Patent Number: 
Section: claims

1. A multileaf collimator comprising:a first set of a plurality of pairs of beam blocking leaves arranged adjacent one another, leaves of each pair in the first set being disposed in an opposed relationship and longitudinally movable relative to each other in a first direction; anda second set of a plurality of pairs of beam blocking leaves arranged adjacent one another, leaves of each pair in the second set being disposed in an opposed relationship and longitudinally movable relative to each other in a second direction generally parallel to the first direction; whereinthe first and second sets of pairs of leaves are disposed in different planes and the first set of pairs of leaves comprises a first quantity of pairs of leaves and the second set of pairs of leaves comprises a second quantity of pairs of leaves wherein the first quantity and the second quantity are different. 2. The multileaf collimator of claim 1 wherein the first quantity is greater than the second quantity by one pair. 3. The multileaf collimator of claim 1 wherein each leaf in the first set is offset from a leaf in the second set by about half a leaf width in a direction generally traverse to the first and second directions. 4. The multileaf collimator of claim 1 wherein the leaves in the first and second sets are supported by one or more movable carriages. 5. The multileaf collimator of claim 1 wherein the leaves in the first and second sets substantially focus on a converging virtual point located substantially at a radiation source. 6. A multileaf collimator comprising:a first set of a plurality of pairs of beam blocking leaves arranged adjacent one another, leaves of each pair in the first set being disposed in an opposed relationship and longitudinally movable relative to each other in a first direction; anda second set of a plurality of pairs of beam blocking leaves arranged adjacent one another, leaves of each pair in the second set being disposed in an opposed relationship and longitudinally movable relative to each other in a second direction generally parallel to the first direction; whereinthe leaves of the first set are disposed in a first level providing first projected widths at an isocenter plane, and the leaves of the second set are disposed in a second level providing, at the isocenter plane, second projected widths that are substantially same as the corresponding first projected widths; andthe leaves in the first level are arranged offset from the leaves in the second level in a direction generally traverse to the first and second directions such that one of the first projected widths offsets about half of corresponding one of the second projected widths at the isocenter. 7. The multileaf collimator of claim 6 wherein the leaves in the first and second sets substantially focus on a converging virtual point located substantially at a radiation source. 8. The multileaf collimator of claim 6 wherein the first projected widths at the isocenter plane are substantially identical and the second projected widths at the isocenter plane are substantially identical. 9. The multileaf collimator of claim 6 wherein the first projected widths at the isocenter plane are variable and the second projected widths at the isocenter plane are variable. 10. The multileaf collimator of claim 6 wherein a quantity of the leaves in the first set is different from a quantity of the leaves in the second set. 11. The multileaf collimator of claim 6 wherein a height of the leaves in the first level is different from a height of the leaves in the second level. 12. The multileaf collimator of claim 6 wherein the leaves in the first and second sets are supported by one or more movable carriages. 13. The multileaf collimator of claim 6 wherein the leaves in the first and second sets have a substantially trapezoidal cross section where the parallel sides of the trapezoidal cross-section have different dimensions. 14. The multileaf collimator of claim 6 wherein at least some of the leaves of the first set have an end portion having an upward and/or downward extended portion.