CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Application No. 2001-353408 filed Nov. 19, 2001. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a position control technology of a gantry in an X-ray CT (Computed Tomography) that performs a scan to take a tomogram of an object to be inspected while moving the gantry. 
     An X-ray CT system has been known whose gantry is mounted on a bogie moving on rails by means of wheels and which performs a scan of an object to be inspected while moving the gantry. 
     Generally, in a rail, its cross section is formed convexly and the convex portion becomes a rail tread surface. A groove corresponding to the width of the rail tread surface is formed on a wheel tread surface along the direction of rolling of the wheel (that is, the wheel tread surface is formed concavely). This groove is engaged with the rail convex portion to enable the wheel to move along the rail to thereby prevent the wheel from coming off the rail. 
     In order to roll the wheels on the rails smoothly, a gap of about 0.5 mm to 1 mm is usually provided between the rail tread surface and the groove of the wheel tread surface. Four wheels of the bogie are machined so that they have the same diameter, but there is a little difference in size between them and the difference in size causes the bogie to move not in a straight line but in a little curved line to the left and right side. As the bogie moves forward, the bogie is deviated-in the lateral direction to narrow the provided gap gradually and when one side of the groove comes into contact with one side of the rail, the bogie can not move further in the deviated direction but is moved straight in the direction of length of the rail forcibly by the groove and the rail. 
     Next, when the bogie moves in the reverse direction, the bogie is deviated reversely in the lateral direction and when the opposite side of the groove comes into contact with the rail, the bogie is forcibly moved straight in the direction of length of the rail. That is, the position of the bogie when the bogie moves forward is deviated in the lateral direction from the position of the bogie when the bogie moves backward. As a result, there is presented a problem that the rotational center of scan rotation is deviated. 
     Moreover, while an rotary encoder is mounted on the shaft of a driven wheel in the related art to detect the position of the bogie by detecting the rotation of the wheel, as described above, because the position of the bogie when the bogie moves forward is deviated in the lateral direction from the position of the bogie when the bogie moves backward, the wheel does not move in the same locus when the bogie moves forward and when the bogie moves backward. For this reason, there is presented a problem that when the bogie stops at a certain position in the direction of length of the rail, the detection result of position of the bogie is different depending on the direction of movement of the bogie before the stop of the bogie. A problem that a large error is caused in detecting the position of the bogie results in a deviation in a scan position in the direction of length of the rail. 
     Moreover, the rotation of the rotary encoder depends on the size of the wheel. For this reason, there is also presented a problem that since the detection accuracy of movement distance of the bogie in the direction of length of the rail depends on the machining accuracy of the size of the wheel, a desired position detection accuracy can not be kept. 
     Further, there is also presented the following problem: while the position of the bogie when the bogie moves back and forth can be recognized by the detection of a limit switch at a retracted position of the gantry, it is necessary for an operator to press a button provided on the gantry to set a zero slice position that is the base of scan (scan base position), which makes operability worse. In particular, this includes a problem that even if changing the scan base position is not required because the predetermined scan base position exists, it is necessary for the operator to input the scan base position manually, which makes operability worse and tends to make an operating miss or to cause a position error. 
     Still further, safety must be sufficiently secured when the gantry is moved. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the present invention to reduce a deviation in the rotational center of scan and a deviation in a scan position in the direction of length of a rail in an X-ray CT system that performs a scan of an object to be inspected while moving a gantry along the rail. 
     It is another object of the present invention to increase accuracy in detecting the position of a gantry in an X-ray CT system that performs a scan of an object to be inspected while moving the gantry along a rail. 
     It is still another object of the present invention to improve operability in setting a scan base position in an X-ray CT system that performs a scan of an object to be inspected while moving a gantry along the rail. 
     It is still another object of the present invention to guarantee safety when a gantry is moved in an X-ray CT system that performs a scan of an object to be inspected while moving a gantry along the rail. 
