Abstract:
Provided herein are devices and methods for supporting and positioning a tracking assembly for mounting variously configured medical devices for prostate imaging, biopsy, and other therapeutic applications. In one aspect, a support assembly provides multiple degrees of freedom for positioning a tracking assembly and mounted probe relative to a patient on a patient support structure such as an examination table or a gurney bed and/or maintaining a tracking assembly and mounted probe in a desired location throughout the rendering of imaging, biopsy, or other therapeutic procedures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/077,017, entitled “Linear Positioning Apparatus,” and having a filing date of Jun. 30, 2008 and also claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/078,164, entitled “Tracking Device to Assist 3-D and 4-D Image Guidance,” and having a filing date of Jul. 3, 2008, the entire contents of both of which are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is directed to systems and methods for positioning and locating a medical instrument relative to a patient. More particulary, the invention relates to an apparatus adapted to securely position a tracker assembly, which tracks 3-D locations of a supported medical device, relative to a patient. 
       BACKGROUND OF THE INVENTION 
       [0003]    According to the National Cancer Institute, an estimated 234,000 new cases of prostate cancer will be diagnosed in the United States alone this year. Of these cases, more than 27,000 deaths due to prostate cancer are expected to occur this year. Following skin cancer, statistics show that prostate cancer is the most common cancer among American men. 
         [0004]    Currently, medical professionals use a transrectal ultrasound-imaging device (TRUS) probe to acquire and guide prostate imaging and biopsy. The TRUS probe is the most widely accepted technique for prostate applications due to its simplicity, high specificity, and real time nature. In such an application, the TRUS probe may be inserted into the rectum of a patient to generate an image. Such images may be utilized to take one or more biopsies from a prostate location of interest and/or implant radioactive seeds at one or more desired locations in a brachy-therapy procedure. The TRUS probe may also be used in conjunction with other medical imaging applications, including cyrotherapy, photo-dynamic therapy, or a combination of these therapies and/or fusion-guided biopsies. With all of these applications, however, precise and repeatable TRUS probe placement and guidance is of utmost importance to achieve accurate imaging and rendering of the applicable therapy. 
         [0005]    Prior art describes a number of methods and devices for assistance in guiding and placing the TRUS probe for imaging, biopsy, and therapy. Generally, these prior art devices utilize several mounting, stepping, and rotating devices for various commercially available TRUS probes. These tracking assemblies are typically stand-alone devices that utilize a combination of linkages and tracking mechanisms for monitoring the spatial position of a supported probe. 
         [0006]    Such tracking assemblies are often not adapted for significant positional adjustment relative to the patient&#39;s body, as located on an examination table or a gurney bed. In order to “pre-fit” or “pre-position” such a tracking assembly to a patent, it has often been necessary to position the patient relative to the tracker assembly. In some instances, such limited tracker movement has impeded the accommodation of patients of varying heights and weights. 
         [0007]    While extending the range of motion of the tracker assembly would apparently alleviate such pre-positioning difficulties, such expanded range of movement raises other difficulties. For instance, many tracking assemblies include a series of rotatively coupled armatures that allow extending and retracting the tracking assembly in one or more dimensions. To expand the range of movement of such devices, the length of the armatures would have to be increased. Considering the limited space often available in medical examination rooms, the armatures on any given tracking assembly would be prohibitively long to accommodate the movement often necessary to preposition the tracking assembly relative to a patient. 
         [0008]    Beyond limited space constraints, increasing armature length to achieve necessary movement may introduce an unacceptable level of distortion into the spatial tracking of the tracking assembly. 
       SUMMARY OF THE INVENTION 
       [0009]    The inventors of the present systems have recognized that solutions for pre-positioning a tracker assembly need not concentrate on the tracking assemblies themselves. Instead, such solutions may focus on a support and positioning device that is adapted to pre-position an entire tracking assembly relative to a patient&#39;s body. Accordingly, various aspects of the present invention provide a support assembly for vertically and horizontally positioning a tracking assembly relative to a patient&#39;s body. As discussed above, such tracking assemblies may be utilized to position medical probes or instruments relative to patient tissue of interest. Such medical devices may include, without limitation, a TRUS probe, a biopsy needle, therapeutic devices, medical imaging devices, etc. 
