Patent Publication Number: US-2016242981-A1

Title: Device for supporting and positioning a patient in a medical equipment

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of prior European Patent Application No. 15156383.0, filed on Feb. 24, 2015, and Luxembourg Patent Application No. 92662, filed on Feb. 24, 2015, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a device for positioning a patient in a medical equipment, in particular a radiation therapy equipment. 
     BACKGROUND 
     A device for positioning a patient in a medical equipment is also called a “patient positioning system (PPS)”. In a radiation therapy equipment, the patient positioning system has to warrant a very precise positioning and orientation of the patient relative to the radiation therapy equipment. Therefore, a modern patient positioning system may include three degrees of freedom for positioning a patient support table in space (generally two degrees of freedom parallel to a horizontal plane, and one degree of freedom parallel to a vertical plane), and three further degrees of freedom for orientation of the patient support table in space (generally three rotational degrees of freedom allowing a back and forward tilting, a top rotation and a rolling movement of the patient support table). All these degrees of freedom are principally motorized using a drive unit with a very high reduction ratio to achieve a slow motorized motion (for safety reasons) and a very precise positioning. 
     In the final irradiation position, the patient to be irradiated is sandwiched between an irradiation nozzle and the patient support table supported by the patient positioning system. There are numerous situations in which it is required to bring the patient rapidly out of this “sandwiched position”, for example, if the patient suddenly suffers a seizure, a respiratory failure or any other problem, or simply for temporarily allowing better access to the patient or to a specific body part of the patient, or if there is any technical failure in the medical equipment or in the patient positioning system. Using the motorized degrees of freedom patient positioning system for this purpose has the disadvantage of being slow, and this is problematic if there is a failure in the patient positioning system itself. Furthermore, if an emergency stop button is pushed when the patient is in the afore-described “sandwiched position”, then all motorized movements are principally disabled, and the patient will remain blocked in this “sandwiched position” until the system gets restarted. 
     To release the patient manually in such situations, most prior art patient positioning systems provide the possibility to manually actuate the drive unit of at least one motorized degree of freedom of the patient positioning system with a dedicated tool, for example a special crank lever. However, because of the very high reduction ratio in the drive unit, this manual actuation of the drive unit is very slow. For example, in some prior art patient positioning systems, more than a thousand rotations of a crank lever are required to manually release a patient. Furthermore, the operators have to be trained to be capable of efficiently using such a dedicated tool for manually releasing the patient, and the dedicated tool must be immediately at hand. Additionally, a manual actuation of the drive unit will generally require a new axis zeroing of the patient positioning system, before being able to reuse the patient positioning system in normal operation. Finally, because of the rather complicated mechanics and kinematics of the robotic wrist, it may be very complicated to act on the latter for releasing the patient out of its sandwiched therapy position 
     In view of the drawbacks in prior art systems, an object of the present disclosure is to provide in a device for supporting and positioning a patient in a medical equipment, a solution for manually actuating at least one of its motorized degree of freedom. 
     SUMMARY 
     Embodiments of the present disclosure provide a device for supporting and positioning a patient in a medical equipment. This device comprises a patient support unit and a positioning mechanism supporting the patient support unit. The positioning mechanism comprises at least one motorized rotary joint member for positioning the patient support unit using a motorized pivoting motion about a pivot axis. In accordance with a first aspect of the disclosure, a rotational release unit is associated with the motorized rotary joint member. This rotational release unit comprises an override bearing and a rotation locking mechanism cooperating with the override bearing. The override bearing is arranged adjacent to or integrated in the in the rotary joint member, so as to be substantially coaxial with the pivot axis, and to allow a free pivoting motion of the positioning mechanism about the pivot axis. The rotation locking mechanism is switchable between a locked state, in which it locks the override bearing in a mechanically defined angular position, and an unlocked state, in which the override bearing is unlocked and the positioning mechanism can freely pivot about the pivot axis, i.e. an operator can manually pivot it about the pivot axis. As the axis of the override bearing is substantially coaxial to the pivot axis of the motorized rotary joint member, the operator has the impression that the motorized rotary joint member can freely rotate about its pivot axis, despite the fact that it is virtually blocked because of a high reduction ratio in its drive unit. Thus, it is possible to pivot the positioning mechanism manually out of a preset angular position, allowing, for example, a better access to the patient or to a specific body part of the patient and to pivot it, thereafter, manually back into the pre-set angular position. It will further be appreciated that a new axis zeroing of the positioning mechanism is not required. 
     In an exemplary embodiment of this device, the override bearing rotatably interconnects a first flange and a second flange, and the rotation locking mechanism is supported by the first flange and includes a locking member. In the locked state of the rotation locking mechanism, the locking member engages the second flange in the mechanically defined angular position, so as to warrant a form-locked transmission of a torque between the two flanges. This form-locked engagement between the locking member and the second flange in the mechanically defined angular position takes place at a radial distance D from the pivot axis, wherein this radial distance D is preferably &gt;100 mm, or &gt;200 mm. It will be appreciated that the greater the distance D is, the better the angular repositioning accuracy is. In the unlocked state of the rotation locking mechanism, the locking member is disengaged from the second flange, so as to allow a free relative rotation between the first flange and the second flange. This embodiment allows good repositioning accuracy after a temporary release of the patient. 
     The locking member may include a locking pin, which is capable of engaging a recess in the second flange in the mechanically defined angular position, so as to warrant a form-locked transmission of a torque between the two flanges. The locking pin may be a tapered locking pin received in a tapered guide hole. Such a tapered system provides an auto-centring function, which may be limited to the direction of the rotational degree of freedom to be blocked. 
