Patent Number: 
Section: claims

1. A filter unit comprising:a filter plate comprising a first filter, a second filter disposed in a first direction with respect to the first filter, and a third filter disposed in a second direction having a predetermined angle from the first direction;a guide plate comprising a guide frame for movement of the filter plate in the first direction and the second direction; anda drive device configured to move the filter plate. 2. A filter unit according to claim 1, wherein the drive device comprises a rotary disc and a single drive motor configured to rotate the rotary disc. 3. A filter unit according to claim 2, wherein the guide plate further comprises a first aperture at the center thereof, the rotary disc comprises a second aperture at the center thereof, and the guide plate and the rotary disc are superimposed one on the other. 4. A filter unit according to claim 1, wherein the rotary disc and the filter plate are connected with each other through a link member. 5. A filter unit according to claim 4, wherein a bushing is attached to the link member and is configured to move along the guide frame. 6. A filter unit according to claim 1, wherein the filter plate defines a square outline such that the first direction and the second direction are orthogonal to each other. 7. An X-ray tube unit having an X-ray tube, a collimator and a filter unit, the filter unit comprising:a filter plate comprising a first filter, a second filter disposed in a first direction with respect to the first filter, and a third filter disposed in a second direction having a predetermined angle from the first direction;a guide plate comprising a guide frame for movement of the filter plate in the first direction and the second direction; anda drive device configured to move the filter plate. 8. An X-ray tube unit according to claim 7, wherein the drive device comprises a rotary disc and a single drive motor configured to rotate the rotary disk. 9. An X-ray tube unit according to claim 8, wherein the guide plate further comprises a first aperture at the center thereof, the rotary disc comprises a second aperture at the center thereof, and the guide plate and the rotary disc are superimposed one on the other. 10. An X-ray tube unit according to claim 7, wherein the rotary disc and the filter plate are connected with each other through a link member. 11. An X-ray tube unit according to claim 10, wherein a hushing is attached to the link member and is configured to move along the guide frame. 12. An X-ray tube unit according to claim 7, wherein the filter plate has defines a square outline and the first direction such that the second direction are orthogonal to each other. 13. An X-ray tube unit according to claim 7, wherein the filter plate further comprises a first aperture formed at the center of the guide plate and a second aperture formed at the center of a rotary disc, the first aperture and the second aperture being in alignment with an axis joining the X-ray tube and a collimator aperture. 14. An X-ray imaging system for radiating X-ray to a subject from an X-ray tube unit and picking up a transmitted image, the X-ray tube unit comprising:an X-ray tube;a collimator; anda filter unit comprising:a filter plate comprising a first filter, a second filter disposed in a first direction with respect to the first filter, and a third filter disposed in a second direction having a predetermined angle from the first direction;a guide plate comprising a guide frame for movement of the filter plate in the first direction and the second direction; anda drive device configured to move the filter plate. 15. An X-ray imaging system according to claim 14, wherein the drive device comprises a rotary disc and a single drive motor configured to rotate the rotary disc. 16. An X-ray imaging system according to claim 15, wherein the guide plate further comprises a first aperture at the center thereof, the rotary disc comprises a second aperture at the center thereof, and the guide plate and the rotary disc are superimposed one on the other. 17. An X-ray imaging system according to claim 14, wherein the rotary disc and the filter plate are connected with each other through a link member. 18. An X-ray imaging system according to claim 17, wherein a bushing is attached to the link member and is configured to move along the guide frame. 19. An X-ray imaging system according to claim 14, wherein the filter plate defines a square outline such that the first direction and the second direction are orthogonal to each other. 20. An X-ray imaging system according to claim 14, wherein the filter plate further comprises a first aperture formed at the center of the guide plate and a second aperture formed at the center of a rotary disc, the first aperture and the second aperture being in alignment with an axis joining the X-ray tube and a collimator aperture.