referring now to the figures , fig1 illustrates a marking device 10 according to one embodiment of the invention for implanting an imaging marker 12 , seen in fig2 , at a predetermined location in a tissue mass . referring additionally to fig3 , the marking device 10 comprises a handle 14 that supports a cannula 16 slidably mounted thereto and a stylet 18 received within the cannula 16 and slidably mounted to the handle 14 . an actuator 20 also mounted to the handle 14 effects movement of the stylet 18 and the cannula 16 to eject the imaging marker 12 from the marking device 10 and to retract the cannula 16 into the handle 14 following ejection of the imaging marker 12 . the imaging marker 12 can be any suitable imaging marker made of any suitable non - bioabsorbable material , bioabsorbable material , or combinations thereof . exemplary materials include , but are not limited to , metals , such as titanium and stainless steel , and polymers , such as polyvinyl alcohol ( pva ), including combinations of such materials examples of suitable imaging markers are disclosed in u . s . pat . nos . 6 , 356 , 782 ; 6 , 371 , 904 ; and 6 , 575 , 991 , which are incorporated herein by reference in their entirety . as best seen in fig3 and 4 , the handle 14 comprises a distal section 30 that terminates in an open distal end 32 and an integral proximal section 34 that terminates at a closed proximal end 36 . the distal section 30 includes a grip area 38 having a reduced outer diameter to accommodate a generally resilient grip 40 that surrounds the grip area 38 and has an aperture 42 to accommodate a trigger mount 44 located in the grip area 38 of the handle 14 . the trigger mount 44 comprises a pair of spaced side walls 46 , each having a pivot aperture 48 , joined by a distal wall 50 and a proximal wall 52 . the walls 46 , 50 , 52 surround a trigger opening 54 in the grip area 38 sized to receive and accommodate movement of the actuator 20 , as will be described in more detail below . the proximal section 34 gradually tapers proximally from the grip area 38 and distally from the proximal end 36 to a hand rest area 56 contoured to support a palm portion of the user &# 39 ; s hand . together , the distal section 30 and the proximal section 34 have an inner surface 60 that defines a generally cylindrical hollow interior 62 . as best seen in fig4 , the inner surface 60 includes a stop 64 that extends radially into the hollow interior 62 from the grip area 38 near the trigger mount 44 . additionally , a pair of diametrically opposed guide grooves 66 , one of which is visible in fig4 , is formed along the inner surface 60 from a proximal side of the trigger mount 44 to the proximal end 36 . the handle 14 further includes a proximal stop 68 projecting into the hollow interior 62 from the proximal end 36 . referring again to fig3 , the distal end 32 of the handle 14 is closed by a handle cap 70 . the handle cap 70 comprises a generally hollow frustoconical body 72 with a distal endwall 74 from which extends a nose 76 sized to slidably receive the cannula 16 . a plurality of resilient prongs 78 project proximally from the body 72 and mate with the distal end 32 of the handle 14 to mount the handle cap 70 to the handle 14 . for convenience of this description , the handle cap 70 is described as being separate from the handle 14 . however , the handle cap 70 can be considered as part of the handle 14 and can even be integrated with the handle 14 . the actuator 20 comprises a cannula mount 90 and a stylet mount 92 that slidably support the cannula 16 and the stylet 18 , respectively , in the handle 14 . as shown in fig5 a - 5c , the cannula mount 90 includes an elongated cannula support shaft 94 integral with a generally orthogonal annular body 96 having an annular distal face 97 . a pair of opposed legs 98 having an arcuate outer surface 100 and a generally straight inner surface 102 extend proximally from the annular body 96 , and each of the legs 98 has a channel 104 formed therein that terminates at a corresponding opening 106 formed through the annular body 96 . the channels 104 are formed in the inner surface 102 so that the channels 104 face one another . the cannula mount 90 further comprises a pair of diametrically opposed resilient prongs 110 positioned between the legs 98 . the resiliency of each prong 110 is enhanced by a u - shaped notch 112 at the juncture between the prong 110 and the annular body 96 . each prong 110 comprises a raised stop 114 disposed distally of a distal flat surface 116 joined to a proximal flat surface 118 by an inclined surface 120 , and each prong 110 terminates