Abstract:
Control of an invasive medical instrument during a medical procedure is achieved using a system that includes magnetic field-based location facilities. Magnetic field sensors are placed in a medical instrument, e.g., a probe, and in an interface device to enable respective positions of the probe and the interface device to be ascertained by a location processor when the sensors are exposed to a magnetic field. The interface device is disposed such that an operator can control the medical instrument and the interface device concurrently. A display device, which can comprise a virtual reality display, is responsive to movements of the interface device as determined by the location processor to control the medical instrument, invoke various functions of the system, e.g., image manipulation, and otherwise facilitate the medical procedure via a graphical user interface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to systems for invasive medical procedures. More particularly, this invention relates to using magnetic fields to track a medical instrument within a living body. 
         [0003]    2. Description of the Related Art 
         [0004]    Magnetic tracking systems for medical application use magnetic fields to detect locations both of points in the patient&#39;s body and of invasive devices, such as catheters and surgical tools, that are in proximity to or inside the body. For this purpose, a magnetic field generator produces a field in and around an area of the body, and sensors in the body and in the invasive device detect the field. A system console receives the sensor signals and displays the location of the invasive device relative to the body. 
         [0005]    For example, commonly assigned U.S. Pat. No. 7,174,201, issued to Govari, et al., and which is herein incorporated by reference, discloses apparatus for performing a medical procedure within a subject, which includes a wireless tag fixed to the tissue and which includes a first sensor coil. A second sensor coil is fixed to a medical device for use in performing the procedure. 
         [0006]    An integral processing and display unit includes a plurality of radiator coils, along with processing circuitry and a display. The radiator coils generate electromagnetic fields in a vicinity of the tissue, thereby causing currents to flow in the sensor coils. The processing circuitry processes the currents to determine coordinates of the tag relative to the medical device. The display is driven by the processing circuitry so as to present a visual indication to an operator of the medical device of an orientation of the device relative to the tag. 
         [0007]    U.S. Pat. No. 5,913,820, issued to Bladen, et al., and which is herein incorporated by reference, discloses methods and apparatus for locating the position, preferably in three dimensions, of a sensor by generating magnetic fields, which are detected at the sensor. The magnetic fields are generated from a plurality of locations and, in one embodiment of the invention, enable both the orientation and location of a single coil sensor to be determined. The system allows an operator to wear small, single coil sensors about his body to enable his movements to be detected and interpreted by a machine without requiring physical contact between the operator and the machine. For example, the positioning system could enable an operator to interact with images on a television or computer screen without the use of a conventional keyboard, mouse or stylus. 
         [0008]    U.S. Pat. No. 6,129,668, issued to Haynor et al., and which is herein incorporated by reference, discloses a device to detect the location of a magnet coupled to an indwelling medical device within a patient using three or more sets of magnetic sensors each having sensor elements arranged in a known fashion. Each sensor element senses the magnetic field strength generated by the magnet and provides data indicative of the direction of the magnet in a three-dimensional space. 
         [0009]    U.S. Pat. No. 6,427,079, issued to Schneider, et al., and which is herein incorporated by reference, discloses a remote location determination system that uses splines of magnetic field values to determine location parameters. The location determination system is used on a laser catheter that is operable to perform myocardial revascularization. An automatic calibration technique compensates for any variations in gain in a sensor and related components. Methods for reducing the effects of eddy currents in surrounding conductive objects are used in electromagnetic position and orientation measurement systems. 
       SUMMARY OF THE INVENTION 
       [0010]    In systems such as the one disclosed in the above-noted U.S. Pat. No. 7,174,201, in order to interact with the console, the system operator, such as a physician, must generally use a conventional user interface device, e.g., a keyboard, mouse or touch screen. The operator may have to disengage from manipulating the invasive device, and move to a different position to work the user interface. Alternatively, he must instruct an assistant to take the necessary actions. 