     According to the present invention, there is provided a gantry system comprising a pair of runway rails and a gantry in an X-ray CT system that can move along the runway rails, characterized by a linear guide rail arranged in the direction along the runway rails and a linear guide block mounted on the gantry and slidably fitted on the linear guide rail. 
     Further, according to the present invention, there is also provided an X-ray CT system for performing a scan of an object to be inspected while moving a gantry, comprising: a storage unit for previously storing a distance from a predetermined check point to an initial scan base position in the direction of movement of the gantry; a measurement unit for measuring a position in the direction of movement of the gantry; a detection unit for detecting that the gantry is moved to the check point; and a setting unit for setting a scan base position based on a position measured by the measurement unit when the gantry is moved to the check point and the distance stored in the storage unit. 
     Still further, according to the present invention, there is provided a method of controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to a predetermined check point, and a storage unit for previously storing a distance from the predetermined check point to an initial scan base position in the direction of movement of the gantry, and that performs a scan of a body to be inspected while moving the gantry, the method comprising: a detection step of detecting that the gantry is moved to the predetermined check point by the detection unit; a measurement step of measuring a position when the gantry is moved to the predetermined check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the measurement step and the distance stored in the storage unit. 
     Still further, according to the present invention, there is provided a program for controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to a predetermined check point, and a storage unit for previously storing a distance from the predetermined check point to an initial scan base position in the direction of movement of the gantry, and that performs a scan of a body to be inspected while moving the gantry, the program comprising the program codes of: a detection step of detecting that the gantry is moved to the predetermined check point by the detection unit; a measurement step of measuring a position when the gantry is moved to the predetermined check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the measurement step and the distance stored in the storage unit. 
     Still further, according to the present invention, there is also provided a storage medium storing a program for controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to a predetermined check point, and a storage unit for previously storing a distance from the predetermined check point to an initial scan base position in the direction of movement of the gantry, and that performs a scan of a body to be inspected while moving the gantry, the storage medium storing the program codes of: a detection step of detecting that the gantry is moved to the predetermined check point by the detection unit; a measurement step of measuring a position when the gantry is moved to the predetermined check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the measuring step and the distance stored in the storage unit. 
     Still further, according to the present invention, there is provided a method of controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to predetermined first and second check points, and a storage unit for previously storing a first distance to show a distance from the first check point to the second check point and a second distance to show a distance from the second check point to an initial scan base position, in the direction of movement of the gantry, and that performs a scan of an object to be inspected while moving the gantry, the method comprising: a first detection step of detecting that the gantry is moved to the first check point by the detection unit; a first measurement step of measuring a position when the gantry is moved to the first check point by the measurement unit; a second detection step of detecting that the gantry is moved to the second check point by the detection unit; a second measurement step of measuring a position when the gantry is moved to the second check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the second measurement step and the second distance stored in the storage unit. 
     Still further, according to the present invention, there is provided a program for controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to predetermined first and second check points, and a storage unit for previously storing a first distance to show a distance from the first check point to the second check point and a second distance to show a distance from the second check point to an initial scan base position, in the direction of movement of the gantry, and that performs a scan of an object to be inspected while moving the gantry, the program comprising the program codes of: a first detection step of detecting that the gantry is moved to the first check point by the detection unit; a first measurement step of measuring a position when the gantry is moved to the first check point by the measurement unit; a second detection step of detecting that the gantry is moved to the second check point by the detection unit; a second measurement step of measuring a position when the gantry is moved to the second check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the second measurement step and the second distance stored in the storage unit. 
