Patent Number: 
Section: claims

1. A multi-leaf collimator for selectively blocking an incident beam of radiation, said collimator comprising:a plurality of independently adjustable leaves comprising a first leaf and a second leaf that is adjacent said first leaf, said first leaf having at least a first portion that extends from a substantially block-shaped second portion of said first leaf between said first leaf and said second leaf; wherein said first portion comprises a first material and said second portion comprises a second material that is different from said first material. 2. The collimator of claim 1 wherein said first portion is substantially block-shaped. 3. The collimator of claim 1 wherein said first portion has a cross-section that is substantially triangular in shape. 4. The collimator of claim 1 wherein said first material has a density that is greater than a density of said second material. 5. The collimator of claim 1 wherein an atomic number for said first material is greater than an atomic number for said second material. 6. The collimator of claim 1 wherein said first material is selected from the group consisting of: tungsten, tungsten alloys, tantalum, tantalum alloys, lead, and lead alloys; and wherein said second material is selected from the group consisting of: steel, brass, zinc, and copper. 7. The collimator of claim 1 wherein said first and second leaves are arranged side-by-side and are moveable in parallel. 8. The collimator of claim 1 wherein said first and second leaves are substantially aligned with each other and are moveable in opposite directions. 9. An apparatus comprising:a source operable for generating a beam of radiation onto a target area; anda multi-leaf collimator disposed between said source and said target area and operable for shielding said target area from said beam, wherein said collimator comprises at least a first pair of independently adjustable leaves that are moveable along a common line; said first pair of leaves including a first leaf having at least a first portion comprising a first thickness of a first material and a second portion comprising a second thickness of a second material that is different from said first material. 10. The apparatus of claim 9 wherein said second portion has a quadrilateral cross-section, wherein said cross-section is orthogonal to said common axis. 11. The apparatus of claim 9 wherein said first portion has a substantially quadrilateral cross-section, wherein said cross-section is orthogonal to said common axis. 12. The apparatus of claim 9 wherein said first portion has a substantially triangular cross-section, wherein said cross-section is orthogonal to said common axis. 13. The apparatus of claim 9 wherein said first thickness is less than said second thickness. 14. The apparatus of claim 9 wherein said first material is a higher density material and said second material is a lower density material. 15. The apparatus of claim 9 wherein said first material is a higher-Z material and said second material is a lower-Z material. 16. The apparatus of claim 9 wherein said first material is selected from the group consisting of: tungsten, tungsten alloys, tantalum, tantalum alloys, lead, and lead alloys; and wherein said second material is selected from the group consisting of: steel, brass, zinc, and copper. 17. The apparatus of claim 9 wherein said first pair of leaves comprises a second leaf separated from said first leaf by a gap, wherein said first material is disposed between said second material and said second leaf and wherein said first portion overlaps a corresponding portion of said second leaf. 18. The apparatus of claim 17 further comprising a second pair of independently adjustable leaves that are moveable along a common line, wherein said first and second pairs are arranged side-by-side and are moveable in parallel to each other. 19. A method of shaping an incident beam of radiation, said method comprising:blocking a first part of said beam using a first thickness of a first material comprising at least a first portion of a first leaf in a multi-leaf collimator; andblocking a second part of said beam using a second thickness of a second material comprising a second portion of said first leaf, said second portion having a substantially block shape, said second material different from said first material and said first thickness less than said second thickness, wherein said first portion extends from said second portion to prevent said first part of said beam from passing through a gap between said first leaf and a second leaf in said multi-leaf collimator. 20. The method of claim 19 wherein said first material has a density that is greater than a density of said second material. 21. The method of claim 19 wherein an atomic number for said first material is greater than an atomic number for said second material. 22. The method of claim 19 wherein said first material is selected from the group consisting of: tungsten, tungsten alloys, tantalum, tantalum alloys, lead, and lead alloys; and wherein said second material is selected from the group consisting of: steel, brass, zinc, and copper. 23. The method of claim 19 wherein said first and second leaves are arranged side-by-side and are moveable in parallel. 24. The method of claim 19 wherein said first and second leaves are substantially aligned with each other and are moveable in opposite directions. 25. The method of claim 19 wherein said first portion has a substantially rectilinear cross-section, wherein said cross-section is orthogonal to the longitudinal axis of said first portion.