Abstract:
An X-ray CT apparatus enables decreasing the degradation of image quality and reducing futile exposure to X-ray in a helical shuttle scan. The X-ray CT apparatus comprises: a gantry rotating device having an X-ray generator for generating the X-ray and an X-ray detector for detecting the generated X-ray and transmitted through a subject; a gantry controller for performing the helical shuttle scan by reciprocally moving the gantry rotating device and the cradle; a scan condition setting device for setting the scan condition of the helical shuttle scan in a predetermined range in the direction of body axis of the subject; an X-ray projection data acquisition device for acquiring the X-ray projection data; a scan controller for controlling the scan as specified in the scan condition; and a image reconstruction device for performing the image reconstruction processing based on the X-ray projection data.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Japanese Patent Application No. 2007-260724 filed Oct. 4, 2007, which is hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to the technology of helical shuttle scan used in an X-ray CT (computed tomography) apparatus. 
         [0003]    As is disclosed in Japanese Unexamined Patent Publication No. 2006-320523, the helical shuttle scan is a method of imaging, in which an X-ray generator and an X-ray detector are rotated while at the same time they are moved reciprocally and relatively in the z direction of a subject, which is the direction of body axis.  FIG. 14  shows the relationship between the scan time and the velocity of the cradle in a helical shuttle scan, and the relationship between the scan time and the relative positions of gantry rotating device and the cradle.  FIG. 14  shows an example of reciprocating motion, between predetermined imaging ranges z 0  and z 1 , in the direction of body axis of the subject, by slowing down prior to stopping at z 0  and z 1 , accelerating after stopping, and maintaining the speed during the intermediate range. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    However, in the helical shuttle scan as described above, due to the difference of the characteristics of the cradle of that apparatus, or due to the difference of the weight of the subject, or due to the velocity fluctuation of the cradle, the helical shuttle scan may not be ideally performed as shown in  FIG. 14 , rather there may occur the positional displacement or timing difference, which causes a difficulty to acquire the X-ray projection data as planned, and consequently becoming the cause of the decrease of image quality. In addition, when setting the imaging time totally as a scan project, the helical shuttle scan may terminate in the middle of the predetermined imaging range in the direction of the body axis of the subject, which may cause the waste exposure. 
         [0005]    The subject of the present invention is to provide an X-ray CT apparatus which allows obtaining of tomographic images in the helical shuttle scan, with less decrease of image quality and less exposure for no use. 
         [0006]    The X-ray CT apparatus in accordance with a first aspect of the present invention comprises: a gantry rotating device including an X-ray generator for generating X-ray, and an X-ray detector for detecting the X-ray generated by the X-ray generator and transmitted through a subject; a cradle for carrying the subject thereon; a gantry controller for performing a helical shuttle scan for relatively reciprocally moving the gantry rotating device and the cradle; a scan condition setting device for setting a scan condition of the helical shuttle scan in a predetermined range in a direction of body axis of the subject; an X-ray projection data acquisition device for acquiring X-ray projection data obtained by the helical shuttle scan; a scan controller for controlling so as to perform the helical shuttle scan as set in accordance with the scan condition; and an image reconstruction device for performing the image reconstruction processing based on the X-ray projection data acquired by the X-ray projection data acquisition device. 
         [0007]    The X-ray CT apparatus in accordance with a second aspect of the present invention provides the X-ray CT apparatus according to the first aspect, wherein the scan condition setting device sets a number of one way or a number of reciprocating of the helical shuttle scan; and the scan controller controls the helical shuttle scan in the predetermined range in the direction of body axis of the subject so as to perform the number of one way or the number of reciprocating set by the scan condition setting device regardless of scan time which is set or estimated at the time of the scan condition setting. 
         [0008]    An X-ray CT apparatus in accordance with a third aspect of the present invention provides the X-ray CT apparatus according to the first aspect wherein the scan condition setting device sets scan time of the helical shuttle scan; and the scan controller controls the scan so as to provide some waiting time in which the move is stopped after the helical shuttle scan has been reached to one end of the predetermined range in the direction of body axis of the subject to start the next scan in the opposite direction after the waiting time, in order to execute a predetermined number of one way or a predetermined number of reciprocating of the helical shuttle scan within the scan time. 
         [0009]    The X-ray CT apparatus in accordance with a fourth aspect of the present invention provides an X-ray CT apparatus according to the third aspect, wherein the X-ray output is minimized during the waiting time. 
         [0010]    An X-ray CT apparatus in accordance with a fifth aspect of the present invention provides the X-ray CT apparatus according to the first aspect wherein the scan condition setting device sets scan time of the helical shuttle scan; and the scan controller starts the scan in the opposite direction in synchronism with the scan in one direction in a predetermined time in the helical shuttle scan, regardless of the predetermined range in the direction of the body axis of the subject in the scan condition. 