         [0010]    The support assembly includes a base that supports a vertical tower. The vertical tower includes a linear positioning assembly. Interconnected to the linear positioning assembly is a horizontal linkage. The linear positioning assembly allows for positioning the horizontal linkage at a desired height. That is, the linear positioning assembly allows for positioning the linkage at a desired horizontal plane. The horizontal linkage includes at least first and second armatures that are connected by at least a first joint. A first end of the linkage interconnects to the linear positioning assembly and a second end of the linkage is a free end that provides a mounting surface for connection to a tracker assembly. The linkage allows for positioning its free distal end and a connected tracker within the horizontal plane. In this regard, after positioning a tracker at a desired height, the tracker may be advanced or retracted relative to a patient. Once a supported tracker is positioned in a desired location, the joint interconnecting the armatures of the linkage may be locked. In this regard, the supported tracker may be flexibly positioned at a desired location relative to a patient prior to performing a medical procedure on the patient. 
         [0011]    Various refinements exist of the features noted in relation to the subject aspect of the present invention. Further features may also be incorporated in the subject aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. For instance, the base element may be adapted for connecting the assembly to a patient support structure such as an examination table or a gurney bed. In another arrangement, the base element may be adapted for positioning on the floor. In such arrangement, the support assembly may be a freestanding unit. 
         [0012]    The horizontal linkage may further incorporate multiple armatures. In such an arrangement, each of the armatures may be interconnected to adjacent armatures utilizing locking joints. In any arrangement, the horizontal linkage is operative to move between a retracted position and an extended position, thus allowing an operator to move a tracking assembly connected to the linkage relative to a patient&#39;s body. 
         [0013]    The locking joints may be formed in an appropriate manner. Such joints may include single degree of freedom joints such as bushings, bearings, and hinges and/or multiple degree of freedom joints such as linear bearings, cylinders, rollers and ball/spherical joints. In one arrangement, single degree of freedom joints are utilized to maintain movement of the linkage within a common horizontal plane. In such an arrangement, two or more joints may permit movement about parallel axes. In any arrangement, it is desirable that the locking joints allow for fixedly positioning the distal end of the horizontal linkage. Such locking joints may be manually locked. Alternatively, the locking joints may be controllably actuated. For instance, the joints may be electromechanically, electromagnetically, hydraulically and/or pneumatically actuated. In one particular arrangement, a remote foot pedal may be utilized to actuate/lock the joints. In a further arrangement, each joint may be individually actuated. 
         [0014]    In one arrangement, the linear positioning assembly includes a passive biasing device to assist in the vertical movement of the horizontal linkage and supported tracker assembly. In such an arrangement, the biasing device may include, without limitation, springs and/or compressed gas cylinders. Such passive devices may assist a user of the support assembly in manually adjusting the height of tracker device connected thereto. 
         [0015]    In another arrangement, the linear positioning assembly may incorporate active biasing devices. Such active devices may include manually controlled devices such as cranks and pulleys as will as electromechanical actuators. In one arrangement, a linear actuator assembly is utilized. Such an assembly may be rodless or may utilize a threaded rod that may be controllably rotated relative to a threaded receptacle. Alternatively such actuators may be electromechanical, hydraulic or pneumatic. In such arrangements, the linear positioning assembly may be formed by a hydraulic/pneumatic cylinder. Further, it will be appreciated that the actuator could use a combination of these technologies. In one arrangement, the linear positioning assembly is controlled via remote switch to allow for hands-free vertical adjustment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a cross-sectional view of a transrectal ultrasound imaging probe as applied to perform prostate imaging, biopsy, or other therapy. 
           [0017]      FIG. 2  illustrates use of a tracking assembly to position an ultrasound imaging prove to perform prostate imaging, biopsy, or other therapy. 
           [0018]      FIG. 3  illustrates a perspective view of one embodiment of a support assembly connected to a patient support structure and interconnected with a tracking assembly. 
           [0019]      FIG. 4A  illustrates a perspective view of the support assembly shown in  FIG. 3  in a retracted position. 
           [0020]      FIG. 4B  illustrates a perspective view of the support assembly shown in  FIG. 3  in an extended position. 
           [0021]      FIG. 5  illustrates a side plan view of another embodiment of a support assembly adapted to be positioned on the floor. 
           [0022]      FIG. 6  illustrates an exploded view of a vertical tower assembly for use with the support assembly shown in  FIG. 5 . 
