Abstract:
A system and method for equally splitting the current supplied to parallel connected strings of LEDs. The system and method includes a current splitting circuit such as a mirror circuit that divides the current substantially equally between two or more parallel connected strings of LEDs. The current splitting circuit ensures that illumination levels of the strings of LEDs are uniform without requiring the strings of LEDs to be binned. The current splitting circuit also allows the strings of LEDs to be dimmed in both pulse width modulation (PWM) and continuous modes.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    The present application relates to the field of radiograph CT, and more particularly, to a CT collimator having a single motor drive system and a radiograph CT system having the CT collimator. 
         [0002]    At present, radiograph CT systems such as X-ray CT system are widely used in various medical institutions for three-dimensional imaging of the regions of interest of the subjects to assist the clinicians to achieve an accurate medical diagnosis of the subjects. 
         [0003]    In a radiograph CT system, a radiation tube generating cone-shaped radiation beams and a detector detecting the radiation beams rotate around a rotation center, wherein the detector is disposed opposite to the radiation tube and consists of detector elements arranged in a matrix form. Projection data generated by the radiation beams transmitting through the subject are collected; based on the collected projection data, an image of the region of interest of the subject is reconstructed; and then the reconstructed CT image is displayed on an image display device. 
         [0004]    In a radiograph CT system, a collimator is generally provided between the radiation tube and the subject to be detected. By adjusting a width of the aperture of the collimator, the width of the radiation beams in a direction parallel to the subject is controlled so as to control a thickness of the scan. 
         [0005]    A conventional collimator generally has at least two different motor drive systems to meet the requirements of multi-slot opening and Z-beam tracking Such a collimator comprises at least two gates or cams, which are driven by at least two different motor drive systems, and hence have higher cost. 
         [0006]    Some newly developed collimators use a single motor drive system to meet the requirements of slot opening and Z-beam tracking For example, a recently developed collimator comprises a plate having a plurality of slots driven by a single motor drive system. Each slot corresponds to a collimator aperture of a different width. Though using a single motor drive system in place of the conventional two drive systems to reduce the cost of the drive system, such a collimator requires converting the rotational motion of the motor into a linear motion and hence the need of such components as lead screw and rails. Therefore, there is a need for a CT collimator and a CT system that, in case of a focus shift of the radiation source due to temperature changes during a CT scan, can automatically correct the position of the collimator aperture and enable the radiation beams to be irradiated to the subject via the collimator directly in a rotational movement manner without departing from the predetermined region of interest so that the detection area of the radiation beams projected to the detector after passing through the subject remains unchanged. 
       SUMMARY OF THE INVENTION 
       [0007]    Embodiments of the present invention provide a CT collimator and a CT system comprising the CT collimator capable of solving the above problems. 
         [0008]    According to a first aspect of the present invention, there is provided a CT collimator. The CT collimator comprises a rotating slot part disposed on a rotation shaft and having a plurality of blades, wherein each blade has a slot of a different width and a radiation beam entering the collimator can only pass via a slot in one of the plurality of blades, wherein an edge of each blade slot along a longitudinal direction of this blade has a convex curved surface structure, and in a vertical plane along a longitudinal direction of each blade slot, the two side edges of the slot are curved, and wherein each blade is arranged to be eccentric to the center of the rotation shaft. 
         [0009]    The CT collimator according to an embodiment of the present invention further comprises a single motor configured to drive the rotating slot part to rotate around the rotation shaft; and an encoder for monitoring an angle of rotation of the rotating slot part around the center of the rotation shaft. 
         [0010]    The CT collimator according to an embodiment of the present invention further comprises a single motor configured to drive the rotating slot part to rotate around the rotation shaft, wherein the single motor is provided with an encoder for monitoring an angle of rotation of the rotating slot part around the center of the rotation shaft. 
         [0011]    In the CT collimator according to an embodiment of the present invention, the curved surface structure of each blade edge comprises two curved lines in a vertical plane along the longitudinal direction of this blade, and a blade slot of this blade allows radiation beams between radiation lines tangent to the two curved lines to pass through. 
         [0012]    In the CT collimator according to the an embodiment of the present invention, the curved surface structure of each blade edge comprises two circular arcs in a vertical plane along the longitudinal direction of this blade, and a blade slot of this blade allows radiation beams between radiation lines tangent to the two circular arcs to pass through. 
         [0013]    In the CT collimator according to an embodiment of the present invention, two circles for the two circular arcs are respectively: when, in the vertical plane, the rotation of a first connecting line and a second connecting line between a maximum shift position to the left of a radiation source and left and right edge points of a radiation detection area of a radiation detector around the center of the rotation shaft relative to a third connecting line and a fourth connecting line between a maximum shift position to the right of the radiation source and the left and right edge points of the radiation detection area reaches a position where there are respective intersections in a blade thickness region between the first connecting line and the third connecting line and between the second connecting line and the fourth connecting line, a first circle that is tangent to the first connecting line and the third connecting line at said position, and a second circle that is tangent to the second connecting line and the fourth connecting line at said position. 
         [0014]    In the CT collimator according to an embodiment of the present invention, each blade has a planar structure, and a width of a slot of each blade gradually increases from the center of the slot to the two ends along the longitudinal direction of the blade. 
         [0015]    In the CT collimator according to an embodiment of the present invention, when each blade has an arc structure whose center of circle is on a focal point of a radiation source outside the collimator when this blade is located in the horizontal position. 
         [0016]    In the CT collimator according to an embodiment of the present invention, when the center of the rotation shaft is not located in an extended region of each blade slot along the thickness direction of the blade, said blade is eccentric to the center of the rotation shaft. 
