Patent Number: 
Section: claims

1. A system for calibrating a radiation imaging device having a radiation source and a radiation beam comprising:a variable thickness filter comprising multiple movable plates for attenuating radiation, the multiple movable plates each having a slit formed therin for allowing the radiation to pass through a movable plate unattenuated when the movable plate is in a first position; anda radiation detector for measuring variations in the attenuated radiation. 2. The system of claim 1, further comprising multiple actuators configured to move each of the multiple movable plates between the first position and a second position, the second position causing the radiation beam to be attenuated in an amount dependent on the total thickness of the movable plates in the second position. 3. The system of claim 2, the multiple actuators being configured to move each of the movable plates between the first position, a second position and a third position. 4. The system of claim 3, the third position causing the radiation beam to be attenuated in an amount that differs from the attenuation caused in the second position. 5. The system of claim 4, the difference in amount of attenuation between the second position and the third position resulting from a difference in material composition of the movable plates. 6. The system of claim 3, the movable plates located in a path of the radiation beam in the second position having a thickness and material composition that differs from the thickness and material composition of the movable plates located in a path of the radiation beam in the third position, attenuation of the radiation beam when the movable plates are in the second and third positions being approximately equal. 7. The system of claim 1, further comprising:a radiation beam flattening filter;a primary collimator;a beam monitoring detector; anda secondary collimator. 8. The system of claim 1, the multiple movable plates have thicknesses which increase incrementally by double the thickness of an immediately preceding movable plate, a first plate being one-quarter of an inch in thickness. 9. The system of claim 1, each of the multiple movable plates being formed of at least one of the following materials from the group consisting of: copper, tungsten, lead, and steel. 10. The system of claim 2, further comprising multiple rods supporting each of the multiple movable plates and moving in response to the multiple actuators. 11. The system of claim 1, the radiation source being one of an x-ray and gamma ray source. 12. The system according to claim 1, the multiple movable plates have thicknesses which can be combined to provide total thicknesses which increase incrementally by one-quarter of an inch. 13. The system according to claim 7, further including a post-collimator. 14. A method for calibrating an imaging device comprising:directing a radiation beam at a detector;passing the radiation through a variable thickness filter including multiple movable plates, the radiation remaining unattenuated when each of the multiple movable plates is in a first position achieved by aligning a slit in a movable plate with a path of the radiation beam;attenuating the radiation in one or more increments dependent on the total thickness of the multiple movable plates that are in a second position; anddetecting the radiation at the detector. 15. The method of claim 14, further comprising:flattening the radiation beam;detecting the radiation beam at a beam monitoring detector; andcollimating the radiation beam. 16. The method according to claim 14, further comprising moving each of the multiple movable plates by actuating at least one actuator. 17. The method according to claim 16, further comprising:moving the multiple plates to a third position within the path of the radiation beam, the radiation beam being attenuated by the multiple plates in the third position in an amount that differs from the attenuation by the multiple plates in the second position. 18. The method according to claim 16, further comprising:moving the multiple plates to a third position within the path of the radiation beam, the radiation beam being attenuated by the multiple plates in the third position in an amount that is approximately equal to the attenuation by the multiple plates in the second position. 19. A system for calibrating a radiation imaging device having a radiation source and a radiation beam comprising: a variable thickness filter including multiple steps and a slit through a thickest section of the variable thickness radiation filter; anda radiation detector for measuring variations in radiation passing through the variable thickness filter. 20. The system of claim 19, the variable thickness radiation filter including a single block of material. 21. The system of claim 19, the variable thickness filter including multiple conjoined blocks of material to form a continuous block of material. 22. The system of claim 20, the material being selected from a group consisting of copper, steel, tungsten and lead. 23. The system of claim 21, each of the multiple blocks of material being selected from a group consisting of copper, steel, tungsten and lead. 24. The system of claim 19, further comprising:a radiation beam flattening filter;a primary collimator;a beam monitoring detector; anda secondary collimator. 25. The system according to claim 19, the radiation source being attenuated when any one of the multiple steps are in a path of the radiation beam and the radiation beam being unattenuated when the slit is aligned with the path of the radiation beam. 26. The system according to claim 19, the radiation source being one of an x-ray or gamma ray source. 27. The system according to claim 24, further including a post-collimator.