presently preferred embodiments of the invention are shown in the above - identified figures and described in detail below . it should be understood that the appended drawings and description herein are of preferred embodiments and are not intended to limit the invention or the appended claims . on the contrary , the intention is to cover all modifications , equivalents , and alternatives falling within the spirit and scope of the invention as defined by the appended claims . in showing and describing the preferred embodiments , like or identical reference numerals are used to identify common or similar elements . the figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness . as used herein and throughout all the various portions ( and headings ) of this patent , the terms “ invention ”, “ present invention ”, and variations thereof are not intended to mean the claimed invention of any particular appended claim or claims , or all of the appended claims . these terms are used to merely provide a reference point for subject matter disclosed herein . the subject or topic of each such reference is thus not necessarily part of , or required by , any particular claim ( s ) merely because of such reference . accordingly , the use herein of the terms “ invention ”, “ present invention ”, and variations thereof is not intended and should not be used in arriving at the construction or scope of the appended claims . for background purposes , reference will now be made to fig1 , wherein a downhole tool 10 is shown . the illustrated downhole tool 10 is a mechanical packer 12 . the packer 12 is shown including an inner mandrel 18 carrying an upper gage ring 22 , lower gage ring 26 , and a deformable member 28 , as are or become known . the deformable member 28 of this example is an elastomeric member 30 . in some embodiments , there may be numerous such components . thus , the references herein to a component in the singular tense includes embodiments having one or more such component ( s ) ( e . g . the term “ elastomeric member ” means one or more elastomeric member ). in the example shown , the axial position of the elastomeric member 30 and lower gage ring 26 are fixed upon the inner mandrel 18 . however , the inner mandrel 18 , the elastomeric member 30 , and lower gage ring 26 are axially movable relative to the upper gage ring 22 . this enables setting of the illustrated packer 12 , as is or becomes known . it should be understood that these components of the packer 12 , if included , may take any other suitable form and configuration . moreover , the packer 12 may include different or additional components . the inner mandrel 18 of the illustrated example includes a conduit 20 in fluid communication with the bore of a tubing ( not shown ), which is used to convey the tool 10 into a borehole 40 . the tubing ( not shown ) may be coiled tubing , or any other suitable tubing or component ( s ). thus , as used herein and throughout the various portions of this patent , the term “ tubing ” and variations thereof means coiled tubing , jointed drill string elements , or any other desirable component ( s ) capable of deploying a tool , or other device , into a borehole . still referring to fig1 , the illustrated packer 12 is shown in a run - in - hole position in the borehole 40 . while the borehole 40 of fig1 appears vertically - oriented , the present invention is not limited to any particular orientation of the borehole 40 . for example , the tool 10 may be used in a borehole 40 that is non - vertical , such as a “ horizontal ” or “ deviated ” well . in typical use , the exemplary packer 12 is inserted into the borehole 40 via the tubing ( not shown ). to enable such insertion , the outer diameter of the packer 12 is smaller than the inner diameter of the borehole 40 . when the packer 12 is moved into the borehole 40 , an area , or annulus , is thus generally formed between the packer 12 and the borehole wall 42 . this area , or annulus , is referred to herein as the “ extrusion gap ” 44 . after the desired depth of the packer 12 in the borehole 40 is achieved , the elastomeric member 30 is expanded to form a seal between the packer 12 and the borehole wall 42 , creating isolated zones in the borehole 40 above and below the elastomeric member 30 . these borehole zones are referred to herein as the upper and lower zones 46 , 48 , respectively ( see e . g . fig3 ). specifically with reference to the packer 12 of fig1 , the elastomeric member 30 is expanded by imparting axial forces upon it , as is known in the art . for example , the upper gage ring 22 may be secured to the borehole wall 42 . the inner mandrel 18 , elastomeric member 30 , and lower gage ring 26 are then moved upwardly relative to the fixed upper gage ring 22 . this action effectively squeezes the elastomeric member 30 between the upper and lower gage rings 22 , 26 , causing it to expand . the elastomeric member 30 is thus compressed under axial forces , causing it to expand outwardly into the borehole 40 to ultimately engage and form a seal with the borehole wall 42 , as is or becomes known . if the pressure in the upper and lower zones 46 , 48 differs , the elastomeric member 30 is subject to a pressure differential , which may cause the elastomeric member 30 to extrude or deform into the extrusion gap 44 adjacent to it . in such instances , the elastomeric member 30 may become damaged and / or lose its seal with the borehole wall 42 , potentially compromising or disrupting isolation of the upper and lower zones 46 , 48 . the ability to maintain isolation of the zones 46 , 48 may thus be dependent upon the size or width of the extrusion gap 44 adjacent to the elastomeric member 30 . further details of the components , arrangement , and operation of the packer 12 , as well as alternate components and arrangements therefore , are , or will be , known to persons skilled in the art , and can be found in various patents and printed publications , such as , for example , u . s . pat . nos . 