A method including an acquisition, with an acquisition apparatus, of at least one two-dimensional image of arches of a patient, called “updated image”, in actual acquisition conditions, a separator separating lips of the patient in order to improve the visibility of patient's teeth, said separator including a register mark, wherein the representation of the register mark on the updated image is used to recut the updated image and/or to roughly assess the actual acquisition conditions and/or to guide the positioning of the acquisition apparatus at the moment of the acquisition of the updated image and/or to identify a dental situation and/or an action to be achieved by the acquisition apparatus.