the invention described in this application is an aspect of a larger set of inventions described in the following co - pending applications which are commonly owned by the assignee of the present invention , and are hereby incorporated by reference : u . s . provisional patent application no . 60 / 084 , 724 , filed may . 8 , 1998 , entitled “ apparatus and method for intra - organ measurement and ablation ”; and u . s . provisional patent application no . 60 / 084 , 712 filed may . 8 , 1998 , entitled “ a radio - frequency generator for powering an ablation device ”. the ablation apparatus according to the present invention will be described with respect to two exemplary embodiments . referring to fig1 and 2 , an ablation device according to the present invention is comprised generally of three major components : rf applicator head 2 , main body 4 , and handle 6 . main body 4 includes a shaft 10 . the rf applicator head 2 includes an electrode carrying means 12 mounted to the distal end of the shaft 10 and an array of electrodes 14 formed on the surface of the electrode carrying means 12 . an rf generator 16 is electrically connected to the electrodes 14 to provide mono - polar or bipolar rf energy to them . shaft 10 is an elongate member having a hollow interior . shaft 10 is preferably 12 inches long and has a preferred cross - sectional diameter of approximately 4 mm . a collar 13 is formed on the exterior of the shaft 10 at the proximal end . as best shown in fig6 and 7 , passive spring member 15 are attached to the distal end of the shaft 10 . extending through the shaft 10 is a suction / insufflation tube 17 ( fig6 - 9 ) having a plurality of holes 17 a formed in its distal end . an arched active spring member 19 is connected between the distal ends of the passive spring members 15 and the distal end of the suction / insufflation tube 17 . referring to fig2 , electrode leads 18 a and 18 b extend through the shaft 10 from distal end 20 to proximal end 22 of the shaft 10 . at the distal end 20 of the shaft 10 , each of the leads 18 a , 18 b is coupled to a respective one of the electrodes 14 . at the proximal end 22 of the shaft 10 , the leads 18 a , 18 b are electrically connected to rf generator 16 via an electrical connector 21 . during use , the leads 18 a , 18 b carry rf energy from the rf generator 16 to the electrodes . each of the leads 18 a , 18 b is insulated and carries energy of an opposite polarity than the other lead . electrically insulated sensor leads 23 a , 23 b ( fig5 a and 5b ) also extend through the shaft 10 . contact sensors 25 a , 25 b are attached to the distal ends of the sensor leads 23 a , 23 b , respectively and are mounted to the electrode carrying means 12 . during use , the sensor leads 23 a , 23 b are coupled by the connector 21 to a monitoring module in the rf generator 16 which measures impedance between the sensors 25 a , 25 b . alternatively , a reference pad may be positioned in contact with the patient and the impedance between one of the sensors and the reference pad measured . referring to fig5 b , electrode leads 18 a , 18 b and sensor leads 23 a , 23 b extend through the shaft 10 between the external walls of the tube 17 and the interior walls of the shaft 10 and they are coupled to electrical connector 21 which is preferably mounted to the collar 13 on the shaft 10 . connector 21 , which is connectable to the rf generator 16 , includes at least four electrical contact rings 21 a - 21 d ( fig1 and 2 ) which correspond to each of the leads 18 a , 18 b , 23 a , 23 b . rings 21 a , 21 b receive , from the rf generator , rf energy of positive and negative polarity , respectively . rings 21 c , 21 d deliver signals from the right and left sensors , respectively , to a monitoring module within the rf generator 16 . referring to fig5 a , the electrode carrying means 12 is attached to the distal end 20 of the shaft 10 . a plurality of holes 24 may be formed in the portion of the distal end 20 of the shaft which lies within the electrode carrying means 12 . the electrode carrying means 12 preferably has a shape which approximates the shape of the body organ which is to be ablated . for example , the apparatus shown in fig1 through 11 has a bicornual shape which is desirable for intrauterine ablation . the electrode carrying means 12 shown in these figures includes horn regions 26 which during use are positioned within the cornual regions of the uterus and which therefore extend towards the fallopian tubes . electrode carrying means 12 is preferably a sack formed of a material which is non - conductive , which is permeable to moisture and / or which has a tendency to absorb moisture , and which may be compressed to a smaller volume and subsequently released to its natural size upon elimination of compression . examples of preferred materials for the electrode carrying means include open cell sponge , foam , cotton , fabric , or cotton - like material , or any other material having the desired characteristics . alternatively , the electrode carrying means may be formed of a metallized fabric . for convenience , the term “ pad ” may be used interchangeably with the term electrode carrying means to refer to an electrode carrying means formed of any of the above materials or having the listed properties . electrodes 14 are preferably attached to the outer surface of the electrode carrying means 12 , such as by deposition or other attachment mechanism . the electrodes are preferably made of lengths of silver , gold , platinum , or any other conductive material . the electrodes may be attached to the electrode carrying means 12 by electron beam deposition , or they may be formed into coiled wires and bonded to the electrode carrying member using a flexible adhesive . naturally , other means of attaching the electrodes , such as sewing them onto the surface of the carrying member , may alternatively be used . if the electrode carrying means 12 is formed of a metallized fabric , an insulating layer may be etched onto the fabric surface , leaving only the electrode regions exposed . the spacing between the electrodes ( i . e . the distance between the centers of adjacent electrodes ) and the widths of the electrodes are selected so that ablation will reach predetermined depths within the tissue , particularly when maximum power is delivered through the electrodes ( where maximum power is the level at which low impedance , low voltage ablation can be achieved ). the depth of ablation is also effected by the electrode density ( i . e ., the percentage of the target tissue area which is in contact with active electrode surfaces ) and may be regulated by pre - selecting the amount of this active electrode coverage . for example , the depth of ablation is much greater when the active electrode surface covers more than 10 % of the target tissue than it is when the active electrode surfaces covers 1 % of the target tissue . for example , by using 3 - 6 mm spacing and an electrode width of approximately 0 . 