         [0011]    An X-ray CT apparatus in accordance with a sixth aspect of the present invention provides the X-ray CT apparatus according to any one of the first through the fifth aspects, wherein, when the coordinate in the direction of body axis in the helical shuttle scan is different from the position which is set or estimated in the scan condition setting time, the scan controller corrects the position in the direction of body axis so as to be at the position which is set or estimated at the time of scan condition. 
         [0012]    An X-ray CT apparatus in accordance with a seventh aspect of the present invention provides the X-ray CT apparatus according to the sixth aspect, wherein the correction is performed by controlling velocity or acceleration of the helical shuttle scan. 
         [0013]    In the X-ray CT of an eighth aspect of the present invention provides the X-ray CT apparatus according to the second or third aspect, wherein the scan condition setting device sets image quality index value for the tomographic image at each positions of coordinates in the direction of each body axis within the predetermined range in the direction of body axis of the subject; and the X-ray projection data acquisition device acquires the X-ray projection data at the coordinate positions in the direction of body axis so as to have the image quality index value being set in the scan condition setting device. 
         [0014]    An X-ray CT apparatus in a ninth aspect of the present invention provides the X-ray CT apparatus according to the fourth or fifth aspect, wherein the scan condition setting device sets image quality index value for the tomographic images at each coordinate position in the direction of body axis at each time in the scan time; and the X-ray projection data acquisition device acquires the X-ray projection data at each time so as to have the image quality index value which is set by the scan condition setting device. 
         [0015]    In the X-ray CT apparatus in accordance with the present invention, by incorporating a scan controller for performing the control for implementing the helical shuttle scan as the scan condition, the X-ray projection data acquisition may be performed as planned while preventing the onset of the positional displacement and the timing difference with respect to the scan project, allowing imaging the tomographic images with decreased degradation of the image quality and with decreased waste exposure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic block diagram illustrating the X-ray CT apparatus  100  in accordance with a preferred embodiment of the present invention; 
           [0017]      FIG. 2  is a flow chart illustrating the overview of the operation of the X-ray CT apparatus  100  in this preferred embodiment; 
           [0018]      FIG. 3  is a schematic diagram illustrating the speed, position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time in the helical shuttle scan of the positional prioritized control; 
           [0019]      FIG. 4  is a flow chart illustrating the helical shuttle scan of the position prioritized control; 
           [0020]      FIG. 5(   a ) is a schematic diagram illustrating a scout image shown on a scan condition setting display screen;  FIG. 5(   b ) is a schematic diagram illustrating the scan condition setting input display screen in the helical shuttle scan of the position prioritized control; and  FIG. 5(   c ) is a schematic diagram illustrating the scan condition setting input display screen in the helical shuttle scan of the time prioritized control; 
           [0021]      FIG. 6(   a ) is a schematic diagram illustrating the occurrence of a misalignment of the scan range;  FIG. 6(   b ) is a schematic diagram illustrating the scan range when the position correction control is performed; 
           [0022]      FIG. 7(   a ) is a schematic diagram illustrating the X-ray exposure area and the area of image reconstruction; and  FIG. 7(   b ) is a schematic diagram illustrating the area of image reconstruction when the stationary scan is used; 
           [0023]      FIG. 8  is a schematic diagram illustrating the control of the speed and position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time when the stationary scan is added to the helical shuttle scan of the position prioritized control; 
           [0024]      FIG. 9  is a schematic diagram illustrating the control of the speed and position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time in the helical shuttle scan of the time prioritized control; 
           [0025]      FIG. 10  is a flow chart in the helical shuttle scan of the time prioritized control; 
           [0026]      FIG. 11  is a schematic diagram illustrating the control of the speed and position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time when the stationary scan is added to the helical shuttle scan of the time prioritized control; 
           [0027]      FIG. 12  is a schematic diagram illustrating the control of the speed and the position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time when the stationary scan and the X-ray automatic exposure control are added to the helical shuttle scan of the time prioritized control; 
           [0028]      FIG. 13  is a schematic diagram illustrating the control of the speed and the position of the cradle, the X-ray output, and the X-ray tube current with respect to the scan time when the stationary scan and the X-ray automatic exposure control are added to the helical shuttle scan of the time prioritized control; and 
           [0029]      FIG. 14  is a schematic diagram illustrating the control of the speed and the position of the cradle with respect to the scan time in a helical shuttle scan. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Now referring to  FIG. 1  there is shown a schematic block diagram of the arrangement of an X-ray CT apparatus  100  in accordance with the preferred embodiment of the present invention. The X-ray CT apparatus  100  includes an operating console  1 , an imaging table  10 , and a scanning gantry  20 . 
         [0031]    The operating console  1  includes an input device  2  such as a keyboard or a mouse for accepting the input from the operator, a central processing unit  3  for performing the preprocessing, the image reconstruction, and the postprocessing, and a data acquisition device  5  for acquiring the X-ray detector data acquired by the scanning gantry  20 . The operating console  1  further includes a monitor  6  for displaying a tomographic image which is reconstructed from the projection data obtained by the preprocessing of the X-ray detector data, and a storage device  7  for storing such items as the program, the X-ray detector data, the projection data, and the X-ray tomographic image. The input of scan condition is input from the input device  2 , and is stored in the storage device  7 . The imaging table  10  includes a cradle  12  for carrying a subject HB thereon to transport it into and from the opening of the scanning gantry  20 . The cradle  12  moves up and down as well as translates in z direction by means of a motor built in the imaging table  10 . 