           [0023]      FIG. 7  illustrates an exploded view of a horizontal linkage for use with the support assembly shown in  FIG. 5   
           [0024]      FIG. 8A  illustrates a top plan view of the support assembly shown in  FIG. 5  in an extended position. 
           [0025]      FIG. 8B  illustrates a top plan view of the support assembly shown in  FIG. 5  in a retracted position. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of the present disclosure. This disclosure primarily describes the present invention in conjunction with transrectal ultrasound imaging for prostate imaging. That said, one of ordinary skill in the art will appreciate that aspects of the present invention may be applicable to other medical imaging applications. For example, other embodiments of the present invention may be used in conjunction with brachy-therapy, cyrotherapy, photo-dynamic therapy, a combination of these therapies, fusion-guided biopsies, as well as other imaging and therapy applications. In this regard, the following description is presented for purposes of illustration and description only. 
         [0027]    As shown in  FIGS. 1 and 2 , one embodiment of the present invention may be adapted to fixedly position a tracking assembly for mounting a trans-rectal ultrasound (TRUS) probe or biopsy needle for medical imaging, biopsy, or other therapy. The support assembly of the present invention may interface with a tracking assembly such that a supported TRUS probe achieves target scanning of an area of interest within the prostate. 
         [0028]      FIG. 1  illustrates a TRUS probe  10  being utilized to obtain a plurality of two-dimensional ultrasound images of the prostate  12 . As shown, the probe  10  may be operative to automatically scan an area of interest. In such an arrangement, a user may rotate the acquisition end  14  of probe  10  over an area of interest. Accordingly, probe  10  may acquire a plurality of individual images while being rotated over the area of interest. In any arrangement, probe  10  may also include a biopsy gun (not shown) that may be attached to probe  10 . Such a biopsy gun may include a spring driven needle that is operative to obtain a core from desired area within the prostate. 
         [0029]    For imaging purposes, it is desirable to minimize relative movement between the probe  10  and the prostrate  12  (i.e., wobble or any other rotational movement of the probe about a fixed axis for image acquisition). It is also often desirable for probe  10  to remain fixed relative to the prostrate  12  during biopsy or other treatment procedures such that the desired tissue locations may be targeted accurately. To achieve such fixed positioning of probe  105  it is often desirable to interface the probe  10  with a tracking assembly  100 , as shown in  FIG. 2 . Tracking assembly  100  maintains probe  10  in a fixed relative position to the prostate  12  as well as providing location information (e.g., frame of reference information) for use with an acquired image. In this regard, location outputs from tracking assembly  100  may be supplied to a computer and/or imaging device. Likewise, the output of the probe  10  may be provided to the computer and/or imaging device, and the computer and/or imaging device may utilize this information to produce an output (e.g., display) of imaged object (e.g., prostate). 
         [0030]    The present invention may be used to position and fixably support various tracking assemblies of the type discussed above. One exemplary device is set forth in International Application Number PCT/CA2007/0001076, entitled Apparatus for Guiding a Medical Tool. Another is set forth in U.S. application Ser. No. 11/850,482, entitled Tracker Holder Assembly, the contents of which are fully incorporated herein by reference. Specifically, apparatuses are described that may be utilized to securely position such tracking devices and probe assemblies relative a patient support surface such as a gurney bed. Further, the apparatuses allow for movement of a supported tracking assembly in one or more degrees of freedom (DOF) so that a probe/medical device supported by the tracking assembly can be positioned in a desired position relative to a patient. 
         [0031]    As previously discussed, oftentimes it is desirable to position a tracking assembly relative to a patient in a limited space environment such as a medical examination room, which may require a compact positioning/support assembly. The overall design and installation of one embodiment of such a compact support assembly  1 , as attached to a gurney bed, is illustrated in  FIG. 3 . As shown, the support assembly  1  is securely clamped on to a gurney bed using a moveable jaw  11 . A tracking assembly  100  is connected to the top of the support assembly. The support assembly  1  securely connects the tracking assembly  100  and supported probe to the gurney bed while allowing for selective positioning of the tracking assembly relative to the patient in one or more dimensions. 
