Patent Number: 
Section: claims

1. A multi-leaf collimator comprising:a controller and two leaf sets, the two leaf sets being arranged opposite to each other, whereineach of the two leaf sets comprises a plurality of leaves, at least one leaf in the two leaf sets being a small irradiation field leaf, wherein at least one irradiation field hole is provided on the small irradiation field leaf; andthe controller driving the leaves of the two leaf sets is configured to:move the leaves of the two leaf sets so as to form a first irradiation field through which a radioactive beam is permitted to pass, anddrive the small irradiation field leaf to move such that the irradiation field hole moves in a same direction as the small irradiation field leaf and forms a second irradiation field through which the radioactive beam is permitted to pass. 2. The multi-leaf collimator according to claim 1, where the small irradiation field leaf is provided at the central position of the leaf sets. 3. The multi-leaf collimator according to claim 1, wherein the at least one irradiation field hole includes more than one irradiation field hole that is provided on the small irradiation field leaf in an equally-spaced manner. 4. The multi-leaf collimator according to claim 1, wherein the at least one irradiation field hole includes more than one irradiation field hole that has different diameters. 5. The multi-leaf collimator according to claim 1, wherein the at least one irradiation field hole includes more than one irradiation field hole that is provided on the small irradiation field leaf in size order. 6. The multi-leaf collimator according to claim 1, wherein the at least one irradiation field hole includes more than one irradiation field hole that is provided from the center of the small irradiation field leaf to two sides of the small irradiation field leaf in size order sequentially. 7. The multi-leaf collimator according to claim 1, wherein the controller comprises a first control module for controlling and driving the leaves to move along a plane intersecting the radiation beam so as to form the first irradiation field when a desired irradiation field is larger than or equal to the minimum value of the first irradiation field and a second control module for controlling and driving a second irradiation field hole with a desired size in the small irradiation field leaf to move so as to form the second irradiation field when the desired irradiation field is smaller than the minimum value of the first irradiation field. 8. The multi-leaf collimator according to claim 1, wherein widths of the small irradiation field leaves are greater than those of other leaves. 9. A collimation system, comprising:a radioactive source;a pre-collimator; anda multi-leaf collimator; and, whereinthe pre-collimator is arranged below the radioactive source and is configured to guide a radioactive beam emitted by the radioactive source into the multi-leaf collimator; andthe multi-leaf collimator is arranged below the pre-collimator and comprises a controller and two leaf sets, the two leaf sets being arranged opposite to each other, whereineach of the leaf sets comprises a plurality of leaves, at least one leaf in the two leaf sets being a small irradiation field leaf, wherein at least one irradiation field hole is provided on the small irradiation field leaf; andthe controller driving the leaves of the two leaf sets is configured to:move the leaves of the two leaf sets so as to form a first irradiation field through which the radioactive beam is permitted to pass, anddrive the small irradiation field leaf to move such that the irradiation field hole moves in a same direction as the small irradiation field leaf and forms a second irradiation field through which the radioactive beam is permitted to pass. 10. A therapy head, wherein the therapy head comprises;a source body provided with a plurality of radioactive sources;a plurality of pre-collimators; anda plurality of multi-leaf collimators; and, whereinthe plurality of the radioactive sources are arranged below the plurality of the pre-collimators, wherein each of the plurality of the radioactive sources is provided with a corresponding one of the plurality of the pre-collimators and a corresponding one of the plurality of the multi-leaf collimators, and wherein radioactive beams emitted by the plurality of the radioactive sources are focused in a same area through the corresponding pre-collimators and multi-leaf collimators; and,a given one of the multi-leaf collimators comprises a controller and two leaf sets, the two leaf sets being arranged opposite to each other, whereineach of the leaf sets comprises a plurality of leaves, at least one leaf in the two leaf sets being a small irradiation field leaf, wherein at least one irradiation field hole is provided on the small irradiation field leaf; andthe controller driving the leaves of the two leaf sets is configured to:move the leaves of the two leaf sets so as to form a first irradiation field through which a radioactive beam emitted by the radioactive source corresponding to the multi-leaf collimator is permitted to pass, anddrive the small irradiation field leaf to move such that the irradiation field hole moves in a same direction as the small irradiation field leaf and forms a second irradiation field through which the radioactive beam emitted by the radioactive source corresponding to the multi-leaf collimator is permitted to pass.