Abstract:
An occlusive implant delivery assembly includes a rapid response decoupling or detachment mechanism that does not effect significant migration of the implant during release. The assembly includes an occlusive implant device, such as an embolic coil, a pusher or device to carry the implant to the selected location, and an expandable coupling-decoupling mechanism for releasing the implant at the selected site. The mechanical construction provides rapid release times. In addition, the releasing mechanism generally operates without exerting any significant force on the implant, thereby avoiding any significant displacement of the implant during release.

Description:
RELATED PATENT APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 09/034,606, now U.S. Pat. No. 6,238,415, filed on Mar. 3, 1998, which is a continuation of U.S. patent application Ser. No. 08/363,264, now U.S. Pat. No. 5,814,062, filed on Dec. 22, 1994. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to surgical instruments. More particularly, the invention relates to delivery assemblies for delivering an occlusive device, such as an embolic coil, to a selected site within a mammal using an expandable coupling or decoupling mechanism. 
     BACKGROUND OF THE INVENTION 
     The endovascular treatment of a variety of vascular maladies throughout the body is an increasingly more important form of therapy. Catheters have been used to place various treatment materials, devices, and drugs within arteries and veins in the human body. Examples of these devices and their use in such treatments are shown in U.S. Pat. Nos. 5,234,437 and 5,261,916, in which methods and devices for delivery of coils or wires within the human body to sites, such as aneurysms, to occlude those sites are disclosed. Coils, such as those discussed in these documents as well as in U.S. Pat. No. 4,994,069, may be of a regular or helical configuration or assume a random convoluted configuration at the site. The coils normally are made of a radiopaque, biocompatible metal such as platinum, gold, tungsten or alloys of these and other metals. In treating aneurysms, it is common to place a number of coils within the aneurysm. The coils occlude the site by posing a physical barrier to blood flow and by promoting thrombus formation at the site. 
     Coils have typically been placed at the desired site within the vasculature using a catheter and a pusher. The site is first accessed by the catheter. In treating peripheral or neural conditions requiring occlusion, the sites are accessed with flexible, small diameter catheters such as those shown in U.S. Pat. Nos. 4,739,768 and 4,813,934. The catheter may be guided to the site through the use of guidewires (see U.S. Pat. No. 4,884,579) or by flow-directed means such as balloons placed at the distal end of the catheter. Use of guidewires involves the placement of relatively long, torqueable proximal wire sections within the catheter attached to more flexible distal end wire sections designed to be advanced across sharp bends at vessel junctions. The guidewire is visible using x-ray techniques and allows a catheter to be navigated through extremely tortuous vessels, even those surrounded by soft tissue such as the brain. 
     Once the site has been reached, the catheter lumen is cleared by removing the guidewire (if a guidewire has been used), and one or more coils are placed into the proximal open end of the catheter and advanced through the catheter with a pusher. Pushers are wires having distal ends adapted to engage and push the coil through the catheter lumen as a pusher itself is advanced through the catheter. Once the coil reaches the distal end of the catheter, it is discharged from the catheter by the pusher into the vascular site. However, there are concerns when discharging the coil from the distal end of the catheter. For example, the plunging action of the pusher and the coil can make it difficult to position the coil at the site in a controlled manner and with a fine degree of accuracy. Inaccurate placement of the coil can be problematic because once the coil has left the catheter, it is difficult to reposition or retrieve the coil. 
     Several techniques involving Interlocking Detachable Coils (IDCs), which incorporate mechanical release mechanisms and Guglielmi Detachable Coils (GDCs), which utilize electrolytically actuated release mechanisms, have been developed to enable more accurate placement of coils within a vessel. 
     One technique for detaching an embolic coil is shown in U.S. Pat. No. 5,261,916. According to that technique, a coil having an enlarged portion is mated with a pusher having a keyway adapted to receive the enlarged portion of the coil in an interlocking relationship. The joint between the pusher and the coil is covered by a coaxial member. The coaxial member is movable by sliding the member axially. As the coaxial member is moved away from the junction where the coil&#39;s member engages the keyway of the pusher, the coil is freed from the catheter assembly and the pusher may then be removed. 