5 - 2 . 5 mm , delivery of approximately 20 - 40 watts over a 9 - 16 cm 2 target tissue area will cause ablation to a depth of approximately 5 - 7 millimeters when the active electrode surface covers more than 10 % of the target tissue area . after reaching this ablation depth , the impedance of the tissue will become so great that ablation will self - terminate as described with respect to the operation of the invention . by contrast , using the same power , spacing , electrode width , and rf frequency will produce an ablation depth of only 2 - 3 mm when the active electrode surfaces covers less than 1 % of the target tissue area . this can be better understood with reference to fig1 a , in which high surface density electrodes are designated 14 a and low surface density electrodes are designated 14 b . for purposes of this comparison between low and high surface density electrodes , each bracketed group of low density electrodes is considered to be a single electrode . thus , the electrode widths w and spacings s extend as shown in fig1 a . as is apparent from fig1 a , the electrodes 14 a , which have more active area in contact with the underlying tissue t , produce a region of ablation a 1 that extends more deeply into the tissue t than the ablation region a 2 produced by the low density electrodes 14 b , even though the electrode spacings and widths are the same for the high and low density electrodes . some examples of electrode widths , having spacings with more than 10 % active electrode surface coverage , and their resultant ablation depth , based on an ablation area of 6 cm 2 and a power of 20 - 40 watts , are given on the following table : examples of electrode widths , having spacings with less than 1 % active electrode surface coverage , and their resultant ablation depth , based on an ablation area of 6 cm 2 and a power of 20 - 40 watts , are given on the following table : thus it can be seen that the depth of ablation is significantly less when the active electrode surface coverage is decreased . in the preferred embodiment , the preferred electrode spacing is approximately 8 - 10 mm in the horn regions 26 with the active electrode surfaces covering approximately 1 % of the target region . approximately 1 - 2 mm electrode spacing ( with 10 % active electrode coverage ) is preferred in the cervical region ( designated 28 ) and approximately 3 - 6 mm ( with greater than 10 % active electrode surface coverage ) is preferred in the main body region . the rf generator 16 may be configured to include a controller which gives the user a choice of which electrodes should be energized during a particular application in order to give the user control of ablation depth . for example , during an application for which deep ablation is desired , the user may elect to have the generator energize every other electrode , to thereby optimize the effective spacing of the electrodes and to decrease the percentage of active electrode surface coverage , as will be described below with respect to fig1 . although the electrodes shown in the drawings are arranged in a particular pattern , it should be appreciated that the electrodes may be arranged in any pattern to provide ablation to desired depths . referring to fig6 and 7 , an introducer sheath 32 facilitates insertion of the apparatus into , and removal of the apparatus from , the body organ to be ablated . the sheath 32 is a tubular member which is telescopically slidable over the shaft 10 . the sheath 32 is slidable between a distal condition , shown in fig6 , in which the electrode carrying means 12 is compressed inside the sheath , and a proximal condition in which the sheath 32 is moved proximally to release the electrode carrying means from inside it ( fig7 ). by compressing the electrode carrying means 12 to a small volume , the electrode carrying means and electrodes can be easily inserted into the body cavity ( such as into the uterus via the vaginal opening ). a handle 34 attached to the sheath 32 provides finger holds to allow for manipulation of the sheath 32 . handle 34 is slidably mounted on a handle rail 35 which includes a sleeve 33 , a finger cutout 37 , and a pair of spaced rails 35 a , 35 b extending between the sleeve 33 and the finger cutout 37 . the shaft 10 and sheath 32 slidably extend through the sleeve 33 and between the rails 35 a , 35 b . the tube 17 also extends through the sleeve 33 and between the rails 35 a , 35 b , and its proximal end is fixed to the handle rail 35 near the finger cutout 37 . a compression spring 39 is disposed around the proximal most portion of the suction / insufflation tube 17 which lies between the rails 35 a , 35 b . one end of the compression spring 39 rests against the collar 13 on the shaft 10 , while the opposite end of the compression spring rests against the handle rail 35 . during use , the sheath 32 is retracted from the electrode carrying means 12 by squeezing the handle 34 towards the finger cutout 37 to slide the sheath 32 in the distal direction . when the handle 34 advances against the collar 13 , the shaft 10 ( which is attached to the collar 13 ) is forced to slide in the proximal direction , causing compression of the spring 39 against the handle rail 35 . the movement of the shaft 10 relative to the suction / insufflation tube 17 causes the shaft 10 to pull proximally on the passive spring member 15 . proximal movement of the passive spring member 15 in turn pulls against the active spring member 19 , causing it to move to the opened condition shown in fig7 . unless the shaft is held in this retracted condition , the compression spring 39 will push the collar and thus the shaft distally , forcing the rf applicator head to close . a locking mechanism ( not shown ) may be provided to hold the shaft in the fully withdrawn condition to prevent inadvertent closure of the spring members during the ablation procedure . the amount by which the springs 15 , 19 are spread may be controlled by manipulating the handle 34 to slide the shaft 10 ( via collar 13 ), proximally or distally . such sliding movement of the shaft 10 causes forceps - like movement of the spring members 15 , 19 . a flow pathway 36 is formed in the handle rail 35 and is fluidly coupled to a suction / insufflation port 38 . the proximal end of the suction / insufflation tube 17 is fluidly coupled to the flow pathway so that gas fluid may be introduced into , or withdrawn from the suction / insufflation tube 17 via the suction / insufflation port 38 . for example , suction may be applied to the fluid port 38 using a suction / insufflation unit 40 . this causes water vapor within the uterine cavity to pass through the permeable electrode carrying means 12 , into the suction / insufflation tube 17 via