Patent Number: 050540416
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a gantry 20, representative of a "third generation" computed tomography scanner, includes an x-ray source 10 collimated by collimator 38 to project a fan beam of x-rays 22 through imaged object 12 to detector array 14. The x-ray source 10 and detector array 14 rotate on the gantry 20 as indicated by arrow 28, within an imaging plane 60, aligned with the x-y plane of a Cartesian coordinate system, and about the z-axis of that coordinate system. The detector array 14 is comprised of a number of detector elements 16, organized within the imaging plane 60, which together detect the projected image produced by the attenuated transmission of x-rays through the imaged object 12. The fan beam 22 emanates from a focal spot 26 in the x-ray source 10 and is directed along a fan beam axis 23 centered within the fan beam 22. The fan beam angle, measured along the broad face of the fan beam 22, is larger than the angle subtended by the imaged object 12 so that two peripheral beams 24 of the fan beam 22 are transmitted past the body without substantial attenuation. These peripheral beams 24 are received by peripheral detector elements 18 within the detector array 14. Referring to FIG. 2, uncollimated x-rays 19 radiating from the focal spot 26 in the x-ray source 10 (not shown in FIG. 2) are formed into a coarse fan beam 1 by primary aperture 40. The coarse fan beam 21 is collimated into fan beam 22 by means of collimator 38. Referring generally to FIGS. 2, 3(a) and 3(b), collimator 38 is comprised of a cylindrical x-ray absorbing mandrel 39 held within the coarse fan beam 21 on high precision bearings 42 allowing the mandrel 39 to rotate along its axis. The mandrel material is a sintered molybdenum to provide both good x-ray absorbing characteristics and randomly oriented residual stress ensuring dimensional stability after the necessary machining. A plurality of tapered slots 41 are cut through the mandrel's diameter by wire electro-discharge machining and extend along the length of the mandrel 39. The slots 41 are cut at varying angles about the mandrel's axis to permit rotation of the mandrel 39 by approximately 36.degree. to bring each such slot 41 into alignment with the coarse fan beam 21 so as to permit the passage of some rays of the coarse fan beam 21 through the slot 41 to form fan beam 22. Referring to FIG. 3(a) and 3(b), the tapered slots 41 are of varying width and hence the rotation of the mandrel 39 allows the width of the fan beam 22 to be varied between a narrow width (mm) as shown in FIG. 3(b) and wide width (10mm) as shown in FIG. 3(a). The fixed slots 41 ensure dimensional accuracy and repeatability of the fan beam 22. The tolerances on the narrowest slot 41 are +0.001 inches -0.000 inches with proportional tolerances on the larger slots 41. The slots 41 are tapered so that the entrance aperture 43 of each slot 41, when orientated with respect to the coarse fan beam 21, is wider than the exit aperture 5. The exit aperture 45 defines the width of the fan beam 2 and the extra width of the entrance aperture 43 prevents either edge of the entrance aperture 43 from blocking the coarse fan beam 21 during rotation of the mandrel 39 when such rotation is used to control the alignment of the fan beam axis 23 as will be discused in detail below. Referring again to FIG. 2, a stepping motor 48 is connected to one end of the mandrel 39 by coupling 50 that is stiff torsionally but flexible in other directions, and a low backlash brake 80 to be described further below. The stepping motor 48 is operated in the micro-step mode as is known in the art to provide a stepping increment of 50,800 steps per revolution. The stepper motor and controller are commercially available from Oriental Motor and compumotor, respectively. The remaining end of the mandrel 39 is attached to a position encoder 46 which allows accurate positioning of the mandrel by motor 48. The position encoder is of the incremental type, providing 20,000 pulses per revolution and a home or zero pulse used to determine absolute position. Fan beam angle shutters 44 at either ends of the mandrel 39 control the length of the fan beam 22. Referring to FIG. 4, the x-ray source 10 is comprised of a rotating anode 52 held within an evacuated glass tube (not shown) and supported by supporting structure including principally anode shaft 54 which is held on bearings 56 (one shown). The coarse fan beam 21 emanates from focal spot 26 at the surface of the anode 52. The coarse fan beam 21 is collimated by the collimator 38 to form a fan beam 22 as previously described. The plane containing the focal spot 26, the center line of the exit aperture 45, and the centerline of the detector array 14 along the z axis, and thus bisecting the fan beam 22 in the z axis direction, will be termed the "fan beam plane" 62. As previously described, the focal spot 26 may not be aligned with the imaging plane 60 either because of thermal drift of the anode 52 and its supporting structure or because of minor misalignment of the x-ray source 10 during assembly. Referring to FIG. 5, the anode 52 is shown displaced from the imaging plane 60 by misalignment distance 58. The effect of this misalignment is to displace focal spot position away from the imaging plane 60 and to move the the center of the fan beam exposure area 36 in the opposite direction. As a result of the movement of the focal spot 26, as shown in FIG. 5, the exposure area 36 is no longer centered within the imaging plane 60 and the fan beam plane 62 is no longer parallel with the imaging plane 60 but deviates by angle .phi.. Referring to FIG. 6, the collimator 38 may be rotated to restore the fan beam plane 62 to parallel with the imaging plane 60. This correction of the angle of the fan beam plane 62 will be termed "parallelism correction". Alternatively, referring to FIG. 7, the collimator 38 may be rotated so that the exposure area 36 will again be centered at