Patent Publication Number: US-2023149624-A1

Title: System and method for syringe plunger engagement with an injector

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 16/714,980, filed Dec. 16, 2019, which is a continuation of U.S. application Ser. No. 15/334,644, filed Oct. 26, 2016, now U.S. Pat. No. 10,512,721, issued Dec. 24, 2019, which is a continuation of and claims priority to U.S. application Ser. No. 14/925,722, filed Oct. 28, 2015, now U.S. Pat. No. 9,480,797, issued Nov. 1, 2016, the disclosures of which are incorporated in their entirety by this reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates generally to a system including a front-loading or front-installing syringe for use with a fluid injector and, further, to a connection interface for securing a syringe plunger to a piston of the fluid injector and to a method for engaging and disengaging the syringe plunger to and from the piston of the fluid injector. 
     Description of Related Art 
     In many medical diagnostic and therapeutic procedures, a medical practitioner, such as a physician, injects a patient with one or more medical fluids. In recent years, a number of injector-actuated syringes and fluid injectors for pressurized injection of fluids, such as a contrast solution (often referred to simply as “contrast”), a flushing agent, such as saline, and other medical fluids have been developed for use in procedures such as angiography, computed tomography (CT), ultrasound, magnetic resonance imaging (MRI), nuclear medicine, positron emission tomography (PET), and other imaging procedures. In general, these fluid injectors are designed to deliver a preset amount of fluid at a preset flow rate. 
     Various connection interfaces have been developed to facilitate the engagement of a syringe plunger to and from a piston of the fluid injector. In some aspects, the syringe having a retention feature is inserted into a syringe port on the fluid injector by aligning the syringe with a corresponding locking feature provided on the fluid injector. Such alignment also aligns the plunger in the syringe with the piston on the fluid injector such that the piston can engage the plunger and drive the plunger through the syringe barrel to draw fluid into the syringe barrel or deliver fluid from the syringe barrel. In other aspects, upon initial engagement with the plunger, the piston may be rotated, in a clockwise or counter-clockwise direction, until the piston engages a catch on the plunger. In further aspects, the piston has one or more radially-extendable pins that engage a lip on the plunger. 
     Many of the existing connection interfaces have construction that requires a complex piston head with various sensor elements and active engagement structures. There is a need in the art for an improved connection interface that allows for a simpler and easier engagement and disengagement of the syringe plunger to and from the piston of the fluid injector. There is a further need in the art for reducing or eliminating the need for the operator to rotationally align the syringe with the fluid injector to allow for a proper engagement of the syringe plunger with the piston of the fluid injector. There is a further need in the art for reducing the angle of rotation through which the operator rotates the syringe to release the syringe plunger from the piston for removal of the syringe. While various syringe plunger connection interfaces and methods are known in the medical field, improved connection interfaces between the syringe plunger and the piston of the fluid injector and methods for engaging and disengaging the syringe plunger to and from the piston of the fluid injector continue to be in demand. 
     SUMMARY OF DISCLOSURE 
     In view of the disadvantages of the existing connection interfaces between a syringe plunger and a piston of a fluid injector, there is a need in the art for an improved connection interface between a syringe plunger and a piston of a fluid injector that overcomes the deficiencies of the prior art. There is an additional need for improved methods for engaging and disengaging a syringe plunger to and from a piston of a fluid injector to allow easy loading or removal of a syringe to and from a fluid injector. 
     In accordance with some aspects, plunger for use with a syringe may have a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis. The plunger may have at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment protruding toward the distal end of the plunger body and deflectable relative to the first segment, and at least one actuation member associated with the at least one resiliently deflectable retaining member. The at least one actuation member may interact with a piston used to engage the plunger to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston. 
     In accordance with other aspects, at least one alignment member may be associated with the plunger body or the at least one resiliently deflectable retaining member. The at least one alignment member may have an alignment surface for guiding the piston into self-orienting alignment with the plunger. The at least one alignment member may have a plurality of alignment members spaced apart around the plunger longitudinal axis. The plurality of alignment members may be spaced apart at equal radial intervals around the plunger longitudinal axis. The second segment of the at least one resiliently deflectable retaining member may be deflectable radially relative to the first segment away from the plunger longitudinal axis. The at least one resiliently deflectable retaining member may be linearly or curvilinearly contiguous between the first segment and the second segment. The second segment of the at least one resiliently deflectable retaining member may be angled toward the plunger longitudinal axis. 
     In accordance with other aspects, the at least one actuation member may be provided on a surface of the at least one resiliently deflectable retaining member. The at least one actuation member may be at the second segment of the at least one resiliently deflectable retaining member. The at least one actuation member may be angled relative to a plane defined by a body of the at least one resiliently deflectable retaining member. The at least one resiliently deflectable retaining member may have a plurality of resiliently deflectable retaining members spaced apart around the plunger longitudinal axis. The plurality of resiliently deflectable retaining members may be spaced apart at equal radial intervals around the plunger longitudinal axis. A terminal surface of the second segment of the at least one resiliently deflectable retaining member may engage a surface of the piston to releasably lock the plunger with the piston. The terminal surface may be linear. The terminal surface may be perpendicular or angled relative to a direction of the plunger longitudinal axis. 
     In accordance with other aspects, the plunger body may define an interior cavity with a conical-shaped portion at the distal end of the plunger body and a cylindrical-shaped portion at the proximal end of the plunger body. The first segment of the at least one resiliently deflectable member may be attached to an inner surface of the plunger body. The at least one resiliently deflectable retaining member may protrude from the inner surface of the plunger body into the interior cavity. A plunger cover may be disposed over at least a portion of an outer surface of the plunger body. The plunger cover may have a resilient seal disposed around at least a portion of a circumferential sidewall of the plunger cover. The resilient seal may have an inner surface at least partially seated in a groove in the circumferential sidewall of the plunger body and an outer surface adapted to slidably engage with a barrel of a syringe. The actuating member may be a cam member. 
     In accordance with other aspects, a plunger for use with a syringe may have a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis. The plunger may have at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second end protruding toward the distal end of the plunger body and deflectable relative to the first segment and at least one alignment member associated with the plunger body or the at least one resiliently deflectable retaining member. The at least one alignment member may guide a piston into self-orienting alignment with the plunger during engagement of the plunger with the piston. The plunger may have at least one actuation member associated with the at least one resiliently deflectable retaining member. The at least one actuation member may interact with the piston to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston during disengagement of the plunger from the piston. A terminal surface of the second segment of the at least one resiliently deflectable retaining member may engage a surface of the piston to releasably lock the plunger with the piston. The at least one actuation member may be at the second segment of the at least one resiliently deflectable retaining member. The second segment of the at least one resiliently deflectable retaining member may be deflectable radially relative to the first segment away from the plunger longitudinal axis. 
     In accordance with other aspects, a plunger for use with a syringe may have a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis. The plunger may have at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment deflectable relative to the first segment, at least one alignment member associated with the plunger body or the at least one resiliently deflectable retaining member, and at least one actuation member associated with the at least one resiliently deflectable retaining member. The at least one alignment member may guide a piston into self-orienting alignment with the plunger during engagement of the plunger with the piston. The at least one actuation member may interact with the piston to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston during disengagement of the plunger from the piston. The second segment of the at least one resiliently deflectable retaining member may protrude towards the distal end of the plunger body and wherein the second end of the at least one resiliently deflectable retaining member is deflectable radially relative to the first end away from the plunger longitudinal axis. The second end of the at least one resiliently deflectable retaining member may protrude in a direction circumferentially around the longitudinal axis of the plunger body and wherein the second end of the at least one resiliently deflectable retaining member is deflectable circumferentially relative to the first end. 
     In accordance with other aspects, a syringe may have a barrel having a barrel proximal end, a barrel distal end having a discharge nozzle, and a barrel sidewall extending between the barrel proximal end and the barrel distal end. The syringe may have a plunger slidably disposed within the barrel and reciprocally movable between the barrel proximal end and the barrel distal end. The plunger may have a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis, at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment protruding toward the distal end of the plunger body and deflectable relative to the first segment, and at least one actuation member associated with the at least one resiliently deflectable retaining member. The at least one actuation member may interact with a piston used to engage the plunger to radially deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston. 
     Various aspects of the present disclosure may be further characterized by one or more of the following clauses: 
     Clause 1. A plunger for use with a syringe, the plunger comprising: a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis; at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment protruding toward the distal end of the plunger body and deflectable relative to the first segment; and at least one actuation member associated with the at least one resiliently deflectable retaining member, wherein the at least one actuation member interacts with a piston used to engage the plunger to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston. 
     Clause 2. The plunger according to clause 1, further comprising at least one alignment member associated with the plunger body or the at least one resiliently deflectable retaining member, the at least one alignment member having an alignment surface for guiding the piston into self-orienting alignment with the plunger. 
     Clause 3. The plunger according to clause 2, wherein the at least one alignment member comprises a plurality of alignment members spaced apart around the plunger longitudinal axis. 
     Clause 4. The plunger according to clause 3, wherein the plurality of alignment members is spaced apart at equal radial intervals around the plunger longitudinal axis. 
     Clause 5. The plunger according to any of clauses 1-4, wherein the second segment of the at least one resiliently deflectable retaining member is deflectable radially relative to the first segment away from the plunger longitudinal axis. 
     Clause 6. The plunger according to any of clauses 1-5, wherein the at least one resiliently deflectable retaining member is linearly or curvilinearly contiguous between the first segment and the second segment. 
     Clause 7. The plunger according to any of clauses 1-6, wherein the second segment of the at least one resiliently deflectable retaining member is angled toward the plunger longitudinal axis. 
     Clause 8. The plunger according to any of clauses 1-7, wherein the at least one actuation member is provided on a surface of the at least one resiliently deflectable retaining member. 
     Clause 9. The plunger according to clause 8, wherein the at least one actuation member is at the second segment of the at least one resiliently deflectable retaining member. 
     Clause 10. The plunger according to any of clauses 1-9, wherein the at least one actuation member is angled relative to a plane defined by a body of the at least one resiliently deflectable retaining member. 
     Clause 11. The plunger according to any of clauses 1-10, wherein the at least one resiliently deflectable retaining member comprises a plurality of resiliently deflectable retaining members spaced apart around the plunger longitudinal axis. 
     Clause 12. The plunger according to clause 11, wherein the plurality of resiliently deflectable retaining members is spaced apart at equal radial intervals around the plunger longitudinal axis. 
     Clause 13. The plunger according to any of clauses 1-12, wherein a terminal surface of the second segment of the at least one resiliently deflectable retaining member engages a surface of the piston to releasably lock the plunger with the piston. 
     Clause 14. The plunger according to clause 13, wherein the terminal surface is linear. 
     Clause 15. The plunger according to clause 13 or clause 14, wherein the terminal surface is perpendicular or angled relative to a direction of the plunger longitudinal axis. 
     Clause 16. The plunger according to any of clauses 1-15, wherein the plunger body defines an interior cavity with a conical-shaped portion at the distal end of the plunger body and a cylindrical-shaped portion at the proximal end of the plunger body. 
     Clause 17. The plunger according to clause 16, wherein the first segment of the at least one resiliently deflectable member is attached to an inner surface of the plunger body. 
     Clause 18. The plunger according to clause 17, wherein the at least one resiliently deflectable retaining member protrudes from the inner surface of the plunger body into the interior cavity. 
     Clause 19. The plunger according to any of clauses 1-18, further comprising a plunger cover disposed over at least a portion of an outer surface of the plunger body, the plunger cover comprising a resilient seal disposed around at least a portion of a circumferential sidewall of the plunger cover. 
     Clause 20. The plunger according to clause 19, wherein the resilient seal comprises an inner surface at least partially seated in a groove in the circumferential sidewall of the plunger body and an outer surface adapted to slidably engage with a barrel of a syringe. 
     Clause 21. The plunger according to any of clauses 1-20, wherein the actuating member is a cam member. 
     Clause 22. A plunger for use with a syringe, the plunger comprising: a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis; at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second end protruding toward the distal end of the plunger body and deflectable relative to the first segment; and at least one alignment member associated with the plunger body or the at least one resiliently deflectable retaining member, wherein the at least one alignment member guides a piston into self-orienting alignment with the plunger during engagement of the plunger with the piston. 
     Clause 23. The plunger according to clause 22, further comprising at least one actuation member associated with the at least one resiliently deflectable retaining member, wherein the at least one actuation member interacts with the piston to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston during disengagement of the plunger from the piston. 
     Clause 24. The plunger according to clause 23, wherein a terminal surface of the second segment of the at least one resiliently deflectable retaining member engages a surface of the piston to releasably lock the plunger with the piston. 
     Clause 25. The plunger according to clause 23 or clause 24, wherein the at least one actuation member is at the second segment of the at least one resiliently deflectable retaining member. 
     Clause 26. The plunger according to any of clauses 23-25, wherein the second segment of the at least one resiliently deflectable retaining member is deflectable radially relative to the first segment away from the plunger longitudinal axis. 
     Clause 27. A plunger for use with a syringe, the plunger comprising: a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis; at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment deflectable relative to the first segment; at least one alignment member associated with the plunger body or the at least one resiliently deflectable retaining member; and at least one actuation member associated with the at least one resiliently deflectable retaining member, wherein the at least one alignment member guides a piston into self-orienting alignment with the plunger during engagement of the plunger with the piston, and wherein the at least one actuation member interacts with the piston to deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston during disengagement of the plunger from the piston. 
     Clause 28. The plunger according to clause 27, wherein the second segment of the at least one resiliently deflectable retaining member protrudes towards the distal end of the plunger body and wherein the second end of the at least one resiliently deflectable retaining member is deflectable radially relative to the first end away from the plunger longitudinal axis. 
     Clause 29. The plunger according to clause 27 or clause 28, wherein the second end of the at least one resiliently deflectable retaining member protrudes in a direction circumferentially around the longitudinal axis of the plunger body and wherein the second end of the at least one resiliently deflectable retaining member is deflectable circumferentially relative to the first end. 
     Clause 30. A syringe comprising: a barrel having a barrel proximal end, a barrel distal end having a discharge nozzle, and a barrel sidewall extending between the barrel proximal end and the barrel distal end; and a plunger slidably disposed within the barrel and reciprocally movable between the barrel proximal end and the barrel distal end, the plunger comprising: a plunger body having a proximal end, a distal end, and a circumferential sidewall extending between the proximal end and the distal end along a plunger longitudinal axis; at least one resiliently deflectable retaining member having a first segment attached to the plunger body and a second segment protruding toward the distal end of the plunger body and deflectable relative to the first segment; and at least one actuation member associated with the at least one resiliently deflectable retaining member, wherein the at least one actuation member interacts with a piston used to engage the plunger to radially deflect the at least one resiliently deflectable retaining member upon rotation of the plunger relative to the piston. 