holes 17 a , through the tube 17 , and through the suction / insufflation unit 40 via the port 38 . if insufflation of the uterine cavity is desired , insufflation gas , such as carbon dioxide , may be introduced into the suction / insufflation tube 17 via the port 38 . the insufflation gas travels through the tube 17 , through the holes 17 a , and into the uterine cavity through the permeable electrode carrying member 12 . if desirable , additional components may be provided for endoscopic visualization purposes . for example , lumen 42 , 44 , and 46 may be formed in the walls of the introducer sheath 32 as shown in fig5 b . an imaging conduit , such as a fiberoptic cable 48 , extends through lumen 42 and is coupled via a camera cable 43 to a camera 45 . images taken from the camera may be displayed on a monitor 56 . an illumination fiber 50 extends through lumen 44 and is coupled to an illumination source 54 . the third lumen 46 is an instrument channel through which surgical instruments may be introduced into the uterine cavity , if necessary . because during use it is most desirable for the electrodes 14 on the surface of the electrode carrying means 12 to be held in contact with the interior surface of the organ to be ablated , the electrode carrying means 12 may be provide to have additional components inside it that add structural integrity to the electrode carrying means when it is deployed within the body . for example , referring to fig1 , alternative spring members 15 a , 19 a may be attached to the shaft 10 and biased such that , when in a resting state , the spring members are positioned in the fully resting condition shown in fig1 . such spring members would spring to the resting condition upon withdrawal of the sheath 32 from the rf applicator head 2 . alternatively , a pair of inflatable balloons 52 may be arranged inside the electrode carrying means 12 as shown in fig2 and connected to a tube ( not shown ) extending through the shaft 10 and into the balloons 52 . after insertion of the apparatus into the organ and following retraction of the sheath 32 , the balloons 52 would be inflated by introduction of an inflation medium such as air into the balloons via a port similar to port 38 using an apparatus similar to the suction / insufflation apparatus 40 . structural integrity may also be added to the electrode carrying means through the application of suction to the proximal end 22 of the suction / insufflation tube 17 . application of suction using the suction / insufflation device 40 would draw the organ tissue towards the electrode carrying means 12 and thus into better contact with the electrodes 14 . fig1 and 13 show an alternative embodiment of an ablation device according to the present invention . in the alternative embodiment , an electrode carrying means 12 a is provided which has a shape which is generally tubular and thus is not specific to any particular organ shape . an ablation device having a general shape such as this may be used anywhere within the body where ablation or coagulation is needed . for example , the alternative embodiment is useful for bleeding control during laparoscopic surgery ( fig1 ), tissue ablation in the prostate gland ( fig1 ), and also intrauterine ablation ( fig1 and 16 ). operation of the first exemplary embodiment of an ablation device according to the present invention will next be described . referring to fig1 , the device is initially configured for use by positioning the introducer sheath 32 distally along the shaft 10 , such that it compresses the electrode carrying means 12 within its walls . at this time , the electrical connector 21 is connected to the rf generator 16 , and the fiberoptic cable 48 and the illumination cable 50 are connected to the illumination source , monitor , and camera , 54 , 56 , 45 . the suction / insufflation unit 40 is attached to suction / insufflation port 38 on the handle rail 35 . the suction / insufflation unit 40 is preferably set to deliver carbon dioxide at an insufflation pressure of 20 - 200 mmhg . next , the distal end of the apparatus is inserted through the vaginal opening v and into the uterus u as shown in fig6 , until the distal end of the introducer sheath 32 contacts the fundus f of the uterus . at this point , carbon dioxide gas is introduced into the tube 17 via the port 38 , and it enters the uterine cavity , thereby expanding the uterine cavity from a flat triangular shape to a 1 - 2 cm high triangular cavity . the physician may observe ( using the camera 45 and monitor 56 ) the internal cavities using images detected by a fiberoptic cable 48 inserted through lumen 42 . if , upon observation , the physician determines that a tissue biopsy or other procedure is needed , the required instruments may be inserted into the uterine cavity via the instrument channel 46 . following insertion , the handle 34 is withdrawn until it abuts the collar 13 . at this point , the sheath 32 exposes the electrode carrying member 12 but the electrode carrying member 12 is not yet fully expanded ( see fig9 ), because the spring members 15 , 19 have not yet been moved to their open condition . the handle 34 is withdrawn further , causing the shaft 10 to move proximally relative to the suction / insufflation tube 17 , causing the passive spring members 15 to pull the active spring members 19 , causing them to open into the opened condition showing in fig1 . the physician may confirm proper positioning of the electrode carrying member 12 using the monitor 56 , which displays images from the fiberoptic cable 48 . proper positioning of the device and sufficient contact between the electrode carrying member 12 and the endometrium may further be confirmed using the contact sensors 25 a , 25 b . the monitoring module of the rf generator measures the impedance between these sensors using conventional means . if there is good contact between the sensors and the endometrium , the measured impedance will be approximately 20 - 180 ohm , depending on the water content of the endometrial lining . the sensors are positioned on the distal portions of the bicornual shaped electrode carrying member 12 , which during use are positioned in the regions within the uterus in which it is most difficult to achieve good contact with the endometrium . thus , an indication from the sensors 25 a , 25 b that there is sound contact between the sensors and the endometrial surface indicates that good electrode contact has been made with the endometrium . next , insufflation is terminated . approximately 1 - 5 cc of saline may be introduced via suction / insufflation tube 17 to initially wet the electrodes and to improve electrode electrical contact with the tissue . after introduction of saline , the suction / insufflation device 40 is switched to a suctioning mode . as described above , the application of suction to the rf applicator head 2 