Patent Number: 051075296
Section: summary

FIELD OF THE INVENTION The present invention relates generally to the area of x-ray radiography. More particularly, the present invention relates to an equalization system for fluoroscopic and radiographic diagnostic x-ray systems. BACKGROUND OF THE INVENTION Equalization is a common term in the x-ray radiography field which refers to the process of selectively attenuating portions of the x-ray beam that are too intense for the density of a corresponding portion of the patient or object ("subject") exposed to the x-ray. Without equalization, the resulting x-ray image (appearing on either film or a monitor) may have inconsistent overall exposure, manifested as light and dark areas in the image, as a result of corresponding variations in density in the exposed portions of the subject. The effect of equalization is to reduce the intrinsically large dynamic range of the x-ray beam intensities in order to accommodate the dynamic range limitations of the x-ray detector system. The most common detector systems employed in diagnostic x-ray radiography are film and image intensifier-TV systems, both of which have severely limited dynamic range. Equalization is also useful in digital subtraction angiography (DSA). DSA is a known imaging technique where digital radiographic images are obtained both before and after injection of an iodine based dye into the vasculature, and then the two images are subtracted. DSA employs an x-ray image intensifier (fluoroscope) that is optically coupled to a high quality television chain and to a video digitizer. Although the image intensifier has a relatively large dynamic range (i.e., the ratio of the highest allowable signal intensities to the lowest is large), the TV camera presents substantial dynamic range limitations and thus limits the dynamic range of the entire imaging system. It is known to equalize an x-ray radiographic image by selectively attenuating only those areas of the image that are determined to have been overexposed. One such method involves arranging a plurality of filters between the x-ray emitter and the image receptor. The filters are selected and arranged so that only the areas of over-exposure are attenuated. Practice of this method provides acceptable results once the correct combination of filters has been found. However, a serious drawback of this method is that it is cumbersome since filter selection and juxtaposition is a manual process, and can require time consuming trial and error for the correct combination to be found. It is therefore desirable to provide an apparatus and method for performing selective equalization of x-ray radiographic images that is automated and rapid, but yet is simple and relatively inexpensive to implement. The present invention achieves these goals. SUMMARY OF THE INVENTION According to one embodiment of the invention, a plurality of juxtaposed members, such as disks, is provided wherein each disk has an annulus defining a filter region for attenuating electromagnetic radiation such as x-rays. The attenuation provided by the annular filter region of each disk varies throughout at least selected angular portions of the annulus. At least a portion of the annular filter region of each disk overlaps a portion of the annular filter region of all other disks. The members may be embodied as strips or belts. In such case, the surface of the member defines the filter and varies throughout its length. Motive means are operatively coupled to the disks for selectively and independently rotating each disk relative to the other disks and relative to an electromagnetic radiation emitter, such as an x-ray emitter. The emitter is disposed to emit radiation along a path intersected by the overlapping portions of the filter regions. Rotation of a disk thereby alters attenuation of the radiation along the path and received by the image receptor. A control means is operatively coupled to the motive means for controlling the operation of the motive means to alter the attenuation provided by the overlapping filters. According to a preferred embodiment of the invention, the filter of each disk is comprised of a plurality of adjacent and unique preselected attenuation or filtration patterns. Thus, rotation of a disk results in a selected, unique combination of attenuation or filtration patterns along the path, wherein the attenuated radiation has a pattern that corresponds to the selected combination of attenuation patterns. According to yet a further embodiment of the invention, the control means is operative to rotate the disks to a selected position for irradiating a subject to obtain a preliminary non-attenuated image. The control means is operatively coupled to the image receptor to receive and process the preliminary image. The control means determines locations and magnitudes of unsuitable exposures (i.e., overexposures) in the preliminary image and rotates the disks to select one of the unique combinations of attenuation patterns to compensate for the regions of unsuitable exposures. According to yet a further embodiment of the invention, each disk has an area in the annular region that provides substantially constant attenuation to electromagnetic radiation and defines a parked position of the disk. The disks are rotatable to substantially align the parked positions with the path when it is desired to obtain the preliminary image .