Abstract:
A self-expandable mechanical clot retrieval device is designed variously to dislodge, engage and retract blood clot from extremely small and tortuous vasculature. The retrieval device comprises an elongate member and a plurality of ring elements. The ring elements may comprise a plurality of struts and crowns interconnected by a tether formed at a distal end of the elongate member or may be a separate component attached thereto. At least one tether may connect each ring element to the elongate member to restrain the ring element in a collapsed configuration during delivery of the retrieval device through an intravascular microcatheter. The tether connecting the ring element to the elongate member disengages when the retrieval device is positioned at the occluded site and the microcatheter is retracted to allow the self-expandable ring elements to reach an expanded configuration.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of distal mechanical thrombectomy, more particularly, to a self-expandable mechanical clot retrieval device used in the treatment of acute ischemic stroke and pulmonary embolism in a patient. The retrieval device is of extremely small size delivered through an intravascular microcatheter and meticulously designed to remain in a collapsed state while advancing through the microcatheter to remove thrombus or embolus from the blocked arteries. 
     DISCUSSION OF THE PRIOR ART 
     Acute ischemic stroke remains one of the major causes of death and disability worldwide. It refers to stroke caused by thrombosis or embolism creating an acute blockage in vasculature that stops flow of blood and deprives the surrounding brain tissue of oxygen. In the absence of oxygen, the brain cells of the immediate area begin to die and release a cascade of toxic chemicals that threaten brain tissue in the surrounding area. 
     Similarly, pulmonary embolism is a common and potentially fatal condition, creating a sudden blockage in a lung artery caused by any naturally occurring embolus traveling through the arteries of the lung and occluding a small artery that stops the flow of blood and deprives the surrounding lung tissue of oxygen. In the absence of oxygen, the lung tissues in the immediate area succumb to lysis as they are unable to process sufficient oxygen into the blood. 
     Importantly, both acute ischemic stroke and pulmonary embolism can be avoided and treated by removing the acute blockage and restoring blood flow to the affected area. In recent years, significant advances have been made to prevent ischemic stroke and pulmonary embolism through endovascular procedures involving the use of distal mechanical clot retrieval devices on the end of catheters to manually retract clot and fragments of clot. 
     Generally, the distal mechanical devices are delivered endovascularly and advanced by guidewire and microcatheter until the device is located distal of the clot where it expands. Thus force is applied to the distal surface of the clot to dislodge it from the artery wall where it is caught and retracted proximally by the retrieval device. 
     Despite continuous advances made in the art of distal mechanical thrombectomy, there is still much room for improvement in effectively removing acute blockage particularly in engaging and retracting thrombus or embolus from extremely small vasculature. The present invention is an efficient and meticulous approach to improve upon the distal mechanical thrombectomy. 
     SUMMARY OF THE INVENTION 
     Various aspects of the present invention concern a self-expandable mechanical clot retrieval device that effectively and reliably removes thrombus or embolus from blocked vasculature. It is designed variously to dislodge, engage and retract blood clot from extremely small and tortuous vasculature. The retrieval device comprises an elongate member and a plurality of ring elements. The ring elements may comprise a plurality of struts and crowns interconnected by a tether formed at a distal end of the elongate member or may be a separate component attached thereto. In one aspect, at least one tether connects each ring element to the elongate member to restrain the ring element in a collapsed configuration during delivery of the retrieval device through an intravascular microcatheter. The tether connecting the ring element to the elongate member disengages when the retrieval device is positioned at the occluded site and the microcatheter is retracted to allow the self-expandable ring elements to reach an expanded configuration. Preferably, the self-expandable ring elements are formed of shape memory material such as Nitinol. 
     Additional aspects of the present invention concern methods of using the mechanical clot retrieval device described in the previous paragraph. It is used in the treatment of acute ischemic stroke and pulmonary embolism in a patient. The method of removing the blood clot from a blocked artery includes advancing a self-expandable mechanical clot retrieval device having an elongate member and a plurality of self-expandable ring elements arranged at a distal end of the elongate member through the vasculature to a blockage site in the artery. The retrieval device bypasses the clot and is positioned distal to the clot to allow the self-expandable ring elements to reach an expanded configuration in order to engage the clot within expanded ring elements and is proximally retracted from the artery. The method may include retracting the expanded ring elements proximally into a guide catheter or a microcatheter. 
     STATEMENT OF THE INVENTION 
     In a first embodiment of the invention the clot retrieval device comprises an elongate member and a plurality of clot engaging elements at a distal end of the elongate member, the clot engaging elements being self expandable and having a retracted delivery configuration and an expanded deployed configuration; and a biasing element for biasing the clot engaging elements into a retracted delivery configuration against the elongate member; the biasing element being releasable to allow the clot engaging elements to expand into the deployed configuration. 
     The second embodiment of the invention comprises a method of retrieving a clot comprising the steps of: (1) providing an elongate member and a plurality of clot engaging elements at a distal end of the elongate member, the clot engaging elements having a retracted configuration and an expanded configuration: (2) providing a biasing element for retaining the clot engaging elements in the retracted configuration; (3) advancing a microcatheter across a clot; (4) advancing the elongate member with the clot engaging elements in a retracted configuration through the microcatheter so that at least some of the clot engaging elements are distal of the clot; (5) releasing the biasing so that the clot engaging elements expand to the expanded configuration; (6) engaging the clot engaging elements with a clot; and (7) using the clot engaging elements, drawing the clot into a retrieval element. 
     A third embodiment of the invention comprises a luminal prosthesis device comprising: an elongate member and a plurality of luminal prosthetic elements at a distal end of the elongate member; the luminal prosthetic elements being self expandable and having a retracted delivery configuration and an expanded deployed configuration; and a biasing element for biasing the luminal prosthetic elements into a retracted delivery configuration against the elongate member; the biasing element being releasable to allow the luminal prosthetic elements to expand into the deployed configuration. 
     A fourth embodiment of the invention comprises a clot retrieval device comprising: an elongate member and a plurality of clot engaging elements at a distal end of the elongate member, the clot engaging elements being self expandable and having a retracted delivery configuration, a retracted deployed configuration and a fully expanded configurations; and providing a release element for biasing the clot engaging elements in at least one retracted configuration; activation of the release element allowing the clot engaging elements to expand into the fully expanded configuration. 
     In one variation of this embodiment the diameter of the clot engagement elements in the retracted delivery configuration is smaller than the diameter of the clot engagement elements in the retracted deployed configuration and the diameter of the clot engagement elements in the retracted deployed configuration is smaller than the diameter of the clot engagement elements in the fully expanded configuration. 
     In another variation of this embodiment a release element is configured to bias the clot engaging elements in a retracted deployed configuration. The release element may be integral with the clot engagement elements. The release element may be integral with elongate member. The release element may be connected to either the clot engagement elements, the elongate member or to both. The release element may comprise; a snap element, a restraining element, a tether, an interlocking element, an overlapping element of the clot engagement element, an abutment element, a connector element, a housing element, a pair of interacting elements, or a limiting element configured to limit at least one degree of expansion movement. 
     In another variation of this embodiment the clot engagement element comprises a plurality of struts and said release element is configured to limit relative movement between at least a pair of said struts. In another variation of this embodiment the release element is configured to limit relative movement between the clot engagement element and the elongate member. The elongate member may be configured to activate the release element. 
     In another variation of this embodiment the device comprises an activation element said activation element configured to affect the release of the release element. The activation element may comprise an elongate tube, an elongate member, or a tether. 
     A fifth embodiment of the invention comprises a clot retrieval device comprising: an elongate member and a first clot engaging element and a second clot engaging element the first clot engaging element distal of the second clot engaging element, the clot engaging elements being self expandable and having a sheathed delivery configuration, a deployed configuration and a fully expanded configurations; and providing at least one release element for biasing the clot engaging elements in the deployed configuration; activation of the at least one release element allowing the clot engaging elements to expand into the fully expanded configuration. 
     In one variation of this embodiment the sheathed configuration comprises sheathing within the lumen of a microcatheter. In another variation of this embodiment the unsheathing the clot engaging elements allows the clot engaging elements to assume the unsheathed configuration. In another variation of this embodiment the diameter of the clot engaging elements in the deployed configuration is greater than the diameter of the clot engaging elements in the sheathed configuration. In another variation of this embodiment the first clot engagement element assumes the expanded configuration while the second clot engaging element is in the deployed configuration. 
     A sixth embodiment of the invention comprises a method for retrieval of a clot comprising the steps of: (1) providing a clot retrieval device the clot retrieval device comprising an elongate member and a plurality of clot engaging elements at a distal end of the elongate member, the clot engaging elements having a retracted configuration, a partially expanded configuration and an expanded configuration; (2) providing a biasing element for limiting the expansion of the clot engaging elements; (3) advancing a microcatheter across a clot; (4) advancing the elongate member with the clot engaging elements in a retracted configuration through the microcatheter so that at least some of the clot engaging elements are distal of the clot; (5) retracting the microcatheter so as to unsheathe at least one clot engaging element, (6) activating the biasing element so as to trigger the expansion of at least one of said clot engagement elements to the expanded configuration, (7) retracting the clot retrieval device and the clot into a proximal retrieval element. 
     In one variation of this embodiment the method comprises the step of retracting the microcatheter also comprises the step of expanding the at least one clot engagement element to the partially expanded configuration. In another variation of this embodiment the step of activating the biasing element comprises the step of applying an activation force to the biasing element. 
     In another variation of this embodiment the step of activating the biasing element comprises the step of applying a compressive, tensile, twisting or radial force to the biasing element. 
     In another variation of this embodiment the method comprises the step of providing a plurality of clot engagement elements configured such that in the partially expanded configuration a flow lumen extends through the centre of said clot engagement elements, said lumen being larger than the diameter of the microcatheter and smaller than the diameter of the vessel. 
     In another variation of this embodiment the method comprises the step of holding the clot retrieval device steadfast in the vessel for a dwell time period the dwell time period comprising at least one minute. 
     A seventh embodiment of the invention comprises a retrieval device for removing occlusive clot from the vasculature the retrieval device comprising: an elongate member and an expandable body connected to the elongate member, the expandable body comprising a plurality of self-expanding clot engaging elements, each clot engaging element comprising a plurality of struts and a plurality of crowns arranged to form at least one ring, at least one of said plurality of crowns comprising an integral connection to an adjacent ring, a spacing between at least two of said clot engaging elements said spacing comprising a variable spacing in use. 
     In one variation of this embodiment the at least one integral connection comprises a monolithic connection. The at least one integral connection may comprise a connection to a crown of an adjacent ring of the same clot engaging element. The at least one integral connection may comprise a connection to a crown of a ring of an adjacent engaging element. The at least one integral connection may comprise a connection to the elongate member. In another variation of this embodiment the at least one of said plurality of crowns comprises an unconnected crown. In another variation of this embodiment the a pair of adjacent crowns of said expandable body comprise a pair of unconnected crown, the first of said pair of adjacent crowns comprising a proximally facing crown and the second of said pair of adjacent crowns comprising a distally facing crown. The first crown of said pair of adjacent crowns may be located on a first clot engaging element and the second crown of said pair of adjacent crowns may be located on a second clot engaging element. 
     In another variation of this embodiment the clot engaging elements are arranged in series. In another variation of this embodiment the clot engaging elements are arranged in series. 
     In another variation of this embodiment the clot engagement elements are spaced apart. The spacing between at least two of said clot engaging elements may vary during delivery. The spacing between at least two of said clot engaging elements may vary during engagement with the clot. The spacing between at least two of said clot engaging elements may vary during clot disengagement with the vessel wall. The spacing between at least two of said clot engaging elements may vary during withdrawal of the clot through the vasculature. The spacing between at least two of said clot engaging elements may vary during removal of the clot and device into the lumen of a retrieval catheter. The spacing between at least two of said clot engaging elements may vary across the diameter of the clot engaging elements. 
     An eighth embodiment of the invention comprises a retrieval device for removing occlusive clot from the vasculature comprising: an elongate member and an expandable body connected to the elongate member, the expandable body comprising an collapsed delivery configuration and an expanded configuration, the expandable body comprising a plurality of self-expanding clot engaging elements, cut from a tube in a monolithic structure, each clot engaging element comprising a plurality of struts and a plurality of crowns arranged to form a ring, at least one pair of adjacent clot engaging elements being spaced apart by a plurality of connector elements, the pair of adjacent clot engaging elements and the plurality of connectors configured to create a plurality of clot engaging cells, the plurality of connectors comprising a plurality of helical strut. 
     In one variation of this embodiment the plurality of clot engaging cells are configured to scaffold outwardly so as to create a flow lumen through the clot. The plurality of clot engaging cells may be configured to urge clot ingress through the cell openings so that the clot is interlaced with the plurality of engaging cells. 