via the suction / insufflation tube 17 collapses the uterine cavity onto the rf applicator head 2 and thus assures better contact between the electrodes and the endometrial tissue . if the generally tubular apparatus of fig1 and 13 is used , the device is angled into contact with one side of the uterus during the ablation procedure . once ablation is completed , the device ( or a new device ) is repositioned in contact with the opposite side and the procedure is repeated . see . fig1 and 16 . next , rf energy at preferably about 500 khz and at a constant power of approximately 30 w is applied to the electrodes . as shown in fig5 a , it is preferable that each electrode be energized at a polarity opposite from that of its neighboring electrodes . by doing so , energy field patterns , designated f 1 , f 2 and f 4 in fig1 , are generated between the electrode sites and thus help to direct the flow of current through the tissue t to form a region of ablation a . as can be seen in fig1 , if electrode spacing is increased such by energizing , for example every third or fifth electrode rather than all electrodes , the energy patterns will extend more deeply into the tissue . ( see , for example , pattern f 2 which results from energization of electrodes having a non - energized electrode between them , or pattern f 4 which results from energization of electrodes having two non - energized electrodes between them ). moreover , ablation depth may be controlled as described above by providing low surface density electrodes on areas of the electrode carrying member which will contact tissue areas at which a smaller ablation depth is required ( see fig1 a ). referring to fig1 b , if multiple , closely spaced , electrodes 14 are provided on the electrode carrying member , a user may set the rf generator to energize electrodes which will produce a desired electrode spacing and active electrode area . for example , alternate electrodes may be energized as shown in fig1 b , with the first three energized electrodes having positive polarity , the second three having negative polarity , etc . as another example , shown in fig1 c , if greater ablation depth is desired the first five electrodes may be positively energized , and the seventh through eleventh electrodes negatively energized , with the sixth electrode remaining inactivated to provide adequate electrode spacing . as the endometrial tissue heats , moisture begins to be released from the tissue . the moisture permeates the electrode carrying member 12 and is thereby drawn away from the electrodes . the moisture may pass through the holes 17 a in the suction / insufflation tube 17 and leave the suction / insufflation tube 17 at its proximal end via port 38 as shown in fig7 . moisture removal from the ablation site may be further facilitated by the application of suction to the shaft 10 using the suction / insufflation unit 40 . removal of the moisture from the ablation site prevents formation of a liquid layer around the electrodes . as described above , liquid build - up at the ablation site is detrimental in that provides a conductive layer that carries current from the electrodes even when ablation has reached the desired depth . this continued current flow heats the liquid and surrounding tissue , and thus causes ablation to continue by unpredictable thermal conduction means . tissue which has been ablated becomes dehydrated and thus decreases in conductivity . by shunting moisture away from the ablation site and thus preventing liquid build - up , there is no liquid conductor at the ablation area during use of the ablation device of the present invention . thus , when ablation has reached the desired depth , the impedance at the tissue surface becomes sufficiently high to stop or nearly stop the flow of current into the tissue . rf ablation thereby stops and thermal ablation does not occur in significant amounts . if the rf generator is equipped with an impedance monitor , a physician utilizing the ablation device can monitor the impedance at the electrodes and will know that ablation has self - terminated once the impedance rises to a certain level and then remains fairly constant . by contrast , if a prior art bipolar rf ablation device was used together with an impedance monitor , the presence of liquid around the electrodes would cause the impedance monitor to give a low impedance reading regardless of the depth of ablation which had already been carried out , since current would continue to travel through the low - impedance liquid layer . other means for monitoring and terminating ablation may also be provided . for example , a thermocouple or other temperature sensor may be inserted to a predetermined depth in the tissue to monitor the temperature of the tissue and terminate the delivery of rf energy or otherwise signal the user when the tissue has reached a desired ablation temperature . once the process has self terminated , 1 - 5 cc of saline can be introduced via suction / insufflation tube 17 and allowed to sit for a short time to aid separation of the electrode from the tissue surface . the suction insufflation device 40 is then switched to provide insufflation of carbon dioxide at a pressure of 20 - 200 mmhg . the insufflation pressure helps to lift the ablated tissue away from the rf applicator head 2 and to thus ease the closing of the rf applicator head . the rf applicator head 2 is moved to the closed position by sliding the handle 34 in a distal direction to fold the spring members 15 , 19 along the axis of the device and to cause the introducer sheath 32 to slide over the folded rf applicator head . the physician may visually confirm the sufficiency of the ablation using the monitor 56 . finally , the apparatus is removed from the uterine cavity . a second embodiment of an ablation device 100 in accordance with the present invention is shown in fig2 - 37b . the second embodiment differs from the first embodiment primarily in its electrode pattern and in the mechanism used to deploy the electrode applicator head or array . naturally , aspects of the first and second exemplary embodiments and their methods of operation may be combined without departing from the scope of the present invention . referring to fig2 and 22 , the second embodiment includes an rf applicator head 102 , a sheath 104 , and a handle 106 . as with the first embodiment , the applicator head 102 is slidably disposed within the sheath 104 ( fig2 ) during insertion of the device into the uterine cavity , and the handle 106 is subsequently manipulated to cause the applicator head 102 to extend from the distal end of the sheath 104 ( fig2 ) and to expand into contact with body tissue ( fig3 ). referring to fig2 , in which the sheath 104 is not shown for clarity , applicator head 102 extends from the distal end of a length of tubing 108 which is slidably disposed within the