         [0017]    According to an embodiment of the present invention, there is provided a CT system. The CT system comprises a CT collimator according to the first aspect of the present invention, a radiation detection area monitoring unit disposed on a radiation detector, and a collimator controller that selects one of a plurality of blades of the rotating slot part according to a region of interest of a subject to allow a desired radiation beam to be projected to the region of interest of the subject, wherein the radiation detection area monitoring unit monitors, during a CT scan, an offset of the radiation detection area on the radiation detector caused by focus shift of the radiation source as radiation beam is projected to the radiation detector via the selected blade of the CT collimator, and wherein the collimator controller is configured to correct an angle of rotation of the rotating slot part of the CT collimator according to the monitored offset received from the radiation detection area monitoring unit to eliminate the offset of the radiation detection area caused by the focus shift of the radiation source for performing Z-beam tracking 
         [0018]    The CT system according to an embodiment of the present invention is an X-ray CT system. 
         [0019]    In the CT system according to an embodiment of the present invention, the collimator controller comprises a memory or is coupled to a memory. 
         [0020]    In the CT system according to an embodiment of the present invention, a plurality of offsets of the radiation detection area predetermined for each blade in the rotating slot part and a plurality of corresponding correction angles that the rotating slot part is required to rotate for performing Z-beam tracking are stored in the form of a table in the memory. 
         [0021]    In the CT system according to an embodiment of the present invention, the collimator controller is configured to: determine a rotation correction angle of the rotating slot part for the selected blade through a search in said table in said memory according to the monitored offset of the radiation detection area, and perform Z-beam tracking according to the determined rotation correction angle of the rotating slot part and the current angle of the selected blade. 
         [0022]    In the CT system according to an embodiment of the present invention, the collimator controller is configured to: in case of failure to find a corresponding rotation correction angle of the rotating slot part in said table according to the monitored offset of the radiation detection area, search for two rotation correction angles corresponding to two offsets close to the monitored offset of the radiation detection area, and use an average of the two rotation correction angles or an interpolation therebetween as the determined rotation correction angle of the rotating slot part; or in case of failure to find a corresponding rotation correction angle of the rotating slot part in said table according to the monitored offset of the radiation detection area, search for a rotation correction angle corresponding to the shift closest to the monitored offset of the radiation detection area and use it as the determined rotation correction angle of the rotating slot part. 
         [0023]    In the CT system according to an embodiment of the present invention, a plurality of focus shifts of the radiation source predetermined for each blade in the rotating slot part and a plurality of rotation correction angles of the rotating slot part required for Z-beam tracking are stored in the form of a table in the memory. 
         [0024]    In the CT system according to an embodiment of the present invention, the collimator controller is configured to: determine a focus shift of the radiation source according to the monitored offset of the radiation detection area, determine a rotation correction angle of the rotating slot part for the selected blade through a search in said table in said memory according to the determined focus shift, and perform Z-beam tracking according to the determined rotation correction angle of the rotating slot part and the current angle of the selected blade. 
         [0025]    In the CT system according to an embodiment of the present invention, the collimator controller is configured to: in case of failure to find a corresponding rotation correction angle of the rotating slot part in said table according to the determined focus shift, search for two rotation correction angles corresponding to two focus shifts close to the determined focus shift, and use an average of the two rotation correction angles or an interpolation therebetween as the determined rotation correction angle of the rotating slot part; or in case of failure to find a corresponding rotation correction angle of the rotating slot part in said table according to the determined focus shift, search for a rotation correction angle corresponding to the focus shift closest to the determined focus shift and use it as the determined rotation correction angle of the rotating slot part. 
         [0026]    In the CT system according to an embodiment of the present invention, the collimator controller is further configured to, after correction of a rotation angle of the rotating slot part of the CT collimator, compare a latest monitored offset of the radiation detection area received from the radiation detection area monitoring unit with a predetermined threshold, and if the latest monitored offset of the radiation detection area does not exceed the predetermined threshold, then stop the Z-beam tracking; or if the latest monitored offset of the radiation detection area exceeds the predetermined threshold, then perform a new Z-beam tracking until the latest monitored offset of the radiation detection area does not exceed the predetermined threshold. 
         [0027]    In the CT collimator and CT system comprising said CT collimator according to embodiments of the present invention, a plurality of blades having variable slot widths can be provided in the CT collimator according to the needs of a CT scan. An edge of each blade slot along a longitudinal direction of the blade has a convex curved surface structure (namely, in a vertical plane along a longitudinal direction of the blade slot, the two side edges of the slot are curved) so that when a focus shift of the radiation source along a focus shift path occurs as a result of temperature change, by rotating the selected blade about the rotation center eccentric to the blade a correction angle corresponding to the focus shift, the radiation beams reaching the radiation detector via the blade slot are maintained at the same region as the circumstance where focus shift does not occur. Therefore, the CT collimator and CT system according to an embodiment of the present invention eliminate the need to relocate other components such as the radiation detector or the subject when a focus shift of the radiation source occurs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    In the following some exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which like or similar elements are denoted by the same reference numerals, wherein: 
           [0029]      FIGS. 1A and 1B  show a radiograph CT system according to an exemplary embodiment of the present invention; 
           [0030]      FIG. 2  shows a CT collimator according to an exemplary embodiment of the present invention; 
           [0031]      FIGS. 3A ,  3 B,  3 C, and  3 D show an aperture assembly in a CT collimator according to an exemplary embodiment of the present invention; 
           [0032]      FIGS. 4A ,  4 B,  4 C, and  4 D illustrate a method for determining an inner circular arc and an outer circular arc of a blade slot in an aperture assembly in a CT collimator according to an exemplary embodiment of the present invention; 
           [0033]      FIG. 5  shows an offset of the X-ray detection area on the X-ray detector when focus shift of the X-ray tube occurs and no Z-beam tracking is employed in the collimator; and 
           [0034]      FIGS. 6A ,  6 B,  6 C, and  6 D show Z-beam tracking according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    In the following detailed description, exemplary embodiments of the present invention are described with reference to the accompanying drawings. However, it will be appreciated by persons skilled in the art that the present invention is not limited to these exemplary embodiments. 
         [0036]      FIGS. 1A and 1B  show a radiograph CT system  100  according to an exemplary embodiment of the present invention. In an embodiment, the radiograph CT system  100  is an X-ray CT system. 
