X-ray or gamma ray systems or devices – Beam control – Collimator
Reexamination Certificate
2000-04-19
2001-10-09
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Collimator
C074S441000, C074S409000, C411S231000, C378S150000
Reexamination Certificate
active
06301334
ABSTRACT:
TECHNICAL FIELD
The present invention relates to mechanical drive assemblies, and in particular to those which are used to effect precise movement of a slit plate in scanning and imaging devices.
BACKGROUND OF THE INVENTION
X-ray imaging devices generally employ a collimated beam of radiation which is directed from a focal spot through an object to be scanned, such as a live human or animal patient, or a package to be inspected. A bank of radiation-sensitive detectors is located opposite the focal spot, with the object to be scanned between the focal spot and the detectors. The focal spot and detectors may be fixed relative to one another on a gantry which rotates about the object to be scanned. The detectors receive radiation which has passed through the object and which has therefore been attenuated to varying degrees as a function of the density of structures within the object and in the radiation path. The detectors generate signals which correspond to the detected density values, and these signals are used to map the object so that the internal structures can be seen.
The radiation beam is collimated by passage of the beam through a slit in a radiation-opaque plate. Typically the plate contains a number of slits of differing widths, so that the beam can be collimated to different widths. For convenience, the slit plate is generally moved into the desired position by translation in a direction transverse to the direction of the beam (generally referred to as the z-direction).
It is important that the slit through which the radiation beam passes to be collimated be located properly relative to the focal spot and the detectors so as to admit precisely as much radiation as can be detected by the detectors, for maximum data acquisition for each scan of the object. If the slit is not properly aligned with the focal spot and the detectors, some of the radiation pass through the object but may not be received by the detectors. Any radiation which passes through the object without being detected subjects the object to radiation exposure without providing useful imaging data, and this is undesirable.
It is also important that movement of the slit plate be smooth, without backlash, accurate and precise. This is especially critical in scanners which may rotate at speeds greater than one revolution per second.
In prior art scanners, the slit plate is typically mounted on a slide which is adapted for travel along a set of parallel shafts. The slide is driven by a drive mechanism, such as a precision leadscrew, which is mounted to a stepper motor. Misalignment of the shafts, of the slide on the shafts, and binding of the leadscrew, commonly occur because it is difficult to maintain these structures in precise alignment. Because each structure is independently mounted, alignment of all three is a costly and time-consuming undertaking and must be checked and repeated frequently.
It would therefore be an advancement in the art of precision drive assemblies to overcome the disadvantages of the prior art.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a backlash-resistant drive assembly, comprising a base, a first shaft secured to the base, a driver mounted to the base, a second shaft coupled to the driver and substantially parallel to the first shaft, and a carrier adapted for translation in the direction of the first and second shafts. The second shaft is coupled to the driver with a backlash-resistant nut assembly.
In one embodiment, the driver comprises a stepper motor.
The backlash-resistant nut assembly preferably comprises a first nut portion with a threaded bore, and a second nut portion having a threaded bore concentric with the threaded bore of the first nut portion, means for mutual engagement of the first and second nut portions, and a wave washer disposed between the first and second nut portions. The wave washer biases the first and second nut portions away from each other.
In one embodiment, the first nut portion is adapted for fixed engagement with the carrier.
The drive assembly may further include a bearing sleeve disposed over the first shaft and the nonthreaded portion of the second shaft and adapted for rolling engagement with the shafts and the carrier.
According to another aspect of the invention, there is provided a collimator assembly for an x-ray imaging device. The assembly comprises a base containing an aperture for passage of an x-ray beam therethrough, a backlash-resistant drive assembly affixed to the base and including a first shaft secured to the base, a driver mounted to the base, a second shaft coupled to the driver and substantially parallel to the first shaft, and a carrier adapted for translation in the direction of the first and second shafts, wherein the second shaft is coupled to the driver with a backlash-resistant nut assembly, and a slit plate fixedly mounted to the carrier and having a plurality of slits of different widths.
The collimator assembly may further include a mask plate fixed to the base beneath the carrier and over the aperture of the base, wherein the mask plate includes a single slit therein.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the composition and apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.
REFERENCES:
patent: 4962518 (1990-10-01), Spongr et al.
patent: 5054041 (1991-10-01), Hampel
patent: 5732596 (1998-03-01), Erikson et al.
patent: 5839321 (1998-11-01), Parsons
Gillis Jonna A.
McDermott Brian M.
Swain Ronald E.
Tybinkowski Andrew P.
Analogic Corporation
Dunn Drew
McDermott & Will & Emery
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