         [0032]    The scanning gantry  20  includes an X-ray tube  21 , an X-ray controller  22 , a multi X-ray detector  24 , and a data acquisition system. A collimeter  23 , an X-ray beam forming filter  28 , and an X-ray filter  23  are placed between the X-ray tube  21  and the subject HB. The scanning gantry  20  further includes a rotating controller  26  for controlling the revolution of the rotating device  15  having the X-ray tube  21  which rotates around the body axis of the subject HB, and a gantry controller  29  for transmitting and receiving control signals to and from the operating console  1  or the imaging table  10 . The X-ray controller  22  controls the X-ray tube current mA to the X-ray tube  21 . 
         [0033]    The X-ray beam forming filter  28  is a filter for increasing X-ray toward the center of rotation which is the center of imaging, and decreasing the amount of X-ray at the periphery. By using this, the exposure at the body surface of the subject HB which has a similar shape of circle or ellipse may be minimized. 
         [0034]    The central processing unit  3  includes a preprocessor  33 , an image reconstruction device  34 , a scan condition setting device  35 , and a scan controller  36 . 
         [0035]    The preprocessor  33  performs the preprocess such as the correction of the amount of X-ray, which corrects the ununiformity of the sensitivity between channels for the raw data acquired by the data acquisition system  25 , and corrects the decrease of the extreme signal intensity or the signal loss caused by an X-ray absorber more specifically a metal part, and also performs beam hardening processing. 
         [0036]    The image reconstruction device  34  receives the X-ray projection data which has been preprocessed by the preprocessor  33  to reconstruct an image based on the X-ray projection data thus received. The X-ray projection data undergoes the fast Fourier transform (FFT) for transforming to the frequency domain, then is multiplied by the reconstruction function and undergoes the inverted fast Fourier transform. In brief, the reconstruction function overlay process in the real space is performed. Then the image reconstruction device  34  performs the three dimensional backprojection processing to the projection data having the reconstruction function overlaid, to determine the tomographic image (x-y plane) for each direction of body axis (the direction of z axis) of the subject HB. The image reconstruction device  34  then stores the tomographic image in the storage device  7 . 
         [0037]    The scan condition setting device  35  indicates the scan condition of the helical shuttle scan in the predetermined range in the direction of body axis of the subject, for example, acceleration, deceleration, maximum speed, and the like of the cradle  12 , and the operator inputs from the input device  2  any necessary input in order to set the scan condition. The scan condition setting device  35  will be described in greater details later in the section of embodiment described below. 
         [0038]    The scan controller  36  will perform the control for implementing the helical shuttle scan as set by the scan condition. The scan controller  36  also will be described in greater details later in the section of embodiment described below. 
         [0039]    Now referring to  FIG. 2  there is shown a flow chart illustrating the overview of the operation of the X-ray CT apparatus  100 . 
         [0040]    In step P 1  the subject HB is carried on the cradle  12  to align the position. In this step the subject HB carried on the cradle  12  is registered to the reference mark of each part with the center position of the slice light of the scanning gantry  20 . Then the scout image is acquired. In the scout scan the X-ray tube  21  and the multi X-ray detector  24  are fixed immobile while the cradle  12  is rectilinearly moved to perform the data acquisition operation of the X-ray detector data. In this example the scout image are normally taken at the view angle positions of 0 degree and 90 degrees. The right side of  FIG. 2  shows a scout image SC taken at 0 degree around the chest. The imaging position of the tomographic image may be planned from the scout image SC. 
         [0041]    In step P 2  the position and size of the tomographic image to be taken is displayed on the scout image while at the same time the scan condition setting is performed. The dotted line indicated in the scout image indicates the position of the tomographic image. In the X-ray CT apparatus in accordance with this preferred embodiment, the helical shuttle scan may be conducted. The helical shuttle scan is a scan method in which the gantry rotating device  15  is rotated in a manner similar to the helical scan and the cradle  12  is accelerated or decelerated to reciprocate in the positive direction or in the negative direction of the z axis to acquire the X-ray projection data. In this step the scan schedule of the helical shuttle scan in the predetermined range in the direction of body axis of the subject is planned. In this preferred embodiment of the present invention, a plurality of scans including the conventional scan, helical scan and so on may also be performed. The conventional scan is a scan method which obtains the X-ray projection data by rotating the X-ray tube  21  and the multi X-ray detector  24  each time the cradle  12  is moved in the direction of z axis at predetermined intervals. The helical scan is a scan method which acquires the X-ray projection data by rotating the gantry rotating device  15  while at the same time the cradle  12  is moved at a constant speed. 