         [0032]    Specifically, the support assembly allows for selectively positioning the tracking assembly with multiple degrees of freedom (DOF) prior to initiating a medical procedure. As shown, and more fully discussed herein, the design allows for longitudinal motion along the length of the gurney bed, transverse motion relative to the width of the gurney bed (e.g., towards and away from a patient) as well as vertical up and down motion. For instance, it may be desirable to adjust the elevation of the supported tracking assembly along a vertical axis of the support assembly such that a probe supported by the tracking assembly is disposed relative to a centerline of a patient on a support surface. Once in such a position, the limited movement of the tracker assembly is typically sufficient for any further vertical adjustment relative to the patient. Finally, the support assembly allows for rotational motion of the tracking assembly about a vertical axis. In this regard, the support assembly  1  provides four degrees of freedom for selectively positioning the tracker assembly. Once an operator positions the support assembly in a desired location, the support assembly may be locked in place and a medical procedure may be performed. 
         [0033]    In the embodiment illustrated in FIGS.  3  and  4 A-B, support assembly  1  comprises a clamp assembly  23  for securely mounting support assembly  1  at a desired location along an edge  15  of the patient support structure  13 , which defines a reference axis XX′. As shown, the clamp assembly  23  includes a body  27  having a top/base surface  25  and a movable jaw  11 . In use, the top surface of the body is positioned below a frame of patient support structure  13  and the jaw  11  is disposed on an opposing side of the frame. Prior to advancing the jaw  11  relative to the base surface of the clamp assembly, an operator may move support assembly  1  along axis XX′ to position the support assembly  1  relative to a patient. Once positioned at a desired location along the patient support structure  13 , the jaw may be advanced relative to the base surface  25  of the clamp assembly  23  to compress the frame between the jaw  11  and base surface  25 . The jaw  11  is adjusted vertically relative to the base surface  25  by releasing the mechanical friction lock  19 . Once the frame of the patient support structure  13  has been compressed between the jaw  11  and the base surface  25 , the friction lock  19  can be used to secure the clamping device at that particular position. 
         [0034]    Safety lock  21  ensures that support assembly  1  remains fastened to patient support structure  13  as an operator moves support assembly  1  along axis XX′. As shown in  FIGS. 4A-B , safety lock  21  is an L-shaped locking mechanism that comprises a tab  42  that extends toward jaw  11 . Tab  42  is adapted to slip fit with the underside lip  41  ( FIG. 3 ) of the frame of the patient support structure  13 . In addition, safety lock  21  is slidably engaged within a slot  29  such that safety lock  21  may be adjusted to engage patient support structures of varying frame widths. In this embodiment, safety lock  21  ensures that support assembly  1  remains engaged with the underside lip  41  of the patient support structure  13  when the operator loosens the clamp assembly  23  to move support assembly  1  along axis XX′. 
         [0035]    Once the frame is compressed between the jaw  11  and base surface  25  of the clamp assembly  23 , the support assembly  1  may be secured via a mechanical friction lock  19 . In the present embodiment, the clamp locking mechanism is a mechanical lock comprising a set screw (not shown) that extends though a threaded aperture in the base member of the clamp assembly  23  to engage the jaw  11 . This locks the jaw relative to the clamp body  27 . However, it will be appreciated that in other embodiments, the mechanical friction lock  19  may be manual, electromechanical, electromagnetic, hydraulic, pneumatic, or any combination of these technologies. 
         [0036]    As shown in greater detail in  FIGS. 4A-B , support assembly  1  further comprises a hollow sleeve member  16  having a first end  18  and a second end  20 . The first end  18  of sleeve member  16  may be slidably interconnected to clamp assembly  23  via a bearing (not shown) that interfaces with a sliding member  17 . As shown, the sliding member  17  is formed of a beam/strap having a substantially rectangular cross-sectional shape. This strap is received in a corresponding channel  40  formed in the bottom of the body  27  of the clamp assembly  23 . An operator may position sliding member  17  between a fully retracted position and fully extended position along a reference axis YY′, which is perpendicular to reference axis XX′. An operator may then engage slide member locking mechanism  32  to secure sliding member  17 , and thus sleeve member  16 , at a desired position relative to the width of the patient support structure  13  along axis YY′. In the present embodiment, the slide member lock mechanism  32  is a set screw (not shown) that extends through a threaded aperture in the body  27  of the clamp assembly  23 . When advanced, a distal tip of the set screw extends into the channel  40  and engages the sliding member  17 . One of ordinary skill in the art will understand that slide member locking mechanism  32  may, in other embodiments, be manual, electromechanical, electromagnetic, hydraulic, pneumatic, or any combination of these technologies. 