     In another variation of this embodiment the helical struts extend circumferentially through an angle of at least 25 degrees. In another variation of this embodiment the helical struts extend circumferentially through an angle of at least 40 degrees. In another variation of this embodiment the helical struts extend circumferentially through an angle of at least 50 degrees. In another variation of this embodiment the helical struts extend axially a distance of at least 500 micrometers. In another variation of this embodiment the helical struts extend axially a distance of at least 750 micrometers. In another variation of this embodiment the helical struts extend axially a distance of at least 900 micrometers. 
     In another variation of this embodiment the tube from which the expandable body is cut has a diameter larger than the diameter of the expandable body in the collapsed delivery configuration. The tube from which the expandable body is cut may have a diameter equal to or smaller than the expanded diameter of the expandable body. 
     A ninth embodiment of the invention comprises a mechanical clot retrieval device for use in a blood artery, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration inside the artery, wherein the ring elements comprise a radial force biasing ring elements toward an expanded configuration, wherein the radial force is absorbed by the elongate member in the collapsed configuration, wherein the microcatheter is retracted proximally allowing the ring elements reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element. The crown element may comprise an engagement member and a fixed tab. The crown element may comprise an engagement member and an engagement hook. The crown element may comprise an engagement member and a connector. The crown element may comprise an engagement member and an engagement tab. The crown element may engage a neighboring crown element in a snap-fit mechanism. 
     In another variation of this embodiment the snap-fit mechanism restrains the ring elements in the collapsed configuration. The ring elements in the collapsed configuration may be substantially parallel along the longitudinal axis of the elongate member. In another variation of this embodiment the device comprises a proximal fixed collar configured proximally of the ring elements. In another variation of this embodiment the device further comprises a push tube attached to the proximal fixed collar. 
     In another variation of this embodiment the device further comprises a distal moveable collar configured distally of the ring elements. In another variation of this embodiment the device comprises a balloon. In another variation of this embodiment the balloon is delivered in a deflated state. In another variation of this embodiment the device further comprises a ball element. In another variation of this embodiment the device further comprises a pair of pull tethers having distal and proximal ends. In another variation of this embodiment the distal ends are connected to the most proximal ring element. In another variation of this embodiment the distal ends are attached to the connector. In another variation of this embodiment the distal ends are attached to the engagement tab. 
     In another variation of this embodiment the distal ends are attached to the distal movable collar. In another variation of this embodiment the proximal ends extend exterior of the artery. The proximal ends may be manually pulled proximally to disengage the snap-fit mechanism. The proximal ends may be attached to the elongate member. In another variation of this embodiment the balloon is inflated to disengage the snap-fit mechanism. In another variation of this embodiment the ball element is manually retracted proximally to disengage the snap-fit mechanism. 
     A tenth embodiment of the invention comprises a mechanical clot retrieval device for use in an artery, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is attached to the elongate member by connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the elongate member in the collapsed configuration, wherein the microcatheter is retracted proximally allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element. In another variation of this embodiment the proximally facing crown element is a ring proximal crown and distally facing crown element is a ring distal crown. The ring proximal crown may be attached to the elongate member by connecting tethers. The crown element may be engaged with the elongate member in a snap-fit mechanism. The snap-fit mechanism restrains the ring elements in the collapsed configuration. The elongate member is manually retracted to disengage the snap-fit mechanism. In another variation of this embodiment the elongate member is attached to at least one ring element in the expanded configuration. 
     A eleventh embodiment of the invention comprises a mechanical clot retrieval device for use in an artery, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element comprising a plurality of struts is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element. The crown element may be attached to the elongate member by a bonding agent. The bonding agent restrains the ring elements in the collapsed configuration. The bonding agent may be dissolved in vivo allowing the ring elements reach the expanded configuration. The radial force may be absorbed by the bonding agent. 
     In another variation of this embodiment the crown element is attached to the elongate member by a resorbable covering. The re-absorbable covering restrains the ring elements in the collapsed configuration. The re-absorbable covering may be dissolved in vivo allowing the ring elements to reach the expanded configuration. The radial force may be absorbed by the re-absorbable covering. In another variation of this embodiment the device comprises a pair of pull tethers having distal and proximal ends. The distal ends of said tethers are attached to most the proximal ring element. The proximal ends of said tethers extend exterior of the artery. 
     An twelfth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end and a plurality of radial pins, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is advanced through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by radial pins in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment the radial pin has an engagement member. In another variation of this embodiment each strut comprises a crown element. The crown element may comprise a plurality of eyelets. In another variation of this embodiment the distally facing eyelet engages with the radial pin. The distally facing eyelet may fasten the ring elements to the elongate member. 
     In another variation of this embodiment the proximally facing eyelet engages with the radial pins in a snap-fit mechanism. In another variation of this embodiment the snap-fit mechanism restrains the ring elements in the collapsed configuration. In another variation of this embodiment the engagement member is dissolved in vivo to disengage the snap-fit mechanism. In another variation of this embodiment the engagement member is dissolved in vivo allowing the ring elements to reach the expanded configuration. 
     An thirteenth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end, an outer tube having a plurality of slots and a retractable inner core wire, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the inner core wire in the collapsed configuration, wherein the microcatheter is retracted proximally allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element having an eyelet. In another variation of this embodiment a tether is advanced out of the slot to loop through the eyelet and folds back into the slot to loop around the retractable inner core wire. The tether may engage the ring elements with the elongate member in a tether-loop mechanism. In another variation of this embodiment the tether-loop mechanism restrains the ring elements in the collapsed configuration. 
     In another variation of this embodiment the retractable inner core wire is manually retracted proximally to disengage the tether-loop mechanism. The retractable inner core wire may be advanced out of each slot to loop through each eyelet and folds back into the slot to loop inside the outer tube. The retractable inner core wire may engage the ring elements with the elongate member in a tether-loop mechanism. 
     In another variation of this embodiment the tether-loop mechanism restrains the ring elements in the collapsed configuration. In another variation of this embodiment the retractable inner core wire is manually retracted proximally to disengage the tether-loop mechanism. 
     An fourteenth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end and a tube lumen having a plurality of slots, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the microcatheter is retracted proximally allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element having an eyelet. In another variation of this embodiment the device further comprises a plurality of tethers, having distal ends that extend interior of the tube lumen and proximal ends that extend exterior of the artery. Each tether may be advanced through a slot to loop around the eyelet and folded back into the slot attached to the distal end of the elongate member. Each tether may engage the ring elements with the elongate member in a tether-loop mechanism. The tether-loop mechanism may restrain the ring elements in the collapsed configuration. Each tether may be broken to disengage the snap-fit mechanism allowing ring elements to reach the expanded configuration. 
     An fifteenth embodiment of the invention comprises a mechanical clot retrieval device for use in a blood artery, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal collar attached to an abutment tube; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is attached to the elongate member by connecting tethers and is advanced through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element. In another variation of this embodiment the device further comprises a plurality of tethers. Each tether may be looped around the ring element and is attached to the elongate member. 
     Each tether may engage the ring elements with the elongate member in a tether-loop mechanism. The radial force may be absorbed by the tether-loop mechanism. The tether-loop mechanism may restrain the ring elements in the collapsed configuration. In another variation of this embodiment the elongate member may be manually retracted proximally to disengage the tether-loop mechanism. 
     A sixteenth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a plurality of collars configured at the distal end of the elongate member; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element is configured between two collars, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the collars in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements reach the expanded configuration. 
     In one variation of this embodiment the collar is bevel shaped or the collar is recess shaped or the collar is a shaped engagement collar or the collar has a plurality of holes or the collar has a plurality of dove tail grooves. 
     In one variation of this embodiment each strut comprises a crown element. In one variation of this embodiment the crown element has a tab extending axially and radially inward. The crown element may have a pin extending axially and radially inward. The crown element may have a shaped tab extending axially and radially inward and the tab may be bevel shaped. The tab may be square shaped. The tab may engage with the collar in a collar mechanism. 
     In another variation of this embodiment the collar mechanism engages the ring elements with the elongate member. The collar mechanism may restrain the ring elements in the collapsed configuration. In another variation of this embodiment the elongate member is manually retracted proximally to disengage the collar mechanism. 
     In another variation of this embodiment the pin engages with the hole in a collar mechanism. In another variation of this embodiment the collar mechanism engages the ring elements with the elongate member. In another variation of this embodiment the collar mechanism restrains the ring elements in the collapsed configuration. In another variation of this embodiment the elongate member is manually retracted proximally to disengage the collar mechanism. 
     In another variation of this embodiment the shaped tab engages with the dove tail groove in a collar mechanism. In another variation of this embodiment the collar mechanism engages the ring elements with the elongate member. In another variation of this embodiment the collar mechanism restrains the ring elements in the collapsed configuration. In another variation of this embodiment the elongate member is manually retracted proximally to disengage the collar mechanism. 
     A seventeenth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal collar; and a self-expandable ring element configured at the distal end of the elongate member, wherein the ring element, comprising a plurality of struts, is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring element comprises a radial force biasing the ring element towards an expanded configuration, wherein the radial force is absorbed by the proximal collar in the collapsed configuration, wherein the microcatheter is retracted allowing the ring element to reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element having a tab extending axially and radially inward. In another variation of this embodiment the tab engages with the proximal collar in a collar mechanism. In another variation of this embodiment the collar mechanism engages the ring element with the elongate member. In another variation of this embodiment the collar mechanism restrains the ring element in the collapsed configuration. In another variation of this embodiment the elongate member is manually retracted proximally to disengage the collar mechanism. 
     A eighteenth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end and inner lumen having a plurality of openings, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular catheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the microcatheter is retracted allowing the ring elements reach the expanded configuration. 
     In one variation of this embodiment each strut comprises a crown element and an engagement tab. In one variation of this embodiment the device further comprises a plurality of tethers having proximal ends extend exterior of the inner lumen. In one variation of this embodiment each tether is advanced out of each opening and is wrapped around the engagement tab in an eyelet mechanism. In one variation of this embodiment the eyelet mechanism engages the ring elements with the elongate member. In one variation of this embodiment the eyelet mechanism restrains the ring elements in the collapsed configuration. 
     In one variation of this embodiment the radial force is absorbed by the eyelet mechanism. In one variation of this embodiment the tethers are manually pulled proximally to disengage the eyelet mechanism. 
     A nineteenth embodiment of the invention comprises a mechanical clot retrieval device for use in a blood artery, comprising an elongate member having a distal end, a proximal end and a plurality of raised tabs, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements towards an expanded configuration, wherein the radial force is absorbed by the raised tabs in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     The variations described in association with each of the embodiments 1 through to 19 can be applied to the 20 th  through to the 36 th  embodiments (below) and are hereby incorporate as variations to each of the 20 th  through to the 36 th  embodiments. 
     A twentieth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end, a plurality of engagement diameters and a plurality of disengagement diameters, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal abutment tube; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by engagement diameters in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. Each strut may comprise a crown element having a C-shaped engagement tab extending radially inward. The C-shaped engagement tab engages with the engagement diameter to restrain the ring elements along the elongate member. The C-shaped engagement tab engages with the engagement diameter to restrain the ring element in the collapsed configuration. In one variation of this embodiment proximal axial movement of the elongate member disengages the ring element at the disengagement diameter. 
     A twenty first embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end, a proximal end and an inner wire, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal abutment tube; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the elongate member in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty second embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising: an elongate member having a distal end, a proximal end and an inner lumen with a plurality of slots, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal collar; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by slots in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty third embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising: an elongate member having a distal end, a proximal end and a plurality of monofilaments, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal collar; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements in an expanded configuration, wherein the radial force is absorbed by monofilaments in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty fourth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising: an elongate member having a distal end, a proximal end and a plurality of inactivated cuffs configured at the distal end of the elongate member, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a tube; and a plurality of self-expandable ring elements configured over the inactivated cuffs, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements comprise a radial force biasing the ring elements towards an expanded configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty fifth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a proximal collar; a bumper tube; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration; wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty sixth embodiment of the invention comprises a mechanical clot retrieval device for use vasculature, comprising: an elongate member having a distal end, a proximal end and a plurality of restraining loops configured at the distal end of the elongate member, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is connected to the restraining loop by connecting tethers, wherein the ring elements have a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the restraining loops in a collapsed configuration. 
     A twenty seventh embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising: an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end that extends exterior of the artery; a bumper tube; a proximal collar; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element comprising a plurality of struts is interconnected by ring connecting tethers, wherein the ring elements comprise a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the elongate member in a collapsed configuration. 
