JP2009022367A - X-ray ct apparatus and control method of x-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus capable of eliminating the influence of a scattered ray by a simple method without lowering the S/N of images. <P>SOLUTION: A scan control part 60 divides one view period into two periods, changes the tube current values of X-ray tubes 12 and 22 for each period, makes the tube current values of the X-ray tubes 12 and 22 be respectively different values in the same period and makes the X-ray tubes 12 and 22 emit X-rays. An X-ray dose calculation part 52 obtains the X-ray dose of penetrated rays emitted from the X-ray tube 12 to be a pair, penetrated through a subject and detected by an X-ray detector on the basis of the X-ray dose actually detected by the X-ray detector 13 during the one view period, and similarly obtains the X-ray dose of the penetrated rays emitted from the X-ray tube 22 to be a pair, penetrated through the subject and detected by the X-ray detector 23 on the basis of the X-ray dose actually detected by the X-ray detector 23. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a multi-tube type X-ray CT apparatus including a plurality of X-ray tubes and X-ray detectors corresponding to the plurality of X-ray tubes, and in particular, the influence of X-rays scattered by a subject. The present invention relates to an X-ray CT apparatus intended to remove the above.

  The X-ray CT apparatus rotates an X-ray tube and an X-ray detector, which are opposed to each other with the subject interposed therebetween, around the subject, and transmits the X-ray subject irradiated from the X-ray tube. The detected amount (transmission X-ray dose) is detected by an X-ray detector, the detected data is collected as projection data by a data collection device, and the collected projection data is reconstructed by using a computer. A tomographic image of a subject is obtained. For example, the projection data is collected by rotating the X-ray tube and the X-ray detector by 360 ° or (180 ° + fan angle), and the projection data for 360 ° or (180 ° + fan angle) is collected. A tomographic image is obtained based on this.

  In an X-ray CT apparatus called a third generation, an X-ray tube and an X-ray detector are arranged opposite each other with a subject interposed therebetween, and these are arranged around the subject at 360 ° or (180 °). X-ray detector in which a fan-shaped X-ray beam collimated from an X-ray tube is radiated onto the subject while rotating over the + fan angle, and a plurality of detection elements are arranged in an arc shape. X-rays transmitted through are detected.

  A set of detection data detected by an X-ray detector having a plurality of detection elements at a certain angle with respect to the subject is referred to as a view, and the X-ray tube and the X-ray detector are placed around the subject. Collecting multiple views of projection data necessary to rotate and reconstruct an image is called a scan (scsn), and the collection of view data repeated during the scan is called a measurement cycle. A scan in which projection data is collected by rotating the X-ray tube and the X-ray detector (180 ° + fan angle) is referred to as a half scan. Note that scanning is performed for each tomographic plane (slice) to be imaged. A tomographic image of the subject can be obtained by reconstructing the projection data of a plurality of views obtained by one scan using a high-speed arithmetic device or the like.

  For example, when collecting projection data of one view every 1 °, projection data of 360 views (that is, 360 measurement cycles) is obtained in one scan, and an image is re-used using the projection data of 360 views. Will be composed. The resolution of the image obtained by this reconstruction can be increased as the number of views obtained by rotating the X-ray tube and the X-ray detection unit once around the subject increases.

  For example, when 900 views of projection data are collected per scan (1 rotation), 1 view of projection data is collected every 0.4 °, and this is sequentially collected over 360 °. In order to further improve the image quality, there has been proposed one that collects projection data of 1200 views or 1800 views per scan. When collecting projection data of 1200 views per scan, projection data of 1 view is collected every 0.3 °, and when projection data of 1800 views is collected, projection of 1 view is performed every 0.2 °. Data will be collected.

  By the way, in order to make a detailed diagnosis of a circulatory organ such as the heart, there is a demand for reducing the time required for one scan (scan time). However, accelerating the rotation cycle adds an excessive centrifugal force to the rotating parts such as the X-ray tube and the X-ray detector. Therefore, meeting the demand is technically problematic and not easy.

  Therefore, a multi-tube X-ray CT apparatus using a plurality of X-ray tubes and X-ray detectors has been proposed in order to collect projection data of a desired number of views in a short time without increasing the rotation period. ing.

  Here, the configuration of the multi-tube type X-ray CT apparatus will be described with reference to FIG. FIG. 8 is a front view of a rotating mount for explaining the arrangement of the X-ray tube and the X-ray detector. Here, a multitubular X-ray CT apparatus including two X-ray tubes will be described as an example of a multitubular X-ray CT apparatus.

  As shown in FIG. 8, the two X-ray tubes 12 and 22 are arranged at positions shifted by 90 ° around the rotation axis. Here, an example in which the two X-ray tubes 12 and 22 are arranged at positions shifted by 90 ° will be described. However, this example of the angle is an example, and an arrangement other than 90 ° (for example, 120 °) is performed. There are some cases.

  An X-ray aperture (not shown) for defining the X-ray fan angle is provided on the X-ray tube 12 and 22 side of the X-ray tube, respectively, and the X-ray is collimated in a fan shape with a predetermined thickness. A path 6 is formed.

  Two X-ray detectors 13 and 23 are installed so as to correspond to the two X-ray tubes 12 and 22, respectively. The X-ray detectors 13 and 23 may be configured by arranging, for example, 1000-channel detection elements in a row, and a plurality of detection elements are arrayed in each of two orthogonal directions (slice direction and channel direction). They may be arranged to form a two-dimensional X-ray detector.

  The X-ray detectors 13 and 23 are provided with data collection units (DAS) 14 and 24, respectively. The data collection units 14 and 24 have data collection elements arranged in an array like the detection elements of the X-ray detectors 13 and 23, and X-rays (detection) detected by the X-ray detectors 13 and 23. Signal) is collected in correspondence with a data collection control signal output from a scan control unit (not shown). This collected data becomes projection data. The data collection units 14 and 24 convert the current signals of the respective channels of the corresponding X-ray detectors 13 and 23 into voltages, amplify them, and convert them into digital signals.

  Then, the X-ray irradiated from the X-ray tube 12 and transmitted through the subject P is detected by the X-ray detector 13, and the detection signal is amplified by the data collecting unit 14, converted into a digital signal, and collected as projection data. Is done. Similarly, X-rays emitted from other X-ray tubes 22 are collected as projection data by the data collection device 24. The X-ray tubes 12 and 22, the X-ray diaphragm (not shown), the X-ray detectors 13 and 23, and the data collection units 14 and 24 are integrally fixed to the rotating mount 5, and the rotating mount 5 is It is supported by the fixed base 7 so as to rotate about the rotation center O, and is rotated by a rotation function (not shown) (for example, Patent Document 1 and Patent Document 2).

  The X-ray CT apparatus configured as described above collects projection data while rotating the rotating mount 5 under the control of a scan control unit (not shown). That is, when the X-ray tube 22 is rotated from the 0 ° position to the 90 ° position and projection data is collected, the X-ray tube 12 is rotated from the 270 ° position to the 360 ° (0 °) position. During this time, projection data is collected. Therefore, if the rotary mount 5 is rotated by 90 °, projection data for 180 ° can be collected, and the scan time can be shortened without increasing the rotation period of the rotary mount.

  By the way, since the X-ray CT apparatus provided with a plurality of X-ray tubes generates X-rays continuously from the plurality of X-ray tubes, the scattered rays influence each other, and the image quality is low. There is a problem that decreases. Here, the influence of scattered radiation will be described with reference to FIGS. FIG. 9 and FIG. 10 are front views of a rotating mount for explaining scattered X-ray rays.

  For example, a part of X-rays irradiated from the X-ray tube 12 to the subject P is absorbed by the subject P, and X-rays that are not absorbed and pass through the subject P are along the irradiation direction. The light enters a predetermined channel of the X-ray detector 13 and only a small part is scattered and scattered in all directions. A collimator (not shown) is installed on the front surface of the X-ray detector 13 so as to correspond to each channel so that the scattered radiation does not enter the other channels of the X-ray detector 13.

