System for planning the introduction of a needle in a patient's body, comprising: - a needle guide adapted to be coupled to a needle, wherein the needle guide is selected from: (i) a guide in which the needle is adapted to be slidingly arranged, said guide comprising a tracker configured to be tracked by the navigation system; (ii) a guide adapted to be rigidly attached to the needle, said guide comprising a tracker configured to be tracked by the navigation system; and (iii) a tracker configured to be tracked by the navigation system, said tracker being adapted to be arranged inside the needle.; - a navigation system configured for tracking the needle guide with respect to a 3D medical image of the patient; - a processor configured for: - receiving navigation data of the needle guide from the navigation system, - determining a virtual needle having a position and an orientation with respect to the 3D image that are computed based on said navigation data of the needle guide, - detecting at least one inserted needle (2'; 2a', 2b', 2c') as a trace in the 3D medical image, - computing a distance between the virtual needle and the detected needle (2'; 2a', 2b', 2c'), and - determining a representation of said computed distance; - a display coupled to the processor for displaying a representation of the virtual needle and said determined representation of the computed distance between the virtual needle and the at least one detected needle (2'; 2a', 2b', 2c'). - a needle guide adapted to be coupled to a needle, wherein the needle guide is selected from: (i) a guide in which the needle is adapted to be slidingly arranged, said guide comprising a tracker configured to be tracked by the navigation system; (ii) a guide adapted to be rigidly attached to the needle, said guide comprising a tracker configured to be tracked by the navigation system; and (iii) a tracker configured to be tracked by the navigation system, said tracker being adapted to be arranged inside the needle.; (i) a guide in which the needle is adapted to be slidingly arranged, said guide comprising a tracker configured to be tracked by the navigation system; (ii) a guide adapted to be rigidly attached to the needle, said guide comprising a tracker configured to be tracked by the navigation system; and (iii) a tracker configured to be tracked by the navigation system, said tracker being adapted to be arranged inside the needle.; - a navigation system configured for tracking the needle guide with respect to a 3D medical image of the patient; - a processor configured for: - receiving navigation data of the needle guide from the navigation system, - determining a virtual needle having a position and an orientation with respect to the 3D image that are computed based on said navigation data of the needle guide, - detecting at least one inserted needle (2'; 2a', 2b', 2c') as a trace in the 3D medical image, - computing a distance between the virtual needle and the detected needle (2'; 2a', 2b', 2c'), and - determining a representation of said computed distance; - receiving navigation data of the needle guide from the navigation system, - determining a virtual needle having a position and an orientation with respect to the 3D image that are computed based on said navigation data of the needle guide, - detecting at least one inserted needle (2'; 2a', 2b', 2c') as a trace in the 3D medical image, - computing a distance between the virtual needle and the detected needle (2'; 2a', 2b', 2c'), and - determining a representation of said computed distance; - a display coupled to the processor for displaying a representation of the virtual needle and said determined representation of the computed distance between the virtual needle and the at least one detected needle (2'; 2a', 2b', 2c').