10, and a rectangular waveform 30 with a delay time is shown below the tube voltage waveform 10. For reference, a rectangular waveform 20 with no delay time is also shown below the rectangular waveform 30.

[0042] The rectangular waveform 30 is provided with a first delay time TA and a second delay time TB relative to the rectangular waveform 20. Due to space limitations, only two first delay times TA (i.e., the first delay time TA in period 35 and the first delay time TA in period 37) and two second delay times TB (i.e., the second delay time TB in period 36 and the second delay time TB in period 38) are shown in Figure 5.

[0043] The delay time TA of period 35 is the time from time t1 when the kV starts to switch from low to high to time t11 when collection of projection data begins. The delay time TB of period 36 is the time from time t2 when the kV starts to switch from high to low to time t21 when collection of projection data begins. Therefore, the detector assembly 108 detects projection data corresponding to high kV during period 31 (i.e., the period shifted from period 21 by delay times TA and TB).

[0044] Furthermore, the delay time TA of period 37 is the time from time t3 when the kV starts to switch from low to high to time t31 when the collection of projection data starts. Therefore, the detector assembly 108 detects projection data corresponding to low kV during period 32 (i.e., the period shifted from period 22 by delay times TB and TA).

[0045] FIG. 6 shows the effective voltage difference obtained for a rectangular waveform 30. The range of high kV (and voltages close to high kV) included in period 31 can be made wider than that included in period 21. Therefore, the effective voltage value VH can be made larger. Furthermore, the range of low kV (and voltages close to low kV) included in period 32 can be made wider than that included in period 22. Therefore, the effective voltage value VL can be made smaller. Therefore, by providing the first delay time TA and the second delay time TB, the effective voltage difference D can be made larger.

[0046] The specific values ​​of the first delay time TA and the second delay time TB are generally determined as fixed values ​​through experiments or the like during the development stage of a CT system, taking into consideration individual differences and aging degradation of the CT system itself. The delay times determined during this development stage are then used as default values ​​for the first delay time TA and the second delay time TB. However, the optimal delay time varies depending on individual differences in the CT system itself. Furthermore, the optimal delay time changes due to factors such as aging degradation of components used in the CT system. Therefore, if the delay times set as default values ​​are continued to be used, it becomes difficult to obtain high-quality images.

[0047] Therefore, the CT system of this embodiment is configured so that the optimum delay time can be determined even after the CT system is installed in a facility such as a hospital. How to determine the optimum delay time will be described below.

[0048] In this embodiment, as will be described later, an effective voltage value VH corresponding to a high kV and an effective voltage value VL corresponding to a low kV are calculated, and optimal values ​​of the first delay time TA and the second delay time TB are calculated based on these effective voltage values ​​VH and VL. Therefore, in order to calculate the optimal values ​​of the first delay time TA and the second delay time TB, it is necessary to calculate the effective voltage values ​​VH and VL. Therefore, in the following, a method for calculating the effective voltage values ​​VH and VL in this embodiment will first be described. After this explanation, a method for calculating the optimal values ​​of the first delay time TA and the second delay time TB will be described.

[0049] 7 to 12 are diagrams illustrating a method for calculating the effective voltage value in this embodiment. FIG. 7 is an illustration of the detector assembly 108.

[0050] The detector assembly 108 has a plurality of detector elements 202. The detector assembly 108 is provided with a reference region 109. Energy information of X-rays detected in the reference region 109 is used to calculate an index value M, which will be described later. The index value M will be described in detail later. The reference region 109 is provided at a position as far as possible from a center position 120 in the x-direction of the detector assembly 108 so that, during a scan of the object 112, X-rays irradiated from the X-ray tube 104 that do not pass through the object 112 can be detected. In this embodiment, the reference region 109 is provided at one end 121 of the detector assembly 108. Note that the reference region 109 may be provided not only at the one end 121 of the detector assembly 108 but also at the opposite end 122 of the detector assembly 108. In this embodiment, the description will continue assuming that the reference region 109 is provided only at the one end 121 of the detector assembly 108.

[0051] Furthermore, the detector assembly 108 has a detector collimator 123 to reduce image degradation due to scattered rays. The detector collimator 123 is disposed on the X-ray tube 104 side with respect to the detector elements 202.

[0052] FIG. 8 is an enlarged view of the reference area 109 of the detector assembly 108. The detector collimator 123 is provided on the extension of the boundary between adjacent detector elements 202 .

[0053] 8 also shows X-rays 104A and 104B. X-ray 104A represents an X-ray traveling toward detector element 202A among the plurality of detector elements 202 provided in the reference region 109 of the detector assembly 108. X-ray 104B represents an X-ray traveling toward detector element 202B among the plurality of detector elements 202 provided in the reference region 109 of the detector assembly 108.

[0054] The detector assembly 108 includes a filter 124. The filter 124 can be made of, for example, copper, molybdenum, or tungsten. The filter 124 is not disposed on the path of the X-ray 104A, but is disposed on the path of the X-ray 104B. The X-ray 104A is detected by the detector element 202A without passing through the filter 124. On the other hand, the X-ray 104B passes through the filter 124, and the X-ray 104C, whose energy is absorbed by the filter 124, is detected by the detector element 202B. Therefore, the reference region 109 of the detector assembly 108 includes a first sub-region 109B that detects X-rays that have passed through the filter 124 and a second sub-region 109A that detects X-rays that have not passed through the filter 124.

[0055] In this embodiment, as described above, the CT system 100 includes the detector assembly 108 having the filter 124 provided in the reference region 109, and the effective voltage value of the X-ray tube 104 can be calculated based on the X-rays detected in the reference region 109 of the detector assembly 108. However, in order to calculate the effective voltage value, it is necessary to prepare a baseline to be used for calculating the effective voltage value in advance. This baseline can generally be created when installation work is performed to install the CT system 100 in a facility such as a hospital. A method for creating the baseline will be described below.

