Abstract:
An x-ray C-arm device has a C-arm rotatable around an orbital axis proceeding perpendicular to the plane of the C-arm. The C-arm carries an x-ray source and a radiation detector, and the overall center of gravity of the C-arm and the components carried thereby exerts a first torque on the C-arm. A counterbalancing device generates a second torque that at least partially compensates the first torque. The counterbalancing device includes a counterweight that is displaceably coupled to the C-arm by a gearing arrangement.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention concerns a C-arm apparatus.  
         [0003]     2. Description of the Prior Art  
         [0004]     C-arm apparatuses are prevalent today in medical technology. A diagnosis or treatment device is mounted on a C-shaped base body. Due to its shape, the C-arm (and with it the diagnosis or treatment device) can move orbitally around a point of a patient to be examined or treated in order to reach various angle positions between patient and diagnosis or treatment device without having to reposition the patient.  
         [0005]     X-ray devices in which an x-ray source is mounted at one end of the C-arm and an x-ray receiver or image intensifier is mounted at the opposite end are prevalent as diagnosis apparatuses. Such an x-ray C-arm exhibits a not-insignificant dead weight.  
         [0006]     If it is ensured in a C-arm apparatus that, given orbital travel, the diagnosis or treatment device is aligned on the same point at every angle position, this is known as an isocentric C-arm apparatus. Most notably in x-ray C-arms designed in such a way, in which the central ray of the x-ray system proceeds through the isocenter of the arrangement situated on the orbital axis (rotation axis of the orbital motion), the overall center of gravity of the arrangement naturally lies outside of the isocenter (thus radially removed from the orbital axis) due to the weight ratios. The dead weight of the overall arrangement therefore effects a torque on the C-arm. The center of gravity of the arrangement namely gravitates towards its stable equilibrium position, thus the lowest point below the orbital axis that can be reached via the orbital movement.  
         [0007]     Force must thus be applied counter to the intrinsic angular momentum to hold the C-arm in a specific position or given movement. For example, the C-arm must be fixed in a specific position via a suitable braking device at the support device.  
         [0008]     However, it is desirable to achieve a weight compensation at the C-arm such that the C-arm is free of force at every travel position, meaning that no torque whatsoever relative to the rotation axis acts on the C-arm. A number of approaches have previously been pursued in order to effect a weight compensation.  
         [0009]     A first approach is to place the x-ray source and the image intensifier such that the overall center of gravity of C-arm and x-ray device lies on the rotation axis. Due to the heavy x-ray components, as compensation for the weight of the C-arm these must be further offset towards its ends. The central ray of the x-ray system then no longer proceeds through the isocenter of the arrangement, which requires a continuous re-placement of the patient region or of the entire patient to be treated by movement of the C-arm.  
         [0010]     In a second approach the x-ray system is placed such that its central ray passes through the isocenter. Supplementary weights are additionally attached at the C-arm ends in order to again displace the overall center of gravity of the arrangement into the isocenter. However, the heavy supplementary weights significantly increase the total weight of the arrangement and mechanically load the C-arm such that it exhibits an inherent deformation.  
         [0011]     A third approach is to act on the C-arm with brakes and an electrical motor drive such that the torque generated by gravity from the center of gravity of the C-arm is compensated by the electrical drive and the brakes. However, it is hereby a disadvantage that the C-arm requires electrical current for movement. Given a power failure a dangerous situation for the patient could occur since, for example, no access space to said patient can be achieved by movement of the C-arm.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to provide a C-arm apparatus in which the equilibration is improved.  
         [0013]     The object is achieved by a C-arm apparatus, in particular an x-ray C-arm apparatus, with a C-arm that can move around an orbital axis proceeding perpendicular to the C-arm plane. Auxiliary components, in particular an x-ray system including an x-ray source and an image intensifier, are mounted on the C-arm. The overall center of gravity of C-arm and auxiliary components exerts a first torque on the C-arm. The C-arm apparatus includes a compensation device for generation of a second torque that at least partially compensates the first torque. The compensation device includes a counterweight that is displaceably coupled with the C-arm via a gearing arrangement.  