  However, in an X-ray CT apparatus in which a plurality of sets of X-ray tubes and X-ray detectors are installed, each X-ray detector generates scattered radiation based on X-rays emitted from non-paired X-ray tubes. There is a risk of detection.

  As shown in FIG. 9, the X-ray tubes 12 and 22 form an X-ray path 6 having a fan-shaped spread. In this case, the X-ray detector 13 detects X-rays emitted from the X-ray tube 12, and the X-ray detector 23 detects X-rays emitted from the X-ray tube 22. However, scattered radiation 6 a based on the X-rays irradiated from the X-ray tube 22 enters the X-ray detector 13. That is, the incident of the scattered radiation based on the X-rays irradiated from the X-ray tube 12 to the X-ray detector 13 can be eliminated by the action of a collimator (not shown) installed in the X-ray detector 13. Incidence of scattered rays based on the X-rays irradiated from the X-ray tube 22 to the X-ray detector 13 cannot be excluded. Similarly, scattered rays 6a based on the X-rays irradiated from the X-ray tube 12 enter the X-ray detector 23.

  The X-ray dose of scattered radiation detected by the X-ray detectors 13 and 23 will be described with reference to FIG. X-rays irradiated from the X-ray tube 12 pass through the subject P and reach the paired X-ray detector 13, but some X-rays are reflected by the subject P and become scattered rays 6a. The X-ray detector 23 is not paired with the X-ray tube 12. Looking at all channels (all detection elements), the X-ray dose of scattered radiation detected by the X-ray detector 23 is represented by a profile curve 202.

  Similarly, X-rays irradiated from the X-ray tube 22 pass through the subject P and reach the paired X-ray detector 23, but some X-rays are reflected by the subject P and scattered. The line 6a reaches the X-ray detector 13 which is not paired with the X-ray tube 22. Looking at all channels (all detection elements), the X-ray dose of scattered radiation detected by the X-ray detector 13 is represented by a profile curve 102.

  The X-ray dose of scattered radiation is added to the X-ray dose of X-rays (hereinafter sometimes referred to as “transmission rays”) that reaches the paired X-ray detector through the subject P from the X-ray tube. It will be. Therefore, when an image is generated by performing reconstruction processing based on the X-rays detected by each X-ray detector, the image quality of the reconstructed image is degraded due to the influence of scattered radiation.

  Therefore, an attempt has been made to eliminate the influence of scattered radiation caused by an unpaired X-ray tube, and to determine the X-ray dose of transmitted rays irradiated from the paired X-ray tube and transmitted through the subject P. Yes. For example, by capturing a known object, the scattered radiation component detected by each X-ray detector is obtained in advance, the scattered radiation component is used to remove the influence of the scattered radiation, and the transmitted X-ray dose is calculated. Has been proposed to seek.

  In addition, a method has been proposed in which each X-ray detector is controlled so as not to detect scattered radiation by shifting the timing for obtaining the X-ray amount of transmitted rays in each set of X-ray tube and X-ray detector ( For example, Patent Document 3).

  The prior art described in Patent Document 3 will be described with reference to FIG. FIG. 11 is a diagram illustrating the timing of X-ray irradiation and X-ray detection according to the related art. In the prior art, the time required to collect data for one view (1 view period in the figure) is divided into two periods, and only the X-ray tube 22 emits X-rays in the first half and X-rays in the second half. The timing of X-ray irradiation by the X-ray tube 12 and 22 is controlled so that only the tube 12 emits X-rays. That is, by switching ON / OFF of the X-ray irradiation by the X-ray tubes 12 and 22 between the first half and the second half of the 1 view period, the tube current of the X-ray tube 12 is set to 0 of a desired tube current value in the first half. [%], The tube current of the X-ray tube 22 is set to 100 [%] of the desired tube current value, and in the latter half, the tube current of the X-ray tube 12 is set to 100 [%] of the desired tube current value, X-ray The tube current of the tube 22 is set to 0 [%] of the desired tube current value.

  At this time, the X-ray detector 13 detects X-rays while the paired X-ray tube 12 is irradiating X-rays (the second half of the 1 view period), and the X-ray detector 23 is paired. X-rays are detected while the X-ray tube 22 is irradiating X-rays (the first half of the 1 view period). In addition, in FIG. 11, the detection timing by the X-ray detector 13 is shown. As a result, the X-rays detected by the X-ray detectors do not include the scattered radiation component caused by the X-rays from the X-ray tubes that are not paired, and the X-ray detectors are paired. It is possible to detect only the transmitted rays that are irradiated from the X-ray tube and pass through the subject.

JP 2002-172112 A JP 2004-121446 A JP 2004-121446 A

  However, when a substance having a shape, component, or internal structure different from that of a specimen (phantom) photographed in advance is photographed, the X-ray dose of the scattered radiation included in the X-ray detected by the photographing and the scattered radiation obtained in advance. There is a possibility that an error with the X-ray dose increases and the scattered radiation component cannot be removed appropriately.

  Further, in the method described in Patent Document 3, the change in the tube current value of the X-ray tubes 12 and 22 during 1 view period is large, and ON / OFF of X-ray irradiation by the X-ray tubes 12 and 22 is switched at high speed. This involves technical difficulties. For example, when one scan is performed in 0.3 seconds and 1000 views of projection data are acquired in one scan, the period (1 view period) for acquiring projection data of 1 view is 333 [μs]. Therefore, it is necessary to execute ON / OFF switching of X-ray irradiation by the X-ray tubes 12 and 22 in 167 [μs] which is a half time of 1 view period. In such a short time, it is necessary to switch the tube current of the X-ray tube 12 or the X-ray tube 22 from 100 [%] to 0 [%], or from 0 [%] to 100 [%]. With great difficulty.

  Furthermore, in the method described in Patent Document 3, each X-ray detector detects X-rays only in a half period of 1 view period, so the X-ray dose detected by each X-ray detector is extremely small. Therefore, there is a problem that the S / N of the projection data detected by each X-ray detector becomes very bad, and the S / N of the image obtained by reconstruction is deteriorated. Therefore, unless the tube current value is increased, the sensitivity of the X-ray detector is improved, or the sensitivity (gain) of the data acquisition unit (DAS) is not improved, the S / N of the image can be improved. It becomes difficult. For example, in order to improve the S / N of an image, it is necessary to double the tube current value or double the sensitivity of the data acquisition unit (DAS).

  The present invention solves the above-described problems. An X-ray CT apparatus and an X-ray CT apparatus capable of removing the influence of scattered radiation by a simple method without reducing the S / N of an image. An object is to provide a control method.