[0056] FIG. 9 is a flow diagram of a baseline creation method. In step ST1, the reference tube voltages (hereinafter referred to as "reference tube voltages") used to create a baseline are determined. In this embodiment, four reference tube voltages VR1, VR2, VR3, and VR4 are determined as the reference tube voltages. The reference tube voltages VR1 to VR4 can be determined based on the tube voltages actually used in kV switching. In this embodiment, the reference tube voltages are VR1=80 kV, VR2=100 kV, VR3=120 kV, and VR4=140 kV, although other reference tube voltages may also be used.

[0057] In this embodiment, four reference tube voltages VR1 to VR4 are determined as the reference tube voltages used to create the baseline. However, five or more reference tube voltages may be determined as the reference tube voltages used to create the baseline, or three or fewer reference tube voltages may be determined as the reference tube voltages used to create the baseline. Note that if the number of reference tube voltages is too small, the reliability of the accuracy of the effective tube voltage may decrease. On the other hand, if the number of reference tube voltages is too large, data collection takes a long time. Therefore, it is desirable to determine the number of reference tube voltages by taking these factors into consideration. As an example, it is desirable to determine the four reference tube voltages VR1 to VR4 used in this embodiment as the reference tube voltages used to create the baseline. After determining the four reference tube voltages VR1 to VR4, proceed to step ST2.

[0058] In step ST2, the tube voltage of the X-ray tube 104 is set to the reference tube voltage used to create a baseline. Here, of the four reference tube voltages VR1 to VR4 determined in step ST1, the reference tube voltage VR1 (=80 kV) is first selected, and the tube voltage of the X-ray tube 104 is set to the reference tube voltage VR1 (=80 kV). After the reference tube voltage is set, the process proceeds to step ST3.

[0059] In step ST3, a reference tube voltage VR1 (=80 kV) is applied to the X-ray tube 104, causing the X-ray tube 104 to emit X-rays. The X-rays emitted from the X-ray tube 104 are detected by the detector assembly 108. The X-rays detected by the detector assembly 108 are output as analog signals to the DAS 214, as shown in FIG. 10 . The DAS 214 samples the analog data received from the detector assembly 108 and converts the analog data into digital signals for subsequent processing. The digital signals are output to the computer 216.

[0060] In step ST4, the computer processor 217 calculates, based on the digital signal, an index value M that represents the difference between the energy information of the X-rays detected in the first sub-region 109B of the reference region 109 (i.e., the X-rays that have passed through the filter 124) and the energy information of the X-rays detected in the second sub-region 109A of the reference region 109 (i.e., the X-rays that have not passed through the filter 124). A method for calculating the index value M will be described with reference to FIG. 10.

[0061] In this embodiment, the index value M can be calculated by the following formula. M = Sfilt(E) / S(E) (1) where M: index value Sfilt(E): A signal value containing energy information of the X-rays detected by the detector elements 202B of the first sub-region 109B (i.e., the X-rays that have passed through the filter 124). S(E): A signal value containing energy information of the X-rays detected by the detector elements 202A of the second sub-region 109A (i.e., X-rays that have not passed through the filter 124).

[0062] Therefore, the index value M can be expressed as the ratio between the signal value Sfilt(E) containing energy information of X-rays that have passed through the filter 124 and been detected in the first sub-region 109B and the signal value S(E) containing energy information of X-rays that have not passed through the filter 124 and have been detected in the second sub-region 109A.

[0063] The signal value Sfilt(E) can be determined based on signals obtained from one or more of the five detector elements 202B. For example, the signal value Sfilt(E) may be determined based on a representative value (maximum value, integral value, etc.) of a signal obtained from one of the five detector elements 202B, or may be determined based on an average value and / or a total value of representative values ​​(maximum value, integral value, etc.) of signals obtained from two or more of the five detector elements 202B.

[0064] Similarly, the signal value S(E) can be determined based on signals obtained from one or more of the five detector elements 202 A. For example, the signal value S(E) may be determined based on a representative value (maximum value, integral value, etc.) of a signal obtained from one of the five detector elements 202 A, or may be determined based on an average value and / or a total value of representative values ​​(maximum value, integral value, etc.) of signals obtained from two or more of the five detector elements 202 A.

[0065] Therefore, by setting the tube voltage of the X-ray tube 104 to 80 kV and obtaining the signal values ​​Sfilt(E) and S(E) based on the energy of the X-rays detected in the reference region 109 of the detector assembly 108, the index value M (=M80) corresponding to the reference tube voltage VR1 (=80 kV) can be calculated from equation (1).

[0066] As described above, when calculating the index value M80, the tube voltage of the X-ray tube 104 is constant at 80 kV. Therefore, while the gantry is rotating, there is no need to switch the tube voltage between high kV and low kV, and the value of the tube voltage is stable while the gantry is rotating. This allows the index value M80 to be calculated without being affected by the rotation speed of the gantry. Furthermore, the index value M80 is a value calculated based on X-ray energy information. Since the X-ray energy basically depends on the tube voltage of the X-ray tube 104 and does not depend much on the tube current, the index value M80 can be calculated without being affected by the tube current.

[0067] In this way, the index value M80 corresponding to the reference tube voltage VR1 (=80 kV) can be calculated without being substantially affected by the rotation speed of the gantry or the tube current. After calculating the index value M80, the index value M80 is stored in the storage device, and the process proceeds to step ST5.

[0068] In step ST5, it is determined whether to change the reference tube voltage. Here, of the four reference tube voltages VR1 to VR4, only the index value M80 for the reference tube voltage VR1 has been calculated. Therefore, the process returns to step ST2 to calculate the index values ​​M for the remaining reference tube voltages.

[0069] In step ST2, the reference tube voltage of the X-ray tube 104 is set to another voltage value. Here, the reference tube voltage VR2 (=100 kV) is selected from the four reference tube voltages VR1 to VR4 determined in step ST1, and the tube voltage of the X-ray tube 104 is set to the reference tube voltage VR2 (=100 kV).

[0070] In step ST3, a reference tube voltage VR2 (=100 kV) is applied to the X-ray tube 104, causing the X-ray tube 104 to emit X-rays. The X-rays emitted from the X-ray tube 104 are detected by the detector assembly 108. The X-rays detected by the detector assembly 108 are output as analog signals to the DAS 214, as shown in FIG. 5. The DAS 214 samples the analog data received from the detector assembly 108 and converts the analog data into digital signals for subsequent processing. The digital signals are output to the computer 216.