at an outwardly projecting tang 122 adjacent the proximal flat surface 118 . the stylet mount 92 , as shown in fig6 a - 6c , comprises an elongated stylet support shaft 130 integral with a generally orthogonal cylindrical body 132 . the body 132 has a pair of diametrically opposed prong apertures 134 formed therein from a distal face 142 to a proximal face 144 of the body 132 and sized to receive the prongs 110 of the cannula mount 90 . as best viewed in fig6 c , the prong apertures taper from the distal face 142 to the proximal face 144 . the body 132 further includes a pair of guide projections 136 extending radially from the body 132 in radial alignment with the prong apertures 134 . a pair of diametrically opposed prongs 138 extends distally from the body 132 , and each prong 138 terminates in an outwardly projecting tang 140 . the prongs 138 are oriented along a diameter generally orthogonal to a diameter that contains the prong apertures 134 with the stylet support shaft 130 between the prongs 138 . referring back to fig3 , the actuator 20 further comprises a trigger 150 rotatably mounted to the trigger mount 44 of the handle 14 . although the trigger 150 can move relative to the handle 14 , it will be described with respect to the orientation shown in fig3 . the trigger 150 includes a distally facing arcuate finger rest 152 and an irregularly shaped body 154 having a pivot member 156 on each side thereof . the body 154 is bounded by a sloped upper surface 158 extending proximally of the finger rest 152 and joined to a generally straight proximal surface 160 by a fillet 162 . the rest of the body 154 is defined by a curved cam surface having a distal cam surface 166 and a lower cam surface 164 . the distal cam surface 166 extends from the finger rest 152 to the lower cam surface 164 , which terminates at the straight proximal surface 160 . in addition to the cannula mount 90 , the stylet mount 92 , and the trigger 150 , the actuator 20 comprises a pair of biasing members : a cannula mount biasing member 170 and a stylet mount biasing member 172 . according to the illustrated embodiment of the invention , the biasing members 170 , 172 are compression springs . preferably , the cannula mount biasing member 170 tapers from a distal end 174 to a proximal end 176 . the cannula 16 is generally hollow and comprises a distal end 180 defining a tip 182 and a proximal end 184 mounted to the cannula support shaft 94 of the cannula mount 92 . the cannula 16 is preferably sufficiently rigid to permit the direct insertion of the cannula 16 into a tissue mass . alternatively , the cannula 16 can be flexible for use with a probe or the like . the tip 182 is preferably pointed for insertion through skin and into the tissue mass ; however , the tip 182 can optionally be blunt , for example , if the marking apparatus 10 is utilized with a probe or the like . further , the distal end 180 of the cannula 16 can be beveled , as best seen in fig2 , from a bevel proximal edge 179 to a bevel distal edge 181 to define a bevel opening 183 . preferably , the cannula 16 is a 17 - gage ( 0 . 058 inch outer diameter ) cannula , with an inner diameter ranging from 0 . 049 to 0 . 051 inches . furthermore , the distal end 180 of the cannula 16 can be designed for enhanced visibility using common imaging techniques , such as radiography , ultrasonography , and magnetic resonance imaging ( mri ). for example , the distal end 180 can include imageable markings 186 , as seen in fig3 . with continued reference to fig3 and additional reference to fig2 , the cannula 16 slidingly receives the stylet 18 , which comprises a distal end 190 located in the cannula 16 and a proximal end 192 mounted to the stylet support shaft 130 of the stylet mount 92 . prior to use of the marking device 10 , the distal end 190 of the stylet 16 is spaced inwardly from the tip 182 to form a marker recess 194 , as best viewed in fig2 , for housing the imaging marker 12 . when the marking device 10 is in an assembled condition , as shown in fig7 a , the stylet mount 92 is mounted in the hollow interior 62 with the guide projections 136 positioned in general alignment with the guide grooves 66 . the stylet 18 extends from the stylet support shaft 130 , through the cannula support shaft 94 of the cannula mount 90 , which is located distally of the stylet mount 92 in the hollow interior 62 , and out the distal end 32 of the handle 14 through the nose 76 of the handle cap 70 . the cannula mount 90 is positioned so that the cannula 16 also leaves the distal end 32 of the handle 14 through the nose 76 of the handle cap 70 . the relative positioning