A method and apparatus is disclosed, employed in combination with an ultrasonographic system, for assisting in guiding and placing at least one medical instrument into a target tissue during a minimally invasive medical procedure. The apparatus comprises: a reference means; a processing means in communication with the ultrasonographic system; and a mounting means for mounting the reference means in a predetermined relationship to an ultrasonographic transducer. The reference means includes a plurality of apertures arranged in an predefined manner which are sized to permit at least one medical instrument to pass therethrough. The processing means determines the spatial relationship between the target tissue and the reference means and further merges a representation of the plurality of apertures with the ultrasonographic image to form a positioning image. The positioning image assists in the guiding and placement of the at least one medical instrument into a target location by identifying a path to the target location via a selected aperture. Preferably, each aperture is provided with an internal adjustment means to permit minor adjustment to the selected path.