sheath 104 . applicator head 102 includes an external electrode array 102 a and an internal deflecting mechanism 102 b used to expand and tension the array for positioning into contact with the tissue . referring to fig2 a and 25b , the array 102 a of applicator head 102 is formed of a stretchable metallized fabric mesh which is preferably knitted from a nylon and spandex knit plated with gold or other conductive material . in one array design , the knit ( shown in fig2 a and 26b ) is formed of three monofilaments of nylon 109 a knitted together with single yarns of spandex 109 b . each yarn of spandex 109 b has a double helix 109 c of five nylon monofilaments coiled around it . this knit of elastic ( spandex ) and inelastic ( nylon ) yarns is beneficial for a number of reasons . for example , knitting elastic and relatively inelastic yarns allows the overall deformability of the array to be pre - selected . the mesh is preferably constructed so as to have greater elasticity in the transverse direction ( t ) than in the longitudinal direction ( l ). in a preferred mesh design , the transverse elasticity is on the order of approximately 300 % whereas the longitudinal elasticity is on the order of approximately 100 %. the large transverse elasticity of the array allows it to be used in a wide range of uterine sizes . another advantage provided by the combination of elastic and relatively inelastic yarns is that the elastic yarns provide the needed elasticity to the array while the relatively inelastic yarns provide relatively non - stretchable members to which the metallization can adhere without cracking during expansion of the array . in the knit configuration described above , the metallization adheres to the nylon coiled around the spandex . during expansion of the array , the spandex elongates and the nylon double helix at least partially elongates from its coiled configuration . one process which may be used to apply the gold to the nylon / spandex knit involves plating the knit with silver using known processes which involve application of other materials as base layers prior to application of the silver to ensure that the silver will adhere . next , the insulating regions 110 ( described below ) are etched onto the silver , and afterwards the gold is plated onto the silver . gold is desirable for the array because of it has a relatively smooth surface , is a very inert material , and has sufficient ductility that it will not crack as the nylon coil elongates during use . the mesh may be configured in a variety of shapes , including but not limited to the triangular shape s 1 , parabolic s 2 , and rectangular s 3 shapes shown in fig2 a , 27 b and 27 c , respectively . turning again to fig2 a and 25b , when in its expanded state , the array 102 a includes a pair of broad faces 112 spaced apart from one another . narrower side faces 114 extend between the broad faces 112 along the sides of the applicator head 102 , and a distal face 116 extends between the broad faces 112 at the distal end of the applicator head 102 . insulating regions 110 are formed on the applicator head to divide the mesh into electrode regions . the insulated regions 110 are preferably formed using etching techniques to remove the conductive metal from the mesh , although alternate methods may also be used , such as by knitting conductive and non - conductive materials together to form the array . the array may be divided by the insulated regions 110 into a variety of electrode configurations . in a preferred configuration the insulating regions 110 divide the applicator head into four electrodes 118 a - 118 d by creating two electrodes on each of the broad faces 112 . to create this four - electrode pattern , insulating regions 110 are placed longitudinally along each of the broad faces 112 as well as along the length of each of the faces 114 , 16 . the electrodes 118 a - 118 d are used for ablation and , if desired , to measure tissue impedance during use . deflecting mechanism 102 b and its deployment structure is enclosed within electrode array 102 a . referring to fig2 , external hypotube 120 extends from tubing 108 and an internal hypotube 122 is slidably and co - axially disposed within hypotube 120 . flexures 124 extend from the tubing 108 on opposite sides of external hypotube 120 . a plurality of longitudinally spaced apertures 126 ( fig2 ) are formed in each flexure 124 . during use , apertures 126 allow moisture to pass through the flexures and to be drawn into exposed distal end of hypotube 120 using a vacuum source fluidly coupled to hypotube 120 . each flexure 124 preferably includes conductive regions that are electrically coupled to the array 102 a for delivery of rf energy to the body tissue . referring to fig2 , strips 128 of copper tape or other conductive material extend along opposite surfaces of each flexure 124 . each strip 128 is electrically insulated from the other strip 128 by a non - conductive coating on the flexure . conductor leads ( not shown ) are electrically coupled to the strips 128 and extend through tubing 108 ( fig2 ) to an electrical cord 130 ( fig2 ) which is attachable to the rf generator . during use , one strip 128 on each conductor is electrically coupled via the conductor leads to one terminal on the rf generator while the other strip is electrically coupled to the opposite terminal , thus causing the array on the applicator head to have regions of alternating positive and negative polarity . the flexures may alternatively be formed using a conductive material or a conductively coated material having insulating regions formed thereon to divide the flexure surfaces into multiple conductive regions . moreover , alternative methods such as electrode leads independent of the flexures 124 may instead be used for electrically connecting the electrode array to the source of rf energy . it is important to ensure proper alignment between the conductive regions of the flexures 124 ( e . g . copper strips 128 ) and the electrodes 118 a - 118 d in order to maintain electrical contact between the two . strands of thread 134 ( which may be nylon ) ( fig2 ) are preferably sewn through the array 102 a and around the flexures 124 in order to prevent the conductive regions 128 from slipping out of alignment with the electrodes 118 a - 118 d . alternate methods for maintaining contact between the array 102 a and the conductive regions 128 include using tiny bendable barbs extending between the flexures 124 and the array 102 a to hook the array to the conductive regions 128 , or bonding the array to the flexures using an adhesive applied along the insulating regions of the flexures . referring again to fig2 , internal flexures 136 extend laterally and longitudinally from the exterior surface of hypotube 122 . each internal flexure 136 is connected at its distal end to one of the flexures 124 and a