of the cannula mount 90 and the stylet mount 92 in the hollow interior 62 is such that the distal end 190 of the stylet 18 is spaced from the tip 182 to form the marker recess 194 ( fig2 ). in this condition , the cannula 16 is in an extended position where it extends from the distal end 32 of the handle 14 , and the stylet 18 is in a ready position in the extended cannula 16 . as best seen in fig7 b and 7c , the cannula mount 90 and the stylet mount 92 are axially aligned in the hollow interior 62 with the stylet mount prongs 138 partially inserted into the channels 104 of the cannula mount 90 and the stylet support shaft 130 partially received in the cannula support shaft 94 of the cannula mount 90 . additionally , the cannula mount prongs 110 extend through the prong apertures 134 of the stylet mount 92 with the tangs 122 abutting the proximal face 144 of the body 132 and the proximal flat surfaces 118 positioned in the prong apertures 134 . the cannula mount prongs 110 naturally spread apart from one another to facilitate securing the cannula mount 90 to the stylet mount 92 in the position best seen in fig7 b and 7c . additionally , as a result of the natural resiliency of the cannula mount prongs 110 , the stops 114 extend radially outward and abut the stop 64 of the handle 14 to help prevent proximal movement of the cannula mount 90 even though the cannula mount 90 is biased away from the handle cap 70 by the cannula mount biasing member 170 , which abuts the handle cap 70 at the distal end 174 and the annular body 96 at the proximal end 176 . similarly , the stylet mount 92 is biased away from the cannula mount 90 by the stylet mount biasing member 172 , which extends between the annular body 96 and the body 132 of the stylet mount 92 . the interaction between the cannula mount prongs 110 and the stylet mount body 132 prevents retraction of the stylet mount 92 beyond the position shown in fig7 b and 7c . additionally , proximal movement of the stylet mount 92 and the cannula mount 90 is prevented in part by the trigger 150 . the pivot mounts 156 are rotatably received by the pivot apertures 48 to mount the trigger 150 in the trigger mount 44 of the handle 14 . the trigger 150 extends through the trigger opening 54 in a locked condition , shown in fig7 b and 7c , where the proximal surface 160 abuts the proximal wall 52 of the trigger mount 44 to prevent counterclockwise rotation , relative to the orientation of fig7 b and 7c , of the trigger 150 , and the distal cam surface 166 faces the stylet mount 92 . the trigger 150 is movable from the locked position shown in fig7 a - 7c to a stylet advance position shown in fig8 a and 8c to displace the stylet 18 proximally and eject the imaging marker 12 from the marker recess 194 , as shown in fig9 . rotation of the trigger 150 clockwise , relative to the orientation of fig8 a , such as by rotation of the finger rest 152 by the user , causes the cam surface formed by the distal cam surface 166 and the lower cam surface 164 to abut the body 132 of the stylet mount 92 and ride along the proximal face 144 of the body 132 between the cannula mount prongs 110 while pushing the stylet mount 92 distally toward the cannula mount 90 against the bias of the stylet mount biasing member 172 , as best seen in fig8 b . as a result , the tangs 140 of the stylet mount prongs 138 slide along the channels 104 of the cannula mount 90 until the tangs 140 slide through the openings 106 in the annular body 96 and flex outward to abut the distal face 97 of the annular body 96 and thereby secure the stylet mount 92 to the cannula mount 90 together in the position shown in fig8 b . at the same time , the body 132 rides distally along the cannula mount prongs 110 , particularly along the inclined surfaces 120 , as best seen in fig8 d . as the body 132 moves along the prongs 110 , the taper of the prong apertures 134 forces the prongs 110 to flex toward each other at their respective notches 112 so that the stops 114 are no longer in abutting contact with the stop 64 on the interior surface 60 of the handle 14 . distal movement of the stylet mount 92 ceases when the body 132 reaches the distal flat surfaces 116 . in this position , the trigger 150 prevents proximal movement of the cannula mount 90 and the stylet mount 92 by the cannula mount biasing member 170 . because the stylet mount 92 moves distally while the cannula mount 90 remains stationary , the stylet 16 advances distally into the marker recess 194 to an implant position to eject the imaging marker 12 therefrom , as shown in fig9 . preferably , the stylet 18 is sized so that when the stylet 