     These and other features and characteristics of syringes, syringe plungers, and systems having syringes and/or syringe plungers, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only. As used in the specification and the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a top perspective view of a system including a fluid injector and at least one syringe according to an aspect of the present disclosure; 
         FIG.  2    is a side cross-sectional view of a syringe according to one aspect of the present disclosure; 
         FIG.  3 A  is a top perspective view of a plunger according to one aspect of the present disclosure; 
         FIG.  3 B  is a bottom perspective view of the plunger shown in  FIG.  3 A ; 
         FIG.  3 C  is a bottom view of the plunger shown in  FIG.  3 A ; 
         FIG.  3 D  is a side view of the plunger shown in  FIG.  3 A ; 
         FIG.  3 E  is a side cross-sectional view of the plunger shown in  FIG.  3 A ; 
         FIG.  4 A  is a top perspective view of a piston according to one aspect of the present disclosure; 
         FIG.  4 B  is an exploded perspective view of the piston shown in  FIG.  4 A ; 
         FIG.  4 C  is a side view of the piston shown in  FIG.  4 A ; 
         FIG.  5 A  is a side view of a plunger and a piston in an assembled state; 
         FIG.  5 B  is a side cross-sectional view of the plunger and the piston during initial engagement of the piston with the plunger; 
         FIG.  5 C  is a side cross-sectional view of the plunger and the piston prior to full engagement of the piston with the plunger; 
         FIG.  5 D  is a side cross-sectional view of the plunger and the piston during full engagement of the piston with the plunger; 
         FIG.  6 A  is a top perspective view of a plunger according to another aspect of the present disclosure; 
         FIG.  6 B  is a bottom perspective view of the plunger shown in  FIG.  6 A ; 
         FIG.  7 A  is a perspective cross-sectional view of the plunger shown in  FIGS.  6 A- 6 B  and a piston prior to initial engagement of the piston with the plunger; 
         FIG.  7 B  is a side cross-sectional view of the plunger and the piston shown in  FIG.  7 A ; 
         FIG.  8 A  is a perspective cross-sectional view of the plunger and the piston during initial engagement of the piston with the plunger; 
         FIG.  8 B  is a side partial cross-sectional view of the plunger and the piston shown in  FIG.  8 A ; 
         FIG.  9 A  is a perspective cross-sectional view of the plunger and the piston prior to full engagement of the piston with the plunger; 
         FIG.  9 B  is a side cross-sectional view of the plunger and the piston shown in  FIG.  9 A ; 
         FIG.  10 A  is a perspective cross-sectional view of the plunger and the piston during full engagement of the piston with the plunger; 
         FIG.  10 B  is a side cross-sectional view of the plunger and the piston shown in  FIG.  10 A ; 
         FIG.  11 A  is a perspective cross-sectional view of the plunger and the piston during initial disengagement as the plunger is rotated relative to the piston; 
         FIG.  11 B  is a side cross-sectional view of the plunger and the piston shown in  FIG.  11 A ; 
         FIG.  12 A  is a perspective cross-sectional view of the plunger and the piston during disengagement with a locking ring of the piston in a forward position; 
         FIG.  12 B  is a side cross-sectional view of the plunger and the piston shown in  FIG.  12 A ; 
         FIG.  13    is a side cross-sectional view of a first adapter configured for connecting a non-compatible plunger with a piston in accordance with one aspect of the present disclosure; 
         FIG.  14    is a side cross-sectional view of a second adapter configured for connecting a plunger with a non-compatible piston in accordance with one aspect of the present disclosure; 
         FIG.  15 A  is a front perspective view of a piston in accordance with another aspect; 
         FIG.  15 B  is a side view of the piston head shown in  FIG.  15 A ; 
         FIG.  15 C  is a front perspective view of the piston shown in  FIG.  15 A  and a plunger removed from the piston; 
         FIG.  15 D  is a front perspective of the piston and plunger shown in  FIG.  15 C  with the plunger assembled on the piston; 
         FIG.  15 E  is a side cross-sectional view of the plunger and the piston shown in  FIG.  15 D ; 
         FIG.  15 F  is an exploded perspective view of the piston shown in  FIG.  15 A ; 
         FIG.  15 G  is a bottom perspective view of a plunger in accordance with another aspect of the present disclosure; 
         FIG.  16    is a cylindrical plan projection view of the piston and the plunger shown in  FIG.  15 C ; 
         FIG.  17    is a cylindrical plan projection view of a piston and a plunger in accordance with another aspect of the present disclosure; 
         FIG.  18    is a cylindrical plan projection view of a piston and a plunger in accordance with another aspect of the present disclosure; 
         FIG.  19    is a cylindrical plan projection view of a piston and a plunger in accordance with another aspect of the present disclosure; 
         FIG.  20 A  is a top perspective view of a piston in accordance with another aspect; 
         FIG.  20 B  is a top perspective view of the piston shown in  FIG.  20 A  and a plunger in accordance with one aspect of the present disclosure; 
         FIG.  20 C  is a bottom perspective view of the plunger shown in  FIG.  20 B ; 
         FIG.  20 D  is a side cross-sectional view of the plunger shown in  FIG.  20 C ; 
         FIG.  21 A  is a front perspective view of a piston and a plunger in accordance with another aspect of the present disclosure; 
         FIG.  21 B  is a detailed cross-sectional side view of  FIG.  21 A  showing the engagement between the piston and the plunger; 
         FIG.  21 C  is a cross-sectional top view of  FIG.  21 A  showing the engagement between the piston and the plunger; 
         FIG.  22 A  is a top perspective view of a plunger in accordance with another aspect of the present disclosure; 
         FIG.  22 B  is a bottom perspective view of the plunger shown in  FIG.  22 A ; 
         FIG.  22 C  is a cross-sectional side view of the plunger illustrated in  FIG.  22 A  showing the engagement between a piston and the plunger; 
         FIG.  22 D  is a cross-sectional top view of the plunger illustrated in  FIG.  22 A  showing the engagement between a piston and the plunger; 
         FIG.  23 A  is a top perspective view of a plunger in accordance with another aspect of the present disclosure; 
         FIG.  23 B  is a first cross-sectional side view of the plunger illustrated in  FIG.  23 A  showing the engagement between a piston and the plunger; 
         FIG.  23 C  is a second cross-sectional side view of the plunger illustrated in  FIG.  23 A  showing the engagement between the piston and the plunger; 
         FIG.  23 D  is a cross-sectional top view of the plunger illustrated in  FIG.  23 A  showing the engagement between a piston and the plunger; 
         FIG.  24 A  is a top perspective view of a piston and a plunger in accordance with another aspect of the present disclosure; 
         FIG.  24 B  is a side cross-sectional view of the piston and the plunger of  FIG.  24 A  shown in an assembled state; and 
         FIG.  24 C  is a top cross-sectional view of the piston and the plunger shown in  FIG.  24 B . 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, and derivatives thereof shall relate to the components as they are oriented in the drawing/figures. When used in relation to a syringe and/or a plunger, the term “proximal” refers to a portion of a syringe and/or a plunger nearest a fluid injector when a syringe and/or a plunger is oriented for connecting to a fluid injector. The term “distal” refers to a portion of a syringe and/or a plunger farthest away from a fluid injector when a syringe and/or a plunger is oriented for connecting to a fluid injector. The term “radial” refers to a direction in a cross-sectional plane normal to a longitudinal axis of a syringe, a plunger, and/or a piston extending between proximal and distal ends. The term “circumferential” refers to a direction around an inner or outer surface of a sidewall of a syringe, a plunger, and/or a piston. The term “axial” refers to a direction along a longitudinal axis of a syringe, a piston, and/or a piston extending between the proximal and distal ends. The term “self-orienting” means that a piston head or a plunger orients itself to a correct orientation relative to a plunger or piston head, respectively, without a rotational effort by a technician or a fluid injector. The term “curvilinear” refers to a shape of a surface that has one or more curved lines, one or more straight lines with one or more curved lines, and/or one or more straight line segments arranged non-linearly. It is to be understood, however, that the disclosure may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary aspects of the disclosure. Hence, specific dimensions and other physical characteristics related to the aspects disclosed herein are not to be considered as limiting. 
     Referring to the drawings in which like reference characters refer to like parts throughout the several views thereof, the present disclosure is generally directed to a syringe plunger and a connection interface for connecting the syringe plunger to a piston of a fluid injector. Various aspects are directed to syringe plungers that may be connected to and disconnected from the piston. In various aspects, such plungers may be manually, hydraulically, or electrically activated. Furthermore, the present disclosure provides a quick and easy solution for engaging and disengaging the syringe plunger to and from the piston without a specific rotational orientation of the plunger about the plunger longitudinal axis relative to a longitudinal axis of the piston. For example, the piston may be advanced forward until it engages with the plunger, regardless of rotational orientation of the piston relative to the plunger about a longitudinal axis of the syringe, as described herein. In addition, a simple angular rotation of the plunger relative to the piston about a longitudinal axis of the syringe may allow for detachment of the two elements. In certain aspects, the syringe and the plunger may be engaged with a syringe port and piston extending from the syringe port, respectively. Concomitant disengagement of the syringe from the syringe port and the plunger from the piston may require the operator or technician to manually rotate the syringe. According to various aspects, rotation of the syringe may first result in disengagement of the plunger from the piston, as described herein, followed by disengagement of the syringe from the syringe port, for example as described in U.S. patent application Ser. Nos. 14/526,294 and 14/526,395, both of which were filed on Oct. 28, 2014, U.S. Pat. No. 6,652,489, issued Nov. 25, 2003, and U.S. Pat. No. 7,540,856, issued Jun. 2, 2009, the disclosures of each of which are incorporated by reference herein. In specific aspects, since rotation of the syringe also results in rotation of the plunger, due to frictional fit of plunger within the syringe, to affect disengagement of the plunger from the piston prior to disengagement of the syringe from the injector port, the rotational distance that the plunger must turn relative to the piston to cause disengagement of the plunger from the piston must be less than the rotational distance that the syringe must turn relative to the injector port to cause disengagement of the syringe from the port. Various aspects of the present disclosure provide a piston/plunger interface mechanism with features that provide this sequential disengagement requirement. 
     With reference to  FIG.  1   , a fluid injector  10  (hereinafter referred to as “injector  10 ”), such as an automated or powered fluid injector, is adapted to interface with and actuate at least one syringe  12 , each of which may be independently filled with a medical fluid F, such as contrast media, saline solution, or any desired medical fluid. The injector  10  may be used during a medical procedure to inject the medical fluid into the body of a patient by driving a plunger  26  of the at least one syringe  12  with at least one piston. The injector  10  may be a multi-syringe injector, wherein several syringes  12  may be oriented in a side-by-side or other arrangement and include plungers  26  separately actuated by respective pistons associated with the injector  10 . In aspects with two syringes arranged in a side-by-side relationship and filled with two different medical fluids, the injector  10  may deliver fluid from one or both of the syringes  12 . 
     The injector  10  may be enclosed within a housing  14  formed from a suitable structural material, such as plastic or metal. The housing  14  may have various shapes and sizes depending on a desired application. For example, the injector  10  may be a free-standing structure configured to be placed on the floor with a stationary or movable platform. Alternatively, the injector  10  may be configured for placement on a suitable table or support frame. The injector  10  includes at least one syringe port  16  for connecting the at least one syringe  12  to respective piston elements. In some aspects, the at least one syringe  12  includes at least one syringe retaining member for retaining the syringe  12  within the syringe port  16  of the injector  10 . The at least one syringe retaining member operatively engages a locking mechanism provided on or in the syringe port  16  of the injector  10  to facilitate loading and/or removal of the syringe  12  to and from the injector  10 , as described herein. The syringe retaining member and the locking mechanism together define a connection interface for connecting the syringe  12  to the injector  10 . 
     At least one fluid path set  17  may be fluidly connected with the at least one syringe  12  for delivering medical fluid F from the at least one syringe  12  to a catheter, needle, or other fluid delivery device (not shown) inserted into a patient at a vascular access site. Fluid flow from the at least one syringe  12  may be regulated by a fluid control module (not shown). The fluid control module may operate various pistons, valves, and/or flow regulating structures to regulate the delivery of the medical fluid, such as saline solution and contrast, to the patient based on user selected injection parameters, such as injection flow rate, duration, total injection volume, and/or ratio of contrast media and saline. One example of a suitable front-loading fluid injector  10  that may be modified for use with the above-described system including at least one syringe  12  and at least one syringe interface for self-oriented loading and releasable retaining of the at least one syringe  12  with the fluid injector  10  described herein with reference to  FIG.  1    are disclosed in U.S. Pat. No. 5,383,858 to Reilly et al. and U.S. patent application Ser. Nos. 14/526,294 and 14/526,395, both of which were filed on Oct. 28, 2014 which are incorporated by reference in their entirety. Another example of relevant multi-fluid delivery systems that may be modified for use with the present system are found in U.S. Pat. No. 7,553,294 to Lazzaro et al.; U.S. Pat. No. 7,666,169 to Cowan et al.; International Patent Publication No. WO 2012/155035; and United States Patent Application Publication No. 2014/0027009 to Riley et al.; the disclosures of which are incorporated herein by reference. Other aspects may include new fluid injector systems designed to include various aspects of the piston plunger interfaces described herein. 
     Having described the general structure and function of the injector  10 , the at least one syringe  12  will now be discussed in greater detail. With reference to  FIG.  2   , the syringe  12  generally has a cylindrical syringe barrel  18  formed from glass, metal, or a suitable medical-grade plastic. The barrel  18  has a proximal end  20  and a distal end  24 , with a sidewall  19  extending therebetween along a syringe longitudinal axis  15  extending through a center of the barrel  18 . The barrel  18  may be made from a transparent or translucent material and may include at least one fluid verification member  11  (shown in  FIG.  1   ) for verifying a presence of the fluid F within the syringe barrel  18 . A nozzle  22  extends from the distal end  24  of the barrel  18 . The barrel  18  has an outer surface  21  and an inner surface or wall  23  that defines an interior volume  25  for receiving the fluid therein. The proximal end  20  of the barrel  18  may be sealed with the plunger  26  that is slidable through the barrel  18 . The plunger  26  may have a plunger cover  58  that forms a liquid-tight seal against the inner surface  23  of sidewall  19  of the barrel  18  as the plunger  26  is advanced therethrough. Details of various aspects of the plunger body and plunger cover are set forth herein. 
     A drip flange  35  may extend radially outwardly from the outer surface  21  of the syringe barrel  18  relative to the longitudinal axis  15 . The drip flange  35  may extend around at least a portion of the outer circumference of the barrel  18 . The drip flange  35  may prevent fluid that drips from the nozzle  22  from entering the syringe port  16  on the injector  10 . In this manner, the drip flange  35  helps reduce the amount of fluid that may enter the syringe port  16  and jam or otherwise interfere with the connection interface and/or the interior mechanics and electronics of the injector  10 . In some aspects, the drip flange  35  defines a stop surface that delimits the depth at which an insertion section  30  of the syringe  12  may be inserted into the syringe port  16  (shown in  FIG.  1   ). The drip flange  35  may be formed integrally with the barrel  18  or it may be affixed or otherwise secured to the outer surface  21  of the barrel  18  using, for example, a frictional fit and/or an adhesive, welding, or by molding. In other aspects, the drip flange  35  may be formed on the outer surface  21  of the barrel  18  by etching, laser cutting, or machining. 
     With continued reference to  FIG.  2   , the proximal end  20  of the syringe  12  is sized and adapted for being removably inserted in the syringe port  16  of the injector  10  (shown in  FIG.  1   ). In some aspects, the proximal end  20  of the syringe  12  defines the insertion section  30  that is removably insertable into the syringe port  16  of the injector  10  while the remaining portion of the syringe  12  remains outside of the syringe port  16 . In certain aspects, the proximal end  20  of the syringe  12  includes one or more syringe retaining members (not shown) adapted to form a locking engagement with a corresponding locking mechanism in the syringe port  16  of the injector  10  for releasably retaining the syringe  12  in the syringe port  16 . Various retaining members for releasably locking the syringe  12  with the injector  10  are described in, for example, U.S. patent application Ser. No. 14/526,294, filed on Oct. 28, 2014 and entitled “Self-Orienting Syringe and Syringe Interface”, and U.S. patent application Ser. No. 14/526,395, filed on Oct. 28, 2014 and entitled “Self-Orienting Syringe and Syringe Interface”, the disclosures of which are incorporated herein by reference in their entirety. 
     Exemplary syringes suitable for use with the injector  10  depicted in  FIG.  1    and which can be adopted for use with a fluid verification system are described in U.S. Pat. No. 5,383,858 to Reilly et al., which is assigned to the assignee of the present application, the disclosure of which is incorporated by reference in its entirety. Additional exemplary syringes are disclosed in, for example, U.S. Pat. No. 6,322,535 to Hitchins et al. and U.S. Pat. No. 6,652,489 to Trocki et al., both of which are assigned to the assignee of the present application, and the disclosures of which are both incorporated by reference in their entireties. 
     With reference to  FIGS.  3 A- 3 B , the plunger  26  is shown in accordance with one aspect of the present disclosure. The barrel  18  of the syringe  12  is omitted from  FIGS.  3 A- 3 B  for clarity. The plunger  26  includes a plunger body  32  defining a plunger longitudinal axis  34  (shown in  FIG.  3 A ) and having a proximal end  36 , a distal end  38 , and a circumferential sidewall  39  connecting the proximal end  36  and the distal end  38 . The sidewall  39  may have a uniform or non-uniform thickness between the proximal end  36  and the distal end  38 . The sidewall  39  may have a continuous outer surface. In some aspects, the sidewall  39  may have a discontinuous outer surface having one or more portions of sidewall  39  separated by one or more voids or openings. The plunger body  32  may be formed from glass, metal, plastic, or other suitable material, including medical grade versions. 