     Another IDC device for placement of coils is shown in U.S. Pat. No. 5,234,437. This device includes a coil having a helical portion at least one end and a pusher wire having a distal end that is threaded inside on the helical coil by use of a threaded section on the outside of the pusher. The device operates by engaging the proximal end of the coil with a sleeve and unthreading the pusher from the coil. Once the pusher is free, the sleeve may be used to push the coil out into the targeted treatment area. 
     U.S. Pat. No. 5,312,415 discloses the use of a catheter having a constricted or feathered end to retain a number of embolic coils on a guidewire for precise placement using a pusher sheath. 
     Electrolytic coil detachment is disclosed in U.S. Pat. Nos. 5,122,136 and 5,354,295. As disclosed in U.S. Pat. No. 5,122,136, the coil is bonded via a metal-to-metal joint to the distal end of the pusher. The pusher and coil are made of dissimilar metals. The coil-carrying pusher is advanced through the catheter to the site and a small electrical current is passed through the pusher-coil assembly. The current causes the joint between the pusher and the coil to be severed via electrolysis. The pusher may then be retracted leaving the detached coil at an exact position within the vessel. Since no significant mechanical force is applied to the coil during electrolytic detachment, highly accurate coil placement is readily achieved. In addition, the electric current may facilitate thrombus formation at the coil site. The only perceived disadvantage of this method is that the electrolytic release of the coil may require a period of time that may inhibit rapid detachment of the coil from the pusher. 
     Another method of placing an embolic coil is disclosed in U.S. Pat. No. 5,108,407. This patent shows the use of a device in which embolic coils are separated from the distal end of a catheter by the use of heat-releasable adhesive bonds. The coil adheres to the therapeutic device via a mounting connection having a heat sensitive adhesive. Laser energy is transferred through a fiber optic cable which terminates at that connector. The connector becomes warm and releases the adhesive bond between the connector and the coil. Among the drawbacks of this system is that it involves generally complicated laser optic componentry. 
     There is a need to provide alternative mechanical mechanisms for delivering implants, such as embolic coils, that combine accurate positioning capability with rapid implant decoupling response times. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mechanical occlusive implant delivery assembly having a rapid response decoupling or detachment mechanism that does not effect significant migration of the implant during release. The assembly includes an occlusive implant device, such as an embolic coil, a pusher or device to carry the implant to the selected location, and an expandable mechanism that is expanded or contracted to release the implant at the selected site. The invention advantageously incorporates a release mechanism that simply involves unloading a locking force, which is preferably uniformly applied, thereby avoiding the transmission of any significant force to the implant during release. In addition, the locking members preferably have generally, smooth, rounded configurations so that they do not catch and dislodge previously positioned coils upon retraction. 
     According to a first embodiment of the invention, the occlusive implant delivery assembly includes an occlusive implant; a pusher having a proximal section and a distal section; a coupling having first and second portions, the first portion being coupled to the distal section of the pusher and the second portion being coupled to the implant; and an inflatable member having a proximal portion and a distal portion, the proximal portion being coupled to the distal section of the pusher. At least a portion of the inflatable member is disposed in the coupling such that when inflated, it expands the coupling and decouples the coupling from either the implant or the pusher. With this arrangement, rapid implant release times can be achieved with minimal, if any, force being applied to the implant. That is, the hydraulic pressure is only transmitted to the detachment point or juncture between the inflatable member and the implant, and not to the implant. 
     According to another aspect of this embodiment, the inflatable member and coupling are configured so that the hydraulic pressure generated by the inflatable member is applied uniformly to the inner circumferential surface of the coupling. Thus, any force that may be applied to the implant in the radial direction is countered by an equal, but opposite force, thereby avoiding implant displacement during release. In the preferred embodiment, the coupling is cylindrical with an essentially uniform radius and the inflatable member is essentially symmetrical about its longitudinal axis in the inflated and uninflated states. 