         [0014]     Due to the at least partial compensation of the first torque by the second torque, a smaller overall torque generated by gravity acts on the C-arm. Less force is thereby necessary for orbital movement of the C-arm and less retention force via a brake is necessary to secure the C-arm in a specific position. The C-arm is inherently stable (thus weight-compensated) in rotational positions in which first torque and second torque are equal and opposite. For example, a base position can be defined to which the C-arm returns due to gravity as long as no external force is exerted on it, for example with a brake arresting the C-arm is released. If the second torque counteracts the first torque insofar as that the remaining torques are only slight, the C-arm can be effortlessly moved by hand. The contrary torque (second torque) is that generated solely by gravitation acting on the counterweight. No energy feed whatsoever to the C-arm apparatus is thus necessary for the weight compensation. In particular a motor drive for weight compensation at the C-arm is not necessary; the C-arm can thus be moved without power. To increase the operating comfort a motor drive can naturally be provided on the C-arm, such a motor drive, for example, acting on said C-arm in a frictionally-engaged manner but which does not limit the operability of the C-arm given a power failure since it can be brought out of engagement without impairing the movement capability of the C-arm. The compensation device including the counterweight is not mounted on the C-arm itself, which is why the C-arm&#39;s own weight is not increased. The masses to be moved given an orbital travel of the C-arm thus remain as small as possible. A wide range of mechanical embodiments in the form of levers, gearings, cable pulls or shafts that allow a transfer or torques are possible for the compensation device or the movement coupling between counterweight and C-arm.  
         [0015]     In a preferred embodiment of the invention the counterweight is supported such that it can rotate around a rotation axis so that an angle change at the C-arm effects the same angle change at the counterweight. Due to the orbital path of the overall center of gravity of C-arm and auxiliary components around the orbital axis, the intrinsic angular momentum of the C-arm possesses a torque likewise cosine-dependent on its rotation angle. If the counterweight can likewise rotate around a rotation axis, this thus generates a torque that is likewise cosine-dependent on its rotation angle. If the movement coupling between counterweight and C-arm is not executed in a ratio of 1:1, meaning that an angle change at the C-arm effects the same angle change on the counterweight, the cosine-dependencies of both torques are the same. It can thus be achieved that the second torque exerted on the C-arm is always equal in magnitude to the first in the opposite direction; the C-arm is thus weight compensated in every orbital position, i.e. completely weight compensated. Due to the 1:1 translation no variable lifting arms are necessary for this at the counterweight; the construction is simplified. The C-arm is free of forces at every orbital position. A fixing brake for secure arresting of the C-arm must exert only slight force. A slight friction force (for example in the gearing arrangement) on the C-arm or on the counterweight is sufficient that the C-arm stably remains at each position even without additional braking.  
         [0016]     If the counterweight can be moved in the orbital plane (C-arm plane) of the C-arm, a space-saving design of the overall system can be achieved that barely requires an overhang laterally outside of the orbital plane. The mechanical force transfer between the counterweight and C-arm can occur in a simple manner since no angular deflection is required between the movement of the C-arm and that of the counterweight.  
         [0017]     In a further embodiment of the invention the C-arm can be moved around an angulation axis intersecting the orbital axis at right angles. Due to its overall center of gravity, a further torque caused by gravity acts on the C-arm relative to the angulation axis via this additional degree of freedom for the C-arm movement and the auxiliary components. This can also be compensated by the compensation device and the counterweight. This causes the C-arm also to be completely weight-compensated with regard to its angulation axis or to return at least in part to a base position with inherent stability, from which base position it can be moved again with a slight displacement force. The angular weight compensation can be achieved by rotation of C-arm and counterweight in the same or opposite direction around the angulation axis.  
         [0018]     In addition to the orbital weight compensation, the angular weight compensation is particularly simple to realize when the C-arm is movably supported on a support device that includes the compensation device. If the compensation device is introduced directly onto or into the support device, short paths for force transfer and thus a smaller designed space of the overall system result. Given a design that is rigid relative to the angulation axis, the counterweight is, for example, automatically panned as well when the C-arm is panned. A separate mechanism for the angular weight compensation is still not necessary once. Various degrees, up to the complete weight compensation can be realized by suitable dimensioning of the mass and the movement path of the counterweight or its distances relative to the angular rotation axis and orbital rotation axis.  