     Still further, according to the present invention, there is also provided a storage medium storing a program for controlling an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail, a measurement unit for measuring a position in the direction of movement of the gantry, a detection unit for detecting that the gantry is moved to predetermined first and second check points, and a storage unit for previously storing a first distance to show a distance from the first check point to the second check point and a second distance to show a distance from the second check point to an initial scan base position, in the direction of movement of the gantry, and that performs a scan of an object to be inspected while moving the gantry, the storage medium storing the program codes of: a first detection step of detecting that the gantry is moved to the first check point by the detection unit; a first measurement step of measuring a position when the gantry is moved to the first check point by the measurement unit; a second detection step of detecting that the gantry is moved to the second check point by the detection unit; a second measurement step of measuring a position when the gantry is moved to the second check point by the measurement unit; and a setting step of setting a scan base position based on a position measured at the second measurement step and the second distance stored in the storage unit. 
     Still further, according to the present invention, there is also provided an X-ray CT system that includes a gantry mounted on a runway rail and movable along the runway rail and an operating console connected to the gantry and outputting information relating to scan to the gantry and performs a scan of an body to be inspected while moving the gantry, comprising: a first direction unit that is mounted on the gantry and directs the gantry to move; a second direction unit that is mounted on the operating console and directs the gantry to move; a storage unit that stores information of a maximum range in which the gantry is moved by the first direction unit; and a control unit that controls the movable range of the gantry by the second direction unit in accordance with the maximum range in which the gantry is moved by the first direction unit. 
     According to the present invention, in an X-ray CT system that performs a scan of a body to be inspected while moving a gantry along rails, it is possible to reduce a deviation in the rotational center of scan and a deviation in scan position in the direction of length of the rail. 
     Further, according to the present invention, in an X-ray CT system that performs a scan of a body to be inspected while moving a gantry along rails, it is possible to increase the accuracy of detecting the position of the gantry. 
     Still further, according to the present invention, in an X-ray CT system that performs a scan of a body to be inspected while moving a gantry along rails, it is possible to improve operability in setting the scan base position. 
     Still further, according to the present invention, in an X-ray CT system that performs a scan of a body to be inspected while moving a gantry along rails, it is possible to ensure safety when the gantry is moved. 
     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 DRAWINGS 
         FIG. 1  is an external perspective view to show one example of a gantry mounted on rails in an embodiment. 
         FIG. 2  is a side view of the gantry in the embodiment. 
         FIG. 3  is an illustration to show the relationship between wheel and side rail in the embodiment. 
         FIG. 4  is a top perspective view of side rails, a center rail, and the gantry. 
         FIG. 5  is an illustration to show one example of a state where a linear guide block is mounted on a linear guide rail. 
         FIG. 6  is a cross-sectional view taken on a line A—A in FIG.  5 . 
         FIG. 7  is a block diagram to show the constitution of a control board built in a gantry base part in the embodiment. 
         FIG. 8  is an illustration to show one example of an operating panel in the embodiment. 
         FIG. 9  is a flow chart to show a process of initially setting the positional relations of a first limit switch, a second limit switch, and a desired scan base position, respectively. 
         FIG. 10  is a flow chart to show a processing of setting a scan base position in the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Modes for carrying out the invention will be described in detail with reference to the drawings. 
       FIG. 1  is a schematic perspective view to show one example of a gantry in an X-ray CT system mounted on rails in a mode for carrying out the invention. 
     As shown in  FIG. 1 , a gantry  1  has a gantry base part  2  and is mounted on a pair of side rails  3   a  and  3   b . The gantry  1  can move along the side rails  3   a ,  3   b  and while the gantry  1  moves, a scan is performed. Moreover, a center rail  4  is disposed at the central position sandwiched between the side rails  3   a  and  3   b  in the direction along the side rails  3   a  and  3   b . The structure of the center rail  4  will be described later. 
     For the sake of convenience in the following description, let&#39;s define a direction of disposition of the side rails  3   a  and  3   b , that is, a direction in which the gantry moves, as a z-axis direction, a direction along a floor and perpendicular to the z-axis direction as an x-axis direction, and a direction perpendicular to the floor (vertical direction) as a y-axis direction. 
