Abstract:
Surgical apparatus, including a trocar having a tubular member with a tubular member distal end and a tubular member proximal end. The apparatus further includes a tool having a tool proximal end and a tool distal end. The tool is insertably disposed within the tubular member with the tool distal end projecting beyond the tubular member distal end. The apparatus further includes an indicator indicating the tool distal end while the tool distal end projects beyond the tubular member distal end.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to invasive medical procedures, and specifically to tracking items used during such procedures. 
     BACKGROUND OF THE INVENTION 
     Tracking of tools used during an invasive medical procedure, regardless of whether or not the procedure may be classed as minimally invasive, is extremely important. In some cases, the tracking may be performed if the tools have a preset feature, such as a color or shape, and if analysis of an image of the procedure allows the tool to be identified on the basis of the preset feature. However, such a tracking system fails if a tool without the preset feature is used during the procedure. 
     An improved tool tracking system would therefore be advantageous. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides surgical apparatus including: 
     a trocar having a tubular member with a tubular member distal end and a tubular member proximal end; 
     a tool having a tool proximal end and a tool distal end, the tool being insertably disposed within the tubular member with the tool distal end projecting beyond the tubular member distal end; and 
     an indicator indicating the tool distal end while the tool distal end projects beyond the tubular member distal end. 
     Typically, the apparatus further includes a sensor sensing a position of the tool, and the indicator indicates the tool distal end based on a signal of the sensor. The indicator may be an illuminator disposed on the trocar and configured to illuminate the tool distal end based on the signal. Typically, on illumination the tool distal end generates an illumination spectrum different from a tissue spectrum generated by tissue in proximity to the tool. 
     In a disclosed embodiment the indicator is an applicator, disposed on the trocar, configured to apply a marker to the tool upon passage of the tool beyond the tubular member distal end. 
     The marker may consist of a liquid, configured to dry in response to being applied to the tool. Alternatively, the marker consists of a label, configured to be deposited on the tool. The apparatus may include a label remover disposed in the trocar, configured to remove the label after deposition thereof, and upon withdrawal of the tool from the trocar via the tubular member. The label may be configured to be removed from the tool on autoclaving the tool. 
     In an alternative embodiment, the apparatus includes a processor, configured to track the tool in response to the indication of the indicator. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic illustration of an automated tool tracking system, according to an embodiment of the present invention; 
         FIG. 1B  is a schematic illustration of an alternative automated tool tracking system, according to an embodiment of the present invention; 
         FIGS. 2A and 2B  are schematic cross-sectional illustrations of a trocar, according to an embodiment of the present invention; 
         FIGS. 3A and 3B  are schematic cross-sectional illustrations of a trocar, according to an alternative embodiment of the present invention; 
         FIGS. 3C and 3D  are schematic cross-sectional illustrations of a trocar, according to a further alternative embodiment of the present invention; 
         FIGS. 3E and 3F  are schematic cross-sectional illustrations of a trocar, according to a yet further embodiment of the present invention; 
         FIGS. 4A and 4B  are schematic cross-sectional illustrations of a trocar, according to another alternative embodiment of the present invention; 
         FIGS. 4C and 4D  are schematic cross-sectional illustrations of a trocar, according to yet another embodiment of the present invention; 
         FIG. 5  is a flowchart of steps performed in use of a trocar, according to an embodiment of the present invention; and 
         FIG. 6  is a schematic illustration of results of performing the flowchart steps, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     An embodiment of the present invention provides a trocar which is able to automatically track the location of a distal end of a tool. The tracking is performed after the distal end has been introduced into a region, herein assumed to comprise a body cavity, via the trocar, so as to project beyond the distal end of the trocar. An endoscope may be introduced into the body cavity, and acquires an image of the body cavity and the tool distal end. The endoscope forms the image by radiating “endoscope” light, typically white light, from the endoscope. 
     In order to provide the automatic tracking, an indicator is formed in a tubular member of the trocar. Typically, within the scene of the endoscope&#39;s field of view, the indicator operates to delineate the tool distal end location, by enabling the tool distal end to be distinguished from elements of the body cavity. 