         [0042]    In the scan condition setting of the tomographic image the automatic exposure mechanism of the X-ray controller  22  can be used to optimize the amount of exposure to the subject HB. 
         [0043]    In step P 3  through step P 9 , the tomographic image is obtained. In step P 3 , the X-ray projection data is acquired. When the data acquisition is performed by the helical shuttle scan, the X-ray tube  21  and the multi X-ray detector  24  are rotated around the subject HB and the cradle  12  on the imaging table  10  is rectilinearly moved while data acquisition operation is performed for the X-ray detector data. Then, the positional information of the z axis coordinate Ztable (view) the positional information of the z axis coordinate Ztable (view) is added to the X-ray detector data D 0  (view, j, i) (j=1 to ROW, i=1 to CH) represented by the view angle view and the detector row number j and the channel number i. The positional information of the coordinate in z axis can be added at predetermined intervals such as every view or every couple of views of the X-ray projection data, by measuring the position in z direction coordinate by using an encoder in the imaging table  10 , and by transferring the measurement data to the data acquisition system  25 . Although the positional information of the z axis coordinate may be added to the X-ray projection data (X-ray detector data) as have been described above, the information may also be used as another file, which can be associated to the X-ray projection data. 
         [0044]    In step P 4  a preprocessing is performed on the X-ray detector data D 0  (view, j, i) to convert it to the X-ray projection data. More specifically, the offset correction is performed, the logarithm transform is performed, the correction of the amount of X-ray is performed, and then the sensitivity correction is performed. 
         [0045]    In step P 5  the preprocessor  33  performs the beam hardening correction. In this case, to the X-ray projection data D 1  (view, j, i) the beam hardening correction is performed. At this time because the beam hardening correction can be performed independently for every j rows of detectors, if the X-ray tube voltage of each gantry rotating device  15  is different in the scan condition, the difference of the characteristics of the X-ray energy for each row of detectors can be corrected. In this embodiment, the processing of the beam hardening correction is changed corresponding to the profile surface area of the subject HB, or oval ratio. 
         [0046]    In step P 6 , the image reconstruction device  34  performs a z filter overlay processing. In this case, the z filter overlay processing is performed for applying a filtering in the direction of z axis (in the direction of rows) for the X-ray projection data D 11  (view, j, i) which has been subjected to the beam hardening correction. More specifically, after the preprocess in each view angle, a filter having a filter size in the row direction such as 5 rows is applied to the projection data D 11  (view, j, i) which has been subjected to the beam hardening correction. 
         [0047]    In step P 7 , the image reconstruction device  34  performs the reconstruction function overlay processing. More specifically, a Fourier transform for transforming the X-ray projection data into frequency domain is performed, then the reconstruction function is multiplied, and finally the inverse Fourier transform is applied thereto. 
         [0048]    In step P 8 , the image reconstruction device  34  performs a three dimensional backprojection processing. In this example, the three dimensional backprojection processing is applied to the X-ray projection data D 3  (view, j, i) which has been applied to the reconstruction function overlay processing to determine the backprojection data D 3  (x, y, z). The image to be image reconstructed is a plane that is perpendicular to the z axis based on the positional information of the z axis coordinate. The reconstruction area herein below is assumed to be in parallel to the x-y plane. 
         [0049]    In step P 9 , the image reconstruction device  34  performs the postprocessing. To the backprojection data D 3  (x, y, z), such post processing as the image filter overlaying, the image space z filter, the CT value conversion will be applied to obtain a tomographic image. 
         [0050]    In step P 10 , the monitor  6  displays the tomographic image thus image reconstructed. As an example a tomographic image, TM is shown on the right side of  FIG. 2 . 
         [0051]    The scan control of the X-ray CT apparatus described above will be described in greater details herein below by way of example of various embodiments. 
         [0052]    In a first embodiment, a case will be described in greater details in which the operation control of the cradle  12  is performed with the priority given to the scan position at the time when the X-ray projection data is acquired for the helical shuttle scan. Now referring to  FIG. 3 , there is shown a schematic diagram illustrating the change of the scan speed with respect to the scan time t, the z coordinate position, and the current of the X-ray tube being used.  FIG. 4  shows a flow chart in accordance with the preferred embodiment of the present invention. 
         [0053]    In step H 1  shown in  FIG. 4 , the operator will take a scout image. 
         [0054]    In step H 2  the operator inputs the number of passes Pass of the X-ray projection data acquisition (where pass indicates the unit of scan that is comprised of one way scan) on the monitor  6  for the scan condition setting device  35 , and the scan range in z direction Range. The scan condition setting device  35  determines the scan time T at this time and displays it. The scan condition setting device  35  is capable of selecting and changing acceleration Accel, deceleration Decel, maximum speed MaxSpeed and so on of the data acquisition pass in one way direction. 
