Abstract:
For the purpose of providing a method of keeping an iso-center at a fixed position regardless of the tilt of a table top, and a table system having such a function, an apparatus comprises a table top member; lifting means for moving the table top member up and down; forwarding/backing means for longitudinally moving the table top member forward and backward; tilting means for tilting the table top member from a horizontal state; and control means, and the table top member is tilted around an iso-center lying at a spatial position different from a mechanical center of tilt motion of the table top member by the control means controlling vertical displacement of the mechanical center and longitudinal displacement of the table top member according to a tilt angle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Application No. 2002-327795 filed Nov. 12, 2002. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a table control method and table system, and more particularly to a method of tilting a table from a horizontal state, and a table system that can be tilted from a horizontal state. 
     In a conventional medical image capturing apparatus such as an X-ray imaging apparatus, a table system for supporting a subject to be imaged lying thereon is employed. Such table systems include one having a table top that can be arbitrarily tilted from a horizontal state (see, for example, Patent Document 1). 
     [Patent Document 1] 
     Specification and drawings of U.S. Pat. No. 6,353,949B1 (Columns 5–6, FIGS. 1–4). 
     When a desired region in the subject is imaged in the horizontal state and then the same region is imaged with a different tilt of the table top, the center of imaging, i.e., the iso-center, must lie at the same position. However, when the tilt of the table top is changed the iso-center is moved in the aforementioned table system, so that readjustment is required. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method of keeping the iso-center at a fixed position regardless of the tilt of the table top, and a table system having such a function. 
     (1) The present invention, in one aspect thereof for solving the aforementioned problem, is a table control method for tilting a table top from a horizontal state, characterized in comprising: tilting the table top around one point lying at a spatial position different from a mechanical center of tilt motion of the table top by controlling vertical displacement of the mechanical center and longitudinal displacement of the table top according to a tilt angle. 
     (2) The present invention, in another aspect thereof for solving the aforementioned problem, is a table system, characterized in comprising: a table top member; lifting means for moving the table top member up and down; forwarding/backing means for longitudinally moving the table top member forward and backward; tilting means for tilting the table top member from a horizontal state; and control means for tilting the table top member around one point lying at a spatial position different from a mechanical center of tilt motion of the table top member by controlling vertical displacement of the mechanical center and longitudinal displacement of the table top member according to a tilt angle. 
     In the invention of these aspects, since the table top is tilted around one point lying at a spatial position different from a mechanical center of tilt motion of the table top by controlling vertical displacement of the mechanical center and longitudinal displacement of the table top according to a tilt angle, the iso-center can be kept at a fixed position regardless of the tilt of the table top. 
     When horizontal and vertical distances between the aforesaid mechanical, center and the aforesaid one point in the horizontal state of the table top are represented by Ch and Cv, respectively, and a target value of the tilt angle is represented by φ 1 , the amount of the vertical displacement of the mechanical center is preferably defined as: 
                   Y1   =       ⁢     {       (       -   1     *   Sqrt   ⁢           ⁢     (       Ch   2     +     Cv   2       )       )     *                       ⁢       (     Sin   ⁢           ⁢   ϕ   ⁢           ⁢     1   /   Sin     ⁢           ⁢     (       (     180   -     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢   1     )         )     /   2     )       )     *                     ⁢     Sin   ⁢           ⁢     (     90   +     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢     1   /   2       )       -                               ⁢       Sin     -   1       ⁡     (     C   ⁢           ⁢     v   /   Sqrt     ⁢           ⁢     (       Ch   2     +     Cv   2       )       )       )     )     }     +                   ⁢     {       (       -   1     *   Sqrt   ⁢           ⁢     (       Ch   2     +     Cv   2       )       )     *                       ⁢       (     Sin   ⁢           ⁢   ϕ   ⁢           ⁢     1   /   Sin     ⁢           ⁢     (       (     180   -     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢   1     )         )     /   2     )       )     *                     ⁢     Sqrt   ⁢           ⁢     (     1   -     (     Sin   ⁢           ⁢     (     90   +     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢     1   /   2       )       -                                             ⁢       Sin     -   1       ⁢           ⁢     (       Cv   /   Sqrt     ⁢           ⁢     (       Ch   2     +     Cv   2       )       )       )     )     2     )     *   Tan   ⁢           ⁢   ϕ   ⁢           ⁢   1     }     ,                 [     Equation   ⁢           ⁢   7     ]             
 
and the amount of the longitudinal displacement of the table top is preferably defined as:
 
