Patent Publication Number: US-2013251101-A1

Title: X-ray ct apparatus and method for controlling the same

Description:
CROSS-REFERENCE TO THE RELATED APPLICATION 
     This application is based on and claims the benefit of priority from Japanese Patent Applications No. 2012-63176, filed on Mar. 21, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an X-ray CT apparatus and a method for controlling the X-ray CT apparatus. 
     BACKGROUND 
     An X-ray CT apparatus (X-ray computed tomography imaging apparatus) irradiates an examinee such as a patient with X-rays and detects X-rays transmitted through the examinee. A data collecting device of the X-ray CT apparatus then collects X-ray transmission data which is based on the amount of X-rays detected. Thereafter, the X-ray CT apparatus performs reconstruction processing on the X-ray transmission data, and generates a slice image (a tomographic image) of the examinee. An example of such an X-ray CT apparatus which has been developed is configured to image an examinee using an X-ray irradiator and an X-ray detector located opposite each other with the examinee on a table of a bed in between, while rotating them about the body axis of the examinee. 
     In order to change an X-ray radiation field, the X-ray CT apparatus includes an X-ray diaphragm, such as a collimator, below an X-ray tube of the X-ray irradiator. For example, this X-ray diaphragm includes components such as a pair of blades (slit plates) configured to block X-rays and a movement mechanism configured to move the blades in directions away from and toward each other. The X-ray diaphragm changes the X-radiation field by adjusting the width of an opening which is formed by the pair of blades and through which X-rays transmit. 
     Such an X-ray CT apparatus performs imaging by helical scan. In the helical scan, the X-ray irradiator and the X-ray detector are rotated about the body axis of an examinee lying down on a table while the table is moved at a constant speed along the body axis of the examinee, i.e., either in a direction from the feet to the head or in a direction from the head to the feet. 
     In this imaging, provided that the moving speed of the table is constant (within an allowable range), timings for starting an opening operation and a closing operation of the X-ray diaphragm are controlled based on a time period almost proportional to the number of views, namely, the number of rotations (rotational speed) of the X-ray irradiator and the X-ray detector about the body axis of the examinee (a time period converted from the number of views). Note that the opening operation is to move the pair of blades away from each other, while the closing operation is to move them toward each other. 
     However, in variable Helical Pitch (vHP) scan and shuttle helical scan, the moving speed of the table is not constant like in the helical scan described above, but the table moves at a variable speed or shuttles. For this reason, it is difficult to perform control as intended according to the moving speed of the table. As a result, the opening and closing operations of the X-ray diaphragm might be performed too early or too late. 
     For example, assume that only a certain site of an examinee is to be imaged. If the opening operation starts early, the examinee is exposed to radiation more than necessary, and if the opening operation starts late, the amount of X-ray transmission data obtained is insufficient, which makes the resultant image unreliable (incomplete). Similarly, if the closing operation starts early, the amount of X-ray transmission data obtained is insufficient, which makes the resultant image unreliable (incomplete), and if the closing operation starts late, the examinee is exposed to radiation more than necessary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the configuration of an X-ray CT apparatus according to an embodiment. 
         FIG. 2  is a diagram schematically showing the configurations of an X-ray diaphragm and a controller of the X-ray CT apparatus according to the embodiment. 
         FIG. 3  is a diagram schematically showing the configuration of the controller, together with a bed of the X-ray CT apparatus and part of an imaging device. 
         FIG. 4  is a flowchart showing a flow of imaging processing performed by the X-ray CT apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, an X-ray CT apparatus comprises: a table on which an examinee lies down; an X-ray irradiator including an X-ray tube configured to emit X-rays to the examinee on the table and a diaphragm configured to block the X-rays and capable of opening and closing; an X-ray detector configured to detect X-rays transmitted through the examinee on the table after being emitted by the X-ray irradiator and passing through the diaphragm; a rotating gantry configured to support the X-ray irradiator and the X-ray detector; a rotation drive unit configured to rotate the rotating gantry about a body axis of the examinee on the table; a movement drive unit configured to move any one of the table and the rotating gantry in a direction of the body axis of the examinee on the table; a positional information acquirer configured to acquire positional information on a mobile object which is any one of the table and the rotating gantry moved by the movement drive unit; and a controller configured to control opening and closing operations of the diaphragm of the X-ray irradiator based on the positional information acquired by the positional information acquirer. 
