Patent Number: 
Section: claims

1. A filter assembly for a computed tomographic imaging system comprising:first and second endplates at opposite ends of the filter assembly;a first moveable subassembly comprising at least a first x-ray filter and configured to move between said first and second endplates along a z-axis perpendicular to said first endplate;a second moveable subassembly comprising at least a second x-ray filter and configured to move between said first and second endplates along the z-axis perpendicular to said second endplate; andwherein said first moveable subassembly and said second moveable subassembly are independently movable along the z-axis to provide at least a small bowtie x-ray filter, a large bowtie x-ray filter, a medium bowtie x-ray filter, a flat filter, and a closed position for a radiation source positioned in a fixed position relative to said filter assembly, wherein the z-axis is parallel to a rotational axis of the computed tomographic imaging system. 2. A filter subassembly in accordance with claim 1 wherein said first movable subassembly comprises a small bowtie filter and the large bowtie filter, and the second movable subassembly comprises the medium bowtie filter. 3. A filter assembly in accordance with claim 2 wherein the second movable subassembly further comprises at least one flat filter. 4. A filter assembly in accordance with claim 3 wherein said at least one flat filter on said second movable subassembly is configurable to overlap a portion of one of said bowtie filters of said first movable subassembly in at least one position in a range of movement of said first movable subassembly and said second moveable subassembly. 5. A filter assembly in accordance with claim 3 wherein said at least one flat filter is a titanium plate. 6. A filter assembly in accordance with claim 5 wherein said at least one flat filter further comprises a plate of tungsten, and said bowtie filters of said first movable subassembly comprises a combination of aluminum and graphite machined parts and copper, and said bowtie filter of said second movable subassembly further comprises aluminum and graphite machined parts. 7. A filter assembly in accordance with claim 1 wherein said first movable subassembly and said second movable subassembly are independently movable and also simultaneously moveable. 8. A filter assembly in accordance with claim 1 further comprising a first stepper motor with lead screw drive configured to move said first moveable subassembly and a second stepper motor with lead screw drive configured to drive said second movable subassembly. 9. A filter assembly in accordance with claim 8 further comprising hollow tubing between the first and the second endplates configured to be outside an x-ray beam when in use, said hollow tubing having wires therein carrying electrical signals to at least one of said stepper motors. 10. A filter assembly in accordance with claim 8 further comprising a position encoder and a feedback control to provide a home position reference and position feedback. 11. A computed tomographic (CT) imaging system comprising:a rotatable gantry having a gantry opening and a z-axis parallel to a gantry rotational axis;an x-ray filter assembly on the rotatable gantry;a radiation source configured to direct a fan beam of radiation towards an object in the gantry opening through the x-ray filter assembly; anda detector array on the rotatable gantry configured to acquire projection data representative of radiation passing through the object; whereinsaid CT imaging system configured to reconstruct an image of the object utilizing the acquired projection data; andsaid filter assembly further comprising:first and second endplates at opposite ends of the filter assembly;a first moveable subassembly comprising at least a first x-ray filter and configured to move between said first and second endplates along a z-axis perpendicular to said first endplate; anda second moveable subassembly comprising at least a second x-ray filter and configured to move between said first and second endplates along the z-axis perpendicular to said second endplate and perpendicular to a plane of said fan beam of radiation;wherein said first moveable subassembly and said second moveable subassembly are independently movable along the z-axis to provide at least a small bowtie x-ray filter, a large bowtie x-ray filter, a medium bowtie x-ray filter, a flat filter, and a closed position for a radiation source positioned in a fixed position relative to said filter assembly. 12. A CT imaging system in accordance with claim 11 wherein said first movable subassembly comprises a small bowtie filter and the large bowtie filter, and the second movable subassembly comprises the medium bowtie filter. 13. A CT imaging system in accordance with claim 12 wherein the second movable subassembly further comprises at least one flat filter. 14. A CT imaging system in accordance with claim 13 wherein said at least one flat filter on said second movable subassembly is configurable to overlap a portion of one of said bowtie filters of said first movable subassembly in at least one position in a range of movement of said first movable subassembly and said second moveable subassembly. 15. A CT imaging system in accordance with claim 11 wherein said first movable subassembly and said second movable subassembly are independently movable and also simultaneously moveable. 16. A CT imaging system in accordance with claim 11 further comprising a first stepper motor with lead screw drive configured to move said first moveable subassembly and a second stepper motor with lead screw drive configured to drive said second movable subassembly. 17. A CT imaging system in accordance with claim 16 further comprising hollow tubing between the first and the second endplates configured to be outside the x-ray fan beam, said hollow tubing having wires therein carrying electrical signals to at least one of said stepper motors. 18. A CT imaging system in accordance with claim 16 further comprising a position encoder and a feedback control to provide a home position reference and position feedback for the first filter subassembly and the second filter subassembly. 19. A method for filtering a radiation source of a computed tomographic (CT) imaging system for scanning a region of an object using the CT imaging system, said method comprising:passing a fan beam of the radiation source between first and second endplates at opposite ends of a filter assembly, the filter assembly having a first moveable subassembly comprising at least a first x-ray filter configured to move between said first and second endplates along a z-axis perpendicular to said first endplate and the fan beam, and a second moveable subassembly comprising at least a second x-ray filter configured to move between said first and second endplates along the z-axis perpendicular to said second endplate and the fan beam; andmoving at least one of the first moveable subassembly or the second moveable subassembly along the z-axis to interpose a small bowtie x-ray filter, a large bowtie x-ray filter, a medium bowtie x-ray filter, a flat filter, or a closed position between the radiation source and the object, wherein the z-axis parallel to a rotational axis of the CT imaging system. 20. A method in accordance with claim 19 further comprising overlapping at least one flat filter on the second movable subassembly over a portion of a bowtie filter of the first movable subassembly. 21. A filter subassembly in accordance with claim 1 wherein said first endplate is other than said second endplate.