Patent Number: 054328349
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIGS. 1, 2A, 2B and 2C, a patient 1 lies horizontally during scanning on a table 2. X-ray radiation produced by an x-ray source 3 located beneath table 2 is transmitted through patient 1 to a detector 4 having an array of detector positions and located above patient 1. Both x-ray source 3 and detector 4 are supported on a rigid arm 5 which maintains a selected source-to-detector distance and alignment. In this example of the invention, x-ray source 3 has a stationary anode. Adjacent x-ray source 3 is a slit collimator 6 made of a material an x-ray opaque material such as lead or tungsten of sufficient thickness to substantially block x-rays from source 3. One or more selectable slits have been machined into collimator 6 to allow passage of the x-rays therethrough. The preferred embodiment includes a 1 mm wide collimator slit. The x-ray radiation from the x-ray source 3 passes through the slit in the collimator 6 and forms a fan shaped beam of x-rays 3a. The angle subtended by beam 3a and the distance between its origin at the focal spot of the x-ray tube and patient 1 are selected such that beam 3a would not cover the entire cross-section of a typical adult patient at any one time but would cover only a selected portion of the width. In the preferred embodiment, fan beam 3a has a maximum fan angle of 22 degrees, whereas a fan angle of 65 degrees may be required to completely cover patient 1 for whole body analysis. Of course, x-ray beam 3a not only has width (along the X-axis illustrated in the Figures) but also has a thickness along the Y-axis that is defined by the width of the slit in collimator 6 and its distance from the origin of beam 3a. A scan line is defined by the area of the patient irradiated at any one time, i.e. the width and thickness of the x-ray beam over which data is collected at one point in time. A complete pass or scan consists of a set of adjacent scan lines obtained over a period of time such that the entire region of interest has been measured. Opposite x-ray source 3 is detector 4 which in this embodiment comprises approximately 200 detector elements arranged in a linear configuration along the XZ plane which is about 16" long and is about 42" from the origin of beam 3a (42" source-to-detector spacing) and subtends a 22 degree fan angle. The detector elements making up detector 4 are fixed with respect to x-ray source 3. However, both x-ray source 3 and detector 4 can move with respect to patient 1 and table 2. One motion translates fan beam 3a along the patient axis defined by the spine, i.e., in the Y-direction. Another motion rotates beam 3a around the patient. The center of rotation is at a point C (see FIG. 5A) determined by the support arm 5 and the method of rotation employed. In this embodiment, the detectors and x-ray source are mounted to C-arm 5 which rotates on a set of rollers 7. Thus, the center of rotation is determined by the outer radius R of the C-arm, and is not at the origin (focal spot) of beam 3a. Patient 1 lies on a table 2, for example in the supine position. Table 2 can move horizontally along the X-axis as well as vertically along the Z-axis. These motions can be carried out by using a toothed-belt driven by a stepping motor or a DC servo motor, although other implementations such as stepper-motor driven lead-screws can also be employed. To perform a scan, a series of scan lines of data must be acquired. To do this, C-arm 5 carrying the x-ray source 3 and detector 4 is moved along the Y-axis along the length of patient 1. This motion moves detector 4 and x-ray source 3 to form a succession of spatially overlapping scan lines adding up to a scanned rectangular area. The signals produced by the detectors in response to x-rays impinging thereon at successive scan lines are digitized by an analog to digital (A/D) converter and are stored, for example on disk. A computer processes the signals from the A/D converter into density representations and images using the principles disclosed in the prior art discussed in the background section of this disclosure. For body structures of interest such as the spine, hip and wrist, only a single pass of fan beam 3a along the Y-axis is required because typically the area of interest in the patient's body is covered by fan beam 3a as shown in FIG. 2A for the Posteroanterior (PA) spine and in FIG. 2B for the hip. Indeed, a fan beam of only 14 degrees can be sufficient for