Medical system and recording media

Abstract

To provide techniques capable of shorting the time required for calibration.SOLUTION: An embodiment comprises: determining an effective voltage value V1 of a high kV on the basis of energy information of X-rays detected in a first sub-region of a detector assembly and energy information of X-rays detected in a second sub-region of the detector assembly, when the high kV is applied to an X-ray tube; determining an effective voltage value V2 of a low kV on the basis of 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 low kV is applied to the X-ray tube; calculating a correction coefficient required to correct beam hardening on the basis of the effective voltage value V1; and calculating a correction coefficient required to correct beam hardening on the basis of an effective voltage value V2.SELECTED DRAWING: Figure 15

Description

[Technical Field]

[0001] The present invention relates to a medical system that performs beam hardening correction, 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] Furthermore, over time, CT system components deteriorate, causing the beam-hardening correction coefficients used in the CT system to deviate from their ideal values. Therefore, CT systems are periodically calibrated to correct for beam hardening. In particular, calibration for DECT, which uses kV switching, involves performing a calibration scan to collect data corresponding to high and low tube voltages, and the correction coefficients required to correct for beam hardening are calculated based on the collected data.

[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. On the other hand, as described above, kV switching requires switching voltages in a short time. Therefore, when switching tube voltages, for example, from a low to a high tube voltage, even if the high tube voltage reaches the ideal voltage value, the tube voltage is immediately switched back to the low tube voltage, shortening the time during which the ideal voltage value is maintained. This becomes more pronounced as kV switching speeds increase. Therefore, when considering high and low tube voltages in kV switching technology, the effective tube voltage value is considered, rather than the ideal tube voltage value.

[0009] Therefore, when beam hardening correction is performed, a necessary correction coefficient is calculated based on the effective voltage value of the tube voltage.

[0010] On the other hand, the effective voltage value depends on the rotation speed of the gantry and the tube current of the X-ray tube. Therefore, calibration requires that combinations of rotation speed and tube current be determined in advance as presets, a calibration scan be performed to determine the effective voltage value for each preset, and a beam hardening correction coefficient be calculated based on the effective voltage value. Therefore, as the number of presets increases, the number of calibration scans that must be performed also increases, resulting in a problem of longer calibration times.

[0011] Furthermore, since a calibration scan cannot be performed for a combination of rotation speed and tube current that is not preset, it is not possible to obtain an effective voltage value. Therefore, even if a user wishes to perform an examination of a subject using a combination of rotation speed and tube current that is not preset, it is not possible to obtain an effective voltage value, and therefore it is not possible to obtain a beam hardening correction coefficient. This results in a problem in that a scan cannot be performed using a combination of rotation speed and tube current that the user desires. Therefore, a technique that allows for easy determination of the correction coefficients required for beam hardening is desired. [Means for solving the problem]

[0012] 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, 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; calculating a first correction factor required to correct for beam hardening based on the first effective voltage value; and calculating a second correction factor required to correct for beam hardening based on the second effective voltage value; and one or more processors executing A medical system including:

[0013] 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: 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; calculating a first correction factor required to correct for beam hardening based on the first effective voltage value; and calculating a second correction factor required to correct for beam hardening based on the second effective voltage value; It is a storage medium that executes the above. [Effects of the Invention]

