BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed to a subject table for a mammography apparatus of the type having a slot for an X-ray film cassette as well as a locking mechanism for the cassette arranged in the slot that has first and second detents, at least one of the detents being spring-loaded such that the detents lie tightly against the narrow sides of the cassette when a cassette is introduced into the tunnel. 
   2. Description of the Prior Art 
   A subject table having a locking mechanism of this type is shown in the brochure of Siemens AG entitled “Because she deserves the very best MAMMOMAT 3000 Nova”. The locking mechanism therein is an arcuately curved leaf spring that is secured in one of the slot walls with the aforementioned detent disposed at an end thereof. When a cassette held in the slot is released or when replacing a cassette, the operator pushes one of the detents in the direction of the slot wall with one hand, so that the detent is moved away from the narrow side of the cassette. At the same time, the operator pushes the cassette out of the slot by pressing the other narrow side of the cassette into the slot with his/her other hand. A release or replacement of a cassette can ensue only with the use of both hands. 
   SUMMARY OF THE INVENTION 
   An object of present invention is to provide a subject table of the type initially described having a lock mechanism allowing a release or a replacement of a cassette to be implemented in a simple way with the use of only one hand. 
   This object is inventively achieved in a subject table of the type initially described wherein the lock mechanism has at least one arm that is spring-loaded in its longitudinal direction, with one end is provided with the first detent, and wherein the arm is displaceable in its longitudinal direction, and wherein the second detent is arranged to be displaceable perpendicularly to the longitudinal direction of the arm. When releasing a cassette that is clamped in a lock mechanism, the operator pushes the second detent perpendicularly to the longitudinal direction of the arm with one finger of one hand, so that the first detent is moved away from the narrow side of the cassette. The first detent of the arm is spring-loaded by a cassette inserted into the slot, so that it now automatically pushes the cassette out of the slot to such an extent that the operator can grasp it. 
   In an embodiment of the lock mechanism of the invention the lock mechanism has a second arm that is also spring-loaded in its longitudinal direction to which the second detent is attached. As a result of this structure of the lock mechanism, the detents of the highly spring-loaded arms are pressed against both narrow sides of the cassette when a cassette is introduced between the detents. When removing the cassette, the operator can then select from which of the slot openings the cassette should be removed. This ensues by the appropriate detent being removed with one hand from that narrow side of the cassette that the operator wants to eject first, and the spring-loaded detent that presses against the other narrow side of the cassette automatically pushes the cassette out. 
   In another embodiment of the invention, a comparatively sophisticated structure of the lock mechanism is established in that the arms lie in axial alignment with one another, their free ends being rotatably connected to respective, rotatable gearwheels via respective pins. The pins are eccentrically arranged at the gearwheels, and the gearwheels are arranged in the same plane and mesh with one another, and so the arms are tensional in their longitudinal directions with springs. Again, the operator can select, using only one hand, from which slot opening a cassette inserted into the subject table should be automatically pushed out. 
   The lock mechanism of the invention preferably is attached to the base of the slot and is disposed therein, so that only the detents project above the surface of the base. It is thus simple to push a cassette into the slot of the subject table. 


   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a subject table with a part of the lock mechanism of the invention. 
       FIG. 2  is a plan view of a lock mechanism of the invention in the parked position. 
       FIG. 3  is a plan view of a lock mechanism of  FIG. 2  in the active position. 
       FIG. 4  is a plan view of a further embodiment of a lock mechanism of the invention. 
       FIG. 5  is a plan view of another embodiment of a lock mechanism of the invention. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a side view of a subject table 1 that is secured via a shaft  2  to an arm  3  for an X-ray tube (not shown here). The subject table 1 is provided with a slot extending completely through the slot  4 , into which an X-ray film cassette  5  can be introduced.  FIG. 1  also shows one of two blocking devices  23  (to be described in greater detail later) for a lock mechanism (to be described in greater detail below) for the cassette  5 , whereby the lock mechanism is arranged at the base  7  of the slot  4 . The second dent  13  of the lock mechanism also is shown in  FIG. 1  (at a positioned occupied by second detents  35  and  42  in the respective further embodiments). 
