Patent Number: 048295523
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to anti-scatter grid systems for use in X-ray apparati and specifically to anti-scatter grid systems for reconstructing a shadow image of X-rays projected through a subject and filtered through a grid mechanism as if the grid mechanism did not obstruct unscattered irradiation. Despite technical promise prior grid systems have been unable to filter scattered X-ray photon irradiation from interfering with shadow image recordation and simultaneously produce clear and accurate shadow images of a subject being examined. Such prior grid systems typically employ dense arrays of X-ray absorptive strips which unduly obstruct critical unscattered image forming irradiation, cumbersome grid vibration mechanisms, and complex chamical film recording mechanisms. It is an object of the invention, therefore, to provide an anti-scatter grid system which minimizes obstruction of unscattered irradiation. It is a further object of the invention to provide an anti-scatter grid system which efficiently and effectively produces a clear and accurate shadow image of a subject examined with X-ray photon irradiation. SUMMARY OF THE INVENTION In accordance with the invention there is provided in an apparatus for projecting a beam of X-ray photons through a subject at an X-ray photon sensitive detector, an anti-scatter grid system for reducing transmittance to the detector of a portion of the beam scattered by the subject and enhancing recordation of a portion of the beam passing unscattered through the subject to the detector, comprising: a grid disposed between the subject and the detector, the grid including filtration strips filtering scattered X-ray photons from detection by the detector, the strips also obstructing a minority of the unscattered X-ray photons passing through the subject from detection by the detector means; a mechanism for recording an analog image of the subject from the detector, the analog image recorded from photons absorbed by the subject and not detected by the detector, photons having passed through the subject and detected by the detector and photons having passed through the subject, obstructed by the strips and not detected by the detector; a mechanism for converting the recorded analog image into an array of digital data representative of the analog image; and a mechanism for reconstructing the image as if the unscattered X-ray photons were not obstructed from detection by the detector. Preferably the grid strips comprise an X-ray absorptive material, the strips being disposed in a serial spaced array relative to each other between the source and the detector. Most preferably the X-ray absorptive material is selected from the group of metals consisting of tantalum, lead, uranium, and alloys, mixtures, or laminates of at least one of said metals. The digital data is typically stored in an addressable storage mechanism arranged in a plurality of rows and columns forming a grid array of pixels and the mechanism for reconstructing preferably comprises: a mechanism connected to the storage mechanism for retrieving data from selected pixels in the array, the selected pixels being adjacent to pixels corresponding to photon obstructed areas of the analog image; a mechanism connected to the retrieving mechanism for generating values from the retrieved data; and, a mechanism connected to the storage mechanism and the generating mechanism for storing the generated values in the pixels corresponding to the photon obstructed areas of the analog image. The generating mechanism preferably comprises: a counting mechanism for identifying the number of retrieved data values; a summing mechanism for adding the retrieved data values; and, a dividing mechanism for dividing the output of the summing mechanism by the output of the counting mechanism. The counting mechanism preferably includes a mechanism for weighting the selected retrieved data prior to adding. The mechanism for retrieving preferably comprises: a comparison mechanism for comparing the data stored in adjacent pixels; an identifying mechanism connected to the comparison mechanism for identifying adjacent pixels having differences greater than a selected value; and, a defining mechanism connected to the identifying mechanism for defining the photon obstructed pixels by the output of the identifying mechanism. The number of strips per inch and the height and the thickness of the strips are preferably selected to filter a majority of the scattered photons from detection by the detector and to obstruct less than about fourteen percent of the unscattered photons from detection by the detector means. The strips are preferably focussed at the source projecting the X-ray photon beam. The height of the strips is preferably selected to be in the range between about one quarter of an inch and about five inches, and the density of the strips is preferably selected to be in the range between about one and about ten strips per inch. The present invention also provides a method of constructing an X-ray image of a subject which partially absorbs and partially transmits scattered and unscattered X-ray photons, comprising the steps of: projecting a beam of X-ray photons at and through the subject to be examined; aligning an X-ray photon sensitive detector behind the subject to detect the X-ray photons transmitted through the subject; filtering the scattered X-ray photons transmitted through the subject with an X-ray absorptive grid disposed between the subject and the detector, the grid obstructing a minor portion of the unscattered X-ray photons transmitted through the subject from reaching the detector; recording an analog image of the subject from the X-ray photons detected by the detector, the image representing photons absorbed by the subject, photons transmitted through the subject to the detector, and photons transmitted through the subject and obstructed from the detector; converting the analog image to a digital array of data representative of the analog image; and, reconstructing the areas of the image corresponding to the unscattered photons obstructed by the grid mechanism from the digital data. The X-ray absorptive grid preferably comprises a serial array of spaced strips comprising an X-ray absorptive material, the strips being fixedly focussed at the source projecting the X-ray photons. The X-ray absorptive material is preferably selected from the group of metals consisting of tantalum, lead, uranium, and alloys, mixtures, or laminates of at least one of the metals. The digital data is preferably stored in an addressable storage mechanism arranged in a plurality of rows and columns forming a grid array of pixels and the step of reconstructing preferably comprises: retrieving data from selected pixels in the array, the selected pixels being adjacent to pixels corresponding to photon obstructed areas of the analog image; generating values from the retrieved data; and, storing the generated values in the pixels corresponding to the photon obstructed areas of the analog image. Most preferably the step of generating comprises: counting the number of values retrieved from the selected pixels; summing the values retrieved from the selected pixels; and, dividing the summed values of the selected pixels by the number of values retrieved. The step of counting typically comprises weighting the retrieved values of the selected pixels. The step of retrieving typically comprises: comparing the data stored in adjacent pixels; identifying the adjacent pixels having differences greater than a selected value; and, defining the photon obstructed pixels by the output of the step of identifying.