     To reduce friction between the locking pin and the second flange, the pin may have a front surface that has the form of a spherical-dome and/or may be coated with a friction reducing material. Alternatively, the front surface of the locking pin includes a rolling ball, to achieve a rolling contact between this front surface and the second flange. 
     An exemplary embodiment of the rotation locking mechanism has to be powered to switch into the locked state and, if it is unpowered, switches back into the unlocked state, under the action of a passive element, for example a resilient element such as a spring. Thus, a patient may be rapidly released even if no power is available. 
     A detector may be mounted in the recess to detect that the locking pin is in proper engagement with the recess. Such a detector allows to detect prior to the unlocking of the release unit that such unlocking may take place, thereby providing a buffer time to take precautionary measures, such as for example cutting off the medical equipment, before the patient is released. 
     The switching of the rotation locking mechanism from the locked state into the unlocked state may be triggered by simultaneously pushing two release buttons, so that an operator has to use both hands to trigger this switching. 
     An exemplary embodiment of the rotation locking mechanism includes a linear drive for driving a locking member in a locking position. This linear drive is for example electrically, hydraulically or pneumatically powered and may include a passive element, for example a resilient element such as a spring, for urging the locking member out of the locking position, if the linear drive is unpowered. 
     An exemplary embodiment of the rotation locking mechanism includes a pneumatic cylinder and a control valve. The pneumatic cylinder includes a cylinder chamber, a piston, a piston rod and a return spring, the return spring retracting the piston rod into the cylinder chamber when the latter is vented. The control valve is connected to the cylinder chamber. When the control valve is powered, it connects the cylinder chamber to a pressure source. When the control valve is unpowered, it vents the cylinder chamber. 
     In an exemplary embodiment, the rotational release unit is arranged adjacent to the rotary joint member. If the device for supporting and positioning a patient further comprises a support base for the positioning mechanism, the rotational release unit may for example be arranged directly between the support base and the motorized rotary joint member. If the positioning mechanism comprises a support member pivotably supported by the motorized rotary joint member, or a support member pivotably supporting the motorized rotary joint member, then the rotational release unit may be arranged between the motorized rotary joint member and the support member. 
     In an exemplary embodiment, the rotational release unit is arranged in the rotary joint member, for example in a drive unit of the latter. For example, if the motorized rotary joint member includes an annular drive gear that is coaxial with the pivot axis, and a motor unit with a pinion meshing with the annular drive gear for motorizing the rotary joint member, the annular drive gear may be supported by the override bearing of the rotational release unit. Alternatively, the motor unit motorizing the rotary joint member may be supported by the override bearing of the rotational release unit. 
     If the positioning mechanism comprises two motorized rotary joint members defining two substantially vertical pivot axes, a rotational release unit as defined herein may be associated with each of the two motorized rotary joint members. 
     If the motorized rotary joint member has a substantially horizontal pivot axis, a damper or brake may be associated with the positioning mechanism for slowing down a gravity caused pivoting motion of the motorized rotary joint member, when the rotation locking mechanism of the rotational release unit is switched from the locked state into the unlocked state. This damper or brake may be integrated into the rotational release unit, so as to only become effective if the rotational release unit is switched from the locked state into the unlocked state. 
     In an exemplary embodiment, the positioning mechanism is a robotic arm, and the device further includes: an orientation mechanism with at least two motorized rotational degrees of freedom, the orientation mechanism being borne by the robotic arm and bearing the patient support unit; and an translational release unit connected between the orientation mechanism and the patient support unit. This translational release unit may include an XY translation mechanism providing two translational degrees of freedom and a translation locking mechanism cooperating with the XY translation mechanism. This translation locking mechanism is switchable between a locked state, in which it locks the two translational degrees of freedom of the XY translation mechanism in a mechanically defined position, and an unlocked state, in which the two translational degrees of freedom are unlocked. In the unlocked state, this translational release allows a rapid manual release of the patient, simply by pulling and pushing, whereas the preset orientation of the orientation mechanism is not affected. It follows that the initial position and orientation of the patient support unit may be re-established by bringing the XY translation mechanism back into its mechanically defined position. 
     The translation locking mechanism may provide in its locked state, a form-locked locking of the XY translation mechanism in a mechanically defined position, for example, by using a locking pin for each of said two translational degrees of freedom. 
     Embodiments of present disclosure provide a device for supporting and positioning a patient in a medical equipment, comprising: a patient support unit; a robotic arm supporting the patient support unit; and an orientation mechanism with at least two motorized rotational degrees of freedom, the orientation mechanism coupling the robotic arm to the patient support unit. A translational release unit is connected between the orientation mechanism and the patient support unit. This translational release unit includes: an XY translation mechanism providing two translational degrees of freedom; and a translation locking mechanism cooperating with the XY translation mechanism. The translation locking mechanism is switchable between a locked state, in which it locks the two translational degrees of freedom of the XY translation mechanism in a mechanically defined position, and an unlocked state, in which the two translational degrees of freedom are unlocked. In the unlocked state of the translation locking mechanism, the translational release unit allows to manually release the patient by simply pulling and/or pushing directly on the patient support unit. The at least two motorized rotational degrees of freedom of the orientation mechanism remain unaffected, so that the operator is exclusively confronted with a translational movement for freeing the patient. With this system, it becomes for example possible to manually push and/or pull the patient support unit temporarily in a position allowing better access to the patient or to a specific body part of the patient, and to push and/or pull it, thereafter, manually back into its therapy position, which corresponds to the mechanically defined position of the XY translation mechanism in its locked state. A new axis zeroing of the orientation mechanism is generally not required after such an operation. 
