1. A device for detecting a surgical tool in an image, said surgical tool going through an incision made in a patient's skin or one of patient's organs, the device comprising: means for storing shape parameters of the surgical tool; means for determining the position of the projection of the incision in the image plane; and means for determining points or straight lines characteristic of the projection of the surgical tool on the image based on the shape parameters and on the position of the projection of the incision.