Patent Number: 
Section: claims

1. A heel effect compensation filterwhich is configured to have a thickness distribution that uniforms an X-ray intensity angular distribution that is nonuniform in a body axis direction of a subject in an X-ray flux irradiated space,the space being formed by an X-ray flux diverging from an anode in a body width direction of the subject and diverging in a shape of an approximate sector in the body axis direction orthogonal to the body width direction due to the X-ray intensity angular distribution affected by a heel effect, when the X-ray flux generated on the anode by irradiating a thermoelectron beam flux from a cathode to the anode is irradiated on the subject through a wedge filter configured to have a cylindrical concave surface with a curve being formed in the body width direction of the subject, whereinthe thickness distribution is defined by Formula 1:                                          (                                                                                y                    ′                                                                                                                    z                    ′                                                                        )                    =                      (                                                                                                      L                      ⁡                                              (                        θ                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                                                                                                          FFD                      FCD                                        ⁢                                          (                                                                        FCD                          ⁢                                                                                                          ⁢                          tan                          ⁢                                                                                                          ⁢                          θ                                                -                                                                              L                            ⁡                                                          (                              θ                              )                                                                                ⁢                          sin                          ⁢                                                                                                          ⁢                          θ                                                                    )                                                                                            )                          ⁢                                  ⁢                  (                      θ            ≤                                                        cone                ⁢                                                                  ⁢                angle                                                            )                                    (                  Formula          ⁢                                          ⁢          1                )            where, on a plane containing an irradiation axis of the X-ray flux and a beam irradiation axis of the thermoelectron beam flux, the irradiation axis of the X-ray flux is defined as a Y-axis, and an axis orthogonal to the Y-axis at a distance FCD along the Y-axis in a direction of X-ray flux irradiation is defined as a Z-axis; z′ and y′ represent positions in corresponding axial directions with the proviso that an intersection point of the Z-axis and the Y-axis is defined as an origin point; FFD is defined as a predetermined distance along the Y-axis from a position of the anode; θ is defined as a predetermined angle within a range of a cone angle symmetrically diverging from the position of the anode relative to the irradiation axis of the X-ray flux; and La(θ) is defined as a length in a y′ direction at the angle θ. 2. The heel effect compensation filter according to claim 1, wherein the heel effect compensation filter is separable into pieces and a distance in the heel effect compensation filter through which the X-ray flux transmits during usage is equal to the thickness distribution. 3. The heel effect compensation filter according to claim 1, wherein either of an X-ray flux-incoming side transmissive surface and an X-ray flux-outgoing side transmissive surface is configured as a cylindrical convex surface with a curve being formed in the body axis direction of the subject and the other is configured as a flat surface. 4. The heel effect compensation filter according to claim 1, wherein either of an X-ray flux-incoming side transmissive surface and an X-ray flux-outgoing side transmissive surface is configured as a cylindrical convex surface with a curve being formed in the body axis direction and the other is configured as a cylindrical concave surface with a curve being formed in the body width direction orthogonal to the body axis direction. 5. The heel effect compensation filter according to claim 1, which is employed in an X-ray CT scanner having 32 arrays or more of X-ray detectors. 6. An X-ray irradiator in which a thermoelectron beam flux is irradiated from a cathode to an anode and an X-ray flux generated on the anode is irradiated on a subject, whereinthe heel effect compensation filter according to claim 1 is disposed between the anode and the subject at a predetermined distance,the filter being configured to adjust the X-ray intensity angular distribution of the X-ray flux to become uniform that is nonuniform in a body axis direction of the subject in an X-ray flux irradiated space,the space being formed by the X-ray flux diverging from the anode in a body width direction of the subject and diverging in a shape of an approximate sector in the body axis direction orthogonal to the body width direction due to the heel effect. 7. An X-ray CT scanner in which the X-ray irradiator according to claim 6 is employed. 8. A method for X-ray CT imaging which reduces an artifact of image data obtained by an X-ray CT scanner by employing the heel effect compensation filter according to claim 1 in the X-ray CT scanner and reducing a difference in CT value of the image data obtained along a body axis direction.