         [0037]    As shown in  FIGS. 1A and 1B , the X-ray CT system  100  mainly includes three parts: a scan gantry  110 , a support table  116  for positioning a subject  114  to be detected, and an operation console  130 . The scan gantry  110  includes an X-ray tube  102 . X-rays  106  emitted from the X-ray tube  102  pass through a collimator  104  to form an X-ray beam of such shapes as fan shaped beam and cone shaped beam, to be irradiated to a region of interest of the subject  114 . The X-ray beam that passes through the subject  114  is applied to an X-ray detector  112  disposed on the other side of the subject  114 . The X-ray detector  112  has a plurality of two-dimensional X-ray detecting elements in the propagation direction (the signal channel direction) and the thickness Z direction (column direction) of the fan-shaped X-ray beam. 
         [0038]    A data acquisition system (DAS)  124  is coupled to the X-ray detector  112 . The data acquisition system  124  collects the data detected by each of the X-ray detecting elements of the X-ray detector  124  for using as the projection data. The X-ray radiation from the X-ray tube  102  is controlled by an X-ray controller  122 . In  FIG. 1B , the connections between the X-ray tube  102  and the X-ray controller  122  are not shown. 
         [0039]    The data acquisition system  124  collects data related to the tube voltage and tube current applied to the X-ray tube  102  by the X-ray controller  122 . In  FIG. 1B , the connections between the X-ray controller  122  and the data acquisition system  124  are omitted. 
         [0040]    The collimator  104  is controlled by a collimator controller  120 . In an embodiment, the collimator  104  and the collimator controller  120  are two separate components. In an embodiment, the collimator controller  120  may be disposed within the collimator  104 . In  FIG. 1B , the connections between the collimator  104  and the collimator controller  120  are omitted 
         [0041]    Components like the X-ray tube  102 , the collimator  104 , the X-ray detector  112 , the data acquisition system  124 , the X-ray controller  122  and the collimator controller  120  are mounted in a rotating portion  128  of the scan gantry  110 . The rotating portion  128  rotates under the control of a rotation controller  126 . In  FIG. 1B , the connections between the rotating portion  128  and the rotation controller  126  are not shown. 
         [0042]    Under the action of a drive system such as a motor, the support table  116  can be moved together with the subject  114  carried thereon along a longitudinal axis  118  of the subject into an opening  108  of the scan gantry  110 , so that the region of interest of the subject  114  is substantially perpendicular to the X-ray beam irradiated thereon through the collimator  104 . 
         [0043]    The operation console  130  has a central processor  136  such as a computer. A control interface  140  is connected to the central processor  136 . The scan gantry  110  and the support table  116  are connected to the control interface  140 . The central processor  136  controls the scan gantry  110  and the support table  116  via the control interface  140 . 
         [0044]    The data acquisition system  124 , the X-ray controller  122 , the collimator controller  120  and the rotation controller  126  in the scan gantry  110  are controlled via the control interface  140 . In  FIG. 1B  the separate connections between the relevant parts and the control interface  140  are not shown. 
         [0045]    A data acquisition buffer  138  is connected to the central processor  136 . The data acquisition system  124  of the scan gantry  110  is connected to the data acquisition buffer  138 . Projection data collected by the data acquisition system  124  are inputted to the central processor  136  via the data acquisition buffer  138 . 
         [0046]    The central processor  136  uses the projection data inputted from the data acquisition buffer  138  to perform an image reconstruction. In performing image reconstruction, such methods as the filtered back projection method, and three-dimensional image reconstruction method can be used. A storage device  142  is connected to the central processor  136 . The storage device  142  may be used to store data, reconstructed images and procedures for implementing the various functions of the X-ray CT system  100 . 
         [0047]    A display device  132  and an input device  134  are connected to the central processor  136 , respectively. The display device  132  displays the reconstructed images and other information output from the central processor  136 . An operator can input various instructions and parameters to the central processor  136  via the input device  134 . Through the display device  132  and the input device  134 , the operator can achieve an interactive operation of the X-ray CT system  100 . 
         [0048]      FIG. 2  shows a schematic structural diagram of a radiograph CT collimator  104  according to an exemplary embodiment of the present invention. As shown in  FIG. 2 , the collimator  104  includes four main parts: a collimator case  203 , a collimator cover  202 , a filter assembly  201  and an aperture assembly  204 . The aperture assembly  204  selects blades of different slot widths to allow the desired X-ray beam to reach the X-ray detector  112 . The filter assembly  201  filters the X-ray beams from the X-ray tube  104  to eliminate the scattered X-ray beams. The collimator case  203  is used for supporting, fixing and housing various components of the collimator  104 . The collimator cover  202  provides shielding for the collimator  104 . 
         [0049]      FIGS. 3A ,  3 B,  3 C, and  3 D show an exemplary structure of the aperture assembly  204  shown in  FIG. 2 . As shown in  FIG. 3A , the aperture assembly  204  mainly includes three parts: a rotating slot part  2041 , a single motor drive system  2043  driving the rotation of the rotating slot part  2041 , and an encoder  2042  detecting the rotation angle of the rotating slot part  2041 . In an embodiment, the motor drive system  2043  and the encoder  2042  are two separate parts. In an embodiment, the encoder  2042  may be disposed within the motor drive system  2043 . The rotating slot part  2041  and the encoder  2042  can rotate together when driven by the motor drive system  2043 . 
         [0050]    Since the rotating slot part  2041  is directly driven by the motor drive system  2043 , no rails, ball screw or lead screw are needed to convert the rotational motion into a linear motion, thereby simplifying the mechanical structure of the drive system of the collimator  104 . 