[0014] In the present invention, a first effective voltage value of the first tube voltage is determined by irradiating X-rays from the X-ray tube at a first tube voltage and detecting X-rays that have passed through a filter and X-rays that have not passed through the filter with a detector assembly. Furthermore, a second effective voltage value of the second tube voltage is determined by irradiating X-rays from the X-ray tube at a second tube voltage and detecting X-rays that have passed through a filter and X-rays that have not passed through the filter with a detector assembly. Therefore, the effective voltage values ​​of the first tube voltage and the second tube voltage can be determined and correction coefficients corresponding to each effective voltage value can be calculated without changing the combination of operating conditions of the medical system (e.g., the rotation speed of the gantry and the tube current of the X-ray tube) during calibration. This shortens the time required for calibration and enables the subject to be scanned under the combination of operating conditions desired by the user. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a block diagram of a CT system 100 according to the present embodiment. [Figure 2] FIG. 2 is an explanatory diagram of a detector assembly 108. [Figure 3] FIG. 1 is an enlarged view of a reference area 109 of a detector assembly 108. [Figure 4] FIG. 1 is a flow diagram of a method for creating a baseline. [Figure 5] FIG. 10 is an explanatory diagram of a method for calculating an index value M. [Figure 6] FIG. 10 is an explanatory diagram of a baseline creation method. [Figure 7] FIG. 1 shows a baseline 60. [Figure 8] FIG. 7 is a diagram showing calibration data 70 obtained by a calibration scan. [Figure 9] 1 is a flow diagram for generating an image of an object 112. FIG. [Figure 10] FIG. 10 is an explanatory diagram of a method for setting a view number. [Figure 11] FIG. 10 is an explanatory diagram of a method for calculating a first index value M (=MV1) corresponding to a view v1 (high kV). [Figure 12] FIG. 10 is a diagram showing a first effective voltage value V1 corresponding to a first index value MV1. [Figure 13] FIG. 10 is an explanatory diagram of a method for determining correction coefficient data. [Figure 14] FIG. 10 is an explanatory diagram of a method for calculating a second index value M (=MV2) corresponding to view v2 (low kV). [Figure 15] 10 shows a second effective voltage value V2 corresponding to a second index value MV2. [Figure 16] FIG. 10 is an explanatory diagram of a method for determining correction coefficient data. [Figure 17] FIG. 10 is a diagram showing an example of the relationship between the view and the effective voltage value when kV switching is performed. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a description will be given of an embodiment of the invention, but the present invention is not limited to the following embodiment.

[0017] 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.

[0018] 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.

[0019] 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 rapid kV switching method 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.

[0020] 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.

[0021] 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 x-ray detector assembly 108 can acquire projection data for each view.

[0022] 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.

[0023] The computer 216 includes one or more processors 217. Using the one or more processors, the computer 216 outputs 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.).

[0024] 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.

[0025] 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.

[0026] Although FIG. 1 shows only one operator console 220 , more than one operator console may be coupled to computer 216 .

[0027] 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.

[0028] 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).

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 104 between a high tube voltage (hereinafter referred to as "high kV") and a low tube voltage (hereinafter referred to as "low kV").

[0034] However, the X-rays used in CT systems are polychromatic X-rays with a wide energy distribution from low to high energy, rather than monochromatic X-rays with a single energy. This causes the problem of beam hardening artifacts. Therefore, CT systems reconstruct CT images based on correction coefficients required for beam hardening correction.

[0035] However, the optimal value of the correction coefficient depends on the usage of the CT system, so the correction coefficient must be adjusted periodically. Therefore, CT system users perform periodic calibration. In particular, calibration for DECT using kV switching involves alternating between high and low kV tube voltages, detecting X-rays with a detector assembly, and calculating the correction coefficients required to correct for beam hardening based on the X-ray data detected by the detector assembly.

[0036] On the other hand, kV switching requires switching between low and high kV in a short time. Ideally, when the tube voltage is switched, the voltage should change in a rectangular pattern. However, in reality, when the tube voltage is switched, it takes a certain rise time for the tube voltage to reach the desired value. On the other hand, as described above, kV switching requires switching the voltage in a short time. Therefore, when the tube voltage is switched, for example, from low to high kV, even if the high kV reaches the ideal voltage value, the tube voltage is immediately switched back to low kV, so the time during which the ideal voltage value is maintained is shortened. This becomes more noticeable as kV switching speeds increase. Therefore, when considering high and low kV in kV switching technology, the effective voltage value, rather than the ideal voltage value, of the tube voltage is considered.