     According to another embodiment, provided is a method for controlling an X-ray CT apparatus including a table on which an examinee lies down, an X-ray irradiator having an X-ray tube configured to emit X-rays to the examinee on the table and a diaphragm configured to block the X-rays and capable of opening and closing, an X-ray detector configured to detect X-rays which are emitted by the X-ray irradiator, pass through the diaphragm, and then transmit through the examinee on the table, a rotating gantry configured to support the X-ray irradiator and the X-ray detector, a rotation drive unit configured to rotate the rotating gantry about a body axis of the examinee on the table, and a movement drive unit configured to move any one of the table and the rotating gantry in a direction of the body axis of the examinee on the table. The method comprises the steps of: acquiring, by a positional information acquirer, positional information on a mobile object which is any one of the table and the rotating gantry moved by the movement drive unit; and controlling, by a controller, opening and closing operations of the diaphragm of the X-ray irradiator based on the positional information acquired by the positional information acquirer. 
     An embodiment is described with reference to the drawings. 
     As shown in  FIG. 1 , an X-ray CT apparatus (X-ray computed tomography imaging apparatus)  1  according to this embodiment includes a bed  2  on which an examinee P, such as a patient, lies down, an imaging device  3  configured to image the examinee P on the bed  2 , and a control device  4  configured to control the bed  2  and the imaging device  3 . 
     The bed  2  includes a rectangular table  2   a  on which the examinee P is placed and a movement drive unit  2   b  configured to support the table  2   a  and move the table  2   a  in horizontal directions and vertical directions (up and down directions). The movement drive unit  2   b  has a movement mechanism for moving the table  2   a , a drive source for supplying a driving power for moving the table  2   a , and the like. The movement drive unit  2   b  of the bed  2  moves the table  2   a  up to a desired height and then moves the table  2   a  horizontally to transfer the examinee P on the table  2   a  to a desired position. 
     The imaging device  3  includes a rotator  3   a  provided rotatably inside a gantry A, which is a chassis, a rotation drive unit  3   b  configured to rotate the rotator  3   a , an X-ray irradiator  3   c  configured to emit X-rays, a high-voltage generator  3   d  configured to supply the X-ray irradiator  3   c  with a high voltage, an X-ray detector  3   e  configured to detect X-rays transmitted through the examinee P on the table  2   a , and a data collector  3   f  configured to collect the X-rays detected by the X-ray detector  3   e  as X-ray transmission data (X-ray amount distribution data). 
     The rotator  3   a  is a ring-shaped rotational frame (rotating gantry) configured to support components such as the X-ray irradiator  3   c  and the X-ray detector  3   e  and rotate. The rotator  3   a  is held by the gantry A rotatably. The X-ray irradiator  3   c  and the X-ray detector  3   e  are located on the rotator  3   a  at positions opposite each other so that the examinee P on the table  2   a  may be located in between them, and they rotate around the examinee P about the body axis of the examinee P. 
     The rotation drive unit  3   b  is located inside the gantry A of the imaging device  3 . The rotation drive unit  3   b  drives the rotator  3   a  to rotate as controlled by the control device  4 . For example, the rotation drive unit  3   b  rotates the rotator  3   a  in one direction at a predetermined rotation speed based on a control signal sent from the control device  4 . 
     The X-ray irradiator  3   c  is fixed to the rotator  3   a  and includes an X-ray tube  3   c   1  configured to emit X-rays and an X-ray diaphragm  3   c   2 , such as a collimator, configured to narrow the X-rays emitted by the X-ray tube  3   c   1 . Specifically, the X-ray irradiator  3   c  is configured such that X-rays emitted by the X-ray tube  3   c   1  are narrowed by the X-ray diaphragm  3   c   2  so that the examinee P on the table  2   a  may be irradiated with a beam of X-rays having a fan beam shape with a cone angle, e.g., a pyramid shape. 
     The high-voltage generator  3   d  is located inside the gantry A of the imaging device  3 . The high-voltage generator  3   d  is a device for generating a high voltage to be supplied to the X-ray tube  3   c   1  of the X-ray irradiator  3   c , and is configured to step-up or rectify a voltage given by the control device  4  and supply the stepped-up or rectified voltage to the X-ray tube  3   c   1 . To cause the X-ray tube  3   c   1  to generate X-rays as desired, the control device  4  controls the waveform of a voltage to give to the high-voltage generator  3   d , i.e., various conditions such as the amplitude and pulse width. 
