Patent Number: 045286850
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 is a block diagram of an X-ray system that can be used for performing various digital subtraction angiography (DSA) and ordinary digital fluorography procedures. The X-ray tube and the casing containing it are represented by the block marked 10. The tube is energized from an X-ray power supply 11. This power supply is to some extent conventional in that it contains the usual high voltage step-up transformer, rectifiers, selectors for the kilovoltage and current that is to be supplied to the X-ray tube during X-ray exposures and the switching elements for turning the X-ray tube on and off. The electronics for controlling the switching cycles is represented by the X-ray control block marked 12. The X-ray power supply and control is one that has the capability of projecting low and high average photon energy X-ray beam pulses alternately and rapidly for the purpose of performing hybrid DSA and also has the capability of projecting longer duration beams for the purpose of performing ordinary digital fluorography or radiography. A suitable multiple voltage, that is, multiple X-ray energy beam producing system is described in Daniels et al U.S. Pat. No. 4,361,901 which is assigned to the assignee of the present invention. A conventional X-ray beam collimator is represented by the block 13. The collimator is used for the usual purpose of defining the boundaries of the X-ray beam field. One cooperating pair of blades in the collimator are symbolized by the dashed lines 14. Another pair which moves orthogonally to the first pair of blades is not shown. In any event, the blades are driven to positions that cause the proper X-ray beam field to be defined by means of servo motors such as the one marked 15. The servo motors for each pair of blades are usually mounted in the collimator housing. The controls for automatic adjustment of the collimator blade opening to define a beam field that is no larger than the area of the image intensifier or other image receptor that may be used are not shown or described since said controls are conventional. The operator interface that allows the operator to select the kilovoltages and currents for the X-ray tube for the different energy beams used during an exposure sequence is symbolized by the block marked 18 and labelled "selector." The dashed line boundaries of the X-ray beam defined by the collimator 13 are marked 21 and 22. This beam is projected from the focal spot of the X-ray tube through the patient who is represented by the ellipse 23. A blood vessel, which may be the subject of a DSA study is marked 24. In FIG. 1 the block representing the new filter device has the numeral 25. The dashed line within this block is simply to indicate that the X-ray beam will pass through one or more filter elements before penetrating the patient. In accordance with the invention, the filter elements are shifted in or out of the X-ray beam path by using a servo motor 26 which is actually mounted in the filter housing as will be evident later. The servo amplifier and other electronics are represented by the triangle 27. A controller 19 is programmed to provide the control signals to the servo system for acomplishing various filter shifting modes that are pertinent to performing hybrid, that is, temporal and energy subtraction DSA and ordinary DSA. Mode selection is made with terminal 19. In FIG. 1, the emergent X-ray beam that is differentially attenuated by the body and entrains the X-ray image falls on the photocathode 28 of a conventional electronic image intensifier 29. As is known, the image intensifier converts the X-ray image to a bright and minified optical image which is formed on an output phosphor represented by the dashed line 30. A television (TV) camera views the successive optical images obtained, for example, with the high and low energy X-ray beams and converts the images to analog video signals that are conducted to a video signal processor 32 by way of a cable 33. As indicated earlier, for digital subtraction angiography, one type of filter must be in the X-ray beam for a low energy exposure and another type for a high X-ray energy exposure. In any case, the video signal processor 32 converts the analog video signals to digital signals on a frame-by-frame basis and performs all of the weighting, summation and subtracting steps which will result in producing data representative of the image of the blood vessel 24, for example, to the exclusion of substantially all surrounding soft tissue and bone. The final image data is converted to analog video signals for driving a TV monitor 34 which displays the image 24' of the contrast medium containing blood vessel on its screen 35. In FIG. 1, the camera control for achieving the proper time relationships for filter element positioning, TV camera target readout and X-ray exposures is marked 36. The control signals for the camera control are provided from system controller 18 by way of a bus 37. The new filter device will now be described in detail in reference to FIGS. 2-11. Referring to FIG. 8, the bottom 40 of the collimator housing is mountable to the X-ray tube