within the imaging plane 60. Correction of the position of the of the fan beam exposure area 36 with respect to the detector 14 will be termed "z-axis offset correction". In summary, rotation of the collimator 38 may correct for misalignment of the fan beam plane 62 either to make it parallel with the imaging plane 60 or to bring the exposure area 36 into alignment on the detector array 14. As previously discussed, both of these corrections will reduce image artifacts. As discussed, various external forces act on the collimator 38 during the rotation of the gantry 20 shown in FIG. 1. The torque on the mandrel 39, exerted by these forces, is resisted by means of a low backlash brake 80 as shown in FIG. 2. Referring now to FIG. 8, the torque T.sub.s of the stepper motor 48 varies as a function of the angular displacement .alpha. of its shaft 78 around a step position .alpha..sub.o. The torque T.sub.s rises from zero torque at .alpha..sub.0 to positive values (representing counterclockwise torque) as one moves in a clockwise direction away from .alpha..sub.o, and the torque T.sub.s drops from zero torque to negative values (representing clockwise torque) as one moves in a counterclockwise direction away from .alpha..sub.o. This is typical of the torque characteristics of a positioning motor and reflects the positioning action of the motor around at .alpha..sub.o. Referring again to FIG. 1, the collimator mandrel 39 is disposed tangentially to the rotation 28 of the gantry 20 and hence experiences a steady centripetal acceleration and a rotating gravitational acceleration depending on the position and velocity of the gantry 20. The complex cross-section of mandrel 39 prevents it from being perfectly balanced under these varying accelerative forces and hence there exists a small but significant perturbation torque .+-.TP on the mandrel 39 during rotation of the gantry 20. Referring again to FIG. 8, when the stepping motor is energized this perturbation torque .+-.T.sub.p may move the mandrel by as much as .+-..alpha.p before it is resisted by the restoring torque T.sub.s of the stepping motor 48. Referring to FIG. 11, the effect of the perturbation torque .alpha.T.sub.P may be counteracted by means of the low backlash brake 80 comprised of a brake drum 82 affixed to, and coaxial with, the shaft 78 of the stepper motor 48 connected with the mandrel 39. A brake pad 84 attached to an arcuate brake shoe 86 is positioned in sliding contact with the circumference of the brake drum 82 so as to create a frictional countervailing braking torque T.sub.B. The brake shoe 86 is attached to a housing 88 by means of a flexible arm 90 of spring steel. The flexible arm 90 is orientated tangentially to the brake drum 82 to flex only in a radial direction and hence to be unyielding with respect to tangential forces imparted by the friction between the brake drum 82 and the brake lining 84. A biasing spring 92 serves to impart an inward radial force to the brake shoe 86 and brake pad 84 against the circumference of the brake drum 82 and hence to establish the frictional braking torque T.sub.F which may be adjusted by controlling the compression of biasing spring 92 and hence the force imparted by the biasing spring 92 on the brake shoe 86. Referring again to FIG. 9, the braking torque T.sub.B is essentially constant with angle .alpha. and equal to T.sub.F and always opposing the direction of rotation. The braking torque T.sub.B only counteracts the other torques and drops to zero when there is no motion. The braking torque T.sub.B creates the hysteresis curve of FIG. 9 where the torque curve T.sub.S is displaced by .+-.T.sub.F depending on the direction of rotation of shaft 78. With the braking torque T.sub.s, the stepping motor 48 will position its shaft at equilibrium point 100 or 100' removed from .alpha..sub.o depending on the direction which the stepping motor 48 approaches .alpha..sub.o. In the preferred embodiment, the stepper motor always turns in the counterclockwise direction (as viewed from the non-shaft side of the stepper motor) to ensure that its shaft 78 will always stop at equilibrium point 100. When the shaft 78 of the stepper motor 48 has reached position 100, the braking torque T.sub.B and the stepper motor torque T.sub.S are just balanced and the shaft 78 of the stepper motor 48 stops. Nevertheless, the shaft 78 is not immune from perturbation torque T.sub.P which may unbalance this equilibrium in either direction, even if T.sub.P is less than T.sub.F. This displacement is designated .alpha..sub.p ' and .alpha..sub.p " depending on the direction of perturbation. In general, the displacement .alpha..sub.p ' and .alpha..sub.P ", with the brake 80, will be less than the displacement .alpha..sub.p without the brake 80 as shown in FIG. 8. Referring to FIG. 9, once the stepper motor 48 has positioned its shaft 78 at point 100, the power to the stepper motor 48 is reduced to lessen the amount of the stepper motor restoring torque T.sub.s, for small displacement angles .alpha. of shaft 78, to T.sub.s ', where T.sub.s '&lt;&lt;T.sub.F for small angles .alpha.. This reduction of torque T.sub.s may be obtained by reducing the current flowing through the windings of the stepper motor 48 as is understood in the art. Now the braking torque T.sub.s provides a nearly constant resisting force to motion in either direction and will prevent motion of the shaft 78 from perturbing torques T.sub.P so long as -T.sub.F &gt;T.sub.P &gt;T.sub.F. Therefore, somewhat counterintuitively, the braking action is improved by reducing the stepper motor restoring torque T.sub.s to T.sub.s '. The stepper motor 48 is not shut off completely, however, so as to provide resistance to higher perturbation torques than T.sub.P and to prevent shifting of the mandrel 39 by large angles .alpha. . The above description has been that of a preferred embodiment of the present invention. It will occur to those who practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.