         [0011]    Embodiments of the present invention provide new methods and devices for user interaction with a system for medical treatment and/or diagnosis that uses magnetic position tracking. These methods and devices permit the system operator to interact with the console without leaving his normal operating position. In some of these embodiments, the operator is provided with a stylus or other user interface device containing a magnetic sensor, which is linked to the console. The interface device may itself have a dual function as an invasive medical instrument. As long as the stylus is near the patient&#39;s body, the sensor senses the fields generated by the magnetic field generator. In other embodiments, the interface device and the medical instrument generate magnetic fields, which are sensed by an external position sensor. A position processor in the console is thus able to determine the location of the stylus just as it determines the locations of the other elements of the system. The system console displays a cursor on a screen, which moves as the operator moves the stylus. The operator can use this cursor to actuate on-screen controls, to draw lines on the screen, and to mark points and otherwise interact with images and maps that are displayed on the screen. 
         [0012]    In other words, the effect of the stylus and magnetic tracking system is to provide a “virtual touch screen” that the system operator can use conveniently while operating on the patient. 
         [0013]    Some embodiments of the present invention permit the system operator to view a virtual image of an anatomical structure, in the actual location of the structure, using a “virtual reality” or “augmented reality” display, and to use the stylus to interact with the image. For example, the display with which the operator interacts using the stylus may be presented on goggles worn by the system operator. The goggles contain a position sensor, so that the display is registered with the body of the patient. 
         [0014]    An embodiment of the invention provides apparatus for invasive medical operations in the body of a living subject. The apparatus includes one or more field generating elements disposed at known locations for generating magnetic fields at respective frequencies, and a medical instrument adapted for insertion into the body. The medical instrument has a first magnetic position sensor coupled thereto that emits first signals responsively to the magnetic fields. An interface device has a second magnetic position sensor coupled thereto that emits second signals responsively to the magnetic fields. The apparatus includes a position processor operative to receive the first signals and the second signals and to determine respective positions of the interface device and the medical instrument relative to the known locations, responsively to the first signals and the second signals, and a display device operative to display an image responsively to the position of the medical instrument. The display device has a cursor moveable thereon under control of the position processor responsively to changes in the position of the interface device. 
         [0015]    According to an aspect of the apparatus, the display device has a display control that is actuated responsively to a superimposition of the cursor thereon. 
         [0016]    According to another aspect of the apparatus, the display device has a display control that is actuated responsively to a displacement of the interface device generally toward the display device while the cursor is superimposed on the display control. 
         [0017]    According to one aspect of the apparatus, the display device is a virtual reality display device having a third magnetic position sensor that emits third signals responsively to the magnetic fields. 
         [0018]    In yet another aspect of the apparatus, positioning controls are provided for the medical instrument, and the interface device is disposed within reach of an operator of the positioning controls. 
         [0019]    According to a further aspect of the apparatus, the first magnetic position sensor and the second magnetic position sensor comprise at least two sensor coils. 
         [0020]    Other embodiments of the invention provide methods that are carried out by the above-described apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
           [0022]      FIG. 1  is a pictorial illustration of a system for medical imaging using a virtual touch screen, in accordance with a disclosed embodiment of the invention; 
           [0023]      FIG. 2  is a pictorial illustration of a catheter that may be used in the system shown in  FIG. 1 , in accordance with an embodiment of the present invention; 
           [0024]      FIG. 3  is a pictorial illustration of an interface device that may be used in the system shown in  FIG. 1 , in accordance with an alternate embodiment of the invention; 
           [0025]      FIG. 4  is a pictorial illustration of a device that produces a virtual reality display that may be used in the system shown in  FIG. 1 , in accordance with another alternate embodiment of the invention; 
           [0026]      FIG. 5  is a flow chart showing a method for performing invasive medical operations with the assistance of a virtual touch screen, in accordance with a disclosed embodiment of the invention; and 
           [0027]      FIG. 6  is a flow chart showing a method for imaging an anatomical structure on the virtual reality display of  FIG. 4 , in accordance with a disclosed embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the present invention unnecessarily. 
         [0029]    Software programming code, which embodies aspects of the present invention, is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known media for use with a data processing system, such as a diskette, or hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to other computer systems for use by users of such other systems. 