     A twenty eighth embodiment of the invention comprises a mechanical clot retrieval device for use in a tortuous vessel, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the vessel and the proximal end extends exterior of the vessel; and a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements have a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A twenty ninth embodiment of the invention comprises a mechanical clot retrieval device for use in a tortuous vessel, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the vessel and the proximal end extends exterior of the vessel; a plurality of self-expandable ring elements having a first diameter and a second diameter, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration; and a control strut having distal end attached to most proximal ring element and proximal end attached to the elongate member by adhesive bonds, wherein the ring elements have a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the micro catheter in the collapsed configuration, wherein the microcatheter is retracted proximally allowing the ring elements to reach the expanded configuration. 
     A thirtieth embodiment of the invention comprises a mechanical clot retrieval device for use in a tortuous vessel, comprising an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the vessel and the proximal end extends exterior of the vessel; and a plurality of self-expandable ring elements, wherein each ring element, comprising a plurality of struts, is interconnected by ring connecting tethers and is delivered through an intravascular microcatheter in a collapsed configuration; and wherein the ring elements have a radial force biasing the ring elements towards an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted proximally allowing the ring elements reach the expanded configuration. 
     A thirty first embodiment of the invention comprises a mechanical clot retrieval device for use in a tortuous vessel, comprising: an elongate member having a distal end and a proximal end, wherein the distal end extends interior of the vessel and the proximal end extends exterior of the vessel; a plurality of self-expandable ring elements configured at the distal end of the elongate member, wherein each ring element comprises a plurality of struts and is delivered through an intravascular microcatheter in a collapsed configuration; and a plurality of ring connecting tethers having distal and proximal ends, wherein the distal ends are connected to the ring element and the proximal ends are attached to the elongate member, wherein the ring elements have a radial force biasing the ring elements towards an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A thirty second embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an activation tether having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a distal collar, wherein the distal end of the activation tether is attached to the distal collar; and a plurality of self-expandable ring elements configured at the distal end of the activation tether, wherein each ring element, comprising a plurality of struts, is interconnected by bridges and is delivered through a microcatheter in a collapsed configuration, wherein the ring elements have a radial force biasing the ring elements towards an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements reach the expanded configuration. 
     A thirty third embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an activation tether having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a distal collar, wherein the distal end of the activation tether is attached to the distal collar; and a self-expandable ring element configured at the distal end of the activation tether, wherein the ring element comprises a plurality of struts and is delivered through a microcatheter in a collapsed configuration, wherein the ring element has a radial force biasing the ring element toward the expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring element to reach the expanded configuration. 
     A thirty fourth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an activation tether having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a distal collar, wherein the distal end of the activation tether is attached the distal collar; a self-expandable ring element configured at the distal end of the activation tether, wherein the ring element comprises a plurality of struts and is delivered through a microcatheter in a collapsed configuration; and a pair of removal tethers having distal ends attached to the ring element and proximal ends that extend exterior of the artery, wherein the ring element has a radial force biasing the ring element towards an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring element to reach the expanded configuration. 
     A thirty fifth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising an activation tether having a distal end and a proximal end, wherein the distal end extends interior of the artery and the proximal end extends exterior of the artery; a couple of collars; a couple of ring elements configured at the distal end of the activation tether, wherein each ring element comprising a plurality of struts and is delivered through a microcatheter in a collapsed configuration; and a pair of removal tethers having distal ends attached to the ring element and proximal ends attached to the collar, wherein the ring elements have a radial force biasing the ring elements toward an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. 
     A thirty sixth embodiment of the invention comprises a mechanical clot retrieval device for use in vasculature, comprising a pair of activation tethers having distal ends extending interior of the artery and proximal ends extending exterior of the artery; a distal collar, wherein distal ends of the activation tether are attached to the distal collar; and a plurality of self-expandable ring elements configured at distal ends of the activation tethers, wherein the each ring element, comprising a plurality of struts, is interconnected by bridges and is delivered through an intravascular microcatheter in a collapsed configuration, wherein the ring elements have a radial force biasing the ring elements towards an expanded configuration, wherein the radial force is absorbed by the microcatheter in the collapsed configuration, wherein the microcatheter is retracted allowing the ring elements to reach the expanded configuration. The mechanical clot retrieval device in any of the preceding embodiments may comprise a ring element made of shape memory material. The mechanical clot retrieval device in any of the preceding embodiments may comprise a ring element is made of Nitinol. The mechanical clot retrieval device in any of the preceding embodiments may comprise a retrieval device that is made of shape memory material. The mechanical clot retrieval device in any of the preceding embodiments may comprise a retrieval device that is partly made of shape memory material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example with reference to the accompanying drawings, in which: 
         FIG. 1 a    is a diagram illustrating a typical occlusive clot in a cerebral artery; 
         FIG. 1 b    is an enlarged view of a detail of  FIG. 1   a;    
         FIGS. 2 a -2 h    are diagrams illustrating the insertion of a guidewire and the deployment of a clot retrieval device of the invention; 
         FIGS. 3 a -3 e    illustrate a clot retrieval device according to one embodiment of the invention; 
         FIGS. 4 a -4 d    illustrate steps involved in using the device of  FIGS. 3 a   - 3   d;    
         FIGS. 5 a -5 e    illustrate another clot retrieval device of the invention; 
         FIGS. 6 a -6 f    illustrate a further clot retrieval device of the invention; 
         FIGS. 7 a -7 e    illustrate another clot retrieval device of the invention; 
         FIGS. 8 a -8 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 9 a -9 d    illustrate another clot retrieval device of the invention; 
         FIGS. 10 a -10 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 11 a -11 h    illustrate another clot retrieval device of the invention; 
         FIGS. 12 a -12 c    illustrate a further clot retrieval device of the invention; 
         FIGS. 13 a -13 f    illustrate another clot retrieval device of the invention; 
         FIGS. 14 a -14 c    illustrate a further clot retrieval device of the invention; 
         FIGS. 15 a -15 c    illustrate a method for using a clot retrieval device of the invention; 
         FIGS. 16 a -16 d    illustrate another method for using a clot retrieval device of the invention; 
         FIGS. 17 a -17 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 18 a -18 d    illustrate another clot retrieval device of the invention; 
         FIGS. 19 a -18 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 20 a -20 b    illustrate another clot retrieval device of the invention; 
         FIGS. 21 a -21 f    illustrate a further clot retrieval device of the invention; 
         FIGS. 22 a -22 d    illustrate a method for using a clot retrieval device of the invention; 
         FIGS. 23 a -23 c    illustrate a mechanism used in a clot retrieval device of the invention; 
         FIGS. 24 a -24 c    illustrate a further clot retrieval device of the invention; 
         FIGS. 25 a -25 k    illustrate another clot retrieval device of the invention; 
         FIGS. 26 a -26 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 27 a -27 b    illustrate another clot retrieval device of the invention; 
         FIGS. 28 a -28 b    illustrate a further clot retrieval device of the invention; 
         FIGS. 29 a -29 h    illustrate a method for using a clot retrieval device of the invention; 
         FIGS. 30 a -30 e    illustrate another clot retrieval device of the invention; 
         FIGS. 31 a -31 e    illustrate a further clot retrieval device of the invention; 
         FIGS. 32 a -32 d    illustrate another clot retrieval device of the invention; 
         FIGS. 33 a -33 g    illustrate a further clot retrieval device of the invention; 
         FIGS. 34 a -34 j    illustrate another clot retrieval device of the invention; 
         FIGS. 35 a -35 b    illustrate a further clot retrieval device of the invention; 
         FIGS. 36 a -36 d    illustrate another clot retrieval device of the invention; 
         FIGS. 37 a -37 i    illustrate a further clot retrieval device of the invention; 
         FIGS. 38 a -38 d    illustrate another clot retrieval device of the invention; 
         FIGS. 39 a -39 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 40 a -40 e    illustrate a method for using a clot retrieval device of the invention; 
         FIGS. 41 a -41 d    illustrate another method for using a clot retrieval device of the invention; 
         FIGS. 42 a -42 g    illustrate a further clot retrieval device of the invention; 
         FIGS. 43 a -43 h    illustrate another clot retrieval device of the invention; 
         FIGS. 44 a -44 b    illustrate a further clot retrieval device of the invention; 
         FIG. 45  illustrates another clot retrieval device of the invention; 
         FIG. 46  illustrates a further clot retrieval device of the invention; 
         FIG. 47  illustrates another clot retrieval device of the invention; 
         FIGS. 48 a -48 f    illustrate a further clot retrieval device of the invention; 
         FIGS. 49 a -49 f    illustrate another clot retrieval device of the invention; 
         FIGS. 50 a -50 g    illustrate a further clot retrieval device of the invention; 
         FIGS. 51 a -51 f    illustrate another clot retrieval device of the invention; 
         FIGS. 52 a -52 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 53 a -53 f    illustrate a method for using a clot retrieval device of the invention; 
         FIGS. 54 a -54 h    illustrate another method for using a clot retrieval device of the invention; 
         FIG. 55  illustrates another clot retrieval device of the invention; 
         FIG. 56  illustrates a further clot retrieval device of the invention; 
         FIG. 57  illustrates another clot retrieval device of the invention; 
         FIG. 58  illustrates a further clot retrieval device of the invention; 
         FIG. 59  illustrates a further clot retrieval device of the invention; 
         FIG. 60  illustrates another clot retrieval device of the invention; 
         FIGS. 61 a -61 c    illustrate a further clot retrieval device of the invention; 
         FIGS. 62 a -62 b    illustrate another clot retrieval device of the invention; 
         FIGS. 63 a -63 b    illustrate a further clot retrieval device of the invention; 
         FIGS. 64 a -64 b    illustrate another clot retrieval device of the invention; 
         FIGS. 65 a -65 d    illustrate a further clot retrieval device of the invention; 
         FIGS. 66 a -66 c    illustrate another clot retrieval device of the invention; 
         FIGS. 67 a -67 b    illustrate a further clot retrieval device of the invention; 
         FIGS. 68 a -68 b    illustrate another clot retrieval device of the invention; 
         FIGS. 69 a -69 b    illustrate a further clot retrieval device of the invention; and 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Specific embodiments of the present invention are now described in detail with reference to the figures, wherein similar reference numbers indicate identical or functionality similar elements. The terms “distal” or “proximal” are used in the following description with respect to a position or direction relative to the treating physician. “Distal” or “distally” are a position distant from or in a direction away from the physician. “Proximal” or “proximally” or “proximate” are a position near or in a direction toward the physician. 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Although the description of the invention is in the context of treatment of blood arteries such as the carotid and lung arteries, the invention may also be used in any other vessels where it is deemed useful. 
     As shown generally in  FIGS. 1 a -1 b  and 2 a -2 h   , an obstructive clot  2  is occluding a cerebral artery  1  and a guidewire  3  is inserted into the artery  1  using conventionally known techniques. The guidewire  3  is advanced across the clot  2  and then a microcatheter  4  is advanced over the guidewire  3  to a location distal to the clot  2 . The mechanical clot retrieval device  10  of the present invention is advanced through the microcatheter  4  and across the clot  2 . The retrieval device  10  has an elongate member  11  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  12  configured at the distal end of the elongate member. The ring elements  12  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  12  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  11 . When the retrieval device  10  is advanced through the microcatheter  4 , the ring elements are in the collapsed configuration substantially parallel to the longitudinal axis of the elongate member  11 . Each ring element  12  comprising a plurality of struts  13  is made of self expanding material such as nitinol and is interconnected by ring connecting tethers  14 . The microcatheter  4  is retracted after the retrieval device  10  is positioned within or distal of the clot  2  to allow ring elements  12  to deploy and expand. When the ring elements  12  deploy and expand, the struts  13  engage or capture the clot  2  in the ring elements  12  and the retrieval device  10  is retracted proximally out of the artery  1 . 
       FIGS. 3 a -3 d    illustrates one of the preferred embodiments of a mechanical clot retrieval device  20  of the present invention. The retrieval device  20  has an elongate member  21  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  22  configured at the distal end of the elongate member  21 . The ring elements  22  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  22  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  21 . Each ring element  22  comprising a plurality of struts  23  is made of shape memory material Nitinol and is interconnected by ring connecting tethers  24 . Each strut  23  has a crown element  27  with an engagement member  29  and a fixed tab  30 . The crown element  27  holds the neighboring crown element  27  in a snap-fit mechanism  28  to restrain the ring elements  22  in the collapsed configuration substantially parallel to the longitudinal axis of the elongate member  21 . Two pull tethers  25  which have distal ends that are attached to most proximal ring element  22 , and proximal ends that extend exterior of the artery for maneuvering and control by the physician, are present. The snap-fit mechanism  28  restrains the retrieval device  20  in the collapsed configuration when advancing through a microcatheter  4 . 
