Patent Number: 056087764
Section: description

DETAILED DESCRIPTION Referring to FIG. 1, a gantry 10, representative of a "third generation" computed tomography scanner type, includes an x-ray source 12 having a focal spot 14. A fan beam of x-rays 16 are emitted from source 12 towards an object 18 to be imaged. A detector array 20, composed of two rows 22A and 22B of detector elements or cells 24A and 24B and collimators 26, is mounted on a side of gantry 10 opposing x-ray source 12. Source 12 and detector array 20 rotate on gantry 10 as indicated by arrow 28. FIG. 2 illustrates a typical detector sensitivity response curve 30 with respect to detector element 24A. Detector element 24A is shown positioned axially with respect to the z-axis, with the center of detector element 24A indicated by the "0" point on the z-axis and the respective ends, or edges, of detector element 24A located at points "a" and "-a" respectively As clearly shown by curve 30, at both edges of detector 24A, the sensitivity response of detector element 24A is extremely low and has significant variation as function of position. Points "b" and "-b" represent the locations where, moving axially along z-axis towards the detector edges, the detector element sensitivity about begins to fall off. Detector element 24A may not generate highly accurate intensity indicative signals for x-rays received at or between points a and b and points -a and -b, respectively. The detector edge regions are generally defined as the portions of detector element 24A between and including points a and b and points -a and point -b. FIG. 3 illustrates adjacent detector elements 24A and 24B of detector array 20, collimator 26 and x-ray beam 16. Collimator 26 may be constructed of x-ray absorbing material such as sintered molybdenum, and is utilized to substantially block a portion of beam 16 from substantially impinging upon adjacent edges of detectors 24A and 24B. Beam 16 is illustrated as being off-center with respect to the center axis of collimator 26. As described above, such a condition may result from focal spot movement. Regardless of the cause, it can be seen in FIG. 3 that a greater percentage of beam 16 is received in the edge region of detector element 24B when beam 16 is off-center as shown. Such a condition may result in the generation of artifacts. Also, even though collimator 26 may block a portion of beam 16 from impinging upon sections of the edge regions of detectors 24A and 24B, at least some portions of beam 16 are still received within such edge regions. As explained above, such a condition is undesirable. FIG. 4A illustrates projection data sampling in a twin beam scanner having a post-patient collimator with a helical pitch about equal to 1:1, and FIG. 4B illustrates the optimal projection data sampling. As shown in FIG. 4A, double interwoven helixes mapped by the projection data have a significant amount of "overlap". This means that the projection data collected by one detector element in the first row substantially overlaps, or is substantially identical to, the projection data collected by an adjacent detector element in the second row in the next gantry rotation. Such overlap is undesirable because the duplicate projection data does not significantly facilitate generating higher quality images for multiple slices. FIG. 4B illustrates the optimal projection data sampling by adjacent detectors in separate detector rows. Particularly, as shown in FIG. 4B, there is no significant overlap and the projection data collected by the detector elements in the second row staddle the samples by the detector elements in the first row. Providing such sampling increases the amount of projection data collected and should improve the quality of an image reconstructed using such data. FIG. 5 is a schematic representation of an x-ray source 50, an x-ray detector array 52 having at least two adjacent rows of detector cells 54A and 54B separated by a collimator 56, and a dynamic pre-patient collimator 58. An offset detector data acquisition system 60 and a reference detector data acquisition system 62 are coupled to detector array 52, and outputs from such systems 60 and 62 are supplied to a motor controller 64. Motor controller 64 drives a stepper motor 66 which is coupled to pre-patient collimator 58. X-ray source 50 includes an anode 68 and a rotating shaft 70 and operates in a well known manner. For a third generation system, X-ray source 50, pre-patient collimator 58 and detector array 52 would be mounted to the system gantry (not shown) in the one embodiment. Prior to operation, and for a plurality of selectable helical pitches, the particular orientations for components of collimator 58 are determined. For example, the components, which are hereinafter described in more detail, should be oriented in a first configuration for a first helical pitch and in a second configuration for a second helical pitch. The selected orientations may be determined, for example, by experimentation to avoid any significant data overlap. In one form of operation, a beam 72 emitted from source 50 is received at pre-patient collimator, or beam splitter, 58. Splitter 58 is configured, as hereinafter described in more detail, so that input beam 72 is split and forms two output beams 74A and 74B. Beams 74A and 74B are then at least partially attenuated through an object to be imaged, and the transmitted portions of beams 74A and 74B are received by detector array 52. Signals from detector array 52 are collected by an imaging system (not shown) and may be used to reconstruct an image of the object. In addition, signals from array 52 are received by an offset detector data acquisition system 60 and a reference detector data acquisition system 