An assembly for reconstitution of a substance comprising: a fluid injector comprising a fluid reservoir and having a longitudinal axis; a receptacle defining an interior volume and having a longitudinal axis; and a pressure equalizing device connected between the fluid injector and the receptacle having a longitudinal axis, the pressure equalizing device comprising: a container for attachment to the receptacle, to permit pressure equalization between the container and the receptacle, the container comprising a first chamber; a second chamber, and a movable wall or a bellows, disposed within the container between the first chamber and the second chamber; and a fluid transfer member having an access channel, connected between the fluid injector and the interior volume of the receptacle, and a vent channel, connected between the interior volume of the receptacle and the first chamber, wherein when the receptacle is connected to the pressure equalizing device, the vent channel establishes direct fluid communication between the first chamber and the receptacle, wherein the longitudinal axis of the pressure equalizing device, the longitudinal axis of the receptacle, and the longitudinal axis of the fluid injector are aligned with or parallel to each other, characterized in that the movable wall or the bellows defines a central opening, and wherein the fluid transfer member is inserted through the central opening such that the movable wall or the bellows can slide along the fluid transfer member. a fluid injector comprising a fluid reservoir and having a longitudinal axis; a receptacle defining an interior volume and having a longitudinal axis; and a pressure equalizing device connected between the fluid injector and the receptacle having a longitudinal axis, the pressure equalizing device comprising: a container for attachment to the receptacle, to permit pressure equalization between the container and the receptacle, the container comprising a first chamber; a second chamber, and a movable wall or a bellows, disposed within the container between the first chamber and the second chamber; and a fluid transfer member having an access channel, connected between the fluid injector and the interior volume of the receptacle, and a vent channel, connected between the interior volume of the receptacle and the first chamber, a container for attachment to the receptacle, to permit pressure equalization between the container and the receptacle, the container comprising a first chamber; a second chamber, and a movable wall or a bellows, disposed within the container between the first chamber and the second chamber; and a fluid transfer member having an access channel, connected between the fluid injector and the interior volume of the receptacle, and a vent channel, connected between the interior volume of the receptacle and the first chamber, wherein when the receptacle is connected to the pressure equalizing device, the vent channel establishes direct fluid communication between the first chamber and the receptacle, wherein the longitudinal axis of the pressure equalizing device, the longitudinal axis of the receptacle, and the longitudinal axis of the fluid injector are aligned with or parallel to each other, characterized in that the movable wall or the bellows defines a central opening, and wherein the fluid transfer member is inserted through the central opening such that the movable wall or the bellows can slide along the fluid transfer member.