Patent Number: 
Section: claims

1. A collimation system for controlling x-ray exposure during a dynamic pitch helical scan using a translatable table comprising:a collimator positioned between an x-ray source and an object to be scanned configured to shutter an x-ray fan beam generated by the x-ray source to at least one of translate the x-ray fan beam along a z-axis of the scan and vary the width of the x-ray beam along the z-axis; anda collimator controller configured to dynamically position the collimator using at least one of predetermined trajectory of the translatable table and an actual position of the translatable table. 2. A collimation system in accordance with claim 1 further comprising a table position encoder configured to determine an actual position of the translatable table. 3. A collimation system in accordance with claim 2 wherein said collimator controller is configured to determine at least one of a velocity of the translatable table and an acceleration of the translatable table. 4. A collimation system in accordance with claim 1 further comprising a trajectory selector configured to store at least one of a predetermined trajectory and a received trajectory. 5. A collimation system in accordance with claim 1 wherein the trajectory is described by the coefficients of an equation describing the translational position of the translatable table as a function of at least one time and a gantry rotation angle. 6. A collimation system in accordance with claim 5 wherein the coefficients are transmitted to the collimator controller at least one of before the beginning of the scan and during the scan. 7. A collimation system in accordance with claim 5 wherein the translational position of the translatable table is described by:            z      ⁡              (        s        )              =                  Tk        ⁡                  (                      1            -                          cos              ⁡                              (                                  s                  T                                )                                              )                    +                        v          0                ⁢        s            +                        1          2                ⁢                  a          0                ⁢                  s          2                    +              z        0              ,wherein T represents a trajectory selected,k represents a constant,v0 represents an initial velocity,a0 represents an initial acceleration, andz0 represents an initial position. 8. A system for reducing x-ray exposure in a computed tomography system, the computed tomography system including an x-ray source, a detector array comprising a plurality of detector cells extending in a z-axis, and a pre-patient collimator, said system configured to:determine a position of a patient table along the z-axis; andposition said collimator to define a thickness of a fan beam to be directed towards an object to be imaged using at least one of the position of the patient table and a predetermined table trajectory. 9. A system in accordance with claim 8 wherein said system is configured to determine a position of a patient table using a position encoder. 10. A method of controlling x-ray dosage in a computed tomography (CT) system including a patient table translatable along a z-axis at a dynamically changeable rate and a collimator that defines an aperture, said method comprising:receiving data concerning a scan trajectory of the patient table;determining an amount of collimation on the detector using a selected image reconstruction algorithm requirement; andpositioning the collimator aperture such that the determined collimation is maintained during the scan. 11. A method in accordance with claim 10 wherein receiving data concerning a scan trajectory of the patient table comprises receiving a selectable one of a plurality of predetermined scan trajectories. 12. A method in accordance with claim 10 wherein receiving data concerning a scan trajectory of the patient table comprises determining an adaptive scan trajectory determined using scan data received during the scan. 13. A method in accordance with claim 10 wherein receiving data concerning a scan trajectory of the patient table comprises determining an equation that describes the translational position of the patient table as a function of at least one of time and an angle of rotation of a gantry portion of the CT system. 14. A method in accordance with claim 13 wherein receiving data concerning a scan trajectory of the patient table comprises receiving coefficients of the terms of the equation. 15. A method in accordance with claim 10 further comprising describing a translational position of the patient table using:            z      ⁡              (        s        )              =                  Tk        ⁡                  (                      1            -                          cos              ⁡                              (                                  s                  T                                )                                              )                    +                        v          0                ⁢        s            +                        1          2                ⁢                  a          0                ⁢                  s          2                    +              z        0              ,wherein T represents a trajectory selected,k represents a constant,v0 represents an initial velocity,a0 represents an initial acceleration, andz0 represents an initial position. 16. A method in accordance with claim 15 further comprising receiving coefficients T, k, v0, a0, and z0 by a collimator controller. 17. A method in accordance with claim 10 further comprising receiving coefficients of the terms of the equation by a collimator controller. 18. A method in accordance with claim 10 wherein determining an amount of collimation of the detector comprises receiving at least one of a current position of the patient table and a current angular position of the gantry. 19. A method for reducing x-ray exposure in a computed tomography (CT) system, the CT system including a gantry having at least an x-ray source, a detector array comprising a plurality of detector cells extending in a z-axis, a patient table translatable in the Z axis, and a collimator positioned between the x-ray source and the patient table, said method comprising:receiving coefficients of the terms of an equation that describes the translational position of the patient table as a function of at least one of time and an angle of rotation of a gantry portion of the CT system, by a collimator controller;determining an amount of collimation of the detector for an image reconstruction algorithm selected; andadjusting the aperture of the collimator in the Z direction to illuminate a portion of the detector that is used by the reconstruction algorithm. 20. A method in accordance with claim 19 wherein determining an amount of collimation of the detector comprises receiving at least one of a current position of the patient table and a current angular position of the gantry.