         [0019]     The support device can include the compensation device with a housing. The compensation device and support device can thus be accommodated in a housing together with the counterweight. A compact C-arm apparatus thus is achieved with a gearing arrangement that is protected from dust, can emit no detritus and allows a simple cleaning and disinfection of the entire C-arm apparatus in a sterile region, for example the treatment room of a hospital. The moving parts of the C-arm are protected from contact via the housing, to the risk of damage to the operating personnel is significantly limited.  
         [0020]     The C-arm and counterweight can be coupled via a multi-stage gearwheel gearing arrangement mounted on the support device and including components arranged parallel to the orbital plane. The moment translation between the overall center of gravity and counterweight is 1:1. A gearwheel gearing arrangement is designed mechanically very simple and robust. Via the gearwheels, pinions or other gearing arrangement parts arranged in parallel, a planar design of the gearing arrangement is possible relative to the orbital plane. The 1:1 translation is easily achievable due to the multi-stage nature of the gearwheel gearing arrangement and various increases or decreases of gear ratio, so a degree of freedom with regard to mass and lever arm of the counterweight arises given an at least two-stage gearwheel gearing arrangement. For example, for weight reduction of the overall system the counterweight can thus amount to half of the total mass of C-arm and auxiliary components, but act with a lever arm that is twice as long as the lever arm with which the total mass acts on the orbital axis. The torques also then completely cancel each other.  
         [0021]     The gearing arrangement is particularly space-saving the output part of the gearwheel gearing arrangement is an extension arm having an end at which the counterweight is mounted and that exhibits an internal gearing. A nesting of the gearing arrangement and thus the smallest possible design space can be achieved via the internal gearing. Given a level gearing arrangement created by the extension arm, a fine adjustment of the second torque is possible by, for example, a fine adjustment of the length of the extension arm and therewith of the lever arm for the counterweight. The second torque thus can be adjusted such that it is exactly, oppositely equal to the first torque. Should components be exchanged during the lifespan of the C-arm apparatus, the second moment can be adapted to the new weight ratios in the system.  
         [0022]     In a further embodiment of the invention, the gearing arrangement and counterweight include a cavity in which is arranged a cable drum for accommodation of a supply cable for the C-arm. The weight compensation and the cabling of the C-arm (thus of all moving parts, connection cables, hoses etc. of the C-arm) are thus inaccessible from the outside. Hooking or twisting of moving parts in the surroundings of the C-arm thus is prevented. The risk of injury by moving parts is minimized for operating personnel. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a side view of an x-ray C-arm with weight compensation in 90° orbital position and 0° angular position.  
         [0024]      FIG. 2  is a section through the C-arm from  FIG. 1  in the viewing direction of the arrow II.  
         [0025]      FIG. 3  is a mass model of the angularly-moved C-arm from  FIG. 1 .  
         [0026]      FIG. 4  shows the C-arm from  FIG. 1  in 0° orbital position in a representation according to  FIG. 1 .  
         [0027]      FIG. 5  is a mass model of the angularly-moved C-arm from  FIG. 4  in a representation according to  FIG. 3 .  
         [0028]      FIG. 6  shows an alternative embodiment of a support device with a compensation device in a representation according to  FIG. 1 .  
         [0029]      FIG. 7  is a section through the device from  FIG. 6  in the viewing direction of the arrow VII. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]      FIG. 1  shows an x-ray C-arm apparatus  2  having a C-arm  6  supporting the x-ray system  4  and a support device  8  for the C-arm  6 . The stand of the C-arm apparatus  2  supporting the overall arrangement on an axle  10  is not shown.  
         [0031]     The x-ray system  4  has an x-ray source  12  and an x-ray receiver or image intensifier  14 . The central ray  16  of an x-ray cone (not shown) emitted by the x-ray source  12  centrally leaves the x-ray source  12  and centrally strikes the image intensifier  14 .  
         [0032]     The C-arm  6  is supported such that it can move orbitally on a roller bearing  18  which is attached in a fixed manner at the support device  8 . The movement direction of the C-arm  6  on the bearing device  8  is represented by the double arrow  20 . Given such a movement C-arm  6  and x-ray system  4  describe orbital movements around an orbital axis  22  perpendicularly piercing the plane of the drawing in  FIG. 1 . Orbital axis  22  and central ray  16  intersect in the isocenter  24 . In  FIG. 1  the C-arm  6  is situated in the 90° position, meaning that the central ray  16  encompasses an angle  28  of 90° with an angulation axis  26  running horizontally and passing centrally through the bearing axle  10 . Given travel in the direction  20  the C-arm  6  slides along on the rollers  32  of the roller bearing  18  on an orbital contact surface  30  attached to the C-arm  6 .  