The invention described in claim 1 is an X-ray CT apparatus in which a plurality of pairs of an X-ray tube and an X-ray detector are arranged at different angles, and each pair is configured to be rotatable around a subject. One view period for acquiring projection data of one view is divided into a plurality of periods, the tube current values of the X-ray tubes constituting each pair are set to different values for each of the plurality of periods, and the same period The scan control means for irradiating X-rays from the X-ray tubes constituting the respective pairs by setting the tube current values of the X-ray tubes constituting the respective pairs to different values, and the respective pairs Based on the X-ray doses detected by the plurality of X-ray detectors in the plurality of periods, the transmission lines are transmitted from the pairs of X-ray tubes and pass through the subject. And an X-ray dose calculating means for calculating an X-ray dose of each pair of transmission lines, Is an X-ray CT apparatus characterized by comprising: a reconstruction processing means for reconstructing the image data, the based on the amount of X-rays transmitted radiation determined by.
The invention described in claim 7 includes a first detection system including a first X-ray tube and a first X-ray detector, a second X-ray tube and a second X-ray detector. In the X-ray CT apparatus comprising the second detection system, the first X-ray tube and the second X-ray tube simultaneously emit X-rays, and at least the first X-ray tube Scan control means for performing scanning while changing the tube current value of the X-ray tube, and collecting projection data corresponding to the first tube current value and projection data corresponding to the second tube current value; X-ray dose calculation means for obtaining projection data from which scattered radiation from other detection systems is removed based on projection data corresponding to a tube current value and projection data corresponding to the second tube current value; and the X-ray dose calculation Reconstructing means for reconstructing an image in the subject based on the projection data obtained by the means. It is an X-ray CT apparatus according to.
The invention according to claim 8 is a method of controlling an X-ray CT apparatus in which a plurality of pairs of X-ray tubes and X-ray detectors are arranged at different angles, and each pair is configured to be rotatable around a subject. Wherein one view period for acquiring projection data of one view is divided into a plurality of periods, and the tube current value of the X-ray tube constituting each pair is set to a different value for each of the plurality of periods, A scanning step of irradiating X-rays from the X-ray tubes constituting the pairs, with the tube current values of the X-ray tubes constituting the pairs being different from each other during the same period; Each of the X-ray detectors constituting the transmission line is transmitted from the pair of X-ray tubes and transmitted through the subject based on the X-ray dose detected in the plurality of periods, An X-ray dose calculating step for obtaining an X-ray dose of each pair of transmission lines during the period; A reconstruction processing step of reconstructing the image data based on the X-ray dose of the transmission line obtained by serial No. X dose calculating step, a method of controlling the X-ray CT apparatus characterized by including the.

  According to the present invention, it is possible to determine the X-ray dose of transmitted rays by removing the influence of scattered rays based on X-rays irradiated from an unpaired X-ray tube by a simple method. Further, according to the present invention, each X-ray detector can detect X-rays with a sufficient X-ray dose, so that it is possible to remove the influence of scattered radiation without reducing the S / N of the image. Become.

  A configuration of an X-ray CT apparatus according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing a schematic configuration of an X-ray CT apparatus according to an embodiment of the present invention.

  The X-ray CT apparatus according to this embodiment is characterized by the X-ray irradiation timing control of the X-ray tubes 12 and 22 by the scan control unit 60 and the removal of scattered radiation components by the X-ray dose calculation unit 52. Further, the X-ray CT apparatus according to this embodiment may be a single slice X-ray CT apparatus or a multi-slice X-ray CT apparatus. That is, an X-ray detector configured by arranging detection elements in a line may be used, or an X-ray detector in which a plurality of arrays are arranged in an array may be used.

  The X-ray CT apparatus according to this embodiment includes a gantry device 1, a console unit 2, and a bed device 3. The gantry device 1 includes a rotating gantry (gantry) that stores an X-ray tube and an X-ray detector, and collects projection data related to the subject. The projection data is output to the console unit 2 and used for processing such as image reconstruction processing. The couch device 3 includes a couch 31 for placing a subject.

  The gantry device 1 is provided with a plurality of X-ray tubes and a plurality of X-ray detectors paired with them. In the X-ray CT apparatus according to this embodiment, two X-ray tubes 12 and 22 and two X-ray detectors 13 and 23 paired therewith are installed. For example, as shown in FIG. 8, the two X-ray tubes 12 and 22 are arranged at positions shifted by 90 ° around the rotation axis, and two X-ray detectors 13 and 23 that are paired with them are installed. Yes. Thereby, the X-ray tube 12 and the X-ray detector 13 are paired, and the X-ray tube 22 and the X-ray detector 23 are paired. In this embodiment, the X-ray tubes 12 and 22 are arranged so as to be shifted by 90 ° from each other. For example, the X-ray tubes 12 and 22 may be arranged at positions shifted from each other by 120 °. Furthermore, not only two sets of X-ray tubes and X-ray detectors but also three or more sets of X-ray tubes and X-ray detectors may be installed.

  X-ray diaphragms (not shown) for defining the X-ray fan angle are provided on the X-ray tube 12 and 22 side of the X-ray irradiation opening. For example, the X-ray detectors 13 and 23 may be configured by arranging 1000-channel detection elements in a row, and a plurality of detection elements are arranged in two orthogonal directions to form a two-dimensional X-ray detection. A vessel may be configured. The high voltage generators 11 and 21 supply a high voltage for irradiating X-rays to the X-ray tubes 12 and 22 in accordance with a control signal from the scan controller 60.

  The X-ray detectors 13 and 23 are provided with data collection units (DAS) 14 and 24, respectively. The data collection units 14 and 24 have data collection elements arranged in an array similar to the detection elements of the X-ray detectors 13 and 23, and the X-rays detected by the X-ray detectors 13 and 23 are Data is collected according to the data collection control signal output from the scan control unit 60. This collected data becomes projection data. The data collection units 14 and 24 convert the current signals of the respective channels of the corresponding X-ray detectors 13 and 23 into voltages, amplify them, and convert them into digital signals.

  Then, the X-ray irradiated from the X-ray tube 12 and transmitted through the subject P is detected by the X-ray detector 13, and the detection signal is amplified by the data collecting unit 14, converted into a digital signal, and collected as projection data. Is done. Similarly, X-rays emitted from other X-ray tubes 22 are collected as projection data by the data collection device 24. The X-ray tubes 12 and 22, the X-ray diaphragm (not shown), the X-ray detectors 13 and 23, and the data collection units 14 and 24 are integrally fixed to the rotating mount 5, and the rotating mount 5 is It is supported on the fixed base 7 so as to rotate around the rotation center O.

  The gantry driving unit 4 rotates the rotating gantry 5 based on the gantry control signal output from the scan control unit 60. Thereby, the rotary mount 5 is rotated around the rotation center O.

  The scan control unit 60 supplies a rotation control signal to the gantry driving unit 4 in order to stably rotate the gantry 5 at a constant speed during scanning. The scan control unit 60 outputs an X-ray generation control signal for controlling the X-ray generation to the high voltage generation units 11 and 21, and sends a detection control signal indicating the X-ray detection timing to the data collection devices 14 and 24. Output.

  Then, under the control of the scan control unit 60, projection data is collected while rotating the rotating gantry 5. That is, when the X-ray tube 22 is rotated from the 0 ° position to the 90 ° position and projection data is collected, the X-ray tube 12 is rotated from the 270 ° position to the 360 ° (0 °) position. During this time, projection data is collected. Therefore, if the rotating mount 5 is rotated by 90 °, projection data for 180 ° can be collected.

  The X-ray CT apparatus according to this embodiment is characterized in that the X-ray dose irradiated by the X-ray tubes 12 and 22 is controlled by the scan control unit 60. In this embodiment, the scan control unit 60 divides the period for acquiring projection data of one view into a plurality of periods, and sets the tube current values of the X-ray tubes 12 and 22 to different values for each of the plurality of periods. Furthermore, during the same period, the tube current values of the X-ray tubes 12 and 22 are made different from each other, and X-rays are irradiated from the X-ray tubes 12 and 22.

  Here, control of the X-ray dose will be described with reference to FIG. FIG. 2 is a diagram showing tube current values of the X-ray tube according to the embodiment of the present invention. The scan control unit 60 divides the time required to collect one view of data (1 view period in FIG. 2) into two periods having the same time interval, and the tube currents of the X-ray tube 12 and 22 in each period. The value (X-ray dose irradiated by the X-ray tubes 12 and 22) is controlled. That is, the scan control unit 60 controls the timing for switching the tube current values of the X-ray tubes 12 and 22 and the tube current values of the X-ray tubes 12 and 22 in each period.

  For example, the scan control unit 60 periodically changes the tube current value of the X-ray tube 12 between the first half and the second half of the 1 view period. Similarly, the scan control unit 60 changes the tube current value of the X-ray tube 22 between the first half and the second half of the 1 view period. The tube current value is changed periodically. Furthermore, during the same period, the tube current values of the X-ray tubes 12 and 22 are set to different values.