     With continued reference to  FIGS.  3 A- 3 B , the plunger body  32  has an interior cavity  40  defined by a conical-shaped portion  42  at the distal end  38  of the plunger body  32  and a cylindrical-shaped portion  44  at the proximal end  36  of the plunger body  32 . The conical-shaped portion  42  may be monolithically formed with the cylindrical-shaped portion  44 . In some aspects, the conical-shaped portion  42  may be affixed or otherwise secured to the cylindrical-shaped portion  44  of the plunger body  32  using, for example, a frictional fit and/or an adhesive, welding, or by molding. The conical-shaped portion  42  may have a truncated end  46  that has a central opening  48 . In some aspects, the distal end  38  of the plunger body  32  may be partially or fully enclosed such that the plunger  26  does not have a central opening  48 . In other aspects, the distal end  38  of the plunger body  32  may have one or more radial openings, such as the openings  49  shown in  FIG.  20 B . 
     With reference to  FIG.  3 E , the plunger  26  may have a plunger cover  58  with a resilient seal  59  that covers at least a portion of an outer surface  60  of the plunger body  32 . The seal  59  may be a resilient and flexible seal that engages the inner surface  23  of the syringe barrel  18  (shown in  FIG.  2   ) such that the seal  59  seals the interior volume  25  of the syringe barrel  18  in a liquid-tight manner. The plunger cover  58  may be provided separately from the plunger body  32 , or it may be integrally formed with the plunger body  32 , such as by co-molding. In some aspects, the outer surface  60  of the plunger body  32  may have a circumferential groove  62 . At least a portion of the plunger cover  58  may be retained within the circumferential groove  62 . The exterior surface  64  of the seal  59  may have one or more lips, projections, or other sealing elements  66  that engage the inner surface  23  of the syringe barrel  18 . In some aspects, the one or more sealing elements  66  of the seal  59  may be made from an elastomeric material that resiliently engages the inner surface  23  of the syringe barrel  18  (shown in  FIG.  2   ). The at least one extension  56  on the plunger body  32  may prevent the seal  59  from coming out of axial engagement with the syringe  12  as the plunger  26  is moved through the syringe barrel  18 . 
     Referring again to  FIGS.  3 A- 3 B , the plunger  26  may have at least one resiliently deflectable retaining member  68  (hereinafter “retaining member  68 ”) protruding from the plunger body  32 . In some aspects, the at least one retaining member  68  may protrude in a direction from the proximal end  36  toward the distal end  38  of the plunger body  32 . In some aspects, the at least one retaining member  68  may protrude distally and radially inward from an inner surface  52  of the plunger body  32  into the interior cavity  40 . In various aspects, the at least one retaining member  68  may be formed as a cantilever spring element, a coil spring element, or an elastomeric portion of the plunger body  32 . 
     With reference to  FIG.  3 E , the at least one retaining member  68  has a first segment or a first end  70  connected to the plunger body  32  and a second segment or a second end  72  protruding distally from the first end  70 . The second end  72  may deflect or twist relative to the first end  70 . As described herein, the second end  72  may be radially deflectable relative to the first end  70  when the at least one retaining member  68  engages a piston of the fluid injector  10 . In some aspects, the retaining member  68  may be radially deflected outwards, away from the longitudinal axis  34  when deflected by a portion of a piston during the engagement process. The resiliently deflectable retaining member  68  may then move radially inwards under the restoring force of the retaining member  68  to engage a portion of the piston to releasably lock the plunger body  32  with the piston. In some aspects where the retaining member  68  is circumferential with the plunger body  32  (described herein), the second end  72  may be circumferentially deflectable relative to the first end  70 . The first end  70  and the second end  72  may be spaced apart in a direction that extends substantially along a direction of the plunger longitudinal axis  34  of the plunger  26 . The at least one retaining member  68  may be linearly, stepwise, or curvilinearly contiguous between the first end  70  and the second end  72 . In some aspects, one or more retaining members  68  may generally extend in a direction parallel to a direction of the plunger longitudinal axis  34 . In other aspects, one or more retaining members  68  may extend in a direction that is angled relative to the direction of the plunger longitudinal axis  34 . For example, one or more retaining members  68  may be angled at an angle A toward or away from the plunger longitudinal axis  34  from the inner surface  52  of the plunger body  32 . The inner surface  52  of the plunger body  32  may have one or more pockets  69  that are recessed in a radially outward direction into the sidewall  39  to allow for an increased deflection of the second end  72  relative to the first end  70  of the at least one retaining member  68 . 
     With reference to  FIG.  3 A , the at least one retaining member  68  may include a plurality of retaining members  68  spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . The retaining members  68  may be separated from each other by portions of the inner surface  52  of the interior cavity  40 . In aspects where two or more retaining members  68  are provided, the retaining members  68  may be evenly spaced apart from each other. In one exemplary and non-limiting aspect with three retaining members  68  having equal angular center-to-center separation therebetween, such as shown in  FIG.  3 C , each retaining member  68  is separated by 120 degrees from the retaining members  68  adjacent on either side. In another exemplary and non-limiting aspect with six retaining members  68  having equal angular separation therebetween, each retaining member  68  is separated by 60 degrees from the retaining members  68  adjacent on either side. In some aspects, the retaining members  68  may have unequal angular extension and/or unequal angular spacing between the retaining members  68  about the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one retaining member  68  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston, as described herein. In another exemplary and non-limiting aspect with two or more retaining members  68 , each retaining member  68  has a center to center separation from the retaining members  68  adjacent on either side of an integer number of 60 degrees and furthermore each retaining member  68  has an angular extent less than 60 degrees, for example 15 to 45 degrees. This enables the release of the plunger  26  from the piston  88  with a relative rotation of 15 to 45 degrees. Optionally the relative rotation is less than 30 degrees. 
     With reference to  FIG.  3 E , the second end  72  of the retaining member  68  has at least one catch  74 . The at least one catch  74  may be a terminal surface of the second end  72  of the retaining member  68 . As described herein, the at least one catch  74  is shaped to be received within at least a portion of a recess, lip, or ledge on the piston to releasably lock the at least one retaining member  68 , along with the plunger  26 , to the piston with respect to motion in at least one direction. In some aspects, the at least one catch  74  may be linear or curvilinear to fit within a recess, lip, or ledge on the piston to releasably lock the at least one retaining member  68  and resist disconnecting from the piston  88  during reciprocal movement of the plunger  26  through the barrel  18  of the syringe  12  (shown in  FIG.  2   ). In some aspects, the at least one catch  74  may be oriented in a direction substantially perpendicular to a direction of the plunger longitudinal axis  34 . In other aspects, the at least one catch  74  may be angled relative to a direction of the plunger longitudinal axis  34 . The at least one catch  74  may be continuous or discontinuous. In some aspects, the at least one catch  74  may protrude radially inward or outward relative to a body of the retaining member  68 . The at least one catch  74  may be formed integrally with the second end  72  of the at least one retaining member  68  or it may be co-molded, affixed, or otherwise secured to the second end  72  of the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. In other aspects, the at least one catch  74  may be formed on the second end  72  of the at least one retaining member  68  by etching, laser cutting, or machining. 
     At least one retaining member  68  may have at least one actuation member associated therewith. In various aspects, the at least one actuation member on the retaining member  68  of the plunger  26  is configured to interact with a corresponding actuation surface, such as an actuation surface on the piston  88  (shown in  FIG.  4 A ), when the plunger  26  is connected to the piston  88 . The at least one actuation member may interact with the corresponding actuation surface on the piston  88  to cause the at least one retaining member  68  to be released from the piston  88 , such as when it is desired to disconnect the plunger  26  from the piston  88 . Relative rotation between the piston  88  and the plunger  26  about the plunger longitudinal axis  34  causes at least a portion of the actuation member to engage at least a portion of the piston  88 , such as the corresponding actuation surface on the piston  88 . For example, the piston may be substantially fixed against rotation in the direction of the plunger  26 , while the plunger  26  is rotated about the plunger longitudinal axis  34 , by frictional contact with an inner wall of a syringe during rotation of the syringe, to cause the actuation member on the plunger  26  to engage, through rotational contact, the corresponding actuation surface on the piston  88 . Alternatively, the plunger  26  may be fixed, while the piston  88  is rotated about the piston longitudinal axis  115  (shown in  FIG.  4 A ) to cause the actuation surface on the plunger  26  to engage, through rotational contact, the corresponding actuation surface on the piston  88 . As a further alternative, the piston  88  and the plunger  26  may be rotated in opposite directions relative to one another about their respective longitudinal axes, or the piston  88  and the plunger  26  may be rotated in a same direction, such as clockwise or counter-clockwise direction, about their respective longitudinal axes at different rotation rates. The engagement between the actuation member on the plunger  26  and the corresponding actuation surface on the piston may be through sliding contact occurring when the rotational position of the actuation member on the plunger  26  is rotationally aligned with the rotational position of the corresponding actuation surface on the piston  88 . In some aspects, a sliding contact between the actuation member on the plunger  26  and the corresponding actuation surface on the piston  88  during relative rotational movement between the piston  88  and the plunger  26  causes a movement of the at least one retaining member  68  to a disengaged position to release the plunger  26  from the piston  88 . 
     With reference to  FIGS.  3 A- 3 B , the plunger  26  may have at least one actuation member, such as at least one first cam member  78 . In some aspects, the first cam member  78  may be provided directly on the retaining member  68 , or it may be provided on a portion of the plunger body  32  or an extension therefrom such that movement of the cam member  78  causes a corresponding disengagement of the retaining member  68 . In some aspects, the at least one first cam member  78  may be provided between the first end  70  and the second end  72  of the retaining member  68 . The at least one first cam member  78  interacts with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston, as described herein. In some aspects, the position of the at least one first cam member  78  may be selected to allow for an increased radial deflection of the at least one first cam member  78  upon relative rotation between the plunger  26  and the piston. In such aspects, the at least one first cam member  78  may be provided closer to the second end  72  of the retaining member  68 . 
     In some aspects, the at least one retaining member  68  may have the catch  74  formed thereon, while the actuation member, such as the cam member  78 , may be formed as a separate component that interacts with the at least one retaining member  68 . The actuation member, for example, the cam member  78  used to disengage the at least one retaining member  68 , may be a second, separate member attached to the plunger body  32 . Alternatively, the actuation member may be formed on a second separate member which is attached to the plunger body  32  and interacts with the piston  88  and the at least one retaining member  68  to transmit force, motion, or displacement from the piston  88  to the at least one retaining member  68  to deflect it for disengagement from the plunger  26 . 
     In some aspects, the at least one first cam member  78  may protrude at an angle relative to a plane defined by a body of the retaining member  68 . With reference to  FIG.  3 C , the at least one first cam member  78  may be angled at an angle B relative to the plane defined by the body of the retaining member  68 . In various aspects, the at least one first cam member  78  may be formed as a sidewall, lip, extension, or protrusion that is associated with the body of the retaining member  68 . The at least one first cam member  78  may be planar, curved, or a combination thereof. The at least one first cam member  78  may be formed as a continuous surface or a discontinuous surface formed from two or more separate surfaces that together form the at least one first cam member  78 . The at least one first cam member  78  may have an angled engagement surface  83  that interacts with the piston to disengage the plunger  26  from the piston, as described herein. The position of the at least one first cam member  78  between the first end  70  and the second end  72  of the retaining member  68  minimizes the radial protrusion of the at least one first cam member  78  while still allowing a full radial deflection of the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston  88  to allow disengagement of the plunger  26  from the piston  88 , as described herein. In some aspects, the at least one first cam member  78  may be provided on at least a portion of the at least one catch  74 . A plurality of first cam members  78  may be axially spaced apart along a length of the retaining member  68  between the first end  70  and the second end  72 . The at least one first cam member  78  may be formed integrally with the at least one retaining member  68  or it may be affixed or otherwise secured to the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. In other aspects, the at least one first cam member  78  may be formed on the at least one retaining member  68  by etching, laser cutting, or machining. 
     With reference to  FIG.  3 A , according to certain aspects, the plunger  26  may have at least one first alignment member  71  protruding radially inward from the plunger body  32 . In some aspects, the at least one first alignment member  71  may protrude in a direction from the distal end  38  toward the proximal end  36  of the plunger body  32 . In some aspects, the at least one first alignment member  71  may protrude proximally from the inner surface  52  of the interior cavity  40  of the plunger body  32 . 
     With reference to  FIG.  3 E , the at least one first alignment member  71  has a first end  73  connected to the plunger body  32  and a second end  75  protruding radially inward from the first end  73 . The at least one first alignment member  71  is shaped and/or configured for facilitating self-orienting alignment of the piston  88  with the plunger  26 . In some aspects, at least a portion of the at least one first alignment member  71  may extend in a direction that is angled relative to the direction of the plunger longitudinal axis  34 . For example, at least one first alignment member  71  may have a proximal alignment surface  77   a  that is angled at an angle C relative to the longitudinal axis  34  to facilitate positioning of the retaining member  68  during connection of the plunger  26  to a piston. The at least one first alignment member  71  may have a distal alignment surface  77   b  that is angled in a direction opposite to the proximal alignment surface  77   a  to facilitate positioning of the retaining member  68  when the plunger  26  is being disconnected from the piston. The proximal alignment surface  77   a  helps guide the piston  88  into self-orienting alignment with the plunger  26 , as described herein, for example by rotation of a portion of the piston  88  relative to the plunger  26  utilizing a one-way clutch associated with the piston  88 . 
     With reference to  FIG.  3 C , a plurality of first alignment members  71  may be spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . In some aspects, the number of first alignment members  71  may be equal or unequal to the number of retaining members  68 . When equal in number, the first alignment members  71  may be disposed between the retaining members  68  such that each first alignment member  71  has a retaining member  68  on either side of the first alignment member  71 . The first alignment members  71  may be separated from each other by portions of the inner surface  52  of the interior cavity  40 . In aspects where two or more first alignment members  71  are provided, the first alignment members  71  may be evenly spaced apart from each other. In one exemplary and non-limiting aspect with three first alignment members  71  having equal angular separation therebetween, such as shown in  FIG.  3 C , each first alignment member  71  is separated by 120 degrees from the first alignment members  71  adjacent on either side. In another exemplary and non-limiting aspect with six first alignment members  71  having approximately equal angular center-to-center separation therebetween, each first alignment member  71  is separated by 60 degrees from the first alignment members  71  adjacent on either side. In some aspects, the first alignment members  71  may have unequal angular extension and/or unequal angular spacing between adjacent first alignment members  71  about the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one first alignment member  71  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston  88  to allow for alignment of the plunger  26  with the piston  88 , as described herein. The at least one first alignment member  71  may be constituted from or composed of multiple members, optionally separately attached to the inner surface  52  of the plunger body  32 , whose alignment surfaces, both real and virtual, interact to perform the alignment function. Similarly, an at least one retaining member  68  may be constituted from or composed of multiple members, optionally separately attached to the inner surface  52  of the plunger body  32 , whose catches  74  interact to perform the engagement, connection, attachment, or retention functions. Similarly, an at least one actuation member may be constituted from or composed of multiple members, optionally separately attached to the inner surface  52  of the plunger body  32 , which interact to provide the activation of the at least one retaining member  68  to perform the disengagement, disconnect, separation, or release function. Optionally, the at least one alignment member  71 , the at least one retaining member  68 , and/or the at least one actuation member may be attached to each other and interact or transmit force directly from one to another, or optionally may be separate and interact with each other through surfaces between them that transmit force, motion, or deflection from one to another, directly or indirectly, or through their action on the plunger body  32 . Similarly, the force, motion, or deflection for activation to accomplish release of a retaining member  68  may occur through the movement of or force transmitted from a second retaining member  68  or intermediate linkage or mechanism. 
     Referring to  FIG.  4 A , a piston  88  (shown in FIG. 4 A as only the distal portion of piston  88 ) is configured to interact with the plunger  26  (shown in  FIG.  3 A ) to releasably lock the plunger  26  such that the plunger  26  can be driven reciprocally within the barrel of the syringe  12  (shown in  FIG.  2   ). The piston  88  is extendible and retractable from the housing  14  of the fluid injector  10  (shown in  FIG.  1   ) via a powered means (not shown) preferably contained within housing  14 . The powered means may include, for example, an electric motor, a hydraulic system, or a pneumatic system, including appropriate gearing (not shown). As known in the art, the fluid injector  10  also may include a controller (not shown) for controlling operation of the powered means and thereby controlling operation of the piston  88 . 
