JP5972577B2 - X-ray computed tomography system - Google Patents

Description

  Embodiments described herein relate generally to an X-ray computed tomography apparatus.

  Head CT perfusion analysis is used in an X-ray computed tomography apparatus. Head CT perfusion analysis is useful for hemodynamic diagnosis of acute cerebral infarction. In perfusion analysis, X-rays are repeatedly irradiated to a subject into which a contrast medium has been injected, so that the exposure dose of the subject or engineer inevitably increases.

  The X-ray generation cycle is set in advance in the scan planning stage after predicting a change in the contrast agent concentration over time, and is not changed after the scan starts. For this reason, the X-ray generation cycle does not match the actual blood contrast medium concentration, and X-rays may be continuously generated during a period in which the CT value change is small. Therefore, even if the X-ray generation period is set at the planning stage, the exposure dose of the subject cannot be sufficiently reduced.

JP 2009-207876 A

  An object is to provide an X-ray computed tomography apparatus capable of reducing the exposure dose.

The X-ray computed tomography apparatus according to the present embodiment includes an X-ray tube that generates X-rays and a high voltage that generates a high voltage applied to the X-ray tube to generate X-rays from the X-ray tube. A generator, an X-ray detector that detects X-rays generated from the X-ray tube and transmitted through the subject, a support mechanism that supports the X-ray tube rotatably about a rotation axis, and the X-ray detector a generating unit that occurs in the first CT image and the second CT image based on an output from, the time rate of change of the CT values between the first CT image and the second CT image and time change rate calculating section for calculating, for anda generation period determination unit which determines the generation period of the X-rays from the X-ray tube according to the time rate of change.

The figure which shows the structure of the X-ray computed tomography apparatus which concerns on this embodiment. The figure for demonstrating the X-ray generation period, X-ray continuation time, and X-ray stop time which are utilized in this embodiment. The other figure for demonstrating the X-ray generation period, X-ray continuation time, and X-ray stop time which are utilized in this embodiment. The figure which shows the X-ray generation timing in the contrast scan which concerns on this embodiment with a CT value change curve and a CT value change rate curve. The figure which shows the typical flow of the contrast scan performed under control of the system control part of FIG. The figure for demonstrating the change timing of the X-ray stop time and X-ray generation period performed in step S9 of FIG. 5 taking a continuous scan as an example. The figure for demonstrating the change timing of the X-ray stop time and X-ray generation period performed in step S9 of FIG. 5 taking an intermittent scan as an example. The figure which shows the typical example of each CT value change curve of the artery area | region and vein area | region based on the modification 1 of this embodiment.

  The X-ray computed tomography apparatus according to this embodiment will be described below with reference to the drawings.

  Assume that the scanning method according to the present embodiment is a contrast scan in which a subject into which a contrast medium is injected is repeatedly scanned with X-rays. The scan site according to this embodiment can be applied to any site where blood vessels such as the head, chest, abdomen, arms, and legs run. For example, a contrast scan of the head is used for head CT perfusion analysis. In contrast scanning, repeated scanning is performed to analyze blood flow dynamics, and CT images are repeatedly generated. Perfusion analysis is performed based on repeatedly generated CT images. As is well known, the contrast scan includes a monitoring scan and a main scan. The monitoring scan is a scan for confirming whether or not the contrast medium injected into the subject has reached the scan site. Transition from the monitoring scan to the main scan is triggered when the contrast agent has sufficiently reached the scan site. The monitoring scan is performed at a lower dose than the main scan to reduce the exposure dose.

  Image reconstruction methods used by the X-ray computed tomography apparatus include a full scan method and a half scan method. In the full scan method, in order to reconstruct a CT image of one slice or one volume, projection data for one round around the subject, that is, about 2π [rad] is required. In the half scan method, projection data for π + α [rad] (α: fan angle) is necessary to reconstruct a CT image of one slice or one volume. In this embodiment, any of the full scan method and the half scan method can be applied.

  FIG. 1 is a diagram showing a configuration of an X-ray computed tomography apparatus 1 according to the present embodiment. As shown in FIG. 1, a gantry 10 and a console 30 are mounted.

  The gantry 10 is mounted with an annular or disk-shaped rotating frame 11. The rotating frame 11 supports the X-ray tube 13 and the X-ray detector 15 so as to be rotatable around the central axis (rotating axis) of the rotating frame 11. An FOV (field of view) is set inside the opening of the rotating frame 11. The rotating frame 11 is connected to the rotation driving unit 17. The rotation driving unit 17 rotates the rotating frame 11 at a constant angular velocity according to control by the scan control unit 43 in the console 30, and rotates the X-ray tube 13 and the X-ray detector 15 around the rotation axis.

  The Z axis is defined as the rotation axis of the rotating frame 11. The Y axis is defined as an axis connecting the X-ray focal point of the X-ray tube 13 and the center of the detection surface of the X-ray detector 15. The Y axis is orthogonal to the Z axis. The X axis is defined as an axis orthogonal to the Y axis and the Z axis. Thus, the XYZ orthogonal coordinate system constitutes a rotating coordinate system that rotates with the rotation of the rotating frame 11.