       FIGS. 4 a -4 d    shows a method of using the retrieval device  20  in accordance with the preferred embodiment as shown in  FIGS. 3 a -3 d   . A guidewire (not shown) is inserted in the cerebral artery  1  and is advanced distally across an obstructive clot  2 . A microcatheter  4  is advanced over the guidewire to a location proximal to the clot and then the retrieval device  20  of the present invention is advanced through the microcatheter  4  to the clot in the collapsed configuration. The retrieval device  20  has an elongate member  21  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  22  configured at the distal end of an elongate member  21 . The ring elements  22  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  22  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  21 . When the retrieval device  20  is advanced through the microcatheter  4 , the ring elements are in collapsed configuration substantially parallel to the longitudinal axis of the elongate member  21 . Each ring element  22  comprises a plurality of struts  23  and is made of shape memory material nitinol and is interconnected by ring connecting tethers  24 . The microcatheter  4  is retracted when the retrieval device  20  is positioned within or distal of the clot  2  to allow ring elements  22  to reach the expanded configuration. The pull tethers  25  attached to the retrieval device  20  allow the physician to manually disengage the snap-fit mechanism  28 . The ring elements  22  engage or capture the clot  2  from the distal surface of the clot  2  and retract the clot  2  proximally towards the microcatheter  4 . The microcatheter  4  is re-advanced to reduce the expanded configuration of the proximal end of the device  20  in order to retrieve it more easily from the artery  1 . 
       FIGS. 5 a -5 d    shows a mechanical clot retrieval device  40  of the present invention which is a slight variant of the embodiment of the retrieval device as shown in  FIGS. 3 a -3 d   . The retrieval device  40  has an elongate member  41  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  42  configured at the distal end of the elongate member  41 , and a proximal collar  51 . The ring elements  42  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  42  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  41 . Each ring element  42  comprising a plurality of struts  43  and is made of shape memory material Nitinol and is interconnected by ring connecting tethers  44 . Also present are a pair of pull tethers  45 , having distal ends which are attached to the most proximal ring element  42  and the proximal ends are attached to the elongate member  41 . Each strut  43  has a crown element  47  with an engagement member  49  and an engagement hook  50 . Each crown element  47  holds the neighboring crown element  47  in a snap-fit mechanism  48  which restrains the ring elements  42  in the collapsed configuration substantially parallel to the longitudinal axis of the elongate member  41 . The snap-fit configuration  48  restrains the retrieval device  40  in the collapsed configuration when advancing through the microcatheter  4 . 
     Another embodiment of the mechanical clot retrieval device of the present invention, which is similar to the one shown in  FIGS. 3 a -3 d   , is illustrated in  FIGS. 6 a -6 f   . The retrieval device  60  has an elongate member  61  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  62  configured at the distal end of the elongate member  61 . The ring elements  62  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  62  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  61 . Each ring element  62 , comprising a plurality of struts  63 , is made of shape memory material Nitinol and is interconnected by ring connecting tethers  64 . Each strut  63  has a crown element  67  with an engagement member  68 . Each engagement member  68  holds the neighboring crown element  67  in a snap-fit mechanism which restrains the ring elements  62  in the collapsed configuration substantially parallel to the longitudinal axis of the elongate member  61 . A pair of pull tethers  65  are attached, having distal ends which are attached to each connector (not labeled) of the crown element  67 , and proximal ends which extend exterior of the artery for manipulation and control by the physician. The snap-fit mechanism restrains the retrieval device  60  in the collapsed configuration as it is advancing through the microcatheter  4 . The pull tethers  65  may be used to manually disengage the snap-fit mechanism and allow the retrieval device  60  reach the expanded configuration. 
     Another embodiment of the mechanical clot retrieval device of the present invention, which is largely similar to the one shown in  FIGS. 3 a -3 d   , is illustrated in  FIGS. 7 a -7 f   . The retrieval device  80  has an elongate member  81  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  82  configured at the distal end of the elongate member  81 . The ring elements  82  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  82  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  81 . Each ring element  82 , comprising a plurality of struts  83 , is made of shape memory material Nitinol and is interconnected by ring connecting tethers  84 . Each strut  83  has a crown element  87  with an engagement member  89  and an engagement tab  88 . Each crown element  87  holds the neighboring crown element  87  in a snap-fit mechanism which restrains the ring elements  82  in the collapsed configuration substantially parallel to the longitudinal axis of the elongate member  81 . A pair of pull tethers  85  is attached, having distal ends which are attached to each engagement tab  88  of the crown element  87 , and proximal ends which extend exterior of the artery for manipulation and control by the physician. The snap-fit mechanism restrains the retrieval device  80  in the collapsed configuration as it is advancing through the microcatheter  4 . The pull tethers  85  may be used to manually disengage the snap-fit mechanism and allow the retrieval device  80  to reach the expanded configuration. 
     Another preferred embodiment of the mechanical clot retrieval device  100  of the present invention is illustrated in  FIGS. 8 a -8 d   . The retrieval device  100  includes an elongate member  101  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  102  arranged at the distal end of the elongate member  101 , a fixed proximal collar  111  and a distal moveable collar  110 . The ring elements  102  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  102  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  101 . Each ring element  102  comprising a plurality of struts  103  is made of shape memory material nitinol and is interconnected by ring connecting tethers  104 . Each strut  103  has a hook lock  107  with first lock  108  and second lock  109 . Each hook lock  107  holds the neighboring hook lock  107  in a snap-fit mechanism to restrain the ring elements  102  in the collapsed configuration substantially parallel to the axis of the elongate member  101 . There are pair of pull tethers  105  attached, having distal ends which are attached to the distal collar  110 , and proximal ends that extend exterior of the artery for maneuvering and control by the physician. The snap-fit mechanism restrains the retrieval device in the collapsed configuration when advancing through the microcatheter  4 . The distal collar  110  is pulled proximally by the pull tether  105  to manually disengage and allow ring elements  102  reach an expanded configuration. 
       FIGS. 9 a -9 d    shows a mechanical clot retrieval device  120  of the present invention, which is a slight variant of the embodiment of the retrieval device  100  as shown in  FIGS. 8 a -8 d   ; the only difference is use of a balloon  125  in place of the distal moveable collar  110 . The retrieval device  120  has an elongate member having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  122  configured at the distal end of the elongate member  121 , a proximal collar  126  and a balloon  125 . The ring elements  122  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  122  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  121 . Each ring element  122  comprises a plurality of struts  123  and is made of shape memory material nitinol and is interconnected by ring connecting tethers  124 . Each strut  123  has a hook lock  127  with a first lock  128  and a second lock  109 . Each hook lock  127  holds the neighboring hook lock  127  in a snap-fit mechanism to restrain the ring elements  122  in the collapsed configuration. The balloon  125  is in a collapsed state when the device  20  is advanced through the microcatheter  4 . The balloon  125  is inflated to disengage and allow ring elements  122  to reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device  140  of the present invention is largely similar to the one shown in  FIGS. 8 a -8 d   , and is shown in  FIGS. 10 a -10 d   ; with the main difference being the use of a ball element  145  in place of the distal movable collar  110 . The retrieval device  140  has an elongate member  141  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  142  configured at the distal end of the elongate member  141 , a proximal collar  150  attached to a push tube  146 , and a ball element  145 . The ring elements  142  are self-expandable from a collapsed configuration to an expanded configuration and feature a radial force that biases the ring elements  142  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  141 . Each ring element  142 , comprising a plurality of struts  143 , is made of shape memory material nitinol and is interconnected by ring connecting tethers  144 . Each strut  143  has a hook lock  147  with a first lock  148  and a second lock  149 . Each hook lock  147  holds the neighboring hook lock  147  in a snap-fit mechanism to restrain the device  140  in the collapsed configuration. The ball element  145  is manually pulled proximally through the device  140  to force apart the ring elements  142  which subsequently form the expanded configuration. 
       FIGS. 11 a -11 h    shows another preferred embodiment of a mechanical clot retrieval device of the present invention. The retrieval device  160  includes an elongate member  161  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  162  attached to the elongate member  161  by connecting tethers  164  at connection points  165 . The ring elements  162  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  162  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  161 . Each ring element  162 , comprising a plurality of struts  163 , is made of shape memory material nitinol. Each strut  163  features a crown element  169 , which includes a proximal ring crown  167  and a distal ring crown  166 . The connecting tether  164  connects the proximal ring crown  167  with the elongate member  161 . The crown element  169  engages with the elongate member  161  in a snap-fit mechanism to restrain the ring elements  162  in the collapsed configuration. The snap-fit mechanism  168  restrains the retrieval device  160  in the collapsed configuration as it advances through a microcatheter  4 . The microcatheter  4  is retracted proximally to disengage and allow the ring elements  162  to reach the expanded configuration. The elongate member  161  is manually retracted to disengage the snap-fit mechanism. 
     Another embodiment of a mechanical clot retrieval device of the present invention is illustrated in  FIGS. 12 a -12 c   . A guidewire is inserted inside a cerebral artery  501  using conventional techniques. The guidewire is advanced across an obstructive clot  502  and then an intravascular microcatheter  504  is advanced over the guidewire to a location proximal to the clot  502 . The retrieval device  520  has an elongate member having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  522  configured at the distal end  526  of the elongate member  521 . The ring elements  522  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  522  towards the expanded configuration. Each ring element  522 , comprising a plurality of struts  523 , is made of shape memory material nitinol and is interconnected by ring connecting tethers  524 . Each strut  523  has a crown element  527  attached to the elongate member  521  by a bonding agent  528  to restrain the ring elements  522  in the collapsed configuration. The radial force is absorbed by the bonding agent  528  to restrain the retrieval device  520  in the collapsed configuration when the device is advancing through the microcatheter  504 . The bonding agent  528  is dissolved in vivo to disengage and allow the ring elements  522  to reach the expanded configuration. The device has a pair of pull tethers  525 , having distal ends which are attached to most proximal ring element  522  and proximal ends that extend out of the artery for maneuvering and control by the physician. 
       FIGS. 13 a -13 f    shows a slight variant embodiment of a mechanical clot retrieval device of the present invention, largely similar to the one shown in  FIGS. 12 a -12 c   . The difference between the two is the use of a soluble covering  548  in place of the bonding agent  528 . The retrieval device  540  has an elongate member, having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  542  configured at the distal end  546  of the elongate member  541 . The ring elements  542  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force biasing the ring elements  542  towards the expanded configuration. Each ring element  542 , comprising a plurality of struts  543 , is made of shape memory material nitinol and is interconnected by ring connecting tethers  544 . Each strut  543  has a crown element  547  attached to the elongate member  541  by a re-absorbable covering  548  to restrain the ring elements  542  in the collapsed configuration. The radial force is absorbed by the re-absorbable covering  548  to restrain the retrieval device  540  in the collapsed configuration when it is advancing through the microcatheter (not shown). The re-absorbable covering  548  is dissolved in vivo to disengage and allow the ring elements  542  to reach the expanded configuration. The device features a pair of pull tethers  545 , having distal ends which are attached to the most proximal ring element  542  and proximal ends that extend out of the artery for maneuvering and control by the physician. 
     Another preferred embodiment of a mechanical clot retrieval device of the present invention is shown in  FIGS. 14 a -14 c   . The retrieval device  1020  includes an elongate member  1021  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of radial pins  1028 , and a plurality of ring elements  1022  configured at the distal end  1026  of the elongate member  1021 . The ring elements  1022  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases ring elements  1022  towards the expanded configuration. Each ring element  1022 , comprising a plurality of struts, is made of shape memory material nitinol and each strut  1023  contains a crown element  1027  with an eyelet  1024 . The distal eyelet  1024  engages with the radial pin  1028  by an engagement element  1029  to fasten the ring elements to the elongate member, whereas proximal eyelets  1024  engage with engagement members  1029  of radial pins  1028  in a snap-fit mechanism to restrain the ring elements  1022  in the collapsed configuration. The ring elements  1022  may be connected to the elongate member  1021  by ring connecting tethers (not shown). The snap-fit mechanism restrains the retrieval device  1020  in the collapsed configuration when advancing through the microcatheter (not shown). 
       FIGS. 15 a -15 c  and 16 a -16 d    show a method for using a mechanical clot retrieval device  1040  in accordance with the preferred embodiment shown in  FIGS. 14 a -14 c   . A guidewire  1003  is inserted in a cerebral artery  1001  and is advanced across an obstructive clot  1002 . An intravascular microcatheter  1004  is advanced over the guidewire  1003  to a location proximal to the clot  1002  and then a mechanical clot retrieval device  1004  is advanced through the microcatheter  1004  to the clot  1002  in a collapsed configuration. The retrieval device  1004  includes an elongate member  1041  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of radical pins  1048 , and a plurality of ring elements  1042  arranged at the distal end  1046  of the elongate member  1041 . The ring elements  1042  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  1042  toward the expanded configuration. Each ring element  1042 , comprising a plurality of struts  1043 , is made of shape memory material nitinol and each strut features a crown element  1047  with an eyelet  1044 . The eyelets  1044  engage with the radial pins  1048  to attach the ring elements to the elongate member. The radial force is absorbed by radial pins to restrain the ring elements  1042  in the collapsed configuration. When microcatheter  1004  is retracted the ring elements  1042  are able to expand and disengage eyelets  1044  from pins  1048  and are thus freed to expand towards the fully expanded configuration. The ring elements  1042  engage or capture the clot and the device can then be retracted proximally and removed from the artery. In one embodiment the ring elements are connected to the elongate member  1041  by tether elements  1045  as shown in  FIGS. 16 c    and  16   d.    
