Patent Number: 054855004
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a digital X-ray imaging system and method, and more particularly to a digital X-ray imaging system and method suitable for imaging a large field of view such as a chest. Conventional methods of taking an image of a subject by transmitting therethrough an X-ray are grouped into a method using an X-ray film and a digital radiography (hereinafter abbreviated as "DR") method of taking a digital X-ray image. The DR method is expected to improve a diagnosis performance by using image processing, and can electronically record, store, and search X-ray images. There are known various chest DR methods, including a film digitizing method in which an image taken on an X-ray film is digitized, a storage phosphor digital method in which storage phosphor is used in place of an X-ray film and a latent image is scanned by a laser beam to read it and form a visualized image, a scanography method in which a combination of a one-dimensional X,ray beam and a one-dimensional detector array is used, and an X-ray image intensifier-television camera-method (herein after abbreviated as "X-ray II-TV method) in which uses a combination of an X-ray image intensifier (hereinafter abbreviated as "X-ray II") for converting an X-ray image into an optical image and amplifying it and a television camera for converting the amplified optical image into electric signals. Of these DR methods, the X-ray II-TV method is also called a real time DR method, has the function of immediately displaying and storing a taken image, and has the shortest time required for taking and processing one image. Therefore, a success or failure of imaging can be judged at once, and this method is suitable for mass screening because of a short time required for one test. It is also suitable for urgent check, and routine as well as accurate diagnosis at hospitals because it takes a short time to obtain the results of imaging and it is possible to provide the functions of sequential imaging, dynamic imaging, fluoroscopy, and the like as well as quick diagnosis. The methods other than the X-ray II-TV method require 30 seconds or longer for imaging and reading image data. A technique disclosed, for example, in Electromedica Vol. 60(1992), No. 1, pp. 2-5 is known in which the X-ray II-TV method is used to obtain an X-ray image at an imaging target region wider than the field of view of the X-ray detector, by divisionally imaging the region a plurality of times while changing a relative position of the X-ray detector and the subject. According to this technique, a subject lies on a bed in a dorsal position, and the X-ray source and X-ray detector are moved in unison in one direction, for example, in parallel with the longitudinal direction of the subject. An angiogram of lower extremities, for example, is divisionally imaged a plurality of times, and a plurality of obtained images are joined together to display them as a one complete image. Although the X-ray II-TV method has the above-described superior advantages, conventional DR methods have been accompanied with the problem that one of a field of view and a spatial resolution is inferior to the other methods. It is technically difficult to manufacture an X-ray II which can take an image of a large view field at a ultra high resolution. As an X-ray II having a filed of view as large as about 40 cm * 40 cm necessary for imaging a chest, there is known an X-ray II having a field of view of 47 cm described in Radiology Vol. 171, No. 2 (May, 1989), pp. 297-307). This X-ray II however has a spatial resolution inferior to other methods. Another problem associated with an X-ray II is that the more the position goes apart from the center of the field of view, the more the spatial resolution at the position lowers. Therefore, when lungs are imaged, the central area of the X-ray II having a high resolution images the central region of the mediastinum, whereas the peripheral area of the X-ray II having a lower resolution images the lung field. With the technique disclosed in Electromedica Vol. 60 (1992), No. 1, pp. 2-5, images (photographs) of a target region divisionally taken a plurality of times are cut and pasted to join them together, and they are not image-processed by a computer. Therefore, image densities at areas around joining lines are discontinuous so that the image quality of a vascular system near the areas around joining lines is poor. In addition, this technique does not take into consideration imaging a plurality of regions of interest at the central area of the X-ray II having a higher resolution. With the above described divisional imaging method, X-ray beams transmitting through a subject are not parallel beams but diverging beams. Since the X-ray source and X-ray detector are moved relative to the subject, diverging X-ray beams passing through the same position of a subject have different incident angles when imaging the subject a plurality of times. This different incident angle of an X-ray beam generates a positioning error of the subject image in its depth direction, being unable to correctly join a plurality of images. This problem will be detailed with reference to FIG. 12. An X-ray source 3 and an X-ray detector 16 are moved in unison in parallel to a subject 17 at a dorsal position. As described previously, X-ray beams passing through the subject 17 are diverging beams. Therefore, when