Patent Number: 
Section: claims

1. A system for controlling X-ray beams in an X-ray CT system having an X-ray generating source, an X-ray detecting section, a first filter unit disposed closest to said detecting section, and a second filter unit disposed closest to said source and between said source and said first filter unit, said first filter unit having a thin central portion and thick end portions to reduce X-ray attentuation at said central portion and enhance X-ray attenuation at the end portions, wherein said second filter unit is a flat plate having therein at least two filters, each having different X-ray transmission properties; and further comprising positioning means for selectively moving said second filter unit so that said X-ray beams are selectively directed through a selected one of said at least two filters and then through said first filter unit. 2. The system of  claim 1 , wherein said positioning means comprises a motor, and a gear attached to an edge of said flat plate of said second filter unit, and means for energizing said motor and engaging said gear to selectively drive said flat plate and cause a selected one of said at least two filters of said second filter unit to filter said X-ray beams. claim 1 3. The system of  claim 1 , wherein one of said at least two filters of said second filter unit is used to scan a head of a subject, and a second of said at least two two filters of said second filter unit is used to scan an abdomen of said subject, and wherein said first and second filters of said second filter unit have X-ray transmission factors F1 and F2, respectively, with F2 being less than F1. claim 1 4. The system of  claim 3 , wherein said second filter unit further comprises a third filter for use in scanning a thorax of said subject, said third filter having an X-ray transmission factor F3 which is less than X-ray transmission factor F2. claim 3 5. The system of  claim 4 , wherein said first, second, and third filters of said second filter unit have different thicknesses. claim 4 6. The system of  claim 1 , further comprising an operating means for supplying instruction signals to said source, said detecting section, and said positioning means to perform reconstruction processing for an X-ray tomographic signal based on data obtained from X-ray beams supplied through said first and second filter units and detected by said detecting section. claim 1 7. The system of  claim 6 , wherein said operating means comprises: claim 6 display means for displaying a menu for selecting a scan region of a subject, and at least one field for inputting size of said subject;  decision means for deciding which of said at least two filters of said second filter unit is to be used based on the selected scan region and input size of said subject; and  means for supplying an instruction indicating the filter selection decided by said decision means to said positioning means. 8. The system of  claim 7 , further comprising: claim 7 storage means for storing respective correction data corresponding to said at least two filters of said second filter unit;  correction means for correcting data represent result of a scan transferred using correction data corresponding to the filter decided by said decision means; and  wherein said operating means causes reconstitution of an X-ray tomographic image using data corrected by said correction means. 9. A method for controlling X-ray beams in an X-ray cT system having an X-ray generating source, an X-ray detecting section, a first filter unit disposed closest to said detecting section, and a second filter unit disposed closest to said source and between said source and said first filter unit, said first filter unit having a thin central portion and thick end portions to reduce X-ray attenuation at said central portion and enhance X-ray attenuation at the end portions, wherein said method comprises the steps of: providing as said second filter unit a flat plate having therein at least two filters, each having different X-ray transmission properties; and selectively moving said second filter unit while keeping said first filter unit fixed so that said X-ray beams are selectively directed through a selected one of said at least two filters of said second filter unit and then through said first filter unit. 10. The method of  claim 9 , comprising the steps of: claim 9 providing said positioning means with a motor and a gear attached to an edge of said flat plate of said second filter unit; and  engaging said motor to said gear to selectively drive said flat plate and cause a selected one of said at least two filters of said second filter unit to filter said X-ray beams. 11. The method of  claim 9 , further comprising the steps of: claim 9 providing said first and second filter units with X-ray transmission factors F1 and F2 respectively, with F2 being less than F1;  using one of said at least two filters of said second filter unit to scan a head of a subject; and  using a second of said at least two filters of said second filter unit to scan an abdomen of said subject. 12. The method of  claim 11 , further comprising the steps of: claim 11 providing said second filter unit with a third filter having an X-ray transmission factor F3 which is less than F2; and  scanning a thorax of said subject using said third filter. 13. The method of  claim 12 , wherein said first, second and third filters of said second filter unit have different thicknesses. claim 12 14. The method of  claim 9 , further comprising the steps of; claim 9 supplying instruction signals to said source, said detecting section, and said positioning means;  performing reconstruction processing for an X-ray tomographic signal based on data obtained from X-ray beams supplied through said first and second filter units and detected by said detecting section. 15. The method of  claim 14 , further comprising the steps of: claim 14 displaying a menu for selecting a scan region of a subject, and at least one field for inputting size of said subject;  deciding which of said at least two filters of said second filter unit is to be used based on the selected region and input size of said subject; and  supplying an instruction indicating the filter selection decided by said deciding step to said positioning step. 16. The method of  claim 15 , further comprising the steps of: claim 15 storing respective correction data correspond to said at least two filters of said second filter unit;  correcting data representing result of scan transferred using correction data to the filter decided by said deciding step; and  causing reconstruction of an X-ray tomographic image using data corrected by said correcting step. 17. A storage medium for storing a program code for providing instruction signals to a system for controlling X-ray beams in an X-ray CT system having an X-ray generating source, an X-ray detecting section, an operating device, a first filter unit disposed closest to said detecting section, and a second filter unit disposed closest to said source and between said source and said first filter unit, said first filter unit having a thin central portion and thick end portions to reduce X-ray attentuation at said center 1 portion and enhance X-ray attentuation at the end portions, wherein said second filter unit is a flat plate having therein at least two filters, each having different X-ray transmission properties; and positioning means for selectively moving said second filter unit so that said X-ray beams are selectively directed through a selected one of said at least two filters and then through said first filter unit, wherein said CT system performs reconstruction processing for an X-ray tomographic image based on data transferred from said detecting section, said storage medium storing a program code of a display step of displaying a menu for selecting a scan region of a subject, a program code of a decision step of deciding which of said at least two filters is to be used based on a selected scan region, and a program code of a step of supplying an instruction indicating the filter which is selected at said deciding step.