       FIG. 2  is a side view of the gantry  1  shown in FIG.  1 . As shown in  FIG. 1 , wheels rolling on the side rails are provided in the gantry base part  2  (wheels  5 ,  6  rolling on the side rail  3   a  are shown in FIG.  2 ). This enables the gantry  1  to move in the z-axis direction. 
       FIG. 3  is an illustration to show the relationship between the wheel  5  and the side rail  3   a . As shown in  FIG. 3 , the side rail  3   a  is formed convexly in its cross section and its protruding portion becomes a rail tread surface. Moreover, a groove corresponding to the width of the rail tread surface is formed on the wheel tread surface of the wheel  5  along the direction in which the wheel rolls. That is, the wheel tread surface is formed concavely. Then, the groove is engaged with the rail&#39;s protruding portion to enable the wheel  5  to move along the rail  3   a  to thereby prevent the wheel from coming off the rail. This holds true also for the other wheels. 
       FIG. 4  is a top perspective view to show the side rails  3   a  and  3   b , the center rail  4 , and the gantry  1  which are shown in FIG.  1 . 
     As described above, the wheels  5 ,  6  are set on the side rail  3   a  whereas wheels  7 ,  8  are set on the other side rail  3   b . The wheels  6  and  8  are connected to each other by a shaft  9  and are driven by a motor  11  via a gear  10  (both of them are mounted in the gantry base part  2 ). That is, the wheels  6 ,  8  are driving wheels and the wheels  5 ,  7  are driven wheels. 
     A linear guide rail  41  is mounted on the center rail  4  in the z-axis direction. Then, two linear guide blocks  42 ,  43  mounted on the bottom surface of the gantry base part  2  are mounted on the linear guide rail  41  so that they can slide on the linear guide rail  41 .  FIG. 5  shows one example of a state in which the linear block  42  is mounted on the linear guide rail  41  and  FIG. 6  shows a cross-sectional view taken on a line A—A in FIG.  5 . Referring to this cross-sectional view makes it clear that the linear guide block  42  is fitted on the linear guide rail  41  with bearings  45   a ,  45   b  interposed between their side surfaces. Such structure enables the linear guide block  42  to slide along the linear guide rail  41 . This holds true also for the linear guide block  43 . 
     Moreover, in order to absorb errors in height of the side rails  3   a ,  3   b  and the center rail  4 , respectively, the linear guide block  42  is mounted on the bottom surface of the gantry base part  2  with a gap of about several millimeters in the y-axis direction so that it can slide thereon. 
     Measuring the position of the gantry  1  in the z-axis direction (also simply referred to as a position detection) in the embodiment is performed by the use of a linear encoder. First, a linear scale  50  of the linear encoder is mounted on the center rail  4  along the linear guide rail  41 . Further, a pickup sensor  51  is mounted on the gantry base part  2  so as to oppose the linear scale  50 . The outputs of this pickup sensor  51  (A-phase and B-phase encoder signals) are sent to a control board  100  (see  FIG. 4 ) and the position of the gantry  1  in the z-axis direction can be measured based on this outputs. 
     Since the two linear guide blocks  42 ,  43  mounted on the bottom surface of the gantry base part  2  are fitted in this manner on the linear guide rail  41  mounted on the center rail  4  so that they can freely slide, the deviation of the gantry  1  in the x-axis direction with respect to the movement of the gantry  1  in the z-axis direction is forcibly limited. Therefore, even if the gaps between the grooves of the wheels  5 ,  6 ,  7 ,  8  and the rails  3   a ,  3   b  apply forces to the gantry  1  in the x-axis direction as the gantry  1  moves or apply a force for changing the direction of the gantry  1  gradually to the gantry  1  as the gantry  1  moves, the gantry  1  is not deviated by the forces but can be moved correctly straight. 
     This can reduce a deviation in the rotational center of scan and a deviation in a scan position in the z-axis direction. 