     In one embodiment, the indicator is an illuminator which radiates light, typically but not necessarily within the visible spectrum, onto the tool distal end when it projects beyond the trocar distal end. The illuminator light is typically selected to have a different spectrum from the spectrum of the endoscope light returning from the body cavity. A processor analyzes the endoscope image, and determines a location of the tool distal end using the differences in spectra of the light from the tool distal end and from the body cavity. 
     In an alternative embodiment, the indicator is an applicator disposed on the tubular member of the trocar. The applicator may be configured to apply a liquid to the tool as the tool enters the body cavity. The liquid is selected to be quick-drying, so that it forms a solid on the tool distal end. In addition, the solidified liquid is colored to have a spectrum, when illuminated by the endoscope light, that allows it to be easily distinguished from the body cavity. As for the embodiment described above, a processor analyzes the endoscope image and determines a location of the tool distal end using the differences in spectra. 
     Alternatively, the applicator may be configured as a label dispenser, which applies a label to the tool distal end as the tool enters the body cavity. The label is colored so that, when illuminated by the endoscope light, it has a spectrum that is different from the body cavity spectrum. Alternatively or additionally, the label may have a predefined shape. The processor analyzes the endoscope image to find the label (and thus the tool distal end), using either the differences in spectra or by looking for the predefined shape. 
     DETAILED DESCRIPTION 
     Reference is now made to  FIG. 1A , which is a schematic illustration of an automated tool tracking system  10 , according to an embodiment of the present invention. System  10  may be used in an invasive medical procedure, typically a minimally invasive procedure, on a body cavity  12  of a patient in order to track the location of a tool in the body cavity. By way of example, in the present description the body cavity is assumed to be the abdomen of a patient, and body cavity  12  is also referred to herein as abdomen  12 . However, it will be understood that system  10  may be used on substantially any body cavity, such as the bladder or the chest. 
     System  10  is controlled by an endoscope module  22 , comprising a processor  16  communicating with a memory  18 . Endoscope module  22  also controls the operation of an endoscope  32 . In addition, endoscope module  22  comprises a tool tracker module  20 , which is implemented in software within the endoscope module. The tool tracker module uses processor  16  and memory  18  in order to perform its functions, described below. 
     Typically, module  20  comprises a video object tracker which is configured to receive a video signal from the endoscope system. Module  20  is configured to be aware of the characteristics of an indication used, as described in more detail below, in system  10 . The characteristics include, for example, a type and a setting of the indication. For example, if the indication comprises a label the characteristics typically include a label color and a label shape. Tool tracker module  22  is also configured to control, as required, illuminators and/or valves and/or other functional elements of a trocar, as described in more detail below. The configuration of module  20  may be via wireless transmission or through manual input by an operator of system  10 . 
     Endoscope module  22  may also comprise other modules, such as a cavity illumination module, an image processing module, and a zoom/pan module, which may be used by processor  16 . The processor uses software stored in memory  18 , in the form of the modules referred to above as well as in other forms, to operate system  10 . Results of the operations performed by processor  16  may be presented to a medical physician operating system  10  on a screen  24 , which typically displays an image of body cavity  12  undergoing the procedure, and/or a graphic user interface to the physician. The software may be downloaded to processor  16  in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     To perform a procedure, the physician inserts trocars into abdomen  12  to penetrate an abdomen wall  26 . Herein, the physician is assumed to insert a first trocar  28  and a second trocar  30 . Once inserted, the physician is able to pass items required for the procedure through respective tubular members of the trocars into abdomen  12 . Thus, endoscope  32  may be passed through a tubular member  34  of trocar  28 . Endoscope module  22  provides illumination for the endoscope and displays an image acquired by the endoscope on screen  24 . The physician typically uses the endoscope to view the interior of abdomen  12 . 
     Trocar  30  has a tubular member  36 , and the physician passes a tool  40  through the tubular member so that it enters body cavity  12 . Trocar  30  is operated by tool tracker module  20 , and the structure of the trocar, and its functioning as tools such as tool  40  are passed via member  36  into and out of cavity  12 , are described with respect to  FIGS. 2A and 2B  below. 