casing. The focal spot of the X-ray tube is indicated by the point 41 from which the X-ray beam 21, 22 diverges. The details of the collimator 13 are not shown except that one pair of blades 14 is illustrated to demonstrate their position relative to the focal spot and the filter device which is generally designed by the numeral 25. The top of the collimator constitutes a base plate 42 for the filter device. It has an aperture 43 through which the X-ray beam is projected toward the patient. Refer now to FIG. 2 for a brief description of the filter elements. One of the elements is marked 1 and consists of a metal strip of filtering material which, by way of example and not limitation, may be copper or iron that would be in the X-ray beam which high energy X-ray exposures are being made. For the sake of example, assume copper is used. This copper strip 1, whose configuration can also be seen in the FIG. 11 exploded view, has a hole 50 which is overlaid by another planar filter element 2 which may be in the beam when low X-ray energy exposures are being made. Filter element 2 is comprised of gadolinium in an actual embodiment although other materials might be used depending on the photon energy ranges one might want to attenuate or transmit for a particular X-ray procedure. For instance, elements in the atomic number range of 58 to 71 might be used. In any event, the two different planar filter elements 1 and 2 are arranged adjacent each other and lie substantially in the same plane and they are joined to form a unitary member 4. Referring further to FIG. 2, one may see that the third filter element 3 is arranged in a plane that parallels the plane of the unitary member 4 comprised of filter elements 1 and 2. Filter element 3 is composed of a filter material that filters out X-ray photons whose energy are so low that they would only be absorbed by the body. Aluminum is used in one actual embodiment for element 3. Note in FIG. 2 that the unitary member 4 comprised of filter elements 1 and 2 has an edge turned up to provide a drive prong or lug 51. The upper filter element 3, on the other hand, has two pairs of downwardly projecting pins. One pair of pins is comprised of pin 52 shown in FIG. 2 and 53 which is visible in FIG. 11 and the other pair of pins is comprised of pin 54, shown in FIG. 2, and pin 55 which is visible in FIG. 11. If, as in FIG. 2, the unitary filter member 4 is translated to its rightmost delimiting position as it presently is, it will be evident that lug 51 would have engaged pins 52 and 53 to pull filter element 3 to the position in which it is depicted in FIG. 2. Thus, the center of element 3 is presently coincident with the central ray 56 of the X-ray beam. It should also be evident in FIG. 5 that if the combined filter member 4 is shifted to the left until lug 51 is just about ready to touch pin 54 that the centers of filters 3 and 1 will coincide with the path of the central ray 56 of the X-ray beam. In FIG. 5 the combined filter member is in an intermediate position between its left and right travel limits. Now it will be evident that if the combined or unitary filter member is shifted further to the left from its FIG. 5 position to its FIG. 3 left limit position, lug 51 will have pushed against pins 54 and 55 so that the filter elements 3 and 1 line up with each other out of the X-ray beam path. In FIG. 3, the center of the low energy filter element 2 is substantially coincident with the central ray 56 of the beam. In FIG. 3, upper filter element 3 is in parked or inactive position. It will be evident now that if the unitary member 4 comprised of filter elements 1 and 2 is shifted to the right from the left limiting position in which it was shown in FIG. 3 to an intermediate position as it is shown in FIG. 4, filter element 2 which is located in the beam in FIG. 3 will be shifted out of the beam as is the case in FIG. 4. In the latter figure, the filter element 1 that is used for making the high energy exposures is then in the beam. In accordance with the invention, the low mass and, hence, low inertia combined filter element can be oscillated at the television frame rate to put one filter element 2 in the beam as in FIG. 3 and then, with a delay of no more than a television frame time put the other filter element 1 in the X-ray beam. Previously mentioned FIG. 5 shows an operating mode wherein initial conditions are set up by translating the combined filter member 4 to its rightmost limit for lug 51 on the member to abut pins 52 and 53 and thereby pull the top filter element 3 to the intermediate position in which it is shown in FIG. 5 as another step in setting up initial conditions. Now when the combined filter member 4 is shifted to the right until lug 51 almost touches pins 52 and 53 filter element 1 will be removed from the X-ray beam and element 3 will remain in the beam. When the combined member is restored to the position in which it is depicted in FIG. 5, the combination of filter elements 3 and 1 is in the X-ray beam. To summarize, FIG. 2 depicts one mode of operation that is applicable to fluorographic techniques where a low atomic number filter material 3 that removes uselessly soft