     Further, it is possible to detect the position of the gantry  1  in the z-axis direction with high accuracy without being affected by the machining accuracy such as the size of the wheel and the direction of an axle. Moreover, since the linear scale  50  is mounted on a lower portion near the center in the x-direction of the gantry  1 , even if a force of torsion is applied to the gantry base part  2  to deform the gantry base part  2 , it is possible to detect the scan position correctly. 
     Next, an operation of moving the gantry  1  will be described. Here, referring to  FIG. 4 , on the right side of paper is provided a table (not shown) on which a body to be inspected is placed. In the following description, in the direction of movement of the gantry  1 , that is, in the z-axis direction, let&#39;s call a direction in which the gantry is brought near to the table as an IN side and a direction in which the gantry is brought away from the table as an OUT side. 
     Further, in  FIG. 4 , on the bottom surface of the gantry base part  2  are mounted a first limit switch S 1  and a second limit switch S 2 . Moreover, a protrusion  52  for operating the first limit switch S 1  is provided on an outside movement limit position (OUT limit position) as a first check point on the center rail  4  and a protrusion  53  for operating the second limit switch S 2  is provided on a predetermined position sandwiched between the protrusion  52  and the table as a second check point on the center rail  4 . These protrusions  52 ,  53  are used in a processing of setting the scan base position that will be described later. 
       FIG. 7  is a block diagram to show the constitution of the control board  100  (see  FIG. 4 ) built in the gantry base part  2 . 
     In  FIG. 7 , a reference numeral  61  denotes a CPU for controlling the movement of the gantry  1  and a reference numeral  62  denotes a ROM for storing the operation processing procedure (program) of the CPU  61  and a reference character  63   a  denotes a RAM functioning as a main storage and a reference character  63   b  denotes a flash memory functioning as an auxiliary storage. A reference numeral  64  denotes an interface for inputting data from an operator console  200  for outputting information relating to scan to the gantry  1 . Reference numerals from  65  to  68  denote a plurality of interfaces for inputting data from an operating panel  20 , the first limit switch S 1 , the second limit switch S 2 , and the pickup sensor  51 . A reference numeral  69  denotes an up/down counter indicating a relative position in the z-axis direction based on the A-phase and B-phase encoder signals from the pickup sensor  51  and a reference numeral  70  denotes a motor driver for performing the driving control of the motor  11 . 
     Before performing a scan, for example, after turning on the power, an operation of moving the gantry  1  once on the IN side and the OUT side by a manual operation to make a check that moving the gantry  1  does not cause a danger is performed. The movement of the gantry  1  by this manual operation can be performed by the use of a movement button provided on the operating panel  20  (see  FIG. 1 ) arranged on the gantry  1 . 
       FIG. 8  shows one example of the operating panel  20 . A reference numeral  21  denotes a first movement button for moving the gantry  1  on the IN side, and a reference numeral  22  denotes a second movement button for moving the gantry  1  on the OUT side, and a reference numeral  23  denotes a setting button for setting the base position of the scan. Moreover, a reference numeral  24  denotes a display part for producing various displays. Of course, buttons for other objects can be provided but only the ones necessary for describing the present invention are shown in the drawing. 
     While the operator presses the first movement button  21  (ON), the gantry  1  is moved on the IN side at a predetermined speed. While the operator presses the second movement button  22  (ON), conversely, the gantry  1  is moved on the OUT side at a predetermined speed. In both the buttons, when the operator presses off the button, the button is immediately turned off to stop the movement of the gantry  1 . 
     The maximum range of movement of the gantry  1  performed by this manual operation is stored in the RAM  63   a . Then, the gantry  1  can be moved by a remote control from the operating console  200  and in this case, the gantry  1  is allowed to move only within the maximum movement range stored in the RAM  63   a.    
     Here, for example, it is also possible to clear the maximum movement range stored in the RAM  63   a  when any one of operations of moving the gantry  1  to a retracted position (OUT-side movement limit position), moving the table, and a predetermined operation on the operating console  200  is performed. However, in order to move the gantry  1  or to perform a scan by a remote control thereafter, it is necessary to move the gantry  1  again by the manual operation to make a check of safety. 
