1. A device for determining the position of an instrument with respect to medical images of a body, the device comprising: a. a calibration support comprising a set of fiducial markers visible on medical images and adapted to be fixed to the skin of a body; b. at least one micro-camera providing video images and mounted on the calibration support in a known position; c. a processing unit connected to the micro-cameras and configured to process the video images from said micro-cameras to determine the position of the instrument with respect to medical images.