x-radiation is fixed in the beam during an exposure of any duration. The second filter operation mode, applicable to hybrid DSA, is demonstrated in FIGS. 3 and 4 taken together wherein the combined filter member 4 is oscillated rapidly to position filter element 1 in the beam momentarily as in FIG. 3 and filter element 2 in the beam within the same TV frame time in which element 1 is removed as in FIG. 4 while in both cases filter element 3 remains parked out of the beam. This is one of the filtering methods used for hybrid DSA. The third filtering mode is also used for DSA and it is demonstrated in FIG. 5 where combined filter member 4 is oscillated to put filter elements 3 and 1 in the beam together for a high X-ray energy exposure and to let only filter element 3 remain in the beam during the low energy exposure. Note that in any of the described modes there will always be at least one of the filter elements 1, 2 or 3 in the X-ray beam to assure that the softest or lowest energy photons which could be absorbed by the body and increase patient dosage without contributing to image formation are filtered out. It should be recognized, however, that in some X-ray equipment, filter 3 might be eliminated and unitary filter member 4 could be used by itself. In such cases the X-ray tube envelope, cooling oil surrounding it and a filter plate, usually aluminum, are relied upon to provide sufficient filtration to remove the soft or low energy X-ray photons. Referring to FIGS. 11 and 7, one may see that the metal substrate sheet that composes filter element 1 of the unitary filter assembly 4 has laterally extending tongues 60, 61 and 62, 63. These tongues fit into the lowermost grooves 64 and 65 in a pair of track members 66 and 67 which serve as guides for controlling the unitary filter element to translate linearly and laterally or transversely of the X-ray beam. The generally planar unitary filter member also has upwardly bent side edges such as the one marked 68 for sliding against the track or guide members 66 and 67 to thereby prevent the unitary filter member from going askew and possibly binding. Still referring to FIG. 11, the upper filter element 3 is planar and its edges 70 ride in the upper grooves 72 and 73 of stationary track or guide members 66 and 67, respectively, in parallelism with the unitary filter member 4. The filter driving mechanism will now be described. Referring to FIGS. 11 and 1, the substrate comprised of the material for element 1 of unitary filter member has a tab or flange 84 extending from it. An endless or closed loop cable 74 is attached to flange 84 by means of a bolt 75 that passes through an eyelet 76 which is also used to join the ends to the cable together. The cable passes around stationary idler pulleys 77, 78 and 79. Idler pulley 77 is mounted for rotation on a fixed bracket 80 and idler pulley 78 is mounted on a fixed bracket 81. Idler pulley 79, on the other hand, is mounted to a movable bracket 82 that is engaged by a lead screw 83 which is rotatable to advance or retract slide bracket 82 and thereby increase or decrease cable tension as required. Cable 74, as best seen in FIGS. 7 and 8, makes several wraps around a pulley 85 which is on the shaft of a servo motor which is marked 26 in FIGS. 7 and 8 as it was in FIG. 1. The servo motor is fixedly mounted on a bracket 86 extending up from the base 40. As shown in FIG. 8, the upper end of the servo motor shaft has a pinion 87 fixed to it and this pinion is meshed with a gear 88 that is on the shaft of a potentiometer 89. The wiper of the potentiometer, not shown, rotates with the servo motor shaft and provides a varying signal that is indicative of the rotational position of the motor and, hence, the position of the filter elements. These signals are interpreted by the system controller 18 so it can put out the proper control signals for achieving one of the three modes of filter operation described earlier. In FIG. 7, the center of the upper filter element 3 is presently coincident with the central X-ray path 56. Lug 51 on the unitary filter member is abutted against pins 52 and 53 as is shown in FIG. 7 and FIG. 9 as well. Filter element 1 is out of the X-ray beam path as is filter element 2 so the conditions for the filter elements in FIG. 7 correspond with those in FIG. 2. The collimator and filter device are variously oriented between vertical and horizontal attitudes on an actual X-ray machine. In the present filter design, the unitary filter element 4 is connected by means of the cable 74 to the motor so the motor will prevent the unitary member from shifting regardless of the attitude in which the filter assembly is disposed. The upper filter element 3, however, might slide in its tracks out of the position in which it should be fixed for a particular procedure. Accordingly, means are provided for assuring that the upper filter element 3 will remain in any position to which it has been forcibly translated. For this purpose, two latch members are provided. One latch member, 95, is mounted for pivoting on a pin 96 which is fixed in the top of track 66. The other latch member 97 is pivotable on a pin 98 which is similarly