Patent Number: 056446144
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system 10 is shown as including a gantry 12 representative of a "third generation" CT scanner. Gantry 12 has an x-ray source 14 that projects a fan beam of x-rays 16 toward a detector array 18 on the opposite side of gantry 12. Detector array 18 is formed by detector elements 20 which together sense the projected x-rays that pass through a medical patient 22. Each detector element 20 produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient 22. During a scan to acquire x-ray projection data, gantry 12 and the components mounted thereon rotate about a center of rotation 24. Rotation of gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detector elements 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38. Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 in gantry 12. Particularly, table 46 moves portions of patient 22 through gantry opening 48. Referring to FIG. 3, and with respect to operation of x-ray source 14, x-ray beam 16 emanates from a focal spot 50 of source 14. X-ray beam 16 is collimated by collimator 52, and collimated beam 16 is projected toward detector array 18 along a fan beam axis 54 centered within fan beam 16. As shown in FIG. 4, detector array 18 is generally curved at a fixed radius from focal point 50. A distance (d) between focal spot 50 and the center of any detector element 20 at a fan beam angle (.alpha.) is the same. Particularly: EQU d(.alpha..sub.o)=d(.alpha..sub.n) where: .alpha..sub.o =fan beam angle at vertical; and PA1 .alpha..sub.n =fan beam angle at any angle offset to vertical. PA1 .alpha..sub.o =is the fan beam angle at vertical; and PA1 .alpha..sub.n =fan beam angle at any angle offset to vertical. PA1 .alpha.=fan beam angle; PA1 Z=position of beam on detector; PA1 f=position of focal spot in z axis; PA1 c=position of collimation point in z axis; PA1 d=source to detector distance; and PA1 s=source to collimation distance. PA1 .alpha.=fan beam angle; PA1 Z=position of beam on detector; PA1 f=position of focal spot in z axis; PA1 c=position of collimation point in z axis; PA1 d=source to detector distance; and PA1 s=source to collimation distance. As described above, known collimators have rectangular, or linear, apertures, or slots. A distance (s) between x-ray source 14 and the collimator aperture changes as a function of fan beam angle (.alpha.). Particularly: EQU s(.alpha..sub.o)&lt;s(.alpha..sub.n) where: As shown in FIG. 5, collimated fan beam 16 collimated by collimator 52 with a rectangular slot or aperture 56 is generally convex as indicated at 58. Particularly, each x-ray in beam 16 impinges upon detector cells 20 in detector array 18 at a z axis location, Z(.alpha.), according to the equation: EQU Z(.alpha.)=(c-f)d(.alpha.)/s(.alpha.)+f where: However, since detector elements 20 are generally rectangular, portions of convex fan beam 16 do not impinge detector elements 20. This unused portion 60 (shaded), however, has been attenuated by patient 22. Patient 22 was thus unnecessarily subjected to the full convex fan beam. Referring to FIG. 6, and in accordance with one embodiment of the present invention, a collimator 70 has a contoured aperture 72 therein. Contoured aperture 72 is curved and receives x-ray beams from x-ray source and emits a generally rectangular beam 74 which impinges upon detector elements 20 in detector array 18. As shown, rectangular beam 74 directly overlaps the rectangular detector element 20. Therefore, in one embodiment, contoured aperture 72 prevents any unused portions of fan-beam 16 from impinging on detector elements 20, and thus eliminates patient exposure to unnecessary x-ray dose. In accordance with another embodiment of the present invention, a collimator aperture is curved according to the following equation: EQU c(.alpha.)=(Z-f)s(.alpha.)/d(.alpha.)+f, where: In accordance with yet another embodiment of the present invention a collimator aperture is contoured for each slice configuration and focal spot size. For example, a cam collimator, as hereinafter described in more detail, may be used to continuously change aperture shape as a function of the aperture size. In accordance with still yet another embodiment of the invention, as shown in FIG. 7, collimator 80 has a collimator aperture 82 that is contoured with a fixed linear ramp. The fixed linear ramp approximates the curvature for a nominal slice configuration. For example, where a distance between x-ray source 14 and patient 22 is 541 mm, a distance between x-ray source 14 and detector element 20 is 949 mm, and a vertical distance between x-ray source 14 and collimator is 162 mm, a ramp slope of 0.2 mm per 100 mm may be used. Linear ramp aperture 82 provides similar patient dose savings and is believed to be easier to manufacture than multiple curved or linear contours. FIG. 8 is a top view of a double cam collimator 100 in accordance with yet another embodiment of the present invention. Collimator 100 includes cams 102 and 104. Cams 102 and 104 are shown as being spaced and generally define edges 106 and 108, respectively, for restricting a beam passing therebetween. Cams 102 and 104 each include bosses 110A, 110B, 110C and 110D extending therefrom. A stepper motor (not shown) would be coupled to at least one boss 110A,110B and 110C,110D of each cam 102 and 104 to control relative movement of such cams 102 and 104. The collimators with contoured apertures as described above restrict collimated fan beam 16 to more closely approximate the size of detector cells 20. By so restricting collimated fan beam 16, unused portions of x-ray beams attenuated by patient 22 are reduced, yet the integrity of data received at detector cells 20 is maintained. From the preceding description of various 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 CT system described herein is a "third generation" system in which both the x-ray source and detector rotate with the gantry. Many other CT systems including "fourth generation" systems wherein the detector is a full-ring stationary detector and only the x-ray source rotates with the gantry, may be used. Moreover, the linear ramp contoured aperture described herein has a ramp slope of 0.2 mm per 100 mm. Many other ramp slopes can be used. Furthermore, the aperture contour may be prefabricated with a specific curvature or slope or, alternatively, the aperture contour may be modified during a scan. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.