       FIG. 1B  is a schematic illustration of an automated tool tracking system  11 , according to an embodiment of the present invention. Apart from the differences described below, the operation of system  11  is generally similar to that of system ( FIG. 1B ), and elements indicated by the same reference numerals in both systems are generally similar in construction and in operation. In contrast to system  10 , in system  11  tool tracker module  20  is implemented in hardware, typically as a stand-alone hardware unit external to endoscope module  22 . While the tool tracker module is configured to communicate and utilize processor  16 , having the tool tracker module as a stand-alone hardware unit allows it to be used with existing endoscope modules. 
       FIGS. 2A and 2B  are schematic cross-sectional illustrations of trocar  30 , according to a first embodiment of the present invention.  FIG. 2A  illustrates the trocar before tool  40  passes through tubular member  36  of the trocar.  FIG. 2B  illustrates the trocar while the tool is passing through tubular member  36 , and when a distal end  42  of tool  40  projects beyond a distal end  44  of the trocar. By way of example, in the description herein tubular member is assumed to be cylindrical, with an axis of symmetry  37 . Optical elements (described in more detail below) that are located at the distal end of trocar  30  are connected to, and controlled by, tool tracker module  20 . For clarity, the connections, typically comprising optical and/or conductive cables, are not shown in the figures. In some embodiments at least some of the connections may comprise wireless connections. 
     At its distal end  44 , trocar  30  comprises at least one illuminator  46  emitting radiation beyond the distal end of the tubular member. If more than one illuminator is used, the illuminators are generally similar in radiative properties. In the description herein, for clarity, different illuminators  46  are identified by appending a letter to the identifying numeral  46  but are referred to collectively as illuminators  46 . 
     By way of example, trocar  30  is assumed to have a first illuminator  46 A and a second illuminator  46 B located at distal end  44 . The two illuminators are inset into a wall  48  of tubular member  36 , and are located on opposite sides of a passageway  50  of the tubular member. Illuminators  46 A and  46 B are typically positioned and inset into wall  48  so that together they project a generally conical pattern of radiation from distal end  44 . 
     However, other numbers and distributions of illuminators  46  may be used in distal end  44 , and an optimal number and distribution of illuminators  46  may be determined by one of ordinary skill in the art, without undue experimentation, in order to project the generally conical radiation pattern. However many illuminators are used, they are configured so that the part of tool  40  illuminated (described below) is visible to endoscope  32 , typically regardless of the relative orientation and displacement of the tool and the endoscope. 
     Illuminators  46  may comprise elements which generate the radiation emitted themselves; for example, the illuminators may comprise light emitting diodes (LEDs). Alternatively or additionally, illuminators  46  may be formed as the distal ends of optic fibers which receive the radiation from respective emitters at the proximal ends of the fibers. In this case the emitters may be LEDs or other radiation sources such as incandescent or fluorescent radiators, or lasers. 
     As described in more detail below, the radiation emitted by illuminators  46  enables tool  40  to be automatically tracked in the image generated by endoscope  32  ( FIGS. 1A ,  1 B). Illuminators  46  thus act as an indicator for the presence of the tool distal end when it projects beyond distal end  44  of the trocar. Typically, endoscope  32  has its own illuminator (not shown in  FIGS. 1A ,  1 B), that is controlled by endoscope module  22 . The light emitted by the endoscope illuminator, herein termed “endoscope light,” is typically white light. The endoscope light enables the endoscope to acquire an image of tissue of wall  26 , such as arteries, veins, and/or other components of the wall. Under the endoscope light the tissue spectrum, corresponding to the color of the imaged tissue, is dependent on the absorption of the endoscope light by the tissue. The tissue spectrum typically comprises varying shades of red. 
     In embodiments of the present invention the spectrum of the radiation emitted by illuminators  46 , herein termed the illumination spectrum, is selected so that tool  40 , when illuminated by the radiation, has good visibility with respect to the tissue spectrum. In other words, there is a significant difference between the two spectra. For example, assuming the tissue spectrum has predominating red wavelengths of approximately 650 nm, the illumination spectrum may be selected to have predominant wavelengths for orange (approximately 600 nm), green (approximately 550 nm), blue (approximately 450 nm), or a color having more than one predominant wavelength, such as purple. For simplicity, in the description herein the illumination spectrum is assumed to comprise visible radiation, and is also herein termed light. However, apart from having the significant difference referred to above, there is no limitation on the spectrum emitted by illuminators, so that, for example, the illumination spectrum may comprise infra-red and/or ultra-violet radiation. 
