| 摘要 |
PROBLEM TO BE SOLVED: To allow X-ray beams to always pass through the center of the clearance of a slit mechanism by rotatably providing two cams mutually provided with elliptic cross sections and prescribed length in the mechanism narrowing the X-ray beams radiated from an X-ray tube to fix a slice width to a subject. SOLUTION: A detector of an operation gantry is provided with a sensor 25 for detecting focus displacement sending a focus displacement detecting signal in proportion to the receiving face area of the X-ray beams from plurally divided faces and its output signal is read into a slit width automatically control part 24. Then, the values of plural focus displacement signals are compared with each other to obtain a displacing amount with respect to an X-ray detecting element to geometrically obtain the distances Ra and Rb between the X-ray beams and a pair of elliptic cylinder type cams 17a and 17b at a slit L based on this displacing amount. Then, the relation of these distance is judged and the rotational amount of the respective cams 17a and 17b for making the axis of the X-ray beams pass through the center of the slit L of each of the comes 17a and 17b is obtained to control respective motors 18 and 19.
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