         [0030]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  20  that tracks and operates a medical instrument within a living body using a virtual touch screen, which is constructed and operative in accordance with a disclosed embodiment of the invention. An operator, for example a physician  22  may use system  20  to obtain medical images using a probe, such as a catheter  23 , which may be inserted into an internal body cavity, such as a chamber of a heart  24  of a subject  26 . Typically, catheter  23  is used for diagnostic or therapeutic medical procedures, such as mapping electrical potentials in the heart or performing ablation of heart tissue. The catheter or other intra-body device may alternatively be used for other purposes, by itself or in conjunction with other treatment devices. The cardiac application described with respect to  FIG. 1  is exemplary. The principles of the invention are applicable to many invasive medical and surgical procedures throughout the body. 
         [0031]    Reference is now made to  FIG. 2 , which is a pictorial illustration of catheter  23 , in accordance with an embodiment of the present invention. The catheter shown is exemplary; many other types of catheters may be used as catheter  23 . Catheter  23  typically comprises positioning controls  27  on a handle  28  to enable the physician to steer, locate and orient, and operate a distal end  29  of catheter  23  as desired. 
         [0032]    A pointing device, e.g., joystick  52  is attached to handle  28 . In some embodiments, handle  28  comprises one or more touch-activated switches, shown as buttons  56 . Alternatively, buttons  56  may be located on joystick  52 . Joystick  52  and buttons  56  are used for controlling system  20 , as described in detail herein below. 
         [0033]    Distal end  29  and joystick  52  include position sensors  32  and  54  respectively, each comprising sensor coils  35  as described herein below. 
         [0034]    In some embodiments, distal end  29  comprises an ultrasonic imaging sensor  39 . Ultrasonic imaging sensor  39  typically transmits a short burst of ultrasound energy and converts the reflected ultrasound into electrical signals, which are transmitted via cables  33  to console  34  ( FIG. 1 ), as is known in the art. 
         [0035]    In some embodiments, distal end  29  also comprises at least one electrode  42  for performing diagnostic functions, therapeutic functions, or both, such as electro-physiological mapping and radiofrequency (RF) ablation. In one embodiment, electrode  42  is used for sensing local electrical potentials. The electrical potentials measured by electrode  42  may be used in mapping the local electrical activity on the endocardial surface. When electrode  42  is brought into contact or proximity with a point on the inner surface of heart  24  ( FIG. 1 ), the electrode measures the local electrical potential at that point. The measured potentials are converted into electrical signals and sent through catheter  23  to an image processor  43  ( FIG. 1 ), which converts the signals into an electro-anatomical map. 
         [0036]    Alternatively, electrode  42  may be used to measure parameters different from the electrical potentials described above, such as various tissue characteristics, temperature, and blood flow. 
         [0037]    Referring again to  FIG. 1 , system  20  comprises a positioning subsystem  30  that measures location and orientation coordinates of distal end  29  of catheter  23 . As used herein, the term “location” refers to the spatial coordinates of an object, the term “orientation” refers to angular coordinates of the object, and the term “position” refers to the full positional information of the object, comprising both location and orientation coordinates. 
         [0038]    In one embodiment, positioning subsystem  30  comprises a magnetic position tracking system that determines the position of distal end  29  of catheter  23 . Positioning subsystem  30  typically comprises a set of external radiators, such as field generating elements, e.g., coils  31 , which are in fixed, known locations external to the subject. Coils  31  generate fields, typically magnetic fields, in the vicinity of heart  24 . 
         [0039]    Referring again to  FIG. 2 , position sensor  32  senses the fields generated by coils  31  and transmits, in response to the sensed fields, position-related electrical signals over cables  33  running through catheter  23  to console  34  ( FIG. 1 ). Alternatively, position sensor  32  may transmit signals to the console over a wireless link. 
         [0040]    In order to determine six positional coordinates (X, Y, Z directions and pitch yaw and roll orientations), position sensor  32  comprises at least two, and preferably three, sensor coils  35 , adapted to the frequency of one of coils  31  as is known in the art. Sensor coils  35  are wound on either air cores or cores of material. The axes of sensor coils  35  should be non-parallel and preferably mutually orthogonal. 
         [0041]    In some applications, where fewer position coordinates are required, only a single sensor coil  35  may be necessary in position sensor  32 . 