[0071] In step ST4, processor 217 uses equation (1) to calculate an index value M100 corresponding to reference tube voltage VR2 and stores index value M100 in the storage device. Then, the process proceeds to step ST5, where it is determined whether to change the reference tube voltage. Here, of the four reference tube voltages VR1 to VR4, index values ​​M80 and M100 for two reference tube voltages VR1 and VR2 have been calculated, but index values ​​for the remaining two reference tube voltages VR3 and VR4 have not yet been calculated. Therefore, the process returns to step ST2 to calculate index values ​​for the remaining reference tube voltages.

[0072] Similarly, the loop of steps ST2 to ST5 is repeatedly executed until the index value M120 corresponding to the reference tube voltage VR3 (=120 kV) and the index value M140 corresponding to the reference tube voltage VR4 are calculated. Once the index value M140 corresponding to the reference tube voltage VR4 is calculated, the process proceeds to step ST5.

[0073] In step ST5, it is determined whether to change the reference tube voltage. Here, index values ​​M80 to M140 corresponding to all four reference tube voltages VR1 to VR4 have been calculated. Therefore, the process proceeds to step ST6.

[0074] In step ST6, a baseline to be used for determining the effective voltage value is created based on the calculated four index values ​​M80 to M140.

[0075] FIG. 11 is an explanatory diagram of a method for creating a baseline. The computer processor 217 associates the reference tube voltages VR1 to VR4 with the index values ​​M80 to M140 on two-dimensional coordinates.

[0076] Point P1 is a point that associates the reference tube voltage VR1 with the index value M80 calculated for the reference tube voltage VR1. Point P2 is a point that associates the reference tube voltage VR2 with the index value M100 calculated for the reference tube voltage VR2. Point P3 is a point that associates the reference tube voltage VR3 with the index value M120 calculated for the reference tube voltage VR3. Point P4 is a point that associates the reference tube voltage VR4 with the index value M140 calculated for the reference tube voltage VR4.

[0077] Then, the processor 217 creates a baseline representing the relationship between the tube voltage and the index value M based on the points P1 to P4. This baseline can be obtained, for example, as a curve that best fits the points P1 to P4. FIG. 12 shows the obtained baseline 60. Data representing this baseline 60 is stored in a storage device of the CT system 100 or an external storage device that can communicate with the CT system 100. The following describes how to use the baseline to find the optimal delay time.

[0078] There are generally two examples where it is desirable to perform work to determine the optimal delay time. The first example is when performing installation work to install a CT system in a facility such as a hospital, when performing maintenance, or when replacing parts. The second example is when a user of the CT system periodically performs simple calibration of the CT system after the CT system installed in the facility has actually started operating. Therefore, in the following explanation, a method for determining the optimal delay time will be described for each of the first and second examples. The first example will be described using installation work to install a CT system in a facility such as a hospital.

[0079] (1) How to determine the optimal delay time when installing a CT system FIG. 13 is a flow chart for determining the optimum delay time during installation of a CT system.

[0080] In step ST11, the processor 217 sets the first delay time TA to the first reference delay time TAr and the second delay time TB to the second reference delay time TBr, as shown in FIG. 14. The first reference delay time TAr is a delay time representing a reference value of the first delay time TA, and the second reference delay time TBr is a delay time representing a reference value of the second delay time TB. The reference delay times TAr and TBr are values ​​determined in advance prior to the installation of the CT system and are stored as default values ​​in a storage device or an external storage device. The reference delay times (TAr, TBr) can be values ​​determined through experiments or the like during the development stage of the CT system, for example. The first reference delay time TAr and the second reference delay time TBr may be the same value or different values. The processor 217 reads the reference delay times TAr and TBr from the storage device, and sets the first delay time TA to the first reference delay time TAr and the second delay time TB to the second reference delay time TBr, as shown in Fig. 14. After setting the reference delay times, the process proceeds to step ST12.

[0081] In step ST12, with the delay times (TA, TB) set to the reference delay times (TAr, TBr), kV switching is performed to alternately switch the tube voltage between high kV and low kV, and X-rays are detected by the detector assembly 108. As an example, the high kV may be 140 kV and the low kV may be 80 kV. The DAS 214 performs predetermined processing, including sampling, on the data including energy information of the detected X-rays to convert them into digital data and output the digital data to a computer.

[0082] In step ST13, the processor 217 of the computer calculates the first effective voltage difference D1 (see FIG. 16) based on the data received from the DAS 214. Note that step ST13 includes steps ST131 to ST133, and each step will be described in order.

[0083] In step ST131, based on the data received from DAS214, the computer's processor 217 determines the signal value Sfilt(E) and the signal value S(E) included in Equation (1), and calculates the index value M (= MHr) corresponding to the high kV and the index value M (= MLr) corresponding to the low kV (see FIG. 15).

[0084] FIG. 15 is an explanatory diagram of a method for calculating the index value M (= MHr) corresponding to the high kV and the index value M (= MLr) corresponding to the low kV.

[0085] Based on the data obtained from the reference region 109 (see FIG. 10) during the period 31 in which high kV data collection is performed, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) of Equation (1), substitutes the determined signal value Sfilt(E) and the signal value S(E) into Equation (1), and calculates the index value M (= MHr) corresponding to the high kV. In the scan of kV switching, when N periods 31 in which high kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 31, or may be determined based on the data obtained in M (< N) of the N periods 31. Substitute the determined signal value Sfilt(E) and the signal value S(E) into Equation (1) to calculate the index value M (= MHr) corresponding to the high kV.

[0086] Further, based on the data obtained from the reference region 109 during the period 32 in which low kV data collection is performed, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) of Equation (1), substitutes the determined signal value Sfilt(E) and the signal value S(E) into Equation (1), and calculates the index value M (= MLr) corresponding to the low kV. In the scan of kV switching, when N periods 32 in which low kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 32, or may be determined based on the data obtained in M (< N) of the N periods 32.

