Spinning filter for X-ray apparatus

A device for filtering X-ray beams in a system comprising an X-ray source, means for controlling said X-ray source to project alternate low and high energy pulsed X-ray beams, means for collimating said beams into a thin fan-shaped configuration and means for producing relative scanning movement between the X-ray source and a body being examined such that a high and a low energy beam can be projected through each layer in a succession of layers in the body, and detector means for producing signals corresponding to differential attenuation of the beams by the body layer during each energy pulse to enable reconstruction of projection images of the body region scanned, said device comprising:

• hollow rotatable drum means,
• a first pair of filter elements mounted on said drum means diametrically opposite of each other and a second pair of filter elements having different filtration properties than the first pair mounted diametrically opposite of each other but displaced nominally 90.degree. from the respective elements in the first pair, said elements having sufficient axial length for the fan-shaped beam to pass through them,
• an ac motor energized at power line frequency said motor having a shaft,
• means for driving said drum means in such manner that one pair of filters will be in the path of the low energy beam pulse when it is caused to occur and the other pair of filters will be in the path of the high energy pulse when it is caused to occur,
• said drum having means comprising a pinion fixed on said motor shaft,
• another shaft journaled for rotation and a gear fixed on the shaft, the pinion being meshed with the gear and the ratio of the gear to the pinion being 4 to 1,
• coupling means having one part mounted to said gear in a manner that permits angular adjustment of the part about the rotational axis of the gear, said coupling means having another part extending axially from the drum means, said parts being detachably interengaged, said other shaft extending through both parts to provide support for said drum means,
• shutter means having alternate circumferentially arranged light transmissive and light opaque regions, said shutter means being mounted on said gear for being adjusted to a selected angular position about the gear and drum means axes,
• a light source and photosensitive means, said shutter means being rotatable across the light beam path from the source to the photosensitive means, said means producing output signals state changes in response to said opaque and said light transmissive regions of the shutter being rotated alternately across the beam path, said means for controlling the X-ray source responding to occurrence of alternate signal states by causing said X-ray source to produce corresponding alternate high and low energy X-ray beam pulses.