CHARACTERISTICS OF AN IMPROVED MAGNETIC-IMPLANT GUIDANCE-SYSTEM

The previous companion paper described the motivation, design, and ear ly experiments of a Magnetic Stereotaxis System. The part of the syste m considered in these papers is a helmet with a roughly cubic array of six superconducting coils used to apply force on small permanent magn et pellets in brain and in brain phantom material. This apparatus will be used to deliver drugs and other therapies directly into deep brain tissues, under control of a computer and fluoroscopic imaging system. Here, we analyze the general stability problems of controlling the cu rrents in the coils for impulsive stepwise motion of the pellet, subje ct to quench avoidance in the superconducting coils, and in the face o f Earnshaw's theorem governing stability in static magnetic fields. We also describe solutions that have been found to the primary difficult ies limiting controlled pellet motion in the studies presented in the companion paper.