  Here, an example of control of the X-ray tubes 12 and 22 by the scan control unit 60 will be described. For example, the scan control unit 60 changes the peak value of the tube current of the X-ray tubes 12 and 22 to a rectangular shape. For example, the scan control unit 60 irradiates the X-ray with the tube current value of the X-ray tube 12 relatively lower than the tube current value of the X-ray tube 22 in the first half of the 1 view period, and the X-ray in the second half. The tube current value of the tube 12 is made relatively higher than the tube current value of the X-ray tube 22 to irradiate X-rays.

  In the example shown in FIG. 2, in the first half of the 1 view period, the tube current value of the X-ray tube 12 is set to 80 [%] of the desired tube current value, and the tube current value of the X-ray tube 22 is set to the desired tube current value. In the latter half, the tube current values of the X-ray tubes 12 and 22 are switched, and the tube current value of the X-ray tube 12 is set to 100 [%] of the desired tube current value. The tube current value is set to 80% of the desired tube current value. As described above, the scan control unit 60 periodically changes the tube current values of the X-ray tubes 12 and 22 in the first half and the second half of the 1 view period, and further, in the same period, the X-ray tubes 12 and 22. The tube current values are different from each other.

  The scan control unit 60 includes the timing for switching the tube current values of the X-ray tubes 12 and 22 and the tube current values of the X-ray tubes 12 and 22 in each period in the X-ray generation control signal, and generates the X-rays generated. The control signal is output to the high voltage generators 11 and 21. Accordingly, the scan control unit 60 controls the timing of switching the tube current values of the X-ray tubes 12 and 22 and the tube current value (X dose) of the X-ray tubes 12 and 22 in each period.

  The timing for switching the tube current values of the X-ray tubes 12 and 22 and the tube current values of the X-ray tubes 12 and 22 are determined in advance as scan conditions and stored in the scan condition storage unit 61. The timing for switching the tube current value and the tube current value can be arbitrarily changed by the operator.

  For example, when one scan is performed in 0.3 seconds and 1000 views of projection data are acquired in one scan, the period (1 view period) for acquiring projection data of 1 view is 333 [μs]. Therefore, switching of the tube current values of the X-ray tubes 12 and 22 is executed at 167 [μs], which is half the time of 1 view period.

  As described above, by controlling the timing of X-ray irradiation by the X-ray tubes 12 and 22, each X-ray detector is an X-ray irradiated by a pair of X-ray tubes, In addition to detecting transmitted X-rays (transmitted rays), X-rays irradiated by a non-paired X-ray tube and reflected by the subject are detected.

  For example, the X-ray detector 13 detects a transmission ray caused by X-rays irradiated by the paired X-ray tube 12 and further scatters caused by X-rays irradiated by the non-paired X-ray tube 22. Detect lines. Similarly, the X-ray detector 23 detects a transmission line caused by the X-rays irradiated by the paired X-ray tube 22, and is further caused by the X-rays irradiated by the non-paired X-ray tube 12. Detect scattered radiation.

  Here, an example of the X-ray dose actually detected by the X-ray detectors 13 and 23 is shown in FIG. FIG. 3 is a diagram showing an example of the X-ray dose detected by the X-ray detector. For example, attention is paid to the X-ray detector 13. A profile curve 100 in FIG. 3 represents the X-ray dose actually detected by the X-ray detector 13. Further, the profile curve 101 represents the X-ray dose of transmitted rays caused by the paired X-ray tube 12, and the profile curve 102 represents the X-ray dose of scattered rays caused by the non-paired X-ray tube 22. Yes. Profile curves 100, 101, and 102 represent X-ray doses of all channels (all detection elements) of the X-ray detector 13. That is, when viewed in all channels (all detection elements), the X-ray dose actually detected by the X-ray detector 13 is represented by the profile curve 100, the X-ray dose of the transmitted ray is represented by the profile curve 101, and the scattered radiation The X-ray dose is represented by the profile curve 102. The X-ray detector 13 is irradiated from the paired X-ray tube 12 and transmitted through the subject P, and is irradiated from the unpaired X-ray tube 22 and reflected by the subject P. Detected X-rays (scattered rays) are detected. Therefore, the X-ray dose (profile curve 100) actually detected by the X-ray detector 13 is obtained by adding the X-ray dose of transmitted rays (profile curve 101) and the X-ray dose of scattered rays (profile curve 102). become.

  Similarly, paying attention to the X-ray detector 23, the profile curve 200 in FIG. 3 represents the X-ray dose that the X-ray detector 23 actually detects. Further, the profile curve 201 represents the X-ray dose of the transmitted radiation caused by the paired X-ray tube 22, and the profile curve 202 represents the X-ray dose of the scattered radiation caused by the non-paired X-ray tube 12. Yes. Profile curves 200, 201, and 202 represent X-ray doses of all channels (all detection elements) of the X-ray detector 23. That is, when viewed in all channels (all detection elements), the X-ray dose actually detected by the X-ray detector 23 is represented by the profile curve 200, the X-ray dose of the transmitted ray is represented by the profile curve 201, and the scattered radiation The X-ray dose is represented by a profile curve 202. The X-ray detector 23 is irradiated from the paired X-ray tube 13 and transmitted through the subject P, and is irradiated from the unpaired X-ray tube 22 and reflected by the subject P. X-rays (scattered rays) are detected. Therefore, the X-ray dose (profile curve 200) actually detected by the X-ray detector 23 is obtained by adding the X-ray dose of transmitted rays (profile curve 201) and the X-ray dose of scattered rays (profile curve 202). become.

  As described above, the X-ray doses detected by the X-ray detectors 13 and 23 include the X-ray dose of transmitted rays and the X-ray dose of scattered rays. In the X-ray CT apparatus according to this embodiment, the preprocessing unit 51 removes the influence of scattered radiation and obtains the X-ray dose of transmitted radiation.

  The preprocessing unit 51 performs sensitivity correction, X-ray intensity correction, and the like on the data detected by the data collection units 14 and 24. The preprocessing unit 51 includes an X-ray dose calculation unit 52 and a calculation condition storage unit 53.

  The X-ray dose calculation unit 52 deletes the X-ray dose of the scattered radiation from the X-ray dose actually detected by the X-ray detector 13 based on the data (X-ray dose) actually detected by the X-ray detector 13 during the 1 view period. Then, the projection data of one view, that is, the X-ray dose of the transmission line in one view is obtained. Similarly, the X-ray dose calculation unit 52 determines the X-rays of the scattered radiation from the X-ray dose actually detected by the X-ray detector 23 based on the data (X-ray dose) actually detected by the X-ray detector 23 during the 1 view period. The dose is deleted and the X-ray dose of the transmission line in one view is obtained. As described above, the X-ray dose calculation unit 52 obtains the X-ray dose of the transmission line detected by the X-ray detectors 13 and 23 based on the data (X-ray dose) detected by the X-ray detectors 13 and 23, respectively. .

  Specifically, the X-ray dose calculation unit 52 is configured to irradiate X-rays with the tube current value of the X-ray tube 12 relatively lower than the tube current value of the X-ray tube 22 (the first half of the 1 view period). ), The X-ray dose (corresponding to the first X-ray dose) detected by the paired X-ray detector 13 and the tube current value of the X-ray tube 12 are made relatively higher than the tube current value of the X-ray tube 22. Based on the difference from the X-ray dose (corresponding to the second X-ray dose) detected by the paired X-ray detector 13 during the X-ray irradiation period (the second half of the 1 view period), the X-ray detector 13 is 1 Obtain the X-ray dose of transmitted rays detected during the view period. Similarly to the X-ray detector 13, the X-ray amount of transmission rays detected by the X-ray detector 23 during one view period is also obtained. Further, when calculating the difference, the X-ray dose calculation unit 52 obtains a difference corresponding to each tube current value of the first X-ray dose and the second X-ray dose, and based on the difference, the X-ray detector during one view period X-ray doses of transmitted rays detected by 13 and 23 are obtained.

