Patent Number: 043897304
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT Reference is initially made to FIG. 2 which is an isometric view of a collimator constructed in accordance with the invention. The collimator is shown therein to comprise a housing 10 having an entrance port 12 and an exit port 14. As is known in the art, the collimator housing is covered with x-ray absorbing material such as lead. One side wall 10a of the collimator is shown as providing a pair of adjustment knobs 22, 24 preferably facing the radiologist and thereby, for the purpose of this disclosure, establishing a convenient frame of reference. Accordingly, when the collimator is positioned over an x-ray table and the knobs 22, 24 are facing the radiologist, the side wall 10a is displaced from the opposite side wall 10c in the direction across the x-ray table (or "cross" direction while the side walls 10b, 10d are offset from each other in the table's longitudinal direction. The cross and longitudinal directions are therefore convenient references when describing the components of the collimator. The entrance port 12 will be conveniently taken to be "above" the exit port 14 for the purpose of this disclosure so that the direction orthogonal to the longitudinal and cross directions can be easily referred to. It should be understood, however, that all of the directions herein are arbitrarily chosen for clarity by assuming a particular collimator orientation and that any interchanging thereof would not depart from the scope of the invention. An entrance shutter arrangement 16 comprising four entrance shutter elements 16a-d protrude outward through the entrance port 12. The upper horizontal edges of the entrance shutter elements 16a-d define the boundaries of an entrance aperture 18. As is known in the art, the collimator housing is adapted for mounting to the tube housing of an x-ray generator so that the entrance shutter 16 is positioned within the recessed x-ray window of the tube-housing to block off-focus x-rays. Off-focus radiation is generated in the case of a rotating anode tube by the continued emission of x-rays from portions on the anode target after they have rotated past the impinging electron beam. Off-focus radiation is greater in the cross direction than in the longitudinal direction. For this reason, and as described hereinbelow, the shutter elements 16a, 16c which block the radiation in the cross direction are adjustable while the longitudinal shutter elements 16b, 16d may be fixed. Directing attention to the exit port 14, there is shown an exit aperture 20 whose cross boundaries are defined by a pair of cross shutter elements 30, 32 and whose longitudinal boundaries are defined by a pair of longitudinal (or "long") shutter elements 40, 42. The cross shutter assembly which includes the cross shutter elements 30, 32 is shown in FIG. 3. FIG. 3 is similar to the isometric view of FIG. 2 but illustrates only the elements of the preferred cross shutter assembly. The edge-defining portions 30b, 32b of the shutter elements 30, 32 are shown to be narrow (e.g.3/8") transversely thin, longitudinally-extending central portions which are respectively supported by upward-extending end portions 30a,c and 32a,c for opposing pivot movement about respective longitudinally-extending axes 34, 36. The shutter elements 30, 32 are supported for pivoting movement by such means as respective pairs 33, 35 of longitudinally spaced rivets. Also mounted for pivoting movement about axes 34, 36 are a pair of generally C-shaped entrance shutter brackets 42, 44 of width and thickness similar to the shutter elements 30, 32. Entrance shutter elements 16a, 16c are respectively affixed to the brackets 42, 44 for pivoting movement therewith. Bracket 44 is pivotably responsive to the pivoting movement of shutter element 30 via tie rod 46. Similarly, bracket 42 is pivotably responsive to the pivoting movement of shutter element 32 via tie rod 48. Shutter elements 30, 32 respectively include intermediate shutter elements 38, 39 of x-ray absorbing material. The intermediate shutter element 38 is located approximately midway between the entrance shutter element 16a and exit shutter element 30b and extends longitudinally between the shutter portions 30a,c. The cross-section of the intermediate element 38 is generally L-shaped with the edge 38a, of the inner leg being aligned with the inner edge of the shutter element portion 30b and edge 17 of the entrance shutter 16a. The intermediate element 38 is securely mounted to the shutter arms 30a,c for pivoting movement therewith about axis 34, the alignment of the foregoing three edges being maintained. The two legs of the intermediate element 38 thereby prevent x-rays from falling outward of the narrow edge-defining shutter portion 30b. In operation, the aperture boundaries defined by the cross shutter assembly are adjusted either manually via knob 22 or electrically via motor 52. As shown in FIGS. 2 and 3, the knob 22 and a free-wheel drive gear 52a within the motor are both operable to rotate a driven gear 54 which, in turn, operates through an arcuate gear segment 56 of a bellcrank arrangement 58 to pivot the shutter arm 32. The bellcrank arrangement 58 is shown to comprise a tie rod 58a coupled at its end between a pair of vertically displaced, perpendicularly oriented projecting members 60, 62. The