  A top plate support mechanism 19 is installed in the vicinity of the rotating frame 11. The top plate support mechanism 19 supports the top plate 21 so as to be movable along the Z axis. The top plate support mechanism 19 supports the top plate 21 so that the long axis of the top plate 21 is parallel to the Z axis. A subject P is placed on the top plate 21. The top board support mechanism 19 is equipped with a top board moving part (motor) 23. The top plate moving unit 23 drives the top plate support mechanism 19 according to the control by the scan control unit 43 and moves the top plate 21 along the Z-axis direction.

  The X-ray tube 13 generates X-rays in response to application of a high voltage and supply of a filament current from the high voltage generator 25. The high voltage generator 25 applies a high voltage to the X-ray tube 13 according to control by the scan controller 43 and supplies a filament current.

  The X-ray detector 15 detects X-rays generated from the X-ray tube 13. The X-ray detector 15 includes a plurality of detection elements arranged in a two-dimensional manner. For example, the plurality of detection elements are arranged along an arc centered on the rotation axis of the rotating frame 11. The arrangement direction of the detection elements along this arc is called a channel direction. A plurality of detection elements arranged along the channel direction is called a detection element array. The plurality of detection element rows are arranged along the row direction along the rotation axis. Each detection element detects X-rays generated from the X-ray tube 13 and generates an electric signal (current signal) corresponding to the detected X-ray intensity. The generated electric signal is supplied to a data acquisition circuit (DAS) 27.

  The data collection circuit 27 collects electrical signals for each view via the X-ray detector 15 under the control of the scan control unit 43. As is well known, the view corresponds to the rotation angle of the rotating frame 11 about the rotation axis Z. In terms of signal processing, the view corresponds to a data sampling point when the rotating frame 11 is rotated. The data collection circuit 27 converts the collected analog electrical signal into digital data. Digital data is called raw data. The raw data is supplied to the console 30 by a non-contact data transmission unit (not shown).

  The console 30 includes a preprocessing unit 31, a reconstruction unit 33, a region of interest setting unit 35, a monitoring unit 37, a CT value change rate calculation unit 39, an X-ray generation cycle calculation unit 41, a scan control unit 43, a display unit 45, and an operation. Unit 47, storage unit 49, and system control unit 51.

  The preprocessing unit 33 performs preprocessing on the raw data from the data collection circuit 27 and generates projection data to be used for reconstruction processing. Examples of the preprocessing include logarithmic conversion, gain correction, nonuniformity correction, and offset correction. The projection data is stored in the storage unit 49 in view units.

  The reconstruction unit 33 generates CT image data related to the subject P based on the projection data. More specifically, the reconstruction unit 33 performs an image reconstruction process on the projection data for the view necessary for reconstructing one slice or one volume of CT image data, thereby obtaining one slice or one volume of CT image. Generate the data. The CT image expresses the spatial distribution of CT values depending on the X-ray absorption coefficient. In the present embodiment, the reconstruction unit 33 repeatedly generates CT image data based on projection data collected repeatedly during a contrast scan. The CT image data is stored in the storage unit 49.

  The region-of-interest setting unit 35 sets a region of interest in the CT image according to an instruction from the user via the operation unit 47 or by image processing. The region of interest is set in order to limit the CT value monitoring target region. Note that when the entire range of the CT image is to be monitored for CT values, the region of interest does not necessarily have to be set. In this case, the region of interest setting unit 35 is unnecessary.

  The monitoring unit 37 monitors the CT value on the repeatedly generated CT image. When the region of interest is set, the monitoring unit 37 monitors the CT value only in this region of interest.

  The CT value change rate calculation unit 39 calculates the time change rate of the CT value on the repeatedly generated CT image (hereinafter referred to as the CT value change rate) every time a CT image is generated. When the region of interest is set, the CT value change rate calculation unit 39 calculates the CT value change rate limited to this region of interest.

  The X-ray generation cycle calculation unit 41 repeatedly calculates an X-ray generation cycle (hereinafter referred to as an X-ray generation cycle) from the X-ray tube 13 according to the CT value change rate. Further, the X-ray generation period calculation unit 41 calculates an X-ray stop time based on the calculated X-ray generation period and the X-ray duration. An X-ray generation cycle, an X-ray duration time, and an X-ray stop time will be described with reference to FIGS.

  As shown in FIG. 2, a continuous scan is performed in a specific phase of the contrast scan. During the contrast scan, the rotating frame 11 continues to rotate at a constant angular velocity. X-rays are repeatedly and continuously generated in units of X-ray duration. The X-ray duration depends on the angular velocity of the rotating frame 11 so that it substantially coincides with the time (scan time) required to acquire projection data necessary to reconstruct a CT image of one slice or volume. Are determined in advance. The x-ray duration is constant over the contrast scan. Here, in the present embodiment, an X-ray tube is generated for the X-ray duration while rotating the rotating frame 11, X-rays transmitted through the subject are detected by the X-ray detector, and one slice or one volume worth is obtained. A group of operation units for collecting projection data necessary to reconstruct a CT image is defined as one CT scan. A CT image is instantly generated by the reconstruction unit based on projection data collected during one CT scan.