         [0055]    For example, a scout image SC as shown in  FIG. 5(   a ) may be displayed on the monitor  6 , such that the scan range “Range” [z 0 , z 1 ] in z direction, the number of passes “Pass” [N], the helical pitch “H-pitch” can be ready to be input on the scan condition setting display screen as is shown in  FIG. 5(   b ). The scan condition setting device  35  is capable of determining the estimated time for one pass t, the time of acceleration t 1 , the time of constant speed t 2 −t 1  as shown in  FIG. 3  based on the acceleration “Accel” [a] (mm/s2) as is predetermined, the deceleration “Decel” [−a] (mm/s2), and the maximum speed of the cradle  12  “MaxSpeed” [v 1 ] (mm/s). In addition, the scan time T can be given by the following equation (equation 1): 
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         [0056]    As can be seen the scan time T can be determined by the acceleration a of the cradle  12 , the deceleration −a, the maximum speed v 1  of the cradle  12 , the scan range in z direction [z 0 , z 1 ], and the number of passes N, and is displayed on the scan condition setting display screen. 
         [0057]    In step H 3 , the data acquisition device  5  acquires the X-ray projection data. The X-ray projection data can be appended with the positional data of z direction coordinate of the moving cradle  12  for every view or for every couple of views. Alternatively, the data acquisition device  5  may also store the positional data file as a separate file. 
         [0058]    In step H 4 , the image reconstruction device  34  will image reconstruct the X-ray projection data thus acquired. The image reconstruction device  34  uses the X-ray projection data and the positional data of z direction coordinate to perform the three dimensional image reconstruction processing for each of the range of acceleration of the cradle  12 , the range of constant speed, and the range of deceleration, respectively, to perform the image reconstruction of the tomographic image which is sequential in the z direction. 
         [0059]    In the three dimensional image reconstruction processing of the helical shuttle scan, the amount of helical pitch varies which advances by one rotation of the X-ray projection data acquisition system for each time during acceleration. However, the X-ray detector channel data of the multi X-ray detector  24  corresponding to each pixel in the image reconstruction plane or the X-ray detector channel data at the vicinity of the point corresponding to each pixel in the image reconstruction plane may be weighted added processed to perform the three dimensional backprojection processing for each pixel in the image reconstruction plane, based on the X-ray beam from the X-ray focal point of the X-ray tube  21 . 
         [0060]    In addition, in the three dimensional backprojection processing of the helical shuttle scan, the image reconstruction plane and the distance of the X-ray focal point vary at a constant speed within the range of the constant speed range, however the image reconstruction plane and the distance of the X-ray focal point will vary in the range of acceleration and in the range of deceleration, which ranges are characteristic to the helical shuttle scan. 
         [0061]    In step H 5 , the monitor  6  displays a tomographic image. 
         [0062]    In step H 6 , the gantry controller  29  determines whether or not the acquisition of the X-ray projection data has been done in the range of specified z direction, and if YES then the process proceeds to step H 7 , and if NO then the process go back to step H 3 . For example as shown in  FIG. 3 , even when the time at which the acquisition of the X-ray projection data has been completed in the range in z direction [z 0 , z 1 ] which is the first outward route, is the completed time which is shorter by Δta than the estimated time t, namely t 3 =t−Δta, the process proceeds to step H 7 . 
         [0063]    There are cases in which the time Δta may be shorter or longer than the estimated time t. It varies due to the weight of the subject to be carried on the cradle  12 , the position in z direction coordinate of the cradle  12 , or the characteristics of each cradle  12 . In any cases the gantry controller  29  will reverse the direction to start the acquisition of the X-ray projection data in the next one way direction at the time at which the X-ray projection data has been acquired of the range [z 0 , z 1 ] in z direction specified at the time of scan condition setting. In other words the gantry controller  29  is controlled with priority on the position. The X-ray output Output will therefore continue to be always turned on. 
         [0064]    In step H 7 , the gantry controller  29  determines whether or not there is a next acquisition pass of the X-ray projection data, and if YES then the process proceeds to step H 3 , otherwise if NO then the step proceeds to step  118 . In this case also similar to the case as have been described above, if there is a homeward route of the acquisition of the X-ray projection data even if the estimated time t has not elapsed, then the process will go back to step H 3  without waiting for the time, and perform a next acquisition of the X-ray projection data. 
         [0065]    In step H 8 , the monitor  6  performs a cine display of the tomographic images consecutive in z direction, or the three dimensional images in a manner of time series. For example, the monitor  6  performs a cine display by using the three dimensional images consecutive in z direction as four dimensional images for the number of acquisition passes of the X-ray projection data to display by using MIP (maximum intensity projection) display at a constant interval, or by volume rendering (VR) image. 
         [0066]    Next, another case will be described in greater details in which the acquisition range of the X-ray projection data in each pass is misaligned. For example, in  FIG. 6(   a ), by iteratively repeating the acquisition range of the X-ray projection data L 1 =z 1 −z 0 , although it is intended to acquire the X-ray projection data of L 1 , there exists actually the overshoot by Δz in the outward route. However in the homeward route, the acquisition range will become L 2  if there is an overshoot of Δz, and consequently there are risks to accumulate the mismatches in z direction. 