transverse ribbon 138 extends between the distal portions of the internal flexures 136 . transverse ribbon 138 is preferably pre - shaped such that when in the relaxed condition the ribbon assumes the corrugated configuration shown in fig2 and such that when in a compressed condition it is folded along the plurality of creases 140 that extend along its length . flexures 124 , 136 and ribbon 138 are preferably an insulated spring material such as heat treated 17 - 7 ph stainless steel . the deflecting mechanism is preferably configured such that the distal tips of the flexures 124 are sufficiently flexible to prevent tissue puncture during deployment and / or use . such an atraumatic tip design may be carried out in a number of ways , such as by manufacturing the distal sections 124 a ( fig2 ) of the flexures from a material that is more flexible than the proximal sections 124 b . for example , flexures 124 may be provided to have proximal sections formed of a material having a modulus of approximately 28 × 10 6 psi and distal sections having a durometer of approximately 72d . alternatively , referring to fig3 , the flexures 124 may be joined to the internal flexures 136 at a location more proximal than the distal tips of the flexures 124 , allowing them to move more freely and to adapt to the contour of the surface against which they are positioned ( see dashed lines in fig3 ). given that uterine sizes and shapes vary widely between women , the atraumatic tip design is further beneficial in that it allows the device to more accurately conform to the shape of the uterus in which it is deployed while minimizing the chance of injury . the deflecting mechanism formed by the flexures 124 , 136 , and ribbon 138 forms the array into the substantially triangular shape shown in fig2 , which is particularly adaptable to most uterine shapes . as set forth in detail below , during use distal and proximal grips 142 , 144 forming handle 106 are squeezed towards one another to withdraw the sheath and deploy the applicator head . this action results in relative rearward motion of the hypotube 120 and relative forward motion of the hypotube 122 . the relative motion between the hypotubes causes deflection in flexures 124 , 136 which deploys and tensions the electrode array 102 a . the ablation device according to the second embodiment includes a measurement device for easily measuring the uterine width and for displaying the measured width on a gauge 146 ( fig2 ). the measurement device utilizes non - conductive ( e . g . nylon ) suturing threads 148 that extend from the hypotube 122 and that have distal ends attached to the distal portion of the deflecting mechanism ( fig2 ). as shown in fig2 , threads 148 are preferably formed of a single strand 150 threaded through a wire loop 152 and folded over on itself . wire loop 152 forms the distal end of an elongate wire 154 which may be formed of stainless steel or other wire . referring to fig3 , wire 154 extends through the hypotube 122 and is secured to a rotatable bobbin 156 . the rotatable bobbin 156 includes a dial face 158 preferably covered in a clear plastic . as can be seen in fig3 , dial face 158 includes calibration markings corresponding to an appropriate range of uterine widths . the bobbin is disposed within a gauge housing 160 and a corresponding marker line 162 is printed on the gauge housing . a torsion spring 164 provides rotational resistance to the bobbin 156 . expansion of the applicator head 102 during use pulls threads 148 ( fig2 ) and thus wire 154 ( fig2 ) in a distal direction . wire 154 pulls against the bobbin 156 ( fig3 ), causing it to rotate . rotation of the bobbin positions one of the calibration markings on dial face 158 into alignment with the marker line 162 ( fig3 b ) to indicate the distance between the distal tips of flexures 124 and thus the uterine width . the uterine width and length ( as determined using a conventional sound or other means ) are preferably input into an rf generator system and used by the system to calculate an appropriate ablation power as will be described below . alternately , the width as measured by the apparatus of the invention and length as measured by other means may be used by the user to calculate the power to be supplied to the array to achieve the desired ablation depth . the uterine width may alternatively be measured using other means , including by using a strain gauge in combination with an a / d converter to transduce the separation distance of the flexures 124 and to electronically transmit the uterine width to the rf generator . the most optimal electrocoagulation occurs when relatively deep ablation is carried out in the regions of the uterus at which the endometrium is thickest , and when relatively shallower ablation is carried out in areas in which the endometrium is shallower . a desirable range of ablation depths includes approximately 2 - 3 mm for the cervical os and the cornual regions , and approximately 7 - 8 mm in the main body of the uterus where the endometrium is substantially thicker . as discussed with respect to the first embodiment , a number of factors influence the ablation depth that can be achieved using a given power applied to a bipolar electrode array . these include the power supplied by the rf generator , the distance between the centers of adjacent electrodes (“ center - to - center distance ”), the electrode density ( i . e ., the porosity of the array fabric or the percent of the array surface that is metallic ), the edge gap ( i . e . the distance between the edges of adjacent electrode poles ), and the electrode surface area . other factors include blood flow ( which in slower - ablating systems can dissipate the rf ) and the impedance limit . certain of these factors may be utilized in the present invention to control ablation depth and to provide deeper ablation at areas requiring deeper ablation and to provide shallower regions in areas where deep ablation is not needed . for example , as center - to - center distance increases , the depth of ablation increases until a point where the center to center distance is so great that the strength of the rf field is too diffuse to excite the tissue . it can been seen with reference to fig3 that the center to center distance d1 between the electrodes 118 a , 118 b is larger within the region of the array that lies in the main body of the uterus and thus contributes to deeper ablation . the center to center distance d2 between electrodes 118 a , 118 b is smaller towards the cervical canal where it contributes to shallower ablation . at the distal end of the device , the shorter center to center distances d3 extend between top and bottom electrodes 118 b , 118 c and 118 a , 118 d and again contribute to shallower ablation . naturally , because the array 102 a expands to accommodate the size of the uterus in which it is deployed , the dimensions of the array 102 a vary . one embodiment of the array 102 a includes a range of widths of at least approximately 2 . 