     Each degree of freedom is for example embodied by a linear stage, comprising a platform and a base, which are joined by a linear guide or bearing element, in such a way that the platform is restricted to guided linear motion with respect to the base. 
     In an exemplary embodiment, each stage comprises a separate translation locking mechanism. 
     In an exemplary embodiment, the translation locking mechanism comprises a locking pin providing in its locked state a form-locked locking in said mechanically defined position. The locking pin may be a tapered pin, which is capable of engaging a tapered guide hole, so as to provide, in said mechanically defined position, an auto-centering function in the direction of the translational degree of freedom to be blocked. 
     In an exemplary embodiment, the rotation locking mechanism has to be powered to switch into the locked state and, if it is unpowered, it switches into the unlocked state. Thus it becomes possible to release the patient even if there is no power for operating the rotation locking mechanism. 
     The translation locking mechanism may include a linear drive for driving a locking member in a locking position. The linear drive is electrically, hydraulically or pneumatically powered, and includes a passive element, such as a spring, for urging the locking member out of the locking position, if the linear drive is unpowered. 
     An exemplary embodiment of the rotation locking mechanism includes a pneumatic cylinder with a cylinder chamber, a piston, a piston rod and a return spring. The return spring retracts the piston rod into the cylinder chamber when the latter is vented. A control valve is connected to the cylinder chamber. This control valve connects the cylinder chamber to a pressure source when the control valve is powered, and vents the cylinder chamber when the control valve is unpowered. 
     The switching of the translation locking mechanism from the locked state into the unlocked state may be triggered by simultaneously pushing two release buttons, which may be arranged so that an operator has to use two hands to trigger this switching. 
     In an exemplary embodiment, the orientation mechanism includes three motorized rotational degrees of freedom, for controlling: a pitch angle, which allows a backward and forward tilting of the patient support table; a top rotation angle, which allows a planar swiveling of the patient support table; and a roll angle, which allows a side-to-side pivoting of the patient support table. In this case, the two translational degrees of freedom of the translation release unit are parallel to a plane that is perpendicular to the axis of the top rotation angle. 
     The XY translation mechanism may be centered on the axis of the top rotation angle, when it is in its locked state. With regard to its centered position, the XY translation mechanism provides a degree of freedom of +/−x according to the X-axis and of +/−y according to the Y axis, wherein the absolute values of x and y are both in the range of 300 mm to 800 mm. 
     In an exemplary embodiment, the robotic arm includes at least one motorized rotary joint member for positioning the patient support unit using a motorized pivoting motion about a pivot axis. A rotational release unit is in this case may be associated with the motorized rotary joint member. This rotational release unit comprises: an override bearing arranged adjacent to or in the in the motorized rotary joint member so as to be substantially coaxial with the pivot axis, and to allow a free pivoting motion of the positioning mechanism about the pivot axis; and a rotation locking mechanism cooperating with the override bearing, the rotation locking mechanism being switchable between a locked state, in which it locks the override bearing in a mechanically defined angular position, and an unlocked state, in which the override bearing is unlocked and the positioning mechanism can freely pivot about the pivot axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The afore-described and other features, aspects and advantages of the present disclosure will be described with hereafter with reference to the figures, wherein: 
         FIG. 1  is a schematic elevation view of an exemplary device for supporting and positioning a patient in a medical equipment. 
         FIG. 2  is a schematic section of an exemplary embodiment of a motorized rotary joint member with an associated rotational release unit. 
         FIG. 3  is a schematic section of an exemplary embodiment of a motorized rotary joint member with an associated rotational release unit. 
         FIG. 4  is a schematic section of an exemplary embodiment of a motorized rotary joint member with an associated rotational release unit. 
         FIG. 5  is a schematic section of an exemplary embodiment of a motorized rotary joint member with an associated rotational release unit. 
         FIG. 6  is perspective view of an exemplary XY-translational release unit. 
         FIG. 7A  is a schematic diagram illustrating an exemplary locking mechanism of the release unit in a locked state. 
         FIG. 7B  is a schematic diagram illustrating the locking mechanism of the release unit of  FIG. 7A  in a unlocked state. 
         FIG. 8  is a schematic diagram showing an exemplary locking pin and a cooperating recess of the locking mechanism of the release unit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a device  10  for supporting and positioning a patient in a medical equipment, for example, a radio therapy equipment schematically represented by an irradiation nozzle  12 . It will however be appreciated that the device  10  can also be used for supporting and positioning a patient in other medical equipment. In general, a device in accordance with the present disclosure provides precise motorized positioning of a patient in a treatment position and permits bringing this patient rapidly out of this treatment position. 
     The device  10  shown in  FIG. 1  includes a patient support unit  14 , which is normally a patient support table, also called a patient couch, but may also be a treatment chair or the like. In  FIG. 1 , this patient support table  14  is located below the irradiation nozzle  12 . A patient to be irradiated (not shown) lies on the patient couch  14 , sandwiched between the irradiation nozzle  12  and the patient couch  14 . It will be noted that there are many situations in which it will be required to bring the patient rapidly out of this “sandwiched position”. 