     Another preferred embodiment of a mechanical clot retrieval device of the present invention is illustrated in  FIGS. 17 a -17 d   . The retrieval device  1520  includes an elongate member  1521  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery, an outer tube  1530  with a plurality of slots  1532  and a retractable inner core wire  1531  which has a proximal end that extends exterior of the artery, and a plurality of ring elements  1522  configured at the distal end of the elongate member  1521 . The ring elements  1522  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  1522  toward the expanded configuration. Each ring element  1522  comprising a plurality of struts  1523  is made of shape memory material nitinol and is interconnected by ring connecting tethers  1525 . Each strut  1523  has a crown element  1527  with an eyelet  1524 . A tether is advanced out of the slot  1532  to loop  1528  through the eyelet  1524  and advanced back in the slot  1532  to loop  1526  around the core wire  1531  to engage each ring element  1522  to the elongate member  1521  in a tether-loop mechanism. The radial force is absorbed by the tether-loop mechanism to restrain the retrieval device  1520  in the collapsed configuration when advancing through a microcatheter (not shown). The core wire  1531  is retracted proximally thus disengaging and allowing each ring element  1522  reach the expanded configuration. 
       FIGS. 18 a -18 c    depict a mechanical clot retrieval device  1540  of the present invention which is a variant of the retrieval device  1520  as illustrated in  FIGS. 17 a -17 d   .  FIG. 18 d    is a view of a cross-section through line  1553  of  FIG. 18 b   . The retrieval device  1540  includes an elongate member  1541  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery, an outer tube  1550  with a plurality of slots  1552  and a retractable inner core wire  1551  having proximal end  1549  that extends exterior of the artery, and a plurality of ring elements  1542  arranged at the distal end of the elongate member  1541 . The ring elements  1542  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  1542  toward the expanded configuration. Each ring element  1542 , comprising a plurality of struts  1543 , is made of shape memory material nitinol and is interconnected by ring connecting tethers (not shown). Each strut  1543  has a crown element  1547  with an eyelet  1544 . The core wire  1551  is advanced out of each slot  1552  to loop  1546  through each eyelet  1544  and advanced back in the slot  1552  to loop inside the outer tube  1550  to engage each ring element  1542  to the elongate member  1541  in a tether-loop mechanism. The radial force is absorbed by the tether-loop mechanism to restrain the retrieval device  1540  in the collapsed configuration when advancing through a microcatheter (not shown). The core wire  1551  is retracted in order to disengage and allow each ring element  1542  reach the expanded configuration. 
       FIGS. 19 a -19 d    represents a mechanical clot retrieval device of the present invention which is yet another slightly variant embodiment of the retrieval device  1520  as illustrated in  FIGS. 17 a -17 d   . The retrieval device  1560  includes an elongate member  1561  having a distal end  1569  that extends interior of the artery, a proximal end that extends exterior of the artery and a tube lumen  1570  with a plurality of slots  1571 , and a plurality of ring elements  1562  configured at the distal end of the elongate member  1561 . The ring elements  1562  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  1562  toward the expanded configuration. Each ring element  1562 , comprising a plurality of struts  1563 , is made of shape memory material nitinol and is interconnected by ring connecting tethers  1565 . Each strut  1563  has a crown element  1568  with an eyelet  1564 . A plurality of tethers  1566  is positioned within the lumen  1570  and is advanced out of each slot  1571  to loop  1567  each eyelet  1564  and is attached at a distal end  1569  of the elongate member  1561  to engage each ring element  1562  to the elongate member  1561  in a tether-loop mechanism. The radial force is absorbed by the tether-loop mechanism to restrain the retrieval device  1560  in the collapsed configuration when advancing through a microcatheter (not shown). The tethers  1566  are broken or cut to disengage and allow each ring element  1562  reach an expanded configuration. 
       FIGS. 20 a -20 b    shows a mechanical clot retrieval device  1580  of the present invention. The retrieval device  1580  includes an elongate member  1581 , a distal end  1569  that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  1582  configured at the distal end of the elongate member  1581  and a proximal collar  1589  attached to an abutment tube  1588 . The ring elements  1582  are self-expandable from a collapsed configuration to an expanded configuration and feature a radial force that biases the ring elements  1582  toward the expanded configuration. Each ring element  1582 , comprising a plurality of struts  1583 , is made of shape memory material nitinol and is attached to the elongate member  1581  by connecting tethers  1585 . Each strut  1583  has a crown element  1587 . A tether is wrapped around each ring element  1582  and is attached to the elongate member  1581  by a tether loop  1584  to engage each ring element  1582  to the elongate member  1581  in a tether-loop mechanism. The radial force is absorbed by the tether-loop mechanism to restrain the retrieval device  1580  in the collapsed configuration when it is advancing through a microcatheter (not shown). The elongate member  1581  is retracted proximally thus sliding the tether loop  1584  off the ring element  1582  and disengaging and therefore allowing each ring element  1582  reach the expanded configuration. 
     Another preferred embodiment of a mechanical clot retrieval device of the present invention is illustrated in  FIGS. 21 a -21 f   . The retrieval device  2020  includes an elongate member  2021  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  2022  configured at the distal end of the elongate member  2021 , and a plurality of beveled  2029  or recessed  2028  collars  2026  configured at distal and proximal ends of each ring element  2022 . The ring elements  2022  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  2022  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by collars  2026 . Each ring element  2022 , comprising a plurality of struts  2023 , is made of shape memory material nitinol and is interconnected by tethers (not labeled) and attached to the elongate member  2021  by ring connecting tethers  2025 . Each strut  2023  includes a crown element  2027  with a tab  2024  extending axially and radially inward and may be bevel or square shape in shape. The tab  2024  engages with the collar  2026  to restrain the ring element  2022  to the elongate member  2021  in a collar mechanism. The collar mechanism restrains the retrieval device  2020  in the collapsed configuration when advancing through a microcatheter (not shown). The elongate member  2021  is retracted proximally to disengage it and allow each ring element  2022  reach the expanded configuration. 
       FIGS. 22 a -22 d    shows a method for using the device  2020  in accordance with the preferred embodiment shown in  FIGS. 21 a -21 f   . A guidewire (not shown) is inserted in a cerebral artery  2000  and is advanced across an obstructive clot  2001 . An intravascular microcatheter  2003  is advanced over the guidewire to a location proximal to the clot  2001  and then a mechanical clot retrieval device  2020 , sheathed within a bumper tube  2030 , is advanced through the microcatheter  2003  to the clot  2001  in a collapsed configuration. The microcatheter  2003  is retracted when the retrieval device  2020  is located distal of the clot. It is unsheathed to allow ring elements  2022  to reach the expanded configuration. The elongate member  2021  of the retrieval device  2020  allows the physician to manually disengage the collar mechanism and the ring connecting tether  2025  attached to the elongate member  2021 . The ring elements  2022  engage or capture the clot  2001  from the distal surface of the clot  2001  and retract the clot  2001  proximally to the microcatheter  2003 . 
       FIGS. 23 a -23 c    depicts a mechanism used in a mechanical clot retrieval device  2040  for engaging a tab  2044  with a collar  2046  in a slight variant with the preferred embodiment shown in  FIGS. 21 a -21 f   . The retrieval device  2040  includes an elongate member  2041  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a ring element  2042  configured at the distal end of the elongate member  2041  and a proximal collar  2046 . The ring element  2042  is self-expandable from a collapsed configuration to an extended configuration and has a radial force that biases the ring element  2042  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the proximal collar  2046 . The ring element  2042 , comprising plurality of struts  2043 , is made of shape memory material nitinol and each strut  2043  has a crown element  2047  with a tab  2044  extending axially and radially inward. The tab  2044  is shaped to engage with the collar  2046 , having a recess to restrain the ring element  2042  to the elongate member  2041  in a collar mechanism. The collar mechanism restrains the retrieval device  2040  in the collapsed configuration when advancing through a microcatheter (not shown). The elongate member  2041  is retracted proximally to disengage it and allow each ring element  2042  reach the expanded configuration. 
       FIGS. 24 a -24 c    shows a mechanical clot retrieval device  2060  of the present invention which is a slightly variant embodiment of the retrieval device  2020  as illustrated in  FIGS. 21 a -21 f   . The retrieval device  2060  includes an elongate member  2061  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  2062  configured at the distal end of the elongate member  2061 , and a plurality of collars  2066  having a plurality of holes  2068  configured proximally of each ring element  2062 . The ring members  2062  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  2062  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by collars  2066 . Each ring element  2062 , comprising a plurality of struts  2063 , is made of shape memory material nitinol and is interconnected by ring connecting tethers  2069 . Each strut  2063  has a crown element  2067  with a pin  2064  extending axially and radially inward. The pin  2064  engages with the hole  2068  to restrain the ring elements  2062  to the elongate member  2061  in a collar mechanism. The device includes a pair of cables  2065  having distal ends which are attached to the most proximal ring element  2062  and proximal ends that extend exterior of the artery for manipulation and control by the physician. The collar mechanism restrains the retrieval device  2060  in the collapsed configuration when it is advancing through a microcatheter  2003 . The cables  2065  are manually pulled proximally to disengage the collar mechanism and allow each ring element  2062  reach the expanded configuration. 
       FIGS. 25 a -25 k    represents a mechanical clot retrieval device  2080  of the present invention which is another slightly variant embodiment of the retrieval device  2020  as illustrated in  FIGS. 21 a -21 f   . The retrieval device  2080  includes elongate member  2081   a  distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  2082  configured at the distal end of the elongate member  2081 , and a plurality of shaped engagement collars  2085  having a plurality of dove tail grooves  2088  configured proximally of each ring element  2082 . The ring elements  2082  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  2082  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by collars  2085 . Each ring element  2082 , comprising a plurality of struts  2083 , is made of shape memory material nitinol and is attached to the elongate member  2081  by tethers  2089 . Each strut  2083  has a crown element  2087  with a shaped tab  2084  extending axially and radially inward. The shaped tab  2084  engages with the shaped engagement collar  2085  to restrain the ring element  2082  to the elongate member  2081  in a collar mechanism. The collar mechanism restrains the retrieval device  2080  in the collapsed configuration when it is advancing through a microcatheter (not shown). The elongate member  2081  is manually retracted proximally to disengage it and allow each ring element  2082  reach the expanded configuration. 
       FIGS. 26 a -26 c    represents a slightly variant embodiment of the retrieval device  2080  as represented in  FIGS. 25 a -25 k   , having additional ring connecting tethers  2101  interconnecting each ring element  2082 . 
     Another preferred embodiment of a mechanical clot retrieval device  2520  of the present invention is illustrated in  FIGS. 27 a -27 b   . The retrieval device  2520  includes an elongate member  2521  having a distal end that extends interior of the artery, a proximal end that ends interior of the artery and an inner lumen  2529  with a plurality of openings (not labeled), and a plurality of ring elements  2522  configured at the distal end of the elongate member  2521 . The ring elements  2522  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases ring elements  2522  toward the expanded configuration. Each ring element  2522 , comprising a plurality of struts  2523 , is interconnected by ring connecting tethers  2525 . Each strut  2523  has a crown element  2527  and an engagement tab  2524 . A plurality of tether loops  2526  having proximal ends that extend exterior of the lumen  2529  is advanced out of each opening (not opening) to circle the engagement tab  2524  and engage each ring element  2522  to the elongate member  2521  in an eyelet mechanism. The radial force is absorbed by the eyelet mechanism to restrain the retrieval device  2520  in the collapsed configuration when it is advancing through a microcatheter (not shown). The tethers are manually pulled proximally to disengage them and allow each ring element  2522  to reach the expanded configuration. 
       FIGS. 28 a -28 b    represents a mechanical clot retrieval device  2540  of the present invention which is a slightly variant embodiment of the retrieval device  2520  as illustrated in  FIGS. 27 a -27 b   . The retrieval device  2540  includes an elongate member  2541  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of raised tabs  2548 , and a plurality of ring elements  2542  configured at the distal end of the elongate member  2541 . The ring elements  2542  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  2542  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by raised tabs  2548 . Each ring element  2542 , comprising a plurality of struts  2543 , is interconnected by ring connecting tethers  2545 . Each strut  2543  has a crown element and an engagement tab  2544 . The engagement tab  2544  engages with the raised tab  2548  to restrain the ring element  2542  to the elongate member  2541  in an eyelet mechanism. The eyelet mechanism restrains the retrieval device  2540  in the collapsed configuration when it is advancing through a microcatheter (not shown). The elongate member  2541  is manually retracted proximally to disengage it and allow each ring element  2542  to reach the expanded configuration. 