6 , 257 , 339 ; 4 , 862 , 961 ; and 4 , 665 , 977 , each of which is incorporated herein by reference . the above description of the illustrated packer 12 and its operation is provided for illustrative purposes only and is not limiting upon the present invention . moreover , the present invention , embodiments of which will be described below , is not limited to use with packers , but can be incorporated in , or associated with , any tool , or device , having a deformable member disposable across an area , or which is used to separate two or more zones . thus , the type , operation , components and arrangement of the packer 12 , or other tool 10 , and the environment within which it is used are in no way limiting upon the present invention . referring now to fig2 , an example anti - extrusion device 50 in accordance with the present invention is shown associated with the tool 10 . the illustrated anti - extrusion device 50 includes an expandable member 54 that is expandable into the extrusion gap 44 to reduce the width , or size , of the gap 44 on at least one side of the deformable member 28 . the illustrated expandable member 54 is located adjacent to and up - hole of the elastomeric member 30 and , as shown in fig3 , is expandable entirely across the extrusion gap 44 , thus abutting , or engaging , the borehole wall 42 . this embodiment is thus capable of closing off or blocking the extrusion gap 44 up - hole of the elastomeric member 30 and may be desirable , for example , when the upper zone 46 of the borehole 40 has , or is expected to have , a lower pressure than the lower zone 48 . it should be understood , however , that the expandable member 54 may instead extend only partially into , or across , the extrusion gap 44 , reducing its width . further , if desired , the anti - extrusion device 50 may include multiple expandable members ( not shown ) on either side , or both sides , of the deformable member 28 . for example , it may be desirable to locate the expandable member 54 on the downhole side of the deformable member 28 when the pressure in the upper zone 46 is expected to be higher than the pressure in the lower zone 48 . referring specifically to fig2 , the expandable member 54 may take any suitable form and configuration . for example , the expandable member 54 may be an annular spring - acting member 56 that is radially expandable into the extrusion gap 44 around the circumference of the tool 10 after the tool 10 is positioned as desired in the borehole 40 . in the embodiment shown , the expandable member 54 is a spring - acting member 56 disposed around the inner mandrel 18 between the elastomeric member 30 and the upper gage ring 22 . the illustrated spring - acting member 56 is a wave spring 58 , as is or becomes known . an example embodiment of a wave spring 58 is shown in fig4 . however , the present invention is not limited to the inclusion of a wave spring 58 . any other suitable expandable member 54 may be used . for example , the expandable member 54 may include one or more bellville washers ( not shown ) or compression spring 76 ( fig6 ). the example expandable member 54 of fig2 and 3 is a separate component of the anti - extrusion device 50 . however , the expandable member 54 may instead be molded into , integral with , or attached to the deformable member 28 , or another component . further , the illustrated expandable member 54 has a continuous , or unbroken , outer surface proximate to the deformable member 28 to prevent extrusion of the deformable member 28 into the expandable member 54 itself . if desired , the expandable member 54 may be contained in , or include , a cover or carrier ( not shown ), such as a flexible elastomeric , or high - elasticity rubber , sheath . such configuration may be desirable , for example , to assist in preserving and / or excluding debris from the expandable member 54 during use , providing a continuous outer surface of the expandable member 54 , or any combination thereof . however , the continuous outer surface and use of a cover may not be included in some embodiments of the invention . referring again to fig2 , the expandable member 54 is shown associated with a carrier 62 . the illustrated carrier 62 is a piston - shaped load ring 64 engaged around the inner mandrel 18 in a fixed axial position . the load ring 64 includes a body 66 and a shoulder 68 . the body 66 carries the wave spring 58 , while the shoulder 68 engages , and applies axial forces to the elastomeric member 30 . however , the carrier 62 need not be a load ring 64 having such features , but can be any suitable device for carrying the wave spring 58 and / or engaging the elastomeric member 30 . yet further , if desired , a different component may be used for engaging the deformable member 28 . moreover , a carrier 62 or other such component ( s ) may not be included in some embodiments . now referring to fig5 , the anti - extrusion device 50 may include a centralizer 72 useful for assisting in centering the expandable member 54 on the tool 10 . in the particular embodiment shown , the centralizer 72 includes a body 73 and shoulder 75 and is used in place of the carrier 62 . the body 73 carries the wave spring 58 , while the shoulder 75 engages and applies axial forces to the elastomeric member 30 . the example centralizer 72 also includes a relatively steep - angled cone surface 74 , which assists in maintaining the wave spring 58 centered on the inner mandrel 18 throughout operations . however , the centralizer 72 can take any other suitable form