  As shown in FIG. 3, an intermittent scan is performed in a specific phase in the contrast scan. X-rays are repeatedly and intermittently generated in units of X-ray duration. When the X-ray duration time elapses, the generation of X-rays is stopped for the X-ray stop time in order to reduce the exposure dose. When the X-ray stop time elapses from the end of X-ray generation, X-rays are generated again for the X-ray duration time. The total time of the X-ray duration and the X-ray stop time is the X-ray generation cycle. The X-ray generation cycle is also called an X-ray imaging interval. The X-ray stop time Tr is calculated based on the X-ray generation period Ts and the X-ray duration Td according to the following equation (1).

Tr = Ts−Td (1)
In the case of continuous scanning as shown in FIG. 2, the X-ray stop time Tr is 0, and the X-ray generation period Ts is equal to the X-ray duration Td.

  The scan control unit 43 controls the rotation drive unit 17, the high voltage generation unit 25, and the data collection circuit 27 according to the scan sequence of the contrast scan according to the present embodiment. Specifically, the scan control unit 43 controls the rotation driving unit 17 so that the rotating frame 11 rotates at a constant angular velocity during the contrast scan. The scan control unit 43 controls the high voltage generation unit 25 in real time so that X-rays are generated from the X-ray tube 13 in accordance with the X-ray generation cycle. Further, the scan control unit 43 controls the data collection circuit 27 so that projection data is collected for each view during the contrast scan. The scan control unit 43 also controls the top plate moving unit 23 to move the top plate 21 along the Z axis. The contrast scan according to the present embodiment can be applied to both a dynamic scan in which a plurality of CT scans are executed while the top plate 21 is stationary, and a helical scan in which a plurality of CT scans are executed while the top plate 21 is moved. It is. The helical scan according to the present embodiment can be applied to a normal helical scan in which the top plate 21 is moved only in one direction and a helical scan in which the top plate 21 is reciprocated (so-called helical shuttle scan). In order to simplify the following description, it is assumed that the top plate 21 is stationary in the contrast scan according to the present embodiment.

  The display unit 45 displays the CT image generated by the reconstruction unit 33 on the display device. As the display device, for example, a CRT display, a liquid crystal display, an organic EL display, a plasma display, or the like can be used as appropriate.

  The operation unit 47 receives various commands and information input from the user via the input device. As an input device, a keyboard, a mouse, a switch, or the like can be used.

  The storage unit 49 stores projection data and CT image data. The storage unit 49 stores a control program for the X-ray computed tomography apparatus 1. This control program is for causing the system control unit 51 to execute a control function for performing a contrast scan according to the present embodiment.

  The system control unit 51 functions as the center of the X-ray computed tomography apparatus 1. Specifically, the system control unit 51 reads out a control program stored in the storage unit 49 and develops it on the memory, and controls each unit according to the developed control program.

  Next, an operation example of the X-ray computed tomography apparatus 1 in the contrast scan performed under the control of the system control unit 51 will be described.

  First, an outline of a contrast scan according to the present embodiment will be described with reference to FIG. FIG. 4 is a diagram showing the X-ray generation timing in the contrast scan according to this embodiment together with the CT value change curve and the CT value change rate curve. The CT value change curve is a time change curve of the CT value. The CT value change rate curve is a time change curve of the CT value change rate. As shown in FIG. 4A, the contrast agent is injected into the subject, and the CT value in the scan region increases as the contrast agent flows into the scan region. While the CT value is increasing, the CT value change rate takes a positive value as shown in FIG. The contrast medium will eventually flow out of the scan site. As shown in FIG. 4A, the CT value in the scan region decreases as the contrast agent flows out of the scan region. While the CT value is decreasing, the CT value change rate takes a negative value as shown in FIG. As time passes, the outflow of the contrast agent becomes slower. As shown in (a) of FIG. 4, when the outflow of the contrast agent becomes gentle. The CT value almost does not change, and the CT value change rate approaches 0 as shown in FIG.

  In observing the blood flow dynamics, it is better that the time resolution is high in a period when the CT value change rate is high, and the time resolution is not high in a period where the CT value change rate is low. In the present embodiment, the X-ray generation cycle is calculated in real time according to the CT value change rate. The X-ray generation cycle is defined to be inversely proportional to the CT value change rate. That is, in the period when the CT value change rate is large, the X-ray generation period is small, in other words, continuous scanning is performed. On the other hand, in the period when the CT value change rate is small, the X-ray generation cycle is large and intermittent scanning is performed. The scan control unit 43 controls the high voltage generation unit 25 in real time so that X-rays are generated according to the X-ray generation cycle. Thereby, the X-ray generation cycle can be changed according to the actual blood flow dynamics, and the time interval of the CT scan can be changed in real time.