5 - 4 . 5 cm , a range of lengths of at least approximately 4 - 6 cm , and a density of approximately 35 %- 45 %. the power supplied to the array by the rf generator is calculated by the rf generator system to accommodate the electrode area required for a particular patient . as discussed above , the uterine width is measured by the applicator head 102 and displayed on gauge 146 . the uterine length is measured using a sound , which is an instrument conventionally used for that purpose . it should be noted that calibration markings of the type used on a conventional sound device , or other structure for length measurement , may be included on the present invention to allow it to be used for length measurement as well . the user enters the measured dimensions into the rf generator system using an input device , and the rf generator system calculates or obtains the appropriate set power from a stored look - up table using the uterine width and length as entered by the user . an eprom within the rf generator system converts the length and width to a set power level according to the following relationship : where p is the power level in watts , l is the length in centimeters , w is the width in centimeters , and 5 . 5 is a constant having units of watts per square centimeter . alternatively , the user may manually calculate the power setting from the length and width , or s / he may be provided with a table of suggested power settings for various electrode areas ( as determined by the measured length and width ) and will manually set the power on the rf generator accordingly . referring again to fig2 and 22 , the handle 106 of the rf ablation device according to the second embodiment includes a distal grip section 142 and a proximal grip section 144 that are pivotally attached to one another at pivot pin 166 . the proximal grip section 144 is coupled to the hypotube 122 ( fig2 ) via yoke 168 , overload spring 170 and spring stop 172 , each of which is shown in the section view of fig3 . the distal grip section 142 is coupled to the external hypotube 120 via male and female couplers 174 , 176 ( see fig3 a and 32b ). squeezing the grip sections 142 , 144 towards one another thus causes relative movement between the external hypotube 120 and the internal hypotube 122 . this relative sliding movement results in deployment of the deflecting mechanism 102 b from the distal end of the sheath and expansion of the array 102 a to its expanded state . referring to fig3 a and b , rack 180 is formed on male coupler 174 and calibration markings 182 are printed adjacent the rack 180 . the calibration markings 182 correspond to a variety of uterine lengths and may include lengths ranging from , for example , 4 . 0 to 6 . 0 cm in 0 . 5 cm increments . a sliding collar 184 is slidably disposed on the tubing 108 and is slidable over male coupler 174 . sliding collar 184 includes a rotating collar 186 and a female coupler 176 that includes a wedge - shaped heel 188 . a locking spring member 190 ( fig3 b and 35 ) extends across an aperture 192 formed in the proximal grip 144 in alignment with the heel 188 . when the distal and proximal handle sections are squeezed together to deploy the array , the heel 188 passes into the aperture 192 . its inclined lower surface gradually depresses the spring member 190 as the heel moves further into the aperture 192 . see fig3 a and 36b . after passing completely over the spring member , the heel moves out of contact with the spring member . the spring member snaps upwardly thereby engaging the heel in the locked position . see fig3 c . a release lever 194 ( fig3 ) is attached to the free end of the spring member 190 . to disengage the spring lock , release lever 194 is depressed to lower spring member 190 so that the inclined heel can pass over the spring member and thus out of the aperture 192 . referring again to fig3 a and 32b , sliding collar 184 is configured to allow the user to limit longitudinal extension of the array 102 a to a distance commensurate with a patient &# 39 ; s predetermined uterine length . it does so by allowing the user to adjust the relative longitudinal position of male coupler 174 relative to the female coupler 176 using the rotating collar 186 to lock and unlock the female coupler from the rack 180 and the male coupler 174 . locking the female coupler to the rack 180 and male coupler 174 will limit extension of the array to approximately the predetermined uterine length , as shown on the calibration markings 182 . once the uterine length has been measured using a conventional sound , the user positions sliding collar 184 adjacent to calibration marks 182 corresponding to the measured uterine length ( e . g . 4 . 5 cm ). afterwards , the user rotates the collar section 186 to engage its internally positioned teeth with the rack 180 . this locks the longitudinal position of the heel 188 such that it will engage with the spring member 190 on the proximal grip when the array has been exposed to the length set by the sliding collar . the handle 106 includes a pair of spring assemblies which facilitate controlled deployment and stowage of the array 102 a . one of the spring assemblies controls movement of the grips 142 , 144 to automatically stow the array 102 a into the sheath 104 when the user stops squeezing the grips 142 , 144 towards one another . the other of the spring assemblies controls the transverse movement of the spring flexures 124 to the expanded condition by limiting the maximum load that can be applied to the deployment mechanism 102 b . fig3 shows the distal and proximal grips 142 and 144 in partial cross - section . the first spring assembly for controlled stowage includes a handle return mandrel 196 that is slidably disposed within the proximal grip 144 . a compression spring 198 surrounds a portion of the return mandrel 196 , and a retaining ring 200 is attached to the mandrel 196 above the spring 198 . a spring stop 202 is disposed between the spring 198 and the retaining ring . the lowermost end of the return mandrel 196 is pivotally engaged by a coupling member 204 on distal grip 142 . relative movement of the grips 142 , 144 towards one another causes the coupling member 204 to pull the return member downwardly with the proximal grip 144 as indicated by arrows . downward movement of the mandrel 196 causes its retaining ring 200 and spring stop 202 to bear downwardly against the compression spring 198 , thereby providing a movement which acts to rotate the grips 142 , 144 away from one another . when tension against the grips 142 , 144 is released ( assuming that heel 188 is not locked into engagement with spring member 190 ) the grips rotate apart into the opened position as the compression spring 198 