         [0033]     In addition to the orbital movement of the C-arm  6  relative to the support device  8 , C-arm  6 , bearing device  8  and the axle  10  attached thereon can be panned around the angulation axis  26  in the direction of the arrow  36  in a journal bearing belonging to the stand (not shown) of the C-arm apparatus  2 . In any orbital and angular panning position of the C-arm apparatus  2 , orbital axis  22  and central ray  16  intersect at right angles and penetrate the isocenter  24  (which is stationary) as long as the stand foot of the C-arm  2  is stationary.  
         [0034]     The total mass of C-arm  6  and x-ray system  4  at its overall center of gravity  38  is symbolically represented as a virtual total mass  40  with mass M. The force of gravity  42  acting on the total mass  40  effects a torque  46  on the C-arm  6  relative to the orbital axis  22  via the virtual lever arm  44  of the length L extending from the isocenter  24  to the overall center of gravity  38 . The torque  46  in  FIG. 1  is T=M·L. If the C-arm  6  is orbitally panned from the position shown in  FIG. 1 , the torque cosinusoidally decreases with the corresponding rotation angle since the force of gravity  42  no longer acts at a right angle to the lever arm  44 .  
         [0035]     A compensation device  9  is comprised in the support device  8  (see also  FIG. 2  in this regard). The compensation device  9  has a gearwheel  48 , two parallel extension arms  50  and a counterweight  52  connecting the extension arms  50  at the ends in a U-shape. The gearwheel  48  has a shaft  54  supported on the housing  56  of the support device  8 , a crown gear  58  centrally attached on the shaft  54  and two pinions  60  attached near the shaft ends. The teeth (situated radially outwards) of the crown gear  58  engage teeth  104  permanently attached on the contact surface  30 . The extension arms  50  with their ends situated opposite the counterweight  52  are fastened on shafts  62  parallel to the shaft  54  and orbital axis  22 , which shafts  62  are supported on the housing  56  such that they can rotate. C-shaped recesses  64  are present near the shafts  62  and concentric to these. The radially outer edges of the recess  64  are provided with inner gearings  66  in which the pinions  60  engage. The counterweight  52  exhibiting the mass 2M is fastened on the free ends of the extension arms  50 . The virtual lever arm  68  of the counterweight  52  amounts to L/2 relative to the shafts  62 , such that via the force of gravity  74  this generates a torque  76  of T=2M·L/2=M·L which is equal in magnitude to the torque acting on the C-arm and exhibits the identical cosine dependency of the rotation angle.  
         [0036]     By the movement coupling between teeth  104 , crown gear  58 , pinion  60 , inner teeth  66  and extension arm  50 , an orbital travel of the C-arm  6  in the direction  20  effects an orbital panning of the counterweight  52  around the shaft  62  in the direction of the arrow  69 . The gearing is selected such that an angle change of the angle  28  effects the same (according to amount) angle change of the angle  70  between center longitudinal axis of the extension arms  50  and the perpendicular  72 . Moreover, in  FIG. 1  the gearing arrangement (thus the compensation device  9 ) is adjusted such that a 90° position of the angle  28  corresponds to a 90° position of the angle  70 .  
         [0037]     Without application of a counterforce, i.e., without the compensation device  9  coupled to the gearing  104 , due to gravity the C-arm  6  would slide downwards in the direction of the arrow  49  into the support device  8  until the angle  28  is 0° and the overall center of gravity  38  finds a stable equilibrium position below the isocenter  24  in the direction of gravity.  
         [0038]     Relative to the orbital axis  22 , the compensation device generates the equal and opposite torque on the C-arm  6 . The torque  76  is transferred to the gearwheel  56  via the inner teeth  66  and the pinion  60  and via this gearwheel  56  to the C-arm  6  via the teeth  104  of the contact surface  30  and thus counteracts the torque  46 . In  FIG. 1  the dimensions of the gear components are matched to one another such that this translation from torque  76  to torque  46  amounts to one to one. The two torques are equal and opposite and, in fact for each movement angle of the C-arm  6  cancel to zero on the orbital axis  22 . The C-arm  6  is thus completely weight-compensated with regard to the orbital movement  20  and remains free of torque at every rotation position.  