     The X-ray detector  3   e  is fixed to the rotator  3   a  at a position opposite the X-ray irradiator  3   c . The X-ray detector  3   e  converts X-rays transmitted through the examinee P on the table  2   a  into electric signals and sends them to the data collector  3   f . As the X-ray detector  3   e , a multi-layered, multichannel X-ray detector can be used. The multilayered, multichannel X-ray detector is configured with X-ray detection elements configured to detect X-rays and arranged in lattice. Specifically, a channel is formed by multiple (e.g., several hundreds to several thousands of) X-ray detection elements arranged in a channel direction (i.e., a direction about the body axis of the examinee P), and multiple (e.g., 16 or 64) rows of such a channel are arranged in a slice direction (i.e., in a direction of the body axis of the examinee P). 
     The data collector  3   f  is located inside the gantry A of the imaging device  3 , and configured to collect the electrical signals sent from the X-ray detector  3   c  as X-ray transmission data (X-ray amount distribution data), and send this X-ray transmission data to the control device  4 . 
     The control device  4  includes a controller  4   a  configured to control each unit, an image processor  4   b  configured to perform various kinds of image processing on the X-ray transmission data, a storage unit  4   c  configured to store various programs, various kinds of data, and the like, an operation unit  4   d  configured to receive an operation inputted by the user, and a display unit  4   e  configured to display images. The controller  4   a , the image processor  4   b , the storage unit  4   c , the operation unit  4   d , and the display unit  4   e  are electrically connected to each other via a bus line  4   f.    
     Based on the various programs and data stored in the storage unit  4   c , the controller  4   a  controls units such as the movement drive unit  2   b  of the bed  2  and the rotation drive unit  3   b  and the high-voltage generator  3   d  of the imaging device  3 . In addition, the controller  4   a  controls the diaphragm  3   c   2  of the X-ray irradiator  3   c , and also controls display of various images, such as slice images (tomographic images) and scanograms (positioning images), on the display unit  4   e . For example, a central processing unit (CPU) or the like can be used as the controller  4   a.    
     The image processor  4   b  performs various types of image processing, such as preprocessing for obtaining projection data from the X-ray transmission data sent from the data collector  3   f , image reconstruction processing for performing image reconstruction on the projection data, and scanogram generation processing for generating scanograms. For example, an array processor or the like can be used as this image processor  4   b.    
     The storage unit  4   c  is a storage device configured to store various programs, various kinds of data, and the like. Examples of the various kinds of data include slice images and scanograms. For example, a read-only memory (ROM), a random access memory (RAM), a hard disk (magnetic disk device), a flash memory (semiconductor disk device), or the like can be used as the storage unit  4   c.    
     The operation unit  4   d  is an input unit configured to receive various operations inputted on the input unit  4   d  by a user, such as instructing imaging, displaying an image, switching between images, and making various settings. For example, input devices such as a keyboard, a mouse, and a control lever can be used as the operation unit  4   d.    
     The display unit  4   e  is a display device configured to display various types of images, such as an X-ray image and a scanogram of the examinee P and an operation screen. For example, a liquid crystal display, a CRT-based display, or the like can be used as the display unit  4   e.    
     In the X-ray CT apparatus  1  configured as above, in response to an operation inputted on the operation unit  4   d  by the user, the movement drive unit  2   b  inserts the table  2   a  on which the examinee P is placed into the frame-shaped rotator  3   a , and moves the examinee P on the table  2   a  in its body axis direction (e.g., in a direction from the feet to the head). In the X-ray CT apparatus  1 , while the table  2   a  is moved, the rotation drive unit  3   b  rotates the rotator  3   a  so that the X-ray irradiator  3   c  and the X-ray detector  3   e  rotate about the body axis of the examinee P on the table  2   a . During this rotation of the rotary  3   a , the examinee P is imaged by the X-ray irradiator  3   c  irradiating the examinee P with X-rays and by the X-ray detector  3   e  detecting X-rays transmitted through the examinee P (i.e., this imaging is performed by the helical scan). In this event, in the X-ray CT apparatus  1 , the data collector  3   f  collects electrical signals from the X-ray detector  3   e  as projection data, and the image processor  4   b  processes the projection data and saves the processed X-ray image (slice image) in the storage unit  4   c . The X-ray image is displayed on the display unit  4   e.    
