A unit, assembly and device suitable for a medical localization device suitable for localizing a region in a body with a distal hollow needle includes a tube extending from a proximal first end to a distal second end. The first end is designed to be in communication with a fluid filled container. The unit includes a first part of sensor means. The first part is a sensor interface, in which the sensor interface is provided to the tube between the first end and the second end. The sensor interface is suitable for observation of the pressure in a fluid in the tube that is in communication with the fluid in the container. The unit includes a closing device distally with respect to the sensor interface for varying the pressure in the fluid in the tube, in which the sensor interface is a membrane.