  Next, specific processing contents by the X-ray dose calculation unit 52 will be described with reference to FIG. FIG. 4 is a conceptual diagram showing a method for calculating the X-ray dose in this embodiment. For example, attention is paid to the X-ray detector 13. The X-ray detector 13 detects X-rays when X-rays are irradiated only from the X-ray tube 12 by setting the tube current value of the paired X-ray tube 12 to 100% of the desired tube current value. The dose is X dose a. In other words, the X-ray dose a represents the X-ray dose of the transmitted rays caused by the X-rays irradiated from the X-ray tube 12 paired with the X-ray detector 13. Further, when the tube current value of the X-ray tube 22 is set to 100% of the desired tube current value and X-rays are irradiated only from the X-ray tube 22, the X-ray dose detected by the X-ray detector 13 is detected. Is the X-ray dose b. That is, the X-ray dose b represents the X-ray dose of scattered radiation caused by X-rays emitted from the X-ray tube 22 that does not pair with the X-ray detector 13.

As described above, when the X-ray dose a and the X-ray dose b are defined, the first half of the 1 view period shown in FIG. 2, that is, the tube current value of the X-ray tube 12 is set to 80% of the desired tube current value. FIG. 4 shows the X-ray doses actually detected by the individual detection elements of the X-ray detector 13 when X-rays are irradiated with the tube current value of the tube 22 set to 100% of the desired tube current value. As shown, when the X-ray dose is c1, the X-ray dose c1 is expressed by the following equation (1).
Formula (1)
X dose c1 = X dose b (scattered X dose) + 0.8 × X dose a (transmitted X dose)

Further, in the latter half of the 1 view period shown in FIG. 2, that is, the tube current value of the X-ray tube 12 is set to 100% of the desired tube current value, and the tube current value of the X-ray tube 22 is set to the desired tube current value. Assuming that the X-ray dose actually detected by each detection element of the X-ray detector 13 is X dose c2 as shown in FIG. (2)
Formula (2)
X dose c2 = 0.8 × X dose b (scattered X dose) + X dose a (transmitted X dose)

According to the X-ray dose calculation concept shown in FIG. 4 using the above formulas (1) and (2), the X-ray dose a of transmitted rays that should be detected by the individual detection elements of the X-ray detector 13 is It is represented by Formula (3).
Formula (3)
X dose a = 5/9 × (5 × X dose c2-4 × X dose c1)

Since the X-ray dose a represents the X-ray dose of the transmitted rays detected by the individual detection elements of the X-ray detector 13 in half the time of the 1-view period, the individual detection of the X-ray detector 13 during the 1-view period. Assuming that the X-ray dose of transmitted rays detected by the element is X-ray dose A, the X-ray dose A is expressed by the following equation (4).
Formula (4)
X dose A = 2 × X dose a

Therefore, when the X-ray dose A is expressed using X-ray doses c1 and c2 that are actually detected by the individual detection elements of the X-ray detector 13, the following equation (5) is obtained from equations (3) and (4). expressed.
Formula (5)
X dose A = 10/9 × (5 × X dose c2-4 × X dose c1)

  The X-ray dose c1 in the equation (5) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 13 in the first half of the 1 view period, and the X-ray dose c2 is the X-ray in the second half of the 1 view period. The X-ray dose actually detected by each detection element of the detector 13 is represented. Therefore, according to the equation (5), it is possible to obtain the X-ray dose of the transmitted radiation that does not include the X-ray dose of the scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X-ray detector 13. Become. The X-ray dose c1 corresponds to the “first X-ray dose” of the present invention, and the X-ray dose c2 corresponds to the “second X-ray dose” of the present invention.

Further, the X-ray dose of transmission rays that should be detected by the X-ray detector 23 in the 1 view period can be obtained by the same method as the calculation method of the X-ray dose A of transmission rays that should be detected by the X-ray detector 13. For example, in the first half of the 1view period shown in FIG. 2, the X-ray dose actually detected by each detection element of the X-ray detector 23 is set as the X-ray dose d1, and the individual detection of the X-ray detector 23 is performed in the second half of the 1view period. Assuming that the X-ray dose actually detected by the element is X-ray dose d2, if the X-ray dose B is the X-ray dose B of the transmitted rays detected by the individual detection elements of the X-ray detector 23 during the 1 view period, the X-ray dose B is (6)
Formula (6)
X dose B = 10/9 × (5 × X dose d1-4 × X dose d2)

  The X-ray dose d1 in the equation (6) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 23 in the first half of the 1view period, and the X-ray dose d2 is the X-ray in the second half of the 1view period. The X-ray dose actually detected by each detection element of the detector 23 is shown. Therefore, according to the equation (6), it is possible to obtain the X-ray dose of the transmitted radiation that does not include the X-ray dose of the scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X-ray detector 23. Become. The X-ray dose d1 corresponds to the “first X-ray dose” of the present invention, and the X-ray dose d2 corresponds to the “second X-ray dose” of the present invention.

  Expression (5) is stored in the calculation condition storage unit 53 as a condition for calculating the X-ray dose of the transmission line to be detected by the X-ray detector 13, and the expression (6) is detected by the X-ray detector 23. It is stored as a condition for calculating the X-ray dose of the transmitted ray to be transmitted.

  When the X-ray dose calculation unit 52 receives the data (X-ray dose) actually detected by the X-ray detectors 13 and 23, the X-ray dose calculation unit 52 performs X in one view period according to the above formula (5) stored in the calculation condition storage unit 53. The X-ray dose A of the transmission line that should be detected by the individual detection elements of the line detector 13 is calculated, and further, the individual detection elements of the X-ray detector 23 detect during one view period according to the above equation (6). Calculate the X-ray dose B of the supposed transmitted ray. Then, the X-ray dose calculation unit 52 calculates X-ray doses A and B of transmission rays that should be detected by the individual detection elements of the X-ray detectors 13 and 23 during each view period.

  Note that the X-ray dose calculation unit 52 is constituted by a CPU, for example. Then, the X-ray dose calculation program is stored in a storage unit (not shown), and the CPU reads the X-dose calculation program from the storage unit and executes it, thereby executing the function of the X-ray dose calculation unit 52 and during each view period. Thus, the X-ray doses A and B of the transmitted rays that should be detected by the X-ray detectors 13 and 23 are obtained.

  The projection data that has been subjected to processing such as sensitivity correction by the preprocessing unit 51 is output to the reconstruction processing unit 54.

  The reconstruction processing unit 54 reconstructs image data by performing back projection processing on the projection data that has been subjected to the correction processing by the preprocessing unit 51. Thereby, tomographic image data of the subject is generated. The image data output from the reconstruction processing unit 54 is temporarily held in the image storage unit 55. The projection data output from the preprocessing unit 51 to the reconstruction processing unit 54 has image data (tomographic image data) in which the influence of scattered radiation is reduced because the X-ray dose of scattered radiation is removed.

  The image processing unit 56 performs various image processing on the image data in accordance with an operator's instruction input with an input device (not shown). The image processing unit 56 performs, for example, volume rendering processing, MPR processing, and the like to generate three-dimensional image data and MPR image data (arbitrary slice image data), and outputs the generated data to the display control unit 57. The display control unit 57 displays an image based on the image data output from the image processing unit 56 on a display unit 58 such as a liquid crystal display or a CRT.

  The couch 31 includes a couch top for placing a subject and a couch base that supports the couch top. The couch top can be moved by the couch driving unit 32 in the body axis direction (slice direction) of the subject. The bed base can move the bed top plate in the vertical direction by the bed driving unit 32.