         [0042]    Position sensor  54 , which is located in the joystick  52 , preferably in the handle, is similar to position sensor  32 . Position sensor  54  senses the fields generated by coils  31 , and is used to determine the position of the handle of joystick  52  including its angular orientation in space. Position sensor  54  requires at least one sensing coil, and preferably has three coils. 
         [0043]    Referring again to  FIG. 1 , console  34  comprises a position processor  36  that calculates the location and orientation of distal end  29  of catheter  23  based on the signals sent by position sensor  32  ( FIG. 2 ). Position processor  36  typically receives, amplifies, filters, digitizes, and otherwise processes signals from catheter  23 . System  20  and position processor  36  may also be realized as elements of the CARTO XP EP Navigation and Ablation System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765, and suitably modified to execute the principles of the pre-sent invention. 
         [0044]    Some position tracking systems that may be used in embodiments of the present invention are described, for example, in U.S. Pat. Nos. 6,690,963, 6,618,612 and 6,332,089, and U.S. Patent Application Publications  2004 / 0147920  and  2004 / 0068178 , all of which are incorporated herein by reference. 
         [0045]    In some embodiments, image processor  43  uses the electrical signals received from ultrasonic imaging sensor  39  ( FIG. 2 ) and positional information received from position sensor  32  in distal end  29  of catheter  23  to produce an image of a target structure of the subject&#39;s heart. The images may be enhanced using electrical information derived from electrode  42 . 
         [0046]    In other embodiments, image processor  43  may not produce a medical image, but may merely produce an image of distal end  29  of catheter  23  overlaid on a representation of subject  26 , or may simply show the position of distal end  29  with respect to a target within the subject, in order to assist physician  22  with a medical procedure. 
         [0047]    Images produced by image processor  43  are output on a display device  44 . For example,  FIG. 1  shows an image  46  of part of heart  24 . System  20  typically provides display controls, for example a GUI (Graphical User Interface), comprising windows, icons and menus, for manipulating and viewing images produced by image processor  43 . An interface device is used to move a cursor  48  on display device  44 . 
         [0048]    In one embodiment the interface device comprises joystick  52  ( FIG. 2 ), which is within reach of physician  22  when he is using operating controls  27 . For example, in a medical procedure involving realtime image processing, rotation of the joystick may continuously control a parameter such as the edge threshold in an edge detection algorithm. Other joystick motions and button commands may be user-assigned in order to control other aspects of the operation of the system  20 . As physician  22  moves joystick  52 , the location of position sensor  54  is tracked by the position processor  36  ( FIG. 1 ) transmitted to console  34 , where it is registered on the display  44 . The position processor  36  translates joystick movements into movements of cursor  48  on display device  44 . 
         [0049]    Alternatively, the interface device may be a separate device, distinct from catheter  23  or any other medical device. Reference is now made to  FIG. 3 , which is a diagram of an exemplary interface device  60  for use with system  20  ( FIG. 1 ), in accordance with an alternate embodiment of the invention. Interface device  60  may be a wand or stylus, and is shaped to be easily graspable and manipulable by physician  22  ( FIG. 1 ). Interface device  60  comprises position sensor  54  and buttons  56 , as described above. Position sensor  54  senses magnetic fields produced by coils  31  ( FIG. 1 ) and transmits, in response to the sensed fields, position-related electrical signals over cables  63  to console  34 . Alternatively, position sensor  54  may transmit signals to the console over a wireless link. In this way, system  20  is able to determine the position of interface device  60 . 
         [0050]    A 3-dimensional spatial region  61  including screen  62  of display  40  is mapped by the position processor  36  to a spatial region  67  near or including device  60 . A displacement of device  60  in the region  67  that changes its XY coordinates in coordinate system  65  produces a corresponding movement of a cursor on the screen  62 . When the device  60  is displaced so as to change its Z-coordinate and intersect virtual plane  70 , physical contact with the screen  62  is emulated. This event stimulates the graphical user interface of the display  40  as though a physical touch screen were contacted at a point corresponding to the XY coordinate of the intersection in the plane  70 . 
