Patent Number: 
Section: claims

1. A multi-leaf collimator for a radiotherapy apparatus, comprising at least one array of elongate leaves spaced in a lateral direction and a plurality of motors, each leaf being driven by one of the motors of the plurality of motors, each motor connected to one of the leaves via a drive means so as to extend or retract the leaf in a longitudinal direction, the multi-leaf collimator comprising a sub-frame on which at least a subset of the plurality of motors are mounted, the sub-frame being mounted at a location spaced from the leaf array in a direction transverse to the lateral and longitudinal directions, the drive means including a plurality of elongate threaded drives which are disposed longitudinally, each elongate threaded drive being driven by one of the motors of the subset of motors and being operatively connected to a leaf thereby to drive that leaf, wherein some of the subset of the plurality of motors mounted on the sub-frame are mounted at a first longitudinal end of the elongate threaded drives, and a remainder of the subset of the plurality of motors are mounted at a second longitudinal end of the elongate threaded drives. 2. The multi-leaf collimator according to claim 1 in which the elongate threaded drives are neighboured on either lateral side by a first elongate threaded drive driven by one of the motors mounted at the same longitudinal end and a second elongate threaded drive driven by one of the motors mounted at the opposite longitudinal end. 3. The multi-leaf collimator according to claim 1 in which the elongate threaded drives are mounted in the subframe at one of a plurality of spacings from the at least one array of elongate leaves. 4. The multi-leaf collimator according to claim 1 in which the elongate threaded drives are mounted in the subframe at one of two spacings from the at least one array of elongate leaves, laterally neighbouring elongate threaded drives being mounted at alternating spacings. 5. The multi-leaf collimator according to claim 1 in which the elongate threaded drives are mounted within a bore in the subframe. 6. The multi-leaf collimator according to claim 1, wherein the subframe is an upper subframe and further comprising a lower subframe, the lower subframe being mounted at a location spaced from the leaf array in an opposite direction to that of the upper subframe and on which a remainder of the plurality of motors are mounted. 7. The multi-leaf collimator according to claim 6 in which the plurality of the motors mounted on the lower subframe are mounted at a first longitudinal end of the elongate threaded drives and a remainder are mounted at a second, opposing, longitudinal end of the elongate threaded drives. 8. The multi-leaf collimator according to claim 7 in which the elongate threaded drives are neighboured on either lateral side by a first elongate threaded drive driven by one of the motors mounted at the same longitudinal end and a second elongate threaded drive driven by one of the motors mounted at the opposite longitudinal end. 9. The multi-leaf collimator according to claim 5 in which the elongate threaded drives are mounted in the subframe at one of two spacings from the leaf, laterally neighbouring threaded drives being mounted at alternating spacings. 10. The multi-leaf collimator according to claim 6 in which the elongate threaded drives are mounted within a bore in the lower subframe. 11. The multi-leaf collimator according to claim 1 in which the elongate threaded drives are leadscrews. 12. The multi-leaf collimator according to claim 6 in which half of the leaves are driven via the upper subframe and half are driven via the lower subframe. 13. The multi-leaf collimator according to claim 6 in which adjacent leaves in the array are driven alternately via the upper subframe and via the lower subframe. 14. The multi-leaf collimator according to claim 1 in which the leaves are mounted in a machined guide thereby to allow longitudinal motion. 15. The multi-leaf collimator according to claim 14 in which the subframe is mounted on the machined guide. 16. The multi-leaf collimator according to claim 1 in which the leaves are of a first material that is substantially radio-opaque and including a drive coupling attached to the rear of each leaf via which the leaves are driven, the drive coupling being of a second material. 17. The multi-leaf collimator according to claim 16 in which the drive couplings are driven from an elongate edge thereof. 18. The multi-leaf collimator according to claim 1 in which the drive means further includes a threaded member on the threaded drives. 19. The multi-leaf collimator according to claim 18 in which an operative connection between the threaded member and the leaf acts to restrain rotation of the threaded member about the threaded drive. 20. The multi-leaf collimator according to claim 18 in which the threaded member urges a projecting lug. 21. The multi-leaf collimator according to claim 20, in which the leaves are of a first material that is substantially radio-opaque and including a drive coupling attached to the rear of each leaf via which the leaves are driven, the drive coupling being of a second material, and in which the projecting lug engages with a recess on the drive coupling. 22. The multi-leaf collimator according to claim 20 in which the lug includes spaced flanges positioned to lie adjacent a leaf edge. 23. The multi-leaf collimator according to claim 20 in which the lug has a length in a direction parallel to the threaded member that is at least 50% of a length of the lug transverse to the threaded member. 24. The multi-leaf collimator according to claim 20 in which the lug is held in machined slot in the subframe. 25. The multi-leaf collimator according to claim 24 in which the machined slot has non-parallel sides. 26. The multi-leaf collimator according to claim 1 in which the leaves are arranged in groups, each leaf of a group being identically oriented and driven in unison by the same drive means. 27. The multi-leaf collimator according to claim 26 in which each group consists of two leaves. 28. A multi-leaf collimator for a radiotherapy apparatus, comprising at least one array of elongate leaves spaced in a lateral direction and a plurality of motors, each leaf being driven by one of the motors of the plurality of motors, the plurality of motors being connected to the plurality of elongate leaves via a drive means so as to extend or retract the leaves in a longitudinal direction, the multi-leaf collimator further comprising:an upper sub-frame on which at least a first subset of the plurality of motors are mounted, the upper sub-frame being mounted at a location spaced from the leaf array in a direction transverse to the lateral and longitudinal directions; anda lower sub-frame on which at least a second subset of the plurality of motors are mounted, the lower sub-frame mounted at a location spaced from the leaf array in an opposite direction to that of the upper sub-frame;the drive means comprising a plurality of elongate threaded drives which are disposed longitudinally, each elongate threaded drive being driven by one of the motors of the plurality of motors and being operatively connected to a leaf thereby to drive that leaf.