An engagement mechanism associated with a reciprocally movable piston of a fluid injector is 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. The engagement mechanism has 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. The engagement mechanism further has a drive mechanism for moving the plurality of engagement elements between the first position and the second position.