         [0051]      FIG. 3B  shows an exemplary structure of the rotating slot part  2041  shown in  FIG. 3A , and provides a cross-sectional view of the rotating slot part  2041  in a direction perpendicular to a longitudinal direction of the rotating slot part. As shown in  FIG. 3B , the rotating slot part  2041  includes a rotation shaft  2044  and a plurality of blades  2045  fixed to the rotation shaft  2044  and rotating together with the rotation shaft  2044 . Each blade has a slot of a different width and is provided with a shielding material to block the undesired X-ray beam entering the collimator  104 , so that the X-ray beam can only pass via the slot in the blade for being irradiated to the region of interest of the subject  114 . 
         [0052]    The rotating slot part  2041  as shown in  FIG. 3B  has four blades and therefore four different slot widths. The four blades may be disposed around the rotation shaft  2044  in an evenly spaced or unevenly spaced manner. The number of blades on the rotating slot part  2041  can be determined according to actual needs, and can be set to, for example, 2, 3, 4, 5, etc. 
         [0053]    In an embodiment, each blade has a planar structure, and as shown in  FIG. 3C , the width of the slot of each blade gradually increases from the center of the slot to the two ends along the longitudinal direction of the blade. In an embodiment, each blade and the blade slot have an arc structure whose center of circle, like the arc structure of the X-ray detector  112  disposed on the other side of the subject  114 , is on the focal point of the X-ray tube  102 . 
         [0054]    The two edges of each blade slot has a convex curved surface structure along the longitudinal direction of the blade. As shown in  FIG. 3D , the edges of the slot of each blade  2045  has a curved shape in the cross sectional view along a thickness direction perpendicular to the longitudinal direction of the blade. Each blade is disposed to be eccentric to the rotation center of the rotating slot part  2041 , so that the blade rotation center is not in an extended region of the slot along the thickness direction of the blade. As described below, by configuring the edges of each blade blot as having a convex curved surface structure, and allowing each blade to be eccentric to the rotation center of the rotating slot part  2041 , it is possible to maintain the X-ray detection area on the X-ray detector  112  unchanged through adjustment of the rotating angle of the blade when the focus of the X-ray tube  102  shifts. 
         [0055]    When carrying on a CT examination on the subject  114 , the operator selects a slot width of the aperture assembly  204  of the collimator  104  via the input device  134 . A control command is sent from the central processor  136  to the collimator controller  120 . Under the action of the collimator controller  120 , the motor drive system  2043  causes the blade of the rotating slot part  2041  having the desired slot width to rotate to a substantially horizontal position so that said blade is substantially perpendicular to the central X-ray beam emitted from the X-ray tube. Thus, X-ray beams entering the collimator  104  can only be irradiated to the region of interest of the subject  114  through the slot of said blade and pass through the subject  114  for being projected to the X-ray detector  112 , thereby forming an X-ray detection area. 
         [0056]    During operation, the focus of the X-ray tube  102  will shift as the tube temperature changes. Where the position of the selected blade of the collimator  104  remains unchanged, as compared with the circumstance where no focus shift takes places, a corresponding shift will occur to the X-ray beam irradiated to the subject  114  via the blade slot, which eventually results in a relatively large offset of the X-ray detection area on the X-ray detector  112 . 
         [0057]    As shown in  FIG. 5 , in case of no focus shift of the X-ray tube  102  during a CT scan, the X-ray detection area between the left edge D 1  and the right edge D 2  of the X-ray beam projected to the X-ray detector  112  via the slot of the selected blade of the collimator  104  is represented by A 1 . When the focus of the X-ray tube shifts due to temperature change during a CT scan along a focus shift path to the left edge of the focus shift path, the change of the X-ray detection area for the X-ray beam projected to the X-ray detector  112  via the slot of the selected blade is represented by A 4 . As shown, the right edge of the X-ray detection area of the X-ray detector  112  offsets from point D 2  to point D 4 . Similarly, when due to temperature change the focus of the X-ray tube  102  shifts along a focus shift path to the right edge of the focus shift path, the change of the X-ray detection area for the X-ray beam projected to the X-ray detector via the blade slot is represented by A 5 . As shown, the left edge of the X-ray detection area of the X-ray detector  112  offsets from point D 1  to point D 3 . Therefore, when focus shift occurs to the X-ray tube  102 , if the position of the selected blade of the collimator  104  is not corrected, namely, not to perform Z-beam tracking, then the detection area for the X-ray beam projected to the X-ray detector  112  via the selected blade of the collimator  104  will deviate from the X-ray detection area A 1  when focus shift does not occur to the X-ray tube  102 . 
         [0058]    In an embodiment, the curved lines of the curved surface structure of each blade slot edge in the cross-sectional view along a thickness direction perpendicular to the longitudinal direction of the blade as shown in  FIG. 3D  are circular arcs, wherein the circular arc close to the rotation center of the blade is referred to as an inner circular arc, and the circular arc away from the rotation center of the blade is referred to as an outer circular arc. 
         [0059]    In the following, embodiments of the present invention will be further explained by taking the example where the curved lines of the curved surface structure of each blade slot edge in a vertical plane along the longitudinal direction of the blade slot are circular arcs. The skilled person will appreciate that the curved lines of the curved surface structure of each blade slot edge in a vertical plane along the longitudinal direction of the blade slot may be elliptical arcs or any other curved lines. 
         [0060]    In an embodiment, an unequal angle tracking method as described below with reference to  FIGS. 4A ,  4 B,  4 C, and  4 D can be used to determine the inner and outer circular arcs of the blade slot. For convenience of explanation, a cross-sectional view of the blade slot center along a thickness direction perpendicular to the longitudinal direction of the blade is used as an example for illustration. 
         [0061]      FIG. 4A  shows exemplary focus positions FP 1 -FP 5  of the X-ray tube  102  and edge lines L 1 -L 10  of the X-ray beam when the X-ray beam emitted from the X-ray tube  102  at these focus positions is projected to the X-ray detector  112  via the blade slot, wherein focus positions FP 1  and PF 5  represent the largest shifts of the focus of the X-ray tube  102  along the focus shift path. The focus shift path and range of the X-ray tube are determined by its structure and size. 