[0037] Therefore, when correcting beam hardening, the necessary correction coefficient is determined based on the effective voltage value.

[0038] On the other hand, the effective voltage value depends on the rotation speed of the gantry and the tube current of the X-ray tube 104. For this reason, in calibration, combinations of rotation speed and tube current must be prepared in advance as presets, a calibration scan must be performed to determine the effective voltage value for each preset, and beam hardening must be corrected based on the effective voltage value. For example, if four rotation speeds, r1, r2, r3, and r4, are considered as rotation speeds and A1, A2, A3, and A4 are considered as tube currents, the following 16 combinations of rotation speed and tube current must be determined in advance as presets, and a calibration scan must be performed for each of the 16 presets.

[0039] (r1,A1), (r1,A2), (r1,A3), (r1,A4) (r2,A1), (r2,A2), (r2,A3), (r2,A4) (r3,A1), (r3,A2), (r3,A3), (r3,A4) (r4,A1), (r4,A2), (r4,A3), (r4,A4)

[0040] Therefore, as the number of presets increases, the number of calibration scans that must be performed also increases, resulting in a problem that calibration takes a long time.

[0041] Furthermore, since the effective voltage value generally differs for each preset, the effective voltage value calculated for the 16 combinations described above cannot be used for a combination other than the 16 combinations described above. Therefore, if the combination of rotation speed and tube current desired by the user is not included in the 16 combinations described above, a problem occurs in that the scan cannot be performed under the user's desired conditions.

[0042] Therefore, in order to address the above-mentioned problem, the inventors have conducted extensive research and come up with a method for calculating the effective voltage value in a simple manner. In the following, the method for calculating the effective voltage value in this embodiment will be described.

[0043] 2 to 7 are explanatory diagrams of a method for calculating an effective voltage value in this embodiment. FIG. 2 is an illustration of the detector assembly 108. 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.

[0044] The detector assembly 108 also includes a filter 123 to reduce image degradation due to scattered radiation. The detector collimator 123 is disposed on the X-ray tube 104 side relative to the detector elements 202.

[0045] FIG. 3 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 . 3 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.

[0046] 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.

[0047] 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, a baseline to be used for calculating the effective voltage value must be prepared 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, or when work is performed to replace the X-ray tube. A method for creating the baseline will be described below.

[0048] FIG. 4 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.

[0049] 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.

[0050] 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.

[0051] 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 data 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 data for subsequent processing. The digital data is output to the computer 216.

[0052] In step ST4, the computer processor 217 calculates, based on the digital data, 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. 5.

[0053] 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).

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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).

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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).

[0062] 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 data 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 data for subsequent processing. The digital data is output to the computer 216.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] FIG. 6 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 a two-dimensional coordinate system.

[0068] 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.

[0069] 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 determined, for example, as a curve that best fits the points P1 to P4. FIG. 7 shows the determined 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.

[0070] After determining the baseline 60, a calibration scan is performed for each of the above-mentioned reference tube voltages VR1 to VR4 to obtain calibration data necessary for correcting beam hardening. Fig. 8 shows calibration data 70 obtained by the calibration scan.

[0071] The calibration data 70 includes a plurality of reference correction coefficient data 71 to 74. The horizontal axis of the calibration data 70 represents the channel number of the detector assembly 108, and the vertical axis represents the correction coefficient.

[0072] The reference correction coefficient data 71 is correction coefficient data corresponding to the reference tube voltage VR1 (=80 kV), and includes correction coefficients obtained for each channel of the detector assembly 108. The reference correction coefficient data 72 is correction coefficient data corresponding to the reference tube voltage VR2 (=100 kV) and includes correction coefficients obtained for each channel of the detector assembly 108. The reference correction coefficient data 73 is correction coefficient data corresponding to the reference tube voltage VR3 (=120 kV), and includes correction coefficients obtained for each channel of the detector assembly 108. The reference correction coefficient data 74 is correction coefficient data corresponding to the reference tube voltage VR4 (=140 kV) and includes correction coefficients obtained for each channel of the detector assembly 108.