the geometry of this embodiment to fully illuminate these body areas with x-rays. FIG. 2C shows the positioning for a lateral scan of the spine in which the view is orthogonal to the standard PA spine view. To attain this position, a series of movements of C-arm 5 and table 2 are required to ensure that the table and C-arm clear each other. In this embodiment, table 2 is moved along the X-axis and the Z-axis appropriately while C-arm 5 is rotated about an Y-axis passing through point C until the desired lateral position is reached. Whole body analysis requires that the entire body be illuminated with x-rays. Referring to FIG. 3A, a fan beam 3b of approximately 65 degrees can be suitable for completely illuminating the entire cross-section of patient 1. As illustrated in FIG. 3B, this fan beam can be simulated by utilizing multiple passes from a smaller, 22 degree fan beam 3a as long as all of the fan beams emanate from the same focal spot location to maintain the focal spot to patient body relationship. With a fan beam 3a of 22 degrees and the nominal dimensions of the system in this embodiment, three passes along the Y-axis can be made to cover the entire patient 1. Thus, data from passes 1, 2 and 3 from the smaller fan beam 3a can be added together using a computer to provide data that is substantially equivalent to data that would have been obtained if one large fan beam 3b had been used. To provide the smaller fan beams implies rotation of fan beam 3a with the focal spot thereof as the center of rotation. With fan beam 3a in a vertical orientation as in the middle position of fan 3a in FIG. 3B, fan beam 3a for pass 1 is rotated 21.5 degrees from the vertical while fan beam 3a for pass 3 is rotated -21.5 degrees from the vertical. The data from the 0.5 degrees of overlap is blended, e.g., by progressively using more of the data from the next pass as one moves in angle toward the next pass, using for example principle known in second generation CT technology. FIG. 3C shows an enlargement of the area designated P in FIG. 3B, where beams 3a for passes 1 and 2 overlap spatially. Fan beam 3a is slightly wider than the required 21.5 degrees so that there is an overlap of 0.5 degrees between the two passes. The overlapping areas imply that at least two different elements of detector 4 have measured the x-rays attenuated through the same body area. If rotation of beam 3a around its focal spot is possible, implementation of the multiple passes is relatively easy because the only required motion between passes is rotation. However, in the preferred embodiment, the center of rotation C does not coincide with the focal spot. To overcome this, in accordance with the invention the focal spot is made the effective center of rotation through motion of table 2. Referring to FIGS. 4A, 4B and 4C, the three views depict the relative position of table 2 and C-arm 5 for the three passes in the preferred embodiment. Collimator 6 is not shown in these views. Each position maintains constant the spacing between the focal spot of beam 3a and table 2 as well as the location of a vertical intercept from the focal spot to table 2. In FIG. 5A the geometry of pass 1 in relation to pass 2 is detailed. In pass 1, patient 1 lies supine on patient table 2 at position P1 with the focal spot of x-ray source 3 located at F1. In this position, only the left side of patient 1 is illuminated with x-rays within fan beam 3a. If C-arm 5 could now be rotated about the focal spot, the conditions of pass 2' would be achieved in which the central part of the patient would be illuminated. However, the focal spot rotates about the center of rotation of C-arm 5 located at C with a radius R. A rotation through an angle of -.theta. about a pivot axis at point C attains the positioning of pass 2 in which the focal spot is located at F2. To maintain the focal spot of beam 3a as the effective center of rotation, patient table 2 moves to position P2 (without moving patient 1 relative to table 2) in which the spatial relations between F1 and P1 are identical to the spatial relations between F2 and P2, i.e., a vertical drawn from the focal spot intersects patient table 2 at the same point and extends over the same distance. To attain position P2 requires two motions of table 2, one over a distance DX along the X-axis and another over a distance DZ along the Z-axis. These two motions can be consecutive or concurrent. These distances DX and DZ correspond to the differences in X and Z coordinates for focal spot positions F1 and F2. Referring to-FIG. 5B, where the terms are graphically defined, the distances DX and DZ are given by the relationships: EQU DX=(X2-X1)=R[cos .phi.