         [0039]     All components of the support device  8  and of the compensation device  9  of course have mass. If one excludes the counterweight  52  (strictly speaking together with the likewise movable extension arms  50  which also alter the center of gravity upon movement—for simplicity in the following only the counterweight  52  is discussed) from this consideration, support device  8  and compensation device  9  together have a virtual total mass  78  at the overall center of gravity  80 . Since the support device  8  and compensation device  9  can be moved synchronously with the C-arm  6  around the angulation axis  26 , the total mass  78  effects a torque around the angulation axis  26 , the virtual lever arm  86  being the radial distance of the overall center of gravity  80  from the angulation axis  26 .  
         [0040]      FIG. 2  shows the C-arm apparatus  2  from  FIG. 1  in the direction of the arrow II in the section at the level of the shaft  62 . The C-arm  6  exhibits an approximately semi-circular hollow section cavity, whereby its wall  100  flattens on the side facing the bearing device  8  and is recessed in a central region  102 . There the wall  100  is directed towards the inside of the C-arm  6  and forms the contact surface  30  on which the rollers  32  borne on the housing  56  run. The teeth  104  in which the teeth of the crown gear  58  engage are centrally affixed on the contact surface  30 . The crown gear  58  is mounted in a fixed manner on the shaft  54  together with the pinions  60 , whereby the shaft  54  is supported at one end in the housing  56  such that it can rotate. The extension arms  50  supporting the counterweight  52  are likewise supported in the housing  56  via the shafts  62  such that they can rotate. The inner gearing  66  on the recess  64  engages both pinions  60 . In the housing  56  a receptacle  106  for rotatable bearing of the bearing axle  10  (not shown in  FIG. 2 ) is provided at the end of the bearing device  8  situated opposite the C-arm  8 .  
         [0041]     The crown gear  58  runs approximately centrally in the housing  56 , whereby the extension arms  50  and pinions  60  are situated near the inner border of the housing  56 . Free spaces  108  thus arise for acceptance of additional components (not shown) of the C-arm apparatus  2  such as cable guides, drive motors or the like. The mechanically-stable connection between the extension arms  50  is achieved via the counterweight  52 . A cover plate  110  seals the inner chamber of the housing  56  from the receptacle  106 .  
         [0042]      FIG. 3  shows the plan view of the mass and lever ratios of the C-arm apparatus  2  from  FIG. 1  in the direction of the arrow III. However, the C-arm in the shown 90° orbital position is additionally angularly tilted by approximately 45° relative to the perpendicular  72  in the counter-clockwise direction around the angulation axis  26 .  
         [0043]      FIG. 3  is a schematic drawing in which the entire arrangement of the counterweight  52  is shown as a single real component. All other real components of the C-arm  6  and of the x-ray system  4  are represented by their virtual total mass  40 . All real components of the support device  8  and of the compensation device  9  with the exception of the counterweight  52  are represented by their virtual total mass  78 .  
         [0044]     Due to the angular tilting orbital axis  22  and central ray  16  are likewise tilted by approximately 45° relative to  FIG. 1 . The angulation axis  26  furthermore runs horizontal and perpendicularly penetrates the plane of the drawing in  FIG. 2 . Orbital axis  22 , central ray  16  and angulation axis  26  furthermore intersect in the isocenter  24 .  
         [0045]     In the orbital 90° position of the C-arm  6 , the total mass  40  with its overall center of gravity  38  lies on the angulation axis  26  and exerts no torque relative to this. Angular torques are generated only by the gravitational forces  84  and  74  acting on the total mass  78  and on the counterweight  52  with the lever arms  86  and  88  with regard to the angulation axis  26 . The lever arm  88  is hereby the radial distance of the center of gravity  53  of the counterweight  52  from the angulation axis  26  (which, due to the 90° position (angle  70 ) of the counterweight here corresponds to the distance of the shaft  62  from the angulation axis  26 ). The lever arm  86  is the radial distance of the overall center of gravity  80  from the angulation axis  26 .  
         [0046]     For the following torque consideration, the cosine dependency of the torques on the angulation angle (which here is always the same for all considered values) is omitted for simplicity.  