       FIGS. 29 a -29 h    shows a method for using a mechanical clot retrieval device  2560  in accordance with the preferred embodiment shown in  FIGS. 27 a -27 b  and 28 a -28 b   . A guidewire  2502  is inserted in a cerebral artery  2500  and is advanced through an obstructive clot  2501 . An intravascular microcatheter  2503  is advanced over the guidewire  2502  to a location distal to the clot  2501  and then the retrieval device  2560  is advanced through the microcatheter  2503  to the clot  2501  in a collapsed configuration. The microcatheter  2503  is retracted when the retrieval device  2560  is positioned distal of the clot  2501  to allow a plurality of ring elements  2562  to reach an expanded configuration. The ring elements  2562 , comprising a plurality of struts  2563 , are interconnected by ring connecting tethers  2565 . A tether  2564  is attached at a distal end of the retrieval device  2560  and its proximal end extends out of the artery. The elongate member  2561  and tether  2564  allows the physician to manually disengage eyelet mechanism. The ring elements  2562  engage or capture the clot  2501  from the distal end of the clot  2501  and retract the clot  2501  proximally to the microcatheter  2504 . 
     Another embodiment of a mechanical clot retrieval device  3020  of the present invention is illustrated in  FIGS. 30 a -30 e   . The retrieval device  3020  includes an elongate member  3021  designed to have a distal end that extends interior of the artery, a proximal end that extends exterior of the artery, a plurality of engagement diameters  3028  and a plurality of disengagement diameters  3029 , a plurality of ring elements  3022  configured at the distal end of the elongate member  3021 , and a proximal abutment tube  3031 . The ring elements  3022  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3022  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the engagement diameters. Each ring element  3022  comprising a plurality of struts  3023  is interconnected by ring connecting tethers  3025 . Each strut  3023  has a crown element  3027  with a C-shaped engagement tab  3030  extending radially inward. The C-shaped engagement tab  3030  engages with the engagement diameter  3028  to restrain the ring element  3022  to the elongate member  3021  in the collapsed configuration when it is advanced through a microcatheter (not shown). The device features a pair of control tethers  3026  having distal ends which are attached to the crown element  3027  of the most proximal ring element  3022 . The axial movement of the elongate member  3021  disengages each ring element  3022  at the disengagement diameter  3029  and allows each ring element  3022  to reach the expanded configuration. 
       FIGS. 31 a -31 e    shows a mechanical clot retrieval device  3040  of the present invention which is a slightly variant embodiment of the retrieval device  3020  as illustrated in  FIGS. 30 a -30 e   . The retrieval device  3040  includes an elongate member  3041  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and an inner wire  3051 , a plurality of ring elements  3042  arranged at a distal end of the elongate member  3041 , and a proximal abutment tube  3052 . The ring elements  3042  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3042  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member having inner wire  3051 . Each ring element  3042  comprising a plurality of struts  3042  is interconnected by ring connecting tethers  3045 . Each strut  3043  has a crown element  3047  with a C-shaped engagement tab  3050  extending radially inward. The C-shaped engagement tab  3050  engages with the elongate member  3041 . The inner wire  3051  restrains the retrieval device  3040  in the collapsed configuration when it is advanced through a microcatheter (not shown). The device features a pair of control tethers  3046  having distal ends which are attached to the crown element  3047  of the most proximal ring element  3042  and proximal ends which are attached to the abutment tube  3052 . The withdrawal of the inner wire  3051  disengages it and allows each ring element  3042  reach the expanded configuration. 
       FIGS. 32 a -32 d    illustrates the mechanical clot retrieval device  3060  of the present invention. The retrieval device  3060  includes an elongate member  3061  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and an inner lumen  3071  with a plurality of slots  3070 , a plurality of ring elements  3062  configured at the distal end of the elongate member  3061 , and a proximal collar  3066 . The ring elements  3062  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3062  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by slots  3070 . Each ring element  3062 , comprising a plurality of struts  3063 , is interconnected by ring connecting tethers  3065 . Each strut  3063  has a crown element  3067  with a stepped engagement tab  3064  extending longitudinally and radially. The engagement tab  3064  engages with the slot  3070  to restrain the ring elements  3062  to the elongate member  3061  and to restrain the retrieval device  3060  in the collapsed configuration when it is advanced through a microcatheter (not shown). A pair of control tethers  3068  is attached from the crown element  3067  of the most proximal ring element  3062 , to the proximal collar  3066 . The proximal collar  3066  is manually pulled proximally to disengage it and allow the ring elements  3062  to reach an expanded configuration. 
       FIGS. 33 a -33 g    represents another embodiment of a mechanical clot retrieval device of the present invention. The retrieval device  3080  includes an elongate member  3081  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of monofilaments  3088 , a plurality of ring elements  3082  configured at the distal end of the elongate member  3081 , and a proximal collar  3086 . The ring elements  3082  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force biasing the ring elements  3082  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the monofilaments. Each ring element  3082  comprising a plurality of struts  3083  is interconnected by ring connecting tethers  3085 . Each strut  3082  has a crown element  3087  with a C-shaped engagement tab  3084 . The C-shaped engagement tab  3084  engages with the monofilament  3088  to restrain the ring element  3082  to the elongate member  3081  in a snap-fit mechanism. The snap-fit mechanism restrains the retrieval device  3080  in the collapsed configuration when it is advancing through a microcatheter (not shown). The device contains a pair of control tethers  3089  having distal ends which are attached to the crown element  3087  of the most proximal ring element  3082  and proximal ends which are attached to the collar  3086 . The elongate member  3081  is manually retracted to disengage it and allow each ring element  3082  reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is shown in  FIGS. 34 a -34 j   . The retrieval device  3100  includes an elongate member  3101  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of inactivated cuffs  3108   a  configured at the distal end of the elongate member  3101 , a plurality of ring elements  3102  configured over the cuff  3108   a , and a tube  3106 . Each cuff  3108   a  is made of electro-active polymer which swells to an activated state  3108   b  when current is applied. The ring elements  3102  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force biasing the ring elements  3102  toward the expanded configuration. Each ring element  3102  comprising a plurality of struts  3103  is interconnected by ring connecting tethers  3105 . Each strut has a crown element  3107 . A pair of control tethers  3104  are attached to the crown element  3107  of the most proximal ring element  3102 , having proximal ends that extend out of the artery. A ring restraining tether  3109  is wrapped around each ring element  3102  restraining it to the elongate member  3101  in the collapsed configuration when it is advanced through a microcatheter (not shown). In the collapsed configuration, the radial force is absorbed by the ring restraining tethers  3109 . The current is applied through the restraining tether  3109  to allow each cuff  3108   a  to reach activated state  3108   b  and then they are manually pulled proximally to disengage them and allow each ring element  3102  reach the expanded configuration. 
       FIGS. 35 a -35 b    represents another embodiment of a mechanical clot retrieval device of the present invention. The retrieval device  3120  includes an elongate member  3121  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3122  configured at the distal end of the elongate member  3121 , a proximal collar  3129 , and a bumper tube  3126 . The ring elements  3122  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force biasing the ring elements  3122  toward the expanded configuration. Each ring element  3122 , comprising a plurality of struts  3123 , is interconnected by ring connecting tethers  3125 . Each strut  3123  has a crown element  3127  with an eyelet  3128 . A tether loop  3130  engages each ring element  3122  to the elongate member  3121  to restrain the retrieval device  3120  in the collapsed configuration when it is advancing through a microcatheter  3003 . In the collapsed configuration, the radial force is absorbed by tether loops  3130 . The device contains a pair of control tethers  3124 , having distal ends which are attached to the crown element  3127  of the most proximal ring element  3122  and proximal ends which extend out of the artery. The elongate member  3121  is manually retracted proximally to disengage it and allow each ring element  3122  reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is shown in  FIGS. 36 a -36 d   . The retrieval device  3140  includes an elongate member  3141  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3142  arranged at the distal end of the elongate member  3141 , a bumper tube  3146  and a proximal collar  3149 . The ring elements  3142  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force biasing ring elements toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  3141 . Each ring element  3142  comprising a plurality of struts  3143  is interconnected by ring connecting tethers  3145 . Each strut  3143  has a crown element  3147  with an engagement tab  3148  having an eyelet  3144 . The elongate member  3141  passes through each eyelet  3144  engaging each ring element  3142  to the elongate member  3141  to restrain the retrieval device  3140  in the collapsed configuration when it is advancing through a microcatheter  3003 . The elongate member  3141  is manually retracted proximally to disengage it and allow each ring element  3142  reach the expanded configuration. 
       FIGS. 37 a -37 i    represents a mechanical clot retrieval device of the present invention which is a slightly variant embodiment of the retrieval device  3140  as shown in  FIGS. 36 a -36 d   . The retrieval device  3160  includes an elongate member  3161  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3162  configured at the distal end of the elongate member  3161 , a bumper tube (not labeled) and a proximal collar (not labeled). The ring elements  3162  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force biasing the ring elements  3162  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  3161 . Each ring element  3162  comprising a plurality of struts  3163  is interconnected by ring connecting tethers (not labeled). Each strut has a crown element  3167  with an engagement tab  3166  and an eyelet  3168 . The elongate member  3161  passes through each eyelet  3168  engaging each ring element  3162  to the elongate member  3161  to restrain the retrieval device  3160  in the collapsed configuration when it is advancing through a microcatheter  3003 . The elongate member  3161  is manually retracted proximally to disengage and allow each ring element  3162  reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is illustrated in  FIGS. 38 a -38 d   . The retrieval device  3200  has an elongate member  3201  having a distal end that extends interior of the artery, a proximal end that extends exterior of the artery and a plurality of restraining loops  3206  configured at the distal end of the elongate member  3201 , and a plurality of ring elements  3202  arranged at the distal end of the elongate member  3201 . The ring elements  3202  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3202  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the restraining loops  3206 . Each ring element  3202  comprising a plurality of struts  3203  is connected to the loop  3206  by connecting tethers  3205 . Each strut  3203  is having a crown element  3207 . The device features a pair of control tethers  3204 , having distal ends which are attached to the crown element  3207  of the most proximal ring element  3202 , and proximal ends that extend out of the artery. The loops  3206  engage the ring elements  3202  to the elongate member  3201  in an offset loop mechanism. The offset loop mechanism restrains the retrieval device  3200  in the collapsed configuration when it is advanced through a microcatheter (not shown). The elongate member  3201  is manually retracted proximally to disengage it and allow each ring element  3202  to reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device  3300  of the present invention is illustrated in  FIGS. 39 a -39 d   . The retrieval device  3300  has an elongate member  3301  having a distal end that extends interior of the artery and a proximal end that extend exterior of the artery, a plurality of ring elements  3302  arranged at the distal end of the elongate member  3301 , a bumper tube  3306 , and a proximal collar  3308 . The ring elements  3302  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by the elongate member  3301 . Each ring element  3302 , comprising a plurality of struts  3303 , is interconnected by ring connecting tethers  3305 . Each strut  3303  has a crown element  3307 . The device has comprises a pair of control tethers  3304 , having distal ends which are attached to the crown element  3307  of the most proximal ring element  3302  and proximal ends that are attached to the bumper tube  3306 . The crown element  3307  of each strut  3303  is confined around the elongate member  3301  to restrain the ring element  3302  to the elongate member  3301  when it is advanced through a microcatheter  3309 . The elongate member  3301  is retracted proximally to disengage it and allow each ring element  3302  to reach the expanded configuration. 
       FIGS. 40 a -40 e  and 41 a -41 d    shows a method of advancing a mechanical clot retrieval device  3405  in accordance with a preferred embodiment of the present invention in a tortuous vessel  3400 . A guidewire  3402  is inserted in the vessel  3400  and is advanced through an obstructive clot  3401 . An intravascular microcatheter  3403  is advanced over the guidewire to a location distal to the clot  3401  and then the retrieval device  3405  is advanced through the microcatheter  3403  to the clot  3401  in a collapsed configuration. The microcatheter  3403  is retracted to allow the plurality of ring elements (not labeled) to reach an expanded configuration. The ring elements (not labeled) contain a plurality of struts (not labeled). Each ring element is connected to the next ring element (not labeled) by a ring connecting tether  3406 . A control tether  3407  is attached to the strut (not labeled) of the proximal ring element having a proximal end that extends out of the patient to allow physician to manually maneuver the retrieval device  3405 . The retrieval device  3405  engages or captures the clot  3401  from the distal end of the clot  3401  and retracts the clot  3401  proximally into the microcatheter  3403 . 
