Many irradiation systems use attenuation to modulate a radiation beam in an effort to keep radiation at safe levels to tissues which are to be protected from harmful radiation. The radiation beam irradiating a target in an object is attenuated between the object surface and the target. The target dose distribution perpendicular to the beam may be non-uniform due to spatially non-uniform attenuation, e.g., when the object surface is not perpendicular to the beam.
FIG. 1 illustrates an attenuating wedge 1 used in the prior art. The attenuating wedge 1 is placed in front of an object 2 that has a target 3, which causes the total attenuation of a radiation beam 4 to be the sum of the wedge attenuation and that of the object. The wedge is thus used for compensating target non-uniform dose distribution.
Since object attenuation depends on beam/object orientation, a different wedge may be required for different orientations. Continuously varying orientations, such as during an arc treatment, may require rapid wedges exchange which is technically and economically prohibitive.
U.S. Pat. No. 6,836,535 to Toth et al. describes a filtering apparatus for a CT or x-ray imaging system. The filtering apparatus moves along an axis with respect to an attenuation pattern of a subject during an imaging session to reduce radiation exposure to sensitive anatomical regions of the subject. In one embodiment, the filtering apparatus includes a flexible bladder containing attenuating fluid positioned between an upper plate and a lower plate. The bladder is sufficiently flexible such that the attenuating fluid contained therein may be modulated or manipulated to define the desired attenuation profile. The bladder may contain attenuating liquid, gelatin, beads, or the like. The upper plate is fabricated from a flexible x-ray transparent material such as plastic that, in response to an applied force, alters the shape of the flexible bladder. The lower plate supports the flexible bladder and is fabricated from a solid x-ray transparent material. The flexible bladder and lower and upper plates are each fabricated from an x-ray transparent material so that x-rays are attenuated primarily by the attenuating fluid rather than the bladder or plates.