Beam limiting unit for radiographic apparatus

A method and a unit(1) for limiting an X-ray beam on opposite sides of the X-ray beam is proposed. The unit (1) comprises: - two blades (5, 6) disposed on opposite sides of the X-ray beam and having inner edges, which delimit the X-ray beam on said opposite sides, and - an actuator (12) linked to both blades (5, 6) and moving both of them in an approaching direction reducing the space between the inner edges of the blades (5, 6), and in a withdrawing direction enlarging the space between the inner edges of the blades (5, 6), wherein - one of the blades (5) is linked to the actuator (12) through an uncoupling device system (9) which allows a relative movement of the one blade (5) with respect to a holder which supports the one blade (5) and is engaged with the actuator (12), and wherein - the motion of the one blade (5) in approaching direction and/or withdrawing direction is impedable causing an asymmetric motion of the blades with respect to an initial position of a symmetry axis of a space between the inner edges of the blades (5, 6)