     There are three imaging modes herein: a helical scan mode, a variable helical pitch scan mode, and a shuttle helical scan mode. In the helical scan mode, the examinee P is imaged while the table  2   a  is moved at a constant speed (within an allowable speed range) in one direction along the body axis of the examinee P (e.g., from the feet to the head). In the variable helical pitch scan mode, the moving speed of the table  2   a  is changed for, for example, a site targeted for the imaging. In the shuttle helical scan mode, the examinee P is imaged while the table  2   a  is moved with its moving direction being changed alternately between two directions along the body axis of the examinee P (e.g., in a direction from the feet to the head and in a direction from the head to the feet). The X-ray CT apparatus  1  is thus capable of performing X-ray imaging by various imaging modes. 
     Next, the X-ray diaphragm  3   c   2  described earlier is described in detail. 
     As shown in  FIG. 2 , the X-ray diaphragm  3   c   2  includes a pair of blades  11  and  12  configured to narrow a beam of X-rays by blocking them, a blade moving mechanism  13  configured to move the blades  11  and  12  in such directions as to change the opening width of a slit S which is an opening for narrowing the beam (in directions of the body axis of the examinee P), two encoders  14  and  15  configured to check the positions of the blades  11  and  12 , respectively, and original-position sensors  16  and  17  configured to determine the original positions of the blades  11  and  12 , respectively. 
     The blade moving mechanism  13  includes two shafts  13   a  and  13   b  configured to respectively guide the blades  11  and  12  in such directions as to change the opening width of the slit S, and two motors  13   c  and  13   d  which are drive sources for moving the blades  11  and  12 , respectively. For example, pulse motors can be used as the motors  13   c  and  13   d . The motors  13   c  and  13   d  are electrically connected to the controller  4   a , and are driven as controlled by the controller  4   a . As the blade moving mechanism  13 , a feed-screw mechanism can for example be used. 
     The encoders  14  and  15 , which are electrically connected to the controller  4   a , detect the amounts of movement of the blades  11  and  12 , respectively, and input the detected amounts to the controller  4   a . Upon detection of the blades  11  and  12 , the original-position sensors  16  and  17 , which are electrically connected to the controller  4   a , input detection signals to the controller  4   a . Thus, the controller  4   a  can know the zero points of the encoders  14  and  15 . 
     In the X-ray diaphragm  3   c   2  thus configured, first, the motors  13   c  and  13   d  are driven at a predetermined timing, such as before imaging, to move the pair of blades  11  and  12  to the positions of the original-position sensors  16  and  17 , respectively, so that the controller  4   a  can know the zero points of the encoders  14  and  15 . Then, as controlled by the controller  4   a , the X-ray diaphragm  3   c   2  performs an opening operation for moving the blades  11  and  12  to opening positions where the opening width of the slit S is at a preset value or a closing operation for moving the blades  11  and  12  to closing positions where the opening width is zero. The X-ray radiation field for the X-ray detector  3   e  can be adjusted by changing the above preset value of the opening width. 
     Note that various types of X-ray diaphragm can be used as the X-ray diaphragm  3   c   2 , besides the above described X-ray diaphragm configured to change the size of the opening formed by the blades  11  and  12 , which are X-ray blocking plates made of lead or the like, by moving the blades  11  and  12  in directions away from and toward each other. 
     Next, the controller  4   a  mentioned above is described in detail. 
     As shown in  FIG. 3 , the controller  4   a  includes a position trigger detector  21  configured to detect a timing for starting the opening or closing operation of the X-ray diaphragm  3   c   2  based on positional information on the moving table  2   a , a speed detector  22  configured to detect a moving speed of the table  2   a  based on the positional information on the moving table  2   a , and an open/close controller  23  configured to control driving of the motors  13   d  and  13   c  of the X-ray diaphragm  3   c   2 . 
     Note that the movement drive unit  2   b  of the bed  2  has a positional information acquirer  2   b   1  configured to acquire the positional information on the table  2   a  which is a mobile object and to output the acquired positional information to the position trigger detector  21  and the speed detector  22 . An encoder, for example, can be used as the positional information acquirer  2   b   1 . The encoder is attached to, for example, the motor of the movement drive unit  2   b.    
     The position trigger detector  21  receives the positional information on the table  2   a  from the positional information acquirer  2   b   1  of the bed  2 , and then detects a timing for starting the opening or closing operation based on a comparison between the acquired positional information on the table  2   a  and a start position for the opening or closing operation of the X-ray diaphragm  3   c   2 . Then, the position trigger detector  21  outputs a detection signal notifying of the start timing to the open/close controller  23 . For example, a comparator can be used as the position trigger detector  21 . 