(Operation)
Next, a series of operations (control method of the X-ray CT apparatus) by the X-ray CT apparatus according to the embodiment of the present invention will be described with reference to FIG. FIG. 5 is a flowchart showing a series of operations by the X-ray CT apparatus according to the embodiment of the present invention.

(Step S01)
First, the scan control unit 60 performs scanning while periodically changing the tube current values of the X-ray tubes 12 and 22 every predetermined time according to the scan conditions stored in the scan condition storage unit 61. For example, according to the switching timing of the tube current value shown in FIG. 2, the scan control unit 60 performs scanning by controlling the tube current values of the X-ray tubes 12 and 22.

(Step S02)
When the X-ray dose calculation unit 52 receives data (X-ray dose) actually detected by the X-ray detectors 13 and 23 from the data collection units 14 and 24, the X-ray dose calculation unit 52 follows the calculation conditions stored in the calculation condition storage unit 53. Find projection data for each view. For example, the X-ray dose calculation unit 52 obtains the X-ray dose of transmission rays to be detected by the individual detection elements of the X-ray detector 13 according to the above equation (5), and further detects the X-ray according to the above equation (6). The X-ray dose of transmission rays to be detected by the individual detection elements of the vessel 23 is obtained. Then, for each view, the X-ray dose calculation unit 52 obtains the X-ray dose of transmission rays to be detected by the individual detection elements of the X-ray detectors 13 and 23, and obtains projection data of each view.

(Step S03)
The reconstruction processing unit 54 generates image data by performing a reconstruction process on the projection data processed by the preprocessing unit 51.

(Step S04)
The image data generated by the reconstruction processing unit 54 is subjected to predetermined image processing by the image processing unit 56, and an image such as a tomographic image is displayed on the display unit 58.

  As described above, according to the X-ray CT apparatus according to this embodiment, the individual detection elements of the X-ray detectors 13 and 23 are irradiated from unpaired X-ray tubes based on the X-ray dose actually detected. In addition, the influence of scattered rays based on the X-rays can be removed, and the X-ray dose of transmitted rays can be obtained. Therefore, the X-ray dose of the transmission line can be obtained by a simple method.

  Further, according to the X-ray CT apparatus according to this embodiment, the switching of the tube current values of the X-ray tubes 12 and 22, which has been difficult with the conventional technique (for example, the conventional technique described in Patent Document 3), is also overcome. be able to. That is, in the X-ray CT apparatus according to this embodiment, since the amount of change in the tube current value of the X-ray tube 12 and 22 during 1 view is small, the tube of the X-ray tube 12 and 22 is easier than the prior art. It is possible to realize switching of the current value. For example, in the X-ray CT apparatus according to this embodiment, the tube current value of the X-ray tube 12 or 22 is set to 100 [%] to 80 [%] or 80 [%] to 100 [%] of the desired tube current value. Therefore, the tube current value can be switched more easily than in the prior art.

  For example, even if one scan is performed in 0.3 seconds and 1000 views of projection data are collected, according to the X-ray CT apparatus according to this embodiment, the tube currents of the X-ray tubes 12 and 22 Value switching can be realized.

  Furthermore, according to the X-ray CT apparatus according to this embodiment, an image with little deterioration of S / N can be obtained. In the prior art (for example, the prior art described in Patent Document 3), each X-ray detector detects X-rays only in a half period of 1 view period, so the X-ray dose detected by each X-ray detector is It was extremely small. Therefore, in the prior art, unless the tube current value of the X-ray tube 12 or 22 is doubled, the tube current value of the X-ray tube 12 or 22 is set to 100 [%] of the desired tube current value in the 1 view period. It is not possible to obtain the same X-ray dose as that obtained when maintaining the above.

  In contrast, in the X-ray CT apparatus according to this embodiment, the tube current value of the X-ray tube 12 and 22 is kept at 80 [%] of the desired tube current value in the 1 view period. An X-ray dose approximately 90 [%] of the X-ray dose obtained when the tube current values of the tube tubes 12 and 22 are maintained at 100 [%] is obtained. Therefore, the S / N degradation of the projection data detected by the X-ray detectors 13 and 23 can be reduced, and the S / N degradation of the reconstructed image can be suppressed small. .

  In this embodiment, the peak value of the tube current of the X-ray tubes 12 and 22 is changed to a rectangular shape, but the peak value may be changed to a sine wave shape. In this case, the scan control unit 60 uses the X-ray tube 12, 22 so that the maximum value or the minimum value of the sinusoidal tube current value becomes the center of a half period (one collection period) of the 1 view period. Control the timing of irradiation. As described above, even when the tube current values of the X-ray tubes 12 and 22 are controlled, the calculation method of the X-ray dose of the transmission line according to the above formulas (1) to (5) is used. From the X-ray dose actually detected by the individual detection elements of the line detectors 13 and 23, the X-ray dose of the transmission line can be obtained.

  In this embodiment, an X-ray CT apparatus including two sets of X-ray tubes and an X-ray detector has been described, but an X-ray CT including three or more sets of X-ray tubes and an X-ray detector. Even in the case of an apparatus, by applying the method for calculating the X-ray dose of transmission lines according to the above formulas (1) to (5), X-rays of transmission lines can be obtained from the X-ray doses actually detected by the respective X-ray detectors. The dose can be determined.

[Modification]
Next, an X-ray CT apparatus according to a modification will be described with reference to FIGS. FIG. 6 is a conceptual diagram showing a method for calculating the X-ray dose in the first modification. FIG. 7 is a conceptual diagram showing a method for calculating the X-ray dose in Modification 2. In the modification, the scan control unit 60 periodically changes the tube current value of one of the X-ray tubes 12 and 22 during the first half and the second half of the 1 view period, and the tube for the other X-ray tube Scan without changing the current value.

(Modification 1)
In the first modification, the scan control unit 60 changes the peak value of the tube current of the X-ray tube 22 to a rectangular shape. On the other hand, the scan control unit 60 controls the tube current value of the X-ray tube 12 to be constant. For example, in the first half of the 1 view period, the scan control unit 60 sets the tube current value of the X-ray tube 12 to 100 [%] of the desired tube current value, and sets the tube current value of the X-ray tube 22 to the desired tube current value. In the latter half, the tube current value of the X-ray tube 22 is switched, and the tube current value of the X-ray tube 12 is set to 100 [%] of the desired tube current value. The tube current value is set to 80% of the desired tube current value. Thus, the scan control unit 60 periodically changes the tube current value of the X-ray tube 22 between the first half and the second half of the 1 view period.

  The scan control unit 60 includes the timing for switching the tube current value of the X-ray tube 22 and the tube current value of the X-ray tubes 12 and 22 in each period in the X-ray generation control signal, and the X-ray generation control signal Is output to the high voltage generators 11 and 21. Thereby, the scan control unit 60 controls the timing of switching the tube current value of the X-ray tube 22 and the tube current value (X dose) of the X-ray tubes 12 and 22 in each period.

  The timing for switching the tube current value of the X-ray tube 22 and the tube current values of the X-ray tubes 12 and 22 are determined in advance as scan conditions and stored in the scan condition storage unit 61. The timing for switching the tube current value and the tube current value can be arbitrarily changed by the operator.