     FIG. 2  shows a plan view of a lock mechanism  43  for a cassette  5 . As described above, the lock mechanism  43  is attached to the base  7  of the slot  4 . The lock mechanism  43 , which is shown in  FIG. 2  in a parked position, has two arms  10 ,  11  arranged in axial alignment with one another and loaded their longitudinal direction by respective springs  8 ,  9 . The outer ends of the arms  10 ,  11  are provided with respective detents  12 ,  13 . One end of the spring  8  is connected to the arm  10  via a hook  14  projecting therefrom, and the other end of the spring  8  is connected to the base  7 . One end of the spring  9  is connected to the arm  11  via a hook  15  projecting therefrom, and the other end of the spring  9  is connected to the base  7 . The inner ends of the arms  10 ,  11  are rotatably connected via respective shaft pins  20 ,  21  to respective gearwheels  16 ,  17  rotatable around respective shafts  18 ,  19 . The pins  20 ,  21  of the arms  10 ,  11  are eccentrically arranged at the gearwheels  16 ,  17 . The gearwheels  16 ,  17  are arranged in the same plane and mesh with one another. The lock mechanism  43  is contained within the base  7  of the slot  4 , so that only the detents  12 ,  13  project above the surface of the base  7 . 
   Insertion of a cassette  5  into the subject table 1 can ensue from either opening at the opposite ends of the slot  4 . Upon insertion of the cassette  5  from the side that is the right side in the plane of the drawing, the detent  13  is pushed aside, so that the cassette  5  can be pushed into the slot  4 . The arm  11  is thus rotated around the pin  21 . Blocking devices  22 ,  23  (described later) that prevent such a motion of the arms  10 ,  11  when a cassette  5  is disposed in the slot  4 , allows such a motion when the lock mechanism  43  is in the parked position. The cassette  5  is first pushed into the slot  4  until one narrow side  24  thereof reaches the detent  12  of the arm  10 . When the cassette  5  is manually introduced farther into the slot  4 , the arm  10  is displaced in its longitudinal direction. Simultaneously therewith the arm  10 , by its engagement with the gearwheel  16  via the pin  20  rotates the gearwheel  16  in a counter-clockwise direction, causing the spring  8  to be tensioned. At the same time, the gearwheel  17  is rotated to exactly the same extent, but clockwise. Thus the arm  11  and the detent  13  (since the arm  11  engages the gearwheel  17  via the shaft  21 ) is displaced in its longitudinal direction to exactly the same extent a the arm  10 , so the spring  9  of the arm  11  also is tensioned. When the distance between the detents  12  and  13  is large enough, the cassette  5  is placed such that the detent  12  lies resiliently against one narrow side  24  of the cassette  5  and the detent  13  lies resiliently against the other narrow side  25  of the cassette  5 . This is shown in FIG.  5 . 
   Upon introduction of the cassette  5  from the side that is the left side in the plane of the drawing, the detent  12  is displaced such that the cassette  5  can be pushed into the slot  4 . The sequence that has already been described then ensues mirror-inverted. 
   Given a cassette  5  introduced in the subject table 1, the blocking device  22 ,  23  (as shown in  FIG. 3 ) respectively block the detents  12 ,  13  of the arms  10 ,  11 , so that they cannot be rotated around their shafts  20 ,  21 . Each blocking device  22 ,  23  that is shown in  FIG. 1  is composed of a spring-loaded pushbutton  26  having a free end provided with a tongue  27  that lies against the arms  10 ,  11  when a cassette  5  has been introduced in the subject table 1. 
   When the cassette  5  is to be released or replaced such that the cassette  5  is pushed out from the right side in the plane of the drawing, the button  26  of the blocking device  23  is pressed down with a finger, so that the tongue  27  comes to lie under the arm  11 . At the same time, the detent  13  is displaced with another finger, i.e. the arm  11  is rotated around the pin  21 , so that the detent  13  is removed from the narrow side  25  of the cassette  5 . In this position of the detent  13 , the detent  12  pushes the cassette  5  from the slot  4  with the spring-loaded arm  10  to such an extent that the operator can grasp the cassette  5 . 