  FIG. 5 is a diagram illustrating a typical flow of a contrast scan performed under the control of the system control unit 51 according to the present embodiment. First, as shown in FIG. The system control unit 51 causes the region of interest setting unit 35 to perform setting processing (step S1). In step S <b> 1, the region-of-interest setting unit 35 sets a region of interest at a position in accordance with an instruction from the user via the operation unit 47 in the CT image related to the scan region of the subject acquired in advance by scanography or the like. Since the region of interest is set to monitor whether or not the contrast medium has reached the scan site, the region of interest is set to include a pixel region corresponding to the blood vessel. Note that the region of interest may be automatically set by existing image processing. The region of interest may be set at one place or may be set at a plurality of places. However, in order to simplify the following description, it is assumed that the region of interest is set in one place.

  When step S <b> 1 is performed, the system control unit 51 stands by for an instruction to start a monitoring scan via the operation unit 47. When the user is ready to start monitoring, the user inputs a monitoring start instruction via the operation unit 47. The contrast agent may be injected before or after the start instruction is input. However, in order to specifically describe the following, it is assumed that the contrast agent is injected into the subject before the start instruction is input.

  When a monitoring start instruction is input, the system control unit 51 causes the scan control unit 43 to perform a monitoring scan (step S2). In step S <b> 2, the scan control unit 43 controls the rotation drive unit 17, the high voltage generation unit 25, and the data collection circuit 27 for the monitoring scan. The rotation driving unit 17 rotates the rotating frame 11 at a constant angular velocity. The high voltage generator 25 repeatedly generates a relatively low dose of X-rays from the X-ray tube 13. The data acquisition circuit 27 reads an electrical signal from the X-ray detector 15 for each view, and supplies digital data corresponding to the read electrical signal to the console 30. The digital data supplied to the console 30 is converted into projection data by the preprocessing unit 31, and CT image data is generated by the reconstruction unit 33 based on the projection data.

  During the monitoring scan, the system control unit 51 causes the monitoring unit 37 to monitor the CT value (step S3). In step S3, the monitoring unit 37 specifies the CT value in the region of interest in real time based on the repeatedly generated CT image. Statistical values of CT values of a plurality of pixels constituting the region of interest are specified as CT values in the region of interest. For example, an average value, an intermediate value, or a total value of CT values of a plurality of pixels is used as the statistical value. A CT value in the region of interest is specified each time a CT image is generated.

  When step S3 is performed, the system control unit 51 causes the monitoring unit 37 to perform determination processing (step S4). In step S4, the monitoring unit 37 determines whether or not the CT value in the region of interest has reached the first threshold value. The first threshold is a threshold for shifting from the monitoring scan to the main scan. The first threshold value can be set to an arbitrary value by the user via the operation unit 47.

  When it is determined in step S4 that the CT value in the region of interest has not reached the first threshold value (step S4: NO), the system control unit 51 causes the scan control unit 43 to continue the monitoring scan, and the monitoring unit 37 repeats steps S3 and S4.

  If it is determined in step S4 that the CT value in the region of interest has reached the first threshold (step S4: YES), the system control unit 51 causes the scan control unit 43 to shift from the monitoring scan to the main scan. (Step S5). In step S5, the scan control unit 43 controls the rotation drive unit 17, the high voltage generation unit 25, and the data collection circuit 27 for the main scan. The rotation driving unit 17 rotates the rotating frame 11 at a constant angular velocity, similarly to the monitoring scan. The high voltage generator 25 generates X-rays with a high dose from the X-ray tube 13 as compared with the monitoring scan. The data acquisition circuit 27 reads an electrical signal from the X-ray detector 15 for each view, and supplies digital data corresponding to the read electrical signal to the console 30. The digital data supplied to the console 30 is converted into projection data by the preprocessing unit 31, and CT image data is generated by the reconstruction unit 33 based on the projection data. In the first CT scan after shifting to the main scan, X-rays are generated according to a preset X-ray generation cycle. For example, in the first CT scan, it is preferable that the X-ray generation period coincides with the X-ray duration, that is, the continuous scan is performed.

  When step S5 is performed, the system control unit 51 causes the CT value change rate calculation unit 39 to perform calculation processing (step S6). In step S6, the CT value change rate calculation unit 39 calculates the time change rate of the CT value of the region of interest on the repeatedly generated CT image for each CT scan. The time change rate is the latest CT with respect to the time difference between the scan time of the latest CT image and the scan time of the CT image generated before this latest CT image (hereinafter referred to as the previous CT image). It is defined by the ratio of the CT value change between the image and the previous CT image. The CT value change is equal to the CT value difference between the latest CT image and the previous CT image. In other words, the CT value change rate is a time derivative of the CT value. The previous CT image is, for example, a CT image generated one time before the latest CT image. In this case, the time difference between the scan time of the latest CT image and the scan time of the previous CT image coincides with the X-ray generation cycle. As the scan time, any one of the CT scan start time, intermediate value, and end time is preferably adopted. The previous CT image is not limited to the CT image generated one time before the latest CT image, and any CT image can be applied as long as it is a CT image generated before the latest CT image. .

