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 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 movable about the pivot pin in a radially inward direction to engage 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.本發明揭示一種與一流體注射器之一可往復移動活塞相關聯之接合機構，其經構形以將一接合部分可釋放地接合於一滾動膜片注射筒之一近端處，該滾動膜片注射筒具有經構形以在由該活塞作用於其上時可以自動滾動之一撓性側壁。該接合機構具有複數個接合元件，該複數個接合元件可相對於該注射筒之該接合部分，圍繞一樞軸銷，在一第一位置與一第二位置之間可逆且可樞轉地移動，在該第一位置處，該複數個接合元件自該注射筒之該接合部分脫離，在該第二位置處，該複數個接合元件可在一徑向向內方向圍繞該樞軸銷樞轉地移動以與該注射筒之該接合部分接合。該接合機構進一步具有用於使該複數個接合元件在該第一位置與該第二位置之間移動之一傳動機構。