     The patient support table  14  is supported by a positioning mechanism  16 , here a robotic arm, which is itself supported by a support base  18  on a floor  20 , in a pit or on any kind of external support structure. Here, the robotic arm  16  comprises three arm members  22   1 ,  22   2 ,  22   3  (generally referred to as support members). The first arm member  22   1  is connected to the support base  18  using a first motorized rotary joint member  24   1 , which allows a motorized pivoting motion of the first arm member  22   1  relatively to the support base  18  and about a first pivot axis  26   1 , which is substantially vertical. The second arm member  22   2  is connected to the first arm member  22   1  using a second motorized rotary joint member  24   2 , which allows a motorized pivoting motion of the second arm member  22   2  relatively to the first arm member  22   1  and about a second pivot axis  26   2 , which is substantially parallel to the first pivot axis  26   1  (i.e. the second pivot axis  26   2  is vertical too). The first arm member  22   1  and the second arm member  22   2  allow to adjust the horizontal X, Y coordinates of the patient support unit  14 . The third arm member  22   3  is connected to the second arm member  22   2  using a third motorized rotary joint member  24   3 , which allows a motorized pivoting motion of the third arm member  22   3  relative to the second arm member  22   2  and about a third pivot axis  26   3 , which is substantially horizontal. This third arm member  22   3  allows a raising or lowering of the patient support unit  14 , i.e. to adjust the vertical Z-coordinate of the patient support unit  14 . Alternatively, the robotic arm includes for example, a vertical translational degree of freedom, for adjusting the vertical Z-coordinate of the patient support unit  14 , and two rotational degrees of freedom about two parallel vertical axis, for adjusting the horizontal X, Y coordinates of the patient support unit  14 . 
     The robotic arm  16  supports the patient support table  14  using an orientation mechanism  28 , which is also called a “robotic wrist”. This orientation mechanism  28  allows to adjust the orientation of the patient support table  14  according to three rotational degrees of freedom, which are called: pitch angle  30  (allowing a back and forward tilting of the patient support table  14 ), top rotation angle  32  (allowing a planar swiveling of the patient support table  14 ), and roll angle  34  (allowing a side to side pivoting of the patient support table  14 ). 
       FIG. 2  schematically illustrates the mechanical layout of an exemplary embodiment of a motorized rotary joint member  24  with a rotational release unit  36 . The motorized rotary joint member  24  extends between a first flange  40  and a second flange  42 . The first flange  40  bears a spacing structure  44 . The second flange  42  is connected to the spacing structure  44  of the first flange  40  using a joint bearing  46 . The latter defines an axis of rotation forming the pivot axis  26  of the final motorized rotary joint member  24 , i.e. the axis about which a motorized pivoting motion of the support member  22  connect to the second flange  42  will take place. 
     Reference  48  identifies a tubular drive shaft, which is supported by the second flange  42 , and which supports an annular drive gear  50  coaxially with the pivot axis  26 . A motor unit  52  is fixed on the first flange  40  and includes a pinion  54 , which meshes with the annular drive gear  50  for pivoting the second flange  42  about the pivot axis  26 . If the pivoting motion is limited to an angle of less than 360°, the annular drive gear  50  too may be an annular drive gear segment of less than 360°. The motor unit  52  generally comprises an electric motor and a gearbox designed to achieve slow motorized pivoting motion and a very precise angular positioning. As a consequence of the very high reduction ratio, which is due to the gearbox and to the large diameter annular drive gear  50  (for example, a typical diameter of this drive gear would be in the range of 300 mm to 800 mm) cooperating with the relatively small diameter pinion  54 , it will be difficult to rotate by hand any arm member  22  connected to the second flange  42 . 
     Associated with the motorized rotary joint member  24  is a rotational release unit  36 . The latter mainly comprises an override bearing  60  and a rotation locking mechanism  64  cooperating with the override bearing  60 . The override bearing  60  is arranged axially adjacent to the rotary joint member  24 , so as to be substantially coaxial with the pivot axis  26 . In  FIG. 2 , the override bearing  60  is connected between an auxiliary flange  66  and the first flange  40  of the motorized rotary joint member  24 . 
     The rotation locking mechanism  64  is switchable between a locked state, in which it locks the override bearing  60  in rotation in a mechanically defined angular position, and an unlocked state, in which the override bearing  60  is unlocked, so that the first flange  40  can freely rotate relative to the auxiliary flange  66 . In  FIG. 2 , this rotation locking mechanism  64  is supported by the auxiliary flange  66  and includes a locking pin  68 . In the locked state, which is shown in  FIG. 2 , the locking pin  68  is engaged with a corresponding recess  70  in the first flange  40 , so as to warrant a form-locked transmission of a torque between the auxiliary flange  66  and the first flange  40  in the mechanically defined angular position. In the unlocked state, the locking pin  68  is disengaged from the first flange  40 , so as to allow a free relative rotation between the first flange  40  and the auxiliary flange  66 . As the axis of the override bearing  60  is substantially coaxial to the axis of the joint bearing  46 , the operator has the impression that the motorized rotary joint member  24  can now freely rotate about its pivot axis  26 , despite the fact that it is blocked because of the afore-mentioned high reduction ratio in the drive unit  50 ,  52 . It will be noted that the amplitude of free pivot movement is normally limited by limit stops to an angle of less than +/−180° measured from the mechanically defined angular position. 
     In the robotic arm  16 , the auxiliary flange  66  is for example connected to the support base  18  or to an arm member  22 . The second flange  42  is connected to another arm member  22 . If the motor unit  52  is stopped and locked prior to switching the rotational release unit  36  into its unlocked state, the arm member  22  connected to the second flange  42  can be manually pivoted out of a specific angular position, and can thereafter be easily brought back into said specific angular position, by manually pivoting it back, until the locking pin  68  engages again with the recess  70  in the first flange  40 . Thus it is possible to pivot the arm member  22  manually out of a preset angular position, for example for allowing better access to the patient or to a specific body part of the patient, and then to pivot it manually back again into the pre-set angular position with great angular accuracy. It will be noted that the angular repositioning accuracy is better, the greater the distance D between the locking pin  68  and the pivot axis  26  is. Assuming for example that this distance D is 300 mm, a play of 0.05 mm of the locking pin  68  in the recess  70  results in an angular play of less than 0.01°. The distance D will preferably be greater than 150 mm. 
