Patent Number: 043897304
Section: summary

This invention relates to x-ray generating equipment and, more particularly, to collimators which operate to controllably vary the aperture through which the emerging x-rays pass. As is known in the art, collimators are adapted for mounting to the tube housing of an x-ray generator and are positioned to receive a generally expanding conical x-ray beam via an entrance shutter. The entrance shutter is typically positioned within a recess in the tube housing that includes a transparent window to the x-ray. X-rays are generated at a focal spot on an anode target of the tube in response to the impingement of electrons emanating from the tube cathode. Those x-rays passing through the window are referred to as "useful rays" while the remaining rays are absorbed by x-ray absorbing material, such as lead, which lines the housing. Collimators accordingly use x-ray absorbing shutter elements to controllably vary an exit aperture and thereby variably define the beam boundaries on both the x-ray film and the patient. By limiting the cross sectional impingement area of x-rays on the patient to the area being examined, the patient is protected from a needless over-exposure to x-rays. By limiting the film exposure to sharply defined area, a plurality of adjacent images may be formed on a single piece of film. The shutter elements are conventionally arranged as two orthogonally disposed pairs of opposingly moving plates. One pair of shutters, conveniently referred to as the "cross shutters" have aperture-defining edges which are parallel to the length of the x-ray table forming part of the overall x-ray system. The second pair of shutters, conveniently referred to as the "long shutters", have edges which are transverse to the length of the table. The "cross shutter" edges opposingly move across the table to adjust the cross-spaced image area boundaries. Similarly, the "long shutter" edges move opposingly in the longitudinal direction to adjust the longitudinally-spaced boundaries. One concern with collimator design relates to its weight; the collimator typically hangs from an x-ray tube housing which is supported from the ceiling by an overhead support; in addition, some applications require the collimator to be oriented in a way which results in its sideways projection from the tube housing. The consequential loading of the support bearings, which permit such orientation, is a source of concern. Since the x-ray absorbing material of the collimator is typically lead, the weight of the device increases rapidly with size. The size and weight of the collimator are also important with respect to its mobility. The inertia associated with a large collimator adversely affects the ability of the radiologist to precisely position the unit above the region to be imaged. This is particularly troublesome when compensatory movement by the patient is precluded because of pain or unconsciousness. Size is additionally important in terms of interchangeability, in that a compact collimator may fit on the tube housings of many manufacturers within the different spaces allotted. In addition to the foregoing design goals, cost and reliability dictate that the coupling mechanism between the shutters and the shutter-adjusting knobs or motors be as uncomplicated as possible. In an article entitled Diagnostic X-Ray Beam Collimation (Cathode Press; Vol. 23, No. 1, pgs 36-42 (1966)), the contents of which are hereby incorporated by reference, several types of shutter elements are shown and described. In providing a background description of the art, the author describes the shutter elements' length and width as being directly proportional to their distance from the focal spot of the x-ray tube. The reference teaches that, with the shutters moving laterally across the beam, the full open shutter position establishes the collimator housing size in the direction of movement as twice the shutter size; since the shutter size is essentially the width of the x-ray beam in the plane of movement, the housing size may alternatively be described as being twice the beam width. In order to reduce the size of the housing, the author of the foregoing reference illustrates and describes a number of alternative shutter element configurations, two of which are depicted in FIGS. 1A and 1B herewith. FIG. 1A illustrates a so-called louvered shutter mechanism including so-called "far" shutters D which are located as far from the focal spot as practical to enhance boundary definition, and intermediate shutters C. The intermediate shutters C, being located nearer to focal spot than shutters D, block the outer portion of useful radiation, thereby permitting the lateral dimension of the shutters B, D to be decreased as shown. Although reducing the size of the collimator housing, the louvered mechanism illustrated in FIG. 1A is shown to provide a non-linear aperture adjustment; that is to say, the distance of aperture change .DELTA.S for a given angular rotation, .DELTA..theta., will be decreased as the shutter elements D, move inward. A second type of shutter mechanism illustrated in the foregoing reference is shown in FIG. 1B herein. Referred to in the reference as a folding shutter, the mechanism comprises a pair of transversely and opposingly movable shutter elements F, G, each of which folds at approximately its center, as the elements move transversely outward. While the folding shutter configuration provides the linearity lacking in the louver mechanism of FIG. 1A, it is not clear from the reference how radiation would be prevented from escaping through the fold, which is presumably a hinge arrangement. Additionally, a similar shutter arrangement in the orthogonal direction (to define the remaining exit aperture boundaries) would interfere with the movement of shutters F, G unless its plane of movement transverse to the beam was located above the uppermost reach of the outward edges of shutter F, G. The foregoing shutter arrangement would have an adverse effect on boundary definition, however. As is known in the art, the sharpness of boundary definition increases with distance from the focal spot since the sharpest definition occurs when the blocking surface is closest to the imaged object. For this reason, and as stated in the reference, the exit shutter should be as far from the focal spot as possible. By placing the second shutter above the uppermost reach of shutters F, G boundary definition would be diminished. In addition to the negative impact on boundary definition, the foregoing arrangement would create dissimilar linearity of movement for each shutter mechanism; the relationship (.DELTA.S/.DELTA..theta.) would differ significantly for the two shutter mechanisms because of the difference in distance between the respective pivot axes and the edge-defining surfaces of each mechanism. Accordingly, the adjustment mechanism would need to take the linearity differences into account resulting in complicated linkages between the adjustment knob and/or motor and the shutters. SUMMARY OF THE INVENTION The collimator described herein overcomes the forementioned limitations by employing two different shutter mechanisms which co-operatively reduce the required size of the collimator housing while providing an essentially linear shutter control; that is to say, that the amount of aperture change per degree of adjustment remains essentially constant throughout the span of shutter movement. Additionally, the collimator described herein provides essentially the same linearity for both shutters and a simple adjustment mechanism with resultant savings in size as well as cost. The collimator described herein is: adapted for mounting to an x-ray source to adjustably define the longitudinal and transverse boundaries of radiation which emerges from the source in a direction generally orthogonal to both the longitudinal and transverse directions and has a housing having an inlet port and an outlet port aligned therewith; a first shutter assembly including a first pair of transversely narrow, longitudinally extending, x-ray absorbing shutter elements adjacent the outlet port and supported for opposing rotational movement about respective longitudinally extending axes adjacent the inlet port, the first shutter element pair having respective transversely inner, longitudinally extending edge portions which define the transverse radiation boundaries; a second shutter assembly including a second pair of transversely extending, x-ray absorbing shutter elements oppositely adjacent the first pair with respect to the outlet port and supported for opposing rotational movement about respective transversely-extending axes adjacent the inlet port, the second pair of shutter elements having respective, longitudinally inner, transversely extending inner edge portions which define the longitudinal radiation boundaries and further include respective x-ray blocking surfaces extending longitudinally outward from the edge portions towards longitudinally-outward ends thereof; guide means for opposingly rotating the outer ends of the blocking surfaces about the inner ends thereof as the second pair of shutter elements are rotated; first means for opposingly rotating the first pair of shutter elements and second means for opposingly rotating the second pair of shutter elements.