member 62 pivots about an axis 68. The bellcrank 58 functions to transmit the adjusting movement of gear segment 56 around the corner in the housing to the shutter arm 32 and to transform the rotational movement of the gear 54 into a pivoting movement of the arm 32 about axis 36. The pivoting movement of shutter arm 32 is transmitted to shutter arm 30 via a pair of mating gear segments 64, 66 respectively extending from the shutter arms 30a, 32a. The gear segments 64, 66 are arcuate segments circumferentially disposed about axes 34, 36 respectively. The pivoting movement of shutter arm 30 is coupled to the bracket 44 via a tie rod 46. Bracket 44 responsively pivots about axis 36 to move shutter element 16c. Similarly, bracket 42 is coupled to shutter arm 32c for responsive pivoting movement about axis 34 via a tie rod 48. Having described the cross shutter mechanism, attention is turned to the longitudinal shutter mechanism illustrated in FIGS. 2 and 4. The longitudinal shutter assembly includes longitudinal shutter elements 40, 42. The edge defining portions 40b, 42b are perpendicularly oriented with respect to the cross shutter elements 30b, 32b and immediately above them, preferably 1/2". The edge-defining portions 40b, 42b are transversely thin elements, extending in the cross direction and are respectively supported by upward-extending end portions 40a,c and 41a,c for opposing pivotable movement about respective cross-extending axes 68, 69. As with the cross shutter assembly the longitudinal shutter elements 40, 42 are supported by such means as rivets. As illustrated in FIGS. 2 and 4, the adjustment of the "long" shutter assembly includes a driven gear 88 which is operable by either electrical or manual means such as a motor 90 and knob 24 respectively. The driven gear 88 meshes with a arcuate segment 92 affixed to one end portion 42c of shutter element 42. Rotation of the driven gear 88 and the consequential pivoting of the shutter element 42 causes corresponding and opposing pivoting of shutter element 40 via arcuate gear segments 94, 96. The toothed surfaces of the gear segments 94, 96 are circumferentially disposed about axes 69, 68 respectively. The "long" shutter elements 40, 42 include a generally planar sheet of x-ray absorbing material 98, 100 respectively. The element 100 extends in the cross direction between the end portions 42a, 42c of the shutter element 42 and is coupled for pivoting movement at its inner end 100a by means of a pair of rivets 102 which define a pivot axis 104. The outer edge 100b of the x-ray absorbing element 100 is provided with a pair of roller elements 106, 108 which respectively engage guides 110, 112. It can be appreciated from FIGS. 2 and 4 that the planar element 100 will pivot counter clockwise about axis 104 as the shutter element 42 is moved longitudinally outward. The planar element 98, of course, operates similarly. Because off-focus radiation in the longitudinal direction is minimal, the entrance shutter elements 16b, 16d may conveniently be fixed rather than movable with the long shutter mechanism. Aperture adjustment by the illustrated shutter mechanisms are essentially linear for two reasons. First, the pivot arms (i.e. portions 30a,c, 32a,c, 40a,c, 42a,c) of the edge-defining elements are sufficiently long so that the translation path of the exit shutter elements are essentially straight. Stated another way, the angular variation .DELTA..theta., experienced by the shutter arms falls within the linear portion of the tangent .theta. curve which correlates S to .theta.. In the preferred embodiment, the pivot arm is approximately 43/4 inches for a translation path of 1 3/16". Secondly, the pivot axes 34, 36, 68, 69 are located generally over the midpoint of translation to reduce non-linearities at the ends of travel. In practice, the axes are preferably located just inside the midpoint, since as a practical matter, aperture settings will usually not include the outer fringes of translation path. It may be appreciated by those skilled in the art that the foregoing embodiment provides a compact collimator which has substantially eliminated the problems heretofore associated with the complex adjustment mechanisms. Because the x-ray absorbing material is typically lead, the reduction in size translates into a reduction of weight and, consequentially, a reduction in the loading of the x-ray tube head. The compact size is shown to result from the use of thin edge-defining elements for the cross-shutter assembly, with small intermediate shutter elements. The "long" shutter mechanism, in turn, is configured to work in close relationship with the cross mechanism without interferring with its operations. While the foregoing Description of a Preferred Embodiment is sufficient to enable one skilled in the art to practice the invention, it is apparent that many variations and modifications are possible. Further, as previously indicated, the identification of the directions as "longitudinal", "cross", and "transverse" as well as relative positions such as "above" and "below" have been arbitrarily choosen for the sake of clarity. It is accordingly intended that the invention not be limited by these terms and that claims appended hereto be interpreted as broadly as permissible in view of the prior art so that to include all equivalents of the embodiment described herewithin: