X-ray table with bucky elevator

Abstract

Diagnostic quality of the image on an x-ray film in a cassette beneath a movable patient supporting top of an x-ray table is improved by providing Bucky and cassette tray elevating means in the Bucky carriage. When a radiograph is to be taken, the Bucky is elevated to close proximity with the bottom of the table top to thereby reduce the object to film distance. The Bucky is lowered to provide clearance under the table top frame for exchanging film cassettes.

Description

BACKGROUND OF THE INVENTION

This invention relates to improvements in diagnostic x-ray tables and is particularly concerned with improving radiographic quality by reducing the distance between the anatomy of a patient supported on the x-ray table top and the film beneath the table top on which an x-ray image is recorded.

Diagnostic x-ray tables usually comprise an open metal frame which is covered with an x-ray transmissive panel or top for supporting a patient during x-ray examination. In one class of x-ray tables, the x-ray tube is supported over the table top and the radiographic film is contained in a cassette under the table top. The cassette is carried in a Bucky which is translatable longitudinally of the table top in the table body in a direction opposite of x-ray tube translation for tomographic procedures and so it may be positioned to coincide with the x-ray beam for other procedures.

As is known in the x-ray arts, the term Bucky designates a device for supporting a film cassette in a plane transverse to the x-ray beam which penetrates the patient and the x-ray table top. Usually the device will accept cassettes of various sizes. The device also usually incorporates a Potter-Bucky diaphragm or other grid in a plane between the x-ray table top and the cassette. The grid intercepts scattered radiation which does not originate at the x-ray tube focal spot and comprises alternate strips of x-ray opaque and x-ray permeable material. The grid is set into motion during radiographic exposures to diffuse the shadow lines which the opaque strips would otherwise cast. In some Bucky devices there is a sensor interposed between the grid and cassette for sensing the x-ray dosage received at the film. A tray which supports the film cassette and the components which support the grid above the tray are collectively called a Bucky herein. The Bucky is in a movable carriage in the present case but it may be supported independently in some cases.

It is often customary to support the top frame in a manner that permits it and the patient supported thereon to be moved longitudinally and laterally of the x-ray table body. Because this movement has to be executed without interference, it has been customary to mount the Bucky carriage at a level sufficiently far below the table top to permit the top frame to translate over it. As a result, it has been necessary for the plane of the film in the cassette to be on the order of three inches from the bottom of the table top in many existing tables.

As is well known, it is desirable to have the x-ray film plane as close as possible to the patient on the table top to minimize enlargement, and more importantly, unsharpness of the image on the radiographic film. Enlargement results from the x-ray beam radiating from a focal spot on the x-ray tube target and diverging conically so that the image of any increment of the patient's anatomy is displaced laterally and longitudinally on the film beneath the patient. The displacement and, hence, enlargement becomes greater as the distance between the film and patient is increased.

Unsharpness results from the x-ray tube focal spot having finite dimensions rather than being a point source. Thus, the edge of any increment of a patient's anatomy receives a ray from all areas of the focal spot which leads to the increment being imaged on the film with an umbra and a penumbra that manifests as unsharpness. This effect can be reduced by positioning the x-ray film plane closer to the object and to the focal spot. As indicated, in conventional x-ray tables the object-to-film distance was fixed by design and was minimized only to the extent permitted by the need for having a table top frame clear the Bucky carriage when moving the table top bilaterally over the film cassette supported in the carriage.

One compromise that has been used to shorten the table top or object-to-film distance has been to depress the center part of the table top so the patient can be brought as close as possible to the film plane. However, a depression in the top has the disadvantage of making it difficult to transfer a seriously ill patient to and from the table top from a patient cart.

SUMMARY OF THE INVENTION

An object of the present invention is to enable minimizing the distance between the film plane under the table top and a patient supported on an x-ray table top.

A further object is to support the Bucky and film cassette on the Bucky carriage in such manner that the Bucky and film cassette can be elevated in between the top supporting frame members to proximity with the bottom of the table top.

A still further object is to provide for lowering the Bucky and film cassette to enable its withdrawal and replacement with convenience equal to that of prior table designs without elevatable Bucky and cassette trays.

How the foregoing and other more specific objects of the invention are achieved will appear in the ensuing more detailed description of a preferred embodiment of the invention taken in conjunction with the drawings.

In general terms, the improved x-ray table is characterized by having a Bucky carriage mounted for longitudinal movement within the table body under the table top. A vertically movable Bucky is mounted within the carriage. The Bucky has an independent tray for supporting the film cassette. Means are provided for elevating the Bucky when a radiograph is to be taken and for lowering the Bucky when the cassette tray is withdrawn for changing cassettes.

How the foregoing and other more specific objects of the invention are achieved will be evident in the ensuing description of an illustrative embodiment of the invention taken in conjunction with the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a diagnostic x-ray table in which the new film positioning system is used;

FIG. 2 is an enlarged perspective view of the x-ray table body with parts omitted and with the patient supporting top displaced so as to exhibit the top supporting frame and Bucky carriage;

FIG. 3 is a plan view of the Bucky carriage and cassette tray with the overlying Bucky grid removed;

FIG. 4 is a fragmentary vertical and lateral sectional view taken on a line corresponding with 4--4 in FIG. 3;

FIG. 5 is a bottom view of the Bucky carriage showing the drive mechanism for raising and lowering the film tray and grid;

FIG. 6 is an upright rear elevational view of the carriage;

FIG. 7 is a view of the elevating mechanism drive cables taken on a line corresponding with 7--7 in FIG. 5; and

FIG. 8 is a partial transverse section taken through the table assembly to show the carriage and associated parts in side elevation.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 depicts a typical diagnostic x-ray table in which the new Bucky raising and lowering device may be used. This is a nontilting table but the device may also be used in the tilting type. Moreover, although the herein described x-ray table includes an integral overhead x-ray tube support means it will be understood that the new Bucky elevating system may be used in an x-ray table that has an independently supported x-ray tube. The illustrated table comprises a body 10 on which there is a patient supporting top assembly 11. The top assembly essentially comprises a rectangular frame, which will be described later, on which there is a planar x-ray transmissive panel 12 for supporting the patient. Behind the table is a column 13 which is mounted for translation longitudinally of the table top and for rotation about a vertical axis. The mounting which permits rotation of column 13 and the tracks which permit its translation are not shown since they may be conventional. The tracks may be located in the rear part 14 of housing 10.

A vertically movable arm 15 extends laterally over the table top 11. An x-ray tube assembly 16 is mounted on the free end of the arm and a conventional x-ray beam collimator 17 is supported from the tube assembly. The x-ray tube is energized by electric cables 18 and 19. The tube, not shown in detail, is conventional in that it has a high energy electron beam focused on a target. X-rays radiate from the focal spot and are projected in a beam through collimator 17 toward table top 12 and through any object or patient supported thereon. In this case the x-ray image is recorded on film which is in a cassette supported in a Bucky in a carriage underneath the table top in the region identified by the numeral 20. The Bucky and cassette tray and affiliated parts will be described in detail hereinafter.

The x-ray table top assembly 11 is mounted for limited longitudinal and lateral translation with respect to table body 10. FIG. 2 shows more details of the table top assembly 11 and other components within body 10. FIG. 2 shows a table top assembly supporting carriage identified generally by the reference numeral 25 and comprised of four bar members 26, 27, 28 and 29 which are joined at their corners by any suitable means to form a rectangle. Top carriage 25 is adapted for translating laterally of body 10 by suitable roller supports, not visible in FIG. 2, which are of a known type and can be devised by a skilled mechanical design. The table top assembly 11 is shown fragmentarily with its leading end about to be assembled on laterally movable carriage 25. Top assembly 11 is adapted for translating longitudinally on carriage 25. For this purpose, the top assembly 11 has a longitudinally extending channel 30, comprising part of the table top supporting frame, which has inwardly extending flanges 31 and 32 which serve as a track for guiding the table top on laterally movable carriage 25. The rear of top assembly 11 has a track corresponding with track 30 although it is not visible in FIG. 2. Carriage 25, as can be seen on front bar member 27, has rollers such as 33, 34 and 35. The top flange 32 of channel 30 bears on rollers 33 and 35 which extend above the top plane of bar 27. The bottom flange 31 of channel 30 is engaged by center roller 34 such that there is no vertical free play between top assembly 11 and carriage 25. Of course, when the assembly is complete, top 11 is substantially coextensive with rectangular carriage 25 as it appears in FIG. 1 but there are stops, not shown, for limiting the amount of longitudinal travel of the top 11 with respect to carriage 25 and also stops, not shown, for limiting lateral travel of table top carriage 25.

In FIG. 2, the Bucky carriage, which is designed in accordance with the invention to enable shortening the distance between the radiographic film beneath table top panel 12 and a patient supported thereon, is designated generally by the reference numeral 40. By means which will be described in detail hereinafter, Bucky carriage 40 is adapted for moving to a limited extent longitudinally within the table body. For instance, by means which are not shown in detail, the table is adapted for conducting tomography which is an x-ray procedure involving moving Bucky carriage 40 and film longitudinally in one direction underneath table top 12 while the x-ray tube 16 is moved in an opposite direction above the table top. The motion above the table top is obtained by translating column 13 which in combination with a coupling bar 36 causes tube arm 15 to rotate on its horizontal axis. This conventional tomographic coupling bar 36 couples the Bucky carriage 40 in effect to the tube arm 15. Longitudinal driving of column 13 brings about the opposite motions of the film and Bucky carriage 40 and x-ray tube 16. As is well known, in tomographic procedures, the fulcrum or rotational axis of the coupling bar is positioned at the same height as the required object plane so that all planes above and below the plane containing the rotational axis are blurred on the film while the plane containing the rotational axis remains in focus and produces a relatively sharp image on the film.

Attention is now invited to FIGS. 3, 4 and 8 for an explanation of how the Bucky carriage 40 is constructed and mounted within the x-ray table body. In these figures, it is evident that the Bucky carriage 40 comprises an open top box-like member having upstanding sides 46 and 47, an upstanding rear 48 and a front 49 with a front opening 41 for admitting and withdrawing a film tray 42 from the front of the x-ray table for exchanging cassettes therein. The bottom of carriage 40 is identified by the number 50 and is particularly evident in FIG. 8. The bottom 50 has a central opening 51, see FIG. 3, which, when there is no film in the carriage, permits an x-ray image to be intercepted on the sensitive surface of an image amplifier 52 that is shown in phantom in FIG. 8.

The manner in which Bucky carriage 40 is mounted for longitudinal movement within the x-ray table will now be described. Refer to FIG. 8 where an end view of the Bucky carriage 40 taken in a longitudinal direction may be seen. At the rear, the Bucky carriage 40 is supported on rollers such as 53 which run on a horizontal axis between the flanges of a stationary longitudinally extending channel 54. The channel is provided with some hardened steel strips 55 for the rollers 53 to run on without wearing the track and to provide frictionless free-rolling motion. A lip 56 extending downwardly from channel 54 is engaged between another pair of rollers 57 and 58 which are mounted for rotation about vertical axis on a bracket 59 which is fastened to Bucky carriage 40. Extending rearwardly from the back wall 48 of Bucky carriage 40 is an arm 60 on which a force is applied for translating the Bucky carriage longitudinally within the table. This may be done with the bar 61 which is shown fragmentarily in FIG. 8. Arm 60 and, hence, Bucky carriage 40 is further guided by rollers 62 running on a track 63 which is right angular in cross section. The front end of Bucky carriage 40 has a bracket 64 on its bottom to support the shaft of a roller 65 which cooperates with a stationary channel member 66 constituting a track. There may be several such front rollers. Thus, it is evident that Bucky carriage 40 may translate bidirectionally longitudinally of and under the table top under the influence of a longitudinal force applied to arm 60.

In FIG. 8 one may see that carriage 40 contains a Bucky 70. The bottom 71 of the Bucky 70 is shown in hidden lines in FIG. 8 and the substantially imperforate backwall thereof is marked 72. The front wall of the Bucky 70 is open in the region 73. Bucky 70 supports the reciprocable Bucky grid 45 which was briefly described hereinbefore. The grid oscillating mechanism is not shown but may be a known type. This grid, being part of the Bucky, raises and lowers with the Bucky so as to advance and recede with respect to the bottom surface of the patient supporting top 12, thereby maintaining constant spacing between the grid 45 and cassette 80. In FIG. 8 the Bucky is shown in elevated position in which case it extends upwardly between the channel tracks or side frame members 31 and 31' on which the table top 12 is supported. There is sufficient clearance at the front and rear ends, the right ends in FIG. 8, of the Bucky 70 to allow table top assembly 11 to be shifted manually laterally of the table body.

The bottom 71 of the Bucky 70 has a central hole 74 which aligns with hole 51 in the bottom of the Bucky carriage so that a limited area of the subject being radiographed can be intercepted on the image amplifier 52. Image amplifier 52 may be a conventional type for converting an x-ray image to an optical image. The optical image appears on a phosphor 75 and this image may be viewed through an objective lens 76 and a mirror 77 which directs the image to a television camera 78. A monitor, not shown, is used to display the television image produced by camera 78.

In FIG. 8 one may see that a manually movable cassette tray 42 extends into the Bucky 70 through its open front region 73. Tray 42 supports a film cassette 80 which is outlined in FIG. 8. The plane of the radiographic film within the cassette is shown as a hidden dashed line 81. In a commercial embodiment, the film plane 81 is only about 1.2 inches from the top surface of patient supporting panel 12 when the Bucky 70 is elevated in Bucky carriage 40. This dimension may be increased if an optional x-ray sensing device, not shown, is installed between Bucky grid 45, which is at a fixed height within the Bucky 70, and the cassette tray 42. When the Bucky 70 is lowered, tray 42 and cassette 80 supported thereby are lowered with it to facilitate withdrawal of the film tray 42 from under table top frame channel 30 by gripping handle 43 and drawing the tray toward the front of the table. In this manner, cassettes may be interchanged in the table.

In FIG. 3, one may see that there are longitudinally spaced apart laterally extending channel tracks 82 and 83 for supporting cassette tray 42 in Bucky 70. The front ends of tracks 82 and 83 are visible in FIG. 2. On cassette tray 42 are clamping members 84 and 85 which clamp cassette 80 in approximately center position and are linked together by means which are not shown. The clamping members move toward and away from each other in respect to guide slots 86 and 87. Movement of the clamping members 84 and 85 is effected by longitudinal shifting of knob 44. The mechanism for driving the clamping members is conventional and is omitted for the sake of clarity as indicated earlier. Tray 42 has a partition wall 88, see FIG. 3, for defining with end wall 89 a compartment 90 for cables and other electrical components, not shown.

A significant feature of the structure is that Bucky 70 and tray 42 thereon can be raised and lowered within Bucky carriage 40 so as to place the film plane 81 as close as possible to table top 12 prior to radiography. In this design the Bucky constitutes a support means for the laterally movable cassette tray means, a cassette thereon, and the grid. The mechanism for raising and lowering the Bucky 70 and tray 42 to accomplish this purpose will now be described.

As can be seen in FIGS. 3, 4 and 8, there are four lift pins 91 to 94 projecting from the side walls of elevatable Bucky 70. Pins 92 and 93 extend through straight vertical slots 95 and 96, respectively, in the side wall 46 of cassette carriage 40. Pins 92 and 93 are engaged by bell cranks 97 and 98, respectively. Bell crank 97 pivots on a pin 99 which is mounted on the outside of side wall 46 of Bucky carriage 40. Bell crank 98 is similarly mounted for pivoting on a pin 100. Bell cranks 97 and 98 are provided with slotted holes 101 and 102, respectively, by means of which the bell cranks are engaged with lift pins 92 and 93 and, hence, Bucky 70. Counterclockwise rotation of bell cranks 97 and 98 will restore Bucky 70 to its uppermost position in which it is depicted in FIG. 8.

The bell cranks 97 and 98 on this end of the Bucky carriage 40 are connected together by a link 110. Opposite ends of link 110 are pivotally connected at 111 and 112 to bell cranks 97 and 98, respectively. It is evident from inspection of FIG. 8 in particular that when link 110 is shifted to the left from the position in which it is shown, bell cranks 97 and 98 will rock clockwise jointly to lower Bucky 70 in carriage 40. Pins 92 and 93 travel in slots 95 and 96. When link 110 is shifted to the right again to the position in which it is shown in FIG. 8, the bell cranks will rock counterclockwise and elevate Bucky 70 again. The force for shifting link 110 lengthwise is applied by means of a centrally located pin 113.

The bell crank and linkage construction at the opposite side of the Bucky carriage 40 is similar and symmetrical to that described in the preceding paragraph. In FIG. 3 it is evident that lift pins 91 and 94 on the opposite end cooperate with bell cranks 114 and 115 which are connected together by a link 116. The link 116 also has a pin 117 for shifting it lengthwise.

A mechanism for shifting links 110 and 116 to raise and lower Bucky 70 will now be described primarily in reference to FIGS. 5-7. FIG. 5 shows an inverted or bottom view of Bucky carriage 40. When properly upright, the rollers 53 and 54 on which the Bucky carriage are supported would be to the rear of the table.

In this example, links 110 and 116 are shifted and Bucky 70 is thereby raised and lowered by a motor 120 mounted on the bottom of Bucky carriage 40. Motor 120 is supported on a bracket 121 which is fastened to the bottom 50 of the Bucky carriage. FIG. 6 is an elevation view of the Bucky carriage as it would appear from the rear when it is installed in its normal upright position. Motor 120 has a shaft 122 on which there is a worm gear 123 that engages a worm wheel 124 which is fixed on a shaft 125. Shaft 125 has two axially adjacent cable drums fixed 126 and 127 on it. The lowermost cable drum 127 has the end of a cable 128 running half-way around it. Cable 128 has its end fastened approximately at the point marked 129 inside of the drum. Cable 128 may be considered continuous from its starting point on drum 127 and running to the right and back around the bottom of the carriage to return as an end 128'". End 128'" also makes a half-turn around drum 127 and is fastened at the same point 129 at which the end of cable portion 128 is fastened. A turnbuckle type device including an internally threaded body 131 and cooperating screw 132 can be turned to establish proper tension in the cable run. It is evident from FIGS. 5 and 7 that cable section 128 runs across the foremost or rear portion of carriage 25 and makes a bend around a corner mounted sheave 133 whereupon it runs along the side of the carriage and is marked 128'. It then passes around another corner mounted sheave 134 whereupon it crosses over another cable, to be described, and around a corner mounted sheave 135 which is beneath another sheave 145 on a common axis. The crossover is exhibited in FIG. 7. After passing around sheave 135, the continuation of the cable is marked 128" which then goes around the sheave 136. These sheaves are also on a common axis of rotation. After passing around sheave 136, the continuation of the cable is marked 128'" where it continues to its anchor point 129 on drum pulley 127. The side section 128" is fastened to pin 117 which is carried by bell crank operating link 116. One may see in FIG. 5 that if drum pulley 127 is rotated counterclockwise, cable section 128 will pay out and section 128' " will be taken in and placed in tension so as to shift link 116 at the left forward or toward the observer.

The other drum pulley 126 also has the end of a separate cable section 140 wrapped half way around it and anchored at approximately the point 141. In FIG. 5, the section of cable 140 extending to the right has a turnbuckle 141 installed in it for maintaining proper cable tension. Cable 140 section passes around a sheave 142 which is underneath sheave 133 and on the same axis, and the continuation of cable 140 is marked 140'. The cable section 140' is fastened to pin 113 in bell crank operating link 110 on the right side of the carriage as it appears in FIG. 5. Section 140' continues and passes around a sheave 143 which is on the same axis of rotation of sheave 134. Cable 140' then crosses over cable 128' as is evident from FIG. 7. Cable 140' continues around sheave 145 after which the ensuing cable section is marked 140". This section is not fastened to pin 117 but merely passes it. Cable section 140" then passes around the uppermost sheave 148 in a pair of sheaves on a common axis which includes a sheave 147. After passing around sheave 148, cable 140 is marked 140'" whereupon it makes its terminal half turn around drum pulley 126 and is anchored at point 141. It will be evident from inspection of FIG. 5 that when motor 120 drives drum pulley 126 counterclockwise along with pulley 127, cable section 140 will be placed in tension and cable will pay out in the direction of section 140'". Because cable section 140' is fastened to pin 113 of link 110, the latter link will be urged toward the observer simultaneously with link 117. This translational movement of link 110 will rock bell cranks 97 and 98 in unison with bell cranks 114 and 115 to thereby cause Bucky 70 to raise in the cassette carriage 40. The double cable arrangment involving cables 128 and 140 assures that the Bucky 70 will be raised and lowered steadily. Further steadiness and equilibrium of the Bucky 70 is assured by use of a spring balancing system including a pair of springs 150 and 151 which have their corresponding inner ends fastened to a fixed stud 152. The free end of spring 150 is connected to a cable 153 which runs around a sheave 154 and attaches at point 155 to pin 113 in operating link 110. The other spring 151 is attached to a tension cable 156 and it runs around sheave 147 whereupon it is attached at 157 to pin 117 in operating link 116. It will be evident that springs 150 and 151 tend to keep the system in equilibrium and balance and aid the motor in lifting the Bucky 70 within Bucky carriage 40. They also tend to absorb shock when the Bucky 70 is lowered.

Although the new concept of enabling elevation of Bucky in a Bucky carriage to get the film closer to the object being radiographed has been exemplified in a system using a motor to raise and lower the Bucky, it will be appreciated by those skilled in the art that manual means can be employed as well. For instance, it is contemplated in some embodiments to provide a manually accessible mechanism including a lever, not shown, for actuating bell crank operating links 110 and 116. Now that the concept of raising the Bucky film prior to radiography has been suggested, it should be within the purview of a skilled designer to produce other mechanisms for raising and lowering the Bucky and film. Thus, although a preferred embodiment of the invention has been described in detail, such description should be considered illustrative rather than limiting for the invention may be variously embodied and should be limited only by interpretation of the claims which follow.

Claims

We claim:

1. In diagnostic x-ray apparatus including an x-ray permeable table top arranged for cooperating with an x-ray tube which is positionable on one side of said top for accommodating a diagnostic subject between said tube and table top, means supporting said table top for bidirectional lateral and longitudinal movements in a plane to thereby enable a subject on said top to be positioned selectively, said supporting means including frame means extending away from said top and defining a space that is closed about its perimeter and has an open bottom, said frame means supporting said top on a side remote from the patient supporting surface thereof, a carriage means mounted for moving longitudinally relative to said table top beneath it and independently thereof, the improvement comprising:

a. support means on said carriage means, said support means being selectively movable between a first position remote from said table top and a second position closer to said table top,

b. cassette tray means mounted on said support means for being moved laterally to enable withdrawal thereof for exchanging film cassettes, said tray means being constructed and arranged to support a film cassette, said tray means supporting a film cassette within said space defined by said frame means when said support means is in said second position and said tray means supporting said cassette below said frame means and said space when said support means is in said first position,

c. withdrawal of said cassette tray means being interfered with by said table top frame means when said support means is in its second position and being clear of said frame means when in its first position, and

d. selectively operable means for moving said support means between its first and second positions relative to said carriage means.

2. The invention set forth in claim 1 including:

a. grid means mounted on said support means in substantial parallelism with said tray means, said grid means being interposed between said cassette tray means and said table top and being movable with said support means jointly with said cassette tray means toward and away from said table top.

3. The invention set forth in claim 1 wherein:

a. said selectively operable means includes link means mounted pivotally on said carriage means and engaged with said support means, and

b. means for actuating said link means to pivot in selectively opposite directions to thereby effect said position changes of said support means.

4. The invention set forth in claim 3 including:

a. motor means mounted on said carriage means, and

b. means for coupling said motor to said actuating means.

5. The invention set forth in claim 1 including:

a. longitudinally spaced apart pairs of elements each of which are pivotally mounted to said carriage means and the elements in a pair being spaced laterally from each other,

b. means for engaging each of said elements with said support means,

c. link means connecting said elements in each pair, respectively, for pivoting jointly, and

d. selectively operable actuating means operatively connected with said respective link means to translate said link means and thereby pivot said elements and move said support means.

6. The invention set forth in claim 3 wherein said actuating means comprises:

a. motor means mounted on said carriage means,

b. drum means rotatable in selectively opposite directions by said motor means,

c. sets of a plurality of sheave means supported on said carriage means,

d. a first flexible element having its opposed ends attached to said drum means and running on one set of said sheave means, said first flexible element being engaged intermediate its ends with one of said link means, and

e. a second flexible element having its opposite ends attached to another of said drum means and running on another set of said sheave means, said second flexible element being engaged intermediate its ends with the other of said link means.

7. In diagnostic x-ray apparatus including an x-ray permeable table top arranged for cooperating with an x-ray tube which is positionable on one side of said top for accommodating a diagnostic subject between said tube and table top, means supporting said table top for bidirectional lateral and longitudinal movements in a plane to thereby enable a subject on said top to be positioned selectively, said supporting means including frame means supporting said top and defining a space on a side of said table top remote from the patient supporting surface thereof, a carriage means mounted for moving longitudinally relative to said table top beneath it and independently thereof, the improvement comprising:

a. support means on said carriage means, said support means including a rear member, longitudinally spaced apart side members and a front member, said front member having an opening therein,

b. longitudinally spaced apart track means extending from said opening in a lateral direction between said side members,

c. cassette tray means including cassette clamping means for supporting a film cassette, said tray means being supported on said track means and being withdrawable through said opening to permit exchanging film cassettes clear of said table top, and

d. means for selectively moving said support means between raised and lowered positions in respect to said carriage means, withdrawal of said tray means being prevented by said table top frame means interfering with said tray means when it is raised with said support means but said cassette tray means and a film cassette thereon being closer to said table top when in raised position for improving radiographic quality, said tray means being withdrawable without interference when said support means is in its lower position,

e. radiographic grid support means supported from said support means between said cassette tray means and said table top, said grid support means being constructed and arranged to support a grid in substantial parallelism with said tray means, and

f. said grid means support means being movable with said support means to maintain a constant distance between said tray means and grid means when said support means is in either its raised or lowered position.