     According to another embodiment of the invention the implant delivery assembly comprises an occlusive implant having a tubular portion; a pusher having a proximal section and a distal section; and
         an inflatable member having a first portion coupled to the distal section of the pusher and a second portion disposed in the tubular portion of the implant such that upon inflation of the inflatable member the implant and member tend to separate. More specifically, the coil slides off of the inflatable member. In addition to causing minimal post delivery migration of the implant, this construction provides an advantageously simple one-piece decoupling mechanism, which can be readily manufactured.       

     According to another aspect of this embodiment, the inflatable member and tubular portion also are configured as described above so that the hydraulic pressure generated by the inflatable member is applied uniformly to the inner circumferential surface of the tubular portion. In the preferred embodiment, the inner surface of the tubular portion is essentially symmetrical about its longitudinal axis and the inflatable member is essentially symmetrical about its longitudinal axis when inflated or deflated to provide an essentially uniformly distributed force to the inner circumference of the tubular section. 
     According to yet a further embodiment of the invention, the implant delivery assembly comprises an occlusive implant having a tubular portion; a pusher having a proximal section and a distal section; a core member slidably disposed within the pusher and extending into the tubular portion; and a locking member releasably coupled to the coil and core member. With this construction the release mechanism is simply mechanically expanded to interlock the implant to the pusher and relaxed to release the implant. 
     In a first configuration, the locking member comprises an elastomeric ring, such as an O-ring, and the core member includes a locking portion and a tapered portion adjacent thereto. The diameter of the core member exceeds the inner diameter of the ring such that when the ring is positioned on the locking portion it expands and frictionally locks the tubular portion thereto. On the other hand, the tapered portion tapers to a diameter that allows the ring to contract. In the preferred embodiment, the tapered portion is less than or equal to the inner diameter of the ring when the ring is in its relaxed state. When the core member is retracted, the tapered portion becomes positioned within the ring and allows the ring to radially contract and release the tubular portion and, thus, the implant, as the locking member returns to its relaxed state. 
     In another configuration, the locking member comprises a flexible sleeve and the core member extends into the sleeve and is secured thereto. The sleeve is configured so that when axially compressed, it expands radially against the inner surface of the tubular portion and frictionally locks the implant thereto. The core member is retracted to compress the sleeve against a restraint, expand it radially and lock the implant to the delivery assembly. When it is desired to release the implant, the core member is advanced to remove the axial compression and radially contract the sleeve. 
     These configurations advantageously eliminate the need for auxiliary hydraulics. 
     The above is a brief description of some of the features and advantages of the present invention. Other features, advantages and embodiments of the invention will be apparent to those skilled in the art from the following description, accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a catheter apparatus constructed according to a general embodiment of the present invention; 
         FIG. 2  is an enlarged, fragmentary view of an occlusive implant delivery assembly, constructed according to the principles of the present invention, disposed within a catheter; 
         FIG. 3  is a perspective view of the coupling mechanism that forms part of the release mechanism shown in  FIG. 2 ; 
         FIG. 4  is an enlarged, fragmentary view of the implant delivery assembly of  FIG. 2  with the implant positioned at a desired location; 
         FIG. 5  is a further view of the implant delivery assembly shown in  FIG. 4  with the coupling of the release mechanism expanded to unlock and release the implant from the pusher; 
         FIG. 6  shows the release mechanism deflated and retracted from the implant location; 
         FIG. 7  is an enlarged, fragmentary view of another embodiment of the implant delivery system of the present invention with the release mechanism in a locked state; 
         FIG. 8  illustrates the release mechanism of  FIG. 7  in an unlocked state; 
         FIG. 9  is a further embodiment of the release mechanism of the present invention; 
         FIG. 10  is a further view of the release mechanism shown in  FIG. 9  showing the mechanism actuated to release the coil therefrom; 
         FIG. 11  is a further view of the release mechanism shown in  FIGS. 8 and 9  illustrating the implant fully detached from the mechanism; 
         FIG. 12  is an enlarged, fragmentary view of yet another embodiment of the release mechanism of the present invention showing the mechanism in a locked state; 
         FIG. 13  is a further view of the release mechanism of  FIG. 12  illustrating the mechanism in an unlocked configuration; 
         FIG. 14  is yet a further embodiment of the release mechanism of the present invention illustrating the mechanism in a locked configuration; and 
         FIG. 15  is a further view of the release mechanism of  FIG. 14  showing the mechanism in an unlocked configuration and the implant released therefrom. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in detail, wherein like numerals indicate like elements, several embodiments of an occlusive implant delivery assembly are shown according to the principles of the present invention. The various embodiments employ an expandable mechanism, which is expanded or contracted, to decouple and release the implant at the desired site. Although variously configured implants can be used in conjunction with the assembly of the present invention, an embolic coil type implant will be described for purposes of example. 
