Patent Number: 
Section: claims

1. An X-ray generating apparatus comprising:an X-ray generating tube including:a cathode including an electron emitting source having an electron emitting member,an anode comprising:a target having a target layer receiving an electron beam emitted from the electron emitting source and generating an X-ray, and a support member supporting the target layer and transmitting the X-ray,wherein the electron emitting source and the target layer are disposed to face each other, anda tubular backward shielding member having a connection portion connected to the periphery of the support member and protruded from the connection portion toward the electron emitting source;a collimator having a plurality of movable blades which form an opening for defining an angle for extracting the X-ray at the opposite side of the electron emitting source side of the target; andan adjusting device connected to the plurality of movable blades, and configured to vary an opening diameter of the opening,wherein the target layer is distant from the connection portion via a separate portion so as to reduce an off-focal radiation due to re-entering electrons to the target layer from the tubular backward shielding member or the target layer, andwherein the collimator is configured to be controlled by the adjusting device so as to reduce an off-focal radiation due to the tubular backward shielding member and the separate portion. 2. The X-ray generating apparatus according to claim 1, wherein the separate portion is located annularly around the target layer. 3. The X-ray generating apparatus according to claim 1, wherein the target layer includes at least a target material made of a metal of an atomic number of 42 or greater, andthe backward shielding member includes the target material. 4. The X-ray generating apparatus according to claim 1, wherein, between the periphery of the target layer and the periphery of the support member, the target has a region at which the support member is exposed in the separate portion. 5. The X-ray generating apparatus according to claim 1, wherein the target has an electrode formed to be sequentially arranged within an area from the periphery of the target layer to the periphery of the support member, and the electrode is made of a material having a lower atomic number than the atomic number of the target material. 6. The X-ray generating apparatus according to claim 5, wherein the target is electrically connected to the backward shielding member via the electrode. 7. The X-ray generating apparatus according to claim 1, wherein a diameter of the target layer defined by the periphery of the target layer is larger than an irradiation diameter of the electron beam to be formed on the target layer. 8. The X-ray generating apparatus according to claim 7, wherein the X-ray generating tube includes a forward shielding member which has a tube outgoing opening for allowing a part of the X-ray emitted from the target toward the front of the target to pass through and located between the target and the collimator, and the inner diameter of the tube outgoing opening is larger than the irradiation diameter. 9. The X-ray generating apparatus according to claim 8, wherein a minimum value of an opening diameter of the collimator is at least equal to or less than 2A+2×(B−A)×(C+D)/C, where 2A is a diameter of an electron beam irradiated on the target layer, 2B is a tube outgoing diameter, C is a distance between a virtual plane for defining an inner diameter of the tube outgoing opening and the target layer, and D is a distance between a virtual plane for defining an opening diameter of the collimator, and the virtual plane for defining the inner diameter of the tube outgoing opening. 10. The X-ray generating apparatus according to claim 8, wherein a maximum value of an opening diameter of the collimator is at least equal to or less than −2B+2×(A+B)×(C+D)/C, where 2A is a diameter of an electron beam irradiated on the target layer, 2B is a tube outgoing diameter, C is a distance between a virtual plane for defining an inner diameter of the tube outgoing opening and the target layer, and D is a distance between a virtual plane for defining an opening diameter of the collimator, and the virtual plane for defining the inner diameter of the tube outgoing opening. 11. The X-ray generating apparatus according to claim 9, wherein the opening has a rectangular opening shape having one side of a length P, and the other side of a length Q, a minimum value of the opening diameter to be defined by the mechanism for varying the opening diameter of the opening corresponds to a minimum value of the length of P or Q not longer than the other one. 12. The X-ray generating apparatus according to claim 10, wherein the opening has a rectangular opening shape having one side of a length P, and the other side of a length Q, a maximum value of the opening diameter to be defined by the mechanism for varying the opening diameter of the opening corresponds to a maximum value of the length of P or Q not shorter than the other one. 13. The X-ray generating apparatus according to claim 1, wherein, among the movable blades, a movable blade configured to define a larger opening diameter is to be located at a position closer to the target as compared to movable blades defining a smaller opening diameter. 14. A radiograph system comprising:the X-ray generating apparatus according to claim 1, andan X-ray detector configured to detect an X-ray emitted from the X-ray generating apparatus and transmitted through a subject. 15. The radiography system according to claim 14, further comprising:an opening diameter instruction unit connected to the adjusting device, and configured to instruct an opening diameter determined based on a size of an exposure field to the adjusting device. 16. The radiography system according to claim 15, further comprising:an imaging field acquisition unit having an optical camera configured to define an imaging field toward the X-ray detector, and configured to determine a size of the exposure field based on an image acquired by the optical camera, and send the size of the exposure field to the opening diameter instruction unit. 17. The radiography system according to claim 15, further comprising:a sighting optical system located between a forward shielding member and the collimator, and configured to provide a virtual exposure field to an operator or a subject, and send the size of the exposure field determined based on the virtual exposure field to the opening diameter instruction unit. 18. The radiography system according to claim 17, wherein the sighting optical system includes at least a mirror configured to transmit the X-ray and reflect visible light, and a visible light source configured to emit visible light to the mirror. 19. The X-ray generating apparatus according to claim 1, wherein the tubular backward shielding member is protruded backwardly from the connection portion along a tube axis.