Patent Number: 054328349
Section: summary

BACKGROUND OF THE INVENTION This invention relates to x-ray analysis systems and methods. X-rays or gamma-rays can be used to measure the density and distribution of bone in the human body in order to help health professionals assess and evaluate projected bone mineral density, which in turn can be used to monitor age-related bone loss that can be associated with diseases such as osteoporosis. Additionally or alternatively, similar procedures can be used to measure non-bone related body content such as body fat and muscle. In general, in bone densitometry a patient is placed on a table while a radiation source irradiates the patient. An x-ray detector is positioned on the opposite side of the patient from the source to detect the radiation transmitted through the patient. The x-ray source and detector are usually mechanically linked by a structure such as a C-arm to ensure alignment of source and detector. Both x-ray tubes and isotopes have been used as a source of the radiation. In each case, the radiation from the source is collimated to a specific beam shape prior to reaching the patient to thereby restrict the field of x-ray or gamma radiation to the predetermined region of the patient opposite which are located the detectors. In the case of using x-rays, various beam shapes have been used in practice including fan beam, pencil beam and cone or pyramid beam shapes. The shape of the beam and the shape of the detector system correspond. The detector in a fan beam system typically is a linear array of detectors. Some examples of the actual detectors which make up the array are the relatively low cost silicon photo diodes coupled with a scintillation material and the relatively high cost photo multiplier tubes coupled with scintillation material. In both cases, the cost of the detector system and associated electronics increases substantially with increasing numbers of detectors. By means of mechanically moving the source/detector system relative to the patient, the fan beam of x-rays can be scanned in a direction normal to the plane defined by the boundaries of the fan beam angle to produce a rectangular analysis area. The width of this rectangular area is defined by the width of the fan beam when it passes through the patient. Of course, it is desirable that the scanned area include the desired region of analysis. Typical regions of analysis in bone densitometry include the spine, hip and wrist, scanned individually. They can be covered individually within a reasonable time by a fan beam that has a relatively narrow angle in a single pass or, alternatively, by a pencil beam scanning a raster pattern. Another analysis region is termed "oblique hip" in which the hip is viewed at an angle relative to the horizontal and vertical directions. This can be desirable for optimizing the projection angle through the femoral neck. However, current techniques typically require patient leg positioning which can result in imprecise measurements on repeated scans of the same patent. Another analysis region is referred to as "whole body" in which the entire patient body is scanned and analyzed for bone density and possibly also for "body composition" or the percentages of fat and muscle in the body. Known whole body procedures have utilized pencil beam scans of the patient, using a relatively narrow beam of radiation and a single detector scanning the whole body in a raster scan. However, such a scan takes a considerable length of time. If the whole body is to be scanned in a single pass with a fan beam of radiation, the fan angle would have to be considerably greater than that required for other typical analysis such as hip, spine or wrist analysis. This implies that the detector array must also be substantially wider than an array for wrist, hip or spine analysis if whole body analysis is to proceed by using only pass of the beam over the patient. Alternatively, one could utilize multiple longitudinal passes of the patient body using a smaller and less expensive detector array but the data from the multiple passes would need to be merged without artifacts especially at the boundaries between passes. Known system of this type are manufactured by the assignee hereof under the tradenames QDR-2000, QDR-1500, QDR-1000plus, and QDR-1000. The following commonly owned U.S. Pat. Nos. pertain to such systems and are hereby incorporated by reference herein: 4,811,373, 4,947,414, 4,953,189, 5,040,199, 5,044,002; 5,054,048, 5,067,144, 5,070,519, 5,132,995 and 5,148,455; and 4,986,273 and 5,165,410 (each assigned on its face to Medical & Scientific Enterprises, Inc. but now commonly owned). Other bone densitometry systems are believed to be offered by the Lunar Corporation of Madison, Wis. (see, e.g., the system which is believed to be offered under the tradename Expert and U.S. Pat. No. 5,228,068, neither of which is admitted to be prior art against this invention). SUMMARY OF THE INVENTION According to one important aspect of the invention, an x-ray analysis system comprises an x-ray source which generates and projects at least one x-ray beam along a plane transverse to a patient's long axis, and detector array arranged on the opposite side of the patient to detect x-rays so as to produce signals corresponding to the amount of x-rays transmitted through the patient. The detector array together with and in fixed relation to the x-ray source is movable relative to the patient in a scanning direction normal to the beam plane through a multiplicity of scan line positions called a "pass" or a j"scan." The detector outputs from multiple passes or scans for different areas of the patient can be combined to yield one equivalent pass or scan of a larger area for subsequent analysis. Preferred embodiments of this aspect of the invention include one or more of the following features: The x-ray source comprises an x-ray tube and a slit collimator that produces a fan beam. The detector array includes a linear array of evenly spaced discrete detectors. The x-ray source and detector array are translatable in a direction normal to the detector array to produce a movement in the scanning direction. In addition, the x-ray source and detector array are rotatable to fixed angular positions to produce scanning beams at other angles which cover other areas of the patient or to cover the same general area at different angles. The patient is on a table which is movable in at least the vertical direction and one horizontal direction to allow the focal spot of the x-ray tube to remain in a fixed vertical distance from the table. This makes it possible to combine the detector output from scans at different angles into an accurate composite x-ray image because in accordance with one aspect of the invention, the fan beams for each pass or scan can all emanate from a common focal spot location, i.e., the focal spot for each fan beam can be exactly in the same location relative to the patient. Therefore, the projection through bone and other tissue seen at adjacent beam boundaries can be sufficiently similar to allow for an image seamed from multiple pass fan beams to be the same or equivalent to an image that could have been derived from a single pass with a fan beam wide enough to encompass the entire width of the patient. An exemplary and non-limiting method in accordance with the inventions comprises placing a patient on a table movable in a vertical direction and in a direction along the width of the patient, irradiating the patient with a fan beam of x-rays subtending an angle that includes substantially less than the body width of the patient; receiving x-rays from the source within the angle subtended by the fan beam after passage thereof through the patient at a number of radiation detecting position arrayed within said angle; scanning the fan beam and the detector along the length of the patient in successive scans; selectively moving the table both along the width of the patient and vertically between successive scans to thereby scan the patient from different angles while maintaining a selected vertical distance between an origin of the fan beam and the table; and integrating the detector outputs from the successive scans into a single whole-body image. In addition to this process, or instead of this process, the vertical distance between the origin of the x-rays and the table, and the angle of the beam to the patient, can be changed and the beam can be scanned relative to the patient to obtain detector outputs for a lateral view and/or an oblique hip view without moving the patient relative to the table.