     The operation of the assembly generally comprises the steps of (1) advancing a catheter through a vessel lumen, for example, to the vicinity of the site to be occluded (e.g., an aneurysm, vascular malformation, or arterial venous fistula), (2) advancing the implant delivery assembly through and beyond the catheter to the location, and (3) radially expanding or contracting the release mechanism to detach the implant from the assembly. 
     Referring to  FIG. 1 , a catheter apparatus  2  suitable for guiding the occlusive implant delivery assembly and providing actuation pressure for the hydraulically actuated release mechanism embodiments is shown. Catheter apparatus  2  generally includes a catheter  4 , syringe  6  and sidearms (adapters)  8 A and  8 B. Catheter  4  generally comprises an elongate tubular member having proximal and distal end portions  10  and  12 . The catheter is preferably between about 50-300 cm in length, and typically between about 60-200 cm in length. The catheter also is designed for accessing a vessel site at which, for example, vasoocclusion is desired. For example, the vessel site may be within a small diameter vessel having 2-5 mm lumen diameter and accessible by way of a tortuous vessel path which may involve sharp vessel turns and multiple vessel branches. In that case, the catheter preferably has a small diameter, flexible construction with a lumen diameter of less than about 40 mil, and preferably between about 8-30 mil. Catheters of this type, which are typically used for accessing deep brain vascular sites, are commercially available. 
     The elongated tubular member or catheter  4  is secured at its proximal end  10  to sidearm  8 A, which is of conventional design for introducing fluids or apparatus into the catheter. The end of proximal section  32  of pusher  26 , which will be described in more detail below, extends through sidearm  8 A and is coupled to the distal or downstream end of sidearm  8 B. Sidearm  8 B, which is otherwise essentially similar in construction to sidearm  8 A, can include a tubular extension  14  that surrounds a portion of the pusher as shown in FIG.  1 . Mandrel  54 ,  56  or  68 , which extends through the pusher, as will be discussed below in connection with  FIGS. 9-15 , extends through one tube of sidearm  8 B. The discharge tip of syringe  6 , which is used in conjunction with the embodiments shown in  FIGS. 2-11  is fluidly coupled to the other tube of sidearm  8 B and, thus, the inner lumen of pusher  26  through which the aforementioned mandrels extend. 
     Syringe  6  is of conventional construction and includes a cylindrical barrel  18  and a plunger  20  that is reciprocally mounted therein. A stopcock  22  preferably is provided in the discharge piece of the syringe for opening or closing the fluid connection between the syringe and pusher lumen. Alternatively, the stopcock can be provided in a connector (not shown) that couples the discharge piece of the syringe to sidearm  8 B. When the stopcock is in the closed position, the decoupling or release mechanism of the implant delivery assembly will not be inadvertently actuated, thereby avoiding wrongly positioning the implant within the body as a result of such accidental discharge of liquid from the syringe into the catheter. 
