Patent Number: 
Section: claims

1. In an X-ray imaging system comprising an X-ray tube disposed to project an X-ray beam, and a detector spaced apart from said X-ray tube for receiving X-rays of said beam within an Active Imaging Area, collimator apparatus comprising: a first collimator positioned along the path of said beam, said first collimator being adjustable and operable to symmetrically collimate said beam for any adjustment of said collimator; and  a second collimating device comprising a collimator blade fixably positioned to lie in the path of a portion of said symmetrically collimated beam to limit first selected rays of said symmetrically collimated beam reaching said detector to a nonadjustable first boundary, defining a first edge of said Active Imaging Area, while allowing second selected rays of said symmetrically collimated beam reaching said detector to define a second edge of said Active Imaging Area which is in parallel spaced relationship with said first edge, and which is determined by selective adjustment of said first collimator. 2. The apparatus of  claim 1  wherein: claim 1 said first collimator comprises two adjustable collimator blades respectively positioned so that said beam passes therebetween. 3. The apparatus of  claim 2  wherein: claim 2 said collimator blades of said first collimator are adjustable to vary the width of said collimated beam in a vertical plane, while said collimated beam remains vertically symmetrical. 4. The apparatus of  claim 2  wherein: claim 2 said X-ray tube and said detector are respectively mounted to enable vertical movement therebetween, in order to selectively position said detector in relation to said upper and lower boundaries of said rays reaching said detector, said upper and lower boundaries comprising the upper and lower edges, respectively, of said Active Imaging Area. 5. An X-ray imaging system comprising: an X-ray tube disposed to protect an X-ray beam having a center line;  a detector spaced apart from said X-ray tube for receiving X-rays of said beam within an Active Imaging Area;  an adjustable first collimator means positioned along the path of said beam for collimating said beam so that said collimated beam is symmetrical with respect to said center line, in a specified plane, for any adjustment of said first collimator means; and  a second collimator means, comprising a collimator blade fixably positioned to lie in the path of a portion of said symmetrically collimated beam, for limiting first selected rays of said symmetrically collimated beam reaching said detector to a nonadjustable first boundary defining a first edge of said Active Imaging Area, said fixably positioned collimator blade allowing second selected rays of said collimated beam reaching said detector to define a second edge of said Active Imaging Area which is in parallel, spaced relationship with said first edge. 6. The apparatus of  claim 5  wherein: claim 5 said first collimator means comprises a pair of collimator blades which are adjustable to vary the width of said collimated beam in a vertical plane, while said collimated beam remains vertically symmetrical. 7. The apparatus of  claim 6  wherein: claim 6 said fixably positioned collimator blade lies in the path of an upper portion of said symmetrically collimated beam, to prevent X-rays of said upper portion from reaching said detector. 8. The apparatus of  claim 7  wherein: claim 7 said X-ray tube and said detector are respectively mounted to enable vertical movement therebetween, in order to selectively position said detector in relation to said first and second edges, which comprise upper and lower edges, respectively, of said Active Imaging Area. 9. In an arrangement comprising an X-ray tube disposed to project an X-ray beam having a center-line towards a detector spaced apart therefrom, in order to receive X-rays on the detector within an Active Imaging Area, a method for adjusting edges of the Active Imaging Area comprising the steps of: performing a first collimating operation on said projected beam to apply a first upper boundary and a first lower boundary to said beam, said first upper and lower boundaries being selectively adjustable; and  performing a second collimating operation on said projected beam to apply a nonadjustable second upper boundary to said beam, said nonadjustable upper boundary determining the upper edge of said Active Imaging Area, and said adjustable first lower boundary determining the lower edge of said Active Imaging Area. 10. The method of  claim 9  wherein: claim 9 said first upper boundary and said first lower boundary are symmetrical to each other, with respect to said center line of said beam, for any adjustment of said first upper and lower boundaries. 11. The method of  claim 10  wherein: claim 10 said X-ray tube and said detector are respectively mounted to enable vertical movement therebetween. 12. The method of  claim 11  wherein: claim 11 said method includes the step of establishing said vertical movement between said X-ray tube and said detector in order to bring the upper edge of said Active Imaging Area and the upper edge of said detector into substantial alignment with each other. 13. The method of  claim 12  wherein said method includes the steps of: claim 12 placing a patient between said tube and said detector so that the upper edge of said Active Imaging Area is proximate to the chest region of said patient; and  adjusting said first lower boundary of said beam to position the lower edge of said Active Imaging Area above the abdominal region of said patient. 14. The method of  claim 13  wherein: claim 13 said X-ray tube is disposed to move vertically, in response to movement of said detector, to maintain a specified relationship between said center line of said beam and a center point on said detector.