       FIGS. 42 a -42 d , 43 a -43 h  and 44 a -44 b    illustrates the mechanical clot retrieval device  3420  of the present invention. The retrieval device  3420  includes an elongate member  3421  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, and a plurality of ring elements  3422  configured at the distal end of the elongate member  3421 . The ring elements  3422  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3422  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3420  is delivered inside the artery. Each ring element  3422 , comprising a plurality of struts  3423 , is interconnected by ring connecting tethers  3425 . Each strut  3423  has a crown element  3427  with or without a shaped tab  3428  or an eyelet  3426 . The crown element  3427  is connected to neighboring crown element  3427  by the tether  3425  which may be looped around tab  3428  or passed through eyelet  3426 . 
     The crown element  3427  which is without the shaped tab  3428  or the eyelet  3426  may be connected to neighboring crown element  3427  by an adhesive bond  3429 . Crown-to-crown  3427  connection restrains the retrieval device  3420  in the collapsed configuration when advanced through the intravascular microcatheter (not shown). The device features a pair of removal tethers  3424 , having distal ends which are attached to the crown element  3427  of the most proximal ring element  3422 , and proximal ends that extend out of the artery. The microcatheter is retracted proximally and tether  3424  is manually pulled proximally to disengage it and allow each ring element  3422  reach the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 45 . The retrieval device  3440  has an elongate member  3441  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3442  having a first diameter and a second diameter, and a control strut  3444  having distal end attached to proximal ring element  3442  and proximal end attached to the elongate member  3441  by adhesive bonds  3446 . The ring elements  3442  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3442  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3440  is delivered inside the artery. Each ring element  3442 , comprising a plurality of struts  3443 , is connected by ring connecting tethers  3445 . Each strut  3443  has a crown element  3447 . The crown element  3447  is connected to neighboring crown element  3447  by the tether  3445 . Crown-to-crown connection restrains the retrieval device  3440  in the collapsed configuration when it is advanced through the microcatheter (not shown). The microcatheter is retracted proximally and each ring element  3442  disengages to create a zigzag stent surface for improved clot engagement. 
       FIG. 46  represents a mechanical clot retrieval device  3460  of the present invention which is a slightly variant embodiment of the retrieval device as illustrated by  FIG. 46 . The retrieval device  3460  has an elongate member  3461  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3462  attached at the distal end of the elongate member and a control strut  3464  having a distal end attached to the most proximal ring element  3462  and a proximal end attached to the elongate member  3461  by adhesive bonds  3466 . The ring elements  3462  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3460  is delivered inside the artery. Each ring element  3462 , comprising a plurality of struts  3463 , is connected by ring connecting tethers  3465 . Each strut  3463  has a crown element  3467 . The crown element  3467  is connected to the neighboring crown  3467  by a connecting strut  3468  to restrain the retrieval device  3460  in the collapsed configuration when it is advanced through the microcatheter (not shown). The microcatheter is retracted proximally and each ring element  3462  disengages to create a zigzag stent surface for improved clot engagement. 
       FIG. 47  represents a mechanical clot retrieval device  3480  of the present invention which is another variant embodiment of the retrieval device as illustrated in  FIG. 46 . The retrieval device  3480  has an elongate member  3481  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3482  and a control strut  3484  having a distal end attached to the proximal ring element  3482  and a proximal end attached to the elongate member  3481 . The ring elements  3482  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3482  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter, through which the retrieval device  3480  is delivered inside the artery. Each ring element  3482 , comprising a plurality of struts  3483 , is connected by ring connecting tethers  3485 . Each strut  3483  has a crown element  3487 . The crown element  3487  is connected to the neighboring crown  3487  by the tether  3485  and adhesive bond  3486 . Crown-to-crown connection restrains the retrieval device  3480  in the collapsed configuration when it is advanced through a microcatheter (not shown). The microcatheter is retracted and each ring element  3482  disengages to create a zigzag stent surface for improved clot engagement. 
     Another preferred embodiment of a mechanical clot retrieval device of the present invention is illustrated in  FIGS. 48 a -48 f   . The retrieval device  3500  includes an elongate member  3501  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3502 , and a pair of removal tethers  3504  having distal ends attached to the proximal ring element  3502  and proximal ends attached to the elongate member  3501 . The ring elements  3502  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3502  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3403 , through which the retrieval device  3500  is delivered inside the artery. Each ring element  3502 , comprising a plurality of struts  3503 , is interconnected by ring connecting tethers  3505 . Each strut  3503  has a crown element  3507 . The distally facing crown element  3507  is connected to the neighboring proximally facing crown  3507  by the tether  3505 . Crown-to-crown connection restrains the retrieval device  3500  in the collapsed configuration when advanced through the microcatheter  3403 . The microcatheter  3403  is advanced inside a torturous artery  3400  and is positioned distal of an obstructive clot  3401 . The microcatheter  3403  is retracted to allow each ring element  3502  reach the expanded configuration and engage or capture the clot  3401  from the distal end of the clot  3401  and the device is then retracted proximally into the microcatheter  3403 . 
       FIGS. 49 a -49 f    represents a mechanical clot retrieval device of the present invention which is a slightly variant embodiment of the retrieval device  3500  as illustrated in  FIGS. 48 a -48 f   . The retrieval device has an elongate member  3521  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3522 , and a control strut  3524  having a distal end attached to the proximal ring element  3522  and a proximal end attached to the elongate member  3521 . The ring elements  3522  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3522  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3403  through which the retrieval device  3520  is delivered inside the artery. Each ring element  3522 , comprising a plurality of struts  3523 , is connected by ring connecting tethers  3525 . Each strut  3523  has a crown element  3527 . The distally facing crown element  3527  is connected to neighboring proximally facing crown  3527  by the tether  3525 . Crown-to-crown  3527  connection restrains the retrieval device  3520  in the collapsed configuration when advanced through the microcatheter  3403 . The microcatheter  3403  is advanced over a guidewire (not shown) inside a torturous artery  3400  and positioned distal of an obstructive clot  3401 . The microcatheter  3403  is retracted proximally to allow each ring element  3522  reach the expanded configuration and engage or capture the clot  3401  from the distal end of the clot  3401  and the device is then retracted proximally into the microcatheter  3403 . 
       FIGS. 50 a -50 g    shows a mechanical clot retrieval device  3540   a  of the present invention which is another slightly variant embodiment of the retrieval device  3500  as illustrated in  FIGS. 48 a -48 f   . The retrieval device  3540   a  includes an elongate member  3541  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3542  configured at the distal end of the elongate member  3541  and a plurality of connecting tethers  3545  having distal ends attached to the ring element  3542  and proximal ends attached to the elongate member  3541 . The ring elements  3542  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3542  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3403  through which the retrieval device  3540   a  is delivered inside the artery. Each ring element  3542  comprising a plurality of struts  3543 . Each strut  3543  has a crown element  3547 . The distally facing crown element  3547  is connected to neighboring proximally facing crown  3547  by the tether  3545 . Crown-crown  3547  connection restrains the retrieval device  3540   a  in the collapse configuration when advanced through a microcatheter  3403 . The microcatheter  3403  is advanced over a guidewire (not shown) inside a tortuous artery  3400  and positioned distal of an obstructive clot  3401 . The microcatheter  3403  is retracted to allow each ring element  3542  to reach the expanded configuration and engage or capture the clot  3401  from the distal base of the clot  3401  and the device is then retracted proximally to the microcatheter  3403 . 
     Another embodiment of a mechanical clot retrieval device  3560  of the present invention is illustrated in  FIGS. 51 a -51 f   . The retrieval device  3560  includes an elongate member  3561  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3562  configured at the distal end of the elongate member  3561 , a plurality of connecting tethers  3565  having distal ends attached to the ring element  3562  and proximal ends attached to the elongate member  3561 , and a pair of removal tethers  3564  having distal ends attached to the most proximal ring element  3562  and proximal ends that extend out of the artery. The ring elements  3562  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3562  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3403  through which the retrieval device  3560  is delivered inside the artery. Each ring element  3562  comprises a plurality of struts  3563 . Each strut  3563  has a crown element  3567 . The connecting tether  3565  engages each ring element  3562  to the elongate member  3562  to restrain the retrieval device  3560  in the collapsed configuration when it is advanced through a microcatheter  3403 . The microcatheter  3403  is advanced over a guidewire (not shown) inside a tortuous artery  3400  and positioned distal of an obstructive clot  3401 . The microcatheter  3403  is retracted to allow each ring element  3562  reach the expanded configuration and engage or capture the clot  3401  from the distal base of the clot  3401  and retracted proximally to the microcatheter  3403 . 
       FIGS. 52 a -52 d    represents a mechanical clot retrieval device  3580  of the present invention which is a slightly variant embodiment of the retrieval device  3560  as illustrated in  FIGS. 51 a -51 f   . The retrieval device  3580  has an elongate member  3581  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3582  configured at the distal end of the elongate member  3581  and a plurality of connecting tethers  3585  having distal ends attached to the ring member  3582  and proximal ends attached to the elongate member  3581 . The ring elements  3582  are self-expandable from a collapsed configuration to an expanded configuration and comprise a radial force that biases the ring elements  3582  towards the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3403  through which the retrieval device  3580  is delivered inside the artery. Each ring element  3582  comprises a plurality of struts  3583 . Each strut  3583  has a crown element  3587 . Each ring element  3582  is wrapped around the elongate member  3581  to restrain the retrieval device  3580  in the collapsed configuration when it is advanced through a microcatheter  3403 . The microcatheter  3403  is advanced over a guidewire (not shown) inside a tortuous artery and positioned distal of an obstructive clot  3401 . The microcatheter  3403  is retracted to allow each ring element  3582  to reach the expanded configuration and engage or capture the clot  3401  from the distal base of the clot (not shown) and then the device is retracted proximally into the microcatheter  3403 . 
       FIGS. 53 a -53 f    shows a method of using a mechanical clot retrieval device  3600  of the present invention in the embodiment as illustrated in  FIGS. 52 a -52 d   . A guide catheter  3601  is inserted inside the artery and a guidewire (not shown) is advanced within the catheter  3601  bypassing an occlusive clot (not shown). A microcatheter  3602  is advanced over the guidewire (not shown) to a location distal to the clot (not shown) and then the retrieval device  3600  is advanced through the microcatheter  3602  to the clot (not shown) in a collapsed configuration. The microcatheter  3602  is retracted to allow ring elements  3604  to reach an expanded configuration. The ring elements  3604  are self-expandable from the collapsed configuration to the expanded configuration and each ring element comprises plurality of struts  3605 . Each strut  3605  has a crown element  3607 . The ring elements  3604  are attached to the elongate member  3603  by connecting tethers  3606  and are wrapped around the elongate member  3603  to restrain the retrieval device  3600  in the collapsed configuration when it is advanced through the microcatheter  3602 . The ring elements  3604 , when expanded, engage or capture the clot (not shown) from the distal base of the clot (not shown) and retract the clot (not shown) proximally to the microcatheter  3602 . The retrieval device  3600  returns to the collapsed configuration while retracting into the guide catheter  3601  and is easily removed from the artery. 