     If the embodiment of  FIG. 2  is used for the rotary joint member  24   1  in  FIG. 1 , the auxiliary flange  66  is connected to the support base  18 , and the second flange  42  is connected to the first arm member  22   1 . The rotational release unit  36  is thus arranged between the support base  18  and the motorized rotary joint member  24 . If the rotation locking mechanism  64  is switched into its unlocked state, the first arm member  22   1 , can be freely rotated by hand about the pivot axis  26   1 . 
     If the embodiment of  FIG. 2  is for example used for the rotary joint member  24   2  in  FIG. 1 , the auxiliary flange  66  may be connected to the first arm member  22   1 , and the second flange  42  is connected to the second arm member  22   2 . The rotational release unit  36  is thus arranged between the first arm member  22   1  and the motorized rotary joint member  24 . If the rotation locking mechanism  64  is switched into its unlocked state, the second arm member  22   2 , can be freely rotated by hand about the pivot axis  26   2 . 
       FIG. 3  schematically illustrates an exemplary motorized rotary joint member  24  associated with a rotational release unit  36 ′, comprising an override bearing  60 ′ and an auxiliary flange  66 ′. The motorized rotary joint member  24  is identical to that of  FIG. 2 . Here, the override bearing  60 ′ now connects the auxiliary flange  66 ′ to the second flange  42  of the motorized rotary joint member  24 . The joint bearing  46  and the override bearing  60  are located very closely together, which provides constructional advantages in many cases. As in  FIG. 2 , as the axis of the override bearing  60 ′ is substantially coaxial to the axis of the joint bearing  46 ′, one has the impression that—in the unlocked state of the rotational release unit  36 ′—the motorized rotary joint member  24  can freely rotate about its pivot axis  26 , despite the fact that it is indeed blocked because of the aforementioned high reduction ratio in the drive unit. It will be noted that the embodiment of  FIG. 3  also warrants a similar repositioning accuracy as the embodiment of  FIG. 2 . 
     If the embodiment of  FIG. 3  is used for the rotary joint member  24   1  in  FIG. 1 , the auxiliary flange  66 ′ is connected to the second arm member  22   2 , and the first flange  40  is connected to the support base  18 . If it is used for the rotary joint member  24   2 , the auxiliary flange  66 ′ is connected to the second arm member  22   2 , and the first flange  40  is connected to the first arm member  22   1 . 
       FIG. 4  shows an exemplary embodiment of a motorized rotary joint member  24 ″ associated with a rotational release unit  36 ″, which is now integrated in the motorized rotary joint member  24 ″. More particularly, the rotational release unit  36 ″ is mounted between the second flange  42 ″ and the annular drive gear  50 ″. The override bearing  60 ″ of the rotational release unit  36 ″ is for example, mounted on a flange  80 ″ that is fixed to the second flange  42 ″, so that the axis of rotation of the override bearing  60 ″ is coaxial to the joint bearing  46 ″. The tubular drive shaft  48 ″ bearing the annular drive gear  50 ″ comprises a flange  82 ″, by means of which it is supported by the override bearing  60 ″. The rotational release unit  36 ″ further comprises a rotation locking mechanism  64 ″ that is mounted for example, on a flange  84 ″ of the tubular drive shaft  48 ″ (alternatively, the rotation locking mechanism  64 ″ can also be mounted on the flange  80 ″ fixed to the second flange  42 ″). It follows, that if the rotation locking mechanism  64 ″ is in its locked state, it locks the tubular drive shaft  48 ″ with the annular drive gear  50 ″ in rotation relatively to the second flange  42 ″, so that the motor unit  52 ″ can rotate the second flange  42 ″ about the pivot axis  26 ″. If the rotation locking mechanism  64 ″ is switched into its unlocked state, the second flange  42 ″ can freely rotate about the coaxial axes of the override bearing  60 ″ and the joint bearing  46 ″, whereas the annular drive gear  50 ″ is blocked by the motor unit  52 ″. 
     The first flange  40 ″ is for example, connected to the support base  18  or to an arm member  22 . The second flange  42 ″ is generally connected to another arm member  22 . If the motor unit  52 ″ is stopped and locked prior to switching the rotational release unit  36 ″ into its unlocked state, the arm member  22  connected to the second flange  42 ″ can be manually pivoted about the pivot axis  26 ″ out of a specific angular position, and can thereafter be easily brought back into said specific angular position, by manually pivoting it back, until the locking pin engages again with the recess in the flange  80 ″. Consequently, after a temporary unlocking of the release unit  36 ″, the embodiment of  FIG. 4  achieves substantially the same repositioning accuracy as the embodiments of  FIGS. 2 and 3 . 
       FIG. 5  shows an exemplary embodiment of a motorized rotary joint member  24 ′″ associated with a rotational release unit  36 ′″, which is also integrated in the motorized rotary joint member  24 ′″. More particularly, the rotational release unit  36 ′″ now includes an override bearing  60 ′″ that is supported on the first flange  40 ′″ and that supports the motor unit  52 ′″ via a motor support flange  86 ′″. It follows that, if the rotation locking mechanism  64 ′″ is switched into is unlocked state, the second flange  42 ′″ can be freely rotated, together with the annular drive gear  50 ′″, the motor unit  52 ′″ (whose pinion  54 ′″ is blocked in rotation) and the motor support flange  86 ′″. The first flange  40 ′″ will however remain unaffected by this manual rotation of the second flange  42 ′″. Also the embodiment of  FIG. 5  achieves, after a temporary release, substantially the same repositioning accuracy as the embodiments of  FIGS. 2 and 3 . 
     The override bearings  60 ″ and  60 ′″ may generally be less expensive than the override bearings  60  and  60 ′, because the load constraints are less demanding. Indeed, whereas the override bearings  60  and  60 ′ have to be dimensioned essentially for the same loads as the joint bearing  46 , the override bearing  60 ″ in  FIG. 4  has to support only the tubular drive shaft  48 ″ with the annular drive gear  50 ″, and the override bearing  60 ′″ in  FIG. 5  has to support only the motor unit  52 ′″. However, the embodiments of  FIGS. 4 and 5  require a relatively precise alignment of the axes of rotation of the override bearing  60 ″,  60 ′″ with the joint bearing  46 ″,  46 ′″, whereas in the embodiments of  FIGS. 2 and 3 , there are no such precise alignment constraints for the axes of rotation of the override bearing  60 ,  60 ′ with the joint bearing  46 ,  46 ′. In the embodiments of  FIGS. 2 and 3 , alignment constraints for these axes of rotation are only imposed by the design of the rotary joints in the outer casing of the robotic arm  16 . Consequently, in the embodiments of  FIGS. 2 and 3 , alignment constraints for the axes of rotation of the joint bearing and the override bearing may be reduced and even be entirely eliminated by an adequate design of the rotary joints in the outer casing of the robotic arm  16 . 