         [0067]    To correct this misalignment, the gantry controller  29  is capable of correcting the scan position at the time when it has terminated the acquisition range of a one side. For example, the gantry controller  29  may perform the positional correction control RP at the time when the acquisition pass of the X-ray projection data in the direction of one way as shown in  FIG. 6(   b ). Even when the acquisition pass of the X-ray projection data misaligns for each reciprocating by Δz as shown, the tomographic image may be reconstructed correctly and accurately by using the positional information in z direction, which information is appended to every view or every couple of views. 
         [0068]    In this manner the gantry controller  29  may perform the accurate control in a plurality of numbers of passes by prioritizing the position in z direction, and by controlling the position alignment at the time of completion of both outward and homeward routes with respect to the range in z direction [z 0 , z 1 ] specified at the time of scan condition setting. 
         [0069]    In addition, as shown in  FIG. 7(   a ), in case in which the movable range of the gantry rotating device relative to the cradle  12  is assumed to be L, and the X-ray aperture at the center of rotation is assumed to be 1), the range D/2 at both ends of the movable range L will be the range of X-ray exposure NonA which is never used. On the other hand, as shown inn  FIG. 7(   b ), by performing imaging of conventional scan at both ends of the movable range L of the gantry rotating device, as referred to as stationary scan cScan, the range capable of image reconstruction will be L+D. More specifically, the range of image reconstruction UseA will be capable of image reconstruction to the end of range D/2 of both sides of the movable range L, which in turn will improve the efficiency of use of the X-ray. The conventional scan which is as the stationary scan cScan will rotate by approximately 180 to 360 degrees at the scan position. 
         [0070]    Now referring to  FIG. 8 , there is shown a schematic diagram illustrating the control of moving speed v(t) of the cradle  12  of the helical shuttle scan, the position z(t) of the cradle  12 , the output timing of X-ray, and the X-ray tube current A(z(t)) by the X-ray controller  22  in case of the position prioritized control with the stationary scan cScan. As can be seen from the figure, the helical shuttle scan is capable of scanning by the position prioritized control with the stationary scan cScan. 
         [0071]    Also the X-ray tube current A(z(t)) will be almost “0” or will be “0” at the ends z 0 , z 1  of the movable range L in case in which the stationary scan cScan of  FIG. 3  is not performed. However the X-ray tube current when the stationary scan cScan of  FIG. 8  is configured will not be “ 0 ” at both ends z 0 , z 1  of the movable range L, so that the X-ray controller  22  will perform the stationary scan cScan by decreasing the X-ray tube current value to mA 1 . 
         [0072]    As can be appreciated from the foregoing description the helical shuttle scan in case of the position prioritized control as shown in this embodiment acquires the X-ray projection data in the pass of outward direction by prioritizing the coordinate position in z direction. The gantry controller  29  may improve the iterative accuracy of reciprocating by performing the position alignment control at both ends of the movable range. Furthermore the gantry controller  29  may effectively use the X-ray by providing the stationary scan cScan at both ends of the movable range. 
         [0073]    In a second embodiment a case is shown in which the operation of the helical shuttle scan is controlled with the time prioritized control. Now referring to  FIG. 9  there is shown a schematic diagram illustrating the changes of the scan speed with respect to the scan time t, the position of the z coordinate, and the X-ray tube current being used.  FIG. 10  shows a flow chart in accordance with the present embodiment. 
         [0074]    In step H 21 , the operator will take a scout image. 
         [0075]    In step H 22 , the operator inputs the scan time T and the scan range Range on the scan condition setting display screen. The scan condition setting device  35  at this time determines and displays the number of passes of the acquisition of the X-ray projection data from the input data. 
         [0076]    For example, the monitor  6  displays the scout image SC as shown in  FIG. 5(   a ) on the scan condition setting display screen. The operator will review the scout image while at the same time he or she inputs the scan time T, the scan range in z direction “Range” [z 0 , z 1 ], the helical pitch “H-pitch” from the display screen for prioritizing the input of the scan time as shown in  FIG. 5(   c ). The scan condition setting device  35  at this time is capable of determining, the estimated time t of one pass for one way direction, the time of acceleration t 1 , and the time of constant speed t 2 −t 1  as shown in  FIG. 9 , based on the acceleration “Accel” [a] (mm/s2} and the deceleration “Decel” [−a] (mm/s2) of the cradle  12  previously determined, and the maximum speed “MaxSpeed” [v 1 ] (mm/s) of the cradle  12 . 
         [0077]    Also, the scan time T should be a multiple number of the estimate time t for one pass which is one way direction. Due to this, the scan condition setting device  35  will adjust the number of passes so as to be always an integer. 
         [0078]    In step H 23 , the data acquisition device  5  will perform the acquisition of the X-ray projection data. For example, the speed v(t) of the cradle  12  will be accelerated at a constant acceleration a, and accelerated till the maximum speed v 1 , the acquisition of the X-ray projection data at the constant speed is performed from the time t 1 , the speed is decelerated with the deceleration −a from the time t 2  and finally the speed will be 0 at the time t 3 . The data acquisition device  5  is capable of appending to the X-ray projection data the scan position information for every view or for every couple of views. Further, the data acquisition device  5  may store the scan position data file as a separate file. 
