Michael A. Lawson

Characteristics of an improved magnetic-implant guidance system

The previous companion paper (see ibid., vol. 42, no. 8, p.793, 1995) described the motivation, design, and early experiments of a Magnetic Stereotaxis System. The part of the system considered in these papers is a helmet with a roughly cubic array of six superconducting coils used to apply force on small permanent magnet 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, the authors analyze the general stability problems of controlling the currents in the coils for impulsive stepwise motion of the pellet, subject to quench avoidance in the superconducting coils, and in the face of Earnshaws theorem governing stability in static magnetic fields. The authors also describe solutions that have been found to the primary difficulties limiting controlled pellet motion in the studies presented in the companion paper.<<ETX>>

Goniometric motion controller for the superconducting coil in a magnetic stereotaxis system

For the purpose of positioning the 80 kg, 2 T superconducting coil/cryostat used in the development version of a magnetic stereotaxis system, a four degree-of-freedom goniometer has been designed, built, and tested. Computer-controlled, stepping motor actuators enable movement of the coil via either joystick, keyboard, or translator-module keypad commands. An integral arrangement of counterweights and counterbalances minimizes the overall weight and size of the goniometer while maintaining static and dynamic stability during operation. As much of the structure as possible has been made of nonmagnetic materials (mostly aluminium) to minimize distortion of the superconducting coils field. The design principles for the goniometer, the essential features of its construction, and its performance characteristics and limitations are discussed, along with a strategy for performing precision magnetic stereotaxis procedures with an arrangement of static superconducting coils.<<ETX>>

Low-dose magnetic-field-immune biplanar fluoroscopy for neurosurgery

The imaging chain of a bi-planar fluoroscopic system is described for a new neurosurgical technique: the Video Tumor Fighter (VTF). The VTF manipulates a small intracranially implanted magnet, called a thermoseed, by a large external magnetic field gradient. The thermoseed is heated by rf-induction to kill proximal tumor cells. For accurately guiding the seed through the brain, the x-ray tubes are alternately pulsed up to four times per second, each for as much as two hours. Radio-opaque reference markers, attached to the skull, enable the thermoseeds three dimensional position to be determined and then projected onto a displayed MRI brain scan. The imaging approach, similar to systems at the University of Arizona and the Mayo Clinic, includes a 20 cm diameter phosphor screen viewed by a proximity focused microchannel plate image intensifier coupled via fiberoptic taper to a solid state camera. The most important performance specifications are magnetic field immunity and, due to the procedure duration, low dosage per image. A preliminary arrangement designed in the laboratories yielded usable images at approximately 100 (mu) R exposure per frame. In this paper, the results of a series of studies of the effects of magnetic fields on microchannel plate image intensifiers used in the image detection chain are presented.