and configuration . for example , the centralizer 72 may be used in addition to a carrier 62 , or may be part of another component . moreover , a centralizer 72 may not be included in some embodiments . referring to fig3 , the expandable member 54 may be expanded into the extrusion gap 44 with any suitable technique . for example , when the expandable member 54 is a spring - acting member 56 , axial forces may be used to expand and allow the retraction of the spring - acting member 56 . in the particular embodiment shown , axial forces are placed upon the wave spring 58 when the inner mandrel 18 is moved relative to the anchored upper gage ring 22 to expand the elastomeric member 30 and set the packer 12 . as the inner mandrel 18 is drawn upwardly , the carrier 62 and wave spring 58 are sandwiched between the upper gage ring 22 and elastomeric member 30 . the wave spring 58 is compressed , causing its outer diameter to expand around its circumference , the wave spring 58 thus extending into the extrusion gap 44 . since the exemplary wave spring 58 has a continuous outer surface ( as shown in fig4 ), the elastomeric member 30 should not extrude into the wave spring 58 . in another aspect of the invention , the anti - extrusion device 50 may , if desired , be designed to assist the elastomeric member 30 in maintaining its position , or seal , in the borehole 40 . in the embodiment of fig3 , for example , when the wave spring 58 is expanded and the elastomeric member 30 is set , the wave spring 58 maintains spring force upon the carrier shoulder 68 , which , in turn , applies generally continuous axial force upon the elastomeric member 30 . such forces maintained on the elastomeric member 30 by the anti - extrusion device 50 may be sufficient to retain the elastomeric member 30 in a set , or sealingly engaged , state during typical use conditions . in an exemplary embodiment , the anti - extrusion device 50 may be designed to assist in maintaining an isolation seal at the elastomeric member 30 at high differential pressures , such as over 6000 psi , and high temperatures , such as over 300 ° f ., or within large extrusion gaps 44 at lower temperatures and pressures . it should be understood , however , that this aspect may not be included in some embodiments . referring again to fig2 and 3 , in yet another aspect of the invention , the expandable member 54 may , if desired , be retractable from an expanded to a non - expanded position , so as to allow movement of the tool 10 within or out of the borehole 40 and / or redeployment of the tool 10 and expandable member 54 within the same or other boreholes 40 . in the embodiment shown , for example , when the packer 12 is disengaged from the borehole wall 42 by lowering the inner mandrel 18 , axial forces on the expandable member 54 are released . the expandable member 54 moves to its original , or a substantially non - expanded , state , thus retracting out of the extrusion gap 44 and removing axial forces placed upon the elastomeric member 30 thereby . in a non - expanded state , the expandable member 54 should not catch on the borehole wall 42 or otherwise hinder the mobility of the tool 10 . further , the exemplary anti - extrusion device 50 should leave no debris or residual material in the borehole 40 that could disrupt movement of the tool 10 therein . the tool 10 is thereafter movable within and from the borehole 40 , and redeployable without obstruction caused by the anti - extrusion device 50 . if desired , the anti - extrusion device 50 may be designed so that the expandable member 54 in a non - expanded state is set back from the outermost diameter of the tool 10 , such as , for example , the outer diameter of the upper and / or lower gage rings 22 , 26 . in such instance , other components of the tool 10 will generally protect the expandable member 54 in a non - expanded state from damage during movement into , within , and out of the borehole 40 . the expandable member 54 may thereafter be redeployed by repeating the expansion process as described above or another suitable technique . it should be understood , however , that retraction and redeployment of the expandable member 54 may not be included in some embodiments . fig6 and 7 illustrate another embodiment of an anti - extrusion device 50 in accordance with the present invention . in this example , the expandable member 54 is a compression spring 76 used with a cone - shaped ring 80 that is disposed around the inner mandrel 18 . the ring 80 includes a ramp - shaped , or angled , surface 82 upon which the compression spring 76 is disposed and a shoulder 84 engageable with the elastomeric member 30 . when deploying the packer 12 of this embodiment , generally axial forces placed upon the compression spring 76 push the spring 76 along the ramped surface 82 of the ring 80 . the first coil 86 of the spring 76 partially unwinds and the outer diameter of the spring 76 is forced to expand into the extrusion gap 44 . also , the expanded spring 76 places forces upon the ring shoulder 84 , which , in turn , maintains generally axial force upon the elastomeric member 30 to assist in retaining it in a set , or sealingly engaged , position during use . upon removal of axial forces on the exemplary compression spring 76 , the spring 76 returns to its nominal , or non - expanded , state enabling movement of the tool 10 and redeployment of the expandable member 54 . the compression spring 76 and ring 80 may , if desired , be sized to achieve the desired expansion of the spring 76 . for example , the compression spring 76 may be formed with a relatively small length and its spring element 77 tightly wound . for another example , the angled surface 82 of the ring 80 may have a specific desired angle , such as approximately