An instrument guide is described for mounting an invasive instrument such as a biopsy needle to an imaging probe, controlling its position, monitoring its position, and/or predictively displaying its position on a user display of the medical imaging system. A plurality of substantially rigid segments are hingeably connected to the probe, to an instrument handle, and to each other such that movement of the biopsy needle is restricted to within the imaged plane. However, substantial freedom of movement within the imaged plane is provided such that the instrument may be inserted into the patient over a wide range of angles. In one preferred embodiment, angle detectors are provided at each segment intersection and measurements provided for computing and displaying the instrument position and orientation on the user display. The instrument guide/position monitor is preferably made with low-cost components such that it is disposable after a single use. A predictive user display is provided in which the throw of a spring-loaded instrument is shown on the user display, the throw corresponding to the space that the instrument will occupy after a spring trigger is activated.