Patent Number: 
Section: claims

1. An X-ray CT apparatus, comprising:a plurality of X-ray tubes;a plurality of X-ray detectors corresponding to said plurality of X-ray tubes, respectively, each X-ray tube and corresponding X-ray detector forming a pair;a support mechanism which supports the X-ray tubes and the X-ray detectors to allow the X-ray tubes and the X-ray detectors to rotate about a single rotation axis;a reconstruction unit which reconstructs image data on the basis of outputs from the X-ray detectors; anda plurality of filters corresponding to said plurality of X-ray tubes, each filter having a characteristic in which an X-ray path length changes as a function substantially equal to an inverted Gaussian curve from the rotation center to two ends of an X-ray beam so as to reduce scattering radiation originating from X-rays generated by an X-ray tube of a first pair, but detected by an X-ray detector of a second, different pair;wherein said each filter has a characteristic in which an intensity of X-rays transmitted through the filter exceeds 90% of a maximum intensity at the rotation center at a middle portion less than 10 cm away from the rotation center, and an intensity at a peripheral portion more than 20 cm away from the rotation center is less than 50% of the maximum intensity at the rotation center. 2. An apparatus according to claim 1, further comprising a mechanism which attaches/detaches the filters. 3. An apparatus according to claim 1, which further comprises a plurality of slits which are respectively provided for said plurality of X-ray tubes and limit X-ray thicknesses, and in whichthe filter is placed between the X-ray tube and the slit. 4. An apparatus according to claim 1, wherein said each filter further has a characteristic in which an intensity of X-rays transmitted through said each filter changes as a function substantially equal to a Gaussian curve from the rotation center to two ends of an X-ray beam. 5. An apparatus according to claim 1, wherein said each filter further has a characteristic in which an intensity of X-rays transmitted through said each filter and a cylindrical homogeneous phantom changes curvilinearly from the rotation center to two ends of an X-ray beam. 6. An apparatus according to claim 1, wherein said each filter has a curved surface shape with a thickness of a central portion being small, and a thickness gradually increasing toward peripheral portions. 7. An X-ray CT apparatus, comprising:a plurality of X-ray tubes;a plurality of X-ray detectors corresponding to said plurality of X-ray tubes, respectively, each X-ray tube and corresponding X-ray detector forming a pair;a support mechanism which supports the X-ray tubes and the X-ray detectors to allow the X-ray tubes and the X-ray detectors to rotate about a single rotation axis;a reconstruction unit which reconstructs image data on the basis of outputs from the X-ray detectors; anda plurality of filters corresponding to said plurality of X-ray tubes and each have a characteristic in which an intensity of X-rays transmitted through the filter exceeds 90% of a maximum intensity at the rotation center at a middle portion less than 10 cm away from the rotation center, and an intensity at a peripheral portion more than 20 cm away from the rotation center is less than 50% of the maximum intensity at the rotation center so as to reduce scattering radiation originating from X-rays generated by an X-ray tube of a first pair, but detected by an X-ray detector of a second, different pair. 8. A filter provided for an X-ray tube of an X-ray CT apparatus, which has a characteristic in which an X-ray path length changes as a function equal to an inverted Gaussian curve from a center to two ends. 9. The filter of claim 8, wherein the filter has a characteristic in which an intensity of X-rays transmitted through the filter and a cylindrical homogenous phantom gradually change such that the intensity at the ends of the X-ray beam is 20%-80% of the intensity at a rotation center. 10. A filter provided for an X-ray tube of an X-ray CT apparatus, which has a characteristic in which an intensity of X-rays transmitted through the filter exceeds 90% of a maximum intensity at a center throughout a middle portion less than 10 cm away from the center, and an intensity throughout a peripheral portion more than 20 cm away from the center is less than 50% of the maximum intensity at the center.