         [0062]    As shown in  FIG. 4A , when the focus of the X-ray tube  102  is at FP 1 , the blade slot allows X-ray beams between lines L 1  and L 6  to pass through; when the focus of the X-ray tube is at FP 2 , the blade slot allows X-ray beams between lines L 2  and L 7  to pass through, and in the same manner, when the focus of the X-ray tube is at FP 5 , the blade slot allows X-ray beams between lines L 5  and L 10  to pass through. 
         [0063]    The width of the blade slot and the positions of the inner and outer circular arcs thereof can be determined based on the requirement on the width of the X-ray detection area of the X-ray detector  112  during the CT scan, as well as the position and size of the various components of the CT system. 
         [0064]    Specifically, as shown in  FIG. 4B , a two-dimensional coordinate system YOZ is established using the rotation center O of the blade as the center of coordinate, wherein the horizontal line of the rotation center O is the OZ axis of the two-dimensional coordinate system. In the established two-dimensional coordinate system YOZ, the X-ray detection area of the X-ray detector  112  (namely, the edge points D 1  and D 2  of the X-ray detection area as shown in  FIG. 6A ), the position of the blade slot, the focus position of the X-ray tube  102  and the maximum shift position are known. Therefore, it is possible to determine, when the focus of the X-ray tube  102  is at the right maximum shift position FP 5 , the position of the X-ray line L 10  that arrives at the left edge point D 1  (see  FIG. 6A ) of the X-ray detection area of the X-ray detector  112  via the left edge point of the blade slot, and the position of the X-ray line L 5  that arrives at the right edge point D 2  (see  FIG. 6A ) of the X-ray detection area of the X-ray detector  112  via the right edge point of the blade slot in the two-dimensional coordinate system YOZ, and it is also possible to determine, when the focus of the X-ray tube  102  is at the left maximum shift position FP 1 , the position of the X-ray line L 6  that arrives at the left edge point D 1  of the X-ray detection area of the X-ray detector  112  via the left edge point of the blade slot, and the position of the X-ray line L 1  that arrives at the right edge point D 2  of the X-ray detection area of the X-ray detector  112  via the right edge point of the blade slot in the two-dimensional coordinate system YOZ. 
         [0065]    Still referring to  FIG. 4B , maintain the positions of the X-ray lines L 10  and L 5  transmitted via the left and right edge points of the blade slot when the focus of the X-ray tube  102  is at the right maximum shift position FP 5  unchanged and maintain the blade in a horizontal position, rotate the X-ray lines L 6  and L 1  transmitted via the left and right edge points of the blade slot when the focus of the X-ray tube  102  is at the left maximum shift position FP 1  an angle of B around the blade rotation center O to enable lines L 6  and L 10  to have an intersection in the region along the thickness direction of the blade and to enable lines L 1  and L 5  to also have an intersection in the same region, as shown in  FIG. 4C . If, during the rotation of the lines L 6  and L 1  around the blade rotation center O, the intersection of the lines L 6  and L 10  and the intersection of the lines L 1  and L 5  are not in the region along the thickness direction of the blade, then the thickness of the blade and the eccentricity between the blade slot and the rotation center O can be adjusted until the intersection of the lines L 6  and L 10  and the intersection of the lines L 1  and L 5  are in the region along the thickness direction of the blade, as the lines L 6  and L 1  rotate around the blade rotation center O. 
         [0066]    Thereafter, as shown in  FIG. 4D , a predetermined radius Ro is used to set an outer circular arc near the intersection of lines L 6  and L 10  as described above, wherein the circle where the outer circular arc resides is tangent to the X-ray lines L 6  and L 10 ; similarly, a predetermined radius Rn is used to set an inner circular arc near the intersection of lines L 1  and L 5 , wherein the circle where the inner circular arc resides is tangent to the X-ray lines L 1  and L 5 , and wherein the width of the blade slot is determined by the inner and outer circular arcs as set above. The radii Ro and Rn of the outer and inner circular arcs may be set according to, for example, the requirement on the curvature of the outer and inner circular arcs. The radii Ro and Rn of the outer and inner circular arcs are appropriately selected so that the diameters of the circles where the outer and inner circular arcs reside are not less than the blade thickness. Besides, when relatively large values are selected for Ro and Rn, the outer and inner circular arcs should have an appropriate curvature so that the edges of the blade slot along the longitudinal direction have a convex curved surface structure. 
         [0067]    In an embodiment, an equal angle tracking method similar to the above-described unequal angle tracking method or other similar methods may be used to determine the curved lines of the curved surface structure of each blade slot edge in a vertical plane along the longitudinal direction of the blade slot. 
         [0068]    In an embodiment, after determining the shape of the curved surface structure of the edge of each blade slot edge in a vertical plane along the longitudinal direction of the blade slot, such as the inner and outer curved lines of the inner and outer circular arcs, the inner and outer curved lines are extended from the center position to the two ends of the slot in accordance with the shape of the slot edge, to enable the edges of the blade slot along the longitudinal direction to have convex curved surface structures, so that the X-rays projected on the X-ray detector  112  will have equal width in the Z direction. In an embodiment, the blade and the slot thereof may be divided into several slot segments, and the above-described unequal angle tracking method or equal angle tracking method is used to determine for each slot segment, the shape of the inner and outer curved lines (such as the inner and outer circular arcs) of the slot segment in the vertical plane along the longitudinal direction of the blade slot. Then the inner and outer curved lines are extended to said slot segment along the edge of the blade on which the slot segment is located, so as to form a convex curved surface structure on the blade slot edge of each slot segment, and finally form a convex curved surface structure for the edge of the entire blade slot along the longitudinal direction of the blade. In this way, the X-rays projected on the X-ray detector  112  will have equal width in the Z direction. 