     Typically, a sensor  52  is located in tubular member wall  48 , at distal end  44 , the sensor serving to activate illuminators  46  on entry of tool  40  via passageway  50  into distal end  44 , and to deactivate the illuminators when the tool is no longer in the passageway. The sensor may be in any convenient form known in the art. For example, the sensor may be a mechanical switch, or alternatively it may be a photoelectric switch. 
     The description above refers to one trocar  30  configured for automated tool tracking. Embodiments of the present invention also comprise multiple trocars, generally similar to trocar  30 , being used simultaneously, so that at any given instance two or more tools may be present in a given body cavity. In the case of multiple trocars, the illumination from each trocar is set to be distinguishable. 
     The distinguishing feature of the illumination may comprise configuring each illuminator to have a different illumination spectrum, corresponding to wavelength multiplexing. For example, if three trocars are used the respective illuminators may be predominantly orange, green, and blue. Alternatively, the illumination may be time multiplexed, i.e., illuminators  46  may be pulsed. Further alternatively, the endoscope light may be included in the time multiplexing, and this type of time multiplexing could be used to enhance the visibility of the tool when only one trocar is used. Yet further alternatively, a combination of wavelength and time multiplexing may be used to differentiate or distinguish the illumination from multiple trocars. 
       FIGS. 3A and 3B  are schematic cross-sectional illustrations of a trocar  130 , according to a second embodiment of the present invention. Apart from the differences described below, the operation of trocar  130  is generally similar to that of trocar  30  ( FIGS. 1 ,  2 A and  2 B), and elements indicated by the same reference numerals in both trocars  30  and  130  are generally similar in construction and in operation.  FIG. 3A  illustrates trocar  130  before tool  40  passes through tubular member  36  of the trocar.  FIG. 3B  illustrates trocar  130  while the tool is passing through tubular member  36 , and when distal end  42  of tool  40  projects beyond distal end  44  of the trocar. 
     In contrast to trocar  30 , trocar  130  does not have illuminators  46  to act as an indicator. Rather, distal end  44  of trocar  130  comprises one or more generally similar tubes  132  within wall  48 , the tubes exiting from the wall at respective tube openings  134 . Openings  134  are located, so that as measured along axis  37  of member  36 , the openings are proximal with respect to sensor  52 . By way of example, in the present description there are assumed to be two tubes  132 , located approximately on opposite sides of passageway  50 . Typically, before tube openings  134  there are respective open/close valves  136 . Processor  16  sets valves  136  to be open or closed in response to a signal from sensor  52 . 
     However many tubes  132  are used, they are configured so that at least some of the markers (described below) from the openings that are applied to the tool are visible to endoscope  32 , typically regardless of the relative orientation and displacement of the tool and the endoscope. 
     Prior to insertion of tool  40  into trocar  130 , valves  136  are closed, and tubes  132  are filled with a liquid. The liquid is selected to be a fast-drying liquid, i.e., a liquid which on exposure to the atmosphere becomes solid, and so is able to act as a marker  138  of the tool upon which it has been deposited. In addition, the liquid is selected so that the marker produced on drying has a marker spectrum which allows the marker, under illumination from the endoscope light, to have good visibility with respect to the tissue of wall  26 . In other words, there is a significant difference between the marker spectrum and the tissue spectrum (as exemplified above for the illumination and tissue spectra in the description of trocar  30 ). 
     On insertion of tool  40  into trocar  130 , so that the distal end of the tool passes sensor  52  and projects beyond the distal end of the trocar, the sensor generates a signal. In response to the signal, processor  16  opens valves  136 , so that the liquid in tubes  132  is deposited on the distal end of the tool. Tubes  132  thus act as applicators of the liquid, and the tubes may also be referred to herein as applicators  132 . The liquid dries on the tool, forming marker  138 . In the embodiment described herein, wherein there are two applicators  132 , marker  138  is in two sections. Typically, openings  134  and the amount of liquid stored in applicators  132  are configured so that at least part of marker  138  is visible from any direction with respect to distal end  42  of the tool. 