 Y   3 =(−1*Sqrt( Ch   2   +Cv   2 ))*(Sin φ 1 /Sin((180−Abs(φ 1 ))/2))*Sqrt(1−(Sin(90+Abs(φ 1 /2)−Sin −1 ( Cv /Sqrt( Ch   2   +Cv   2 )))) 2 )/Cos φ 1 ,  [Equation 8]
 
so that accuracy of keeping of the iso-center may be improved.
 
     When distances between the aforesaid mechanical center and a point of action and a fulcrum of an actuator, which has the point of action and fulcrum that move up and down along with the mechanical center for tilting the table top by extension/contraction of its length, are represented by C 5  and C 6 , respectively, the length of the actuator in the horizontal state of the table top is represented by C 7 , and an angle at the mechanical center subtending the point of action and the fulcrum in the horizontal state of the table top is represented by φ 2 , an amount-of change in the length of the actuator is preferably defined as:
 
 Y   4 ={Sqrt(( C   6 −( C   5 *Cos(φ 2 −φ 1 ))) 2 +( C   5 *Cos(φ 2 −φ 1 ))  2 )}− C   7 ,  [Equation 9]
 
so that accuracy of tilting of the table top may be improved.
 
     The longitudinal displacement of the table top is preferably defined within predetermined limits so that a mechanism for the displacement may be simplified. 
     Therefore, the present invention provides a method of keeping the iso-center at a fixed position regardless of the tilt of the table top, and a table system having such a function. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an apparatus in accordance with one embodiment of the present invention. 
         FIG. 2  shows a horizontal state of a table top. 
         FIG. 3  shows a tilt state of the table top. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  FIG. 1  shows a block diagram of a table system. The apparatus is an embodiment of the present invention. The configuration of the apparatus represents an embodiment of the apparatus of the present invention. The operation of the apparatus represents an embodiment of the method of the present invention. 
     As shown in  FIG. 1 , the present apparatus comprises a table top  100 . A subject P is rested on the table top  100  in a supine position. The table top  100  is driven by a forwarding/backing mechanism  20 , a lifting mechanism  30 , and a tilting mechanism  40  to allow longitudinal forward/backward movement, vertical up/down movement, and tilting with respect to a horizontal direction, respectively. The forward/backward movement, up/down movement and tilting of the table top  100  are detected by sensors  22 ,  32  and  42 , respectively. 
     The table top  100  is an embodiment of the table top member of the present invention. The forwarding/backing mechanism  20  is an embodiment of the forwarding/backing means of the present invention. The lifting mechanism  30  is an embodiment of the lifting means of the present invention. The tilting mechanism  40  is an embodiment of the tilting means of the present invention. 
     The forwarding/backing mechanism  20 , lifting mechanism  30  and tilting mechanism  40  are controlled by a control section  50 . Detected signals from the sensors  22 ,  32  and  42  are input to the control section  50 . For the control section  50 , a computer is employed, for example. The control section  50  is an embodiment of the control means of the present invention. 
     The control section  50  is connected with a display section  60  and an operating section  70 . The display section  60  and operating section  70  are used by an operator to interactively operate the present apparatus. 
       FIG. 2  shows a schematic configuration of the present apparatus along with a gantry for X-ray imaging. As shown, a gantry  200  supports an X-ray emitting section  202  and an X-ray receiving section  204  that face each other, by an arc-shaped arm  206 . The arm  206  is supported by a stand  208 . 
     The gantry  200  has an iso-center O in a space between the X-ray emitting section  202  and the X-ray receiving section  204 . The iso-center Ocorresponds to a center of the arc of the arm  206 . By moving the arm  206  along the arc by a feed mechanism incorporated in the stand  208 , the X-ray emitting section  202  and the X-ray receiving section  204  rotate around the iso-center O while maintaining their facing relationship. The iso-center O lies at a height C 4  from the floor. 
     The table top  100  is supported by a support base  110 . In  FIG. 2 , a horizontal state of the table top  100  is shown. The support base  110  incorporates therein the forwarding/backing mechanism  20 , lifting mechanism  30 , tilting mechanism  40 , and sensors  22 ,  32  and  42  shown in  FIG. 1 . The control section  50 , display section  60  and operating section  70  are housed in a console (not shown), which is installed at an appropriate position, such as in an operator room. 
     The support base  110  has an axis of rotation AX of the table top  100 . The table top  100  is allowed to rotate around the axis of rotation AX in a horizontal plane. The position of the support base  110  is represented by the position of the axis of rotation AX. The support base  110  lies at a horizontal distance C 1  from the iso-center O of the gantry  200 . 
     The height of the table top  100  from the floor is X 1 . The height X 1  changes with up/down movement of the table top  100 . The table top  100  is at a state of being moved forward by a distance X 3  from a reference position S toward the gantry  200 . The movement toward the gantry will be referred to as forward movement, and movement away from the gantry as backward movement hereinbelow. 
     The distance allowed for backward movement from the reference position S is limited to C 8 . Therefore, the maximum distance allowed for backward movement from the state of the table top  100  being moved forward by a distance X 4  from the reference position, as shown, is X 3 +C 8 . 
     A schematic configuration of the tilting mechanism  40  in the support base  110  is indicated by a triangle ABC. The vertex A represents a mechanical center of tilting movement of the table top  100 . The vertex B represents a point of action of driving force of an actuator for the tilting mechanism  40 . The vertex C represents a fulcrum of the actuator. 
     The actuator is one that is extendable in length, such as an electromotive ball screw, and its extension and contraction change the distance between the fulcrum C and point of action B, thereby tilting the table top  100  around A. The lifting mechanism  30  moves up/down such a tilting mechanism  40  and forwarding/backing mechanism  20  along with the table top  100 . 
     In the tilting mechanism  40 , the distance between A and B is C 5 , and that between A and C is C 6 . If the table top  100  remains in the horizontal state, the distance between B and C is C 7 , and an angle at the mechanical center A subtending the point of action B and fulcrum C is φ 2 . 
     The mechanical center A lies at a horizontal distance C 2  from the axis of rotation AX toward the gantry  200 . The distance from the mechanical center A to the surface of the table top  100  is C 3 . Thus, the mechanical center A lies at a horizontal distance Ch and at a vertical distance Cv from the iso-center O. Ch and Cv are given by the following formulae, respectively:
 