[0087] After calculating the index value M (=MHr) corresponding to the high kV and the index value M (=MLr) corresponding to the low kV, the process proceeds to step ST132.

[0088] In step ST132, the processor 217 determines a first effective voltage value corresponding to high kV based on the index value M (=MHr), and determines a second effective voltage value corresponding to low kV based on the index value M (=MLr) (see FIG. 16).

[0089] FIG. 16 is an explanatory diagram of a method for determining an effective voltage value. The processor 217 reads the baseline 60 stored in the storage device. Then, based on the baseline 60, the processor 217 determines a first effective voltage value VH (=VHr) corresponding to the high kV index value M (=MHr) and a second effective voltage value VL (=VLr) corresponding to the low kV index value M (=MLr) (see FIG. 17).

[0090] FIG. 17 is a diagram specifically illustrating a method for determining the effective voltage values ​​VHr and VLr using the baseline 60. The baseline 60 represents the relationship between the index value M and the tube voltage. Therefore, the processor 217 can identify a first effective voltage value VH (=VHr) corresponding to the high-kV index value M (=MHr) based on the baseline 60. This effective voltage value VHr is determined as the effective voltage value corresponding to the high kV.

[0091] Furthermore, the processor 217 can identify a second effective voltage value VL (=VLr) corresponding to the low kV index value M (=MLr) based on the baseline 60. This effective voltage value VLr is determined as the effective voltage value corresponding to the low kV.

[0092] 16, the tube voltage waveform 10 shows the effective voltage values ​​VHr and VLr determined using the baseline 60. After determining the effective voltage values ​​VHr and VLr, the process proceeds to step ST133.

[0093] In step ST133, processor 217 calculates a first effective voltage difference D1 representing the difference between a first effective voltage value VH (=VHr) corresponding to a high kV and a second effective voltage value VL (=VLr) corresponding to a low kV. As shown in FIG. 16, the first effective voltage difference D1 is D1=VHr-VLr. The effective voltage difference D1 is stored in the storage device in association with the effective voltage values ​​(VHr, VLr) and the reference delay times (TAr, TBr). After calculating the first effective voltage difference D1, the process proceeds to step ST14.

[0094] In step ST14, processor 217 determines whether kV switching has been performed with a delay time longer than the reference delay time. Here, kV switching has been performed with the delay time set to the reference delay time, but kV switching has not been performed with a delay time longer than the reference delay time. Therefore, the process proceeds to step ST15.

[0095] In step ST15, the processor 217 sets the delay time to a time longer than the reference delay time (see FIG. 18).

[0096] FIG. 18 is a diagram showing an example in which the delay time is set to a time longer than the reference delay time. 18 shows a tube voltage waveform 10, and below the tube voltage waveform 10 is shown a rectangular waveform 70 with a delay time longer than the reference delay time. For reference, below the rectangular waveform 70 is also shown a rectangular waveform 30 with a delay time set to the reference delay time.

[0097] In the rectangular waveform 70, the delay time TA is set to a delay time TAL that is longer than the first reference delay time TAr by Δa1. The delay time TB is set to a delay time TBL that is longer than the second reference delay time TBr by Δb1. The long delay times TAL and TBL may be the same or different. After the delay times are set to times (TAL, TBL) that are longer than the reference delay times (TAr, TBr), the process returns to step ST12.

[0098] In step ST12, with the delay times (TA, TB) set to times (TAL, TBL) longer than the reference delay times (TAr, TBr), kV switching is executed in which the tube voltage is alternately switched between high kV and low kV, and the detector assembly 108 detects X-rays. DAS 214 performs predetermined processing including sampling processing on the data including the detected X-ray energy information, converts it into digital data, and outputs it to a computer.

[0099] In step ST13, the processor 217 of the computer calculates a second effective voltage difference D2 (see FIG. 20) based on the data received from DAS 214. Specifically, the second effective voltage difference D2 is calculated as follows.

[0100] In step ST131, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) included in equation (1) based on the data received from DAS 214, and calculates the index value M (= MHL) corresponding to high kV and the index value M (= MLL) corresponding to low kV (see FIG. 19).

[0101] FIG. 19 is an explanatory diagram of a method for calculating the index value M (= MHL) corresponding to high kV and the index value M (= MLL) corresponding to low kV.

[0102] The processor 217 determines the signal value Sfilt(E) and the signal value S(E) of equation (1) based on the data obtained from the reference region 109 (see FIG. 10) during the period 71 when high kV data collection is performed, substitutes the determined signal value Sfilt(E) and the signal value S(E) into equation (1), and calculates the index value M (= MHL) corresponding to high kV. In the scan of the kV switching, when N periods 71 in which high kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 71, or may be determined based on the data obtained in M (<N) periods 71 out of the N periods 71.

[0103] Further, during period 72 when low kV data collection is performed, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) of Equation (1) based on the data obtained from the reference region 109, substitutes the determined signal value Sfilt(E) and the signal value S(E) into Equation (1), and calculates the index value M (= MLL) corresponding to the low kV. Note that in the scan of kV switching, when N periods 72 in which low kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 72, or may be determined based on the data obtained in M (< N) of the N periods 72.

[0104] After calculating the index value M (= MHL) corresponding to the high kV and the index value M (= MLL) corresponding to the low kV, the process proceeds to step ST132.

[0105] In step ST132, the processor 217 determines a first effective voltage value corresponding to the high kV based on the index value M (= MHL), and determines a second effective voltage value corresponding to the low kV based on the index value M (= MLL) (see FIG. 20).

[0106] FIG. 20 is an explanatory diagram of a method for determining the effective voltage value. The processor 217 determines a first effective voltage value VH (= VHL) corresponding to the high kV index value M (= MHL) and a second effective voltage value VL (= VLL) corresponding to the low kV index value M (= MLL) based on the baseline 60. Since the method for determining the effective voltage value is the same as the method described while referring to FIG. 17, detailed description thereof is omitted. The tube voltage waveform 10 in FIG. 20 shows the effective voltage value VHL and the effective voltage value VLL determined using the baseline 60. After determining the effective voltage value VHL and the effective voltage value VLL, the process proceeds to step ST133.