       FIGS. 3C and 3D  are schematic cross-sectional illustrations of a trocar  150 , according to a third embodiment of the present invention. Apart from the differences described below, the operation of trocar  150  is generally similar to that of trocar  130 , as described above, and elements indicated by the same reference numerals in both trocars  130  and  150  are generally similar in construction and in operation.  FIG. 3C  illustrates trocar  150  before tool  40  passes through tubular member  36  of the trocar, and  FIG. 3D  illustrates the trocar and the tool after distal end  42  of the trocar has passed sensor  52 . 
     In contrast to its location in trocar  130 , in trocar  150  sensor  52  is located approximately at the same distal position, i.e., at the same location measured with respect to axis  37  of the trocar, as openings  134 . The change of location of the sensor means that markers  138  are located in different positions for trocars  130  and  150 : for trocar  130  markers  138  are located on tool  40  more proximally than their location for trocar  150 . 
       FIGS. 3E and 3F  are schematic cross-sectional illustrations of a trocar  170 , according to a fourth embodiment of the present invention. Apart from the differences described below, the operation of trocar  170  is generally similar to that of trocar  150 , and elements indicated by the same reference numerals in trocars  150  and  170  are generally similar in construction and in operation.  FIG. 3E  illustrates trocar  170  before tool  40  passes through tubular member  36  of the trocar, and  FIG. 3F  illustrates the trocar and the tool after distal end  42  of the trocar has passed sensor  52 . 
     In trocar  170 , rather than locating valves  136  in wall  48 , the valves are located outside the wall. Typically, as illustrated in  FIGS. 3E and 3F , valves  136  are located in proximity to a proximal end of trocar  170 , so that the valves are outside cavity  12 . 
       FIGS. 4A and 4B  are schematic cross-sectional illustrations of a trocar  230 , according to a fifth embodiment of the present invention. Apart from the differences described below, the operation of trocar  230  is generally similar to that of trocar  30  ( FIGS. 1 ,  2 A and  2 B), and elements indicated by the same reference numerals in both trocars  30  and  230  are generally similar in construction and in operation.  FIG. 4A  illustrates trocar  230  before tool  40  passes through tubular member  36  of the trocar, and  FIG. 4B  illustrates the trocar and the tool after distal end  42  of the trocar has passed sensor  52 . 
     In contrast to trocar  30 , trocar  230  does not have illuminators  46 , but rather has one or more applicators  232  configured as label dispensers, and also referred to herein as label dispensers  232 . Herein, by way of example, there are assumed to be two label dispensers located on opposite sides of passageway  50 . 
     However many label dispensers are used, they are configured so that at least some of the labels (described below) from the dispensers that are deposited on the tool are visible to endoscope  32 , typically regardless of the relative orientation and displacement of the tool and the endoscope. 
     The label dispensers are activated, generally as described above for valves  136 , by a signal from sensor  52 . When activated each label dispenser  232  deposits a respective label  234  on the distal end of tool  40 . As for marker  138 , labels  234  are colored or dyed to have a label spectrum that, under illumination from the endoscope light, is easily distinguished from the tissue spectrum of wall  26 . Alternatively or additionally, the labels have a predefined shape, such as being circular or rectangular, that is identifiable by processor  16  in the image produced by the endoscope. The labels typically have a fast drying adhesive that is able to cement the labels to the tool within the conditions provided by body cavity  12 , for the length of time of the procedure being performed. 
     Labels  234  may be permanent or removable. Typically, if tool  40  is a disposable tool, then labels  234  may be formed to be permanent. If tool  40  is to be reused, i.e., the tool is not considered as disposable, then labels  234  may be configured as removable labels. In one embodiment the label adhesive is resistant to body temperature conditions, but is soluble in hot water. In an alternative embodiment, the labels are removed on sterilization by autoclaving of the tool, the high temperature of the autoclaving causing the label adhesive to melt. Use of such a label, i.e., one requiring sterilization for removal, acts as a check that sterilization has been performed, i.e., a tool with a label has not been sterilized. 