         [0037]    To position an attached tracking assembly vertically relative to a patient, a shaft  22  having a top end  24  and a bottom end  26  may be disposed within sleeve member  16 . When disposed within sleeve member  16 , shaft  22  defines a reference axis ZZ′ ( FIG. 3 ) that is transverse to both reference axes XX′ and YY′ ( FIGS. 4A-B ). In use, the sleeve member  16  is typically oriented vertically such that reference axis ZZ′ defines a vertical reference axis. The shaft  22  is linearly coupled to sleeve member  16  via a sleeve bearing (not shown) that facilitates linear movement of shaft  22  along reference axis ZZ′ between a fully retracted position (see  FIG. 4A ) and a fully extended position (see  FIG. 4B ). 
         [0038]    In the fully retracted position, shown in  FIG. 4A , the top end  24  of the shaft  22  extends minimally above the second end  20  of the sleeve member  16 . When extended, the top end  24  of the shaft  22  may extend significantly above the top end of the sleeve member. In the illustrated embodiment, the shaft  22  is operative to move at least five inches. Those of ordinary skill in the art will readily understand that the linear movement of shaft  22  along reference axis ZZ′ may be actuated manually, electromechanically, hydraulically, pneumatically, or through any combination of these technologies. In the present embodiment, the shaft  22  is spring biased. A compression spring (not shown) may be disposed within sleeve member  16  such that the spring is operative to exert a force between the shaft  22  and the sleeve member  16 . In this configuration, the spring assists an operator of the support assembly  1  in manually adjusting the height of mounted tracking assembly  100 . It will be appreciated that the spring may comprise a mechanical coil or helical compression spring or it could be a gas cylinder. 
         [0039]    To rotatively position an attached tracking assembly about reference axis ZZ′, shaft  22  may be rotatively coupled to sleeve member  16  via a surface bearing  36 . The top end  24  of shaft  22  may be fixably coupled to the tracking assembly via a mounting surface  30 . Specifically, a bottom side  31  of mounting surface  30  may be fixably coupled to the top end  24  of shaft  22 , while a top side  33  of mounting surface  30  may be fixably coupled to tracking assembly  100  ( FIG. 3 ) such that tracking assembly  100  and shaft  22  may rotate together about reference axis ZZ′. A shaft locking mechanism  38  may secure shaft  22  and tracking assembly  100  at a desired vertical and angular position. As discussed above, one of ordinary skill in the art will readily understand that the shaft locking mechanism  38  may be manual, electromagnetic, electromechanical, hydraulic, pneumatic, or a combination of these technologies. 
         [0040]    In an alternative embodiment, tracking assembly  100  may be rotatively coupled to mounting surface  30  via a surface bearing such that tracking assembly  100  rotates independently of, and about, shaft  22 . 
         [0041]    FIGS.  5  through  8 A-B illustrate the overall design of another embodiment of a support assembly for positioning the tracking assembly  100  relative to a patient. In the embodiment shown, a support assembly  50  may be positioned on the floor or any other stable surface located next to patient support structure  13 . Further, the support assembly  50  allows for achieving precise linear positioning within multiple degrees of freedom prior to initiating a medical procedure. As more fully discussed below, support assembly  50  comprises a vertical tower assembly that, with the aid of a custom linear actuator assembly, provides vertical positioning relative to the height of the patient support structure and/or the centerline of a patient. This embodiment also incorporates a robotic horizontal linkage that interfaces the supported tracking assembly and the vertical tower assembly and allows for selectively positioning the tracking assembly within the horizontal plane. In this regard, the horizontal linkage comprises two armatures that are rotatively coupled through two electromagnetic locking joints. By engaging and disengaging the electromagnetic clutches associated with the locking joints, the horizontal linkage provides rotational motion about one or both of the electromagnetic locking joints. This allows the horizontal linkage to advance and retract in the horizontal plane, providing for positioning of the supported tracking assembly and probe with a great deal of motion within an additional degree of freedom. Once an operator positions the support assembly in a desired location and locks the horizontal linkage, a supported tracker assembly is securely positioned and a medical procedure may be performed. 