[0073] The calibration data 70 including these reference correction coefficient data 71 to 74 is stored in a storage device of the CT system 100 or an external storage device that can communicate with the CT system 100.

[0074] In this way, when the CT system 100 is installed, the baseline 60 and the reference correction coefficient data 71 to 74 corresponding to the reference tube voltages VR1 to VR4 can be obtained.

[0075] Next, an example of a method for acquiring an image of the subject 112 using the baseline 60 and calibration data 70 will be described.

[0076] First, a scan of the object 112 is performed using the CT system 100. The detector assembly 108 detects the x-rays and outputs them to the DAS 214. The DAS 214 samples the analog data received from the detector assembly 108 and converts the analog data to digital data for further processing. The digital data is output to the computer 216. The computer's processor 217 generates an image of the object 112 based on the digital data (see FIG. 9).

[0077] FIG. 9 is a flow diagram for generating an image of the object 112 . In step ST11, the processor 217 sets the view number of the view for which a correction coefficient is to be calculated. FIG. 10 is an explanatory diagram of a method for setting the view number. In this embodiment, it is assumed that projection data of n views v1 to vn is acquired per one rotation of the X-ray tube. The processor 217 identifies the view for which a correction coefficient is to be calculated from the views v1 to vn, and sets the view number of the identified view as the view number of the view for which a correction coefficient is to be calculated. Here, the view number v=1 of the view v1 is set as the view number of the view for which a correction coefficient is to be calculated. Therefore, the processor 217 sets v=1. After setting the view number, the process proceeds to step ST12.

[0078] In step ST12, the processor 217 determines an effective voltage value for each view based on the signal value including the energy information of X-rays detected in the first sub-region 109B and the signal value including the energy information of X-rays detected in the second sub-region 109A. Here, since v=1 is set, consider the case where the effective voltage value corresponding to the view v1 of the view number v=1 is determined. The view v1 may be a view corresponding to a high kV or a view corresponding to a low kV. Here, the view v1 is assumed to be a view corresponding to a high kV.

[0079] Since step ST12 includes steps ST121 and ST122, steps ST121 and ST122 will be described in order.

[0080] In step ST121, the processor 217 calculates the first index value M (=MV1) corresponding to the view v1 (high kV) using equation (1) (see FIG. 11).

[0081] FIG. 11 is an explanatory diagram of a method for calculating the first index value M (=MV1) corresponding to the view v1 (high kV). The processor 217 determines the signal values ​​Sfilt(E) and S(E) of equation (1) based on the data obtained from the reference region 109. The signal value Sfilt(E) is a first signal value including energy information of the X-rays detected in the first sub-region 109B, and the signal value S(E) is a second signal value including energy information of the X-rays detected in the second sub-region 109A. The processor substitutes the signal values ​​Sfilt(E) and S(E) into equation (1) to calculate a first index value M (=MV1) corresponding to the view v1 (high kV). After calculating the first index value MV1, the process proceeds to step ST122.

[0082] In step ST122, processor 217 determines a first effective voltage value corresponding to the first index value MV1. FIG. 12 shows the first effective voltage value V1 corresponding to the first index value MV1. Processor 217 reads baseline 60 from the storage device and identifies the first effective voltage value V1 corresponding to the first index value MV1 based on baseline 60. V1 is, for example, 132 kV. After determining the effective voltage value, the process proceeds to step ST13.

[0083] In step ST13, the processor 217 calculates correction coefficient data corresponding to the high-kV view v1 based on the first effective voltage value V1 determined in step ST12 (see FIG. 13).