     The joint bearings  36 ,  36 ′,  36 ″,  36 ′″ and the override bearings will normally be rolling contact bearings  60 ,  60 ′,  60 ″,  60 ′″ selected in function of the specific construction and operating conditions. It will further be understood that the mechanical structures shown in  FIG. 2-5  have been simplified to better show the basic concepts underlying the present invention. In practice, the motorized rotary joint member  24 ,  24 ″,  24 ′″ will for example contain more than one joint bearing  46 ,  46 ″,  46 ′″. Furthermore, the arrangement and mounting of the bearings  46 ,  46 ″,  46 ′″ has to be properly designed, duly considering design loads, bearing torques, dimensions and materials, required alignment and rotation precision etc. The same applies to the rotational release units  36 ,  36 ′,  36 ″,  36 ′″ and to the override bearings  60 ,  60 ′,  60 ″,  60 ′″. 
       FIG. 6  shows an exemplary translational release unit  90  connected between the orientation mechanism  28  (the robotic wrist  28 ) and the patient support table  14 . The translational release unit  90  basically comprises an XY translation mechanism providing two translational degrees of freedom. Each degree of freedom is for example embodied by a linear stage  94 ,  96 , comprising in a known manner a platform and a base, joined by some form of guide or linear bearing, in such a way that the platform is restricted to guided linear motion with respect to the base. The platform of the linear stage  94  supports the base of the linear stage  96 , and the platform of the linear stage  96  supports the patient support table  14 , so as to form two translational degrees of freedom that are perpendicular to one another. 
     The patient support table  14  is borne by the XY translation mechanism, so that its X-axis extends parallel to the length of the patient support table  14 , and its Y-axis extends parallel to the width of the patient support table  14 . Both linear stages  94 ,  96  may be free-moving, i.e. they do not include any mechanism or motor for moving the platform relative to the base. Movement of the patient support table  14  is achieved by manually pushing or pulling the patient support table  14 . 
     A translation locking mechanism (not seen in  FIG. 6 ) cooperates with the XY translation mechanism, wherein it is switchable between a locked state, in which it locks the two linear stages  94 ,  96 , and an unlocked state, in which the two linear stages  94 , and  96  are unlocked (see also the description of  FIGS. 7A, 7B and 8 ). If the two linear stages  94  and  96  are unlocked, they allow a free planar translation movement of the patient support table  14  parallel to a plane that is perpendicular to the axis  32 ′ of the top rotation angle  32  of the orientation mechanism. Accordingly, an operator may push or pull the patient support table  14  according to any direction perpendicular to the axis  32 ′ of the top rotation angle  30 . When the two linear stages  94 ,  96  are locked, they are both centred, preferably in a form-locked manner, on the axis  32 ′ of the top rotation angle  32 . With regard to this centred position, the XY translation mechanism provides a degree of freedom of +/−x according to the X-axis and of +/−y according to the Y-axis, wherein the absolute values of x and y are preferably in the range of 300 mm to 800 mm. Each linear stage  94 ,  96  may include a damper or brake for slowing down a gravity caused motion of the patient support unit, if the translation locking mechanism is switched from is locked state in its unlocked state. 
       FIGS. 7A and 7B  are schematic diagrams further illustrating an exemplary locking mechanism  100  that may be used for a rotational release unit  36  or a translational release unit  90  as described hereinbefore.  FIG. 7A  shows the locking mechanism  100  in its locked status, and  FIG. 7B  in its unlocked status. This locking mechanism  100  is mounted between two flanges  102  and  104 , which are mechanically interconnected either by a rotating bearing element, in case of a rotational release unit, or by a linear bearing element, in case of a translational release unit. In  FIGS. 7A and 7B , this rotating bearing element or linear bearing element is schematically represented by a crossed box  105 , which generically stands for a relative movement bearing element. 
     The locking mechanism  100  shown in  FIGS. 7A and 7B  comprises a linear actuator  106  bearing a locking pin  108 . In the locked status, the locking pin  108  engages a recess  110  in the second flange  104 , thereby locking the two flanges  102  and  104  in rotation or in translation, to warrant a form-locked transmission of a torque or a force between them. 
     The linear actuator  106  shown in  FIGS. 7A and 7B  may be a pneumatic cylinder, including a cylinder chamber  112 , a piston rod  114  bearing the locking pin  108 , and a return spring  118 . The return spring  118  retracts the piston rod  114  into the cylinder chamber  112 , when the latter is vented. Pressurizing the cylinder chamber  112  moves the piston rod  114  out of the cylinder chamber  112  and compresses the return spring  118 . The pneumatic cylinder  106  is controlled by a control valve  122 , schematically represented by a conventional graphic symbol. This control valve  122  comprises for example at least three ports and two valve positions. In the first valve position (shown in  FIG. 7B ), the first port is closed and the second port is internally connected to the third port. In the second valve position (shown in  FIG. 7A ), the first port is internally connected to the third port, and the second port is closed. A valve spring  124  urges the valve  122  into its first position, i.e. the rest position. A valve actuator  126  urges, if powered, the valve  122  into the second position. The valve actuator  126  may be connected to an uninterruptible power supply (not shown), i.e. a power supply with battery backup. When the connection between the valve actuator  126  and the uninterruptible power supply is interrupted, for example by pushing a release button (or alternatively two release buttons mounted in parallel), the valve spring  124  urges the valve  122  into its first position. 
     Externally, the first port of the valve  122  is connected to a pressurized air source  120 , the second port is vented (i.e. connected to atmosphere) and the third port is connected to the cylinder chamber  112 . Consequently, when the valve  122  is in the first position (see  FIG. 7B ), the cylinder chamber  112  is vented, and when the valve  122  is in the second position (see  FIG. 7A ), the cylinder chamber  112  is pressurized. 
     Instead of using such a pneumatic cylinder as actuator for the locking pin  108 , one may also use a linear drive that is hydraulically or electrically powered. Furthermore, instead of using a linear actuator  106  with a locking pin  108  axially engaged into a recess  110 , one may also use a pivoting mechanism that is capable of pivoting a locking member, between a locked-position, in which it engages a cooperating locking element on the second flange  104 , to provide a form-locked force transmission in the direction of relative movement of the two flanges  102 ,  104 . The pneumatic cylinder  106  (or possibly another linear drive), the axially actuated locking pin  108  and the recess  110  provide a relatively simple, cost effective and reliable solution. 
