An engagement mechanism associated with a reciprocally movable piston of a fluid injector and configured for releasably engaging an engagement portion at a proximal end of a rolling diaphragm syringe having a flexible sidewall configured for rolling upon itself when acted upon by the piston, wherein the engagement mechanism comprises: a plurality of engagement elements reversibly and radially movable relative to the engagement portion of the syringe between a first position, where the plurality of engagement elements are disengaged from the engagement portion of the syringe, and a second position, where the plurality of engagement elements are engaged with the engagement portion of the syringe; and a drive mechanism for moving the plurality of engagement elements between the first position and the second position, characterized in that the drive mechanism comprises: an outer piston sleeve fixed relative to the piston of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, wherein the abutment section is operatively engaged with the plurality of engagement members; and an inner piston sleeve movably received within the outer piston sleeve and connected with the abutment section such that movement of the inner piston sleeve causes movement of the abutment section, wherein the inner piston sleeve is movable by a piston rod movably coupled to the piston, wherein movement of the piston in a proximal direction moves the outer piston sleeve proximally relative to the abutment section, and wherein proximal movement of the outer piston sleeve relative to the abutment section moves the plurality of engagement members from the first position to the second position. wherein the engagement mechanism comprises: a plurality of engagement elements reversibly and radially movable relative to the engagement portion of the syringe between a first position, where the plurality of engagement elements are disengaged from the engagement portion of the syringe, and a second position, where the plurality of engagement elements are engaged with the engagement portion of the syringe; and a drive mechanism for moving the plurality of engagement elements between the first position and the second position, characterized in that the drive mechanism comprises: an outer piston sleeve fixed relative to the piston of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, wherein the abutment section is operatively engaged with the plurality of engagement members; and an inner piston sleeve movably received within the outer piston sleeve and connected with the abutment section such that movement of the inner piston sleeve causes movement of the abutment section, wherein the inner piston sleeve is movable by a piston rod movably coupled to the piston, a plurality of engagement elements reversibly and radially movable relative to the engagement portion of the syringe between a first position, where the plurality of engagement elements are disengaged from the engagement portion of the syringe, and a second position, where the plurality of engagement elements are engaged with the engagement portion of the syringe; and a drive mechanism for moving the plurality of engagement elements between the first position and the second position, characterized in that the drive mechanism comprises: an outer piston sleeve fixed relative to the piston of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, wherein the abutment section is operatively engaged with the plurality of engagement members; and an inner piston sleeve movably received within the outer piston sleeve and connected with the abutment section such that movement of the inner piston sleeve causes movement of the abutment section, wherein the inner piston sleeve is movable by a piston rod movably coupled to the piston, an outer piston sleeve fixed relative to the piston of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, wherein the abutment section is operatively engaged with the plurality of engagement members; and an inner piston sleeve movably received within the outer piston sleeve and connected with the abutment section such that movement of the inner piston sleeve causes movement of the abutment section, wherein the inner piston sleeve is movable by a piston rod movably coupled to the piston, wherein movement of the piston in a proximal direction moves the outer piston sleeve proximally relative to the abutment section, and wherein proximal movement of the outer piston sleeve relative to the abutment section moves the plurality of engagement members from the first position to the second position.