 Ch=C   1 − C   2 , and   [Equation 10]
 
 Cv=C   4 − X   1   +C 3 .   [Equation 11]
 
       FIG. 3  shows a state in which the table top  100  is tilted. The tilt angle is φ 1 . This state corresponds to a case in which imaging is conducted with the head of the subject slightly lowered in angiography of the head, for example. 
     In such a condition, although the tilting of the table top  100  is tilting around the mechanical center A from the mechanical viewpoint, it is achieved as tilting around the iso-center O from the geometrical viewpoint. Specifically, tilting of the table top  100  is conducted so that the tilting will not change the length and the position in the table top  100  of a perpendicular dropped from the iso-center O and meeting with the table top  100  in the horizontal state. 
     To enable such tilting, the control section  50  controls the tilting means  40  to tilt the table top  100  around the mechanical center A, and also controls the lifting mechanism  30  and forwarding/backing mechanism  20  to change the vertical position of the mechanical center A and the position of the table top  100  in the forward/backing direction according to the tilt angle φ 1 . 
     The control section  50  calculates an amount of change Y 1  in the vertical position of the mechanical center A by the formula below, and controls the lifting mechanism  30  to make the amount of change in the vertical position of the mechanical center A agree with Y 1 : 
                   Y1   =       ⁢     {       (       -   1     *   Sqrt   ⁢           ⁢     (       Ch   2     +     Cv   2       )       )     *                       ⁢       (     Sin   ⁢           ⁢   ϕ   ⁢           ⁢     1   /   Sin     ⁢           ⁢     (       (     180   -     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢   1     )         )     /   2     )       )     *                     ⁢     Sin   ⁢           ⁢     (     90   +     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢     1   /   2       )       -                               ⁢       Sin     -   1       ⁡     (     C   ⁢           ⁢     v   /   Sqrt     ⁢           ⁢     (       Ch   2     +     Cv   2       )       )       )     )     }     +                   ⁢     {       (       -   1     *   Sqrt   ⁢           ⁢     (       Ch   2     +     Cv   2       )       )     *                       ⁢       (     Sin   ⁢           ⁢   ϕ   ⁢           ⁢     1   /   Sin     ⁢           ⁢     (       (     180   -     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢   1     )         )     /   2     )       )     *                     ⁢     Sqrt   ⁢           ⁢     (     1   -     (     Sin   ⁢           ⁢     (     90   +     Abs   ⁢           ⁢     (     ϕ   ⁢           ⁢     1   /   2       )       -                                             ⁢       Sin     -   1       ⁢           ⁢     (       Cv   /   Sqrt     ⁢           ⁢     (       Ch   2     +     Cv   2       )       )       )     )     2     )     *   Tan   ⁢           ⁢   ϕ   ⁢           ⁢   1     }     .                 [     Equation   ⁢           ⁢   12     ]             
 