         [0051]    Icons and menus (not shown) on the display  40  are actuated by superimposing the cursor on them. In an alternate embodiment, the icons and menus are actuated by passing the cursor over them while pressing one of buttons  56 . This causes an electrical signal to be transmitted along cables  33  to console  34 , where the processor interprets the signal to activate the icon or menu. The tracking of a pointing device for a GUI is well known in the art, and is not described further here. 
         [0052]    Similarly, physician  22  may move cursor  48  from a first position to a second position, in order to draw a corresponding line via the GUI from the first position to the second position, mark points using buttons  56 , and otherwise interact with images and maps that are displayed on the display device. 
         [0053]    In some embodiments of the invention, the images are displayed on a virtual reality display rather than a conventional display monitor. Reference is now made to  FIG. 4 , which is a pictorial illustration of a device that produces a virtual reality display, in accordance with an alternate embodiment of the invention. 
         [0054]    Virtual reality goggles  100  comprise at least one, and typically two, display devices  105 , supported by a frame  110 , constructed so that physician  22  ( FIG. 1 ) may wear goggles  100  with display devices  105  in front of his eyes. Display devices  105  show virtual images, for example, of a part of heart  24  ( FIG. 1 ) and distal end  29  of catheter  23  ( FIG. 2 ), as described herein below. Alternatively, display devices  105  may be transparent, or partially transparent, in order to provide augmented reality images in which the virtual images are superimposed on the body of subject  26  ( FIG. 1 ). 
         [0055]    Methods for display of virtual reality and augmented reality images are well known in the art. An exemplary disclosure is U.S. Pat. No. 6,695,779, issued to Sauer et al., which is incorporated herein by reference. 
         [0056]    Goggles  100  comprise a position sensor  132 , similar to position sensor  32 , which senses magnetic fields produced by coils  31  ( FIG. 1 ) and transmits, in response to the sensed fields, position-related electrical signals to console  34  ( FIG. 1 ), using a wireless transmitter  140 . Wireless transmitter  140  may also be used as a receiver for images to be displayed on display devices  105 . Alternatively, the transmitter may be wired to the console. 
         [0057]    Position sensor  132  is similar to position sensor  32 , but may comprise a miniaturized position sensor, for example as described in U.S. Pat. No. 6,201,387, issued to Govari, which is incorporated herein by reference. 
         [0058]    Alternatively, position sensor  132  may comprise a wireless position sensor. A suitable device is described in U.S. Patent Application Publication No. 2005/0099290, which is incorporated herein by reference. In this case, wireless transmitter  140  acts solely as a receiver for images from image processor  43  ( FIG. 1 ). 
         [0059]    Further alternatively, position sensor  132  may transmit signals to the console over a cable (not shown). However, this alternative is less convenient. Similarly, images to be displayed on display devices  105  may be received over cables (not shown). Because the positions of display devices  105  are fixed in relation to position sensor  132 , system  20  is able to determine the positions of each of display devices  105 . Using the information provided by the position sensor  132 , the position processor  36  ( FIG. 1 ) can register the virtual reality display with the body of the patient. In this manner, the operator can view an image of an organ superimposed on an image of the patient&#39;s body in the proper position and orientation, and can use the device  60  ( FIG. 3 ) to interact with the images as described above. 
         [0060]    Alternatively, as shown in  FIG. 4 , each of display devices  105  may be attached to its own position sensor  132 . This allows greater flexibility of movement of the goggles, since the relative positions of display devices  105  need not be constant. Although  FIG. 4  shows each position sensor  132  connected to a separate wireless transmitter  140 , a single wireless transmitter  140  may be used. 
         [0061]    The virtual reality image may be manipulated using many combinations of interface devices such as joystick  52  or interface device  60 , as described above. As conditions of the medical procedure change, some embodiments may become less convenient than others. For example, some phases may be hazardous, e.g., taking place under conditions of radiation exposure, and requiring hands-off actuation of the medical instrument on the part of the physician  22 . In such cases the use of goggles  100  may be preferable. In other situations, the lighting conditions in the operatory may be unsuitable for use of goggles  100 . 
         [0062]    In an alternate embodiment, position sensors  32 ,  54 ,  132  may be replaced by radiators, e.g., coils, that generate magnetic fields, which are received by sensors outside the subject&#39;s body. The external sensors generate the position-related electrical signals. 