  Thus, the previous CT image may be a single CT image corresponding to a single scan time, or may be a plurality of CT images respectively corresponding to a plurality of scan times. When the previous CT image is a plurality of CT images, the CT value change rate may be calculated based on a moving average of the time change rates of the plurality of previous CT images. In addition, the scan time of the previous CT image may be set to an average value or an intermediate value of the scan times of a plurality of previous CT images, the previous scan time, or the like.

  When step S6 is performed, the system control unit 51 causes the X-ray generation cycle calculation unit 41 to perform calculation processing (step S7). In step S7, the X-ray generation cycle calculation unit 41 calculates an X-ray generation cycle and an X-ray stop time corresponding to the CT value change rate calculated in step S6. The X-ray generation cycle is defined to be inversely proportional to the CT value change rate. For example, the X-ray generation cycle Ts [sec] is calculated based on the absolute value R of the CT value change rate according to (2) below.

Ts = A * 1 / R (2)
Here, the conversion coefficient A is a CT value change rate when the X-ray generation period Ts is 1 sec. As shown in the equation (2), the larger the absolute value of the CT value change rate, the smaller the X-ray generation cycle Ts. Conversely, the smaller the absolute value of the CT value change rate, the greater the X-ray generation period Ts. The conversion coefficient A can be set to an arbitrary value via the operation unit 47 by the user.

  In step S7, the X-ray generation period calculation unit 41 calculates the X-ray stop time from the X-ray generation period and the X-ray duration according to the above-described equation (1).

  When step S7 is performed, the system control unit 51 causes the scan control unit 43 to perform determination processing (step S8). In step S8, the scan control unit 43 changes whether or not to change the set value of the X-ray stop time to the value of the X-ray stop time calculated in step S7. Specifically, it is determined whether or not the difference between the set value of the X-ray stop time and the X-ray stop time calculated in step S7 is smaller than a preset second threshold value. When the difference is smaller than the second threshold, it is determined not to change the X-ray stop time, and when the difference is greater than the second threshold, it is determined to change the X-ray stop time.

  When it is determined that the X-ray stop time is to be changed (step S8: YES), the system control unit 51 causes the scan control unit 43 to perform a change process (step S9). In step S9, the scan control unit 43 changes the set value of the X-ray stop time to the value calculated in step S7. Thereby, the X-ray generation cycle is changed with the change of the X-ray stop time. Note that the method for changing the X-ray generation cycle according to the present embodiment is not limited to the method for changing the X-ray generation cycle in accordance with the change of the X-ray stop time, and even if the X-ray generation cycle is changed directly. good. The change of the X-ray generation cycle will be described in detail later.

  When step S9 is performed, the system control unit 51 proceeds to step S5 and causes the scan control unit 43 to perform the next CT scan. Similarly, steps S5 to S9 are repeated for the next CT scan. Specifically, after calculating the X-ray stop time in step S7, the scan control unit 43 does not generate X-rays from the X-ray tube 13 for the X-ray stop time calculated in step S7. And the high voltage generator 25 is controlled so that X-rays are generated from the X-ray tube 13 in accordance with the X-ray generation cycle calculated in step S7 when the X-ray stop time has elapsed.

  Hereinafter, the details of the X-ray stop time and the change timing of the X-ray generation cycle will be described with reference to FIGS. First, the details of the change timing of the X-ray generation cycle in the case of continuous scanning will be described with reference to FIG. As shown in FIG. 6, it is assumed that the CT scan CS is continuously repeated. A CT image is generated based on the projection data collected by the first CT scan CS1 performed continuously. Based on the CT image and the like for the CT scan CS1, the X-ray generation cycle Ts1 and the X-ray stop time Tr1 are calculated at the calculation timing T1. Here, it is assumed that the X-ray stop time Tr1 is zero. The calculation timing T1 is after a predetermined time such as the reconstruction time, the calculation time of the X-ray generation period, the calculation time of the X-ray stop time has elapsed after the CT scan CS1 ends. The second CT scan CS2 is started immediately after the end of the CT scan CS1. That is, the calculation timing T1 overlaps with the CT scan CS2.

  As shown in FIG. 6, according to the X-ray stop time Tr1 = 0 calculated at the calculation timing T1, the third CT scan CS3 is started immediately after the CT scan CS2 ends. A CT image is generated based on the projection data collected by the CT scan CS2. Based on the CT image and the like for the CT scan CS2, the X-ray generation period Ts2 and the X-ray stop time Tr2 are calculated at the calculation timing T2. The calculation timing T2 overlaps with the third CT scan CS3. Here, it is assumed that the X-ray stop time Tr2 is Tr2> 0. After the CT scan CS3 ends, the X-rays are stopped for the X-ray stop time Tr2 calculated at the calculation timing T2. Then, the fourth CT scan CS4 is started when the X-ray stop time Tr2 has passed since the CT scan CS3 has ended.