     With respect to the XY translation mechanism, each linear stage  94 ,  96  may have its own locking mechanism  100 . For example, the linear actuator  106  is fixed to an element of the base (which forms the first flange  102 ) and the locking pin  108  engages a recess in an element of the platform (which forms the second flange  104 ). 
     As long as the linear actuator  106  is powered, the locking pin  108  remains in the recess  110 , providing a form-locked coupling between the two flanges  102  and  104 . If the linear actuator  106  is unpowered, the return spring  118  (or another passive element) withdraws the locking pin  108  from the recess  110 , thereby opening the coupling between the two flanges  102  and  104 . 
     To re-establish a form-locked coupling between the two flanges  102  and  104 , the linear actuator  106  is powered (i.e. the pneumatic cylinder is for example pressurized) to press the locking pin  108  with a front surface  132  against the surface of the second flange  104  into which the recess  110  opens.  FIG. 8  shows the locking pin  108  in this position (the linear actuator  106  itself is not shown in  FIG. 8 , but his action is indicated by an arrow). By manually moving the flange  104  relative to the flange  102  in the direction of the arrow  128 , the recess  110  can be brought in alignment with the locking pin  108 . To reduce friction between the front surface  132  of the locking pin  108  and the second flange  104 , this front surface  132  may have the form of a spherical dome and/or may be coated with a friction reducing material. Alternatively, the front surface  132  of the locking pin  108  may also include a rolling ball, to achieve a rolling contact between the front surface  132  of the locking pin  108  and the second flange  104 . The second flange  104  is provided with contact path having a surface quality adapted for a sliding contact, respectively a rolling contact with the front surface  132 . When the locking pin  108  is aligned with the recess  110 , the linear actuator  106  presses the locking pin  108  into the recess  110 . To facilitate alignment of the locking pin  108  and the recess  110 , the recess  110  may have a cone-shaped opening, as shown in  FIG. 8 . 
     The first flange  102  may only have to bear the linear actuator  106 . It may consequently have a relatively small extension in the direction of the relative movement of the two flanges  102 ,  104 . The second flange  104  may have to bear the recess for receiving  108  the locking pin  108  and form the (circular or linear) contact path for the front surface  132  of the locking pin  108 . Its minimum extension in the direction of the relative movement of the two flanges  102 ,  104  is consequently determined by the length of this contact path, i.e. the extent of free relative movement the rotational release unit  36  or the translational release unit  90  shall provide. 
     In case of a rotational movement, the flange  104  does not have to be a planar annular flange (as shown in the drawings) or an angular segment of such a planar annular flange. It may also be a cylindrical flange or a segment of such a cylindrical flange. In case of a cylindrical flange  104 , the longitudinal axis of the locking pin  108  will be perpendicular to the axis of the rotational movement. (In the embodiments shown in the drawings, the longitudinal axis of the locking pin  108  is parallel to the axis of the rotational movement). 
     Reference number  134  points to a detector that is mounted in the recess  110  to detect that the locking pin  108  is in proper engagement with the recess  110 . This detector  134  may for example be a pressure sensitive switch that is capable of monitoring an axial contact pressure of the locking pin  108  in the recess  110 . A decrease of this axial contact pressure below a pre-set pressure may then trigger an alarm and/or be incorporated a security interlocking system of the positioning device  10  and/or of the medical equipment. Monitoring the axial contact pressure of the locking pin  108  in the recess  110  allows detection, prior to the unlocking of the release unit, that such unlocking may take place. 
     As further seen in  FIG. 8 , the locking pin  108  (or the piston rod  114  shown in  FIGS. 7A &amp; 7B ) may be guided (at least perpendicularly to the direction of movement that has to be locked) in a guide bushing  130  of the first flange  102 , to avoid actuator  106  being subjected to forces, when the locking pin  108  transfers a torque or a force from the first flange  102  to the second flange  104 . 
     The fit between the locking pin  108  and the recess  110  in the direction of the movement that has to be locked (i.e.: in case of a rotational movement locking, the direction tangential to the trajectory of the locking pin  108 ; and in case of a linear movement locking, the direction parallel to the respective X-axis or Y-axis) will strongly influence the positional accuracy of the repositioning. Consequently, whereas the fit between the locking pin  108  and the recess  110  in the direction of movement shall be relatively small (e.g. smaller than 1 mm, and preferably smaller than 0.1 mm), there may be an important clearance in the direction perpendicular to force transmission (i.e. in  FIG. 8 , in the direction perpendicular to the sheet). This important clearance perpendicular to force transmission makes the introduction of the locking pin  108  into the recess  110  easier. 
     Instead of using a cylindrical locking pin  108  (as shown in  FIG. 8 ), it is also possible to use a tapered locking pin received in a tapered guide hole (similar to a machine tapers used for securing cutting bits and other accessories to a machine tool&#39;s spindle, as for example a so-called Morse taper system or another known taper system). Such a taper system may provide an auto-centring function, wherein it is generally preferable to limit the auto-centring function in the direction of the rotational or translational degree of freedom to be blocked. 
     The switching of the rotation or translation locking mechanism from the locked state into the unlocked state may take place according to the “two hands principle”, i.e. the operator has to use both hands to simultaneously push two release buttons to trigger this switching. These release buttons may be arranged close to the rotational release unit, respectively close to the translational release unit with whom they are associated. Alternatively or additionally, the device may include release buttons simultaneously releasing all motorized rotational degrees of freedom, or simultaneously releasing all motorized rotational degrees of freedom with a vertical pivot axis. 
     LIST OF REFERENCE NUMERALS 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                  10 
                 device for supporting and positioning a patient 
               