[0084] FIG. 13 is an explanatory diagram of a method for determining correction coefficient data. The processor 217 interpolates the correction coefficient data 75 corresponding to the first effective voltage value V1 based on the first effective voltage value V1 and the plurality of reference correction coefficient data 71-74. The correction coefficient data 75 is correction coefficient data corresponding to the first effective voltage value V1 and includes correction coefficients obtained for each channel of the detector assembly 108. In this manner, the first correction coefficient data 75 corresponding to the first effective voltage value V1 can be obtained. After the correction coefficient data 75 is obtained, the process proceeds to step ST14.

[0085] In step ST14, the processor 217 determines whether the view number v is v=n. In this case, v=1, so the process proceeds to step ST15.

[0086] In step ST15, the processor 217 increments the view number v. Here, since the view number v was v=1, v=1 is incremented to v=2. After the view number v is incremented, the process returns to step ST12.

[0087] In step ST12, processor 217 determines an effective voltage value for each view based on a signal value including X-ray energy information detected in first sub-region 109B and a signal value including X-ray energy information detected in second sub-region 109A. Here, since v=2 is set, an effective voltage value corresponding to view v2 with view number v=2 is calculated. View v2 is a view corresponding to low kV. In step ST12, the effective voltage value is calculated as follows.

[0088] In step ST121, the processor 217 calculates the second index value M (=MV2) corresponding to the view v2 (low kV) using equation (1) (see FIG. 14).

[0089] FIG. 14 is an explanatory diagram of a method for calculating the second index value M (=MV2) corresponding to the view v2 (low kV). The processor 217 determines the signal values ​​Sfilt(E) and S(E) of equation (1) based on the data obtained from the reference region 109. The signal value Sfilt(E) is a third signal value including energy information of the X-rays detected in the first sub-region 109B, and the signal value S(E) is a fourth signal value including energy information of the X-rays detected in the second sub-region 109A. The processor substitutes the signal values ​​Sfilt(E) and S(E) into equation (1) to calculate a second index value M (=MV2) corresponding to view v2 (low kV). After calculating the second index value MV2, the process proceeds to step ST122.

[0090] In step ST122, processor 217 determines a second effective voltage value corresponding to the second index value MV2. FIG. 15 shows the second effective voltage value V2 corresponding to the second index value MV2. Processor 217 identifies the second effective voltage value V2 corresponding to the second index value MV2 based on baseline 60. For example, V2 is 85 kV. After determining the effective voltage value, the process proceeds to step ST13.

[0091] In step ST13, the processor calculates correction coefficient data corresponding to the low kV view v2 based on the effective voltage value determined in step ST12 (see FIG. 16).

[0092] FIG. 16 is an explanatory diagram of a method for determining correction coefficient data. The processor 217 interpolates the correction coefficient data 76 corresponding to the second effective voltage value V2 based on the second effective voltage value V2 and the plurality of reference correction coefficient data 71-74. The correction coefficient data 76 is correction coefficient data corresponding to the second effective voltage value V2 and includes correction coefficients obtained for each channel of the detector assembly 108. In this manner, the second correction coefficient data 76 corresponding to the second effective voltage value V2 can be obtained. After the correction coefficient data 76 is obtained, the process proceeds to step ST14.

[0093] In step ST14, the processor 217 determines whether correction coefficient data has been calculated for all views. Here, correction coefficient data 75 (see FIG. 13) for view v1 and correction coefficient data 76 (see FIG. 16) for view v2 have been calculated, but correction coefficient data for other views have not yet been calculated. Therefore, the process proceeds to step ST15.

[0094] In step ST15, the processor 217 increments the view number v. Here, since the view number v was v=2, v=2 is incremented to v=3. After the view number v is incremented, the process returns to step ST12.

[0095] Similarly, steps ST12 to ST15 are repeatedly executed until correction coefficient data is obtained for each of all views v1 to vn during one rotation of the X-ray tube 104. Then, when correction coefficient data for all views v1 to vn corresponding to one rotation of the X-ray tube 104 has been obtained, the process proceeds to step ST16.