     The control section  50  also calculates an amount of change Y 3  in the position of the table top  100  in the forward/backing direction by the formula below, and controls the forwarding/backing mechanism  20  to make the amount of change in the position of the table top  100  in the forward/backing direction agree with Y 3 :
 
 Y   3 =(−1*Sqrt( Ch   2   +Cv   2 ))*(Sin φ 1 /Sin ((180−Abs(φ 1 ))/2))*Sqrt(1−(Sin(90+Abs(φ 1 /2)−Sin −1 ( Cv /Sqrt( Ch   2   +Cv   2 )))) 2 )/Cos φ 1 .  [Equation 13]
 
     The control section  50  further calculates an amount of change Y 4  in the length of the actuator for tilting the table top  100  up to the tilt angle φ 1  by the formula below, and controls the actuator to make the amount of change in the length of the actuator agree with Y 4 :
 
 Y   4 ={Sqrt(( C   6 −( C   5 *Cos(φ 2 −φ 1 ))) 2 +( C   5 *Cos(φ 2 −φ 1 )) 2 )}− C   7 .  [Equation 14]
 
     In the formulae, the sign of the tilt angle φ 1  is defined as (+) for a tilt in a direction of lifting up of an end of the table top  100  adjacent to, the gantry  200 , and as (−) for a tilt in a direction of lowering. The sign of Y 1  is defined as (+) in a direction of lifting up of the table top  100 , and as (−) in a direction of lowering. The sign of Y 3  is defined as (+) in a direction of backward movement of the table top  100 , and as (−) in a direction of forward movement. The sign of Y 4  is defined as (+) in a direction of extension of the actuator, and as (−) in a direction of contraction. 
     To calculate Y 1 , Y 3  and Y 4  by the formulae above, the tilt angle φ 1  of the table top  100  is input to the control section  50  by a user via the operating section  70 . Moreover, the horizontal distance Ch and vertical distance Cv between the mechanical center A and iso-center O are input to the control section  50  by the user via the operating section  70 . However, values that are directly input are fixed values C 1 , C 2 , C 3  and C 4  that are known beforehand, and Ch and Cv are calculated by the control section  50  using the formulae (10) and (11), respectively, from those fixed values and the detected height value X 1  of the table top  100  in the horizontal state. 
     By such control based on Y 1 , Y 3  and Y 4 , the table top  100  can be tilted around the iso-center O. Thus, within the subject laid on the table  100 , the position of the iso-center O can be prevented from changing from the horizontal state of the table top  100 . Therefore, imaging can be conducted with the imaging center always kept the same regardless of tilt. 
     If the value of Y 3  obtained by calculation is larger than the maximum value X 3 +C 8  allowed for backward movement, the value of Y 3  is set to X 3 +C 8  regardless of the calculated value. This prevents excessive force from being applied to a stopper, for example, in attempting to move the table top  100  beyond the maximum value allowed for backward movement. Moreover, configuration of the forwarding/backing mechanism  20  can be simplified. 
     Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.