[0107] In step ST133, the processor 217 calculates a second effective voltage difference D2 representing the difference between the first effective voltage value VH (=VHL) corresponding to the high kV and the second effective voltage value VL (=VLL) corresponding to the low kV. As shown in FIG. 20, the second effective voltage difference D2 is D2=VHL-VLL. The second effective voltage difference D2 is stored in the storage device in association with the effective voltage values ​​(VHL, VLL) and the long delay times (TAL, TBL). After calculating the second effective voltage difference D2, the process proceeds to step ST14.

[0108] In step ST14, the processor determines whether kV switching was performed with a delay time longer than the reference delay time. In this case, kV switching was performed with a delay time longer than the reference delay time, so the processor proceeds to step ST16.

[0109] In step ST16, the processor determines whether kV switching has been performed with a delay time shorter than the reference delay time. Here, kV switching with a delay time longer than the reference delay time has been performed, but kV switching with a delay time shorter than the reference delay time has not been performed. Therefore, the processor proceeds to step ST17.

[0110] In step ST17, the processor 217 sets the delay time to a time shorter than the reference delay time (see FIG. 21).

[0111] FIG. 21 is a diagram showing an example in which the delay time is set to a time shorter than the reference delay time. 21 shows a tube voltage waveform 10, and below the tube voltage waveform 10 is shown a rectangular waveform 80 with a delay time shorter than the reference delay time. For reference, below the rectangular waveform 80 is also shown a rectangular waveform 30 with a delay time set to the reference delay time.

[0112] In the rectangular waveform 80, the delay time TA is set to a delay time TAS that is shorter than the reference delay time TAr by Δa2. The delay time TB is set to a delay time TBS that is shorter than the reference delay time TBr by Δb2. The short delay times TAS and TBS may be the same or different. After the delay times (TAS, TBS) are set to times shorter than the reference delay times (TAr, TBr), the process returns to step ST12.

[0113] In step ST12, with the delay times (TA, TB) set to times (TAS, TBS) shorter than the reference delay times (TAr, TBr), kV switching is performed to alternately switch the tube voltage between high kV and low kV, and X-rays are detected by the detector assembly 108. The DAS 214 performs predetermined processing, including sampling processing, on data containing energy information of the detected X-rays to convert them into digital data and output the digital data to a computer.

[0114] In step ST13, the processor 217 of the computer calculates the third effective voltage difference D3 (see FIG. 22) based on the data received from the DAS 214. Specifically, the third effective voltage difference D3 is calculated as follows.

[0115] In step ST131, the processor 217 determines the signal values ​​Sfilt(E) and S(E) included in equation (1) based on the data received from the DAS 214, and calculates the index value M (=MHS) corresponding to high kV and the index value M (=MLS) corresponding to low kV (see Figure 22).

[0116] FIG. 22 is an explanatory diagram of a method for calculating the index value M (=MHS) corresponding to high kV and the index value M (=MLS) corresponding to low kV.

[0117] During the period 81 when high-kV data collection is performed, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) of Equation (1) based on the data obtained from the reference region 109 (see FIG. 10), substitutes the determined signal value Sfilt(E) and the signal value S(E) into Equation (1), and calculates the index value M (= MHS) corresponding to the high kV. In the scan of kV switching, when N periods 81 in which high-kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 81, or may be determined based on the data obtained in M (< N) of the N periods 81.

[0118] Also, during the period 82 when low-kV data collection is performed, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) of Equation (1) based on the data obtained from the reference region 109, substitutes the determined signal value Sfilt(E) and the signal value S(E) into Equation (1), and calculates the index value M (= MLS) corresponding to the low kV. In the scan of kV switching, when N periods 82 in which low-kV data collection is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 82, or may be determined based on the data obtained in M (< N) of the N periods 82.

[0119] After calculating the index value M (= MHS) corresponding to the high kV and the index value M (= MLS) corresponding to the low kV, the process proceeds to step ST132.

[0120] In step ST132, processor 217 determines a first effective voltage value VH (=VHS) corresponding to the high-kV index value M (=MHS) and a second effective voltage value VL (=VLS) corresponding to the low-kV index value M (=MLS) based on the baseline 60. The method for determining the effective voltage values ​​is the same as the method described with reference to FIG. 17, so a detailed description will be omitted. The tube voltage waveform 10 in FIG. 22 shows the effective voltage values ​​VHS and VLS determined using the baseline 60. After determining the effective voltage values ​​VHS and VLS, the process proceeds to step ST133.

[0121] In step ST133, the processor 217 calculates a third effective voltage difference D3 representing the difference between a first effective voltage value VH (=VHS) corresponding to a high kV and a second effective voltage value VL (=VLS) corresponding to a low kV. The third effective voltage difference D3 is D3=VHS-VLS, as shown in FIG. 22. The third effective voltage difference D3 is stored in the storage device in association with the effective voltage values ​​(VHS, VLS) and the short delay times (TAS, TBS). Therefore, the storage device stores data sets Q1, Q2, and Q3, as shown in FIG. 23.

[0122] The data set Q1 includes a first effective voltage difference D1, and effective voltage values ​​(VHr, VLr) and reference delay times (TAr, TBr) associated with the first effective voltage difference D1 (see FIG. 16). The data set Q2 includes a second effective voltage difference D2, and effective voltage values ​​(VHL, VLL) and long delay times (TAL, TBL) associated with the second effective voltage difference D2 (see FIG. 20). The data set Q3 includes a third effective voltage difference D3, and effective voltage values ​​(VHS, VLS) and short delay times (TAS, TBS) associated with the third effective voltage difference D3 (see FIG. 22).

[0123] After the third effective voltage difference D3 is calculated, the process proceeds to step ST14.

[0124] In step ST14, processor 217 determines whether kV switching was performed with a delay time longer than the reference delay time. In this case, kV switching was performed with a delay time longer than the reference delay time, so the process proceeds to step ST16.

[0125] In step ST16, processor 217 determines whether kV switching was performed with a delay time shorter than the reference delay time. In this case, kV switching was performed with a delay time shorter than the reference delay time, so the process proceeds to step ST18.