In the first modification, as in the above-described embodiment, the preprocessing unit 51 removes the influence of scattered radiation and obtains the X-ray dose of transmitted radiation. Here, specific processing contents by the X-ray dose calculation unit 52 will be described with reference to FIG. For example, attention is paid to the X-ray detector 13. The definitions of the X-ray dose a and the X-ray dose b are the same as those in the above embodiment. In the first half of 1 view period, that is, the tube current value of the X-ray tube 12 is set to 100 [%] of the desired tube current value, and the tube current value of the X-ray tube 22 is set to 100 [%] of the desired tube current value. If the X-ray dose actually detected by each detection element of the X-ray detector 13 is X-ray dose c1 as shown in FIG. 6, the X-ray dose c1 is expressed by the following equation (7). expressed.
Formula (7)
X dose c1 = X dose b (scattered X dose) + X dose a (transmitted X dose)

Further, in the latter half of the 1 view period, that is, the tube current value of the X-ray tube 12 is set to 100 [%] of the desired tube current value, and the tube current value of the X-ray tube 22 is set to 80 [% of the desired tube current value. ], When the X-ray dose actually detected by the individual detection elements of the X-ray detector 13 is X-ray dose c2 as shown in FIG. 6, the X-ray dose c2 is expressed by the following equation (8). ).
Formula (8)
X dose c2 = 0.8 × X dose b (scattered X dose) + X dose a (transmitted X dose)

According to the X-ray dose calculation concept shown in FIG. 6 using the above formulas (7) and (8), the X-ray dose a of transmitted rays that should be detected by the individual detection elements of the X-ray detector 13 is It is represented by Formula (9).
Formula (9)
X dose a = 5 × X dose c2-4 × X dose c1

Since the X-ray dose a represents the X-ray dose of the transmitted rays detected by the individual detection elements of the X-ray detector 13 in half the time of the 1-view period, the individual detection of the X-ray detector 13 during the 1-view period. Assuming that the X-ray dose of transmitted rays detected by the element is X-ray dose A, the X-ray dose A is expressed by the following equation (10).
Formula (10)
X dose A = 2 × X dose a
= 10 × X dose c2-8 × X dose c1

  The X-ray dose c1 in the equation (10) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 13 in the first half of the 1 view period, and the X-ray dose c2 is the X-ray in the second half of the 1 view period. The X-ray dose actually detected by each detection element of the detector 13 is represented. Therefore, according to the equation (10), it is possible to obtain the X-ray dose of the transmitted ray that does not include the X-ray dose of the scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X detector 13. .

Further, the X-ray dose of transmitted rays that the X-ray detector 23 should detect in the 1 view period is also obtained in the same manner as the X-ray dose A. For example, in the first half of the 1 view period, the X-ray dose actually detected by the individual detection elements of the X-ray detector 23 is X1 d1, and in the latter half of the 1 view period, the individual detection elements of the X-ray detector 23 are actually Assuming that the X-ray dose to be detected is X-ray dose d2, if the X-ray dose B is the X-ray dose B of the transmitted rays detected by the individual detection elements of the X-ray detector 23 during the 1 view period, ).
Formula (11)
X dose B = 10 × X dose d1-10 × X dose d2

  The X-ray dose d1 in the equation (12) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 23 in the first half of the 1view period, and the X-ray dose d2 is the X-ray in the second half of the 1view period. The X-ray dose actually detected by each detection element of the detector 23 is shown. Therefore, according to the equation (11), it is possible to obtain the X-ray dose of the transmitted ray that does not include the X-ray dose of the scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X-ray detector 23. Become.

  Expression (10) is stored in the calculation condition storage unit 53 as a condition for calculating the X-ray dose of the transmission line to be detected by the X-ray detector 13, and the expression (11) is detected by the X-ray detector 23. It is stored as a condition for calculating the X-ray dose of the transmitted ray to be transmitted.

  When the X-ray dose calculation unit 52 receives the data (X-ray dose) actually detected by the X-ray detectors 13 and 23, the X-ray dose calculation unit 52 performs X in one view period according to the above equation (10) stored in the calculation condition storage unit 53. The X-ray dose A of transmission rays that should be detected by the individual detection elements of the line detector 13 is calculated, and further, the individual detection elements of the X-ray detector 23 detect during one view period according to the above equation (11). Calculate the X-ray dose B of the supposed transmitted ray. Then, the X-ray dose calculation unit 52 calculates X-ray doses A and B of transmission rays that should be detected by the individual detection elements of the X-ray detectors 13 and 23 during each view period.

  Then, the reconstruction processing unit 54 reconstructs image data by performing back projection processing on the projection data that has been subjected to the correction processing by the preprocessing unit 51. Since this projection data has the X-ray dose of scattered radiation removed, image data with reduced influence of scattered radiation is generated.

  According to the first modification, the same effects as those of the above embodiment can be obtained. Furthermore, according to Modification 1, since the tube current value of the X-ray tube 12 is made constant and only the tube current value of the X-ray tube 22 is periodically switched, data collection and tube current value (X dose) are performed. There is an effect that it is technically easy to synchronize the timing of the switching.

(Modification 2)
Next, Modification 2 will be described. In the first modification, the scan control unit 60 changed the peak value of the tube current of the X-ray tube 22 to a rectangular shape, but the peak of the tube current of the X-ray tube 12 instead of the X-ray tube 22. The value may be changed to a rectangular shape. For example, in the first half of the 1 view period, the scan control unit 60 sets the tube current value of the X-ray tube 12 to 100 [%] of the desired tube current value, and sets the tube current value of the X-ray tube 22 to the desired tube current value. In the latter half, the tube current value of the X-ray tube 12 is switched so that the tube current value of the X-ray tube 12 is 80% of the desired tube current value. The tube current value is set to 100% of the desired tube current value. In this way, the scan control unit 60 periodically changes the tube current value of the X-ray tube 12 between the first half and the second half of the 1 view period.

Here, specific processing contents by the X-ray dose calculation unit 52 will be described with reference to FIG. For example, attention is paid to the X-ray detector 13. The definitions of the X-ray dose a and the X-ray dose b are the same as those in the above embodiment. The first half of 1 view period, that is, the tube current value of the X-ray tube 12 is set to 100 [%] of the desired tube current value, and the tube current value of the X-ray tube 22 is set to 100 [%] of the desired tube current value. If the X-ray dose actually detected by the individual detection elements of the X-ray detector 13 is X-ray dose c1 as shown in FIG. 7, the X-ray dose c1 is expressed by the following equation (12). expressed.
Formula (12)
X dose c1 = X dose b (scattered X dose) + X dose a (transmitted X dose)

Further, in the latter half of the 1 view period, that is, the tube current value of the X-ray tube 12 is set to 80 [%] of the desired tube current value, and the tube current value of the X-ray tube 22 is set to 100 [% of the desired tube current value. ], When the X-ray dose actually detected by each detection element of the X-ray detector 13 is X-ray dose c2 as shown in FIG. 7, the X-ray dose c2 is expressed by the following equation (13). ).
Formula (13)
X dose c2 = X dose b (scattered X dose) + 0.8 × X dose a (transmitted X dose)

According to the X-ray dose calculation concept shown in FIG. 7 using the above equations (12) and (13), the X-ray dose a of transmission rays that should be detected by the individual detection elements of the X-ray detector 13 is It is represented by Formula (14).
Formula (14)
X dose a = 5 × X dose c1-5 × X dose c2

Since the X-ray dose a represents the X-ray dose of the transmitted rays detected by the individual detection elements of the X-ray detector 13 in half the time of the 1-view period, the individual detection of the X-ray detector 13 during the 1-view period. When the X-ray dose of transmitted rays detected by the element is X-ray dose A, the X-ray dose A is expressed by the following equation (15).
Formula (15)
X dose A = 2 × X dose a
= 10 × X dose c1-10 × X dose c2

  The X-ray dose c1 in the formula (15) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 13 in the first half of the 1 view period, and the X-ray dose c2 is the X-ray in the second half of the 1 view period. The X-ray dose actually detected by each detection element of the detector 13 is represented. Therefore, according to the equation (15), it is possible to obtain the X-ray dose of the transmitted ray that does not include the X-ray dose of the scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X-ray detector 13. Become.

