A guidance system for assisting with the insertion of a needle into a patient body is disclosed. The guidance system utilizes ultrasound imaging or other suitable imaging technology. In one embodiment, the guidance system comprises an imaging device including a probe for producing an image of an internal body portion target, such as a vessel. One or more sensors are included with the probe. The sensors sense a detectable characteristic related to the needle, such as a magnetic field of a magnet included with the needle. The system includes a processor that uses data relating to the sensed characteristic to determine a 3-D position of the needle. The system includes a display for depicting the position of the needle. In other embodiments, a guidance system for guiding rigid or other medical instruments is disclosed, together with various example implementations thereof.