[0096] In step ST16, the processor 217 determines whether all correction coefficient data necessary for image reconstruction have been determined. For example, when the X-ray tube 104 has moved on to its second rotation and projection data for the second rotation has been collected, correction coefficient data must be calculated for the projection data for the second rotation of the X-ray tube 104. Therefore, when the X-ray tube has moved on to its second rotation, the process proceeds to step ST11, where the view number is reset to v=1, and steps ST12 to ST15 are executed for each view for the second rotation of the X-ray tube 104. Similarly, steps ST11 to ST15 are executed every time the X-ray tube 104 makes one rotation. Then, if the processor determines in step ST16 that all correction coefficient data necessary for image reconstruction by the X-ray tube 104 have been determined, the process proceeds to step ST17.

[0097] In step ST17, the processor 217 reconstructs an image based on the correction coefficient data obtained for each view. In this way, the flow of FIG. 9 ends.

[0098] In this embodiment, the detector assembly 108 of the CT system has a filter 124 (see FIG. 5). 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 sub-region 109A (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 sub-region 109B (detector element 202B). Then, an index value M is calculated using a signal value Sfilt(E) containing information on the energy of the X-ray that has passed through the filter 124 and a signal value S(E) containing information on the energy of the X-ray that has not passed through the filter 124. This index value M is used to calculate an effective voltage value.

[0099] In this embodiment, as a preparation for calculating an effective voltage value based on an index value M, a baseline 60 (see FIG. 7) is created that defines the correspondence between the index value M and the effective voltage value. When creating the baseline 60, four reference tube voltages VR1 to VR4 are set, and four index values ​​M80 to M140 corresponding to the four reference tube voltages VR1 to VR4 are calculated. The baseline 60 is then created based on the four reference tube voltages VR1 to VR4 and the four index values ​​M80 to M140. This baseline 60 is stored in the storage device (or an external storage device) of the CT system. The index values ​​M80 to M140 are values ​​that are calculated by maintaining the tube voltages at constant values ​​of VR1 (= 80 kV), VR2 (= 100 kV), VR3 (= 120 kV), and VR4 (= 140 kV), respectively. Therefore, the index values ​​M80 to M140 can be calculated without switching the tube voltage between high kV and low kV, and each of the index values ​​M80 to M140 can be calculated without being affected by the gantry rotation speed. Furthermore, each of the index values ​​M80 to M140 is a value calculated based on X-ray energy information. X-ray energy basically depends on the tube voltage of the X-ray tube 104 and does not depend much on the tube current, so the index values ​​M80 to M140 can be calculated without being affected by the tube current. Therefore, each of the index values ​​M80 to M140 is not substantially affected by the gantry rotation speed or tube current, and therefore it is not necessary to calculate an index value for each combination of gantry rotation speed and tube current, and one index value can be calculated for one reference tube voltage. In this embodiment, four index values ​​M80 to M140 can be calculated for four reference tube voltages VR1 to VR4 in this way, so that a baseline 60 representing the correspondence between tube voltages and index values ​​can be created without considering many combinations of gantry rotation speeds and tube currents. Therefore, when imaging a subject using kV switching technology, the effective voltage value can be identified for each view (each projection data) from the baseline 60 by calculating the index value M expressed by equation (1) based on the X-rays detected in the reference region 109 during imaging of the subject.Therefore, correction coefficient data can be interpolated for each effective voltage value based on the reference correction coefficient data 71 to 74 (see FIG. 8) stored in the storage device. As described above, since the effective voltage value can be specified for each view (each projection data), correction coefficient data can be calculated for each view, resulting in high-quality images being reconstructed. In addition, since there is no need to calculate correction coefficient data for many combinations of gantry rotation speed and tube current, the work time required for calibration can be shortened. Furthermore, since correction coefficient data can be calculated for each effective voltage value, scans can be performed using a combination of rotation speed and tube current desired by the user.