   When the operator wants to eject the cassette  5  from the left side in the plane of the drawing, the button for the blocking device  22  is pressed. Subsequently, the detent  12  is removed from the narrow side  24  of the cassette  5  is the described way, and the detent  13  of the spring-loaded arm  11  pushes the cassette  5  out to such an extent that the operator can grasp it. 
     FIG. 4  shows a simplified embodiment of a lock mechanism  44  of the invention. Here, the lock mechanism has two arms  30 ,  31  arranged in axial alignment with one another and loaded by springs  28 ,  29 . The arms  30 ,  31  are disposed in the base  7  of the slot  4  and in respective cavities  32 ,  33 . The cavities  32 ,  33  are provided for stabilizing the movements of the arms  30 ,  31 .  FIG. 4  shows that each cavity  32 ,  33  is wedge-shaped, so that the outer ends of the arms  30 ,  31  are partly laterally displaceable. The outer ends of the arms  30 ,  31  are provided with detents  34 ,  35  that project up above the surface of the base  7 . When loading a cassette  5 , for example, the detent  35  and thus the arm  31  as well, is pushed against one edge  36  of the cavity  33 , so the cassette  5  can now be introduced into the slot  4  of the subject table 1. The cassette  5  is pushed in so far that one narrow side thereof reaches the detent  34  of the arm  30 , and so that the other narrow side of the cassette  5  comes to lie against the detent  35  of the arm  31 . The force of the spring  28  of the arm  30  is then partly transmitted to the arm  31 , and the detents  34 ,  35  lie against the narrow sides of the cassette  5  with equally distributed spring power when a cassette  5  has been introduced. Dot-dash contours of the detents  34 ,  35  in  FIG. 4  indicate their positions when a cassette  5  has been inserted and held. The operator can then select the slot opening from which the cassette  5  should be ejected, by causing one of the detents  34 ,  35  to be removed from the cassette  5 , as described in conjunction with  FIGS. 1 and 2 . 
     FIG. 5  shows an extremely simple embodiment of a lock mechanism  37  of the invention. This lock mechanism  37  has only one arm  39  tensioned with a spring  38  and having a detent  41 , whereby the arm  39  and the spring  38  are let into the floor  7  of the tunnel. A part of the arm  39  is arranged in a cavity  40  in order to stabilize the arm  39 . The lock mechanism  37  also has a second detent  42  that is displaceable perpendicular to the longitudinal direction of the arm  39 . When a cassette  5  is loaded, the detent  42  is brought into a position that is indicated with dot-dash contours of the detent  42 . The cassette  5  can now be pushed into the slot  4  for the subject table 1 ( FIG. 1 ) and, as in conjunction with exemplary embodiments that have already been described, pushes the arm  39  until the cassette  5  has reached a position between the detent  41  and the detent  42  (which has now sprung back) to engage a narrow side of the cassette  5 . Given a lock mechanism of this type, the cassette  5  can be pushed out from only one slot opening. When the detent  42  is removed from the cassette  5 , the spring power of the detent  41  pushes the cassette  5  from that slot opening at which the detent  42  is located. 
   Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.
A subject table for a mammography apparatus has a slot for an X-ray film cassette extending therethrough, as well as a locking mechanism for the cassette arranged in the slot. The locking mechanism has a spring-loaded arm carrying a first detent, and a second detent, at least one of the detents being spring-loaded such that the detents lie tightly against the narrow sides of the cassette when a cassette is introduced into the slot. For simplified removal or replacement of a cassette, the arm is displaceable in its longitudinal direction, and the second detent is displaceable perpendicularly to the longitudinal direction of the arm. When the second detent is removed from the cassette by an operating manipulation, the spring loading of the first detent causes the cassette to be ejected from the slot to a sufficient extent to allow it to be grasped by the operator.