         [0047]     Due to the size of the total mass  78  of M/2 and the lever arm  86  of the length H, the total mass  78  effects a torque T=M/2·H=M·H/2. The lever arm  88  is therefore dimensioned at H/4. The counterweight  52  thus generates a torque T=2M·H/4=M·H/2 and compensates so the torque of the total mass  78  amounts to zero. All torques with regard to the angulation axis  26  are thus compensated and the C-arm apparatus  2  is also weight-compensated with regard to this axis. It remains free of forces at every arbitrary angulation position.  
         [0048]      FIG. 4  shows the C-arm apparatus  2  from  FIG. 1  with C-arm  6  panned downwards by 90°, thus in the 0° position. The central ray  16  then coincides with the angulation axis  26  so that the angle  28  amounts to 0°. The overall center of gravity  38  is located in the direction of gravity, thus vertically below the isocenter  24 , which is why the C-arm  6  also assumes a stable equilibrium position without contrary torque. The compensation device  8  likewise exerts no torque on the C-arm  6 . The counterweight  52  is namely located in an unstable equilibrium position relative to the shaft  62 , meaning the angle  70  amounts to 180° relative to the perpendicular  72 . The extension arms  50  are thus pivoted upwards by 90° relative to  FIG. 1 .  
         [0049]     The extension arm  50  is for the most part executed massively and possesses a not-insignificant weight which is likewise to be taken into account as a compensation mass for the weight compensation. Strictly speaking, as already mentioned above the extension arm  50  thus counts towards the total weight  52  given weight considerations. The remaining gearing arrangement parts are designed such that the position of the overall center of gravity  80  does not change in relation to  FIG. 1  given their movement since the counterweight  52  (together with the extension arm  50 ) is itself excluded from this consideration.  
         [0050]     Given movement of the C-arm  6  from the position according to  FIG. 1  into the position according to  FIG. 4 , the C-arm  6  performs a 90° pan. Due to the engagement of the gearing  104  in the crown gear  58 , the gearwheel  4  hereby performs approximately one and a half rotations in the direction of the arrow  90  due to the gearing arrangement ratio between the two effective radii of the gearings (radial distance of the gearing  104  from the orbital axis  22  at the diameter of the crown gear  58 ). The pinion  60  passes through the same angle change as the crown gear  58 . Due to the interaction of the pinion  60  with the inner gearing  66  and the gear reduction associated with this (diameter of the pinion  60  at radial distance of the inner gearing  66  from the shaft  62 ), the extension arm  50  passes through the same 90° angle change as the C-arm  6 , meaning that the overall gearing arrangement ratio of the gearing arrangement is 1:1. At arbitrary intermediate positions between  FIG. 3  and  FIG. 4 , the cosine dependencies of the torques generated by the total mass  40  and the counterweight  52  are therefore equal, which is why the C-arm  6  is weight-compensated at every arbitrary orbital angle position, even beyond those cases shown in Figures.  
         [0051]      FIG. 5  shows the C-arm apparatus  2  from  FIG. 3  in the 0° orbital position in the direction of the arrow V, but angularly tilted by approximately 60° in the counter-clockwise direction in contrast to  FIG. 3 . As in  FIG. 3  (with the exception of the counterweight  52 ), again only the virtual total masses  40  and  78  of the arrangement are shown. The total mass  78  with unchanged distance relative to the angulation axis  26  hereby again generates the same torque T=M/2·H=M·H/2 with the omission of the cosine dependencies. Since the center of gravity  28  now no longer lies on the angulation axis  26 , an additional torque with lever arm  92  also arises relative to the angulation axis  26  due to the total mass  40 . The lever arm  92  corresponds to the radial distance of the center of gravity  38  from the angulation axis  26 , which in the shown example corresponds to equal to the lever arm  44  from  FIG. 1 . The additional torque T=M·L herewith arises. T=M·H/2+M·L thus acts relative to the angulation axis  26  due to the masses  78  and  40 .  
         [0052]     Since the counterweight  52  is now moved on its extension arm  50 , its lever arm now amounts (relative to the angulation axis  26 ) to the sum of the distance  88  of the shaft  62  from the angulation axis  26  and the lever arm  68 , namely the distance of the counterweight  52  from the shaft  62 . The contrary torque due to the counterweight  52  is accordingly T=2M·(H/4+L/2)=M·H/2+M·L, which precisely corresponds to the sum of the other two torques.  