     As discussed above, the implant delivery assembly, which is generally designated with reference numeral  24  in  FIG. 1 , is guided through catheter  4  towards the intended vasoocclusion site. Occlusive implant delivery assembly  24  generally comprises a pusher or elongated carrier member  26 , a coil type occlusive implant  28  and a decoupling or release mechanism for releasing the implant from the assembly. Although coil  28  is shown in the drawings as a uniform diameter helical coil wire, it may have other configurations. It is important, however, that the coil be dimensioned to be able to be advanced through a catheter that is sized to access the desired site. The coil may be made of radiopaque, biocompatible metal such as platinum, gold, tungsten, stainless steel or alloys of these metals. Preferably, the coil comprises platinum, gold, tungsten or alloys of these metals so that its location at the site may be readily viewed radiographically. 
     For use in occluding peripheral or neural sites, the coils will typically be made of 0.05 to 0.15 mm diameter platinum wire that is wound to have an inner diameter of 0.15 to 0.96 mm with a minimum pitch (i.e., the windings are close or tight). The length of the wound wire (i.e., the coil) will normally be in the range of 0.5 to 60 cm, and preferably 0.5 to 40 cm. For wires intended for use in vessels with diameters of about 2 mm and smaller, the coil has a preferred length of about 0.5 to 20 cm. The coil can have any shape. For example, it can be formed so that it takes an essentially linear configuration in which it may be advanced through the catheter and assume a randomly oriented configuration, such as helical, after it is released from the catheter and in a relaxed state as disclosed in U.S. Pat. No. 4,994,069, which is hereby incorporated herein by reference. 
     Referring to  FIGS. 2-6 , a first embodiment of the occlusive implant delivery assembly, will be described. The delivery assembly shown in  FIGS. 2-6  generally comprises a pusher or elongated carrier member  26 , coil  28  and coupling  30 . The pusher preferably has a tubular construction to provide a lumen for fluidly coupling a source of pressurized fluid, such as syringe  6 , and an inflatable member utilized in decoupling the coil from the pusher, as will be described in more detail below. Pusher  26  also preferably has a proximal section that is rigid enough to facilitate torque transmission to the distal portion of the pusher. The distal section of the pusher may be constructed to be more flexible than the proximal portion to facilitate navigation of the distal section into very tiny vessels encountered in the brain, for example. 
     In the preferred embodiment, proximal tubular section of pusher  26  (designated with reference numeral  32 ) is a metal tube, preferably a stainless steel tube, and the distal section of pusher  26 , section  34 , comprises a coil  36 , which is wrapped in a flexible, elastomeric film  38  to fluidly seal the spaces between the coil windings. Film  38  also overlaps section  34  to seal the juncture between section  34  an coil  36 . Film  38  can be in the form of shrinkwrap and, thus, applied to coil  36  and proximal section  34  with conventional shrinkwrap techniques. Coil  36  and, thus, distal coiled section  34  is secured to the proximal tubular section  32  by welding, soldering, brazing, or adhesive. 
     Alternatively, a more simple pusher configuration may be used in which the pusher comprises a rigid plastic tube which can be ground with a tapered distal section to achieve the desired flexibility. Suitable materials for this configuration include PEEK and polyimide. The inner diameter of the distal section in this configuration preferably is significantly less than the outer diameter of the proximal section to which the balloon can attached (e.g., glued). In a preferred embodiment, the lumen, which provides for fluid flow between the source of pressurized fluid and the balloon, has a diameter of about 0.007 inch throughout its length and the distal section has an outer diameter of about 0.014 inch. The outer diameter of the proximal section depends on the application. For a 3 French catheter, the outer diameter of the proximal section may be about 0.016 to 0.018 inch. Although particular pusher configurations have been described, it should be understood that other configurations may be used without departing from the scope of the invention. 