  Next, the case of intermittent scanning will be described with reference to FIG. Assume that the first CT scan CS1 is performed as shown in FIG. A CT image is generated based on the projection data collected by the CT scan CS1. Based on the CT image and the like for the CT scan CS1, the X-ray generation cycle Ts1 and the X-ray stop time Tr1 are calculated at the calculation timing T1. The calculation timing T1 is after a predetermined time such as the reconstruction time, the calculation time of the X-ray generation period, the calculation time of the X-ray stop time has elapsed after the CT scan CS1 ends. After the CT scan CS1 ends, the X-rays are stopped for the X-ray stop time Tr0 calculated before the CT scan CS1. Then, the second CT scan CS2 is started when the X-ray stop time Tr0 has passed since the end of the CT scan CS1.

  As shown in FIG. 7, X-rays are stopped for the X-ray stop time Tr1 after the CT scan CS2 ends. A CT image is generated based on the projection data collected by the CT scan CS2. Based on the CT image and the like for the CT scan CS2, the X-ray generation period Ts2 and the X-ray stop time Tr2 are calculated at the calculation timing T2. The calculation timing T2 is also after a predetermined time has elapsed since the CT scan CS2. The third CT scan CS3 is started when the X-ray stop time Tr1 has elapsed since the end of the CT scan CS2. It repeats similarly about the following CT scans. X-ray duration is constant. Therefore, the X-ray generation period Ts1 elapses when the X-ray stop time Tr1 and the subsequent X-ray duration Td elapse.

  As described above, the X-ray generation cycle and the X-ray stop time calculated using the CT image for the current CT scan are reflected because there is a time lag between the calculation timing and the CT scan end timing. Because it occurs, it is the next CT scan after the current CT scan, that is, the CT scan two times after the current CT scan.

  This is the end of the description of the details of the change timing of the X-ray stop time and the X-ray generation cycle.

  As shown in FIG. 5, when it is determined in step S8 that the X-ray stop time is not changed (step S8: NO), the system control unit 51 causes the monitoring unit 37 to perform determination processing (step S10). In step S10, the monitoring unit 37 determines whether or not the CT value in the region of interest on the latest CT image has reached the third threshold value. The third threshold is a threshold for ending the main scan. The third threshold can be set to an arbitrary value via the operation unit 47 by the user. In addition, as a 3rd threshold value, it is good to set to about 80 HU, for example. It is empirically known that when the CT value in the region of interest falls to about 80 HU, it can be estimated that the contrast agent has completely flowed from the scan site.

  When it is determined in step S10 that the CT value in the region of interest has not reached the third threshold value (step S10: NO), the system control unit 51 causes the scan control unit 43 to continue the main scan, and step S5 -S9 is repeatedly performed.

  When it is determined in step S10 that the CT value in the region of interest has reached the second threshold value (step S10: YES), the system control unit 51 causes the scan control unit 43 to end the main scan. Specifically, the scan control unit 43 controls the high voltage generation unit 25 so as to end the generation of X-rays from the X-ray tube 13 and sets the rotation driving unit 17 so as to end the rotation of the rotating frame 11. And control the data collection circuit 27 to end the data collection. This completes the contrast scan.

  Note that the X-ray stop time or the X-ray generation cycle can be arbitrarily changed or set to a desired value by the user via the operation unit 47 during the contrast scan. In this case, the user refers to the CT value change curve of the region of interest displayed on the display unit 45 and inputs a numerical value of the X-ray stop time or the X-ray generation cycle via the operation unit 47. When the numerical value of the X-ray stop time is input, the scan control unit 43 changes the input numerical value to the set value of the X-ray stop time. Then, the X-ray generation cycle is calculated according to the above equation (1) based on the set value of the changed X-ray stop time and the X-ray duration. When the numerical value of the X-ray generation cycle is input, the scan control unit 43 changes the input numerical value to the set value of the X-ray generation cycle. Next, the X-ray generation cycle calculation unit 41 calculates the X-ray stop time according to the above equation (1) based on the set value of the X-ray generation cycle and the X-ray duration. Then, the scan control unit 43 controls the high voltage generation unit 25 so that X-rays are not generated from the X-ray tube 13 for the calculated X-ray stop time, and sets the X-ray generation cycle after the X-ray stop time has elapsed. The high voltage generator 25 is controlled so that X-rays are generated from the X-ray tube 13 according to the value.

  This is the end of the description of the operation example of the X-ray computed tomography apparatus 1 according to the present embodiment.

  As described above, the X-ray computed tomography apparatus 1 includes the CT value change rate calculation unit 39, the X-ray generation cycle calculation unit 41, and the scan control unit 43. The CT value change rate calculation unit 39 repeatedly calculates the time change rate of the CT value on the repeatedly generated CT image every time the CT image is generated. The X-ray generation cycle calculation unit 41 repeatedly calculates the X-ray generation cycle according to the time change rate. The scan control unit 43 controls the high voltage generation unit 25 in real time so that X-rays are generated from the X-ray tube 13 according to the X-ray generation cycle.