     The start position for the opening or closing operation of the X-ray diaphragm  3   c   2  described above is set in advance according for example to an imaged area, such as which part of the examinee to image (e.g., an organ or a site), determined at the time of imaging planning or the like. 
     The speed detector  22  receives the positional information from the positional information acquirer  2   b   1  of the bed  2 , and then derives the moving speed of the table  2   a  from the positional information on the table  2   a  acquired. More specifically, the speed detector  22  calculates the amount of movement of the table  2   a  per unit time from the positional information on the moving table  2   a  acquired successively and thus obtains the moving speed of the table  2   a . Then, the speed detector  22  outputs data on the moving speed of the table  2   a  to the open/close controller  23 . 
     Based on the detection signal (trigger signal) outputted from the position trigger detector  21  and the data on the moving speed of the table  2   a  outputted from the speed detector  22 , the open/close controller  23  controls the opening or closing operation of the X-ray diaphragm  3   c   2  (an operation for moving the pair of blades  11  and  12  in the directions of the body axis of the examinee P), in other words, controls the motors  13   d  and  13   c  of the X-ray diaphragm  3   c   2 . 
     For example, upon receipt of the detection signal for the opening operation, the open/close controller  23  starts the opening operation of the X-ray diaphragm  3   c   2 , i.e., starts rotating the motors  13   d  and  13   c  in such directions as to move the pair of blades  11  and  12  away from each other. Further, the open/close controller  23  controls the motors  13   d  and  13   c  with an electronic gear multiplied by a multiplication/division rate obtained based to the data on the moving speed of the table  2   a.    
     Upon receipt of the detection signal for the closing operation, the open/close controller  23  starts the closing operation of the X-ray diaphragm  3   c   2 , i.e., starts rotating the motors  13   d  and  13   c  in such directions as to move the pair of blades  11  and  12  toward each other. As is similar to the case of the opening operation, the open/close controller  23  controls the motors  13   d  and  13   c  with an electronic gear multiplied by a multiplication/division rate obtained based to the data on the moving speed of the table  2   a.    
     Note that the position trigger detector  21 , the speed detector  22 , and the open/close controller  23  may be configured by hardware such as electric circuits, or may be configured by software such as programs executing their functions, or may be configured by a combination of both. 
     Next, a description is given of imaging processing performed by the X-ray CT apparatus  1 . Note that the variable helical pitch scan mode and the shuttle helical scan mode are set as the imaging modes, and imaging is performed by either the variable helical pitch scan or the shuttle helical scan. 
     As shown in  FIG. 4 , first, it is determined based on positional information on the table  2   a  whether or not a trigger for starting the opening operation of the X-ray diaphragm  3   c   2  is turned on (Step S 1 ). This step is repeated until the trigger for starting the opening operation is turned on (NO in Step S 1 ). 
     Specifically, in Step S 1 , the trigger for starting the opening operation is determined as being turned on when the open/close controller  23  receives a detection signal (trigger signal) for the opening operation from the position trigger detector  21 . 
     When it is determined in Step S 1  that the trigger for starting the opening operation of the X-ray diaphragm  3   c   2  is turned on (YES in Step S 1 ), the opening operation of the X-ray diaphragm  3   c   2  is started (Step S 2 ), and the opening operation of the X-ray diaphragm  3   c   2  is performed based on the positional information on the table  2   a  (Step S 3 ). 
     Specifically, in Step S 2 , the opening operation of the X-ray diaphragm  3   c   2  is started, i.e., an operation for rotating the motors  13   d  and  13   c  by a predetermined amount in such directions as to move the pair of blades  11  and  12  away from each other is started. Further, in Step S 3 , the motors  13   d  and  13   c  (speed of the opening operation) are controlled by an electronic gear multiplied by a multiplication/division rate obtained based on a moving speed of the table  2   a  which is derived from the positional information on the table  2   a.    
     Next, after the processing in Step S 3 , it is determined based on the positional information on the table  2   a  whether or not a trigger for starting the closing operation of the X-ray diaphragm  3   c   2  is turned on (Step S 4 ). This step is repeated until the trigger for starting the closing operation is turned on (NO in Step S 4 ). 
     Specifically, in Step S 4 , the trigger for starting the closing operation is determined as being turned on when the open/close controller  23  receives the detection signal (trigger signal) for the closing operation from the position trigger detector  21 . 