18 is in the implant position , the distal end 190 of the stylet 18 extends to at least the bevel proximal edge 179 at the bevel opening 183 to ensure that the imaging marker 12 reaches the bevel opening 183 for ejection from the marker recess 194 . the stylet distal end 190 can also extend to a position between the bevel proximal edge 179 and the bevel distal edge 181 ( i . e ., a center point of the bevel and the bevel opening 183 ). further , the stylet distal end 190 can extend to near the tip 182 of the cannula 16 ( i . e ., the bevel distal edge 181 ) or beyond the tip 182 to ensure complete ejection of the imaging marker 12 from the marker recess 194 . continuing rotation of the trigger 150 beyond the stylet advance position to a cannula release position shown in fig1 a and 10c retracts the cannula 16 and the stylet 18 into the hollow interior 62 . while the trigger 150 rotates to the cannula release position , the cam surface formed by the distal cam surface 166 and the lower cam surface 164 rides off of the proximal face 144 of the stylet mount body 132 so that the trigger 150 no longer prevents proximal movement of the stylet mount 92 and the cannula mount 90 in the handle 14 . with the trigger 150 no longer an obstacle and the stops 114 on the cannula mount prongs 110 no longer abutting the stop 64 , the cannula mount biasing member 170 forces the cannula mount 90 and thereby the stylet mount 92 , which is fixed to the cannula mount 90 by the stylet mount prongs 138 and the cannula mount prongs 110 , proximally within the hollow interior 62 toward the proximal end 36 of the handle 14 . movement of the cannula mount 90 and the stylet mount 92 ceases when the stylet mount 92 abuts the proximal stop 68 with the prongs 110 on the cannula mount 90 straddling the stop 68 , as best viewed in fig1 b and 10d . during this movement , the guide projections 136 enter the guide grooves 66 formed in the interior surface 60 of the handle 14 to prevent rotation of the stylet mount 92 and thereby the cannula mount 90 . proximal movement of the cannula mount 90 and the stylet mount 92 retracts the cannula 16 and the stylet 18 into the hollow interior 62 to a retracted position and a withdrawn position , respectively . preferably , when the cannula 16 is in the retracted position , the entire cannula 16 , including the tip 182 , is received within the handle 14 as seen in fig1 a and 10c . similarly , the entire stylet 18 is preferably received within the handle 14 when in the withdrawn position , but it within the scope of the invention for the distal end 190 to project from the nose 76 when the stylet 18 is in the withdrawn position . in operation , a practitioner grasps the marking device 10 at the hand rest area 56 and inserts the cannula tip 182 through the skin of patient &# 39 ; s body into a tissue mass 196 . an exemplary tissue mass 196 is a breast , as shown in fig1 . the practitioner places the tip 182 typically under the guidance of aforementioned imaging systems at or near a predetermined site 198 , such as a biopsy site , as shown in fig1 . once the cannula tip 182 is at the predetermined site 198 , the practitioner places an index finger on the finger rest 152 of the trigger 150 and rotates the trigger 150 to move the trigger 150 from the locked position of fig7 a - 7c to the stylet advance position of fig8 a - 8d to move the stylet 18 from the ready position of fig7 a - 7c to the implant position of fig8 a - 8d and eject the imaging marker 12 from the marker recess 194 at the predetermined site 198 , as illustrated in fig1 . the practitioner continues to the rotate the trigger 150 to the cannula release position of fig1 a and 10b to simultaneously move the cannula 16 from the extended position of fig7 a - 7c and 8 a - 8 d to the retracted position of fig1 a - 10d and the stylet 18 from the implant position of fig8 a - 8d to the withdrawn position of fig1 a - 10d to remove the cannula 16 and the stylet 18 from the tissue mass 196 as they retract into the handle 14 . preferably , rotation of the trigger 150 between the locked , stylet advance , and cannula release positions is substantially continuous such that the practitioner can implant the imaging marker 12 and retract the cannula 16 into the handle 14 in effectively a single operational step . with the cannula 16 retracted into the handle 14 , the practitioner cannot accidentally stab himself or herself , others assisting with the procedure , or the patient with the marking device 10 following implantation of the imaging marker 12 . a marking device 210 according to another embodiment of the invention for implanting