Biplanar X-Ray Methods for Stereotactic Intraoperative Localization in Deep Brain Stimulation Surgery

Surgical Strategy for Directional Deep Brain Stimulation

Deep brain stimulation (DBS) is a well-established treatment for drug-resistant involuntary movements. However, the conventional quadripole cylindrical lead creates electrical fields in all directions, and the resulting spread to adjacent eloquent structures may induce unintended effects. Novel directional leads have therefore been designed to allow directional stimulation (DS). Directional leads have the advantage of widening the therapeutic window (TW), compensating for slight misplacement of the lead and requiring less electrical power to provide the same effect as a cylindrical lead. Conversely, the increase in the number of contacts from four to eight and the addition of directional elements has made stimulation programming more complex. For these reasons, new treatment strategies are required to allow effective directional DBS. During lead implantation, the directional segment should be placed in a "sweet spot," and the orientation of the directional segment is important for programming. Trial-and-error testing of a large number of contacts is unnecessary, and efficient and systematic execution of the programmed procedure is desirable. Recent improvements in imaging technologies have enabled image-guided programming. In the future, optimal stimulations are expected to be programmed by directional recording of local field potentials.

Monte Carlo Simulation of Errors for N-localizer Systems in Stereotactic Neurosurgery: Novel Proposals for Improvements

Introduction: Frame-based stereotaxis has been widely utilized for precise neurosurgical procedures throughout the world for nearly 40 years. The N-localizer is an integral component of most of the extant systems. Analysis of targeting errors related to the N-localizer has not been carried out in sufficient detail. We highlight these potential errors and develop methods to reduce them. 

Methods: N-localizer systems comprising three and four N-localizers of various geometries were analyzed using Monte Carlo (MC) simulations. The simulations included native and altered geometric dimensions (Width [W] x Height [H]). Errors were computed using the MC simulations that included the x- and y-axes of vertically oriented rods, that altered the W/H ratio, and that added a fourth N-localizer to a three N-localizer system. 

Results: The inclusion of an overdetermined system of equations and the geometries of the N-localizer systems had significant effects on target errors. Root Mean Square Errors (RMS-e) computed via millions of MC iterations for each study demonstrated that errors were reduced by (1) inclusion of the x- and y-coordinates of the vertically oriented rods, (2) a greater triangular area enclosed by the diagonal fiducials of the N-localizer system (stereotactic triangle), (3) a larger W/H ratio, and (4) an N-localizer system that comprised four N-localizers.

Conclusion: Monte Carlo simulations of Root Mean Square error (RMS-e) is a useful technique to understand targeting while using N-localizer systems in stereotactic neurosurgery. The application of vertical rod positions enhances computational accuracy and can be performed on any N-localizer system. Keeping the target point within the stereotactic triangle enclosed by the diagonal rods can also reduce errors. Additional optimizations of N-localizer geometry may also reduce potential targeting errors. Further analysis is needed to confirm these findings which may have clinical importance. 

Coordinate Systems for Navigating Stereotactic Space: How Not to Get Lost

All stereotactic neurosurgical procedures utilize coordinate systems to allow navigation through the brain to a target. During the surgical planning, indirect and direct targeting determines the planned target point and trajectory. This targeting allows a surgeon to precisely reach points along the trajectory while minimizing risks to critical structures. Oftentimes, once a target point and a trajectory are determined, a frame-based coordinate system is used for the actual procedure. Considerations include the use of various coordinate spaces such as the anatomical (\begin{document}{X}\end{document}), the frame (\begin{document}{X'}\end{document}), the head-stage (\begin{document}{X''}\end{document}), and an atlas. Therefore, the relationships between these coordinate systems are integral to the planning and implementation of the neurosurgical procedure. Although coordinate transformations are handled in planning via stereotactic software, critical understanding of the mathematics is required as it has implications during surgery. Further, intraoperative applications of these coordinate conversions, such as for surgical navigation from the head-stage, are not readily available in real-time. Herein, we discuss how to navigate these coordinate systems and provide implementations of the techniques with samples.