               
                   
                  12 
                 nozzle of medical equipment 
               
               
                   
                  14 
                 patient support unit 
               
               
                   
                  16 
                 robotic arm (positioning mechanism) 
               
               
                   
                  18 
                 support base 
               
               
                   
                  20 
                 floor 
               
               
                   
                  22 
                 support member (arm member); 
               
               
                   
                  24 
                 motorized rotary joint member 
               
               
                   
                  26 
                 pivot axis 
               
               
                   
                  28 
                 orientation mechanism (robotic wrist) 
               
               
                   
                  30 
                 pitch angle 
               
               
                   
                  32 
                 top rotation angle 
               
               
                   
                  34 
                 roll angle 
               
               
                   
                  36 
                 rotational release unit 
               
               
                   
                  40 
                 first flange of 24 
               
               
                   
                  42 
                 second flange of 24 
               
               
                   
                  44 
                 spacing structure 
               
               
                   
                  46 
                 joint bearing 
               
               
                   
                  48 
                 tubular drive shaft 
               
               
                   
                  50 
                 annular drive gear 
               
               
                   
                  52 
                 motor unit 
               
               
                   
                  54 
                 pinion 
               
               
                   
                  60 
                 override bearing 
               
               
                   
                  64 
                 rotation locking mechanism 
               
               
                   
                  66 
                 auxiliary flange 
               
               
                   
                  68 
                 locking member or pin 
               
               
                   
                  70 
                 recess 
               
               
                   
                  80″ 
                 flange of 36″ 
               
               
                   
                  82″ 
                 flange of 36″ 
               
               
                   
                  84″ 
                 flange of 36″ 
               
               
                   
                  90 
                 translational release unit 
               
               
                   
                  94 
                 linear stage (X-axis) 
               
               
                   
                  96 
                 linear stage (Y-axis) 
               
               
                   
                 100 
                 locking element 
               
               
                   
                 102 
                 first flange of 100 
               
               
                   
                 104 
                 second flange of 100 
               
               
                   
                 105 
                 relative movement bearing e 
               
               
                   
                 106 
                 linear actuator/pneumatic cylinder 
               
               
                   
                 108 
                 locking pin 
               
               
                   
                 110 
                 recess 
               
               
                   
                 112 
                 cylinder chamber 
               
               
                   
                 114 
                 piston rod 
               
               
                   
                 118 
                 spring 
               
               
                   
                 122 
                 control valve 
               
               
                   
                 120 
                 pressurized air source 
               
               
                   
                 124 
                 valve spring 
               
               
                   
                 126 
                 valve actuator 
               
               
                   
                 128 
                 arrow, indicating the direction of movement 
               
               
                   
                 130 
                 guide bushing 
               
               
                   
                 132 
                 front surface of 108 
               
               
                   
                 134 
                 detector