     Alternatively, in some embodiments respective label removers  236  may be incorporated in distal end  44 , removers  236  being configured to physically remove labels  234  when tool is withdrawn via tubular member  36  from cavity  12 . Processor  16  may be configured to activate label removers  236  in response to a signal from sensor  52  indicating that distal end  42  is not occupying a section of passageway  50  in proximity to the sensor. 
       FIGS. 4C and 4D  are schematic cross-sectional illustrations of a trocar  250 , according to a sixth embodiment of the present invention. Apart from the differences described below, the operation of trocar  250  is generally similar to that of trocar  230 , and elements indicated by the same reference numerals in both trocars  230  and  250  are generally similar in construction and in operation.  FIG. 4C  illustrates trocar  250  before tool  40  passes through tubular member  36  of the trocar, and  FIG. 4D  illustrates the trocar and the tool after distal end  42  of the trocar has passed sensor  52 . 
     In trocar  250  sensor  52  is located approximately at the same distal position, i.e., at the same location measured with respect to axis  37  of the trocar, as label dispensers  232 . The change of location of the sensor means that labels  234  are located in different positions for trocars  230  and  250 . As illustrated in the figures, for trocar  230  labels  234  are located on tool  40  more proximally than their location for trocar  250 . 
       FIG. 5  is a flowchart of steps performed in use of a trocar, and  FIG. 6  is a schematic illustration of results of performing the flowchart steps, according to embodiments of the present invention. For clarity, the description of the flowchart assumes, except where otherwise stated, that tool  40  is inserted into body cavity  12  via trocar  30  ( FIGS. 1A ,  1 B). Those having ordinary skill in the art will be able to adapt the description, mutatis mutandis, for one or more other trocars implemented according to the principles of the present invention, such as trocars  130  and  230 . 
     In an initial step  300  of the flowchart, endoscope  32  is inserted into body cavity  12  via trocar  28 . Trocar  30  is then inserted into the body cavity, and tool  40  is inserted into tubular member  36  of trocar  30 . 
     In an activate indicator step  302 , sensor  52  registers that the tool has passed the sensor, so that tool distal end  42  projects beyond trocar distal end  44 , and the sensor transfers a corresponding signal to processor  16 . The processor activates illuminators  46 , to act as indicators of the location of tool  40  by illuminating the tool. For trocar  130 , the processor activates valves  136  so that liquid flows through openings  134 , the dried liquid on tool  40  acting as an indication of the tool location. For trocar  230 , the processor activates label dispensers  232  to apply labels  234  to tool  40 , the labels acting as an indication of the tool location. 
     In an imaging step  304 , endoscope  32  acquires an image  350  of tissue of body cavity  12  ( FIG. 6 ), using the endoscope light to illuminate the cavity. Assuming distal end  42  of tool is in the field of view of the endoscope, the acquired image includes an image of the distal end. The acquired image also includes an indication  352  of the distal end location provided by the indicators described above. Thus, for trocar  30 , the indication comprises illumination spectrum light reflected or diffused from the surface of distal end  42  of the tool. For trocar  130  the indication comprises endoscope light, after it has been reflected or diffused from the surface of marker  138 , and for trocar  230  the indication comprises endoscope light, after it has been reflected or diffused from the surface of labels  234 . 
     In an image analysis step  306 , processor  16  analyzes the acquired image to isolate the indication light in the image. The analysis uses the fact that the indication light has significantly different characteristics, i.e., a different spectrum, than the light from the body cavity tissue. Typically the analysis is on a pixel by pixel basis, to identify a group of contiguous pixels having a spectrum corresponding to the expected indication light. In the case of trocar  30  the expected spectrum is that of illuminators  46 ; for trocars  130  and  230  the expected spectrum is that from marker  138  or labels  234  under illumination by the endoscope light. 
     In a final, location, step  308 , the processor outputs coordinates of the identified group of pixels for further use by system  10  and/or for systems coupled to system  10 . As a first example, the processor may mark a location  354  ( FIG. 6 ), corresponding to tool distal end  42 , in image  350  formed on display  24 . As a second example, the processor may output the coordinates to a robotic system configured to automatically manipulate instruments such as endoscope  32  and trocar  30 . Those having ordinary skill in the art will be aware of other uses for the coordinates output by the processor, and all such uses are assumed to be comprised within the scope of the present invention. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and 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 subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.