         [0053]     Due to the mass and length ratios in the C-arm apparatus  2 , this is thus completely weight-compensated in every orbital and angular position.  
         [0054]      FIG. 6  shows an alternative embodiment for the bearing device  8  with compensation device  9 . Instead of the teeth  104 , a toothed belt  112  is directed on the contact surface  30  of the C-arm  6 . This toothed belt  112  is permanently connected with the C-arm  6  at the end of the C-arm  6  that is not visible in  FIG. 6  and lies on the contact surface  30  on nearly the entire C-arm length. Only in a region  116  situated between deflection rollers  114   a  and  114   b  is the toothed belt  112  directed away from the contact surface  30 . It runs from the deflection roller  114   a  over a cable drum  118   a  and a deflection roller  114   c  back to the deflection roller  114   b  on the C-arm  6 . The cable drum  118   a  comprises a circumferential-side gearing  120   a  in which the toothed belt  112  engages. Two pinions  124  are attached near the shaft ends on a shaft  122  passing through the cable drum  118   a  and borne on the housing  56 . These pinions  124  respectively engage in the gearing of two crown gears  128  born on a shaft  126  near the shaft ends. Another extension arm  132  that supports a counterweight  130  on its free end is attached at one end to the shaft  126 .  
         [0055]     Feed lines (not shown) are wound on the cable drums  118   b , which feed lines lead from the cable drum  118   a  along the C-arm  6  to the image intensifier  14  (not visible in  FIG. 6 ) and from the cable drum  118   b  to the x-ray source  12  (not visible).  
         [0056]     Given an orbital travel of the C-arm  6  on the support device  8  in the direction of the arrow  49 , the toothed belt  112  is directed over the roller arrangement described above in the direction of the arrow  134  and hereby displaces the cable drum  118   a  into rotation in the direction of the arrow  136 . The cable drum  119   a  winds the feed cable approaching from the image intensifier  14  in the direction  49 . The roller  118   a  displaces the cable drum  118  into rotation in the direction  138 , which in turn unwinds the feed cable (not shown) and releases it in the direction  49  of the x-ray source  12 . Simultaneously with the cable drum  118   b , the cable drum  118   a  displaces the gearwheel  128  into motion in the direction  140  via the pinion  124 . The extension arm  132  and the counterweight  130  are hereby simultaneously panned around the axis  126 .  
         [0057]     The dimensioning of the mass and lever ratios is executed corresponding to the embodiment according to  FIG. 1  through  FIG. 5 . Merely altered values of total mass  78  and position of the center of gravity  80  (and therewith of the lever arm  86 ) of the support device  8  and compensation device  9  would lead to a different measurement for the lever arm  88 , thus to a different placement of the shaft  62 . The gearing arrangement ratio of the individual gear speeds is again tuned such that an angle change given orbital travel of the C-arm  6  effects the same angle change of the counterweight  103 . The moment ratios in  FIG. 6  thus correspond to those in  FIG. 1 . The mass of the counterweight  52  of 2M is merely split up into two partial masses of M each of the two counterweights  130 .  
         [0058]      FIG. 7  shows the arrangement from  FIG. 6  in the viewing direction of the arrow VII in the section above the shaft  126 . In contrast to the exemplary embodiment according to  FIG. 5 , essentially the cable drums  118   a  and  118   b  are located in the inside  108  of the bearing device  8 . The pinions  124  connected with the cable drum  118   a  are mounted on both sides outside of the bearing device  8 , as well as the gearwheels  128 , extension arms  132  and counterweights  130 . The synchronous movement of the counterweights  130  is ensured via the rigid connection through the axis  126 . A disadvantage of the embodiment according to  FIG. 1  through  FIG. 5  is that the feed lines (not shown) running in part in the surroundings of the C-arm apparatus  2  interfere and are accident-prone since these easily get caught or twist. Instead of panning outside on the C-arm apparatus  2 , the counterweights  130  in the embodiment according to  FIG. 6  and  FIG. 7  are significantly less disruptive and can, if applicable, be housed in an additional housing (not shown) entirely surrounding the compensation device  9 , whereby a C-arm apparatus  2  entirely closed from the outside results in turn.  
         [0059]     Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventor to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of his contribution to the art.