an imaging marker 212 at a predetermined location in a tissue mass is illustrated in fig1 - 17 . referring particularly to fig1 , the marking device 210 comprises a handle 214 that supports a cannula 216 slidably mounted thereto and a stylet 218 received within the cannula 216 and slidably mounted to the handle 214 . an actuator 220 also mounted to the handle 214 effects proximal movement of the cannula 216 to expose the imaging marker 212 and to retract the cannula 216 and the stylet 218 into the handle 214 after exposing the imaging marker 212 . the handle 214 of the marking device 210 is similar to the handle 14 of the first embodiment marking device 10 and comprises a distal section 230 that terminates in an open distal end 232 and an integral proximal section 234 that terminates at a closed proximal end 236 . a trigger mount 238 in the distal section 230 comprises a pivot mount 240 for mounting a portion of the actuator 220 , as will be described in more detail below . together , the distal section 230 and the proximal section 234 have an inner surface 242 that defines a generally cylindrical hollow interior 244 . the handle 214 comprises a pair of stops 245 extending radially inward from the inner surface 242 in the hollow interior 244 . the handle 214 further includes a proximal stop 268 projecting into the hollow interior 244 from the proximal end 236 . the hollow interior 244 is closed at the distal end 232 of the handle 214 by a handle cap 246 . the handle cap 246 comprises a generally hollow frustoconical body 248 with a distal endwall 250 from which extends a nose 252 sized to slidably receive the cannula 216 . for convenience of this description , the handle cap 246 is described as being separate from the handle 214 . however , the handle cap 246 can be considered as part of the handle 214 and can even be integrated with the handle 214 . the actuator 220 comprises a cannula mount 260 and a stylet mount 262 that slidably support the cannula 216 and the stylet 218 , respectively , in the handle 214 . the cannula mount 260 includes an elongated cannula support shaft 264 integral with a generally orthogonal annular body 266 with a peripheral wall 274 having a trigger detent 272 formed therein . a pair of legs 269 having a sloped terminal cam surface 270 extends proximally from the annular body 266 . the stylet mount 262 comprises an elongated stylet support shaft 276 integral with a generally orthogonal body 278 having a pair of diametrically opposed resilient arms 280 extending distally from the body 278 on opposite sides of the stylet support shaft 276 . each arm 280 comprises a proximal sloped stop surface 282 near the body 278 and terminates at an inclined cam follower surface 284 . with continued reference to fig1 , the actuator 220 further comprises a trigger 288 pivotally mounted to the trigger mount 238 of the handle 214 . the trigger 288 includes a proximal finger rest 290 and a downwardly extending projection 292 integral with the finger rest 290 . in addition to the cannula mount 260 , the stylet mount 262 , and the trigger 288 , the actuator 220 comprises a biasing member 293 . according to the illustrated embodiment of the invention , the biasing member 293 is a compression spring . the cannula 216 is generally hollow and comprises a distal end 294 defining a tip 296 and a proximal end 298 mounted to the cannula support shaft 264 of the cannula mount 260 . the tip 296 is preferably pointed for insertion through skin and into the tissue mass ; however , the tip 296 can optionally be blunt , for example , if the marking apparatus 210 is utilized with a probe or the like . preferably , the cannula 216 is a 17 - gage ( 0 . 058 inch outer diameter ) cannula , with an inner diameter ranging from 0 . 049 to 0 . 051 inches . furthermore , the distal end 294 of the cannula 216 can be designed for enhanced visibility using common imaging techniques , such as radiography , ultrasonography , and magnetic resonance imaging ( mri ), similar to the first embodiment cannula 16 . the cannula 216 slidingly receives the stylet 218 , which comprises a distal end 295 located in the cannula 216 and a proximal end 297 mounted to the stylet support shaft 276 of the stylet mount 262 . prior to use of the marking device 210 , the distal end 295 of the stylet 216 is spaced inwardly from the tip 296 to form a marker recess 299 for housing the imaging marker 212 . when the marking device 210 is in an assembled condition , as shown in fig1 , the stylet mount 262 is mounted in the hollow interior 244 adjacent the stops 245 with the stop surfaces 282 on the arms 280 abutting the stops 