         [0042]    The embodiment shown in FIGS.  5  through  8 A-B may also provide for operator controls (e.g., push buttons and limit switches) to control the overall length of travel in both the vertical and horizontal planes to ensure operational safety and ease of use. To further ensure safety and ease of use, as well as to provide a compact and neat packaging design, the components of the support assembly  50  are configured to route and manage several wires or cables, including, for example, power cords for the tracking assembly, an associated computer and/or monitor, and network cabling. 
         [0043]    In the embodiment shown in FIGS.  5  through  8 A-B, support assembly  50  comprises a base element  52 , a vertical tower assembly  60 , and a horizontal linkage  90  adapted to fixably support tracking assembly  100 , as shown in  FIG. 5 . Base element  52  and vertical tower assembly  60  are moveably coupled to a horizontal linkage  90  via a linking cuff  54 . 
         [0044]      FIG. 6  illustrates an exploded view of vertical tower assembly  60 . In this embodiment, vertical tower assembly  60  comprises a vertical sleeve member  62  having a vertical slot  84 , and a linear actuator assembly  64  is mounted within vertical sleeve member  62  via a top mounting block  66  and a bottom mounting block  68 . The vertical slot  84  is configured to allow access for the linear actuator assembly  64  to fixably connect with the horizontal linkage  90 . When connected with the linear actuator assembly  64 , horizontal linkage  90  moves upward and downward vertically in conjunction with the movement of the linear actuator assembly, as detailed below. 
         [0045]    Linear actuator assembly  64  comprises a threaded shaft  70 , an actuator  72 , a threaded receptacle  74 , an inner sleeve member  80 , two low friction sleeve bearings  76 ,  78 , and a vertical sleeve connecting block  82 . Specifically, threaded shaft  70  is rotatively suspended between top mounting block  66  and bottom mounting block  68 , defining a reference axis AA′. Actuator  72  is mounted within vertical sleeve member  62  to threaded shaft  70  and is adapted to rotate threaded shaft  70  relative to threaded receptacle  74 . Those of ordinary skill in the art will readily understand that actuator  72  may be an electromechanical device, such as a servo motor, or it may be manual, hydraulic, pneumatic, or a combination of these technologies. Those skilled in the art will also recognize that threaded receptacle  74  may be a nut or any other internally threaded fastener known in the art. 
         [0046]    Threaded receptacle  74  may be coupled to inner sleeve member  80  such that when threaded shaft  70  rotates, threaded receptacle  74  and inner sleeve member  80  move between a bottom position and a top position along reference axis AA′. Low friction sleeve bearings  76 ,  78  may attach to inner sleeve member  80  to facilitate smooth linear motion between inner sleeve member  80  and vertical sleeve member  62 . Exemplary low friction sleeve bearings may be purchased from Igus® Inc., or any other suitable bearing manufacturer as is generally known in the art. 
         [0047]    Limit switches (not shown) are located at both ends of the linear actuator assembly  64  at the top mounting block  66  and the bottom mounting block  68 . The limit switches are generally maintained in an open state. As the inner sleeve member  80  reaches its top or bottom positions, it contacts the limit switches, cutting power to the actuator  72  and providing a smooth and safe stop. 
         [0048]    As shown in  FIG. 5 , a linking cuff  54  interconnects vertical tower assembly  60  with horizontal linkage  90 . As discussed in greater detail below, this embodiment of horizontal linkage  90  comprises two armatures and two electromagnetic locking joints. Rotating the armatures about one or both joints allows for the compact retraction and significant extension of the armature. 
         [0049]    Through linking cuff  54 , vertical tower assembly  60  is able to move horizontal linkage  90  vertically relative to reference axis AA′, which, in turn, also moves a supported tracking assembly  100  vertically in relation to reference axis AA′. In greater detail, a proximal flange  88  of vertical sleeve connecting block  82  fixably connects to inner sleeve member  80  through vertical slot  84  within vertical sleeve member  62 . At least one distal aperture  89  of vertical sleeve connecting block  82  mates with linking cuff  54 , which in turn connects to horizontal linkage  90 . 
         [0050]    While the vertical tower assembly  60  allows for vertically positioning the tracking assembly  100  to a desired height (i.e., horizontal plane), the horizontal linkage  90  allows for selectively positioning the tracker assembly  100  within the horizontal plane. In this regard, the linkage  90  includes various rotatively coupled armatures that allow at least for significant in-plane movement of the distal end of the horizontal linkage, as will be discussed in greater detail below. 