  With the above configuration, the X-ray generation cycle can be changed in real time according to the CT value change that changes in real time during the contrast scan. Accordingly, the X-ray generation cycle is high in the period with a low CT value change rate where high time resolution is not required, so that the X-ray generation cycle is large in the period with a high CT value change rate in which high time resolution is required. It can be changed in real time during the contrast scan to be smaller. Therefore, according to the present embodiment, an optimal X-ray generation period corresponding to a change in contrast agent concentration in real time during a contrast scan, such as a dynamic scan, a helical scan, a helical shuttle scan, etc. X-rays can be generated. Further, a contrast scan can be executed with an optimal dose for each subject or each scan region.

  Thus, the X-ray computed tomography apparatus according to the present embodiment can realize a reduction in exposure dose.

[Modification 1]
In the above embodiment, the number of regions of interest is one. However, this embodiment is not limited to this. The X-ray computed tomography apparatus 1 according to Modification 1 sets two or more regions of interest. For example, when a plurality of parts having different blood flow dynamics are simultaneously CT-scanned, a region of interest may be set for each of the plurality of parts. In order to perform the following description specifically, it is assumed that the region of interest is set to an arterial region and a vein region by the region of interest setting unit 35. In the following description, components having substantially the same functions as those of the present embodiment are denoted by the same reference numerals, and redundant description will be given only when necessary.

  FIG. 8 is a diagram illustrating a typical example of the CT value change curves of the arterial region and the venous region. As shown in FIG. 8, the peak of the CT value change differs between the arterial region and the vein region. Specifically, the peak of the CT value change is earlier in the arterial region than in the venous region. Therefore, by setting a region of interest in each of the arterial region and the venous region, a contrast scan can be performed with an X-ray generation cycle suitable for both the arterial blood flow dynamics and the venous blood flow dynamics. The region of interest may be set by the region-of-interest setting unit 35 in accordance with an instruction from the user via the operation unit 47, or may be set by the region-of-interest setting unit 35 by image processing.

  Specifically, in step S6 of FIG. 5, the CT value change rate calculation unit 39 calculates the CT value change rate for each region of interest. In step S <b> 7, the X-ray generation cycle calculation unit 41 calculates an X-ray generation cycle for each CT value change rate. Then, the X-ray generation cycle calculation unit 41 calculates a single X-ray generation cycle suitable for both the arterial region and the vein region based on the two X-ray generation cycles. For example, the average value, minimum value, or intermediate value of the X-ray generation cycle for the arterial region and the X-ray generation cycle for the venous region is applied as a single X-ray generation cycle suitable for both the arterial region and the venous region. Is done. The scan control unit 43 controls the high voltage generation unit 25 so that X-rays are generated from the X-ray tube 13 according to the calculated single X-ray generation cycle.

  Thus, the X-ray computed tomography apparatus 1 according to the first modification can achieve a reduction in exposure dose.

[Modification 2]
In the present embodiment, the X-ray generation cycle is calculated every 1 CT scan during the main scan. However, this embodiment is not limited to this. The X-ray computed tomography apparatus according to Modification 2 performs a continuous scan without calculating the X-ray generation period in a predetermined period. Hereinafter, an X-ray computed tomography apparatus according to Modification 2 will be described. In the following description, components having substantially the same functions as those of the present embodiment are denoted by the same reference numerals, and redundant description will be given only when necessary.

  In the second modification, the system control unit 51 performs a continuous scan from the start of the main scan. Then, the system control unit 51 measures the elapsed time from the start of the main scan, and repeatedly determines whether or not the elapsed time has elapsed for a predetermined threshold time. Then, the system control unit 51 changes the X-ray generation cycle according to the CT value change rate when the elapsed time reaches the threshold time. That is, before the elapsed time reaches the threshold time, the scan control unit 43 performs continuous scanning. In continuous scanning, the X-ray generation period is set to the X-ray duration. That is, the X-ray stop time is set to zero. After the elapsed time reaches the threshold time, the CT value change rate calculation unit calculates the CT value change rate, the X-ray generation cycle calculation unit 41 calculates the X-ray generation cycle, and scans, as in the present embodiment. The controller 43 performs CT scan according to the X-ray generation cycle.

  For example, the threshold time may be set to a value empirically determined so that continuous scanning is performed in a time zone in which the flow of contrast medium is fast. The threshold time can be set to an arbitrary value by the user via the operation unit 47.

  Thus, the X-ray computed tomography apparatus 1 according to Modification 2 can achieve a reduction in exposure dose.

[Modification 3]
In the present embodiment, the main scan is terminated when the CT value in the region of interest is smaller than the third threshold value. However, this embodiment is not limited to this. In Modification 3, when the CT value change rate in the region of interest reaches the fourth threshold value, the main scan ends. Hereinafter, an X-ray computed tomography apparatus 1 according to Modification 3 will be described. In the following description, components having substantially the same functions as those of the present embodiment are denoted by the same reference numerals, and redundant description will be given only when necessary.

