Patent Number: 053965345
Section: description

DETAILED DESCRIPTION OF THE INVENTION Turning to FIG. 1, a front plan view of one shutter assembly 10 is provided. A corresponding side plan view is shown in FIG. 2 and a rear plan view is shown in FIG. 8. The frame 12 defines an opening 13 with two opposed interior edges 14, 16 that extend generally parallel to the direction of motion for the two shutter members 20 and 30. For example, the opening 13 can be substantially rectangular in shape, although other geometric shapes having at least two opposing edges could be employed. The plane of the frame opening 13 is usually substantially normal to the axis of the x-ray beam being collimated. The shutter members 20 and 30 are made of a radio-opaque material such as lead. In the preferred embodiment, the shutter members 20, 30 are molded from plastic to simplify fabrication and then selected portions are sheathed with a layer of lead 21 and 31, as shown in FIGS. 1 and 7. FIGS. 4 and 5 show a rear view of the shutter assembly with the rear portion of the frame 12 removed to reveal the interior channel 18 extending about a portion of the periphery of the frame opening 13. FIG. 6 is a corresponding front view of the rear portion of the frame that was removed in FIGS. 4 and 5. The channel 18 can extend completely around the periphery of the frame opening 13, or alternatively, the channel 18 need only extend about a portion of the periphery of the frame opening 13, as will be discussed below. The channel holds an elongated flexible band 40 that can slide freely within the channel 18 along at least a portion of the periphery of the frame opening 13. Although the preferred embodiment of the frame 12 shown in the drawings employs an interior channel 13 to hold the flexible band 40, it should be understood that other alternative embodiments are feasible. For example, the channel could be formed partially or completely on the exterior of the frame. It would also be possible to eliminate large sections of the channel, for example, by suspending the flexible band between rollers mounted at the interior corners of the frame opening. However, an interior channel offers the advantages of minimizing accumulation of dust and dirt in the channel and on the flexible band, and can be used to provide additional support and guidance for the ends of the shutter members as they slide within the frame opening. In the preferred embodiment, the band 40 is made of a relatively thin strip of flexible plastic. Alternatively, the band can be made of a thin flexible metal strip, rubber, natural fiber, or other flexible materials, either singly or in a composite structure. In addition, the band can be fabricated as a cable, belt, single strand, or chain. A drive means 50, such as a stepper motor, is employed to control the position of the flexible band 40 in the channel 18. In the particular embodiment shown in the drawings, the stepper motor 50 drives a worm gear 51 that exerts a translational force on a sliding block 52 attached to the band 40, as shown most clearly in FIGS. 1, 2, and 4. Other types of drive means, such as a linear actuator motor, a solenoid, or a stepper motor with a cam arrangement, could be readily substituted. In particular, a servo-motor having a cog or gear attached to its drive shaft could be used to engage a series of notches or holes extending along the band. Both of the shutter members 20 and 30 are attached to the flexible band 40 through slots 24 and 34 in the opposing edges 14 and 16, respectively, of the frame 12. As shown in FIGS. 3, 4, and 5, one end 22 of the first shutter member 20 is inserted through a slot 24 in the upper portion of left interior edge of the frame and secured to the flexible band 40. Similarly, the opposite end 32 of the second shutter member 30 is inserted through a slot 34 in the lower portion of the right interior edge of the frame and secured to the flexible band 40. In the preferred embodiment, the remaining ends 23 and 33 of the shutter members 20 and 30 slidably engage tracks 25 and 35, respectively, extending along the corresponding interior edges of the frame. The tracks 25 and 35 are shown most clearly in the lower portion of FIG. 3 and the right portion of FIG. 7. In particular, the first track 35 extends along the lower portion of the first interior edge 14 of the frame 12 and slidably engages the second end 33 of the second shutter member 30. Similarly, the second track 25 extends along the upper portion of the second interior edge 16 of the frame 12 and slidably engages the second end 23 of the first shutter member 20. In other words, the first track 35 is located across the frame opening 13 from the second slot 34, and the second track 25 is located across the frame opening from the first slot 24. These tracks 25 and 35 provide added structural support and help maintain accurate alignment for the shutter members 20 and 