     It is possible to ensure safety by limiting the movable range of the gantry  1  by the remote control in this manner. 
     Next, a processing of setting the scan base position in the embodiment will be described. 
     The scan base position can be set by moving the gantry  1  and pressing the setting button  23  at a desired position. As described above, however, there is presented the problem that even if the scan base position is the same for each scan, the setting operation by the setting button  23  (manual setting operation) needs to be performed every time, which makes operability worse. In the case where the manual setting operation is performed every time, a setting error or a setting miss may be caused. 
     Thus, in the embodiment, the scan base position is automatically set as far as no abnormality is found in the above-described checking operation performed by moving the gantry  1  by the manual operation after turning on the power. 
     A processing of setting the scan base position in the embodiment will be described in detail by the use of a flow chart shown in FIG.  9  and FIG.  10 . 
       FIG. 9  is a flow chart to show a process for setting the positional relations of the first limit switch S 1 , the second limit switch S 2 , and the desired scan base position, respectively. It is preferable to perform this process when the system is adjusted. Further, the above-mentioned desired scan base position is usually determined in accordance with the installation environment of the system (such as size of a room). 
     First, the operator keeps pressing the second movement button  22  to move the gantry  1  to the OUT limit position (step S 1 ). The fact that the gantry is moved to the OUT limit position can be detected by the fact that the first limit switch S 1  is put into contact with the protrusion  52  to be switched from OFF to ON. At this time, the gantry is forcibly stopped. 
     Next, the operator presses the first movement button  21  to move the gantry  1  toward the desired scan base position (step S 2 ). During the movement of the gantry  1 , a position z 1  where the first limit switch S 1  is switched from ON to OFF (first check point) is detected and further a position z 2  where the second limit switch S 2  is put into contact with the protrusion  53  to be switched from OFF to ON (second check point) is detected, and the distance Ds between z 1  and z 2  is stored in the flash memory  63   b  (step S 3 ) 
     Thereafter, when the gantry  1  comes to a desired position as the scan base position, by the manual operation, that is, by setting the setting button  23 , the desired position z 0  is set as the scan base position (step S 4 ). 
     Then, the distance Dr between the position z 2  where the second limit switch S 2  is switched from OFF to ON and z 0  is stored in the flash memory  63   b  (step S 5 ). 
     In this manner is finished the process for initially setting the positional relations of the first limit switch S 1 , the second limit switch S 2 , the desired scan base position, respectively, which is preferably performed when the system is adjusted. 
       FIG. 10  is a flow chart to show a processing of setting the scan base position which is performed in a checking operation performed by moving the gantry  1  by the manual operation after turning on the power or the like. A program corresponding to this flow chart is stored in the ROM  62  and is executed by the CPU  61  after turning on the power. 
     First, at step S 11 , it is judged based on the input of the second movement button  22  whether a movement command to the OUT side is given or not. When the second movement button  22  is pressed down, the routine advances to step S 12  and the gantry  1  starts to move to the OUT side. Here, in the case where the second movement button  22  is separated from the operator to be turned off, at this time, the present process is finished. 
     At step S 13 , it is monitored whether the first limit switch S 1  is turned ON or not during the movement of the gantry  1 . Then, when the first limit switch S 1  is turned ON, the routine advances to step S 14  where the movement of the gantry  1  is stopped. 
     Next, at step S 15 , it is judged based on the input of the first movement button  21  whether a movement command to the IN side is given or not. When the first movement button  21  is pressed down, the routine advances to step S 16  and the gantry  1  starts to move to the IN side. Here, also in the case where the first movement button  21  is separated from the operator to be turned off, at this time, the present process is finished. 
     At step S 17 , it is monitored whether the first limit switch S 1  is switched from ON to OFF or not. Then, when the first limit switch S 1  is turned OFF, at step S 18 , a position Z 1 ′ at this time is stored in the RAM  63   a.    