         [0079]    The estimated time t of one pass at this time should be set slightly longer than the calculated time of one pass determined by the scan condition setting device  35 . The estimated time t is set to a scan time in which the scan always fits therewithin by taking into account the scan position of the cradle  12 , the difference of the characteristics, or the difference of the weight of the subject HB. The estimated time t should be set such that the scan time may leave a remainder for one pass by Δta, Δtb, Δtc and Δtd. Therefore the estimated time t will be one pass time which always terminates. The scan time T will become N·t if the number of passes is assumed to be N, and the accurate time can be thus determined. In the waiting time Δta, Δtb, Δtc, and Δtd, it is desirable that the X-ray output be turned off so as to suppress the futile exposure. 
         [0080]    In step H 24 , the image reconstruction device  34  will perform the image reconstruction of the X-ray projection data. The image reconstruction device  34  reads the X-ray projection data with the positional information in z direction being appended, or the positional information in z direction in a separate file, thereby to allow generating a tomographic image at the accurate scan position. 
         [0081]    In step H 25 , the monitor  6  displays the tomographic image having been image reconstructed. 
         [0082]    In step H 26 , the gantry controller  29  determines whether or not the X-ray projection data has been acquired in the scan range. If YES then the process proceeds to step H 27 , and if NO then the step goes back to step H 23 . 
         [0083]    In step H 27 , the gantry controller  29  determines whether or not there is a pass of one way direction for the acquisition of the X-ray projection data, if YES then the process proceeds to step  1128 , if otherwise NO then the process proceeds to step H 29 . 
         [0084]    In step H 28 , the gantry controller  29  waits until the estimate time t. Thereafter the process will go back to step H 23 . For example the gantry controller  29  is already reached to z 1  of the scan range at the time t 3  as shown in  FIG. 9 , the control will be such that the table movement and the X-ray output will be ceased to wait until the estimated time t. This waiting time is Δta, Δtb, Δtc, and Δtd. 
         [0085]    In step H 29 , the monitor  6  performs a cine display in time series of the tomographic images consecutive in z direction or of the three dimensional image. The cine display of a three dimensional image is performed as shown similar in the previous first preferred embodiment, by using the MIP or the volume rendering image at a constant interval. In a manner similar to the previous first preferred embodiment, since there are risks that the acquisition range of the X-ray projection data may be misaligned for each pass, the gantry controller  29  will perform the positional correction control RP at the time when the scan range in one way direction has been terminated. 
         [0086]    As can be appreciated from the foregoing description, in the helical shuttle scan in the time prioritized control, the estimated time t which is the pass time of the acquisition of the X-ray projection data will be correct, the scan can be served to the contrast imaging in which the change with time should be observed. 
         [0087]    In the second embodiment as similar to the first preferred embodiment, a stationary scan cScan which is a conventional scan of the revolution by 180 to 360 degrees may be set as shown in  FIG. 11 . Also in this case it is possible to turn off the X-ray output during the waiting time Δta, Δtb, Δtc and Δtd. Therefore the helical shuttle scan in the time prioritized control may more effectively use the X-ray by providing a stationary scan cScan at both ends of the acquisition pass of the X-ray projection data of one way direction to widen the range in z direction of image reconstruction of the tomographic image to the direction of both ends. 
         [0088]    In a third embodiment, an example will be described in greater details in which the X-ray automatic exposure control is performed with the image quality index value being set at the time of scan condition setting. First, the case in which the X-ray automatic exposure control is performed with the position prioritized control described above in the first preferred embodiment will be described in greater details herein below. 
         [0089]    Now referring to  FIG. 12 , there is a schematic diagram illustrating the X-ray automatic exposure control is performed in a helical shuttle scan of the position prioritized control, with the stationary scan cScan being provided. The symbol z(t) in the figure indicates the position of the cradle  12  at each moment (time), v(t) indicates the moving speed, mA(z) indicates the X-ray tube current at the time of turning on and off the X-ray output. 
         [0090]    When the operator introduces the X-ray automatic exposure control, the image quality index value is added on the scan condition setting display screen shown in step  112  of the first preferred embodiment. In this case the operator inputs the number of passes of the acquisition of the X-ray projection data, the range of scan, and the image quality index value on the display screen of the scan condition setting. The scan condition setting device  35  at this time will determine the geometric feature amount from the X-ray projection data of the scout image or from the X-ray profile at the coordinate position in z direction of the scout image SC to determine and set the optimum X-ray tube current so as to be the set image quality index value, to display as numerical value or as a graph. The scan condition setting device  35  also determines and displays the time in this session. 
