Patent Number: 
Section: claims

1. A multi-leaf collimator for a radiotherapy apparatus, comprising:a plurality of elongate leaves mounted in a carriage which is mounted on a substrate, wherein the leaves are independently moveable relative to the carriage in a longitudinal direction, and wherein the carriage is moveable in that direction relative to the substrate,a control apparatus arranged to receive a signal representing desired leaf positions relative to the substrate and being arranged to simultaneously control both the leaf positions relative to the carriage and the carriage positions relative to the substrate, in combination, so as to achieve the signaled leaf positions relative to the substrate, andmeans for sensing the current positions of the leaves relative to the substrate, wherein the control apparatus moves the carriage according to an average difference between the current leaf positions and the signaled leaf positions. 2. The multi-leaf collimator according to claim 1, wherein the control apparatus compares the current leaf positions to the signaled leaf positions and moves the leaves and the carriage accordingly. 3. The multi-leaf collimator according to claim 1, wherein the control apparatus moves each leaf according to a difference between the current leaf position and the signaled leaf position. 4. The multi-leaf collimator according to claim 1, wherein the average difference considered by the control apparatus is measured by taking the difference between the mean of the current leaf positions and the mean of the signaled leaf positions. 5. A method of operating a multi-leaf collimator:the multi-leaf collimator comprising a plurality of elongate leaves mounted in a carriage, the carriage being mounted on a substrate, the leaves being independently moveable relative to the carriage in a longitudinal direction, the carriage being moveable in that direction relative to the substrate, the multi-leaf collimator further comprising means for sensing the current positions of the leaves relative to the substrate;the method comprising the steps of receiving a signal representing desired leaf positions relative to the substrate, and simultaneously controlling both the leaf positions relative to the carriage and the carriage positions relative to the substrate, in combination, so as to achieve those leaf positions relative to the substrate, wherein the carriage is moved according to an average difference between the current leaf positions and the signaled leaf positions. 6. The method according to claim 5, wherein the current leaf positions are compared to the signaled leaf positions and the leaves and the carriage are moved accordingly. 7. The method according to claim 5, wherein each leaf is moved according to a difference between the current leaf position and the signaled leaf position. 8. The method according to claim 5, wherein the average difference considered is measured by taking the difference between the mean of the current leaf positions and the mean of the signaled leaf positions. 9. The method according to claim 5, wherein the step of simultaneously controlling includes minimizing one or more of leaf travel, carriage travel and delivery time.