     With continued reference to  FIG.  4 A , the piston  88  includes a stem  90  connected to the proximal portion of the piston  88  within the injector  10  (shown in  FIG.  10 A ) and a piston head  92  formed on a distal end of the stem  90 . At least a portion of the piston head  92  extends distally from the stem  90 . The piston  88  is constructed from a rigid material, such as metal or plastic that resists deformation. The stem  90  may have a cavity  91  for collecting any fluid that may drip from the syringe. The piston head  92  has a substantially cylindrical structure with a pointed distal end  94  with a cap  95  that is shaped to be received inside at least a portion of the interior cavity  40  (shown in  FIG.  3 A ) of the plunger  26 . In some aspects, a sensing member  79 , such as a pin connected to a sensor, may be provided. The sensing member  79  may extend along a longitudinal axis of the piston  88  and may protrude through at least a portion of the piston head  92 , such as through at least a portion of the cap  95 . The sensing member  79  may be operative for sensing contact with a surface, such as a surface of the plunger  26  and/or the plunger cover  58 , and control a movement of the piston  88  based on the sensed condition. For example, an initial contact between the sensing member  79  and the plunger  26  and/or the plunger cover  58  may cause the pin to be moved in a proximal direction such that it makes contact with the sensor. The sensing member  79  may be biased in an extended position by a resilient element  81  (shown in  FIG.  4 B ), such as a spring. The sensor may be connected to the controller of the injector such that, upon activation of the sensor by the pin, the controller controls the movement of the drive mechanism. For example, the drive mechanism may be stopped or slowed from a first rate to a second, slower rate. 
     The piston head  92  may be rotatable relative to the stem  90 . In some aspects, the piston head  92  may be rotatable in one direction only, such as a clockwise or a counter-clockwise direction, relative to the stem  90 . A one-way rotation mechanism  99  (shown in  FIG.  4 B ), such as a one-way clutch mechanism, may be provided to allow the rotation of the piston head  92  in a first direction only, such as the clockwise or the counter-clockwise direction. The one-way rotation mechanism  99  may be rotatable around a central shaft  101  having a seal  102 , such as an O-ring seal. In some aspects, the one-way rotation mechanism  99  may have a stop that prevents rotation of the piston head  92  in a second direction opposite the first direction, such as the counter-clockwise or the clockwise direction, respectively. In other aspects, the one-way rotation mechanism  99  may be provided on at least a portion of the plunger  26 . 
     With reference to  FIG.  4 C , the piston head  92  has a proximal portion  103  connected to a distal portion  105 . Terminal ends of the proximal and distal portions  103 ,  105  may have a radiused or chamfered edge  107 . At least a portion of the proximal portion  103  has a smaller outer diameter compared to an outer diameter of the distal portion  105  such that a radial lip  109  is formed at a transition between the proximal portion  103  and the distal portion  105 . The radial lip  109  may be continuous or discontinuous around a circumference of the piston head  92 . In some aspects, the radial lip  109  defines a locking ledge  111  for engaging the catch  74  (shown in  FIG.  5 D ) of the at least one retaining member  68  when the plunger  26  is fully seated on the piston head  92 . At least a portion of the retaining member  68  shown in  FIG.  5 D  is shown as being hidden behind the piston head  92  due to the rotational position of the plunger  26  relative to the piston head  92  about the piston longitudinal axis  115 . 
     With continued reference to  FIG.  4 C , the piston head  92  may have at least one second alignment member  113  protruding radially outward from an outer surface of the piston head  92 . The at least one second alignment member  113  may be shaped and/or configured for interacting with the first alignment member  71  (shown in  FIG.  5 B ) of the plunger  26  for facilitating alignment of the piston  88  with the plunger  26  in order to allow for a releasable engagement. At least one second alignment member  113  may extend in a direction that is angled relative to the direction of a piston longitudinal axis  115 . For example, at least one second alignment member  113  may have a guiding surface  117  that is angled at an angle D relative to the piston longitudinal axis  115 . The guiding surface  117  is desirably angled such that the piston head  92  may rotate around the piston stem  90 , for example around the axis of the one-way rotation mechanism  99 , when the proximal alignment surface  77   a  of the first alignment member  71  contacts the guiding surface  117  of the second alignment member  113 . 
     In some aspects, a plurality of second alignment members  113  may be spaced apart radially relative to the piston longitudinal axis  115  along an outer circumference of the piston head  92 . In some aspects, the number of second alignment members  113  may be equal to a total number of retaining members  68  and first alignment members  71  on the plunger  26 . The second alignment members  113  are spaced apart circumferentially such that a retaining member  68  or a first alignment member  71  may be received between adjacent second alignment members  113 . The second alignment members  113  may be separated from each other by portions of an outer surface of the proximal portion  103  and/or the distal portion  105  of the piston head  92 . In aspects where two or more second alignment members  113  are provided, the second alignment members  113  may be evenly spaced apart from each other. In one exemplary and non-limiting aspect with six second alignment members  113  having equal angular separation therebetween, such as shown in  FIG.  4 A , each second alignment member  113  is separated by 60 degrees from the second alignment members  113  adjacent on either side. In some aspects, the second alignment members  113  may have unequal angular extension and/or unequal angular spacing between the second alignment members  113  about the outer surface of the proximal portion  103  and/or the distal portion  105  of the piston head  92 . The radial spacing of the second alignment members  113  relative to the piston longitudinal axis  115  is selected to correspond to or operably interact with an inner shape of the plunger  26  to allow the one or more retaining members  68  to be received into the locking ledge  111 . Locking ledge  111  may be radially perpendicular to the longitudinal axis  115  or in certain aspects locking ledge  111  may be radially angled relative to the longitudinal axis  115 . In certain aspects, locking ledge  111  may be angled with an angle complementary to an angle of the catch  74 , such as the angle of the catch  74  shown in  FIG.  3 E . 
     With reference to  FIG.  4 C , each of the guiding surfaces  117  of the second alignment members  113  define a travel path for guiding the movement of the proximal alignment surface  77   a  of the first alignment member  71  in and out of a recess  119  defined between adjacent second alignment members  113 . The guiding surfaces  117  may be inclined or angled radially and axially relative to the piston longitudinal axis  115  to guide the movement of the proximal alignment surfaces  77   a . The guiding surfaces  117  aid in self-orienting the piston head  92  as the plunger  26  is brought into contact with the piston  88  by guiding the one or more proximal alignment surfaces  77   a  on the plunger  26  into the corresponding recess  119  on the piston head  92 . In this manner, a piston  88  whose piston longitudinal axis  115  is rotationally misaligned with the plunger longitudinal axis  34  and the one or more first alignment members  71  which are initially misaligned relative to the corresponding one or more second alignment members  113  is rotated in a rotational direction of the one-way rotation mechanism  99  and brought in alignment axially and rotationally such that the one or more first alignment members  71  are received within the recess  119  between adjacent second alignment members  113 . In this manner, the one or more retaining members  68  are brought into rotational alignment with the recess  119  having the locking ledge  111  adjacent the recess  119 . 
     The one or more second alignment members  113  may have a bottom surface  121  that is angled relative to the direction of a piston longitudinal axis  115 . For example, the bottom surface  121  may be angled at an angle E relative to the piston longitudinal axis  115 . Angle E may be the same or different than angle D of the guiding surface  117 . In another aspect, bottom surface  121  may be rounded or angled to merge with the proximal wall or proximal end  103 . 
     The piston head  92  further has an actuation surface that interacts with the actuation member on the plunger  26 , such as the first cam member  78 . In some aspects, the actuation surface on the piston head  92  may be a second cam member  98  or a cam following surface. In some aspects, the actuation surface or second cam member  98  cooperates with the first cam member  78  on the at least one retaining member  68  of the plunger  26 , as described herein. The actuation surface or second cam member  98  desirably has a shape that, upon relative rotation between the piston  88  and the plunger  26 , the actuation surface or second cam member  98  engages the first cam member  78  to cause the at least one retaining member  68  to be deflected from the piston head  92  to disengage catch  74  of retaining member  68  from the locking ledge  111  such that the plunger  26  disengages and can be removed from the piston  88 . In some aspects, the actuation surface or second cam member  98  may be formed on the second alignment member  113  on the piston head  92 . The actuation surface or second cam member  98  may be a surface that is aligned with a direction of the piston longitudinal axis  115 . In certain aspects, the actuation surface or second cam member  98  may have a chamfered portion  98   a  to facilitate the initial movement and/or the passing of the first cam member  78  after the retaining member  68  is deflected sufficiently to allow the retaining member to be released. In other aspects, the actuation surface  98  may be radially aligned and parallel to longitudinal axis  115 . 
     The piston  88  is configured to interact with the plunger  26  to releasably lock with plunger  26 , such as shown in  FIG.  3 A . By locking the piston  88  to the plunger  26 , the plunger  26  can be driven reciprocally within the barrel of the syringe  12  (shown in  FIG.  2   ). The actuation surface or second cam member  98  on the piston  88  cooperates with the first cam member  78  on the at least one retaining member  68  of the plunger  26 , to releasably lock the plunger  26  to the piston  88 . Further, due to the distal arrangement and in certain aspects, the radially inwardly angled nature, of the at least one retaining member  68 , the locking force between the locking ledge  111  of piston  88  and the catch  74  of the retaining member  68  is increased due to the compressive force as the piston  88  withdraws the plunger  26  in the proximal direction during a syringe filling process. The locking or engagement of the plunger  26  to the piston  88 , and the unlocking or disengagement of the plunger  26  from the piston  88  will be described herein with reference to  FIGS.  5 A- 5 C . The syringe  12 , shown initially in phantom in  FIG.  5 A  is omitted from the remainder of  FIGS.  5 B- 5 D  for clarity. 
     To engage the plunger  26  with the piston  88 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10  (shown in  FIG.  1   ). Once or while the syringe  12  is inserted into the syringe port  16 , various locking mechanisms (not shown) may be used to retain the syringe  12  within the syringe port  16  to prevent detachment of the syringe  12  from the syringe port  16 . Initially, the plunger  26  may be positioned at the proximal end  20  of the syringe barrel  18 . In some aspects, the plunger  26  is positioned at any axial location between the proximal end  20  and the distal end  24  of the syringe barrel  18 . The piston  88  may then be advanced distally toward the plunger  26  for engagement of the piston head  92  with the plunger  26 . In some aspects, the piston  88  may be advanced distally toward the plunger  26  by way of the powered means operated by a controller. In other aspects, the piston  88  may be advanced distally toward the plunger  26  by manual operation. 
     With reference to  FIG.  5 B , the piston  88  is advanced axially in a distal direction shown by the arrow A. If the piston  88  is rotationally misaligned relative to the plunger  26  about the longitudinal axis  115  such that the first alignment members  71  on the plunger  26  are not in rotational alignment to be received within the recesses  119  (shown in  FIG.  4 C ) on the plunger head  92 , the proximal alignment surface  77   a  of the first alignment member  71  on the plunger  26  contacts the guiding surface  117  of the second alignment member  113  on the piston head  92 . The proximal alignment surface  77   a  and the guiding surface  117  are angled in a same direction relative to the longitudinal axes  34 ,  115  such that continued movement of the piston  88  in a distal direction causes the proximal alignment surface  77   a  to engage the guiding surface  117 . Engagement of the proximal alignment surface  77   a  with the guiding surface  117  causes the piston head  92  to automatically rotate in a free rotation direction of the one-way rotation mechanism  99 . Such rotation of the piston head  92  aligns the first alignment members  71  and the retaining members  68  to be received within the recesses  119  between adjacent second alignment members  113 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . If the piston  88  is rotationally aligned relative to the plunger  26  such that the first alignment members  71  on the plunger  26  are in rotational alignment with the second alignment members  113  on the piston head  92 , the first alignment members  71  and the retaining members  68  on the plunger  26  can be received within the recesses  119  between adjacent second alignment members  113  without rotation of the piston head  92 . 
     With reference to  FIG.  5 C , after aligning the first alignment members  71  and the retaining members  68  to be received within the recesses  119  between adjacent second alignment members  113 , the piston  88  is advanced further in the distal direction. Such movement of the piston  88  in the distal direction, causes the retaining members  68  to initially engage an outer sidewall of the distal portion  105  of the piston head  92 . Continued distal movement of the piston  88  causes the retaining members  68  to deflect radially outward relative to the plunger longitudinal axis  34  from a first, undeflected position, to a second, radially deflected position. The piston  88  is advanced distally until the terminal portion of the second end  72  of each retaining member  68  clears the radial lip  109 . The retaining members  68  then deflect radially inward toward or to their initial undeflected position, for example due to the restoring force built up in the retaining members  68  during radial deflection. As shown in  FIG.  5 D , the catch  74  of at least one retaining member  68  is retained within the locking ledge  111  to prevent disengagement of the plunger  26  from the piston head  92 . Distal movement of the piston  88  may be stopped when the sensing member  79  engages at least a portion of the plunger  26 , such as the plunger cover  58  (shown in  FIG.  3 E ). The plunger  26  resists being disconnected from the piston  88  upon movement of piston  88  in a distal and proximal direction relative to the syringe barrel  18 . In one aspect, the retaining members  68  may be designed such that the compressive forces exerted upon the catch  74  upon movement of piston head  92  in the proximal direction substantially prevent radially outward deflection (or bending) of the retaining members  68 . For example, once the retaining members  68  are locked to the piston head  92 , axial movement of the piston  88  does not introduce a bending moment sufficient to deflect the retaining members  68  radially to cause the plunger  26  to be disconnected from the piston  88 . Proximal movement of the piston  88  causes the at least one retaining member  68  to be loaded in compression between the first end  70  and the second end  72  such that the retaining member  68  may be urged in a radially inward direction, thereby increasing the locking force between the plunger  26  and the piston  88 . 
     To unlock the syringe  12  from the syringe port  16  (shown in  FIG.  1   ) and disengage the plunger  26  from the piston  88 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Because the plunger  26  is substantially free from rotation within the syringe barrel  18  due to a frictional force of the seal  59  with the inner surface  23  of the syringe sidewall  19  (shown in  FIG.  2   ), the rotation of the syringe  12  also causes the plunger  26  to rotate relative to the piston  88 . The free-rotation direction of the one-way rotation mechanism  99  is desirably opposite to the rotation direction of the syringe  12  during the release of the syringe  12  from the syringe port  16 . Rotation of the syringe  12 , and thereby the plunger  26 , about the plunger longitudinal axis  34  engages the first cam member  78  on the plunger  26  with the actuation surface or second cam member  98  on the piston head  92 . Such movement causes a radial deflection of the at least one retaining member  68  away from the piston head  92 . 
     As the at least one retaining member  68  is deflected radially outward relative to the plunger longitudinal axis  34 , the catch  74  is moved out of engagement with the locking ledge  111 . In this position, the at least one retaining member  68  is in a deflected state that allows the plunger  26  to be moved axially relative to the piston  88 . Such axial movement of the plunger  26  can be affected by withdrawing the syringe  12  from the syringe port  16  in a distal direction along the syringe longitudinal axis  15 , by withdrawing the piston  88  in a proximal direction away from the plunger  26 , or both. The plunger  26 , together with the syringe  12 , can then be completely disengaged from the piston  88  and the injector  10 . In some aspects, the piston  88  can be released from the plunger  26  by rotating the piston  88  about its longitudinal axis and retracting the piston  88  in a proximal direction to disengage the at least one retaining member  68  in a manner described herein. 
     With reference to  FIGS.  6 A- 6 B , a plunger  26  and a piston  88  are shown in accordance with another aspect of the present disclosure. The components of the plunger  26  shown in  FIGS.  6 A- 6 B  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  3 A- 3 C . As the previous discussion regarding the plunger  26  generally shown in  FIGS.  3 A- 3 C  is applicable to the aspect of the present disclosure shown in  FIGS.  6 A- 6 B , only the relative differences between the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  and the plunger  26  and piston  88  shown in  FIGS.  6 A- 6 B  are discussed hereinafter. 