         [0069]    As described above, during operation of the X-ray tube  102 , the focus thereof will shift as the temperature changes, thereby causing the X-ray detection area of the detector  112  to deviate from the initial X-ray detection area. Depending on the structure of the X-ray tube  102 , the shift path of the focus of the X-ray tube can be a horizontal line, an oblique line, or other shapes. For simplicity, the following description is based on the example where the shift path of the focus of the X-ray tube is a horizontal line. 
         [0070]    In the collimator  104  according to an embodiment of the present invention, when the focus of the X-ray tube  102  shifts, the collimator controller  120  can control the rotating slot part  2041  in the collimator  104  to rotate a certain angle to correct a route of the X-ray beam passing through the blade slot, so that the convex curved surface structure of the blade edge along the longitudinal direction of the blade can block some of the X-ray beams from the X-ray tube, thereby correcting the X-ray beams projected onto the X-ray detector  112 , such that the X-ray detection area of the X-ray detector  112  remains unchanged when the focus of the X-ray tube  102  has changed. This tracking process is referred to as Z-beam tracking 
         [0071]    Next, reference will made to  FIGS. 6A-6D  to describe the Z-beam tracking process of an embodiment of the present invention in which the collimator  104  is controlled by the collimator controller  120  in a manner such that the X-ray detection area of the X-ray detector  112  remains unchanged when the focus the X-ray tube  102  shifts. 
         [0072]    As shown in  FIG. 6A , when the focus of the X-ray tube  102  is at the center of the focus shift range, the X-ray detection area of the X-rays projected to the X-ray detector  112  via the slot of the selected blade of the rotating slot part  2041  has a right edge point D 2  and a left edge point D 1 , wherein focus shift range of the X-ray tube  102  depends on the structure of the X-ray tube  102 . In an exemplary cross-sectional view of the center of the blade slot along the thickness direction perpendicular to the longitudinal direction of the blade, a two-dimensional coordinate system YOZ is established using the blade rotation center as the origin of coordinate. Based on the size, structure and the positional relationship of the various components of the CT system (including the X-ray tube  102 , the collimator  104 , the rotating slot part  2041  and blades of the collimator  104 , and the X-ray detector  112 ), it is possible to determine the horizontal position of the selected blade of the plurality of blades of the rotating slot part  2041 , the positions of the focus of the X-ray tube  102  and the X-ray detector  112 , and the positions of the left and right edge points D 1  and D 2  in the X-ray detection region of the X-ray detector  112 , in the established YOZ dimensional coordinate system. 
         [0073]    As shown in  FIG. 6B , the right edge point F 0  of the focus shift range of the X-ray tube  102  is selected as the initial reference position of the Z-beam tracking As X-ray beams are projected to the X-ray detector  112  via the slot of the selected blade in the horizontal position, an X-ray detection area defined by the right edge point D 2  and left edge point D 1  is formed. It will be appreciated by persons skilled in the art that it is also possible to select one of other positions of the focus shift range as the initial reference position of the Z-beam tracking, as long as the X-ray detection area required for CT scan formed on the X-ray detector  112  by the X-ray beams passing through the slot of the selected blade are between the points D 1  and D 2 . 
         [0074]    When the focus of the X-ray tube  102  shifts due to temperature change during a CT scan, for example, a left shift p along the focus shift path relative to the initial reference position of the focus as shown in  FIG. 6C , a change will be caused to the X-ray detection area formed by projection of X-ray beams onto the X-ray detector  112  via the blade slot. The change of the X-ray detection area can be determined by an X-ray detection area offset monitoring unit provided on the X-ray detector  112 . 
         [0075]    In an embodiment, upon detection of a change of the X-ray detection area on the X-ray detector  112  by the X-ray detection area offset monitoring unit as compared with the X-ray detection area when the focus of the X-ray tube  102  is at the initial reference position, the blade may be in the horizontal position, which is compared to the position shown in  FIG. 6B , the angle B 1  of the blade is zero. The blade may also be in other locations, for example, in a position as shown in  FIG. 6C , where the angle of the blade is B 1  which may be determined by an encoder  2042  disposed in the collimator  104 . As shown in  FIG. 6C , the new X-ray detection area on the X-ray detector  112  has a right edge point D 3  and a left edge point D 4 , which correspond to two X-rays L 12  and L 22 , respectively. The two X-rays represent two edges of the X-ray beam allowed to pass through the slot between the inner circular arc and the outer circular arc of the selected blade. The X-ray detection area offset monitoring unit may send the determined offset of the X-ray detection area, namely, the distance m between point D 3  and point D 2  and/or the distance n between point D 4  and point D 1 , to the collimator controller  120 . 
         [0076]    The encoder  2042  disposed in collimator  104  can measure an angle of rotation B 1  of the selected blade around the rotation center O and send the measured angle B 1  to the collimator controller  120 . Based on the received angle B 1  and the offset m and/or n of the X-ray detection area, as well as the positional relationships of the selected blade, the rotating slot part  2041 , the X-ray tube  102 , and the X-ray detector  112  in the two-dimensional coordinate system YOZ, the collimator controller  120  can determine the shift p of the focus of the X-ray tube  102  relative to the initial reference position. 
         [0077]    Specifically, in an embodiment, the collimator controller  120  determines the position of the inner circular arc in the two-dimensional coordinate system YOZ based on the angle B 1  measured by the encoder  2042  and the radius of rotation of the selected blade around the rotation center O. Then, based on the offset m of the X-ray detection area determined by the X-ray detection area offset monitoring unit, as well as the positional relationships of the selected blade, the rotating slot part  2041 , the X-ray tube  102 , and the X-ray detector  112  in the two-dimensional coordinate system YOZ, the collimator controller  120  determines a straight line L 12  passing point D 3  and tangent to the inner circular arc in the two-dimensional coordinate system YOZ, wherein the straight line L 12  represents the rightmost X ray of the X-ray beam when the focus of the X-ray tube  102  shifts from the right edge point F 0  to the new position F 1 , and the selected blade is at the rotation angle B 1 . The intersection point of the determined line L 12  and the focus shift path of the X-ray tube  102  is the new position F 1  of the shifted focus of the X-ray tube  102 . 