     Next, at step S 19 , it is monitored whether or not the second limit switch S 2  is switched from OFF to ON during the movement of the gantry  1 . Then, when the second limit switch S 2  is turned ON, at step S 20 , a position z 2 ′ at this time is stored in the RAM  63   a.    
     Next, at step S 21 , it is judged whether or not the error between the distance between the position z 1 ′ when the first limit switch S 1  is switched from ON to OFF and the position z 2 ′ when the second limit switch S 2  is switched from OFF to ON and the distance information Ds stored in the flash memory  63   b  is within a predetermined range. 
     That is, it is judged whether or not the error satisfies the following equation.
 
|(z 2 ′−z 1 ′)−Ds|&lt;T
 
where T designates a predetermined threshold value.
 
     Here, in the case where the above-mentioned equation is not satisfied, a position detection by any one of the first limit switch S 1 , the second limit switch S 2 , and the pickup sensor  51  is judged to be not correct and the routine advances to step S 22  where a predetermined error is displayed on the display part  24 , or an alarm sound may be issued to inform the operator of an error. In any case, in the case of such error, the scan in this state is prohibited. 
     Further, in the case where the above-mentioned equation is satisfied, the routine advances to step S 23  where a position obtained by adding the distance information Dr stored in the flash memory  63   b  to the position z 2 ′ when the second limit switch S 2  is switched from OFF to ON is set as the scan base position and where the obtained base position information is stored in the RAM  63   a.    
     In the case where the base position in each scan is the same, as far as no abnormality is found in the checking operation performed by moving the gantry  1  by the manual operation performed after turning on the power, the scan base position is automatically set, so that operability can be drastically improved and the occurrence of a setting error and a setting miss can be prevented. 
     Further, in the case where an abnormality is caused in a position detection mechanism provided in the system, the abnormality is informed and the scan in this state is prohibited, so that safety can be ensured. 
     Here, while the protrusions  52 ,  53  are provided at the predetermined positions in the direction of movement of the gantry  1  and the predetermined positions are detected by the use of the limit switches S 1 , S 2  that are operated when they come in contact with the protrusions  52 ,  53  in the embodiment described above, the predetermined positions can be detected also by the other means, for example, a combination of an optical sensor and an optical passing slit, a combination of an optical sensor and an optical reflector, a magnetic position detection sensor, a position detection sensor by an electrostatic capacity, a position detection by an image by means of a CCD camera, and the like. 
     Further, while the OUT limit position is made the first check point and a position where the gantry  1  is moved forward by a predetermined distance from the first check point is made the second check point in the embodiment described above, it is also recommended that a sensor for detecting the first check point and a sensor for detecting the second check point are provided independently from each other and that the first check point and the second check point be provided at the positions that are detected by both the sensors at the same time when the gantry  1  is moved to the predetermined position. 
     Further, at step S 5  in  FIG. 9 , the distance Dr from the second check point z 2  to the scan base position z 0  by the manual operation is stored, and at step S 5  in  FIG. 10 , the scan base position is set again by using this Dr, but it is also recommended that the distance Dr′ from the first check point z 1  to the scan base position z 0  by the manual operation be stored at step S 5  in FIG.  9  and that the scan base position be set again by using this Dr′ at step S 23  in FIG.  10 . 
     Still further, while two linear guide blocks  42 ,  43  mounted on the bottom surface of the gantry base part  2  are mounted on the linear guide rail  41  in the embodiment described above, as far as they are fixed with sufficient accuracy so that the direction of the gantry  1  is not changed, it is also recommended that only one guide block be used. 
     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.
For the purpose of reducing a deviation in the rotational center of scan and a deviation in scan position in the direction of length of a rail, a gantry system including a pair of runway rails and a gantry in an X-ray CT system that can move along the runway rails, is characterized by comprising a linear guide rail arranged in the direction along the runway rails and linear guide blocks mounted on the gantry and slidably fitted on the linear guide rail.