1. A method for per-operatively determining the position of a linear instrument with respect to medical images of a body, the method comprising the steps of : a. placing a calibration support comprising fiducial markers on the skin of the body in the intended region of interest; b. acquiring the medical images of the body with calibration support in place; c. detecting the fiducial markers on the medical images to determine the geometric relationship between a coordinate system attached to the medical images and a coordinate system attached to the calibration support; d. connecting rigidly at least one micro-camera to the calibration support at a known position; e. placing a linear instrument in the field of view of the at least one micro-camera; f. detecting in real time the linear instrument in the video images of the at least one micro-camera by searching linear or curvilinear segments in the video images and by constructing a 3D line or curve corresponding to the axis of the linear instrument based on said linear or curvilinear segments; g. calculating in real time the position of the linear instrument in a coordinate system attached to the at least one micro-camera using the calibration of each micro-camera in the coordinate system of the calibration support; and h. transferring the position of the linear instrument in the coordinate system attached to the medical images using the result of step (c).