245 on the handle 214 to prevent proximal movement of the stylet mount 262 in the hollow interior 244 . the stylet 218 extends from the stylet support shaft 270 , through the cannula support shaft 264 of the cannula mount 260 , which is located distally of the stylet mount 262 in the hollow interior 244 , and out the distal end 232 of the handle 214 through the nose 252 of the handle cap 246 . the cannula mount 260 is positioned so that the cannula 216 also leaves the distal end 232 of the handle 214 through the nose 252 of the handle cap 246 . the trigger projection 292 resides in the trigger detent 272 of the cannula mount 260 to prevent proximal movement of the cannula mount 260 by the biasing member 293 , which is positioned between the distal end wall 250 of the handle cap 246 and the body 266 of the cannula mount 260 . the relative positioning of the cannula mount 260 and the stylet mount 262 is such that the former is spaced from the latter in the hollow interior 244 , and the distal end 295 of the stylet 218 is spaced from the tip 296 to form the marker recess 299 . in this condition , the cannula 216 is in an extended position where it extends from the distal end 232 of the handle 214 , and the stylet 218 is in a ready position in the extended cannula 216 . referring now to fig1 , the cannula 216 is slidably movable in the hollow interior 244 relative to the stylet 218 in a proximal direction to expose the imaging marker 212 . retraction of the cannula 216 occurs when the projection 292 of the trigger 288 is removed from the trigger detent 272 on the cannula mount 260 by pivotal movement of the trigger finger rest 290 . when the trigger 288 releases the cannula mount 260 , the biasing member 293 forces the cannula mount 260 and thereby the cannula 216 proximally to a first retracted position , shown in fig1 , to expose the imaging marker 214 . after exposing the imaging marker 212 , the cannula 216 continues to retract into the handle 214 , and the cannula mount 260 contacts the stylet mount 262 , as shown in fig1 . in particular , the proximally extending legs 269 of the cannula mount 260 contact the distally extending arms 280 of the stylet mount 262 . the cam surfaces 270 of the legs 269 contact the corresponding cam follower surfaces 284 of the arms 280 to deflect the arms 280 radially inward . as a result , the stop surfaces 282 deflect inward such that the stops 245 no longer prevent proximal movement of the stylet mount 262 , and the stylet mount 262 and thereby the stylet 218 can retract with the cannula mount 260 and the cannula 218 , as shown in fig1 . preferably , the cannula mount 260 and the stylet mount 262 are spaced to ensure that the distal end 294 of the cannula 216 is adjacent to or extends proximally of the tip 296 for eliminating the marker recess 299 ( i . e ., the stylet distal end 295 is located at or extends beyond the tip 296 ). thus , retraction of the stylet 218 is delayed until the cannula 218 has been sufficiently retracted relative to the stylet 218 . if the marker recess 299 is present during simultaneous retraction of the cannula 216 and the stylet 218 , tissue or the imaging marker 212 can potentially be undesirably drawn in to the marker recess 299 . proximal movement of the cannula mount 260 and the stylet mount 262 retracts the cannula 216 and the stylet 218 into the hollow interior 244 to a second retracted position and a withdrawn position , respectively . proximal movement of the cannula mount 260 and the stylet mount 262 ceases when the stylet mount 262 abuts the proximal stop 268 . preferably , when the cannula 216 is in the retracted position , the entire cannula 216 , including the tip 296 , is received within the handle 214 . similarly , the entire stylet 218 is preferably received within the handle 214 when in the withdrawn position , but it within the scope of the invention for the distal end 295 to project from the nose 252 when the stylet 218 is in the withdrawn position . in operation , a practitioner grasps the marking device 210 and inserts the cannula tip 296 through the skin of patient &# 39 ; s body into a tissue mass . the practitioner places the tip 296 typically under the guidance of aforementioned imaging systems at or near a predetermined site , such as a biopsy site . once the cannula tip 296 is at the predetermined site , the practitioner places an index finger or other suitable finger on the finger rest 290 of the trigger 288 and pivots the trigger 288 to move the trigger 288 from a locked position of fig1 to a cannula retract position of fig1 to move the cannula 218 from the extended position of fig1 to the