An engagement mechanism associated with a reciprocally movable piston of a fluid injector, the piston comprising the engagement mechanism, 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 pivotally movable about a pivot pin 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 pivotally moved about the pivot pin in a radially inward direction to engage 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 a piston rod of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, said abutment section being configured to abut at least a portion of the proximal end of the syringe, wherein the abutment section is operatively engaged with the plurality of engagement elements, such that movement of the abutment section with regard to the outer piston sleeve moves the engagement elements between the first position and the second position; 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 the piston rod movably coupled to the piston, wherein movement of the piston rod 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 elements from the first position to the second position. a plurality of engagement elements reversibly and pivotally movable about a pivot pin 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 pivotally moved about the pivot pin in a radially inward direction to engage 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 a piston rod of the fluid injector; an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, said abutment section being configured to abut at least a portion of the proximal end of the syringe, wherein the abutment section is operatively engaged with the plurality of engagement elements, such that movement of the abutment section with regard to the outer piston sleeve moves the engagement elements between the first position and the second position; 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 the piston rod movably coupled to the piston, an abutment section movably received within the outer piston sleeve at a distal end of the outer piston sleeve, said abutment section being configured to abut at least a portion of the proximal end of the syringe, wherein the abutment section is operatively engaged with the plurality of engagement elements, such that movement of the abutment section with regard to the outer piston sleeve moves the engagement elements between the first position and the second position; 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 the piston rod movably coupled to the piston, wherein movement of the piston rod 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 elements from the first position to the second position.