       FIGS. 54 a -54 h    shows another method of using a mechanical clot retrieval device  3620  of the present invention in accordance with the embodiment as illustrated in  FIGS. 52 a -52 d   . A guidewire  3404  is inserted in a tortuous artery  3400  and advanced through an obstructive clot  3401 . A microcatheter  3403  is advanced over the guidewire  3404  to a location distal to the clot  3401  and then the retrieval device  3620  is advanced through the microcatheter  3403  to the clot  3401  in a collapsed configuration. The microcatheter  3403  is retracted when the retrieval device  3620  is positioned distal of the clot  3401  to allow ring elements  3622  to reach an expanded configuration. The ring elements  3622  are self-expandable from the collapsed configuration to the expanded configuration and are wrapped around the elongate member  3621  to restrain the retrieval device  3620  in the collapsed configuration when advanced through the microcatheter  3403 . A removal tether  3624  is attached to the retrieval device  3620 . The ring elements  3622 , when expanded using an activation element  3626 , engage or capture the clot  3401  from the distal base of the clot  3401  and retract the clot  3401  proximally to the microcatheter  3603 . The retrieval device  3620  returns to the collapsed configuration while retracting into a guide catheter  3627  and is easily removed from the artery  3400 . 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 55 . The retrieval device  3700  has an elongate member  3701  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3702   a  having a larger diameter and a plurality of ring elements having a smaller diameter  3702   b , and a control strut  3704  having distal end attached to proximal ring element  3702   a  and proximal end attached to the elongate member  3701  by adhesive bonds  3706 . The ring elements  3702   a  and  3702   b  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3702   a  and  3702   b  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3700  is delivered inside the artery. Each ring element  3702   a , and  3702   b  comprises a plurality of struts  3703 . Each strut  3703  has a crown element  3707 . The crown element  3707  is connected to neighboring crown element  3707  by the ring to ring connection  3705 . The ring to ring connection  3705  restrains the retrieval device  3700  in the collapsed configuration when it is advanced through the microcatheter (not shown). The microcatheter is retracted proximally and each ring element  3702  disengages to form the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 56 . The retrieval device  3720  has an elongate member  3721  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3722 , and a removal tether  3724  having distal end attached to proximal ring element  3722  and proximal end attached to the elongate member  3721 . The ring elements  3722  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3722  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3720  is delivered inside the artery. Each ring element  3722  comprises a plurality of struts  3723 . The outer mesh  3725  restrains the retrieval device  3720  and limits it to a certain diameter in the expanded configuration. The microcatheter is retracted proximally and each ring element  3722  disengages to form the maximum allowable diameter within the outer mesh. 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 57 . The retrieval device  3740  has an elongate member  3741  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3742 , and a removal tether  3744  having distal end attached to proximal ring element  3742  and proximal end attached to the elongate member  3741 . The ring elements  3742  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3742  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3740  is delivered inside the artery. Each ring element  3742  comprises a plurality of struts  3743 . The tether net cover  3745  restrains the retrieval device  3740  and limits it to a certain diameter in the expanded configuration. The microcatheter is retracted proximally and each ring element  3742  disengages to form the maximum diameter allowed by the tether net cover. 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 58 . The retrieval device  3760  has an elongate member  3761  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3762 , and a control strut  3764  having distal end attached to proximal ring element  3762  and proximal end attached to the elongate member  3761  by adhesive bonds  3766 . The ring elements  3762  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3762  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3760  is delivered inside the artery. Each ring element  3762  comprises a plurality of struts  3763 . Each strut  3763  has a crown element  3767 . The crown element  3767  is connected to neighboring crown element  3767  by the ring to ring connection  3765 . The ring to ring connection  3765  restrains the retrieval device  3760  in the collapsed configuration when it is advanced through the microcatheter (not shown). The microcatheter is retracted proximally and each ring element  3762  disengages to form the expanded configuration. 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 59 . The retrieval device  3780  has an elongate member  3781  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3782 , and a removal tether  3784  having distal end attached to proximal ring element  3782  and proximal end attached to the elongate member  3781 . The ring elements  3782  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3782  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3780  is delivered inside the artery. Each ring element  3782  comprises a plurality of struts  3783 . The tether net cover  3785  restrains the retrieval device  3780  and limits it to a certain diameter in the expanded configuration. The device also comprises a tapered ring element at the distal end of the tether net cover  3785 . The microcatheter is retracted proximally and each ring element  3782  disengages to form the maximum diameter allowed by the tether net cover  3785 . 
     Another embodiment of a mechanical clot retrieval device of the present invention is represented in  FIG. 60 . The retrieval device  3800  has an elongate member  3801  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a plurality of ring elements  3802 , and a control strut  3804  having distal end attached to proximal ring element  3802  and proximal end attached to the elongate member  3801  by adhesive bonds  3806 . The ring elements  3802  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3802  toward the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter (not shown) through which the retrieval device  3780  is delivered inside the artery. Each ring element  3802  comprises a plurality of struts  3803 . Each strut  3803  has a crown element  3807 . The crown element  3807  is connected to neighboring crown element  3807  by the ring to ring connection  3805 . The ring to ring connection  3805  restrains the retrieval device  3800  in the collapsed configuration when it is advanced through the microcatheter (not shown). The ring elements are wrapped in an outer mesh/net cover  3808  which limits the diameter of the ring elements in the expanded configuration. The microcatheter is retracted proximally and each ring element  3802  disengages to form the expanded configuration with the maximum diameter allowed by the outer mesh/net cover  3808 . 
     Another preferred embodiment of a mechanical clot retrieval device  3820  of the present invention is illustrated in  FIGS. 61 a -61 c   . The retrieval device  3820  is advanced through a microcatheter  3821  inside the patient artery and includes an activation tether  3824 , having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a distal collar  3827 , and a plurality of ring elements  3822  configured to the distal end of the activation tether  3824 . The ring elements  3822  are self-expandable from a collapsed configuration to an expanded configuration and have a radial force that biases the ring elements  3822  in the expanded configuration. In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3821  through which the retrieval device  3820  is delivered inside the artery. Each ring element  3822 , comprising a plurality of struts  3823 , is interconnected by bridges  3825 . The microcatheter  3821  is retracted and the activation tether  3824  is manually pulled proximally to compress the device and allow ring elements  3822  to reach the expanded configuration. 
       FIGS. 62 a -62 b    shows a mechanical clot retrieval device  3840  of the present invention which is a slightly variant embodiment of the retrieval device as illustrated in  FIGS. 61 a -61 c   . The retrieval device  3840  is advanced through a microcatheter  3841  inside the artery and has an activation tether  3844 , having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a distal collar  3847 , and a ring element  3846  configured at the distal end of the activation tether  3844 . The ring element  3846  is self-expandable from a collapsed configuration to an expanded configuration and comprises a radial force that biases the ring element  3846  to the expanded configuration. The ring element comprises a plurality of struts  3843 . The micro catheter  3841  is retracted and the activation tether  3844  is manually pulled proximally to compress the device proximally and allow ring elements  3842  to reach the expanded configuration. 
       FIGS. 63 a -63 b    shows a mechanical clot retrieval device  3860  of the present invention which is a slightly variant embodiment of the retrieval device  3840  as illustrated in  FIGS. 62 a -62 b   . The retrieval device  3862  is advanced through a microcatheter  3861  inside the artery and has an activation tether  3864  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a distal collar  3867 , a ring element  3866  configured at the distal end of the activation tether  3864 , and a pair of removal tethers  3862  attached to the ring element  3866  having proximal ends that extend out of the artery. The ring element  3866  is self-expandable from a collapsed configuration to an expanded configuration and comprises a radial force that biases the ring element  3866  to the expanded configuration. In the collapsed configuration, the radial force is absorbed by the microcatheter. The microcatheter  3861  is retracted and the activation tether  3864  is manually pulled proximally to compress the device proximally and allow ring element  3866  to reach the expanded configuration. 
       FIGS. 64 a -64 b    shows a mechanical clot retrieval device  3880  of the present invention which is a slightly variant embodiment of the retrieval device  3860  as illustrated in  FIGS. 63 a -63 b   . The retrieval device  3882  is advanced through a microcatheter  3881  inside the patient artery and has an activation tether  3884  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a couple of collars  3887 , a couple of ring elements  3886  configured at the distal end of the activation tether  3884 , and a pair of removal tethers  3882  having distal ends attached to the ring element  3886  and having proximal attached to the collar  3887 . The ring element  3886  is self-expandable from a collapsed configuration to an expanded configuration and has a radial force that biases the ring element  3886  to the expanded configuration. In the collapsed configuration, the radial force is absorbed by the microcatheter  3881 . Each ring element  3886  comprises a plurality of struts  3883 . The microcatheter  3881  is retracted and the activation tether  3884  is manually pulled proximally to compress the device and to allow ring element  3886  reach the expanded configuration. 
       FIGS. 65 a -65 d    illustrates another preferred embodiment of a mechanical clot retrieval device  3900   a  of the present invention. The retrieval device  3900   a  is advanced through a microcatheter  3901  inside the patient artery and has a pair of activation tethers  3904  having distal ends that extend interior of the artery and proximal ends that extend exterior of the artery, a distal collar  3907 , and a plurality of ring elements  3902  configured to the distal end of the activation tether  3904 . The ring elements  3902  are self-expandable from a collapsed configuration  3900   a  to an expanded configuration  3900   b  and have a radial force that biases the ring elements  3902  to the expanded configuration  3900   b . In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3901  through which the retrieval device  3900   a  is delivered inside the artery. Each ring element  3902 , comprising a plurality of struts  3903 , is connected by bridges  3905 . Each strut includes a tether guide tube  3908 . The distal end of activation tether is passed through the guide tube  3908 . The microcatheter  3901  is retracted and the activation tethers  3904  are manually pulled proximally to compress the device proximally and to allow ring elements  3902  to reach the expanded configuration  3900   b.    
       FIGS. 66 a -66 c    illustrates a mechanical clot retrieval device  3920   a  of the present invention which is a slightly variant embodiment of the retrieval device shown in  FIGS. 61 a -61 c   . The retrieval device  3920   a  is advanced through a microcatheter  3921  inside the patient artery and has a pair of activation tethers  3924  having distal ends that extend interior of the artery and proximal ends that extend exterior of the artery, a distal collar  3927 , and a plurality of ring elements  3922  configured to the distal end of the activation tether  3824 . The ring elements  3922  are self-expandable from a collapsed configuration  3920   a  to an expanded configuration  3920   b  and comprise a radial force that biases the ring elements  3922  in the expanded configuration  3920   b . In the collapsed configuration, the radial force is absorbed by an intravascular microcatheter  3921  through which the retrieval device  3920   a  is delivered inside the artery. Each ring element  3922  comprising a plurality of struts  3923  and a limiting tether  3928  is interconnected by bridges  3925 . The microcatheter  3921  is retracted and the activation tether  3924  is manually pulled proximally to compress the device and to allow ring elements  3922  to reach the expanded configuration  3920   b . The limiting tether  3928  prevents the ring element from expanding beyond a certain diameter. 
       FIGS. 67 a -67 b    shows a mechanical clot retrieval device  3940  of the present invention which is a slightly variant embodiment of the retrieval device  3840  as illustrated in  FIGS. 62 a -62 b   . The retrieval device  3940  is advanced through a microcatheter  3941  inside the patient artery and has an activation tether  3944  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a distal collar  3947 , and a ring element  3946  configured at the distal end of the activation tether  3944 . The ring element  3946  is self-expandable from a collapsed configuration to an expanded configuration and has a radial force that biases the ring element  3946  to the expanded configuration. The ring element comprising a plurality of struts  3943  and a limiting tether  3948 . The microcatheter  3941  is retracted and the activation tether  3944  is manually pulled proximally to compress the device and to allow ring elements  3942  to reach the expanded configuration. 
       FIGS. 68 a -68 b    shows a mechanical clot retrieval device  3960  of the present invention which is a slightly variant embodiment of the retrieval device  3860  as illustrated in  FIGS. 63 a -63 b   . The retrieval device  3962  is advanced through a microcatheter  3961  inside the artery and has an activation tether  3964  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a distal collar  3967 , a ring element  3966  configured at the distal end of the activation tether  3964 , and a pair of removal tethers  3962  attached to the ring element  3966  having proximal ends that extend exterior of the artery. The ring element  3966  is self-expandable from a collapsed configuration to an expanded configuration and has a radial force that biases the ring element  3966  to the expanded configuration. In the collapsed configuration, the radial force is absorbed by the microcatheter  3961 . Each ring element  3966  comprises a plurality of struts  3963  and a limiting tether  3968 . The microcatheter  3961  is retracted and the activation tether  3964  is manually pulled proximally to compress the device and allow ring elements  3966  to reach the expanded configuration. 
       FIGS. 69 a -69 b    shows a mechanical clot retrieval device  3980  of the present invention which is a slightly variant embodiment of the retrieval device  3880  as illustrated in  FIGS. 64 a -64 b   . The retrieval device  3980  is advanced through a microcatheter  3981  inside the artery and has an activation tether  3984  having a distal end that extends interior of the artery and a proximal end that extends exterior of the artery, a couple of collars  3987 , a couple of ring elements  3986  configured at the distal end of the activation tether  3984 , and a pair of removal tethers  3982  having distal ends attached to the ring element  3986 . The ring element  3986  is self-expandable from a collapsed configuration to an expanded configuration and has a radial force that biases the ring element  3986  to the expanded configuration. In the collapsed configuration, the radial force is absorbed by the microcatheter  3981 . Each ring element comprises a plurality of struts  3983  and a limiting tether  3988 . The microcatheter  3981  is retracted and the activation tether  3984  is manually pulled proximally to compress the device and allow the ring elements  3986  reach the expanded configuration. 
     The ring elements used in the above description are made of shape-memory material, preferably Nitinol. Remaining elements of the retrieval device as described herein are preferably formed of a material such as a stainless steel, a nickel-based super alloy, a spring steel alloy, particularly a composition sold under the trademark nitinol. 
     It will be apparent from the foregoing description that, while particular embodiments of the present invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. 
     Modifications and additions can be made to the various embodiments of the invention described herein. For example, while embodiments may refer to particular features, the invention includes embodiments having different combinations of features. The invention also includes embodiments that do not include all of the specific features described. 
     The invention is not limited to the embodiments hereinbefore described which may be varied in construction and detail.