[0100] To specifically explain the effect of this embodiment, FIG. 17 shows an example of the relationship between the view and the effective voltage value when kV switching is performed.

[0101] FIG. 17 shows the effective voltage values ​​corresponding to high kV and low kV for views v400 to v440. The even-numbered views represent effective voltage values ​​corresponding to high kV, while the odd-numbered views represent effective voltage values ​​corresponding to low kV. As can be seen from FIG. 17, the effective voltage values ​​corresponding to high kV vary from view to view. Similarly, the effective voltage values ​​corresponding to low kV also vary from view to view. Therefore, when calculating a correction coefficient for each view using a conventional method, it is necessary to prepare a combination of rotation speed and tube current for each effective voltage value of each view as a preset and perform a calibration scan for each preset. However, this conventional method requires preparing a preset for each effective voltage value and performing a calibration scan, which results in a significant amount of time required for calibration. Therefore, in order to avoid the long calibration time, the number of presets prepared in advance as combinations of rotation speed and tube current is limited to, for example, 16, thereby minimizing the time required for calibration. However, this poses a problem in that if the combination of rotation speed and tube current desired by the user is not included in the above 16 combinations, the subject cannot be scanned under the conditions desired by the user.

[0102] Furthermore, if the number of views in a scan of one preset is 1000, slight variations in the effective voltage value for each view will occur, and there is a problem in that if the calibration data obtained for one preset is used for another preset, variations will occur in the corrected data.

[0103] In contrast to this, in this embodiment, if the index value M is calculated for each view, the effective voltage value corresponding to the index value M can be identified from the baseline 60, and the correction coefficient corresponding to the identified effective voltage value can be easily calculated by interpolation. Therefore, according to this embodiment, the time required for calibration can be shortened, and further, it is possible to obtain the effect that a scan of the subject can be performed with a combination of rotation speed and tube current desired by the user.

[0104] 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 112. [Explanation of symbols]

[0105] 60 Baseline 70 calibration data 71~74 Standard correction coefficient data 71 Standard correction coefficient data 72 Standard correction coefficient data 73 Standard correction coefficient data 74 Standard correction coefficient data 75 Correction coefficient data 76 Correction coefficient data 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 Subject 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 Rotational 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, 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; calculating a first correction factor required to correct for beam hardening based on the first effective voltage value; and calculating a second correction factor required to correct for beam hardening based on the second effective voltage value; one or more processors executing medical systems, including

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

3. Determining the first effective voltage value includes: calculating a first index value based on the first signal value and the second signal value; 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 the third signal value and the fourth signal value; and The medical system of claim 2 , further comprising determining the second effective voltage value based on the second index value.

4. 4. The medical system of claim 3, 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.

5. The one or more processors: The medical system according to claim 1 , 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.

6. 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; 6. The medical system of claim 5, 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.

7. the one or more processors determining first correction coefficient data corresponding to the first effective voltage value; and determining second correction coefficient data corresponding to the second effective voltage value; The medical system according to claim 6 , wherein the medical system executes the following:

8. The medical system of claim 7 , wherein the first correction factor data and the second correction factor data include correction factors obtained for each channel of the detector assembly.

9. a plurality of reference correction coefficient data corresponding to the plurality of reference tube voltages is stored in a storage device included in the medical system or in a storage device accessible by the medical system; the one or more processors calculating the first correction coefficient data based on the first effective voltage value and the plurality of reference correction coefficient data; and calculating the second correction coefficient data based on the second effective voltage value and the plurality of reference correction coefficient data; The medical system according to claim 7 , wherein the medical system executes the following:

10. 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: 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; calculating a first correction factor required to correct for beam hardening based on the first effective voltage value; and calculating a second correction factor required to correct for beam hardening based on the second effective voltage value; A storage medium that executes the above.

Images