Patent Number: RE0361623
Section: summary

FIELD OF THE INVENTION The present invention relates generally to radiographic instruments and more particularly to an apparatus for assembling broad area images from narrow beam radiographic scans. BACKGROUND OF THE INVENTION Scanning radiographic equipment differs from conventional radiography in that it employs a narrowly collimated beam of radiation, typically x-rays formed into, for example, a fan beam, rather than a broad area cone beam. The small beam size used in scanning radiographic equipment allows replacement of an image forming sheet of radiographic film, used with conventional radiographic equipment, with a small area array of detector elements. The detector elements receiving the transmitted radiation produce electrical signals which may be converted to digital values by an analog to digital converter for the later development of an image or for other processing by computer equipment. The ability to quantify the measurement of the transmitted radiation, implicit in the digitization by the analog to digital converter, allows not only the formation of a radiographic "attenuation" image but also the mathematical analysis of the composition of the attenuating material by dual energy techniques. See generally, "Generalized Image Combinations in Dual KVP Digital Radiography", by Lehmann et al. Med. Phys. 8(5) September/October 1981. Such dual energy techniques quantitatively compare the attenuation of radiation at two energies to distinguish, for example, between bone and soft tissue. Dual energy techniques allow the measurement of bone mass, such measurement being important in the treatment of osteoporosis and other bone diseases. The limited area of the beam of radiation used in scanning radiographic systems allows the use of limited area detectors permitting high resolution with relatively lower cost. The limited area of the detectors, however requires that the beam be scanned along several adjacent path if large area images are to be constructed. Typically, a fan beam will be scanned in a raster pattern over the area to be measured, each line of the scan separated by somewhat less than the width of fan beam to ensure complete illumination of the entire volume of the imaged object, with the directions of scanning being generally perpendicular to the direction of the radiation and the plane of the fan beam. Images formed by a scanning radiographic system are potentially more accurate than those produced by a typical broad beam radiograph system. This accuracy arises from the limited divergence, in the scanning direction, of the rays of the fan beam, as compared to a broad area cone beam. This narrow collimation of the fan beam reduces "parallax" in the projected image, particularly of anatomical planar surfaces that are nearly parallel with the plane of the tan beam--such as the superior and interior borders of the vertebrae in the spine when the scanning ..directions.!. .Iadd.direction .Iaddend.is along the superior-inferior axis of the body. Morphological measurements of the vertebrae, and other structures, which benefit from reduced parallax are used to evaluate various dimensions of a vertebra to detect crushing or other deformation that are one element of certain bone diseases such as osteoporosis. See e.g. Minne et al., "A Newly Developed Spine Deformity Index (SDI) to Quantitate Vertebral Crush Factors in Patients with Osteoporosis," Bone and Mineral, 3:335-349 (1988); J. C. Gallagher et al, "Vertebral Morphometry: Normative Data," Bone and Mineral, 4:189-196 (1988); Hedlund et al, "Vertebral Morphometry in Diagnosis of Spinal Fractures," Bone and Mineral, 5:59-67 (1988); and Hedlund et al, "Change in Vertebral Shape in Spinal Osteoporosis," Calcified Tissue International, 44:168-172 (1989). Automatic techniques for morphological measurements of bone are described in U.S. patent application Ser. No. 07/944,626 filed Sep. 14, 1992 and entitled: "Method for Analyzing Vertebral Morphology Using Digital Radiography" assigned to the same assignee as the present application and hereby incorporated by reference. Nevertheless images developed with scanning fan beam equipment can include certain distortions or artifacts. In particular, it has been noted that objects at the interface between two adjacent scan paths contain a blurring or distortion in a direction perpendicular to the scan path. SUMMARY OF THE INVENTION The present invention provides a method and apparatus for constructing broad area images from a sequence of narrow fan beam scans. The invention recognizes that a source of image artifacts in combining narrow, fan beam scans is the varying amount of overlap between the fan beams when the axes of the fan beams are held parallel. This overlap causes some volume elements of the patient to be measured with rays at two different angles. The amount of overlap depends on the height of the structure being imaged, as measured along the path of the fan beams, and thus cannot, in general, be determined or corrected in a two dimensional image. The present invention vanes the angle of the axis of each fan beam so as to create a larger, effective fan beam of arbitrary width and to eliminate any height dependent overlap. The elimination of height dependent overlap ensures that each volume element of the patient is measured by rays at only one angle. Specifically, the invention employs an imaging system having a radiation source directing a fan beam of radiation toward the patient, where the fan beam diverges about a radiation axis, substantially within a beam plane, from a focal spot. A radiation detector opposing the radiation source along the radiation axis receives the diverging beam of radiation after passage through the patient to produce a projection signal indicating the attenuation of the beam of radiation for multiple rays within the beam. The radiation axis may be moved along a first and second path across the patient, the first and second paths being spaced apart and substantially perpendicular to the beam plane. In moving between the first and second paths of the scan, the radiation axis is rotated about the focal spot by a displacement angle, within the beam plane. The signals obtained along the first and second path are then combined to produce a two dimensional projection image. It is thus one object of the invention to reduce image artifacts, caused by combining image data obtained from multiple scannings of a narrow fan beam. Creating a larger, effective fan beam eliminates areas of overlap or produces areas of overlap that, with appropriate projections, are constant regardless of the height of the imaged structures and which therefore can be eliminated by a constant weighting factor applied to the data of the overlapping area. The radiation detector may be a linear array of detector elements, each subtending a first width of the fan beam along the linear array, where the ..projections.!. .Iadd.projection .Iaddend.signals include a plurality of elements signals from each detector element. A projector may be employed to map the element signals to pixels of a non-planar image surface generally normal to the radiation axis, each pixel subtending second widths of the fan beam varying from the first widths. The non-planar image surface may be positioned midway along the height of the patient as measured along the radiation access. It is thus another object of the invention to reduce the distortion caused by the divergence of rays in both the marrow measuring fan beams and the larger, effective fan beam by mapping the element signals to pixels of a non-planar surface so that each such pixel represents rays of the fan beam passing through equal areas of the patient. This reduces variations, for example, in bone mineral density measurements, which are sensitive to distortion in the measured area. It is another object of the invention to reduce the magnitude of ..magnifications.!. .Iadd.magnification .Iaddend.induced errors on the projected image. By positioning the non planar image surface to approximately bisect the body, distance between the imaging plane and any particular structure in the body, such as affects magnifications, is reduced to a minimum. The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a pan hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims herein for interpreting the scope of the invention.