A new method of stereotaxis.

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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 and Clarke (4) sixty years ago, was applied to humans by Spiegel and Wycis (6) twenty years ago, and has been adopted with variations by most sophisticated stereotaxic centers. This system of rectangular coordinates, while fairly accurate, is usually complicated and time-consuming. On the other hand, the system of polar coordinates or its modification was popularized by Cooper (1). While many physicians prefer this latter technic because of its relative simplicity, achieving accuracy of probe-target alignment is often difficult. In the true rectangular coordinate system the probe is always parallel to the frontal and lateral x-ray films so that parallax and geometric distortion problems are reduced. Movements are rectilinear and measured in millimeters. In the polar coordinate system the probe guide is fixed to rotate about a fulcrum over any convenient burr hole. Movements are angular and measured in degrees. Although the latter method appears simple and rapid, difficult problems with parallax and geometric distortion occur (2). Because the probe is not parallel with the x-ray films, angular measurements of probe correction cannot be transferred directly from the x-ray films to the probe guide. Nevertheless, the use of the polar coordinate system of stereotaxis is very appealing. If accuracy of probe-target alignment could be achieved simultaneously in two planes, a very rapid, simple, and accurate method of stereotaxis would be effected. We decided to improve upon the optical concepts of Remond (5) and Woringer, Chambon, and Brain (7) by introducing a biplane television (TV) system. Instead of correcting distorted radiographic measurements for transference to the stereotaxic frame (usual technic), we set about to distort the stereotaxic frame optically so that the radiographic measurements can be transferred directly.