     In one exemplary and non-limiting aspect with  6  locking ledges  111  having equal angular separation therebetween, to mate with the plunger as shown in  FIG.  3 C , each locking ledge  111  is separated by 60 degrees center to center from the locking ledge  111  adjacent on either side. In some aspects, the locking ledge  111  may have unequal angular extension and/or unequal angular spacing between the locking ledge  111  about the outer surface of the piston  88 . In another exemplary and non-limiting aspect with two or more locking ledges  111 , each retaining locking ledge  111  has a center to center separation from the locking ledge  111  adjacent on either side of an integer number of 60 degrees and furthermore each locking ledge  111  has an angular extent less than 60 degrees, for example 15 to 45 degrees. This enables the release of the plunger  26  from the piston  88  with a relative rotation of 15 to 45 degrees. Optionally the relative rotation is less than 30 degrees. The radial spacing of the at least one locking ledge  111  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with radial spacing of the at least one plunger retaining member  68 , as described herein. 
     With reference to  FIGS.  6 A- 6 B , a plunger  260  is shown in accordance with another aspect of the present disclosure. The barrel  18  of the syringe  12  is omitted from  FIGS.  6 A- 6 B  for clarity. The plunger  260  includes a plunger body  320  defining a plunger longitudinal axis  340  and having a proximal end  360 , a distal end  380 , and a circumferential sidewall  390  connecting the proximal end  360  and the distal end  380 . The sidewall  390  may have a uniform or non-uniform thickness between the proximal end  360  and the distal end  380 . The plunger body  320  may be formed from glass, metal, or a suitable medical-grade plastic. 
     With continued reference to  FIGS.  6 A- 6 B , the plunger body  320  has an interior cavity  400  with a conical-shaped portion  420  at the distal end  380  of the plunger body  320  and a cylindrical-shaped portion  440  at the proximal end  360  of the plunger body  320 . The conical-shaped portion  420  may be monolithically formed with the cylindrical-shaped portion  440 . In some aspects, the conical-shaped portion  420  may be affixed or otherwise secured to the cylindrical-shaped portion  440  of the plunger body  320  using, for example, a frictional fit and/or an adhesive, welding, or by molding. The conical-shaped portion  420  may have a truncated end  460  that has a central opening  480 . In some aspects, the distal end  380  of the plunger body  320  may be enclosed. In some aspects, the plunger  260  may have a plunger cover, such as the plunger cover  58  shown in  FIG.  2   , configured for covering at least a portion of an outer surface of the plunger body  320 . 
     With continued reference to  FIGS.  6 A- 6 B , the plunger  260  may have at least one resiliently deflectable retaining member  680  (hereinafter “retaining member  680 ”) protruding from the plunger body  320  in a distal direction. In some aspects, the at least one retaining member  680  may protrude distally and radially inward from an inner surface  520  of the interior cavity  400  of the plunger body  320 . The at least one retaining member  680  has a first segment or a first end  700  connected to the plunger body  320  and a second segment or a second end  720  radially deflectable relative to the first end  700 . As described herein, the second end  720  may be radially deflectable relative to the first end  700  when the at least one retaining member  680  engages a piston of the fluid injector  10  (shown in  FIG.  1   ). The first end  700  and the second end  720  may be spaced apart in a direction that extends substantially along a direction of the plunger longitudinal axis  340  of the plunger  260 . The at least one retaining member  680  may be linearly or curvilinearly contiguous between the first end  700  and the second end  720 . 
     In some aspects, a plurality of retaining members  680  are spaced apart radially from the plunger longitudinal axis  340  along a circumference of the inner surface  520  of the interior cavity  400 . In such aspects, the retaining members  680  are separated from each other by portions of the inner surface  520  of the interior cavity  400 . In aspects where more than one retaining member  680  is provided, the retaining members  680  may be evenly spaced apart from each other. In one exemplary and non-limiting aspect with three retaining members  680  having equal angular separation therebetween, a center of each retaining member  680  is separated by 120 degrees from a center of the retaining members  680  adjacent on either side. In another exemplary and non-limiting aspect with six retaining members  680  having equal angular separation therebetween, a center of each retaining member  680  is separated by 60 degrees from a center of the retaining members  680  adjacent on either side. In some aspects, the retaining members  680  may have unequal angular extension and/or unequal angular spacing between the retaining members  680  about the inner surface  520  of the interior cavity  400 . The radial spacing of the at least one retaining member  680  relative to the plunger longitudinal axis  340  is selected to correspond to features on an outer circumference of the piston, as described herein. 
     In some aspects, one or more retaining members  680  may be parallel with the longitudinal axis  340 . In other aspects, one or more retaining members  680  may be angled relative to the longitudinal axis  340 . For example, one or more retaining members  680  may be angled inward toward the longitudinal axis  340  in a direction from the first end  700  toward the second end  720 . 
     With continued reference to  FIGS.  6 A- 6 B , the second end  720  of the retaining member  680  has at least one catch  740 . The at least one catch  740  may be a terminal surface of the second end  720  of the retaining member  680 . In some aspects, the at least one catch  740  may protrude radially from the retaining member  680 . For example, the at least one catch  740  may protrude radially inward toward the plunger longitudinal axis  340  of the plunger body  320 , or radially outward away from the plunger longitudinal axis  340 . As described herein, the at least one catch  740  is shaped to engage at least a portion of a recess on the piston to releasably lock the at least one retaining member  680  relative to the piston. The at least one catch  740  may be formed integrally with the second end  720  of the at least one retaining member  680  or it may be affixed or otherwise secured to the second end  720  of the at least one retaining member  680  using, for example, a frictional fit and/or an adhesive, welding, or by molding. In other aspects, the at least one catch  740  may be formed on the second end  720  of the at least one retaining member  680  by etching, laser cutting, or machining. 
     With continued reference to  FIGS.  6 A- 6 B , the plunger  260  may have at least one first cam member  780  disposed between the first end  700  and the second end  720  of the retaining member  680 . The at least one first cam member  780  is configured to interact with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  680  upon rotation of the plunger  260  relative to the piston, as described herein. The position of the at least one first cam member  780  between the first end  700  and the second end  720  of the retaining member  680  allows for a greater radial deflection of the at least one first cam member  780  upon relative rotation between the plunger  260  and the piston  880  (shown in  FIGS.  7 A- 7 B ) compared to providing the at least one first cam member  780  at the second end  720 . The at least one first cam member  780  may be parallel with a surface of the retaining member  680 . In some aspects, the at least one first cam member  780  may be angled relative to a surface of the retaining member  680 . 
     In some aspects, the at least one first cam member  780  protrudes radially inward toward the plunger longitudinal axis  340  of the plunger body  320 . In other aspects, the at least one first cam member  780  protrudes radially outward relative to the plunger longitudinal axis  340  of the plunger body  320 . The position of the at least one first cam member  780  between the first end  700  and the second end  720  of the retaining member  680  may minimize the radial protrusion of the at least one first cam member  780  while still allowing a full radial deflection of the at least one retaining member  680  upon rotation of the plunger  260  relative to the piston  880 , as described herein. In some aspects, the at least one first cam member  780  may be provided on at least a portion of the at least one catch  740 . A plurality of first cam members  780  may be axially spaced apart along a length of the retaining member  680  between the first end  700  and the second end  720 . The at least one first cam member  780  may be formed integrally with the at least one retaining member  680  or it may be affixed or otherwise secured to the at least one retaining member  680  using, for example, a frictional fit and/or an adhesive, welding, or by molding. In other aspects, the at least one first cam member  780  may be formed on the at least one retaining member  680  by etching, laser cutting, or machining. 
     With reference to  FIG.  6 A , the at least one first cam member  780  may have at least one tooth  800  configured to engage a corresponding groove on the piston. The at least one tooth  800  on the at least one first cam member  780  is desirably shaped to generally correspond to the corresponding groove on the piston. Each tooth  800  may have a peak  820  leading to a groove  840  for example along a gear surface  860 . The at least one tooth  800  on the at least one first cam member  780  may be the shape of a gear tooth having a spur gear profile or a helical gear profile. While  FIGS.  6 A- 6 B  illustrate one non-limiting aspect of the at least one first cam member  780 , various other shapes are also contemplated. For example, the at least one first cam member  780  of the at least one retaining member  680  may have a generally circular, triangular, square, rectangular, or any suitable polygonal shape or cross-section. In each aspect, the at least first cam member  780  is configured for engaging at least a portion of the piston to cause the at least one retaining member  680  to be deflected from the piston upon rotation of the plunger  260  relative to the piston. In general, the at least one first cam member  780  may have at least one corrugated, rippled, or multilevel surface, either in singular or regularly or irregularly repeating form, that is configured for engaging with the corresponding cam member on the piston of the fluid injector. 
     With continued reference to  FIGS.  6 A- 6 B , the plunger  260  may have at least one first alignment member  825  disposed between the first end  700  and the second end  720  of the retaining member  680 . The first alignment member  825  has a proximal edge or tip  826 , a distal edge  827 , and a proximal alignment surface  828 . The first alignment member  825  cooperates with a second alignment member  1010  (shown in  FIG.  7 A ) on the piston head  920  to rotationally guide, self-align, move, or drive the plunger  260  and piston  880  into the proper rotational alignment for attachment or engagement and for subsequent detachment. The proximal alignment surface  828  may be a continuous surface or may be multiple discrete surfaces which act to provide continuous alignment activation when cooperating with the second alignment member  925  of the piston head  920  during plunger to piston engagement. A first alignment member  825  may optionally be on the retaining member  680 , may be part of the first cam member  780 , and/or may be rigidly associated with another aspect of the plunger  260 . 
     Referring to  FIG.  7 A , a piston  880  is extendible and retractable from the housing  14  of the fluid injector  10  (shown in  FIG.  1   ) via a powered means (not shown) preferably contained within housing  14 . The powered means may include, for example, an electric motor, a hydraulic system, or a pneumatic system, including appropriate gearing (not shown). As known in the art, the fluid injector  10  also may include a controller for controlling operation of the powered means and thereby controlling operation of the piston  880 . 
     With continued reference to  FIG.  7 A , the piston  880  includes a stem  900  and a piston head  920  formed on a distal end of the stem  900 . The piston  880  is construed from a relatively rigid material, such as metal or plastic that resists deformation due to repeated engagement with and disengagement from the plunger  260 . The piston head  920  has a substantially cylindrical structure with a pointed or conical distal end  940  that is configured to be received inside at least a portion of the interior cavity  400  of the plunger  260 . In some aspects, a sensing member  1500 , such as a spring-loaded pin connected to a sensor, may be provided. The sensing member  1500  may extend along a longitudinal axis of the piston  880  and may protrude through at least a portion of the piston head  920 . The sensing member  1500  may be operative for sensing contact with a surface, such as a surface of the plunger  260  and/or the plunger cover  58  (shown in  FIG.  3 E ), and control a movement of the piston  880  based on the sensed condition. For example, an initial contact between the sensing member  1500  and the plunger  260  and/or the plunger cover  58  may cause the pin to be retracted in a proximal direction such that it makes contact with the sensor. The sensor may be connected to the drive mechanism of the piston  880  such that, upon activation of the sensor by the pin, the sensor controls the movement of the drive mechanism. For example, the drive mechanism may be stopped or slowed from a first rate to a second, slower rate. 
     The proximal end  960  of the piston head  920  has an actuation member that interacts with the actuation member on the plunger  260 , such as the first cam member  780 . In some aspects, the actuation member on the piston head  920  may be a second cam member  980 . In some aspects, the second cam member  980  is a tooth of a gear  990  that extends around at least a portion of an outer circumference of the piston head  920 . The second cam member  980  is configured for cooperation with the first cam member  780  on the at least one retaining member  680  of the plunger  260 , as described herein. The second cam member  980  desirably has a shape that, upon self-aligning, relative rotation between the piston  880  and the plunger  260 , engages the first cam member  780  to cause the at least one retaining member  680  to be deflected from the piston head  920  such that the plunger  260  can be removed from the piston  880 . 
     In some aspects, the second cam member  980  may be parallel with the longitudinal axis  340 . In other aspects, the second cam member  980  may be angled relative to the longitudinal axis  340 . For example, the second cam member  980  may be angled toward the longitudinal axis  340  at an angle corresponding to an angle of inclination of the at least one retaining member  680 . In various aspects, regardless of the angular orientation of the at least one retaining member  680 , the first cam member  780  is desirably parallel with the second cam member  980 . 
     With continued reference to  FIGS.  7 A- 7 B , the piston head  920  may have a second alignment member  1010  to interact with the first alignment member  825  (shown in  FIG.  6 B ) on the plunger  260 . The alignment member  1010  has a first edge  927  and a second edge  926 , and a surface  928  between the first edge  927  and the second edge  926 . The longitudinal, radial, and circumferential locations of the first edge  927  and the second edge  926  and contour and extent of surface  928  of the second alignment member  1010  are chosen to interact with the edges and surfaces of the first alignment member  825  to provide a rotational force and motion as the piston  880  and plunger  260  are axially moved together. The piston head  920 , and optionally the collar  950 , may be connected to the piston stem  900  via a one-way rotation mechanism, such as the one-way rotation mechanism  99  discussed herein with reference to  FIG.  4 B , so that the piston  880  and the plunger  260  can rotate into alignment in response to the interaction of the first and second alignment surfaces. Alternatively, if the amount of rotation is small enough, the elasticity or slip of the plunger cover (shown in  FIG.  3 E ) with respect to the syringe barrel may be sufficient to allow the plunger  260  to rotate into alignment with the piston head  920 . 
     With continued reference to  FIGS.  7 A- 7 B , the piston  880  may have a collar  950  surrounding at least a portion of the stem  900  and/or the piston head  920 . The collar  950  may protrude radially outward relative to an outer radial surface of the stem  900  and the piston head  920  such that an annular space  970  is defined between the piston  880  and the collar  950 . The collar  950  may have an open top end and a closed bottom end that is defined by a bottom sidewall  1000  that connects the collar  950  to the stem  900  and/or the piston head  920 . The bottom sidewall  1000  defines a seat  1020  for a first end  1040  of a resiliently elastic member, such as a spring  1060 , that surrounds the stem  900 . In other aspects, the seat  1020  may be provided as a radial flange that protrudes from an outer surface of the stem  900 . The second end  1080  of the spring  1060  engages a proximal end of a movable capture ring  1100 . The capture ring  1100  has a substantially annular shape and surrounds at least a portion of an outer circumference of the stem  900 . In some aspects, at least a portion of an outer diameter of the capture ring  1100  may have a same or larger outer diameter than an outer diameter of the piston head  920 . The spring  1060  biases the capture ring  1100  toward a first radial lip  1120  of the piston head  920 . The capture ring  1100  is movable axially between a first position, where the capture ring  1100  engages the first radial lip  1120  of the piston head  920 , and a second position, where the spring  1060  is compressed and the capture ring  1100  is deflected by at least a portion of the at least one retaining member  680  toward the bottom sidewall  1000  of the collar  950 . In some aspects, the capture ring  1100  may be movable between the first position and the second position when urged by contact with, for example, the first end  700  of the at least one retaining member  680 . A stop member (not shown) may be provided to limit the movement of the capture ring  1100  to the second position. During disengagement of the plunger  260  from the piston  880 , the capture ring  1100  urges the at least one retaining member  680  in a distal direction due to a restoring force of the spring  1060 . In some aspects, the capture ring  1100  may have a grooved radial edge  1530  configured to engage the first cam member  780  of the at least one retaining member  680 . 
     With continued reference to  FIGS.  7 A- 7 B , the piston head  920  further defines a second radial lip  1510  at a distal end of the at least one second cam member  980 . When the plunger  260  is engaged with the piston  880 , the second radial lip  1510  acts as a retention surface for the at least one catch  740  of the at least one retaining member  680 . The piston head  920  may further have guiding grooves  1520  provided distally from the second radial lip  1510 . In some aspects, the guiding grooves  1520  may have a shape that corresponds to the shape of the first cam member  780 . In this manner, the tooth  800  of the first cam member  780  may be guided into the guiding groove  1520  as the plunger  260  and the piston head  920  are moved toward each other. 
     Having described the structure of the plunger  260  and the piston  880  in accordance with one non-limiting aspect of the present disclosure, the engagement and disengagement of the plunger  260  with and from the piston  880  will now be described with reference to  FIGS.  7 A- 12 B . The syringe  12 , shown initially in phantom in  FIG.  7 A  is omitted from the remainder of  FIGS.  7 B- 12 B  for clarity. 