         [0051]      FIG. 7  illustrates an exploded view of horizontal linkage  90 . Horizontal linkage  90  comprises a first armature  101  having a proximal end  102  and a distal end  103  and a second armature  104  having a proximal end  105  and a distal end  111 . It will be appreciated that the distal end  111  of second armature  104  also forms a free distal end of horizontal linkage  90 . 
         [0052]      FIGS. 8A-B  illustrate that one embodiment of horizontal linkage  90  further comprises two electromagnetic locking joints  96 ,  98 , about which first armature  101  and second armature  104  may be rotationally adjusted to provide an expanded motion within the horizontal plane. For instance, first electromagnetic locking joint  96  may be rotated to position the distal end  111  of the second armature  104  such that it is disposed at least partially behind or beside the vertical tower assembly  60  in a collapsed position ( FIG. 8B ). It will be appreciated that moving the first armature  101  and second armature  104  between expanded and retracted positions, as shown in  FIGS. 8A-B , respectively, gives horizontal linkage  90  a significant range of motion within the horizontal plane while taking advantage of a compact design of limited armature length 
         [0053]    The electromagnetic locking joints  96 ,  98  each comprise a joint shaft  106   1-2  having a threaded top end  108   1-2 , respectively. Joint shaft  106   1  is fixably coupled with proximal end  102  of a top surface  113  of first armature  101 , and joint shaft  1062  is fixably coupled with distal end  103  of top surface  113  of first armature  101 . At each electromagnetic locking joint  96 ,  98 , an electromagnetic clutch  114   1-2  and at least two surface bearings  122   1-2 ,  124   1-2  may be positioned about joint shafts  106   1-2 , respectively Surface bearing  122   1-2  may be press fit into machined housings  110 ,  112 , while bearing spacers  109   1-2  may be fixably positioned between surface bearings  124   1-2  and housings  110 ,  112 . Further, electromagnetic clutches  114   1-2  may be affixed to housings  110 ,  112 , such that they selectively engage and disengage joint shafts  106   1-2  to control the rotational motion of first armature  101  and second armature  104  about electromagnetic locking joints  96 ,  98 , respectively. Finally, retaining nuts  120   1-2  may be tightened about the threaded top ends  108   1-2  of joint shafts  106   1-2  in order to compress housings  110 ,  112  against top surface  113  of first armature  101  such that each component of electromagnetic locking joints  96 ,  98  is contained within housings  110 ,  112 . 
         [0054]    When electromagnetic clutches  1141 .. 2  are actuated, they engage joint shafts  106   1-2  such that the free distal end  111  of horizontal linkage  90  is locked in place to maintain the supported tracking assembly  100  in a desired position. One or more operator control devices such as push buttons (not shown) may be mounted alongside the keyboard of a control computer (not shown) that is externally connected to the tracking assembly  100  to provide the electrical/electronic controls for the linear actuator and electromagnetic locking joints/clutches. It will be appreciated that these devices may also be actuated through other types of remote switches such as, for example, a foot pedal (not shown). 
         [0055]    Housing  112  is configured to fixably couple with second armature  104 . One of ordinary skill in the art will readily understand that this, as well as all of the fixed couplings discussed above, may be accomplished via threaded fasteners, rivets, pins, welding, a press fit, an adhesive, or any other method generally known in the art. Moreover, distal end  111  of second armature  104  may be adapted to support tracking assembly  100 , as shown in  FIG. 5 . 
         [0056]    To assist with cable management and routing, the components of vertical tower assembly  60  and horizontal linkage  90  may be configured to allow at least one cable to pass internally through vertical tower assembly  60  and horizontal linkage  90 . In greater detail, vertical sleeve member  63 , top and bottom mounting blocks  66 ,  68 , inner sleeve member  80 , low friction sleeve bearings  76 ,  78 , vertical sleeve connecting block  82 , horizontal linkage connecting block  118 , first armature  101 , second armature  104 , and joint shafts  106   1-2  may be formed such that they allow for the passage of cabling. 
         [0057]    While FIGS.  5  through  8 A-B illustrate an embodiment of support assembly  50  adapted to be positioned on the floor or any other stable surface located near patient support structure  13 , one of ordinary skill in the art will readily understand that support assembly  50  may be configured to attached directly to patient support structure  13  via a clamping device or another suitable method of attachment. 
         [0058]    The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are farther intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in similar or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.