Patent Number: 054188326
Section: summary

FIELD OF THE INVENTION The present invention relates to scanning radiographic equipment and in particular to a method of improving the contrast of images obtained with fan beam scanning systems. BACKGROUND OF THE INVENTION Conventional x-ray radiography records the attenuation of x-ray radiation, over the surface of an image plane, after it has passed through a patient. The attenuation is typically recorded as a pattern or "image" on a sheet of x-ray film. The pattern of attenuation ideally indicates the relative opacity (to x-rays) of the patient along many rectilinear "rays" extending from the x-ray source through the patient to the film. Ideally, each point of the film image indicates the total attenuation caused by internal structures of the patient along a single ray. In practice, however, when x-ray radiation passes through a patient, a certain amount of the radiation is scattered away from its path of incidence. Some of this scattered radiation is still received by the film, although at a point other than where it was originally directed. The scattered radiation causes portions of the x-ray image to receive additional x-ray energy that may not have been attenuated by structure of the patient directly interposed between that portion of the image and the x-ray source. The amount of scatter depends on the material through which the x-rays pass. For example, less scatter is encountered in imaging the lungs, which are of low density, as compared to the mediastinum which is a relatively higher density. The net effect of scatter is that the contrast of the image, the difference between light and dark portions of the image, is degraded. The contrast of an image, all other things being equal, effects the amount of information conveyed by the image. A decrease in contrast may result in the loss of diagnostically important information. Scatter has heightened significance in certain applications, such as dual energy bone densitometry, where the attenuation at each portion of image at two energies is determined quantitatively and mathematically combined to isolate different tissue within the patient. Here small amounts of scatter that might be tolerable on a qualitative basis can cause unacceptable quantitative errors. It has long been known that scatter in conventional radiography may be controlled by the use of a grid consisting of a series of regularly spaced thin plates or lamellae arranged edgewise to allow passage of x-rays only along a straight line path from the x-ray source to the image receptor. Scattered x-rays that do not travel along a straight line path see a much greater area of lead and are preferentially absorbed. The effectiveness of a grid in passing desirable or "primary" x-rays is measured by its "primary transmission" and depends generally on the ratio of the lamellae's thickness to the space between the lamellae, i.e., the "intergrid spacing" and the "lead content" (mg/cm.sup.2) of the grid. Thinner lamellae and greater spacing between the lamellae block fewer primary x-rays. A typical grid may have a primary transmission of approximately 70% and thus there is a significant reduction in total exposure of the film caused simply by the use of the grid. A decrease in exposure of the film, like a decrease in contrast, can reduce the amount of information contained in the image and cause the loss of diagnostically significant details in the image. Accordingly, the use of a grid is not without cost in terms of diagnostic information and the use of a grid is typically considered only when its effect on the reduction of scatter is expected to be significant. The effectiveness of the grid in blocking oblique or scattered x-rays depends generally on the height of the lamellae, as measured along the rays, in proportion to the spacing between the lamellae. Higher lamellae and lamellae that are spaced closer together block more scattered radiation. The height of the lamellae in proportion to their spacing is typically expressed as a "grid ratio". Typical grid ratios are 8:1 and 12:1 meaning that the lamellae are respectively eight or twelve times as high as the spacing between them. Grids having strip densities (lines/mm) of substantially less than 100 lines per inch can often produce objectionable grid lines on the resulting image, the grid lines being the shadows of the lamellae. Moving the grid during the x-ray exposure of the image receptor blurs the grid lines over a larger area thus rendering them fainter and thus less objectionable. With the advent of scanning radiography, where the area x-ray beam is replaced with a highly collimated pencil or fan beam, the problems of scatter have been remarkably reduced. In such systems, the collimated radiation beam is moved in a scanning pattern over an area of the patient to be imaged. Synchronously, a collimating slot is moved to remain opposed to the radiation beam on the opposite side of the patient. Only a portion of the image is exposed at any given time. The effect of the highly collimated radiation beam and the slot is to eliminate the effect of scattered radiation from rays normally present on either side of the collimated beam during the exposure of any given portion of the image. With suitably narrow radiation beams, the problem of scatter from adjacent rays is virtually eliminated. Narrowly collimated radiation beams may create significant tube loading problems. Specifically, in order to provide an acceptably short scanning time the radiation beam must provide no less than a certain minimum fluence. The fluence is generally proportional to both the area of the collimated beam and the power of the x-ray tube. Collimation of the x-ray beam to increasing small areas requires correspondingly greater x-ray tube power and much of that increased power is wasted by the narrow collimation. Thus, in practice, extremely narrow radiation beams may be inefficient or unduly expensive. SUMMARY OF THE INVENTION The present invention provides a scanning radiographic system that provides reduced scatter and acceptable tube loading. The invention recognizes that the use of a grid in addition to the slot of a low-scatter scanning radiographic system provides significant further scatter reduction. This further scatter reduction permits the use of a wider slot without unacceptable increases in scatter thus dramatically lowering tube loading. In particular, the system employs an x-ray source producing a fan beam of rays of x-rays directed toward a patient along a beam axis where the fan beam has a cross-sectional length and width measured in a plane perpendicular to the beam axis. A detector array is positioned to receive the fan beam after the fan beam has passed through the patient and to produce an attenuation signal related to the intensity of the received fan beam. The fan beam and detector are arranged to be scanned over a volume of the patient in a direction perpendicular to the cross-sectional length of the fan beam. A slot, formed of a radio opaque material, has an aperture conforming to the cross-section of the fan beam and is attached to and aligned with the detector array. A grid comprised of a set of radio opaque lamellae extending across the width of the slot is affixed to the slot. It is one object of the invention to provide a scatter controlling scanning radiographic system with acceptable tube loading. The recognition that a grid provides significant increase in scatter reduction to the already low scatter of a scanning radiographic system permits the width of the fan beam to be increased and tube loading to be reduced without detrimental loss of image contrast. The lamellae in the grid may be positioned to extending diagonally across the width of the slot. It is thus another object of the invention to employ the scanning of the scanning radiographic system to eliminate grid lines. By careful selection of the angle of the lamellae within the grid in proportion to the width of the grid and the interlamellae spacing, grid lines may be effectively eliminated. This is true even though the detector and grid have no relative motion whereas eliminating grid lines in a film based system requires the grid be moved with respect to the film. The lamellae may also be tipped so as to align themselves with the rays of the x-ray radiation. Thus it is another object of the invention to provide the foregoing benefits in a grid of high efficiency where a minimum amount of radiation is absorbed by the lamellae. Other objects, advantages, and features of the present invention will become apparent from the following specification when taken in conjunction with the accompanying drawings.