mounted to track 66. Upper filter element 3 has a fixed pin 99 extending from it. This pin is engageable in slots 100 and 101 of the latch members. The end 102 of a flat spring is presently pressing against a straight edge on latch member 97 in FIG. 7 so as to preclude the latch member from rotating and thereby securing filter element 3 in the position in which it is shown. Now if filter element 3 is driven to the left in FIG. 7, pin 99 in slot 100 will cause latch member 97 to rotate 90.degree. clockwise and end up with straight edge 103 of this latch member being pressed by flat spring end 102 to maintain the slot 100 at the proper angle so that when pin 99 comes back, as it will be when filter element 3 is restored to its shown position, the pin can enter the slot 100 and rotate and reset latch 97. When filter element 3 in FIG. 7 is shifted to its leftmost position, of course, its upstanding pin 99 will engage in slot 101 of latch member 95 and cause it to rotate until the other end 104 of the flat spring will be pressing against the presently downwardly extending edge of the latch member 95 in FIG. 7. Thus, the upper filter element 3 is always automatically latched in the X-ray beam or in parked position depending on the operational mode of the filter device that is selected by the user and element 3 will now shift even though it is in other than a horizontal attitude. Other detent means, not shown, could be used to retain filter element 3 at both limits of its travel. For example, spring biased balls, not shown, mounted in sockets on one of the tracks could be pressed into different dents or cavities in the surface of element 3. Alternatively, beveled chips of friction material such as rubber or vinyl, not shown, could be adhered to a surface of element 3 for sliding under tabs, not shown, mounted along the tracks for having the chips run under them to effect frictional gripping. As implied earlier, the third filter element 3 may be eliminated in some applications in which case only the unitary filter element 4 having the two substantially coplanar and adjacent filter materials would be installed. This arrangement would reduce the inertia of the filter system even more than the already small inertia that exists in the three filter element combination which has been described and illustrated. Unitary filter element 4 has small area, is thin and has low weight and low interia so it can be shifted extremely fast with a low power drive system. Of course, element 4 has small area, is thin and also has low weight and inertia. Small area filters can be used because the filter assembly is so compact that it can be located very near to the focal spot of the X-ray tube where the X-ray beam is the narrowest. This permits achieving the objectives of being able to oscillate the filter elements at television frame rates with low power, low noise and no significant vibration. In an actual embodiment, filter element 2, illustrated as gadolinium, is only about two inches square and 5 mils thick so it has trivial mass and inertia. Filter 1, illustrated as copper, is about the same size and about 20 mils thick. Filter 3, illustrated as aluminum, is a little more than 2 inches long overall and about 64 mils thick. The filter thicknesses given are appropriate for dual energy X-ray techniques wherein the kilovoltage applied to the X-ray tube anode is in the range of about 60 kilovolts to 135 kilovolts. Those skilled in the art will appreciate that various filter materials may be substituted for those which are given for illustrative purposes herein. FIG. 6 shows a timing diagram applicable to the new filter device. The diagram represents one cycle of a hybrid DSA exposure sequence. The first interval A is used for preparation and is typically about 33 ms or one TV camera frame time. Interval B is that during which a low energy exposure will be made, for example. This is typically anywhere between one frame or 33 ms to 500 ms or about 15 television frame times. Interval C is 33 ms or 1 TV frame time in an actual embodiment and within this interval filter positions must be exchanged. Concurrently with filter exchanging, the TV camera target is read out for the preceding low energy exposure. In an actual embodiment, 30 ms are allowed for making the filter change and the first three milliseconds of this interval are used by CPU 18 to make the necessary position signal computations. Interval D may be 33 ms or one TV frame time long or it may extend to 500 ms, typically, for making the high X-ray energy exposure in the closely consecutive pair. Then interval E may have a 33 ms or single TV frame time duration minimum to allow for changing filters and reading out the television camera target following the high energy exposure but interval E may be extended to obtain any desired amount of delay before going back to interval A for the next low energy exposure. Although an illustrative embodiment of the new multiple mode translational-type filter device has been described in detail, such description is intended to be illustrative rather than limiting, for the invention may be variously embodied and is to be limited only by the claims which follow.