thirty degrees ( 30 °). for still a further example , the combined size of the largest diameter of the ring 80 along its surface 82 and twice the diameter of the compression spring element 77 may be specifically selected to cause the compression spring 76 to extend into the extrusion gap 44 to a desired , or pre - determined , extent . other than as described above , the characteristics , capabilities , and operation of this embodiment of the anti - extrusion device 50 may , if desired , be generally similar to those described above for the embodiment of fig2 and 3 . referring now to fig8 , the anti - extrusion device 50 of the present invention is illustrated in use with cup packers on a straddle tool 11 such as is used for isolating a borehole interval for treatment . the straddle tool 11 comprises an upper cup packer 14 and a lower cup packer 16 mounted on the inner mandrel 18 by means of mounting rings 15 and 17 , respectively . the wave springs 58 of the anti - extrusion device 50 associated with each cup packer are mounted between the respective mounting ring 15 , 17 and a backup ring 19 , 21 . when the straddle tool 11 is run into the borehole 40 , in the example illustrated in fig8 a borehole 40 having a casing 41 therein , the wave springs 58 are uncompressed so as to pass easily through the casing 41 . the mounting ring 15 of the upper cup packer 14 is secured to the mandrel 18 by a shear member 24 , which may , for example , be in the form of a shear pin or pins , or a shear ring . the backup ring 21 associated with lower cup packer 16 is likewise secured against movement along mandrel 18 by a shear member 25 , which may also , for example , be in the form of a shear pin or pins , or a shear ring . referring now to fig9 , once the straddle tool 11 has been positioned at the proper depth in the borehole 40 , the borehole interval 43 straddled by the packers 14 , 16 may be treated by pressurized fluid injected into the borehole interval 43 from the conduit 20 inside mandrel 18 through a port or ports 23 . the pressurized fluid entering the borehole interval 43 forces the packers 14 , 16 apart and seats them against the casing 41 forming a seal therewith . the force created by the pressurized fluid in the borehole interval 43 causes the shear pin or pins 24 to shear , allowing movement of the packer 14 along the mandrel 18 to compress the wave springs 58 of the anti - extrusion device 50 . likewise , pressurization of the borehole interval 43 causes packer 16 to move along the mandrel 18 until its mounting ring 17 seats against shoulder 27 on mandrel 18 thereby compressing the wave springs 58 of the anti - extrusion device 50 associated therewith . once treatment of the borehole interval 43 is complete , the interval 43 is depressurized in preparation for removal of the straddle tool 11 from the borehole 40 . as illustrated in fig1 , the wave springs 58 will generally remain in the compressed state , thereby presenting an obstruction to removal of the straddle tool 11 when the lower cup packer 16 encounters a restricted diameter section 45 , such as a nipple , in the casing 41 . when this occurs , the force created by the restricted diameter section 45 against the compressed wave springs 58 causes the shear pin or pins 25 to shear , allowing the wave springs 58 to relax until the backup ring 21 seats against the shoulder 29 in the mandrel 18 . as illustrated in fig1 , the straddle tool 11 can then pass through the restricted diameter section 45 of the casing 41 . the present invention includes additional features , capabilities , functions , methods , uses and applications that have not been specifically addressed herein but are , or will become , apparent from the description herein , the appended drawings and claims . preferred embodiments of the present invention thus offer advantages over the prior art and are well adapted to carry out one or more of the objects of the invention . it should be understood that the present invention does not require each of the techniques or acts described above . moreover , the present invention is in no way limited to the above methods . further , the methods described above and any other methods which may fall within the scope of any of the appended claims can be performed in any desired suitable order and are not necessarily limited to the order described herein or listed in the appended claims . yet further , the methods of the present invention do not require use of the particular embodiments shown and described in the present specification , such as , for example , the expandable member 54 of fig2 - 7 , but are equally applicable with any other suitable structure , form and configuration of components . also , it should be understood that the present invention does not require all of the above features and aspects . any one or more of the above features or aspects may be employed in any suitable configuration without inclusion of other such features or aspects . further , while preferred embodiments of this invention have been shown and described , many variations , modifications and / or changes of the apparatus and methods of the present invention , such as in the components , details of construction and operation , arrangement of parts and / or methods of use , are possible , contemplated by the patentee , within the scope of the appended claims , and may be made and used by one of ordinary skill in the art without departing from the spirit or teachings of the invention and scope of the appended claims . all matter herein set forth or shown in the accompanying drawings should thus be interpreted as illustrative and not limiting . accordingly , the scope of the invention and the appended claims is not limited to the embodiments described and shown herein .