the X-ray source 3 is at point A, an X-ray beam 21 incident to the position at point P on the ventral side passes through the position at point Q on the dorsal side, whereas when the X-ray source 3 is at point B, an X-ray beam 22 incident to the position at point P on the ventral side passes through the position at point R on the dorsal side. Since an X-ray image is obtained as an X-ray transmitted image, if the X-ray images taken at positions A and B of the X-ray source 3 are joined by superposing two points P on the ventral side, the joined image is not correct because on the dorsal side, the points Q and R are joined. SUMMARY OF THE INVENTION It is an object of the present invention to provide a real time DR digital X-ray imaging system capable of solving the above problems associated with conventional techniques, and obtaining an X-ray image of a target region having a field of view wider than that of an X-ray detector by correctly joining a plurality of images divisionally taken a plurality of times while changing a relative position between the X-ray detector and an X-ray source. The above object of the present invention can be achieved by a digital X-ray imaging system including an X-ray source, an X-ray slit for limiting the area of an emitted X-ray from the X-ray source, an X-ray grid for shielding a scattered X-ray, an X-ray detection unit for detecting an X-ray transmitted through a subject, a signal processor for acquiring a signal from the X-ray detection unit and processing the signal to obtain an image of the subject, a display unit for displaying the image of the subject obtained through signal processing by the signal processor, and means for controlling to set an imaging view field by changing a relative position between the X-ray detection unit and the subject, the imaging view field setting means setting each of a plurality of regions of interest of the subject to generally a central area of a view field or an X-ray detecting plane of the X-ray detection unit, and setting at least one imaging view field to contain an intermediate region between a plurality of target imaging regions. According to the digital X-ray imaging system of this invention, each of a plurality of imaging target regions is imaged generally at a central area of the view field of the X-ray detection unit. It is therefore possible to obtain the image quality of each target region under the optimum condition. If the subject is a human chest, the right and left lung fields are imaged generally at a central area of the view field of the X-ray detection unit. It is therefore possible to obtain a more correct image of the lung field, improving the diagnosis performance of the lung field. Since the intermediate region between a plurality of target imaging regions is contained in at least one imaging view field, there is no missing part of the view field, allowing to image all necessary regions. By using a high resolution X-ray II-TV system, immediate display and diagnosis are possible, improving a throughput. It is possible to obtain an image having a wider view field than that of the X-ray detection unit and covering both the lungs, by a plurality of radiographic exposures. By joining together a plurality of images, diagnosis is possible by one display unit. By correcting the sensitivity of the X-ray detection unit and the geometric distortion and density of an image, it is possible to realize a digital X-ray imaging system capable of reducing the discontinuity at image joining regions and performing a more precise diagnosis. While the X-ray source and a subject are fixed, the X-ray detection unit has construction in which relative position between the X-ray detection unit and the subject is changed to set the X-ray grid generally perpendicular to a center line passing through the center of an X-ray input plane of the X-ray grid and the. X-ray source during the exposures. Therefore, the incident angle of an X-ray beam at the same position on the subject will not change at each of a plurality of radiographic exposures, generating no image distortion in the subject depth direction. It is therefore possible to correctly join a plurality of X-ray images by correcting the sensitivity non-uniformity of the X-ray detection unit and the geometric distortion in the image regions commonly contained in a plurality of X-ray images. An X-ray grid is mounted at the front of the X-ray detection unit. The X-ray grid covering the imaging view field effective transmits an X-ray and shields the scattered X-ray. As a result, even if the X-ray detection unit is moved, the scattered X-ray can be effectively shielded by the X-ray grid. By using a high resolution X-ray II-TV system, immediate image display and diagnosis become possible, improving a diagnosis throughput. By moving the position of the X-ray slit, unnecessary X-ray exposure to the subject can be avoided. According to the present invention, a wide region such as a lung field can be imaged in real time at a high spatial resolution, providing immediate display and diagnosis. It is therefore possible to easily perform re-imaging and easily change the imaging conditions, permitting precise diagnosis. In the chest DR or the like, an X-ray can be effectively and directly transmitted and the scattered X-ray can be shielded, while shielding the X-ray outside of the imaging view field. Therefore, unnecessary X-ray exposure to the subject can be avoided.