returns to the initial state , stowing the applicator head inside the sheath . the second spring assembly for controlling array deployment is designed to control separation of the flexures . it includes a frame member 178 disposed over yoke 168 , which is pivotally attached to proximal grip 144 . tubing 108 extends from the array 102 a ( see fig2 ), through the sheath 104 and is fixed at its proximal end to the frame member 178 . hypotube 122 does not terminate at this point but instead extends beyond the proximal end of tubing 108 and through a window 206 in the frame member . its proximal end 208 is slidably located within frame member 178 proximally of the window 206 and is fluidly coupled to a vacuum port 210 by fluid channel 212 . hypotube 120 terminates within the frame . its proximal end is fixed within the distal end of the frame . a spring stop 214 is fixed to a section of the hypotube within the window 206 , and a compression spring 170 is disposed around the hypotube between the spring stop 172 and yoke 168 . see fig3 b and 34 . when the distal and proximal grips are moved towards one another , the relative rearward motion of the distal grip causes the distal grip to withdraw the sheath 104 from the array 102 a . referring to fig3 a and 37b , this motion continues until female coupler 176 contacts and bears against frame member 178 . continued motion between the grips causes a relative rearward motion in the frame which causes the same rearward relative motion in external hypotube 120 . an opposing force is developed in yoke 168 , which causes a relative forward motion in hypotube 122 . the relative motion between the hypotubes causes deflection in flexures 124 , 136 which deflect in a manner that deploys and tensions the electrode array . compression spring 170 acts to limit the force developed by the operator against hypotubes 120 , 122 , thus limiting the force of flexures 124 , 136 acting on the array and the target tissue surrounding the array . referring to fig2 , collar 214 is slidably mounted on sheath 104 . before the device is inserted into the uterus , collar 214 can be positioned along sheath 104 to the position measured by the uterine sound . once in position , the collar provides visual and tactile feedback to the user to assure the device has been inserted the proper distance . in addition , after the applicator head 102 has been deployed , if the patient &# 39 ; s cervical canal diameter is larger than the sheath dimensions , the collar 214 can be moved distally towards the cervix , making contact with it and creating a pneumatic seal between the sheath and cervix . in preparation for ablating the uterus utilizing the second exemplary embodiment , the user measures the uterine length using a uterine sound device . the user next positions sliding collar 184 ( fig3 b ) adjacent to calibration marks 182 corresponding to the measured uterine length ( e . g . 4 . 5 cm ) and rotates the collar section 186 to engage its internally positioned teeth with the rack 180 . this locks the longitudinal position of the heel 188 ( fig3 a ) such that it will engage with the spring member 190 when the array has been exposed to the length set by the sliding collar . next , with the grips 142 , 144 in their resting positions to keep the applicator head 102 covered by sheath 104 , the distal end of the device 100 is inserted into the uterus . once the distal end of the sheath 104 is within the uterus , grips 142 , 144 are squeezed together to deploy the applicator head 102 from sheath 104 . grips 142 , 144 are squeezed until heel 188 engages with locking spring member 190 as described with respect to fig3 a through 36c . at this point , deflecting mechanism 102 b has deployed the array 102 a into contact with the uterine walls . the user reads the uterine width , which as described above is transduced from the separation of the spring flexures , from gauge 146 . the measured length and width are entered into the rf generator system 250 ( fig2 ) and used to calculate the ablation power . vacuum source 252 ( fig2 ) is activated , causing application of suction to hypotube 122 via suction port 210 . suction helps to draw uterine tissue into contact with the array 102 . ablation power is supplied to the electrode array 102 a by the rf generator system 250 . the tissue is heated as the rf energy passes from electrodes 118 a - d to the tissue , causing moisture to be released from the tissue . the vacuum source helps to draw moisture from the uterine cavity into the hypotube 122 . moisture withdrawal is facilitated by the apertures 121 formed in flexures 124 by preventing moisture from being trapped between the flexures 124 and the lateral walls of the uterus . if the rf generator 250 includes an impedance monitoring module , impedance may be monitored at the electrodes 118 a - d and the generator may be programmed to terminate rf delivery automatically once the impedance rises to a certain level . the generator system may also or alternatively display the measured impedance and allow the user to terminate rf delivery when desired . when rf delivery is terminated , the user depresses release lever 194 to disengage heel 188 from locking spring member 190 and to thereby allow grips 142 , 144 to move to their expanded ( resting condition ). release of grips 142 , 144 causes applicator head 102 to retract to its unexpanded condition and further causes applicator head 102 to be withdrawn into the sheath 104 . finally , the distal end of the device 100 is withdrawn from the uterus . two embodiments of ablation devices in accordance with the present invention have been described herein . these embodiments have been shown for illustrative purposes only . it should be understood , however , that the invention is not intended to be limited to the specifics of the illustrated embodiments but is defined only in terms of the following claims .
an apparatus and method for use in performing ablation or coagulation of organs and other tissue includes a metallized fabric electrode array which is substantially absorbent and / or permeable to moisture and gases such as steam and conformable to the body cavity . the array includes conductive regions separated by insulated regions arranged to produce ablation to a predetermined depth . following placement of the ablation device into contact with the tissue to be ablated , an rf generator is used to deliver rf energy to the conductive regions and to thereby induce current flow from the electrodes to tissue to be ablated . as the current heats the tissue , moisture leaves the tissue causing the tissue to dehydrate . suction may be applied to facilitate moisture removal . the moisture permeability and / or absorbency of the electrode carrying member allows the moisture to leave the ablation site so as to prevent the moisture from providing a path of conductivity for the current .