     A conventional inflatable balloon  40 , having a construction similar to those used in conventional balloon catheters, is secured to the distal end of coil  36  by adhesive, for example, such that a fluid tight path is formed between the interior of the balloon and the central lumen of pusher  26 , which is formed by proximal and distal sections  32 ,  34  of pusher  26 . 
     Returning to  FIG. 2 , balloon  40  extends into tube  42 , which is also secured to implant coil  28  by welding, soldering, brazing or adhesive. As shown in  FIG. 2 , coupling  30  comprises a tubular member or split tube having slots formed in the axial direction and which open into the end of the tube that is directly coupled to the distal portion of pusher coil  38 . The tube to pusher coupling can be accomplished by a pressure fit, welding, soldering, brazing or adhesive. Slots  42  form multiple segments  44  in tubular coupling  30  and facilitate displacement of those segments to effect release of the coil implant from the pusher, as will be described in more detail below. Although a two slot configuration is shown, other multiples of slots can be used to facilitate displacement of the proximal portion of the coupling as well as other conventional jaw or latch clamping configurations. 
     Tubular coupling  30  can be made from platinum, stainless steel or plastic that is biocompatible with the environment in which the coupling will be placed. The coupling  30  preferably also has a very thin wall of about 0.001 to 0.0003 inches. 
     The implant delivery assembly of  FIGS. 2-6  will be further described by way of the following operative example which is provided merely for exemplary purposes and is not intended to limit the invention to a particular application. 
     A catheter is inserted through the vessel lumen to the site to be occluded (e.g., an aneurysm, vascular malformation, or arteriovenous fistula. Conventional catheter insertion and navigational procedures involving guidewire and/or flow-directed means may be used to access the site with the catheter. Thus, although not shown, catheter  4  may include a guidewire useable therewith to guide the distal end of the catheter toward the desired or selected occlusion site. Guidewires of this type are commercially available, and generally include an elongate wire having a tapered, wire-wound distal end region which is adapted to be advanced through a tortuous vessel path, with the catheter being moved axially along the advanced guidewire. 
     Once the distal end of the catheter is positioned at the selected site (its location may be determined by a coating at the distal end of the catheter with a radiopaque material or otherwise affixing such a material to the distal end of the catheter or incorporating such a material into the distal end of the catheter), the catheter is cleared. For example, if a guidewire has been used to position the catheter, it is withdrawn from within the catheter. 
     Then, the implant delivery assembly, as shown in  FIG. 2 , is introduced into the proximal end portion of catheter  4 , and advanced toward the distal end portion of catheter  4 . The proximal end of pusher  26  is manipulated via sidearm  8 B, to which it is attached, so that coupling  30  and coil implant  28  extend beyond the distal end of the catheter with coupling  30  free of the catheter and the coil positioned exactly at the desired site (FIG.  4 ). Stopcock  22  is then placed in an open position and the plunger of syringe  6  advanced to inflate balloon  40  as shown in FIG.  5 . As balloon  40  is inflated, it further opens split tube or coupling  30 , i.e., segments  44  are displaced radially outward to decouple coupling  30  and coil  28  from pusher  26  without transmitting any significant force to coil  28 . The balloon is then deflated by retracting the plunger in syringe  6 , thereby releasing coupling  30  from balloon  40  so that the pusher can be retracted without altering the position of coil  28 . After the desired number of coils have been placed at the site, the catheter is withdrawn from the vessel. 
     Referring to  FIGS. 7 and 8 , a further embodiment of the release or decoupling mechanism is shown similar to that shown in  FIGS. 2-6 , but in which coupling  30 ′ has its proximal portion fixedly secured to the distal end of coiled portion  34 . In addition, coupling  30 ′ includes end walls  48  at its distal end for overlapping end piece or cap  50  provided at the proximal end of coil implant  28 ′. That is the end walls, which generally form jaws, releasably secure coil  28 ′ to coupling  30 ′ and, thus, releasably secure coil  28 ′ to pusher  26 . Coupling  30 ′ also differs from coupling  30  in that slots  42 ′ are formed in the distal portion of the coupling. Once the coil implant is positioned at the desired location, fluid is introduced through the hollow pusher member and into balloon  40 , as described above, to displace segments  44 ′ radially outward and release coil  28 ′ from coupling  30 ′ (FIG.  8 ). The balloon can then be deflated and the pusher retracted. With this configuration, the coupling is advantageously withdrawn with the pusher. 