     When it is determined in Step S 4  that the trigger for starting the closing operation of the X-ray diaphragm  3   c   2  is turned on (YES in Step S 4 ), the closing operation of the X-ray diaphragm  3   c   2  is started (Step S 5 ), and the closing operation of the X-ray diaphragm  3   c   2  is performed based on the positional information on the table  2   a  (Step S 6 ). 
     Specifically, in Step S 5 , the closing operation of the X-ray diaphragm  3   c   2  is started, i.e., an operation for rotating the motors  13   d  and  13   c  by a predetermined amount in such directions as to move the pair of blades  11  and  12  toward each other is started. Further, in Step S 6 , as is similar to the case of the opening operation, the motors  13   d  and  13   c  (speed of the closing operation) are controlled by an electronic gear multiplied by a multiplication/division rate obtained based on a moving speed of the table  2   a  which is derived from the positional information on the table  2   a.    
     After the processing in Step S 6 , it is determined whether imaging is completed or not (Step S 7 ). The processing proceeds back to Step S 1  to repeat the steps from Step S 1  when the imaging is not completed (NO in Step S 7 ), and the processing ends when the imaging is completed (YES in Step S 7 ). 
     In Step S 7 , the determination of whether imaging is completed or not is made by, for example, judging whether the table  2   a  is at a predetermined imaging complete position or not. As an example, when the imaging mode is the variable helical pitch scan mode or the shuttle helical scan mode, the determination in Step S 7  is made based on a judgment on whether or not the table  2   a  is at the predetermined imaging completion position reached after the variable helical pitch scan or the shuttling helical scan is completed. Then, it is determined that the imaging is completed when it is judged that the table  2   a  is at the predetermined imaging completion position. 
     According to such imaging processing, the opening operation of the X-ray diaphragm  3   c   2  is started at a start timing which is based on a comparison between the positional information on the table  2   a  and the start position for the opening operation of the X-ray diaphragm  3   c   2 . Similarly, the closing operation of the X-ray diaphragm  3   c   2  is started at a start timing which is based on a comparison between the positional information on the table  2   a  and the start position for the closing operation of the X-ray diaphragm  3   c   2 . In addition, the opening operation and the closing operation of the X-ray diaphragm  3   c   2  are controlled according to the moving speed of the table  2   a  derived from the positional information on the table  2   a , and consequently controlled in such a manner as to follow the variable moving speed of the table  2   a . In this way, the opening and closing operations can be controlled accurately independent of the variable (inconstant) moving speed of the table  2   a , i.e., independent of the inconstancy in the trigger for starting the opening and closing operations of the X-ray diaphragm  3   c   2  due to the number of views. Consequently, the timings for starting the opening operation and the closing operation are no longer too early or too late, so as to prevent unnecessary exposure to radiation and to allow reliable (complete) imaging with necessary amount of X-ray transmission data. 
     As described above, according to the embodiment, the positional information acquirer  2   b   1  acquires position information on the mobile table  2   a , and the controller  4   a  controls the opening and closing operations of the X-ray diaphragm  3   c   2  of the X-ray irradiator  3   c  based on the positional information on the table  2   a  acquired. Thus, being controlled based on the positional information on the table  2   a , the opening and closing operations of the X-ray diaphragm  3   c   2  can be controlled accurately not dependent on the change (inconstancy) in the moving speed of the table  2   a . As a result, reduction in exposure to radiation as well as reliable imaging can be achieved. 
     In particular, when the position trigger detector  21  detects a timing for staring the opening or closing operation of the X-ray diaphragm  3   c   2 , the corresponding one of the opening operation and the closing operation is started, and the speed of the opening or closing operation of the X-ray diaphragm  3   c   2  is controlled based on the moving speed of the table  2   a  derived by the speed detector  22 . Thus, the accurate control of the opening/closing operation can be reliably performed, and therefore the exposure to radiation and the reliable imaging can be achieved even more surely. 
     Although the controller  4   a  uses the positional information on the mobile table  2   a  during the imaging in the embodiment described above, the present invention is not limited to this. For example, if the X-ray CT apparatus  1  is of a type where the gantry A, not the table  2   a , is moved by a moving drive unit (including, for example, a rail mechanism, a drive source, a positional information acquirer, and the like) during the imaging, the gantry A, which includes the X-ray irradiator  3   c , the X-ray detector  3   e , the rotator  3   a , and the like, is a mobile object. In such a case, the controller  4   a  uses positional information on the gantry A as the positional information on the mobile object. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.