  As described above, when the CT value change rate is small, high time resolution is not required. Therefore, when the CT value change rate is small, it may be desired to stop the main scan. In accordance with such clinical needs, the monitoring unit 37 according to the modification 3 monitors the CT value change rate calculated by the CT value change rate calculating unit 39 during the contrast scan. Then, the monitoring unit 37 determines whether the CT value change rate has reached the fourth threshold value for the end of the main scan. The fourth threshold is set to an arbitrary value by the user via the operation unit 47. The fourth threshold is set near 0, for example. When the CT value change rate does not reach the fourth threshold value, the scan control unit 43 continues the main scan, and when the CT value change rate reaches the fourth threshold value, the scan control unit 43 ends the main scan. To do.

  As shown in FIG. 4, the CT value change rate instantaneously becomes zero at the peak of the CT value. However, it is not desirable for the main scan to end at the peak of the CT value. Therefore, the monitoring unit 37 may start monitoring the CT value change rate after the CT value has turned to the peak, without monitoring the CT value change rate before the CT value has turned to the peak. As a result, the X-ray computed tomography apparatus 1 according to the modified example 3 prevents the main scan from ending at the moment when the CT value reaches the peak, and at the timing when the contrast agent sufficiently flows out from the scan site. The scan can be terminated.

  Thus, the X-ray computed tomography apparatus 1 according to Modification 3 can achieve a reduction in exposure dose.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... X-ray computed tomography apparatus, 10 ... Gantry, 11 ... Rotating frame, 13 ... X-ray tube, 15 ... X-ray detector, 17 ... Rotation drive part, 19 ... Top plate support mechanism, 21 ... Top plate, 23 DESCRIPTION OF SYMBOLS ... Top plate moving part, 25 ... High voltage generation part, 27 ... Data collection circuit, 30 ... Console, 31 ... Pre-processing part, 33 ... Reconstruction part, 35 ... Region of interest setting part, 37 ... Monitoring part, 39 ... CT Value change rate calculation unit, 41 ... X-ray generation cycle calculation unit, 43 ... scan control unit, 45 ... display unit, 47 ... operation unit, 49 ... storage unit, 51 ... system control unit

Claims (12)

An X-ray tube that generates X-rays;
A high voltage generator for generating a high voltage applied to the X-ray tube to generate X-rays from the X-ray tube;
An X-ray detector for detecting X-rays generated from the X-ray tube and transmitted through the subject;
A support mechanism for rotatably supporting the X-ray tube around a rotation axis;
A generating unit that occurs in the first CT image and the second CT image based on an output from the X-ray detector,
A time change rate calculating unit for calculating a time change rate of a CT value between the first CT image and the second CT image ;
A generation period determination unit which determines the generation period of the X-rays from the X-ray tube according to the time rate of change,
An X-ray computed tomography apparatus comprising: The second CT image is a CT image generated before the first CT image;
The X-ray computed tomography apparatus according to claim 1, wherein the time change rate is a ratio of a CT value change with respect to a time difference between the first CT image and the second CT image.   The X-ray computed tomography apparatus according to claim 2, wherein the generation period is inversely proportional to the time change rate. The time change rate calculation unit is set according to an instruction from a user or by image processing,
The X-ray computed tomography apparatus according to claim 1, wherein the time change rate is calculated only for a single region of interest on the CT image. The time change rate calculation unit is set according to an instruction from a user or by image processing,
Calculating a plurality of time change rates for a plurality of regions of interest on the CT image,
The generation period determining unit determines the generation period according to the plurality of time change rates;
The X-ray computed tomography apparatus according to claim 1. An operation unit for inputting an instruction for changing or setting the generation cycle to a user-desired numerical value,
The X-ray computed tomography apparatus according to claim 1, further comprising:   The X-ray computed tomography apparatus according to claim 1, further comprising a control unit that controls the high-voltage generation unit in real time so that X-rays are generated from the X-ray tube according to the generation period.   When the difference between the latest generation cycle determined by the generation cycle determination unit and a set value is smaller than a threshold value, the control unit is configured to generate the X-ray from the X-ray tube according to the set value. The voltage generator is controlled, and when the difference is larger than the threshold, the high voltage generator is controlled so that X-rays are generated from the X-ray tube according to the latest generation cycle. X-ray computed tomography apparatus.   The X-ray computed tomography apparatus according to claim 8, further comprising an operation unit that inputs an instruction for changing or setting the threshold value to a user-desired numerical value. The generation cycle determination unit calculates an X-ray stop time based on the generation cycle and the X-ray duration time,
After the calculation of the X-ray stop time, the control unit controls the high voltage generation unit so as not to generate X-rays from the X-ray tube for the calculated X-ray stop time, and the X-ray stop time Controlling the high voltage generation unit so that X-rays are generated from the X-ray tube according to the generation cycle, triggered by having passed.
The X-ray computed tomography apparatus according to claim 7. The time change rate calculating unit, said first CT image or the second CT images to be deployed calculated the time rate of change for each generated, X-rays computed tomography apparatus according to claim 1. The generation period determination unit is configured to de San the generation cycle in response to the time rate of change, X-rays computed tomography apparatus according to claim 1.

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