         [0078]    In an embodiment, the collimator controller  120  determines the position of the outer circular arc in the two-dimensional coordinate system YOZ based on the rotation angle B 1  of the selected blade, the radius of rotation of the selected blade around the rotation center O, the position of the inner circular arc and the positional relationship of the inner and outer circular arcs. Then, based on the offset n determined by the X-ray detection area offset monitoring unit, as well as the positional relationships of the selected blade, the rotating slot part  2041 , the X-ray tube  102 , and the X-ray detector  112  in the two-dimensional coordinate system YOZ, the collimator controller  120  determines a straight line L 22  passing point D 4  and tangent to the outer circular arc in the two-dimensional coordinate system YOZ, wherein the straight line L 22  represents the leftmost X ray of the X-ray beam when the focus of the X-ray tube  102  shifts from the right edge point F 0  to the new position F 1 , and the selected blade is at the rotation angle B 1 . The intersection point of the determined line L 22  and the focus shift path of the X-ray tube  102  is the new position F 1  of the shifted focus of the X-ray tube  102 . 
         [0079]    In an embodiment, after the collimator controller  120  determines two focus shift new positions F 1  based on the straight lines L 12  and L 22  in the two-dimensional coordinate system YOZ respectively, an average of the two new positions is used as the final focus shift new position F 1 . 
         [0080]    The collimator controller  120  may, after determining the focus shift p of the X-ray tube  102  along the focus shift path and the rotation angle B 1  of the selected blade, determine a rotation correction angle of the blade needed for eliminating of the offsets m and n of the X-ray detector region on the X-ray detector  112 , and then cause the rotating slot part  2041 , driven by the motor drive system  2043 , to rotate said correction angle about the center of the rotation shaft  2042 , so that the X-ray detection area formed by projection of X rays to the X-ray detector  112  via the slot of the selected blade remains unchanged as the focus of the X-ray tube  102  shifts to a new location F 1  along the focus shift path relative to the initial reference position F 0 , thereby completing Z-beam tracking for focus shift of the X-ray tube  102 . 
         [0081]    Specifically, as shown in  FIG. 6D , in an embodiment, after determining the focus shift p of the X-ray tube  102 , the collimator controller  120  can determine the position of the new position F 1  of the focus in the two-dimensional coordinate system YOZ, and then determine a straight line L 11  passing point F 1  and point D 2  in the two-dimensional coordinate system YOZ based on the determined new position F 1  of the focus and the right edge point D 2  of the initial X-ray detection area on the X-ray detector  112 . When the X-ray detection area obtained on the X-ray detector  112  via the blade slot is remained unchanged as a result of the blade rotating a correction angle about the rotation center O after the focus of the X-ray tube  102  has shifted a distance of p along the focus shift path, the inner circular arc of the slot of the selected blade is tangent to the X-ray at line L 11  in the two-dimensional coordinate system YOZ. Therefore, in the case that the line L 11  is known, the correction angle B that the blade rotates around the rotation center O can be determined based on the positional relationship that the inner circular arc is tangent to line L 11  and such known parameters as the radius of rotation of the blades. 
         [0082]    In an embodiment, after determining the position of the new position F 1  of the focus in the two-dimensional coordinate system YOZ, the collimator controller  120  then determines a straight line L 21  passing point F 1  and point D 1  in the two-dimensional coordinate system YOZ based on the determined new position F 1  of the focus and the left edge point D 1  of the initial X-ray detection area on the X-ray detector  112 . When the X-ray detection area obtained on the X-ray detector  112  via the blade slot is remained unchanged as a result of the blade rotating a correction angle about the rotation center O after the focus of the X-ray tube  102  has shifted a distance of p along the focus shift path, the outer circular arc of the slot of the selected blade is tangent to the X-ray at line L 21  in the two-dimensional coordinate system YOZ. Therefore, in the case that the line L 21  is known, the correction angle B that the blade rotates around the rotation center O can be determined based on the positional relationship that the outer circular arc is tangent to line L 21 , the positional relationship of the inner and outer circular arcs of the blade slot, and such known parameters as the radius of rotation of the blade. 
         [0083]    In an embodiment, the collimator controller  120  may, after determining two correction angles B that the blade rotates around the rotation center O based on the straight lines L 11  and L 21  respectively, use an average of the two values as the final correction angle B for performing the Z-beam tracking 
         [0084]    Alternatively, the collimator controller  120  is further configured to, after completing a first Z-beam tracking for the focus shift of the X-ray tube  102 , compare the offset m and/or n of the X-ray detection area determined in real time by the X-ray detection area monitoring unit with a predetermined threshold. If the offset m and/or n determined in real time do not exceed their respective thresholds, the Z-beam tracking is completed. If the offset m and/or n determined in real time exceed their respective thresholds, then the above procedure of Z-beam tracking may be repeated until the latest offset m and/or n of the X-ray detection area do not exceed their respective thresholds. 
         [0085]    In an embodiment, a plurality of focus shifts p of the X-ray tube  102  along the focus shift path during a CT scan and a plurality of corresponding correction angles B (including the rotation direction) that the selected blade is required to rotate for achieving Z-beam tracking may be predetermined for each blade in the rotating slot part  2041  based on simulation or actual measurements. The plurality of shifts p and the corresponding correction angles B are stored in the form of a table in a memory within the collimator controller  120  or in an external memory coupled to the collimator controller  120  (not shown). When the focus of the X-ray tube  102  shifts along the focus shift path due to temperature change during a CT scan of the subject  114 , the collimator controller  120  can determine the focus shift p of the X-ray tube based on the offset m and/or n of the X-ray detection area determined by the X-ray detection area monitoring unit arranged on the X-ray detector  112 , search in the memory for a blade rotation correction angle B corresponding to the focus shift p of the X-ray tube, and then cause the blade to rotate said correction angle around the rotation center under the action of the motor drive system  2043 , based on the blade rotation correction angle B found and the current angle B 1  of the blade determined by the encoder  2042 , thereby eliminating the offset m and/or n of the X-ray detection area on the X-ray detector  112  and achieving Z-beam tracking 
         [0086]    If the collimator controller  120  fails to find a focus shift p of the X-ray tube in the memory, then it can search for two correction angles B corresponding to two focus shifts close to the focus shift p of the X-ray tube; based on the relationship between the focus shift p of the X-ray tube and the two adjacent focus shifts thereof, a final correction angle B can be determined by performing an interpolation between the two correction angles B found. In an embodiment, if the collimator controller  120  fails to find a focus shift p of the X-ray tube in the memory, a correction angle B corresponding to the focus shift closest to the focus shift p of the X-ray tube can be searched for and used as the final correction angle B. In an embodiment, if the collimator controller  120  fails to find a focus shift p of the X-ray tube in the memory, it can search for two correction angles B corresponding to two focus shifts close to the focus shift p of the X-ray tube, and then use an average of the two correction angles B as the final correction angle B. 