first retracted position of fig1 , whereby the imaging marker 212 becomes exposed to the predetermined site , and the second retracted position of fig1 , whereby the cannula 218 retracts entirely into the handle 214 . as described above , retraction of the cannula 218 from the first retracted position to the second retracted position also retracts the stylet from the ready position of fig1 - 16 to the withdrawn position of fig1 to remove the stylet 218 along with the cannula 216 from the tissue mass as they retract into the handle 214 . as in the first embodiment marking device 10 , with the cannula 216 retracted into the handle 214 , the practitioner cannot accidentally stab himself or herself , others assisting with the procedure , or the patient with the marking device 210 following implantation of the imaging marker 212 . while the embodiments of the marking device 10 , 210 according to invention have been shown and described as comprising an actuator that automatically retracts the cannula and the stylet , such as by force of a biasing element , the actuator can alternatively move the cannula and the stylet manually . for example , the actuator can comprise a slide operably coupled to the cannula mount , and proximal movement of the slide by the user can retract the cannula and the stylet or the cannula alone into the handle . the actuator can comprise a single actuator to effect movement of the cannula and the stylet or , alternatively , individual actuators for the cannula and the stylet . further , while the first embodiment of the marking device 10 according to invention has been shown and described as comprising an actuator that manually advances the stylet to eject the imaging marker from the marker recess , the actuator can alternatively automatically advance the stylet , such as under the force of a biasing member . the marking device according to the invention accurately implants the imaging marker at the predetermined site and , due to the retraction of the cannula into the handle , is safe to use . with the tip of the cannula retracted into the handle , the potential for the practitioner to injure himself / herself or others is eliminated . as a result , the practitioner can focus on the implanting procedure and does not have to be concerned with safety while withdrawing the marking device from the patient . additionally , the actuator is easy to use and facilitates accurate implantation of the imaging marker . while the embodiments of the marking device according to the invention have been shown and described with respect to implanting a single imaging marker , it is within the scope of the invention for the marking device to implant multiple imaging markers , either at the same location or different locations in the tissue mass . additionally , the biasing element of the actuator is not limited to a spring , as shown and described in the above embodiments . it is within the scope of the invention to employ other types of baising elements . for example , the biasing element can comprise a source of compressed gas so that the stylet is advanced with air pressure , and the cannula and the stylet are retracted with a decrease in air pressure . another example of a biasing element is a electric coil powered by a battery , and the coil in an energized size deploys the stylet and in de - energized state retracts the cannula and the stylet . while the invention has been specifically described in connection with certain specific embodiments thereof , it is to be understood that this is by way of illustration and not of limitation , and the scope of the appended claims should be construed as broadly as the prior art will permit .
a marking device includes a handle defining a hollow interior , and a delivery assembly slidably mounted to the handle . the delivery assembly includes a cannula , a stylet assembly , an imaging marker , and an actuator . the cannula has a cannula distal end with a marker recess . the imaging marker is disposed within the marker recess . the actuator is operably coupled to the delivery assembly and is operable to transition the delivery assembly through at least three configurations : a first configuration characterized in that the cannula is positioned in an extended position and the stylet assembly is positioned in a ready position ; a second configuration characterized in that the cannula is positioned in the extended position and the stylet assembly is positioned in an implant position ; and a third configuration characterized in that the cannula is positioned in the retracted cannula position and the stylet is retracted into the handle .