     To engage the plunger  260  with the piston  880 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10 , as described herein. Once the syringe  12  is inserted into the syringe port  16 , various locking mechanisms (not shown) may be used to releasably retain the syringe  12  within the syringe port  16  to prevent detachment of the syringe  12  from the syringe port  16 . Initially, the plunger  260  may be positioned at the proximal end  20  of the syringe barrel  18 . In some aspects, the plunger  260  is positioned at any axial location between the proximal end  20  and the distal end  24  of the syringe barrel  18 . The piston  880  may then be advanced distally toward the plunger  260  for engagement of the piston head  920  with the plunger  260 . In some aspects, the piston  880  may be advanced distally toward the plunger  260  by way of the powered means operated by a controller. In other aspects, the piston  880  may be advanced distally toward the plunger  260  by manual operation. 
     With reference to  FIGS.  8 A- 8 B , the piston  880  is advanced axially in a distal direction such that at least a portion of the pointed or conical distal end  940  of the piston head  920  contacts the at least one retaining member  680  of the plunger  260 . Initially, at least a portion of the piston head  920 , such as the guiding grooves  1520 , contacts the first alignment member  825  (shown in  FIG.  6 B ), for example, on the at least one retaining member  680 . The piston head  920  may be connected to the piston stem  900  via a one-way rotation mechanism, such as the one-way rotation mechanism  99  discussed herein with reference to  FIG.  4 B , so that the piston  880  and the plunger  260  can rotate into self-alignment in response to the interaction of the first and second alignment surfaces. Alternatively, if the amount of rotation is small enough, the elasticity or slip of the plunger cover (shown in  FIG.  3 E ) with respect to the syringe barrel may be sufficient to allow the plunger  260  to rotate into alignment with the piston head  920 . 
     Due to an angled orientation of the at least one retaining member  680  relative to the longitudinal axis, continued axial movement of the piston head  920  relative to the plunger  260  causes the at least one retaining member  680  to be deflected radially outward due to the contact between the at least one retaining member  680  and the outer surface of the piston head  920 . In an aspect having a plurality of retaining members  680 , each of the retaining members  680  may be deflected radially outward relative to the piston head  920 . 
     With reference to  FIGS.  9 A- 9 B , during continued axial movement of the piston  880  in a distal direction, at least a portion of the retaining member  680  engages the distal end of the capture ring  1100 . For example, the first end  700  and/or the first cam member  780  of the retaining member  680  may engage the distal end of the capture ring  1100 . The contact between at least a portion of the retaining member  680  and the distal end of the capture ring  1100  urges the capture ring  1100  against the restoring force of the spring  1060  and away from the first radial lip  1120  of the piston head  920 . 
     With reference to  FIGS.  10 A- 10 B , at least a portion of the retaining member  680 , such as the first end  700  and/or the first cam member  780  of the retaining member  680 , urges the capture ring  1100  proximally against the restoring force of the spring  1060 . The body of the at least one retaining member  680  has an inherent restoring force built up in the material of the at least one retaining member  680  when the at least one retaining member  680  is deflected from its natural, undeflected state to a radially deflected state. Due to this inherent restoring force created within the body of the at least one retaining member  680  during a radial deflection of the at least one retaining member  680 , the second end  720  and/or the catch  740  is snapped radially into the second radial lip  1510 . Such radial movement of the second end  720  and/or the catch  740  also or further engages the first cam member  780  on the plunger  260  with the second cam member  980  on the piston head  920 . Specifically, the peaks  820  of the first cam member  780  are received in the groove of the second cam member  980 , and the groove  840  of the first cam member  780  receives the peaks  1010  of the second cam member  980  (shown in  FIGS.  6 A and  7 A ). In this manner, the gear surface  860  of the first cam member  780  is engaged with the gear surface  1050  of the second cam member  980 . The capture ring  1100  maintains contact with at least portion of the retaining member  680  to urge the second end  720  and/or the catch  740  into contact with the second radial lip  1510 . After retention of the plunger  260  on the piston head  920  by the engagement of the second end  720  and/or the catch  740  in the second radial lip  1510  of the piston head  920 , the plunger  260  resists being disconnected from the piston  880  upon movement of piston  880  in a distal and proximal direction relative to the syringe barrel  18 . In one aspect, the second end  720  and/or the catch  740  may be designed such that the compressive forces exerted upon the second end  720  and/or the catch  740  upon movement of piston  880  in the proximal direction substantially prevents radially outward deflection (or bending) of the catch  740 . For example, once the catch  740  is engaged, axial movement of the piston  880  does not introduce a bending moment which may deflect the catch  740  radially to cause the plunger  260  to be disconnected from the piston  880 . 
     To unlock the syringe  12  from the syringe port  16  (shown in  FIG.  1   ) and disengage the plunger  260  from the piston  880 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Because the plunger  260  is substantially free from rotation within the syringe barrel  18  due to a frictional force between plunger seal  59  and the inner surface  23  of the syringe sidewall  19 , the rotation of the syringe  12  also causes the plunger  260  to rotate relative to the piston  880 . With reference to  FIGS.  11 A- 11 B , rotation of the plunger  260  about its longitudinal axis  340  engages the first cam member  780  on the plunger  260  with the second cam member  980  on the piston head  920 . In particular, rotational movement of the plunger  260  causes the gear surface  860  of the first cam member  780  to move along the gear surface  1050  of the second cam member  980  such that the peaks  820  of the first cam member  780  are moved out of the grooves of the second cam member  980  and toward the peaks  1010  of the second cam member  980 . Such movement causes a radial deflection of the at least one retaining member  680  away from the piston head  920 . The at least one retaining member  680  is at its maximum radial deflection when the peaks  820  of the first cam member  780  on the plunger  260  are positioned over or aligned with the peaks  1010  of the second cam member  980  on the piston head  920 . 
     As the at least one retaining member  680  is deflected radially outward relative to the plunger longitudinal axis  340 , the second end  720  and/or the catch  740  is moved from the second radial lip  1510  of the piston head  920 . As the catch  740  moves out of its engaged position, the capture ring  1100  is advanced in the distal direction under the restoring force of the spring  1060 . As shown in  FIGS.  12 A- 12 B , the distal movement of the capture ring  1100  causes the capture ring  1100  to maintain the position of the at least one retaining member  680  in the radially outward deflected position, for example by moving under at least a segment of the at least one first cam member  780  or another part of the at least one retaining member  680 . In this position, the at least one retaining member  680  is held in a deflected state that allows the plunger  260  to be moved axially relative to the piston  880 . Such axial movement of the plunger  260  can be affected by withdrawing the syringe  12  from the syringe port  16  in a distal direction along the syringe longitudinal axis  15  or by withdrawing the piston  880  in a proximal direction away from the plunger  260 . The plunger  260 , together with the syringe  12 , can then be completely disengaged from the piston  880  and the injector  10 . In some aspects, the piston  880  can be released from the plunger  260  by rotating the piston  880  about its longitudinal axis and retracting the piston  880  in a proximal direction to disengage the at least one retaining member  680  in a manner described herein. 
     With reference to  FIG.  13   , a first adapter  114  may connect with a non-compatible plunger P without the at least one retaining member  68  described herein for removably engaging with the piston  88  of an injector having the piston head  92  with the second cam member  98  in accordance with one of the aspects described herein. In various aspects, the first adapter  114  may be connected to the plunger P for subsequent engagement with the piston  88 . For example, the first adapter  114  may be connected to the non-compatible plunger P releasably or permanently. Such a first adapter  114  may have a connection interface having at least one retaining member  68  with the first cam member  78  in accordance with various aspects described herein. According to another aspect, first adapter  114  may releasably connect with an injector having the piston  88  described herein. The first adapter  114  and the plunger P may be connected prior to connecting to the piston  88 , or the first adapter  114  may be connected to the piston  88  before the plunger P is connected to the first adapter  114 . The first adapter  114  and plunger P may be removed from the piston  88  after use, with the first adapter  114  being disposed of with the plunger P, or being removed from the used plunger P and saved for subsequent use with a different plunger P. Alternatively, the first adapter  114  may be reversibly or non-reversibly connected to piston  88  for use with multiple syringes. 
     In one aspect, a first portion  116  of the first adapter  114  may permanently or releasably receive the plunger P, which is not compatible for use with the piston  88  described herein. The first adapter  114  allows a connection mechanism  118  of the non-compatible plunger P to engage a corresponding attachment mechanism  215  on the first adapter  114  such that the plunger P can be retained on the first adapter  114 . In some aspects, the first adapter  114  may have a separate mechanism for engaging and disengaging the plunger P while the first adapter  114  remains connected to the piston  88 . A second portion  120  of the first adapter  114  may have at least one retaining member  68  in accordance with any of the aspects described herein. In some aspects, the first adapter  114  may have at least one retaining member  68  may have an actuation member, such as a first cam member  78  described herein with reference to  FIGS.  3 A- 3 B ; the first cam member  780  described herein with reference to  FIGS.  6 A- 12 B ; the first cam member  78  shown in  FIGS.  15 A-G ; the first cam member  78  shown in  FIGS.  20 A-D ; the first cam member  78  shown in  FIGS.  21 A-C ; the first cam member  78  shown in  FIGS.  22 A-D ; the first cam member  78  shown in  FIGS.  23 A-D ; and/or the first cam member  78  shown in  FIGS.  24 A-C . The second portion  120  of the first adapter  114  may releasably connect to an injector having the piston  88  with the piston head  92  according to any of the aspects described herein. In this manner, various non-compatible plungers P may be used. The first adapter  114  may non-permanently, permanently, or semi-permanently connect to an injector having the piston  88  with the piston head  92  described herein and allowing plungers P having alternate connection mechanisms to be used with the injector. 
     With reference to  FIG.  14   , a second adapter  122  may connect the plunger  26  with an injector that does not have the piston  88  with the piston head  92  according to any of the aspects described herein. In various aspects, the second adapter  122  may connect to the plunger  26  having at least one retaining member  68  in accordance with any of the aspects described herein for subsequent engagement with a non-compatible piston P′. For example, the second adapter  122  may be connected to the plunger  26  releasably or permanently. Such a second adapter  122  may have a connection interface having features of the piston head  92  in accordance with various aspects described herein. The second adapter  122  and the plunger  26  may be connected prior to connecting to the piston P′, or the second adapter  122  may be connected to the piston P′ before the plunger  26  is connected to the second adapter  122 . The second adapter  122  and plunger  26  may be removed from the piston P′ after use, with the second adapter  122  being disposed of with the plunger  26 , or being removed from the used plunger  26  and saved for subsequent use with a different plunger  26 . 
     In one aspect, a first portion  124  of the second adapter  122  may be configured for permanently or releasably engaging the plunger  26  which is not compatible for use with the piston P′. The second adapter  122  allows a connection mechanism  126  of the non-compatible piston P′ to engage a corresponding connection mechanism  125  on the second adapter  122 . A second portion  128  of the second adapter  122  may have features of the piston head  92  in accordance with aspects described herein. In some aspects, the second portion  128  may have the second cam member  98  described herein with reference to  FIG.  4 A,  15 A,  20 A and/or  24 A  and/or the second cam member  980  described herein with reference to  FIG.  7 A . The second portion  128  of the second adapter  122  may releasably connect to the plunger  26  described herein. In this manner, the plunger  26  may be connected to various non-compatible injectors using the second adapter  122 . 
     Referring to  FIGS.  15 A- 15 G , a piston  88  and a plunger  26  are shown in accordance with another aspect. The piston  88  is configured to interact with the plunger  26  (shown in  FIG.  15 C ) to releasably lock the plunger  26  such that the plunger  26  can be driven reciprocally within the barrel of the syringe  12  (shown in  FIG.  2   ). 
     With continued reference to  FIG.  15 A , the piston  88  includes a stem  90  and a piston head  92  formed on a distal end of the stem  90 . The piston head  92  is construed from a rigid material, such as metal or plastic that resists deformation. The stem  90  may have a cavity  91  for collecting any fluid that may drip from the syringe and an annular collar  93  that surrounds the cavity  91 . One or more buttresses  97  connect the annular collar  93  to the stem  90 . The piston head  92  has a substantially cylindrical structure with a pointed or conical distal end  94  with a cap  95  that is shaped to be received inside at least a portion of the interior cavity  40  (shown in  FIG.  3 A ) of the plunger  26 . In some aspects, a sensing member  79 , such as a pin connected to a sensor, may be provided. The sensing member  79  may extend along a longitudinal axis of the piston  88  and may protrude through at least a portion of the piston head  92 , such as through at least a portion of the cap  95 . The sensing member  79  may be operative for sensing contact with a surface, such as a surface of the plunger  26  and/or the plunger cover  58  (shown in  FIG.  3 E ), and control a movement of the piston  88  based on the sensed condition. For example, an initial contact between the sensing member  79  and the plunger  26  and/or the plunger cover  58  may cause the pin to be retracted in a proximal direction such that it makes contact with the sensor. The sensing member  79  may be biased in an extended position by a resilient element  81  (shown in  FIG.  15 E ), such as a spring. The sensor may be connected to the control mechanism which controls the drive mechanism of the piston  88  such that, upon activation of the sensor by the pin, the controller controls the movement of the drive mechanism. For example, the drive mechanism may be stopped or slowed from a first rate to a second, slower rate. 
     With reference to  FIG.  15 B , the piston head  92  has a proximal portion  103  connected to a distal portion  105 . Terminal ends of the proximal and distal portions  103 ,  105  may have a radiused edge  107 . At least a portion of the proximal portion  103  has a smaller outer diameter compared to an outer diameter of the distal portion  105  such that a radial lip  109  is formed at a transition between the proximal portion  103  and the distal portion  105 . The radial lip  109  may be continuous or discontinuous around a circumference of the piston head  92 . In some aspects, the radial lip  109  defines a locking ledge  111  for engaging the catch  74  of the at least one retaining member  68  when the plunger  26  is fully seated on the piston head  92 . 
     With continued reference to  FIG.  15 B , the piston head  92  may have at least one second alignment member  113  protruding radially outward from an outer surface of the piston head  92 . The at least second alignment member  113  is shaped and/or configured for interacting with the first alignment member  71  of the plunger  26  for facilitating alignment of the piston  88  with the plunger  26  in order to allow for a releasable locking connection of the plunger  26  with the piston  88 . In some aspects, at least a portion of the at least second alignment member  113  may extend in a direction that is angled relative to the direction of a piston longitudinal axis  115 . For example, at least second alignment member  113  may have a guiding surface  117  that is angled at an angle D relative to the piston longitudinal axis  115 . The guiding surface  117  is desirably angled such that the piston head  92  may rotate around the piston stem  90 , for example around an axis of the one-way rotation mechanism  99 , when the proximal alignment surface  77   a  of the first alignment member  71  contacts the guiding surface  117  of the second alignment member  113 . 
     In some aspects, a plurality of second alignment members  113  may be spaced apart radially relative to the piston longitudinal axis  115  along an outer circumference of the piston head  92 . In some aspects, the number of second alignment members  113  may be equal to a total number of retaining members  68  and first alignment members  71  on the plunger  26 . The second alignment members  113  are spaced apart circumferentially such that a retaining member  68  or a first alignment member  71  may be received between adjacent second alignment members  113 . The second alignment members  113  may be separated from each other by portions of an outer surface of the proximal portion  103  and/or the distal portion  105  of the piston head  92 . 
     With continued reference to  FIG.  15 B , each guiding surface  117  of the second alignment members  113  defines a travel path for guiding the movement of the proximal alignment surface  77   a  of the first alignment member  71  in and out of a recess  119  defined between adjacent second alignment members  113  (see  FIG.  15 G ). The guiding surfaces  117  may be inclined or angled radially and axially relative to the piston longitudinal axis  115  to guide the movement of the proximal alignment surfaces  77   a . The guiding surfaces  117  aid in self-orienting the piston head  92  as the plunger  26  (see  FIG.  15 G ) is brought into contact with the piston  88  by guiding the one or more proximal alignment surfaces  77   a  on the plunger  26  into the corresponding recess  119  on the piston head  92 . In this manner, a piston  88  whose piston longitudinal axis  115  is rotationally misaligned with the plunger longitudinal axis  34  and the one or more first alignment member  71  which are initially misaligned relative to the corresponding one or more second alignment members  113  in a rotational direction are brought in alignment axially and rotationally such that the one or more first alignment members  71  are received within the recess  119  between adjacent second alignment members  113 . The one or more second alignment members  113  may have a bottom surface  121  that is angled relative to the direction of a piston longitudinal axis  115 . 
