Patent Number: 052375994
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an X-ray apparatus having an X-ray beam limiting device assembled with an X-ray tube for stereoradiography. In medical examinations such as angiography, stereoradiography has the advantage of being able to obtain stereo images. An X ray apparatus for the stereoradiography has used an X-ray tube having a pair of X-ray focal points positioned a certain distance apart. X-rays are alternately irradiated from the one focal point to the other of the X-ray tube to a living body (a subject) through an X-ray beam limiting device, and the X-ray transmitted through the living body are detected by a film or an image intensifier (referred to as "I.I." hereinafter). An observer can obtain a stereo penetrating image, when his right eye sees an image formed according to the transmitted X-rays from the X-ray focal point and his left eye sees an image formed according to the transmitted X-rays from the X-ray focal point. Japanese Laid-Open No. 60-127698 discloses a example of X-ray beam limiting devices. The above-mentioned X-ray beam limiting device is placed at irradiation-opening side of an X-ray tube having a pair X-ray focal points for one target. The X-ray beam limiting device comprises: a rectangular limiting means for rectangularly shaping the X-rays; compensating filters for compensating a difference in the X-ray absorptions by heart muscles and lungs and which are situated at the X-ray-focal-point side of the rectangular limiting means; a circular limiting blade having two circular holes for shaping the X-rays from the X-ray focal points into circles according to a circular effective detection area (i.e. an input window) of an I.I.; and inside limiting blades for shaping the rectangular irradiation field defined by the rectangular limiting means into individual squares for the X-ray focal points. In normal stereoradiography, a subject contacts the effective detection area. In such contact stereoradiography, the X-ray penetrating image of the subject is detected at an enlargement ratio of 1 to 1, and thus the distance between the X-ray focal points and is 63 mm, which is approximately equal to the distance between the eyes. The blades of the rectangular limiting means and the inside limiting blades are controlled and moved by a stepping motor so that even when the SID (Source-Image Distance) is changed, a square X-ray irradiation field is circumscribed on the circular effective detection area of the I.I. Each of the circular holes has a maximum diameter according to the minimum SID. Then, when the SID is at minimum, the X-rays are shaped by the circular holes into a cone, resulting in a circular X-ray irradiation field which coincides with the circular effective detection area, not in a square X-ray irradiation field. However, when the SID is at maximum or relatively long, a circular X-ray irradiation field on the detection surface resulting from the circular holes would be a circle larger than the exterior of the I.I. As a result, the X-ray is shaped into a pyramid, thus resulting in the square X-ray irradiation field. Four corners of the square X-ray irradiation field may go out of the boundary of the exterior of the I.I. This results in condition in which some of the X-rays are out of the boundary of the exterior of the I.I. and directly leak behind the I.I. Thus, a patient may receive more X-rays than necessary, or other people like an operator may be exposed to the leaked X-rays. To the contrary, when the SID setting range is limited to avoid the leakage behind the I.I., the device fails to provide sufficient information for diagnoses due to a short SID. Presently, there is a demand for a magnifying stereoradiographic device which can both perform high-speed serial stereoradiography (:several frames per second in the case of film photography; several tens of frames per second in the case of I.I. photography) and provide magnified images. For example, for a magnifying stereoradiography with magnification of two in which a subject is placed at the middle between the X-ray focal points and the X-ray detection surface, it is required to use an X-ray tube having an interval between the focal points reduced to approximately 35 mm. As stated above, where the X-ray tube having shorter distances between the focal points is used, the triangular space, in which the X-ray irradiation is not affected, becomes too small to accomodate the conventional horizontally-moving beam limiting means for preventing the above-mentioned x-ray leakage. SUMMARY OF THE INVENTION Thus, it is an object of the present invention to provide an X-ray apparatus having an X-ray beam limiting device which prevents the X-ray beams from directly leaking over the detection surface without restricting the SID or the effective detection area. It is a further object of the present invention to provide an X-ray beam limiting device which prevents the exterior of the detector from increasing its size. These and other objects can be achieved according to the present invention, in one aspect by providing, an X-ray apparatus comprising: an X-ray tube having a pair of X-ray focal points placed a predetermined distance apart from each other; an X-ray detector having a detection surface in which an circular input window is placed; and a X-ray beam limiting device for limiting an X-ray beam irradiated from each of the X-ray focal points of the X-ray tube onto the circular input window of the X-ray detector, wherein the beam limiting device includes an element for limiting the X-ray beams so that an irradiation field of each one of the X-ray beams onto the detection surface can be formed into shape which is circumscribed on the circular input window. Preferably, the limiting element comprises a blade unit consisting of a plurality of blades and being capable of limiting each of the X-ray beams to the polygonal shape which enables the X-ray irradiation field onto the detection surface of the X-ray detector to be circumscribed on the input window and to remain within the detection surface, and a control unit for adjusting each position of the plural blades. The X-ray detector is preferably an image intensifier and the polygonal shape is preferably approximately octagonal. It is preferred that the blade unit comprises a first set of blades projecting a V-shaped aperture having a base to the detection surface, a second set of blades projecting a V-shaped aperture to the detection surface, and a third set of blades projecting a rectangular aperture to the detection surface. Further, the first set of blades and the second set of blades are placed so that the V-shaped aperture having the base projected by one of the first set of blades and the V-shaped aperture projected by one of the second set of blades are faced each other in a longitudinal direction of the rectangular aperture projected by the third set of blades. The three sets of blades are placed, from one side near to the X-ray tube toward another side near to the X-ray detector, in an order of positioning from the first to the third set of blades. It is preferred that the first set of blades are individually rotatable round an axis right to a longitudinal direction of the rectangular aperture. Also, it is preferred that the second set of blades are slidable in a transverse direction right to a longitudinal direction of the rectangular aperture, and the third set of blades are slidable in transverse and longitudinal directions of the rectangular aperture. Further, it is preferred that the control unit is able to adjust each position of the blades in accordance with at least either one of a distance between the X-ray tube and the X-ray detector, and a size of the input window of the X-ray detector. As a result, the irradiation field of the X-ray beams onto the detection surface of the detector can be formed by the first to third set of blades into an almost octagonal shape, which is circumscribed on the input window. And more, when the distance between the X-ray tube and the X-ray detector is changed, it can be kept that the irradiation field is circumscribed on the input window. Thus, the irradiation field can be rounded along the boundary of the input window as possible as it could be, and remains within the detection surface. This prevents x-ray beams from leaking over the X-ray detector.