Patent Number: 
Section: claims

1. An X-ray CT apparatus comprising:a supporter configured to support a patient, capable of moving along the body axis direction of said patient,an imaging part including an X-ray-generator configured to irradiate X-rays while rotating around said body axis and an X-ray-detector configured to detect said X-rays that have permeated said patient,a collimator configured to change the irradiation field of said X-ray to be irradiated,a scan controller configured to control the movement of said supporter and said imaging part, andan image-reconstructer configured to reconstruct image data based on the X-rays that have been detected by said X-ray detector, whereinthe X-ray CT apparatus comprises:a detector configured to detect the amount of movement of said patient by said supporter, anda collimator controller configured to control so as to change the size of the opening of said collimator based on said amount of movement. 2. The X-ray CT apparatus according to claim 1, whereinsaid scan controller is configured to perform helical scanning and to control so that the speed of movement of said supporter changes while collecting said X-rays by the helical scanning. 3. The X-ray CT apparatus according to claim 1, whereinsaid collimator controller is configured to control so as to gradually widen the opening of said collimator at the starting portion of a scanning range by helical scanning. 4. The X-ray CT apparatus according to claim 1, whereinsaid collimator controller is configured to control so as to gradually narrow the opening of said collimator at the end portion of a scanning range by helical scanning. 5. The X-ray CT apparatus according to claim 1, whereinsaid collimator controller comprises:a switching part configured to switch a first mode for controlling the movement of said collimator based on said amount of movement that has been detected by said detector and a second mode for controlling the movement of said collimator without using the amount of movement that has been detected by said detector. 6. The X-ray CT apparatus according to claim 5, whereinsaid first mode is used during helical scanning in which an active collimator is being used, and said second mode is used during scanning other than helical scanning or during helical scanning in which an active collimator is not being used. 7. The X-ray CT apparatus according to claim 1, whereinsaid scan controller is configured to control the movement of said supporter by using a timing pulse and said detector is configured to generate an encoder pulse based on said movement amount of said supporter,further comprising:a pulse-comparing part configured to compare said timing pulse and said encoder pulse, and to issue a warning if there is a difference that exceeds a prescribed value in the pulse cycle.