62. More specifically, as is well known in the art, a z-axis offset detector (not shown) and a reference detector (not shown) may be utilized to detect the fan beam position and supply inputs to systems 60 and 62, respectively. Such fan beam detection is described, for example, in U.S. Pat. No. 4,559,639, X-Ray Detector With Compensation Height-Dependant Sensitivity And Method Of Using Same, issued Dec. 17, 1985 and assigned to the assignee of the present invention, and incorporated herein, in its entirety, by reference. Signals received by DAS 60 and 62 are digitized and output to motor controller 64. Motor controller 64, using such digitized signals, determines whether an adjustment is necessary in order to better position beams 72A and 72B on the respective detector cells 54A and 54B, e.g., not on the detector edge regions. If adjustment is necessary, a control signal is supplied from controller 64 to stepper motor 66. Under the control of controller 64, stepper motor 66 adjusts splitter 58 so that beams 72A and 72B are positioned on cells 54A and 54B as desired. Such adjustment is referred to herein as dynamic adjustment of splitter 58 in that such adjustment may be made during a scan operation. One embodiment of splitter or pre-patient collimator 58 is shown in cross section in FIG. 6. Splitter 58 includes a housing 76, a beam splitting member 78 rotatably secured to housing 76 utilizing a rotatable shaft 80, and first and second collimating members 82A and 82B. Beam splitting member 78 is at least partially positioned between first and second collimating members 82A and 82B and is rotatable relative such members 82A and 82B. Beam splitting member 78 also has an elongate elliptical shape, and collimators 82A and 82B and beam splitting member 78 are constructed of x-ray absorbing material such as molybdenum. Housing 76 includes an input beam opening 84 and output beams opening 86. Rotatable shaft 80 is coupled to stepper motor 66, and motor 66 controls the position of beam splitting member 78 relative to collimators 82A and 82B as described above. First and second collimating members 82A and 82B also may be coupled to stepper motor 66 so that the distance between members 82A and 82B may be adjusted. Specifically, by adjusting beam splitting member 78 and collimating members 82A and 82B to a desired position, the relative positions of output beams 74A and 74B may be adjusted. FIG. 7 illustrates x-rays 74A and 74B impinging upon adjacent detector elements 54A and 54B in a twin beam scanner having dynamic pre-patient collimator 58, and respective detector element sensitivity response curves 30. As shown in FIG. 7, both beams 74A and 74B impinge on detector cells 54A and 54B in the relatively flat region of the detector sensitivity response curve 30 for the respective cells 54A and 54B. Beam splitting member 78 and collimating members 82A and 82B generally are positioned, based on the selected helical pitch, so that optimal sampling, as shown in FIG. 4B, may be obtained. As described above, the preferred positioning for members 78, 82A and 82B may be determined through experimentation for various helical pitches and stored in the motor controller memory. In addition, by dynamically adjusting beam splitting member 78, beams 74A and 74B may be maintained within such flat curve regions during a scan. Such dynamic adjustment provides the desirable result that more accurate projection data may be collected. In addition, such adjustment facilitates eliminating projection data overlap in a helical scan. Further, in one form of operation, no portion of the x-ray attenuated through the object to be imaged is blocked by collimator 56, and no x-rays are received in the detector edge regions. Such a configuration facilitates eliminating projection data discontinuities and accurately calibrating the imaging system. It should be understood, of course, that the shape of beam splitting member 78 and collimators 82A and 82B are not limited to the shapes shown in FIG. 6. For example, member 78 may have many different shapes, including even shapes in which member 78 is not symmetrical about its axis of rotation. Also, although it may be preferable in some configurations, beam splitting member 78 does not necessarily have to be dynamically adjustable. Rather, it is contemplated that sufficient advantages could be obtained by adjusting member 78 prior to a scan and maintaining member 78 in a fixed position for at least one scan. As one specific example of an alternative configuration, it is contemplated that the beam splitting member may have a triangular shape with the apex of the triangular beam splitting member positioned to be the first portion of the splitting member to intercept a transmitted x-ray beam. The axial location, relative to the center axis of symmetry, of the beam splitting member is adjustable so that the relative angular orientation between the output beams is adjustable. Rather than being rotationally adjustable, the triangular shaped beam splitting member is axially adjustable, and such axial adjustment controls certain characteristics, e.g., spacing, of the output beams. Of course, the triangular shaped beam splitting member could also be rotatable to provide additional degrees of adjustment. From the preceding description of several embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. For example, the beam splitter described herein could be utilized with many other computed tomography system, such as "fourth generation" systems in which a stationary detector array extends fully around the gantry bore and only the x-ray source and splitter rotate with the gantry. Accordingly, the spirit and scope of the inventions are to be limited only by the terms of the appended claims.