Patent Number: 
Section: claims

1. A pre-patient collimator for controlling x-ray exposure during a scan with a computed tomography system, the computed tomography system including a patient table translatable along a z-axis, an x-ray source and a detector array, said collimator comprising:a first cam configured to shutter an x-ray fan beam, generated by the x-ray source, in a first direction;a second cam located on an opposite side of a focal point of the x-ray source from the first cam, said second cam configured to shutter the x-ray fan beam in a second direction, the second direction being opposite from the first direction; anda cam drive configured to position at least one of said first cam and said second cam during the scan at a rate proportional to a translation speed of the table. 2. A collimator in accordance with claim 1 wherein said first cam and said second cam are fabricated from a x-ray absorbing material, and said first cam and said second cam are configured to block a portion of an x-ray beam directed towards an object to be imaged to adjust the thickness of the fan beam. 3. A collimator in accordance with claim 2 wherein said cam drive is configured to independently position each said first cam and said second cam. 4. A collimator in accordance with claim 2 wherein said cam drive comprises at least one cam motor. 5. A collimator in accordance with claim 4 wherein said cam drive comprises a cam motor for each said first cam and said second cam. 6. A collimator in accordance with claim 2 wherein said cam drive is configured to modify a fan beam thickness. 7. A system for reducing x-ray exposure in a computed tomography system, the computed tomography system including a patient table translatable along a z-axis, an x-ray source, a detector array comprising a plurality of detector cells extending in the z-axis, and a pre-patient collimator comprising a shutter cam and a tracking cam, said system configured to:determine a speed of a patient table along the z-axis; andposition at least one of the shutter cam and the tracking cam during a scan at a rate proportional to the determined table speed to define a thickness of a fan beam to be directed towards an object to be imaged. 8. A system in accordance with claim 7 wherein said cams are configured to block a portion of an x-ray beam emitted by the x-ray source to adjust the thickness of the fan beam. 9. A method controlling x-ray dosage in a computed tomography system including a patient table translatable along a z-axis and a collimator having a first cam and a second cam that define an aperture, said method comprising:opening the first cam at a rate proportional to a translation speed of the table; andclosing the second cam at a rate proportional to the translation speed of the table. 10. A method in accordance with claim 9 further comprising determining a speed of translation of the table along the z-axis. 11. A method in accordance with claim 9 wherein opening the first cam comprises initiating opening the first cam when a leading edge of an object of interest intersects an edge of an x-ray beam defined by the first cam. 12. A method in accordance with claim 9 wherein opening the first cam comprises opening the first cam at a velocity determined using:  velocity  =            (                        source_to          ⁢          _cam                                      source_to            ⁢            _iso                    +                      SFOV            2                              )        *          table_speed      .       13. A method in accordance with claim 9 wherein opening the first cam comprises initiating opening the first cam when a leading edge of an object of interest is coincident with an edge of an x-ray beam defined by the first cam at a velocity determined by:  velocity  =            (                        source_to          ⁢          _cam                                      source_to            ⁢            _iso                    -                      SFOV            2                              )        *          table_speed      .       14. A method in accordance with claim 9 wherein closing the second cam comprises initiating closing the second cam when a trailing edge of an object of interest intersects an edge of an x-ray beam defined by the second cam. 15. A method in accordance with claim 9 further comprising fully closing the first cam prior to the beginning of a scan. 16. A method in accordance with claim 9 further comprising fully opening the second cam prior to the beginning of a scan. 17. A method in accordance with claim 9 further comprising maintaining the first cam at a position Bcam for a time t0<t<tB, wherein to represents a time when the first view is collected, tB represents a time for a leading edge of an object of interest to intersect the beam edge defined by the first cam when the first cam is fully closed, and wherein Bcam is a fully closed position of the first cam. 18. A method in accordance with claim 9 further comprising maintaining the first cam in a position defined by, Bcam+(t−tB)*vBC, for a time, tB<t<tC, wherein tB represents a time for a leading edge of an object of interest to intersect the beam edge defined by the first cam when the first cam is fully closed, Bcam is a fully closed position of the first cam, and vBC is a velocity of the first cam. 19. A method in accordance with claim 9 further comprising maintaining the first cam in a position defined by, Bcam+(t−tB)*vBC, for a time, tB<t<tC, wherein tB represents a time when a leading edge of an object of interest intersects the beam edge defined by the first cam when the first cam is fully closed, tC represents a time when the object of interest crosses the centerline of the detector (C) Bcam is a fully closed position of the first cam, and vBC is a velocity of the first cam. 20. A method in accordance with claim 9 further comprising maintaining the first cam in a position defined by, Bcam+(tC−tB)*vBC+(t−tC)*vCT, for a time, tC<t<tT, wherein tB represents a time when a leading edge of an object of interest intersects the beam edge defined by the first cam when the first cam is fully closed, tC represents a time when the object of interest crosses the centerline of the detector (C), tT represents a time when the object of interest intersects the beam edge defined by the first cam when the first cam is fully open, Bcam is a fully closed position of the first cam, and vBC and vCT are velocities of the first cam. 21. A method in accordance with claim 9 further comprising maintaining the first cam in a fully open position, Bcam, for a time, t>tT, wherein tT represents a time when the leading edge of the object of interest intersects the beam edge defined by the first cam when the first cam is fully open. 22. A method in accordance with claim 9 further comprising maintaining the second cam in a fully open position, Tcam, for a time, t<tT′, wherein tT′ represents a time when a trailing edge of the object of interest intersects the beam edge defined by the second cam when the second cam is fully open. 23. A method in accordance with claim 9 further comprising maintaining the second cam in a position defined by, Tcam+(tC′−tT′)*vTC′, for a time, tT′>t>tC′, wherein tC′ represents a time when the trailing edge of the object of interest crosses the centerline of the detector (C), tT′ represents a time when the trailing edge of the object of interest intersects the beam edge defined by the second cam when the second cam is fully open, Tcam is a fully open position of the second cam, and vTC is a velocity of the second cam. 24. A method in accordance with claim 9 further comprising maintaining the second cam in a position defined by, Tcam+(tB′−tC′)*vB′C′+(tC′−tT′)*vTC′, for a time, tC′<t<tB′, wherein tB′ represents a time when a leading edge of an object of interest intersects the beam edge defined by the first cam when the first cam is fully closed, tC represents a time when the object of interest crosses the centerline of the detector (C) Bcam is a fully closed position of the first cam, and vBC is a velocity of the first cam. 25. A method in accordance with claim 9 further comprising maintaining the first cam at a position Tcam for a time t>tB′, wherein tB′ represents a time for the trailing edge of the object of interest to intersect the beam edge defined by the second cam when the second cam is fully closed, and wherein Tcam is a fully closed position of the second cam.