[0126] In step ST18, processor 217 optimizes first delay time TA and second delay time TB based on first effective voltage difference D1, second effective voltage difference D2, and third effective voltage difference D3. Specifically, processor 217 determines first delay time TA and second delay time TB when the maximum effective voltage difference is obtained based on first effective voltage difference D1, second effective voltage difference D2, and third effective voltage difference D3. An example of a method for determining first delay time TA and second delay time TB when the maximum effective voltage difference is obtained will be described below.

[0127] The processor 217 reads the data sets Q1 to Q3 from the storage device. Next, the processor 217 optimizes the first delay time TA and the second delay time TB based on the data sets Q1 to Q3 so as to obtain the maximum effective voltage difference, and determines a first optimal delay time TAO representing the optimal value of the first delay time TA and a second optimal delay time TBO representing the optimal value of the second delay time TB. The processor can determine the first optimal delay time TAO and the second optimal delay time TBO based on the effective voltage differences D1 to D3 and the delay times (TAr, TBr), (TAL, TBL), and (TAS, TBS) by, for example, applying a certain constraint (e.g., a constraint that the effective voltage difference and the delay time are expressed by a normal distribution). Alternatively, the processor may identify a data set including the maximum effective voltage difference from the data sets Q1 to Q3, and specify the delay times included in that data set as the optimal delay times (TAO, TBO). For example, if the data set Q2 among the data sets Q1 to Q3 contains the largest effective voltage difference, the long delay time (TAL, TBL) contained in the data set Q2 may be set as the optimum delay time (TAO, TBO).

[0128] In this way, the optimum delay time (TAO, TBO) that is the delay time when the maximum effective voltage difference is obtained can be obtained. The processor registers the optimum delay time (TAO, TBO) in the storage device and ends the flow.

[0129] During CT system installation, in addition to the first effective voltage difference D1 (see FIG. 16) corresponding to the reference delay time (TAr, TBr), a second effective voltage difference D2 (see FIG. 20) corresponding to a delay time (TAr, TBL) longer than the reference delay time (TAr, TBr) and a third effective voltage difference D3 (see FIG. 22) corresponding to a delay time (TAr, TBr) shorter than the reference delay time (TAr, TBr) are calculated. Based on these effective voltage differences D1, D2, and D3, the optimal delay time (TAO, TBO) that provides the maximum effective voltage difference can be determined. Therefore, when actually imaging a subject with a CT examination, projection data corresponding to a high kV and projection data corresponding to a low kV can be acquired based on the optimal delay time (TAO, TBO) that is the optimized delay time obtained in the above flow. Therefore, projection data corresponding to a high kV and projection data corresponding to a low kV can be acquired with the delay time that provides the maximum effective voltage difference, enabling high-quality CT images to be reconstructed.

[0130] 13, kV switching is performed for the reference delay times (TAr, TBr), long delay times (TAL, TBL), and short delay times (TAS, TBS). That is, kV switching is performed for three different delay times. However, two different delay times may be set and kV switching may be performed for two different delay times, or four or more different delay times may be set and kV switching may be performed for four or more different delay times.

[0131] Next, an example will be described in which an optimum delay time is calculated when a user of a CT system periodically performs calibration of the CT system after the CT system installed in a facility has actually started operating.

[0132] (2) How to determine the optimal delay time when a CT system user periodically performs calibration of the CT system. FIG. 24 is a flow diagram of a method for determining the optimum delay time when the user periodically performs calibration.

[0133] In step ST20, the processor 217 accesses the storage device and reads out the latest value of the registered delay time. Figure 25 is an explanatory diagram of the latest value of the registered delay time.

[0134] 25 shows a tube voltage waveform 10 in the upper part, and a rectangular waveform 50 in the lower part. The rectangular waveform 50 is a waveform that represents a period 51 corresponding to a high kV and a period 52 corresponding to a low kV.

[0135] The rectangular waveform 50 shows a first registered delay time TAN and a second registered delay time TBN. The first registered delay time TAN represents the delay time registered as the latest value of the first delay time TA, and the second registered delay time TBN represents the delay time registered as the latest value of the second delay time TB. The delay times TAN and TBN may be the same or different. The effective voltage values ​​VH (=VHN) and VL (=VLN) are the effective voltage values ​​obtained for periods 51 and 52, respectively. The fourth effective voltage difference D4 represents the difference between the effective voltage values ​​VH (=VHN) and VL (=VLN). The registered delay times (TAN, TBN), effective voltage values ​​(VHN, VLN), and fourth effective voltage difference D4 are associated with one another and stored in the storage device as a data set Q4, as shown at the bottom of FIG. 25. The processor 217 accesses the storage device and reads out the first registered delay time TAN and the second registered delay time TBN from the data set Q4. After the processor 217 reads out the registered delay times (TAN, TBN), the process proceeds to step ST21.

[0136] In step ST21, the processor 217 sets the delay time to a delay time different from the registered delay time (TAN, TBN) (see FIG. 26).

[0137] FIG. 26 is a diagram showing delay times that are set to times different from the registered delay times (TAN, TBN). FIG. 26 shows a tube voltage waveform 10, and below the tube voltage waveform 10, a rectangular waveform 90 is shown.

[0138] The rectangular waveform 90 is a waveform that represents a period 91 corresponding to a high kV and a period 92 corresponding to a low kV. The rectangular waveform 90 is provided with a first alternative delay time TAF that is different from the first registered delay time TAN, and a second alternative delay time TBF that is different from the second registered delay time TBN.

[0139] In the rectangular waveform 90, the first alternative delay time TAF is set to a delay time that is longer than the first registered delay time TAN by Δa3. Also, the second alternative delay time TBF is set to a delay time that is longer than the second registered delay time TBN by Δb3. Note that in FIG. 26, the alternative delay times (TAF, TBF) are each longer than the registered delay times (TAN, TBN), but they may also be shorter than the registered delay times (TAN, TBN). After setting the alternative delay times (TAF, TBF), proceed to step ST22.