     The piston head  92  further has an actuation surface that interacts with the actuation member on the plunger  26 , such as the first cam member  78 . In some aspects, the actuation surface on the piston head  92  may be a second cam member  98 . In some aspects, the second cam member  98  cooperates with the first cam member  78  on the at least one retaining member  68  of the plunger  26 , as described herein. The second cam member  98  desirably has a shape that, upon relative rotation between the piston  88  and the plunger  26 , engages the first cam member  78  to cause the at least one retaining member  68  to be deflected from the piston head  92  such that the plunger  26  can be removed from the piston  88 . In some aspects, the second cam member  98  may be formed on or intersect with the second alignment member  113  on the piston head  92 . In certain aspects, the second cam member  98  may have a cam surface  98   a  extending radially outward and parallel to the longitudinal axis  115 . The cam surface  98   a  may be aligned with a direction of the piston longitudinal axis  115 . The second cam member  98  may have a chamfered portion, not shown, to facilitate passing of the first cam member  78  after the retaining member  68  is deflected sufficiently to allow the retaining member to be released. 
     With reference to  FIG.  15 C , the piston  88  is configured to interact with the plunger  26  to releasably lock with plunger  26 , such as shown in  FIG.  15 D . By locking the piston  88  to the plunger  26 , the plunger  26  can be driven reciprocally within the barrel of the syringe  12  (shown in  FIG.  2   ). The second cam member  98  on the piston  88  cooperates with the first cam member  78  on the at least one retaining member  68  of the plunger  26 , to releasably lock the plunger  26  to the piston  88 . 
     With reference to  FIG.  15 F , the piston head  92  may be rotatable relative to the stem  90 . In some aspects, the piston head  92  may be rotatable in one direction only, such as a clockwise or a counter-clockwise direction, relative to the stem  90 . A one-way rotation mechanism  99 , such as a one-way clutch mechanism shown in  FIG.  15 F , may be provided to allow the rotation of the piston head  92  in a first direction only, such as the clockwise or the counter-clockwise direction. The one-way rotation mechanism  99  may be rotatable around a central shaft  101  having a seal  102 , such as an O-ring seal. In some aspects, the one-way rotation mechanism  99  may have a stop that prevents rotation of the piston head  92  in a second direction opposite the first direction, such as the counter-clockwise or the clockwise direction, respectively. In other aspects, the one-way rotation mechanism  99  may be provided on at least a portion of the plunger  26 . 
     With reference to  FIG.  15 G , the at least one first alignment member  71  may be provided directly on one or more of the retaining members  68 . In such aspects, at least one retaining member  68  may have a proximal alignment surface  77   a  provided directly on the body of the at least one retaining member  68 . The first cam member  78  may be also provided directly on the retaining member  68  such that engagement of the cam member  78  causes a corresponding movement of the retaining member  68 , as described herein. Cam member  78  may be provided on a side surface of retaining member  68  or may be provided on an edge of the at least one first alignment member  71  may be provided directly on one or more of the retaining members  68 . 
     With reference to  FIG.  16   , a cylindrical plan projection view of one aspect of the piston  88  and the plunger  26  is shown. If the piston  88  is rotationally misaligned relative to the plunger  26  such that the first alignment members  71  directly on the retaining members  68  on the plunger  26  are not in rotational alignment to be received within the recesses  119  on the plunger head  92 , the proximal alignment surface  77   a  (shown as a dotted line) of the first alignment member  71  on the plunger  26  contacts the guiding surface  117  of the second alignment member  113  on the piston head  92 . Engagement of the proximal alignment surface  77   a  with the guiding surface  117  causes the piston head  92  to automatically rotate in a free rotation direction of the one-way rotation mechanism  99 . Such rotation of the piston head  92  aligns the first alignment members  71  and the retaining members  68  to be received within the recesses  119  between adjacent second alignment members  113 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . If the piston  88  is rotationally aligned relative to the plunger  26 , such as shown in  FIG.  16   , the first alignment members  71  and the retaining members  68  on the plunger  26  can be received within the recesses  119  between adjacent second alignment members  113  without rotation of the piston head  92 . 
     In some aspects, such as shown in  FIG.  17   , the plunger  26  may have one or more first alignment members  71  positioned adjacent to the one or more retaining members  68 . In some aspects, the width of the second alignment member  113  on the piston head  92  in a circumferential direction may be reduced such that the first alignment member  71  and the retaining member  68  may be received in a same space defined between adjacent second alignment members  113  on the piston head  92 . The one or more first alignment members  71  may have a first end  71   a  connected to the body  32  of the plunger and a second end  71   b  that protrudes in a proximal direction which is opposite to the protrusion direction of the second end  72  of the one or more retaining members  68 . The second end  71   b  of the first alignment member  71  may be deflectable in a radial direction relative to the first end  71   a . The first alignment members  71  may further have an angled guide surface, for example defined by an outer surface of the second end  71   b , that cooperates with the second alignment members  113  of the piston head  92  to align the plunger  26  relative to the piston  88 . During the engagement/disengagement process, the second end  71   b  of the first alignment members  71  may be deflected radially outward as it passes over the region defined by the recesses  119  and the second alignment members  113  and is deflected back in a radially inward direction once the second end  71   b  clears the recesses  119 . The one or more first alignment members  71  may have a latching member (not shown) to lock with at least a portion of the piston head  92 , such as the radial lip  109 . 
     In another aspect, such as shown in  FIGS.  18   , the one or more first alignment members  71  may be spaced apart from the one or more retaining members  68  such that each first alignment member  71  and the retaining member  68  is received in a separate space defined between adjacent second alignment members  113  on the piston head  92 . With reference to  FIG.  19   , the at least one first alignment member  71  may be bifurcated and formed from two separate portions. Each portion may be separately attached to the plunger body  32 . In some aspects, the two portions may be joined together at the second end  71   b , such as shown in  FIG.  19   . In various aspects, interaction of the at least one first alignment member  71  on the plunger  26  with the one or more second alignment members  113  on the piston  88  causes self-orientation of the piston  88  such that at least one retaining member  68  is received in the recess  119  on the piston head  92 . 
     With reference to  FIG.  20 A , according to certain aspects, at least a portion of the second alignment members  113 , such as a lower or proximal end of the second alignment members  113  may be connected by a continuous lip  123  that extends continuously around an outer circumference of the piston head  92  at a radial position that may be flush with, radially recessed, or radially protruding relative to an outer surface of the second alignment members  113 . In aspects where two or more second alignment members  113  are provided, the second alignment members  113  may be evenly spaced apart from each other. In one exemplary and non-limiting aspect with six second alignment members  113  having equal angular separation therebetween, each second alignment member  113  is separated by 60 degrees from the second alignment members  113  adjacent on either side. In some aspects, the second alignment members  113  may have unequal angular extension and/or unequal angular spacing between the second alignment members  113  about the outer surface of the proximal portion  103  and/or the distal portion  105  of the piston head  92 . The radial spacing of the at least one second alignment members  113  relative to the piston longitudinal axis  115  is selected to correspond to an inner shape of the plunger  26  (shown in  FIGS.  20 B- 20 D ) to allow the retaining members  68  and the first alignment members  71  to be received between adjacent second alignment members  113 . 
     With reference to  FIGS.  20 C- 20 D , the at least one first alignment member  71  may be provided directly on one or more of the retaining members  68 . In such aspects, at least one retaining member  68  may have a proximal alignment surface  77   a  and a distal alignment surface  77   b  provided directly on the body of the at least one retaining member  68 . The proximal alignment surface  77   a  may be angled relative to the plunger longitudinal axis  34 . In some aspects, the proximal alignment surface  77   a  may be aligned with the guiding surface  117  on the piston  88  (shown in  FIG.  20 A ) such that the proximal alignment surface  77   a  is guided into the recess  119  on the piston head  92  (shown in  FIG.  20 A ). The first cam member  78  may be also provided directly on the retaining member  68  such that engagement of the cam member  78  causes a corresponding movement of the retaining member  68 , for example during rotation of the plunger  26  relative to the piston head  92  as described herein. The retaining members  68  may have at least one catch  74  that it formed on the second end  72  of the retaining member  68 . The at least one catch  74  may be shaped to be received within locking ledge  111  on the piston  88  (shown in FIG. 
       20 A) to lock the plunger  26  axially relative to the piston  88 . In some aspects, the at least one catch  74  may be linear or curvilinear. In some aspects, the at least one catch  74  may be oriented in a direction substantially perpendicular to a direction of the plunger longitudinal axis  34 . In other aspects, the at least one catch  74  may be angled relative to a direction of the plunger longitudinal axis  34 . The at least one catch  74  may be continuous or discontinuous. In some aspects, the at least one catch  74  may protrude radially inward toward the plunger longitudinal axis  34 . With reference to  FIG.  20 B , the distal end  38  of the plunger body  32  may have one or more openings  49  extending through the plunger body  32 . The one or more openings  49  may be spaced apart radially relative to the plunger longitudinal axis  34 . The openings  49  may have equal or unequal angular extension and/or equal or unequal angular spacing between each other. In some aspects, the one or more openings  49  may be formed to facilitate molding of the plunger body  32 . For example, the one or more openings  49  define a path for a molding tool to follow during the molding process of the plunger  26 . 
     With reference to  FIGS.  21 A- 21 C , a plunger  26  and a piston  88  are shown in accordance with another aspect of the present disclosure. The components of the plunger  26  shown in  FIGS.  21 A- 21 C  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  3 A- 3 C . Similarly, the components of the piston  88  shown in  FIGS.  21 A- 21 C  are substantially similar to the components of the piston  88  described herein with reference to  FIGS.  4 A- 4 C . Reference numerals in  FIGS.  21 A- 21 C  are used to illustrate identical components of the corresponding reference numerals in  FIGS.  3 A- 4 C . As the previous discussion regarding the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  is applicable to the aspect of the present disclosure shown in  FIGS.  21 A- 21 C , only the relative differences between the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  and the plunger  26  and piston  88  generally shown in  FIGS.  21 A- 21 C  are discussed hereinafter. 
     With reference to  FIG.  21 A , the plunger  26  may have at least one resiliently deflectable retaining member  68  (hereinafter “retaining member  68 ”) protruding from the plunger body  32 . In some aspects, the at least one retaining member  68  may protrude in a circumferential direction extending around an inner circumference of the inner surface  52  of the interior cavity  40 . In some aspects, the at least one retaining member  68  may extend substantially perpendicularly to a longitudinal axis  34  of the plunger body  32 . In other aspects, the at least one retaining member  68  may be angled in a distal or proximal direction relative to a plane extending perpendicularly to the longitudinal axis  34  of the plunger body  32 . Features such as the at least one alignment members  71  are omitted for clarity. 
     With continued reference to  FIG.  21 A , the at least one retaining member  68  has a first segment or a first end  70  connected to the plunger body  32  and a second segment or a second end  72  extending circumferentially around at least a portion an inner circumference of the plunger body  32  relative to the first end  70 . The second end  72  may deflect and/or twist relative to the first end  70 . As described herein, the second end  72  may be circumferentially and/or radially deflectable toward or away from the inner surface of the plunger body  32  relative to the first end  70 . The first end  70  and the second end  72  may be spaced apart in a direction that extends substantially circumferentially around an inner surface of the plunger body  32 . The at least one retaining member  68  may be linearly, stepwise, or curvilinearly contiguous between the first end  70  and the second end  72 . In some aspects, a plurality of retaining members  68  may be spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . The retaining members  68  may be separated from each other, such as by even or uneven spacing, by portions of the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one retaining member  68  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston, as described herein. 
     With reference to  FIG.  21 B , the second end  72  of the retaining member  68  has at least one catch  74  that is shaped to engage at least a portion of a recess, lip, or ledge on the piston to lock the at least one retaining member  68 , along with the plunger  26 , relative to the piston. In some aspects, the at least one catch  74  may protrude radially inward or outward relative to a body of the retaining member  68 . The at least one catch  74  may be formed integrally with the second end  72  of the at least one retaining member  68  or it may be affixed or otherwise secured to the second end  72  of the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. 
     With reference to  FIG.  21 C , the plunger  26  may have at least one actuation member, such as a first cam member  78  that interacts with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston, as described herein. The at least one first cam member  78  may be provided at the second end  72  of the retaining member  68 . The at least one first cam member  78  may be angled relative to the body of the retaining member  68 . In some aspects, the at least one first cam member  78  may be on at least one surface of a pocket  170  formed on the second end  72  of the at least one retaining member  68 . 
     The plunger  26  may have at least one alignment member, such as the first alignment member  71  shown in  FIG.  3 A  protruding from the plunger body  32 . As described herein, the at least one first alignment member  71  is shaped and/or configured for facilitating self-orienting alignment of the plunger  26  with the piston  88 . 
     To engage the plunger  26  with the piston  88 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10  (shown in  FIG.  1   ), as described herein. If the piston  88  is rotationally misaligned relative to the plunger  26  such that the one or more alignment members on the plunger  26  are not in rotational alignment to be received within the recesses  119  on the plunger head  92 , the one or more alignment members on the plunger  26  contact the guiding surface  117  of the second alignment member  113  on the piston head  92  to rotate the piston head  92  into alignment for connecting to the plunger  26 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . Distal movement of the piston  88  causes the retaining members  68  to circumferentially deflect outward relative to the plunger longitudinal axis  34  from a first, undeflected position, to a second deflected position. The piston  88  is advanced distally until the terminal portion of the second end  72  clears the retaining members  68 , thereby allowing them to deflect circumferentially inward toward or to their initial undeflected position. The catch  74  of at least one retaining member  68  is retained within the locking ledge  111  of the recess  119  or under a locking ledge  111   a  formed on a proximal end of the at least one second alignment member  113  on the piston head  92  to prevent disengagement of the plunger  26  from the piston head  92 , for example by a frictional fit against a second portion of the inner wall of the plunger  26 . In some aspects, such as shown in  FIG.  21 B , at least a portion of the piston  88 , such as the at least one second alignment member  113 , may have a locking ledge  111   a  that engages the catch  74  when the plunger  26  is connected to the piston  88  to prevent the plunger  26  from disconnecting from the piston  88  when the plunger  26  is moved in a proximal direction within the syringe barrel  18  (shown in  FIG.  2   ). 
     To unlock the syringe  12  from the syringe port  16  (shown in  FIG.  1   ) and disengage the plunger  26  from the piston  88 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Rotation of the syringe  12 , and thereby the plunger  26 , about the plunger longitudinal axis  34  engages the first cam member  78  on the plunger  26  with the second cam member  98  on the piston head  92 . Such movement causes a circumferential deflection of the at least one retaining member  68  away from the piston head  92  to unlock the plunger  26  from the piston head  92  and allow the removal of the syringe  12 . 
     With reference to  FIGS.  22 A- 22 D , a plunger  26  and a piston  88  are shown in accordance with another aspect of the present disclosure. The components of the plunger  26  shown in  FIGS.  22 A- 22 D  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  3 A- 3 C . Similarly, the components of the piston  88  shown in  FIGS.  22 A- 22 D  are substantially similar to the components of the piston  88  described herein with reference to  FIGS.  4 A- 4 C . Reference numerals in  FIGS.  22 A- 22 D  are used to illustrate identical components of the corresponding reference numerals in  FIGS.  3 A- 4 C . As the previous discussion regarding the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  is applicable to the aspect of the present disclosure shown in  FIGS.  22 A- 22 D , only the relative differences between the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  and the plunger  26  and piston  88  generally shown in  FIGS.  22 A- 22 D  are discussed hereinafter. 