     Referring to  FIGS. 9-11 , a further embodiment of the invention is shown. This embodiment essentially differs from those described above in that the release or decoupling mechanism simply comprises a balloon. The balloon extends from the pusher with its proximal portion close fit within coil  28 . When it is desired to deploy the coil, the balloon is inflated, and as the balloon expands, the coil slides off the end of the balloon as will be described in more detail below. 
     The decoupling mechanism of  FIGS. 9-11  comprises a balloon  40 ′ having its open end secured to the distal coiled section  34  of pusher  26 , for example, by adhesive. Balloon  40 ′ is packed into the proximal portion of coil  28  such that the balloon frictionally engages the inner surface of coil  28  and secures the coil to the balloon. To enhance the securement between the coil and balloon, the balloon is constructed such that, when in the deflated state, the balloon has a plurality of circumferentially extending ribs  52 , which preferably are configured to have a pitch corresponding to that of the coil so that the ribs can snugly fit between the windings of the coil. The ribs can be formed by placing a mandrel into the balloon, wrapping a thread around the balloon in the regions where the ribs are desired to be located, and then dipping the balloon, mandrel and thread assembly in a reservoir of elastomeric material, such as silicon, to form an outer ribbed elastomeric coating for the balloon. 
     The decoupling mechanism of the embodiment illustrated in  FIGS. 9-11  also preferably includes a mandrel  54  which extends from outside sidearm  8 B through catheter  12  via the interior lumen of pusher  26  and into balloon  40 ′. Mandrel  52  facilitates inserting balloon  40 ′ within coil  28  and preferably is sized to force the outer wall of the balloon against the inner circumferential surface of coil  28  to enhance the interlocking connection between the coil and balloon. 
     In operation, the pusher and the mandrel are advanced through catheter  4  until coil  28  is positioned at the desired location (FIG.  9 ). The mandrel is then retracted or withdrawn from the balloon and the syringe actuated to inflate the balloon  40 ′ as described above (FIG.  10 ). In this case, it is important that mandrel  54  is sized so that when placed in the pusher lumen, sufficient space between the mandrel and the inner surface of the proximal and distal sections  32 ,  34  of pusher  26  is formed. In this manner, the interior of balloon  40 ′ can be fluidly coupled to the syringe  6  when stopcock  22  is in the open position and the mandrel is in the pusher. As the balloon inflates and stretches, the ribs generally flatten and the proximal end of coil  28  slides off the distal end portion of balloon  40 ′. In order to avoid axial displacement of the coil, the balloon can be retracted as it is inflated. Alternatively, the end of the balloon can be positioned where the proximal end of the coil is desired to be finally located. As the balloon inflates, the proximal end of the coil will ultimately be located at the distal end of the balloon. The balloon position can be determined by conventional means such as radiographic techniques. The pusher can then be retracted as shown in FIG.  11  and the balloon deflated. The procedure is repeated if the delivery of additional coils is desired. 
     Referring to  FIGS. 12-15 , further embodiments of the invention are shown in which the release or decoupling mechanism comprises a mechanically expandable or locking member rather than a fluidly inflatable/expandable balloon. The expandable locking member fits within the proximal end of the coil and is radially expanded to grip the inner circumferential surface of the coil. When the expandable member is returned to a generally relaxed state so that its diameter decreases, the coil is released. 