[0140] In step ST22, with the delay time set to another delay time (TAF, TBF), kV switching is performed in which the tube voltage is alternately switched between high kV and low kV, and X-rays are detected by the detector assembly 108. As an example, the high kV may be 140 kV and the low kV may be 80 kV. The DAS 214 performs predetermined processing, including sampling, on the data including energy information of the detected X-rays to convert them into digital data and output the digital data to a computer.

[0141] In step ST23, the computer processor 217 calculates a fifth effective voltage difference D5 based on the data received from the DAS 214 (see FIG. 27).

[0142] FIG. 27 is an explanatory diagram of a method for calculating a fifth effective voltage difference D5. Since step ST23 is the same as step ST13 shown in FIG. 13, it will be briefly described below.

[0143] In step ST131, based on the data received from DAS 214, the processor 217 determines the signal value Sfilt(E) and the signal value S(E) included in Equation (1), and calculates the index value M (= MHF) corresponding to high kV and the index value M (= MLF) corresponding to low kV. In the scan of kV switching, when N periods 91 in which data collection for high kV is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 91, or may be determined based on the data obtained in M (< N) of the N periods 91. Also, in the scan of kV switching, when N periods 92 in which data collection for low kV is performed appear, the signal value Sfilt(E) and the signal value S(E) may be determined based on all the data obtained in the N periods 92, or may be determined based on the data obtained in M (< N) of the N periods 92.

[0144] After calculating the index value M (= MHF) corresponding to high kV and the index value M (= MLF) corresponding to low kV, the process proceeds to step ST132.

[0145] In step ST132, the processor 217 determines the effective voltage value VH (= VHF) corresponding to the index value M (= MHF) of high kV and the effective voltage value VL (= VLF) corresponding to the index value M (= MLF) of low kV based on the baseline 60. After determining the effective voltage value VHF and the effective voltage value VLF, the process proceeds to step ST133.

[0146] In step ST133, the processor 217 calculates a fifth effective voltage difference D5 representing the difference between the effective voltage value VH (=VHF) corresponding to the high kV and the effective voltage value VL (=VLF) corresponding to the low kV. As shown in FIG. 27, the fifth effective voltage difference D5 is D5=VHF-VLF. The fifth effective voltage difference D5, the effective voltage values ​​(VHF, VLF), and the delay times (TAF, TBF) are stored in the storage device as a single data set. FIG. 28 shows the data sets Q4 and Q5 stored in the storage device. The data set Q4 is a data set including the registered delay times (TAN, TBN), the effective voltage values ​​(VHN, VLN), and the fourth effective voltage difference D4 described above in FIG. 25. On the other hand, data set Q5 is a data set that includes another delay time (TAF, TBF) set in step ST21 of the flow of Fig. 24, and the effective voltage values ​​(VHF, VLF) and effective voltage difference D5 obtained in step ST23. After storing data set Q5, the process proceeds to step ST24.

[0147] In step ST24, processor 217 compares the fourth effective voltage difference D4 corresponding to the registered delay time (TAN, TBN) in data set Q4 with the fifth effective voltage difference D5 corresponding to another delay time (TAF, TBF) in data set Q5. If D5>D4, this means that the delay time (TAF, TBF) set in step ST21 can obtain a larger effective voltage difference than the registered delay time (TAN, TBN). Therefore, if D5>D4, it is considered that the delay time (TAF, TBF) set in step ST21 can improve image quality more than the registered delay time (TAN, TBN). Therefore, if D5>D4, the process proceeds to step ST25, where the delay time is updated to another delay time (TAF, TBF) corresponding to the fifth effective voltage difference D5, and the flow ends.

[0148] On the other hand, if D5≦D4, this means that the delay times (TAF, TBF) set in step ST21 have the same or smaller effective voltage difference as the registered delay times (TAN, TBN). Therefore, if D5≦D4, it is considered difficult to improve the image quality using the delay times (TAF, TBF) set in step ST21 compared to the registered delay times (TAN, TBN). For this reason, if D5≦D4, the process proceeds to step ST26, where the registered delay times (TAN, TBN) are maintained without updating the delay times, and the flow ends.

[0149] In the flow shown in FIG. 24, one calibration scan is performed for the delay times (TAF, TBF) set in step ST21, and whether to update the delay time is determined based on the data obtained from the single calibration scan. Therefore, the flow shown in FIG. 24 is suitable for calibration where priority is given to completing the calibration in as short a time as possible, since only one calibration scan is required. For example, a user may be recommended to perform daily calibration approximately once a day. However, since daily calibration is a task that must be performed every day in principle during a user's busy daily work, from the user's perspective, daily calibration is a task that the user wants to complete as quickly as possible. Therefore, by incorporating the flow shown in FIG. 24 into daily calibration, the user can complete the task of updating the delay time with a single calibration scan, thereby reducing the time required for calibration.

[0150] The present invention can be applied not only to full scans but also to half scans, as long as the scan is performed while switching the tube voltage.

[0151] In this embodiment, an example is shown in which the CT system 100 is used as the medical system. However, the present invention is not limited to the CT system 100, and can be applied to systems other than the CT system 100 (for example, a PET-CT system) as long as the medical system irradiates an X-ray source onto a subject. [Explanation of symbols]

[0152] 60 Baseline 100 CT System 102 Gantry 103 Filter section 104 X-ray tube 104A X-ray 104B X-ray 104C X-ray 105 Precollimator 106 X-ray 107 Bore 108 Detector Assembly 109 Reference area 109A Second Sub-area 109B First sub-area 112 specimens 116 tables 118 Table Motor Controller 120 Center position 121 End 122 End 123 Detector collimator 124 filters 202 detector element 202A detector element 202B detector element 206 Rotation axis 210 X-ray controller 212 Gantry motor controller 214 DAS 216 Computer 217 processors 218 Storage device 220 Operator Console 224 PACS 230 Image Reconstructor 232 Display device

Claims

1. an X-ray tube configured to be able to switch a tube voltage applied to the X-ray tube between a first tube voltage and a second tube voltage; a detector assembly for detecting X-rays emitted from the X-ray tube, the detector assembly including a reference region for detecting X-rays and a filter, the reference region including a first sub-region for detecting X-rays that have passed through the filter and a second sub-region for detecting X-rays that have not passed through the filter; one or more processors, setting a first delay time from when the tube voltage is switched to the first tube voltage until when data acquisition starts, and a second delay time from when the tube voltage is switched to the second tube voltage until when data acquisition starts; determining a first effective voltage value of the first tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the first tube voltage is applied to the X-ray tube; determining a second effective voltage value of the second tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the second tube voltage is applied to the X-ray tube; and Calculating an effective voltage difference representing the difference between the first effective voltage value and the second effective voltage value. one or more processors executing Including, the one or more processors a plurality of effective voltage differences are obtained by performing a plurality of steps of setting the first delay time and the second delay time, determining the first effective voltage value, determining the second effective voltage value, and calculating the effective voltage difference, and the first delay time and the second delay time when a maximum effective voltage difference is obtained are obtained based on the plurality of effective voltage differences. A medical system that performs the above.