(cos .theta.-1)+sin .phi. sin .theta.] EQU DZ=(Z2-Z1)=R[sin .phi.(cos .theta.-1)-cos .phi. sin .theta.] Patient table 2 is translated along the Z-axis over a distance DX and along the Z-axis over a distance DZ, where .phi. is the angle that F1 makes with the center of rotation C as the origin and .theta. is the angle of rotation between F1 and F2 which in the preferred embodiment is about -21.5 degrees, with the negative angle denoting a clockwise rotation around C between passes 1 and 2. Similarly, for pass 3, the focal spot is translated by DX and DZ with .theta.=-43 degrees. Although the preferred embodiment uses translations of table 2 along the X-axis and Z-axis to maintain the table/focal spot relationship, other embodiments are possible within the scope of the disclosed inventions without loss of generality. For instance, C-arm 5 can be made movable along the X-axis and the Z-axis while table 2 remains stationary, or table 2 and C-arm 5 can share the translations, i.e., C-arm 5 can move along the X-axis (or the Z-axis) while table 2 moves along the Z-axis (or along the X-axis). As illustrated in FIG. 6, an additional analysis called the "oblique hip" can be performed in accordance with the invention by suitably rotating C-arm 5 and translating patient table 2 along the X-axis and the Z-axis. The actual position can be determined beforehand by performing a "scout" scan which is usually a high speed, low dosage scan for the AP hip. In FIG. 6, F1 is the location of the focal spot of beam 3a, and line a-a' represents the field of radiation in patient 1, at a distance L from the focal spot of beam 3a. For convenience and clarity, patient table 2 is not shown in FIG. 6, but its position can be seen in FIG. 4a. A hip designated H1 is offset from the central ray of beam 3a by a distance D which can be quantitatively determined from the scout scan. Upon rotation of C-arm 5 through an angle .theta. (or 23 degrees in the preferred embodiment) the focal spot is now at F2. Table 2 is translated along the X-axis and the Z-axis while patient 1 remains stationary on table 2 so that the patient's hip is at position H2 which is now located in the central ray F2-H2 of the radiation field b-b' in patient 1. In this geometry, the X and Z translations, DX and DZ, of table 2 made to place the hip at H2 are given by the relationships: EQU DX=R cos .phi.[cos .theta.-1]-sin .phi.[R sin .theta.-L]+D EQU DZ=[R sin .phi.+L][cos .theta.-1]+R cos .phi. sin .theta. where R is the distance of the focal spot F1 from the center of rotation C of the focal spot of beam 3a, and .phi. is the angle of the focal spot F1 with respect to the center of rotation C. The distance L from the focal spot to the hip is estimated as the sum of the known distances from F1 to the table plus the estimated distance from the table to the field a-a'. FIG. 7 illustrates an embodiment in accordance with the invention in block diagram form. Gantry 10 includes the structure illustrated in FIG. 1 as well as a suitable power supply for the x-ray tube and the motors needed to move table 2 and C-arm 5 and to operate collimator 6 in a manner similar to that in said QDR-2000 system. Detector 4 supplies x-ray measurements to A/D convertor and preliminary processor 12 which carries out processing similar to that carried out in said QDR-2000 system. The output of element 12 is supplied to a processor 14 which performs various calculations and forms an image in a manner similar to that used in said QDR-2000 system and, additionally, blends the data from successive scans in a manner similar to that used in second generation CT technology to form whole-body images. Data and images from processor 14 are supplied to a console 16, display 18 and a recording device 20 for purposes and in a manner similar to those in said QDR-2000 system. Two-way arrows connect the elements of FIG. 8 to illustrate the fact that two-way communication can take place therebetween. Conventional elements have been omitted from the Figures and from this description for the sake of conciseness. While a preferred embodiment of the invention has been described in detail, it should be understood that changes and variations will be apparent to those skilled in the art which are within the scope of the invention recited in the appended claims.