Patent Number: 
Section: claims

1. A multi-leaf collimator for collimating a beam of a radiotherapeutic apparatus, comprising a plurality of elongate narrow leaves arranged side-by side and supported in a frame, the frame having upper and lower formations for guiding each leaf and ridges which extend into the leaves on the upper and lower edges of the leaves, thereby to allow the leaves to move in a longitudinal direction, the upper and lower formations being aligned so that the sides of the leaves when fitted are at a non-zero angle to the beam direction, the upper and lower ridges being located on the upper and lower edges of the leaves so that a line joining their centres is at a non-zero angle to the sides of the leaf, tilted relative to the sides, opposite to that of the beam. 2. The multi-leaf collimator according to claim 1 in which the upper formations comprise channels into which the ridges extend. 3. The multi-leaf collimator according to claim 1 in which the lower formations comprise channels into which the ridges extend. 4. The multi-leaf collimator according to claim 2 in which the channels are defined between a pair of ridges. 5. The multi-leaf collimator according to claim 1 in which an outer face of the upper ridges is aligned with a side face of the leaf. 6. The multi-leaf collimator according to claim 1 in which an outer face of the lower ridges is aligned with a side face of the leaf. 7. A radiotherapeutic apparatus, comprising;a source of radiation, anda multi-leaf collimator for shaping the radiation emitted by the source,the multi-leaf collimator comprising a plurality of elongate narrow leaves arranged side-by side and supported in a frame, the frame having upper and lower formations for guiding each leaf and ridges which extend into the leaves on the upper and lower edges of the leaves, thereby to allow the leaves to move in a longitudinal direction, the upper and lower formations being aligned so that the sides of the leaves when fitted are at a non-zero angle to the beam direction, the upper and lower ridges being located on the upper and lower edges of the leaves so that a line joining their centres is at a non-zero angle to the sides of the leaf, tilted relative to the sides in a sense opposite to that of the beam. 8. The radiotherapeutic apparatus according to claim 7 in which the upper formations comprise channels into which the ridges extend. 9. The radiotherapeutic apparatus according to claim 7 in which the lower formations comprise channels into which the ridges extend. 10. The radiotherapeutic apparatus according to claim 8 in which the channels are defined between a pair of ridges. 11. The radiotherapeutic apparatus according to claim 7 in which an outer face of the upper ridges is aligned with a side face of the leaf. 12. The radiotherapeutic apparatus according to claim 7 in which an outer face of the lower ridges is aligned with a side face of the leaf.