30. FIGS. 4 and 5 demonstrate how the shutter members move between an open position (FIG. 4) and a closed position (FIG. 5) as the stepper motor 50 causes the flexible band 40 to translate within the channel 18 relative to the frame 12. The slots 24, 34 and/or tracks 25, 25 can be used to define the maximum range of motion for the shutter members 20, 30 within the frame opening 13. It has been previously noted that the channel 18 can extend either completely or only partially around the periphery of the frame opening 13. The drawings depict an embodiment of the invention in which the channel 18 extends only partially about the frame opening. This same design choice applies to the flexible band 40, which also can extend either partially or completely around the frame opening depending on the selected configuration of the channel 18. If the channel 18 and band 40 extend only partially around the frame opening, the band 40 will pull one shutter member and push the other shutter member depending on the direction of motion of the band 40. The narrow cross-sectional dimensions of the channel 18 prevent the band 40 from buckling while pushing a shutter member. For example, in moving from the open position shown in FIG. 4 to the closed position shown in FIG. 5, the band 40 pushes on the first shutter member 20 and pulls on the second shutter member 30. Conversely, the band 40 pulls on the first shutter member 20 and pushes on the second shutter member 30 to open the shutter aperture. Alternatively, if the band 40 and channel 18 extend completely around the periphery of the frame opening to form a complete loop, the band 40 will simultaneously pull on both shutter members 20 and 30, and thereby eliminate any potential problem of buckling. The control system for regulating the positions of the shutter members 20 and 30 can either operate with positional feedback (closed-loop) or without positional feedback (open-loop). If a stepper motor is employed as the drive means 50, the controller can keep track of the location of the shutter members 20 and 30 by counting the number of electrical pulses it has sent to the stepper motor. However, this does not provide positive verification that the shutter members have, in fact, moved to the desired position. For example, the controller would have no way to know if the stepper motor is broken, the band has broken, or the mechanism is jammed or otherwise nonfunctional. This shortcoming can be addressed by providing the controller with feedback as to the actual positions of the band and/or shutter members. For example, a small rack gear can be attached to the band. This engages the teeth on a pinion gear secured to the shaft of a potentiometer. The controller can then continually measure the resistance of the potentiometer to monitor the position of the band. FIG. 9 shows multiple shutter assemblies 110 and 210 orthogonally stacked along a common axis to provide two-dimensional control of the size and shape of the resulting aperture. FIG. 10 is a side cross-sectional view of the two orthogonally stacked shutter assemblies 110, 210 incorporated into a conventional x-ray generator. Each of the shutter assemblies 110, 210 has essentially the same configuration and components as the shutter assembly 10 previously discussed and shown in FIGS. 1-9. The frame opening of each shutter assembly is substantially orthogonal to a common axis passing through the x-ray source 300. In addition, the shutter assemblies 110 and 210 are oriented substantially orthogonally to one another (i.e., the shutter assemblies are rotated 90 degrees with respect to one another about the common axis). The top shutter assembly 110 provides two shutter members 120 and 130 that define the size of the aperture in the Y direction, and the bottom shutter assembly 210 provides two shutter members 220 and 230 that define the size of the aperture in the X direction. Each pair of shutter members 120, 130 and 220, 230 is independently controllable by the stepper motor 150 or 250 associated with its respective shutter assembly. In an alternative embodiment, a single stepper motor and a single flexible band can be use to position more than two shutter members within a single frame. For example, two pairs of shutter members can be driven by the same band in a square or rectangular frame opening. The first pair of shutter members is attached to the band along the top and bottom interior edges of the frame to regulate the aperture size in the X direction. The second pair of shutter members is attached to the band along the left and right interior edges of the frame to regulate the aperture size in the Y direction. This concept could be further extended to provide three pairs of shutter members attached to opposing pairs of edges in a hexagonal frame, or four pairs of shutter members in an octagonal frame, etc. The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.