Patent Number: 
Section: claims

1. A collimator, comprising:an anti-scatter grid, the anti-scatter grid including an opening, the opening including a plurality of sub-openings arranged along a first direction and a second direction, the opening having a plurality of opening widths along the first direction and a plurality of opening lengths along a second direction, the plurality of opening widths including first opening widths and second opening widths, each of the first opening widths corresponding to one of a plurality of sub-openings at two ends of the collimator in the second direction, each of the second opening widths corresponding to one of a plurality of sub-openings being between the plurality of sub-openings at the two ends of the collimator in the second direction, and the anti-scatter grid including a plurality of protrusions, the plurality of protrusions located in the plurality of sub-openings at the two ends of the collimator in the second direction and extending along the first direction so that each of the first opening widths is smaller than a second opening width, wherein radiation rays generated by a radiation source pass through the opening along a direction substantially perpendicular to the first direction and the second direction and impinge on a detector. 2. The collimator of claim 1, wherein the plurality of sub-openings are configured corresponding to a plurality of detection units of a detector. 3. The collimator of claim 1, wherein the opening is a substantially rectangular opening. 4. The collimator of claim 1, wherein the collimator is made of a material for shielding X-ray. 5. The collimator of claim 1, wherein a length of each of the plurality of protrusions is the same as or smaller than a length of each of the plurality of sub-openings in the second direction. 6. The collimator of claim 1, wherein the plurality of protrusions are removably attached to the body. 7. The collimator of claim 1, wherein a sum of widths of the plurality of sub-openings at one end of the anti-scatter grid in the second direction are smaller than a sum of widths of the plurality of sub-openings within a middle section of the anti-scatter grid in the second direction. 8. The collimator of claim 2, wherein each of the plurality of protrusions partially shields a corresponding detection unit.