In addition, the X-ray dose of transmitted rays that the X-ray detector 23 should detect in the 1 view period is also obtained in the same manner as the X-ray dose A. For example, in the first half of the 1 view period, the X-ray dose actually detected by the individual detection elements of the X-ray detector 23 is X1 d1, and in the latter half of the 1 view period, the individual detection elements of the X-ray detector 23 are actually If the X-ray dose to be detected is X-ray dose d2, and the X-ray dose B is the X-ray dose B of the transmitted rays detected by the individual detection elements of the X-ray detector 23 during the 1 view period, the X-ray dose B is expressed by the following formula (16 ).
Formula (16)
X dose B = 10 × X dose d2-8 × X dose d1

  The X-ray dose d1 in the equation (16) represents the X-ray dose actually detected by the individual detection elements of the X-ray detector 23 in the first half of the 1view period, and the X-ray dose d2 is the X-ray in the second half of the 1view period. The X-ray dose actually detected by each detection element of the detector 23 is shown. Therefore, according to the equation (16), it is possible to obtain the X-ray dose of transmitted rays that do not include the X-ray dose of scattered radiation based on the X-ray dose actually detected by the individual detection elements of the X-ray detector 23. Become.

  Expression (15) is stored in the calculation condition storage unit 53 as a condition for calculating the X-ray dose of the transmission line to be detected by the X-ray detector 13, and the expression (16) is detected by the X-ray detector 23. It is stored as a condition for calculating the X-ray dose of the transmitted ray to be transmitted.

  When the X-ray dose calculation unit 52 receives the data (X-ray dose) actually detected by the X-ray detectors 13 and 23, the X-ray dose calculation unit 52 performs X in one view period according to the above formula (15) stored in the calculation condition storage unit 53. The X-ray dose A of the transmission line that should be detected by the individual detection elements of the line detector 13 is calculated, and further, the individual detection elements of the X-ray detector 23 detect during one view period according to the above equation (16). Calculate the X-ray dose B of the supposed transmitted ray. Then, the X-ray dose calculation unit 52 calculates X-ray doses A and B of transmission rays that should be detected by the individual detection elements of the X-ray detectors 13 and 23 during each view period.

  Then, the reconstruction processing unit 54 reconstructs image data by performing back projection processing on the projection data that has been subjected to the correction processing by the preprocessing unit 51. Since this projection data has the X-ray dose of scattered radiation removed, image data with reduced influence of scattered radiation is generated.

  According to the second modification, the same effect as that of the above embodiment can be obtained. Furthermore, according to the modified example 2, since the tube current value of the X-ray tube 22 is made constant and only the tube current value of the X-ray tube 12 is periodically switched, data collection and tube current value (X dose) are performed. There is an effect that it is technically easy to synchronize the timing of the switching.

1 is a block diagram showing a schematic configuration of an X-ray CT apparatus according to an embodiment of the present invention. It is a figure which shows the tube current value of the X-ray tube which concerns on embodiment of this invention. It is a figure which shows an example of the X-ray dose which an X-ray detector detects. It is a conceptual diagram which shows the calculation method of the X-ray dose in this embodiment. It is a flowchart which shows a series of operation | movement by the X-ray CT apparatus which concerns on embodiment of this invention. It is a conceptual diagram which shows the calculation method of the X-ray dose in the modification 1. It is a conceptual diagram which shows the calculation method of the X-ray dose in the modification 2. It is a front view of the rotation mount for demonstrating arrangement | positioning of an X-ray tube and an X-ray detector. It is a front view of the rotation mount for explaining the scattered radiation of X-rays. It is a front view of the rotation mount for explaining the scattered radiation of X-rays. It is a figure which shows the timing of the X-ray irradiation and X-ray detection which concern on a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11, 21 High voltage generation part 12, 22 X-ray tube 13, 23 X-ray detector 14, 24 Data collection part 51 Pre-processing part 52 X dose calculation part 53 Calculation condition storage part 60 Scan control part 61 Scan condition storage part

Claims (8)

An X-ray CT apparatus in which a plurality of pairs of an X-ray tube and an X-ray detector are arranged at different angles, and each pair is configured to be rotatable around a subject,
One view period for acquiring projection data of one view is divided into a plurality of periods, and the tube current values of the X-ray tubes constituting each pair are made different for each of the plurality of periods. In the scan control means for making the tube current value of the X-ray tube constituting each pair different from each other, and irradiating X-rays from the X-ray tube constituting each pair;
Each of the X-ray detectors constituting each pair is a transmission line that is irradiated from each pair of X-ray tubes and passes through the subject based on the X-ray dose detected in the plurality of periods, X-ray dose calculating means for obtaining an X-ray dose of each pair of transmission lines during the one view period;
Reconstruction processing means for reconstructing image data on the basis of the X-ray dose of transmission rays obtained by the calculation means;
An X-ray CT apparatus comprising:   The X-ray dose calculating means sets the tube current value of a predetermined X-ray tube to be lower than the tube current value of another X-ray tube and irradiates the X-ray with the predetermined X-ray tube. The first X-ray dose detected by the X-ray detector and the tube current value of the predetermined X-ray tube higher than the tube current value of the other X-ray tube, An X-ray detector paired with the predetermined X-ray tube during the one view period based on a difference from the second X-ray dose detected by the X-ray detector paired with the predetermined X-ray tube The X-ray CT apparatus according to claim 1, wherein X-ray doses of transmission rays detected by the X-ray detectors are obtained, and X-ray doses of transmission rays detected by all pairs of X-ray detectors are obtained.   The X-ray dose calculating means is configured to pair with the predetermined X-ray tube during the one view period based on a difference in accordance with each tube current value between the first X-ray dose and the second X-ray dose. The X-ray CT apparatus according to claim 2, wherein an X-ray dose of transmitted rays detected by the line detector is obtained.   The scan control means changes the peak value of the tube current of the X-ray tube constituting each pair in a rectangular shape during the plurality of periods. The X-ray CT apparatus described.   The scan control means changes the peak value of the tube current of the X-ray tube constituting each pair in a sine wave form in the plurality of periods. The X-ray CT apparatus described.   The scan control means changes the tube current value of the X-ray tube constituting each pair between 80 and 100 [%] in the plurality of periods. An X-ray CT apparatus according to any one of the above. A first detection system comprising a first X-ray tube and a first X-ray detector; a second detection system comprising a second X-ray tube and a second X-ray detector; In an X-ray CT apparatus equipped with
X-rays are simultaneously emitted from the first X-ray tube and the second X-ray tube, and at least a tube current value of the first X-ray tube is changed to perform scanning, Scan control means for collecting projection data corresponding to a tube current value and projection data corresponding to a second tube current value;
X-ray dose calculating means for obtaining projection data from which scattered radiation from other detection systems has been removed based on projection data corresponding to the first tube current value and projection data corresponding to the second tube current value;
Reconstructing means for reconstructing an image in the subject based on the projection data obtained by the X-ray dose calculating means;
An X-ray CT apparatus comprising: A control method for an X-ray CT apparatus in which a plurality of pairs of an X-ray tube and an X-ray detector are arranged at different angles, and each pair is configured to be rotatable around a subject,
One view period for acquiring projection data of one view is divided into a plurality of periods, and the tube current values of the X-ray tubes constituting each pair are made different for each of the plurality of periods. A scanning step of irradiating X-rays from the X-ray tubes constituting the pairs, with tube current values of the X-ray tubes constituting the pairs being different from each other;
Each of the X-ray detectors constituting each pair is a transmission line that is irradiated from each pair of X-ray tubes and passes through the subject based on the X-ray dose detected in the plurality of periods, An X-ray dose calculating step for obtaining an X-ray dose of each pair of transmission lines during the one view period;
A reconstruction processing step of reconstructing image data based on the X-ray dose of the transmission line obtained in the X-th dose calculation step;
A control method for an X-ray CT apparatus, comprising:

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