     With reference to  FIG.  22 A , the plunger  26  may have at least one resiliently deflectable retaining member  68  (hereinafter “retaining member  68 ”) protruding from the plunger body  32 . In some aspects, the at least one retaining member  68  may be U-shaped, with a first portion  130  having a first end  132  connected to the plunger body  32  and a second end  134  extending in a direction toward the proximal end of the plunger body  32 . The at least one retaining member  68  may further have a transition portion  136  connected to the second end  134  of the first portion  130 . A first end  138  of a second portion  140  may be connected to the transition portion  136  at an end opposite to the connection of the second end  134  of the first portion  130 . The transition portion  136  extends in a radial direction relative to the longitudinal axis  34  of the plunger body  32  and connects the first portion  130  to the second portion  140 . A second end  142  of the second portion  140  extends toward the distal end of the plunger body  32 . The first portion  130 , the second portion  140 , or both may deflect or twist relative to the plunger body  32 . For example, the second end  134  of the first portion  130  may be deflectable in a radial or circumferential direction relative to the first end  132  and the plunger body  32 . Alternatively, or in addition, the second end  142  of the second portion  140  may be deflectable in a radial or circumferential direction relative to the first end  138 , and therefore, relative to the first portion  130  and the plunger body  32 . In some aspects, a plurality of retaining members  68  may be spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . The retaining members  68  may be separated from each other, such as by even or uneven spacing, by portions of the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one retaining member  68  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston, as described herein. 
     With reference to  FIG.  22 A , the second end  142  of the second portion  140  of the retaining member  68  has at least one catch  74  that is shaped to engage at least a portion of a recess, lip, or ledge on the piston to lock the at least one retaining member  68 , along with the plunger  26 , relative to the piston. In some aspects, the at least one catch  74  may protrude radially inward or outward relative to a body of the retaining member  68 . The at least one catch  74  may be formed integrally with the second end  142  of the second portion  140  of the at least one retaining member  68  or it may be affixed or otherwise secured to the second end  72  of the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. 
     With reference to  FIG.  22 C , the plunger  26  may have at least one first cam member  78  that interacts with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston, as described herein. The at least one first cam member  78  may be provided on the second portion  140  of the at least one retaining member  68 . The at least one first cam member  78  may be angled at an angle B relative to the body of the retaining member  68 . 
     With reference to  FIG.  22 A , the plunger  26  may have at least one first alignment member  71  defined on at least a portion of the at least one retaining member  68 , such as the transition portion  136 . The at least one first alignment member  71  is shaped and/or configured for facilitating self-orienting alignment of the plunger  26  with the piston  88 . In some aspects, at least a portion of the at least one first alignment member  71  may extend in a direction that is angled relative to the direction of the plunger longitudinal axis  34 . For example, at least one first alignment member  71  may have a proximal alignment surface  77   a  that is angled at an angle C relative to the longitudinal axis  34  to facilitate positioning of the retaining member  68  during connection of the plunger  26  to a piston. The proximal alignment surface  77   a  helps guide the plunger  26  into self-orienting alignment with the piston, as described herein. 
     To engage the plunger  26  with the piston  88 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10  (shown in  FIG.  1   ), as described herein. If the piston  88  is rotationally misaligned relative to the plunger  26  such that the one or more alignment members on the plunger  26  are not in rotational alignment to be received within the recesses  119  on the plunger head  92 , the one or more alignment members on the plunger  26  contact the guiding surface  117  of the second alignment member  113  on the piston head  92  to rotate the piston head  92  into alignment for connecting to the plunger  26 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . Distal movement of the piston  88  causes the retaining members  68  to deflect outward relative to the plunger longitudinal axis  34  from a first, undeflected position, to a second, radially deflected position. The piston  88  is advanced distally until the terminal portion of the second end  72  clears the retaining members  68 , thereby allowing them to deflect radially inward toward or to their initial undeflected position. The catch  74  of at least one retaining member  68  is retained within the locking ledge  111  to prevent disengagement of the plunger  26  from the piston head  92 . 
     To unlock the syringe  12  from the syringe port  16  (shown in  FIG.  1   ) and disengage the plunger  26  from the piston  88 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Rotation of the syringe  12 , and thereby the plunger  26 , about the plunger longitudinal axis  34  engages the first cam member  78  on the plunger  26  with the second cam member  98  on the piston head  92 . Such movement causes a deflection of the at least one retaining member  68  away from the piston head  92  to unlock the plunger  26  from the piston head  92  and allow the removal of the syringe  12 . 
     With reference to  FIGS.  23 A- 23 D , a plunger  26  and a piston  88  are shown in accordance with another aspect of the present disclosure. The components of the plunger  26  shown in  FIGS.  23 A- 23 D  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  3 A- 3 C  and other aspects described herein. Similarly, some of the components of the piston  88  shown in  FIGS.  23 A- 23 D  are substantially similar to some of the components of the piston  88  described herein with reference to  FIGS.  4 A- 4 C . Reference numerals in  FIGS.  23 A- 23 D  are used to illustrate identical components of the corresponding reference numerals in  FIGS.  3 A- 4 C . As the previous discussion regarding the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  is applicable to the aspect of the present disclosure shown in  FIGS.  23 A- 23 D , only the relative differences between the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  and the plunger  26  and piston  88  generally shown in  FIGS.  23 A- 23 D  are discussed hereinafter. 
     With reference to  FIG.  23 A , the plunger  26  may have at least one resiliently deflectable retaining member  68  (hereinafter “retaining member  68 ”) protruding from the plunger body  32 . In some aspects, the at least one retaining member  68  may protrude in a proximal direction toward the proximal end of the plunger body  32 . In some aspects, the at least one retaining member  68  may extend substantially parallel to a longitudinal axis  34  of the plunger body  32 . In other aspects, the at least one retaining member  68  may be angled relative to the longitudinal axis  34  of the plunger body  32 . 
     With continued reference to  FIG.  23 A , the at least one retaining member  68  has a first segment or a first end  70  connected to the plunger body  32  and a second segment or a second end  72  extending in a proximal direction relative to the first end  70 . The second end  72  may deflect or twist relative to the first end  70 . As described herein, the second end  72  may be radially deflectable toward or away from the inner surface of the plunger body  32  relative to the first end  70 . The at least one retaining member  68  may be linearly, stepwise, or curvilinearly contiguous between the first end  70  and the second end  72 . In some aspects, a plurality of retaining members  68  may be spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . The retaining members  68  may be separated from each other, such as by even or uneven spacing, by portions of the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one retaining member  68  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston, as described herein. 
     With reference to  FIG.  23 B , the second end  72  of the retaining member  68  has at least one catch  74  that is shaped to engage at least a portion of a recess, lip, or ledge on the piston to lock the at least one retaining member  68 , along with the plunger  26 , relative to the piston. In some aspects, the at least one catch  74  may protrude radially inward or outward relative to a body of the retaining member  68 . The at least one catch  74  may be formed integrally with the second end  72  of the at least one retaining member  68  or it may be affixed or otherwise secured to the second end  72  of the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. 
     With reference to  FIG.  23 C , the plunger  26  may have at least one first cam member  78  that interacts with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston, as described herein. The at least one first cam member  78  may be provided at the second end  72  of the retaining member  68 . The at least one first cam member  78  may be angled at an angle B relative to the body of the retaining member  68 . 
     The plunger  26  may have at least one alignment member, such as the at least one first alignment member  71  protruding from the plunger body  32 . As described herein, the at least one first alignment member  71  is shaped and/or configured for facilitating self-orienting alignment of the plunger  26  with the piston  88 . The at least one first alignment member  71  may be provided adjacent to the at least one retaining member  68 . 
     To engage the plunger  26  with the piston  88 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10  (shown in  FIG.  1   ), as described herein. If the piston  88  is rotationally misaligned relative to the plunger  26  such that the one or more alignment members  71  on the plunger  26  are not in rotational alignment to be received within the recesses  119  on the plunger head  92 , the one or more alignment members  71  on the plunger  26  contact the guiding surface  117  of the second alignment member  113  on the piston head  92  to rotate the piston head  92  into alignment for connecting to the plunger  26 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . Distal movement of the piston  88  causes the retaining members  68  to deflect outward relative to the plunger longitudinal axis  34  from a first, undeflected position, to a second, radially deflected position. The piston  88  is advanced distally until the terminal portion of the second end  72  clears the retaining members  68 , thereby allowing them to deflect radially inward toward or to their initial undeflected position. The catch  74  of at least one retaining member  68  is retained within the locking ledge  111  of the recess  119  or under a locking ledge  111   a  formed on a proximal end of the at least one second alignment member  113  on the piston head  92  (shown in  FIG.  23 B ) to prevent disengagement of the plunger  26  from the piston head  92 . 
     To unlock the syringe  12  from the syringe port  16  (shown in  FIG.  1   ) and disengage the plunger  26  from the piston  88 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Rotation of the syringe  12 , and thereby the plunger  26 , about the plunger longitudinal axis  34  engages the first cam member  78  on the plunger  26  with the second cam member  98  on the piston head  92 . Such movement causes a deflection of the at least one retaining member  68  away from the piston head  92  to unlock the plunger  26  from the piston head  92  and allow the removal of the syringe  12  from the injector  10  (shown in  FIG.  1   ). 
     With reference to  FIGS.  24 A- 24 C , a plunger  26  and a piston  88  are shown in accordance with another aspect of the present disclosure. Some aspects of some of the components of the plunger  26  shown in  FIGS.  24 A- 24 C  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  3 A- 4 C . Other aspects of the components of the plunger  26  shown in  FIGS.  24 A- 24 C  are substantially similar to the components of the plunger  26  described herein with reference to  FIGS.  21 A- 21 C . Similarly, the components of the piston  88  shown in  FIGS.  24 A- 24 C  are substantially similar to the components of the piston  88  described herein with reference to  FIGS.  21 A- 21 C . Reference numerals in  FIGS.  24 A- 24 C  are used to illustrate identical components of the corresponding reference numerals in  FIGS.  4 A- 4 C  or  FIGS.  21 A- 21 C . As the previous discussion regarding the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  is applicable to the aspect of the present disclosure shown in  FIGS.  24 A- 24 C , only the relative differences between the plunger  26  and piston  88  generally shown in  FIGS.  3 A- 4 C  and the plunger  26  and piston  88  generally shown in  FIGS.  24 A- 24 C  are discussed hereinafter. 
     With reference to  FIG.  24 A , the plunger  26  may have at least one resiliently deflectable retaining member  68  (hereinafter “retaining member  68 ”) associated with the plunger body  32 . In some aspects, the at least one retaining member  68  may protrude in a radial direction from the inner surface  52  of the interior cavity  40 . With continued reference to  FIG.  24 A , the at least one retaining member  68  has a first segment or a first end  70  extending through, towards, or into the outside to the plunger body  32 . For example, the first end  70  of the at least one retaining member  68  may extend through an opening  73  extending through the sidewall  39  of the plunger body  32 . The at least one retaining member  68  may have a second segment or a second end  72  extending radially inward to the interior cavity  40  relative to the first end  70 . Features such as the at least one alignment members  71  are omitted for clarity. 
     The first end  70  of the at least one retaining member  68  may contact or abut, either directly or indirectly, a resilient member, for example the resilient inside surface of the plunger cover, such as the plunger cover  58  shown in  FIG.  3 E . Alternatively, or in addition, the first end  70  of the at least one retaining member  68  may contact or abut, either directly or indirectly, an elastic element, such as a metal or plastic spring, or other resilient material which urges the at least one retaining member  68  radially inward. With reference to  FIG.  24 B , the first end  70  of the at least one retaining member  68  may have at least one stop member  171  which limits the radially inward and/or outward travel of the at least one retaining member  68  so that it does not fall out of the plunger body  32 . The stop member  171  may be the elastic element associated the at least one retaining member  68 . The at least one retaining member  68  may be manufactured as a separate piece and inserted into the plunger body  32  before the plunger cover  58  is installed, co-molded with the plunger body  32  using a co-molding process, or may be molded as part of the plunger body  32  with a thin section (not shown) which deflects or breaks upon assembly or deflection of the at least one retaining member  68  in a radially inward or outward direction to allow the desired motion of the at least one retaining member  68 . Optionally, the at least one resilient member  68  can have the stop member  171  or the thin section (not shown) which attaches the at least one retaining member  68  to the plunger body  32 . In some aspects, a plurality of retaining members  68  may be spaced apart radially relative to the plunger longitudinal axis  34  along a circumference of the inner surface  52  of the interior cavity  40 . The retaining members  68  may be separated from each other, such as by even or uneven spacing, by portions of the inner surface  52  of the interior cavity  40 . The radial spacing of the at least one retaining member  68  relative to the plunger longitudinal axis  34  is selected to correspond to or operably interact with an outer shape of the piston, as described herein. 
     With reference to  FIG.  24 B , the second end  72  of the retaining member  68  has at least one catch  74  that is shaped to engage at least a portion of a recess, lip, or ledge on the piston to lock the at least one retaining member  68 , along with the plunger  26 , relative to the piston. In some aspects, the at least one catch  74  may protrude radially inward or outward relative to a body of the retaining member  68 . The at least one catch  74  may be formed integrally with the second end  72  of the at least one retaining member  68  or it may be affixed or otherwise secured to the second end  72  of the at least one retaining member  68  using, for example, a frictional fit and/or an adhesive, welding, or by molding. 
     With reference to  FIG.  24 C , the plunger  26  may have at least one first cam member  78  that interacts with a piston of the fluid injector  10  (shown in  FIG.  1   ) to radially deflect the at least one retaining member  68  upon rotation of the plunger  26  relative to the piston, as described herein. The at least one first cam member  78  may be provided at the second end  72  of the retaining member  68 . The at least one first cam member  78  may be angled relative to the body of the retaining member  68 . 
     The plunger  26  may have at least one alignment member, such as the first alignment member  71  shown in  FIG.  3 A  or  FIG.  23    protruding from the plunger body  32 . As described herein, the at least one first alignment member  71  is shaped and/or configured for facilitating self-orienting alignment of the plunger  26  with the piston  88 . 
     To engage the plunger  26  with the piston  88 , the syringe  12  is first inserted into the syringe port  16  of the fluid injector  10  (shown in  FIG.  1   ), as described herein. If the piston  88  is rotationally misaligned relative to the plunger  26  such that the one or more alignment members on the plunger  26  are not in rotational alignment to be received within the recesses  119  on the plunger head  92 , the one or more alignment members on the plunger  26  contact the guiding surface  117  of the second alignment member  113  on the piston head  92  to rotate the piston head  92  into alignment for connecting to the plunger  26 . In this manner, the piston  88  self-orients itself relative to the plunger  26  such that the plunger  26  may be releasably locked with the piston  88 . Distal movement of the piston  88  causes the retaining members  68  to deflect outward relative to the plunger longitudinal axis  34  from a first, undeflected position, to a deflected position. The piston  88  is advanced distally until the terminal portion of the second end  72  clears the retaining members  68 , thereby allowing them to deflect radially inward toward or to their initial undeflected position. The catch  74  of at least one retaining member  68  is retained within the locking ledge  111  of the recess  119  or the locking ledge  111   a  on a proximal surface of the second alignment members  113  to prevent disengagement of the plunger  26  from the piston head  92 . 
     To unlock the syringe  12  from the syringe port  16  and disengage the plunger  26  from the piston  88 , the syringe  12  is rotated clockwise or counter-clockwise about the syringe longitudinal axis, in a clockwise or counter-clockwise direction, relative to the syringe port  16 . Rotation of the syringe  12 , and thereby the plunger  26 , about the plunger longitudinal axis  34  engages the first cam member  78  on the plunger  26  with the second cam member  98  on the piston head  92 . Such movement causes a deflection of the at least one retaining member  68  radially away from the piston head  92  to unlock the plunger  26  from the piston head  92  and allow the removal of the syringe  12  from the injector  10  (shown in  FIG.  1   ). In an alternative aspect, the catch  74   a  (shown in  FIG.  24 A ) may be on the distal surface of the at least one retaining member  68  to engage within the locking ledge  111  shown in  FIG.  24 A  to prevent disengagement of the plunger  26  from the piston head  92 . The first cam member  78   a  may positioned as indicated in  FIG.  24 A  to enable the catch  74   a  to be released upon relative rotation of the plunger  26  and the piston  88 . 
     In accordance with various alternative aspects of the disclosure, the structural details of the connection interface between the plunger  26  and the piston  88  described herein could be reversed. That is, connection interfaces of any plunger  26  could comprise, for example, the locking mechanism and corresponding features described herein with reference to any piston  88 , while the connection interface on the piston  88  could comprise, for example, the elements described herein as being part of the plunger  26 . 
     Although the disclosure has been described in detail for the purpose of illustration based on what are currently considered to be the most practical and preferred aspects, it is to be understood that such detail is solely for that purpose and that the disclosure is not limited to the disclosed aspects, but, on the contrary, is intended to cover modifications and equivalent arrangements. For example, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any aspect can be combined with one or more features of any other aspect.