Patent Number: 
Section: claims

1. A collimator control method for an X-ray CT apparatus wherein a subject is helically scanned in a direction of a body axis thereof using an X-ray beam formed by a collimator, and image reconstruction is performed based on projection data obtained through an X-ray detector, said method comprising:changing an aperture of the collimator according to a position of a helical scan on the body axis of the subject in the process of progress of the helical scan, said changing comprising:opening the aperture during an acceleration of the progress of the helical scan in the body-axis direction; andclosing the aperture during a deceleration of the progress of the helical scan in the body-axis direction. 2. The collimator control method according to claim 1, wherein the acceleration and the deceleration of the progress of the helical scan in the body-axis direction are linear. 3. The collimator control method according to claim 1, wherein the acceleration and the deceleration of the progress of the helical scan in the body-axis direction are nonlinear. 4. The collimator control method according to claim 1, wherein said position of a helical scan on the body axis of the subject in the process of progress of the helical scan is based on a coordinate of the body axis direction which is measured during the helical scan. 5. The collimator control method according to claim 1, wherein the aperture defines a predetermined opening, a first half of the predetermined opening extending from a center point of the aperture forward along the body axis to a front edge of the aperture and a second half of the predetermined opening extending from the center point of the aperture backwards along the body axis to a back edge of the aperture, said changing comprising:at a start position of the helical scan, opening the first half and closing the second half such that the aperture is at about one-half of the predetermined opening at the first half;at an intermediate position of the helical scan in the direction of the progress of the helical scan, opening the aperture to the predetermined opening by opening the first half and the second half; andat an end position of the helical scan, closing the aperture at the first half such that the aperture is opened by about one-half of the predetermined opening at the second half. 6. The collimator control method according to claim 1, wherein said changing comprises changing the aperture continuously during the helical scan. 7. An X-ray CT apparatus comprising:an X-ray source;a collimator for shaping X-rays emitted from the X-ray source;a control device for controlling the collimator;an X-ray detector disposed so as to be opposed to the X-ray source and the collimator with a subject interposed therebetween; andan image reconstructing device for helically scanning the subject in a direction of a body axis thereof and reconstructing an image on the basis of projection data obtained through the X-ray detector,wherein the control device changes an opening degree of an aperture of the collimator according to a position of a helical scan on the body axis in the process of progress of the helical scan, said control device configured to:open the aperture during an acceleration of the progress of the helical scan in the body-axis direction; andclose the aperture during a deceleration of the progress of the helical scan in the body-axis direction. 8. The X-ray CT apparatus according to claim 7, wherein the acceleration and the deceleration of the progress of the helical scan in the body-axis direction are linear. 9. The X-ray CT apparatus according to claim 7, wherein the acceleration and the deceleration of the progress of the helical scan in the body-axis direction are nonlinear. 10. The X-ray CT apparatus according to claim 7, wherein said position of a helical scan on the body axis of the subject in the process of progress of the helical scan is based on a coordinate of the body axis direction which is measured during the helical scan. 11. The X-ray CT apparatus according to claim 7, wherein the aperture defines a predetermined opening, a first half of the predetermined opening extending from a center point of the aperture forward along the body axis to a front edge of the aperture and a second half of the predetermined opening extending from the center point of the aperture backwards along the body axis to a back edge of the aperture, said control device configured to:at a start position of the helical scan, open the first half and close the second half such that the aperture is at about one-half of the predetermined opening at the first half;at an intermediate position of the helical scan in the direction of the progress of the helical scan, open the aperture to the predetermined opening by opening the first half and the second half; andat an end position of the helical scan, close the aperture at the first half such that the aperture is opened by about one-half of the predetermined opening at the second half. 12. The X-ray CT apparatus according to claim 7, wherein said control device is further configured to change the aperture continuously during the helical scan.