     The decoupling mechanism shown in  FIGS. 12 and 13  generally comprises core wire or actuating member  56  and an elastomeric ring or locking member  60 , such as an O-ring, which is slidably mounted on core wire  56 . Core wire or mandrel  56  includes a proximal locking portion  62 , which preferably has a generally uniform diameter, and a distal tapered or unlocking portion. More specifically, the diameter of the core wire locking portion exceeds the inner diameter of the ring such that when the ring is positioned on the locking portion it expands against the inner circumferential surface of coil  28  and frictionally locks the coil thereto (FIG.  12 ). On the other hand, the tapered portion tapers to a diameter that allows the ring to radially contract and release the coil. In the preferred embodiment, the tapered portion tapers to a diameter that is less than or equal to the inner diameter of the ring when the ring is in its relaxed state. When the core wire is retracted, the tapered portion becomes positioned within the ring and allows the ring to radially contract and release the coil as it returns to its relaxed state (FIG.  13 ). Core wire  56  can be ground to the desired shape as is conventional in the art. 
     In addition, the distal portion of actuating member  56  includes a stop member  66  to ensure that the elastomeric ring  60  does not become detached from the actuating member. Otherwise the ring would become free to migrate in the blood stream, which could result in an embolism. A disc  58  optionally may be secured to the distal end of coil  36  by welding, soldering, brazing or adhesive to simplify retraction of the pusher as will be discussed in more detail below. 
     In operation, ring  60  is positioned on the locking portion of core wire  56  between the core wire and coil  28 . Then, the pusher and core wire are both advanced through catheter  4  so that coil  28  eventually extends beyond the catheter and is positioned at the desired location (FIG.  12 ). Once coil  28  is so positioned, core wire  56  is slowly retracted, causing the tapered distal portion  54  to slide within the opening of ring  60 , thereby allowing the ring to return to its relaxed, unexpanded state. In this state, the ring diameter is significantly less than the inner diameter of coil  28  to facilitate rapid coil release. As the core wire is further retracted, stop member  66 , which has a larger diameter than the inner diameter of ring  60 , catches the ring and carries it as the core wire is completely withdrawn from coil  28  (FIG.  13 ). When disc  58  is incorporated, the entire pusher  26  can be withdrawn by merely retracting actuating member  56  as stop member  66  acts on coil  36  through ring  60  and disc  58  as is apparent from the drawings. 
     Referring to  FIGS. 14 and 15 , a further embodiment of the release or decoupling mechanism is shown. The decoupling mechanism illustrated in these figures generally comprises a core wire or actuating member  68 , disc or retaining member  70  and sleeve or locking member  72 . Sleeve  72  is compressed to expand it in the radial direction and interlock the coil to the pusher assembly (FIG.  14 ). Once in place, it is extended to release the coil therefrom (FIG.  15 ). 
     Core wire  68  extends from sidearm  8 B as shown in FIG.  1 . The distal end portion of core wire  68 , preferably is secured to the distal end of sleeve  72  so that when the core wire  68  is retracted, sleeve  72  is compressed in the axial direction against disc  70  as shown in FIG.  14 . Sleeve  72  preferably is of a material that, upon compression in the axial direction, will expand radially to interlock with coil  28 . Accordingly, sleeve  72  can comprise fabric and, preferably, comprises braided material in which the degree of radial expansion generally depends upon the pitch of the braiding. 
     The actuator is initially positioned as shown in  FIG. 14  with the open end of sleeve  72  compressed against disc  70 . The coil is released from the pusher assembly by simply advancing the core wire  68  as shown in  FIG. 15  while maintaining pusher  26  is a fixed position. Then, pusher  26  and core wire  68  are concurrently retracted so as to maintain sleeve  72  in its extended position, while withdrawing sleeve  72  from coil  28  without placing any significant mechanical force on the coil in either the radial or axial direction. 
     The above is a detailed description of several embodiments of the invention. It is recognized that departures from the disclosed embodiments may be made within the scope of the invention and that obvious modifications will occur to a person skilled in the art. The full scope of the invention is set out in the claims that follow and their equivalents. Accordingly, the claims and specification should not be construed to unduly narrow the full scope of protection to which the invention is entitled.