         [0091]    The geometric feature amount uses for example the surface area of the projection data profile or the oval ratio. The setting of the X-ray tube current will be such that, when the X-ray tube current at each coordinate position in z direction of the helical shuttle scan is assumed to be mA(z), a value of the X-ray tube current is to be set by taking into account the helical pitch HP(z) in the position in z direction, so that the following (equation 2) is constant const. 
         [0000]    
       
         
           
             
               
                 
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         [0092]    In particular, in the range of accelerating speed and decelerating speed of the helical shuttle scan, because the helical pitch HP(z) in each coordinate position in z direction continuously changes, the control of the X-ray tube current value mA(z) is important. 
         [0093]    Within the range where the helical pitch HP(z) is less than 1, a three dimensional image reconstruction is to be conducted by using the X-ray projection data of more than one rotation. Because of this the S/N in this over-scan range will be improved, it is necessary that (equation 2) should be added with the consideration of the over-scan, when the three dimension image reconstruction processing uses the X-ray projection data of r rotations. As an example in which the over-scan is considered, (equation 3) is constant const. 
         [0000]    
       
         
           
             
               
                 
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         [0094]    Here the X-ray of the set X-ray tube current value is accurately emitted by aligning the added coordinate in z direction with the coordinate position in z direction having the X-ray tube current set. 
         [0095]    In the above example, the X-ray automatic exposure control is added to the helical shuttle scan of the first preferred embodiment, however the X-ray automatic exposure control may also be added similarly in the helical shuttle scan of the time prioritized control shown in the second preferred embodiment.  FIG. 13  shows a schematic diagram illustrating the X-ray automatic exposure control in the helical shuttle scan of the time prioritized control when the stationary scan cScan is further provided. 
         [0096]    The scan condition setting device  35  in this case will set a value of the X-ray tube current mA(t) at the moment (time), instead of performing the control of the X-ray tube current value at the position in z direction. The X-ray controller  22  adds the X-ray automatic exposure control to the time prioritized control so as to achieve the optimum X-ray control at each time moment in the helical shuttle scan. The method of determining a value of the X-ray tube current mA(t) at the position in z direction may be allowed to determine by changing the value of the X-ray tube current mA(z) as have been described above to the value of the X-ray tube current mA(t) at each time. 
         [0097]    In a fourth embodiment, the gantry controller  29  performs the positional correction control RP in the first embodiment or in the second embodiment each time a pass terminates, however a misalignment may occur when the scan range is wider. Because of this the gantry controller  29  splits the scan range into a given number of fractions, and performs correction at each scan position so as to obtain a tomographic image without the positional misalignment or the time deviation in a helical shuttle scan of a wider range. 
         [0098]    For example, in the helical shuttle scan with the positional prioritize control, the gantry controller  29  splits the scan range in one way direction into M fractions (where M is a natural number), and the positional alignment is performed for each position in z direction thus split so as to prevent the positional misalignment with respect to the value of the X-ray tube current even when the X-ray automatic exposure control is performed. Therefore, by conducting the scan with an optimum value of the X-ray tube current for the tomographic image at the position in z direction, a tomographic image is generated which is complied with the noise index value which is the image quality index value, in order to obtain a uniform image quality in z direction. 
         [0099]    Also, the gantry controller  29  may adjust by accelerating the speed of the imaging table if there is a positional misalignment and the scan is achieved behind the estimated time of arrival, so as to match at the next check point. In a similar manner to this, if the scan is earlier arrived, the gantry controller  29  may adjust by decelerating the speed of the imaging table. 
         [0100]    Next, in the helical shuttle scan of the time prioritized control, the gantry controller  29  splits the estimated time t in one way direction into M fractions (where M is a natural number) and the positional alignment is performed for each split fraction so as to prevent the misalignment from occurring. If there is a misalignment in the split estimated time, the adjustment is done by accelerating or decelerating the speed of the imaging table in a similar manner described above. 
         [0101]    Although the X-ray CT apparatus  100  in the above embodiments has been described by way of example of the operation of the cradle  12 , the operation may be processed in a similar manner when the imaging table  10  is movable. 
         [0102]    It should be noted here that the present invention is not considered to be limited to the embodiments as have been described above. Although in the embodiments described above, the scanning gantry  20  is not inclined, a similar effect can be achieved in the case of a tilted scan where the scanning gantry  20  is inclined. Although in the embodiments as have been described above, the acquisition of the X-ray projection data is not synchronized with the biological signal, a similar effect can be achieved by synchronizing to a biological signal in particular to the heart-rate signal. 
         [0103]    Furthermore although in the embodiments described above a multi X-ray detector has been described, a flat-panel X-ray detector, or a single row X-ray detector may also be equally used. In this embodiment the helical shuttle scan is achieved by moving the cradle  12  of the imaging table  10  in z direction. However a similar effect can be achieved, relatively by moving the rotating device  15  in the scanning gantry  20  or the scanning gantry  20  itself with respect to the cradle  12 . 
         [0104]    The three dimensional reconstruction method may be any of the three dimensional image reconstruction method by the known Feldkamp method, or may be another method, or may be a two dimensional image reconstruction method.