2. a first reference delay time representing a reference value of the first delay time and a second reference delay time representing a reference value of the second delay time are stored in a storage device included in the medical system or a storage device accessible by the medical system; When a first calibration is performed on the medical system, the one or more processors: calculating a first effective voltage difference representing a difference between the first effective voltage value and the second effective voltage value based on data obtained when the first delay time is set to the first reference delay time and the second delay time is set to the second reference delay time; calculating a second effective voltage difference representing a difference between the first effective voltage value and the second effective voltage value based on data obtained when the first delay time is set to a delay time longer than the first reference delay time and the second delay time is set to a delay time longer than the second reference delay time; calculating a third effective voltage difference representing a difference between the first effective voltage value and the second effective voltage value based on data obtained when the first delay time is set to a delay time shorter than the first reference delay time and the second delay time is set to a delay time shorter than the second reference delay time; and determining the first delay time and the second delay time when a maximum effective voltage difference is obtained based on the first effective voltage difference, the second effective voltage difference, and the third effective voltage difference; The medical system of claim 1 , wherein the medical system executes the following:

3. The medical system according to claim 2 , wherein the first calibration is performed during installation, maintenance, or part replacement of the medical system.

4. The medical system of claim 2 , wherein the first delay time and the second delay time are the same value or different values.

5. a first registered delay time representing a delay time registered as the latest value of the first delay time, a second registered delay time representing a delay time registered as the latest value of the second delay time, and a fourth effective voltage difference corresponding to the first registered delay time and the second registered delay time, When a second calibration is performed on the medical system, the one or more processors: setting the first delay time to a first different delay time different from the first registered delay time, and setting the second delay time to a second different delay time different from the second registered delay time; calculating a fifth effective voltage difference representing a difference between the first effective voltage value and the second effective voltage value based on data obtained when the first delay time is set to the first different delay time and the second delay time is set to the second different delay time; and updating the first delay time to the first different delay time and updating the second delay time to the second different delay time when the fifth effective voltage difference is greater than the fourth effective voltage difference; The medical system of claim 1 , wherein the medical system executes the following:

6. The medical system according to claim 5 , wherein the second calibration is a calibration that is periodically performed by a user.

7. Determining the first effective voltage value includes: determining the first effective voltage value based on a signal value including energy information of X-rays detected in the first sub-region and a signal value including energy information of X-rays detected in the second sub-region; Determining the second effective voltage value includes:

2. The medical system of claim 1, further comprising determining the second effective voltage value based on a signal value including energy information of X-rays detected in the first sub-region and a signal value including energy information of X-rays detected in the second sub-region.

8. Determining the first effective voltage value includes: calculating a first index value based on a signal value including energy information of the X-rays detected in the first sub-region and a signal value including energy information of the X-rays detected in the second sub-region; and determining the first effective voltage value based on the first index value; Determining the second effective voltage value includes: calculating a second index value based on a signal value including energy information of the X-rays detected in the first sub-region and a signal value including energy information of the X-rays detected in the second sub-region; and The medical system of claim 7 , further comprising determining the second effective voltage value based on the second index value.

9. 9. The medical system of claim 8, wherein the first index value and the second index value are calculated by a ratio between a signal value including energy information of X-rays detected in the first sub-region and a signal value including energy information of X-rays detected in the second sub-region.

10. The one or more processors: The medical system according to claim 9 , wherein the first effective voltage value and the second effective voltage value are calculated based on a baseline that associates tube voltages with index values.

11. the baseline is a line created based on a plurality of reference tube voltages and a plurality of index values ​​corresponding to the plurality of reference tube voltages; 11. The medical system of claim 10, wherein each of the plurality of index values ​​is a ratio between a signal value including energy information of X-rays detected in the first sub-region and a signal value including energy information of X-rays detected in the second sub-region.

12. an X-ray tube configured to be able to switch a tube voltage applied to the X-ray tube between a first tube voltage and a second tube voltage; a detector assembly for detecting X-rays emitted from the X-ray tube, the detector assembly including a reference region for detecting X-rays and a filter, the reference region including a first sub-region for detecting X-rays that have passed through the filter and a second sub-region for detecting X-rays that have not passed through the filter; one or more processors; a non-transitory storage medium having stored thereon instructions for execution by the one or more processors of a medical system including: setting a first delay time from when the tube voltage is switched to the first tube voltage until when data acquisition starts, and a second delay time from when the tube voltage is switched to the second tube voltage until when data acquisition starts; determining a first effective voltage value of the first tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the first tube voltage is applied to the X-ray tube; determining a second effective voltage value of the second tube voltage based on energy information of X-rays detected in the first sub-region of the detector assembly and energy information of X-rays detected in the second sub-region of the detector assembly when the second tube voltage is applied to the X-ray tube; and Calculating an effective voltage difference representing the difference between the first effective voltage value and the second effective voltage value. Execute The instructions, when executed by the one or more processors, further cause the one or more processors to: a plurality of effective voltage differences are obtained by performing a plurality of steps of setting the first delay time and the second delay time, determining the first effective voltage value, determining the second effective voltage value, and calculating the effective voltage difference, and the first delay time and the second delay time when a maximum effective voltage difference is obtained are obtained based on the plurality of effective voltage differences. A storage medium that executes the above.

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