Patent Number: 056446144
Section: claims

1. A pre-patient collimator for controlling the shape of a collimated fan beam for use in a computed tomography system, the computed tomography system including a detector array having a plurality of rectangular shaped detector cells and an x-ray source, said collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture contour providing that a fan beam umbra of a beam passing therethrough has a substantially rectangular cross sectional shape. 2. A pre-patient collimator in accordance with claim 1 wherein the system is configured to scan an object, and wherein the dimensions of said contoured aperture may change contour. 3. A pre-patient collimator in accordance with claim 1 wherein said aperture is contoured with a fixed linear ramp. 4. A pre-patient collimator in accordance with claim 3 wherein said fixed linear ramp has a ramp slope of 0.2 mm per 100 mm. 5. A pre-patient collimator in accordance with claim 1 wherein the detector array comprises at least one generally rectangular detector element, and wherein said aperture is contoured so that a fan beam umbra of a beam passing therethrough is substantially fitted to the rectangular detector element. 6. A pre-patient collimator in accordance with claim 1 wherein said collimator is a double cam collimator comprising a first cam and a second cam, at least said first cams being movable relative to said second cam. 7. A pre-patient collimator in accordance with claim 1 wherein said collimator aperture can be adjusted as a function of a focal spot size of the x-ray source. 8. A pre-patient collimator in accordance with claim 1 wherein the system is configured to reconstruct slices of varying configurations, and wherein said aperture is contoured as a function of the slice configuration. 9. A pre-patient collimator for controlling the shape of a collimated fan beam for use in a computed tomography system, the computed tomography system including a detector array and an x-ray source, said collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture having a contour substantially according to: EQU c(.alpha.)=(Z-f)s(.alpha.)/d(.alpha.)+f .alpha.=fan beam angle;  Z=position of beam on detector;  f=position of focal spot in z axis;  c=position of collimation point in z axis;  d=source to detector distance; and  s=source to collimation distance.  .alpha.=fan beam angle;  Z=position of beam on detector;  f=position of focal spot in z axis;  c=position of collimation point in z axis;  d=source to detector distance; and  s=source to collimation distance. 10. In an imaging system including an x-ray source and a detector array having a plurality of rectangular shaped x-ray detector cells, a collimator comprising x-ray absorbing material having an aperture therein for restricting the collimated fan beam, said aperture having a contour to form a generally rectangular collimated fan beam and said aperture contour providing that a fan beam umbra of a beam passing therethrough is substantially fitted to the rectangular detector cells. 11. A collimator in accordance with claim 10 wherein said aperture is contoured according to: EQU c(.alpha.)=(Z-f)s(.alpha.)/d(.alpha.)+f 12. A collimator in accordance with claim 10 wherein said aperture is contoured with a fixed linear ramp. 13. A collimator in accordance with claim 12 wherein said fixed linear ramp has a ramp slope of 0.2 mm per 100 mm. 14. A collimator in accordance with claim 10 wherein the system is configured to scan an object, and wherein the dimensions of said contoured aperture are adjustable. 15. A collimator in accordance with claim 10 wherein said collimator is a cam collimator. 16. A collimator in accordance with claim 10 wherein said collimator aperture can be adjusted as a function of a focal spot size of the x-ray source. 17. A collimator in accordance with claim 10 wherein the system is configured to reconstruct slices of varying configurations, and wherein said aperture is contoured as a function of the slice configuration.