         [0087]    In an embodiment, a plurality of offsets m and/or n of the X-ray detection area on the X-ray detector  112  and a plurality of corresponding correction angles B (including the rotation direction) that the blade is required to rotate for achieving Z-beam tracking may be predetermined based on simulation or actual measurements. The plurality of offsets m and/or n of the X-ray detection area and the corresponding correction angles B are stored in the form of a table in a memory within the collimator controller  120  or in an external memory coupled to the collimator controller  120  (not shown). When the focus of the X-ray tube  102  shifts along the focus shift path due to temperature change during a CT scan of the subject  114 , the collimator controller  120  can search for a corresponding blade rotation correction angle B in the memory based on the offset m and/or n of the X-ray detection area determined by the X-ray detection area monitoring unit arranged on the X-ray detector  112 , and then cause the blade to rotate said correction angle around the rotation center under the action of the motor drive system  2043 , based on the blade rotation correction angle B found and the current angle B 1  of the blade determined by the encoder  2042 , thereby eliminating the offset m and/or n of the X-ray detection area on the X-ray detector  112  and achieving Z-beam tracking 
         [0088]    If the collimator controller  120  fails to find an offset m and/or n of the X-ray detection area determined by the X-ray detection area monitoring unit in the memory, then it can search for two correction angles B corresponding to two offsets close to the offset m and/or n of the X-ray detection area; based on the relationship between the offset m and/or n of the X-ray detection area and the two close offsets thereof, a final correction angle B can be determined by performing an interpolation between the two correction angles B found. In an embodiment, if the collimator controller  120  fails to find an offset m and/or n of the X-ray detection area determined by the X-ray detection area monitoring unit in the memory, a correction angle B corresponding to the offset closest to the offset m and/or n of the X-ray detection area can be searched for and used as the final correction angle B. In an embodiment, if the collimator controller  120  fails to find an offset m and/or n of the X-ray detection area determined by the X-ray detection area monitoring unit in the memory, it can search for two correction angles B corresponding to two offsets close to the offset m and/or n of the X-ray detection area, and then use an average of the two correction angles B as the final correction angle B. 
         [0089]    Returning to  FIG. 5 , when the focus of the X-ray tube  102  shifts along the focus shift path due to temperature change, the collimator controller  120  controls the selected blade in the collimator  104  to rotate a correction angle about the rotation center to eliminate an offset of the X-ray detection area formed by projection of X-rays to the X-ray detector  112  via the slot of the selected blade, so that the X-ray detection area is substantially restored to the initial position when no focus shift takes place. As shown, after performing Z-beam tracking, the X-ray detection area A 2  obtained on the X-ray detector  112  is substantially consistent with the X-ray detection area A 1  before focus shift. Simulation and actual measurements also show that after Z-beam tracking the X-ray detection area A 2  obtained on the X-ray detector  112  has extremely little difference as compared with the X-ray detection area A 1  before focus shift. Accurate tracking is achieved at the D 2  side of the X-ray detection area on the X-ray detector  112 , slight difference is observed in the D 1  side, and accurate tracking is achieved at leftmost and rightmost positions with respect to the angle shift of the X-ray tube. 
         [0090]    The CT collimator according to an embodiment of the present invention uses a single motor drive system to perform slot width selection and Z-beam tracking during a CT scan. As compared with conventional CT collimators using at least two or more motor drive systems, the CT collimator according to an embodiment of the present invention achieves a lower manufacturing cost. By using a single motor drive system to directly drive the rotating slot part of the collimator, the present collimator requires no rails, ball screw or lead screw, and thus has a simpler structure than a conventional collimator, and hence higher reliability and better maintainability. In the CT collimator according to an embodiment of the present invention, according to needs of CT scans, the rotating slot part can be provided with a plurality of blades having different slot widths. The edges of each blade slot along the longitudinal direction of the blade have convex curved surface structures, so that when a focus shift of the radiation source along a focus shift path occurs as a result of temperature change, by rotating the selected blade about a rotation center eccentric to the blade a correction angle corresponding to the focus shift, the radiation beam reaching the radiation detector via the blade slot is maintained at the same region and same width as the circumstance when no focus shift takes place. Therefore, the CT collimator and CT system according to an embodiment of the present invention eliminate the need of extra adjustment of the components such as the radiation detector when a focus shift of the radiation source occurs. 
         [0091]    Although the present invention has been described with reference to specific embodiments, it shall be understood that the present invention is not limited to these specific embodiments. Skilled in the art will appreciate that various modifications, substitutions, changes and so on may be made to the present invention. For example, in the above embodiments one step or component may be divided into multiple steps or components; or, on the contrary, a plurality of steps or components in the above embodiments may be realized in one step or one component. All such variations should be within the scope of protection as long as they do not depart from the spirit of the present invention. In addition, the terms as used in the present specification and claims are not limitative, but descriptive. Moreover, according to actual needs, the entire or part of the features described in one specific embodiment can be incorporated into another embodiment.