Implantation of an electrode or probe into the brain by stereotaxis requires accurate biplane radiography to define the target. The x-ray image of the target may be seen directly (e.g., an ancurysm filled with contrast medium) or, as is usually the case, indirectly by measurements from coordinate points outlined by contrast medium (e.g., in parkinsonism the ventrolateral nucleus of the thalamus midway between the anterior and posterior commissures on the lateral film). The intracranial target usually is approached by a specialized probe passed through a burr hole in the skull. A major problem from a radiologic point of view has been transposition of radiographically distorted measurements from the images on biplane x-ray films to the real stereotaxic frame and hidden target. The many methods, instruments, and calculations used by stereotaxic centers testifies to the lack of a satisfactory stereotaxic technic generally accepted by most centers. The system of rectangular coordinates originated with Horsley ...