         [0063]    Reference is now made to  FIG. 5 , which is a flow chart showing a method for performing invasive medical operations with the assistance of a virtual touch screen, in accordance with a disclosed embodiment of the invention. 
         [0064]    The method begins at an initial step  150 , where the position of distal end  29  ( FIG. 1 ) of catheter  23  is determined, typically using the magnetic fields produced by coils  31  and sensed by position sensor  32  ( FIG. 2 ). Alternatively, as described above, the position of distal end  29  may be determined by external position sensors that detect magnetic fields generated at a fixed position relative to distal end  29 . 
         [0065]    Next, at step  152 , an image, for example image  46 , is acquired and displayed on display  44 . The image may be an image of subject  26 , which may be obtained, for example, using catheter  23 . Alternatively, the image may be an image of distal end  29  overlaid on a representation of subject  26 . Further alternatively, the image may show the position of distal end  29  with respect to a target within the subject. Steps  150  and  152  may be repeated as distal end  29  moves. 
         [0066]    At step  155 , typically performed concurrently with steps  150  and  152 , the position of the interface device is determined, for example by position sensor  54  ( FIG. 2 ). Alternatively, one of position sensors  32 ,  54  may be replaced by a radiator, which is used to as a reference establish coordinates for the system. In this case, the same external sensors are used to detect the positions of the distal end of the catheter and the interface device. 
         [0067]    Next, at step  160 , cursor  48  is positioned on display  44 . The initial position may be predefined or random. 
         [0068]    At step  165 , typically performed after a time delay, or after an interrupt, the position of the interface device is determined, as in step  155 . 
         [0069]    Next, at decision step  170 , it is determined whether the interface device has moved since the previous iteration of step  165 , or step  155  if this is the first iteration. If the determination at determination step  170  is negative, then control proceeds to a decision step  175 , described below. 
         [0070]    If the determination at decision step  170  is affirmative, then control proceeds to step  180 . Cursor  48  is repositioned on display  44  in response to the displacement of the interface device relative to its previous position. Control proceeds to decision step  175 . 
         [0071]    In some embodiments of the invention, display controls, for example a GUI as described above, appear on display  44 . At decision step  175 , it is determined whether the cursor is superimposed on one of the display controls. If the determination at decision step  175  is negative, then control returns to step  165 . 
         [0072]    If the determination at decision step  175  is affirmative, then control proceeds to step  185 . The display control is actuated. This may cause a change in the orientation or scale of the image on display  44 , or other changes to the display of the image or may actuate a function of catheter  23 , according to a computer application that is controlled via the GUI. 
         [0073]    Next at decision step  190 , it is determined whether the procedure is complete. Typically, this is indicated by the actuation of an appropriate display control at step  185 . If the determination at decision step  190  is negative, then control returns to step  165 . 
         [0074]    If the determination at decision step  190  is affirmative, then control proceeds to final step  195 , where the method ends. 
         [0075]    Reference is now made to  FIG. 6 , which is a flow chart showing a method for imaging an anatomical structure on the virtual reality display of  FIG. 4 , in accordance with a disclosed embodiment of the invention. The process steps are shown in a particular linear sequence in  FIG. 6  for clarity of presentation. However, it will be evident that many of them can be performed in parallel, asynchronously, or in different orders. For example, acquiring the image and locating the display devices may be performed in either order, or simultaneously. 
         [0076]    The method begins at initial step  205 , where an image, typically three-dimensional, of a part of an anatomical structure is acquired. For an ultrasound image, this may be performed as described for example, in U.S. Patent Application Publication No. 2006/0241445, which is incorporated herein by reference. 
         [0077]    Next, at step  220 , one or more position sensors  132  ( FIG. 4 ) determine the positions of display devices  105 . The position information is transmitted to console  34 . 
         [0078]    Next, at step  222 , image processor  43  uses position information from step  220  and standard geometrical techniques to obtain, for each of display devices  105 , a 2-dimensional projection of the image. 
         [0079]    At final step  225 , the projections are transmitted to display devices  105  ( FIG. 4 ) and displayed. 
         [0080]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.