Measurement of friction on straight catheters in in vitro brain and phantom material.

As part of our studies on the magnetic stereotaxis system (MSS), a means of delivering therapies to the bulk brain, we have measured the frictional forces on a thin, straight tube used to simulate a catheter. Experiments were done with a spring-loaded, stainless steel tube of 1.9-mm diameter which was passed through 5.5 cm of gelatin phantom or, alternatively, through in vitro calf brain. The dynamic response of the tube to sudden displacement of the outer end of the spring yields estimates of the tube's friction per unit length. Twenty-three runs in the two media were analyzed for the static and dynamic frictional forces exhibited. In these series the static frictional forces were found to be (0.0132 +/- 0.0012) N cm-1 [(1.32 +/- 0.12) g cm-1] of length in the gelatin phantom and (0.0079 +/- 0.0008) N cm-1 [(0.79 +/- 0.08) g cm-1] of length in brain. The kinetic friction coefficient, b, was found to be (8.4 +/- 2.1) N s m-1/cm length of catheter in brain and (16.3 +/- 7.6) N s m-1/cm length of catheter in the phantom material. Based on these figures, the MSS will be capable of moving straight catheters of similar friction that are 20-cm long at rates of displacement of 0.02 to 0.05 cm s-1 in the white and grey matter of the brain. Future studies will evaluate the forces arising from curved paths. Unanswered questions remain as to the mechanical difference between in vivo and in vitro brain, between animal and human brain, and the involvement of sulci in practical paths of motion.

Functional design features and initial performance characteristics of a magnetic-implant guidance system for stereotactic neurosurgery.

A helmet with a roughly cubic array of six super-conducting coils is used to apply force on a small permanent magnet pellet in brain or in brain phantom material. This apparatus, called the Magnetic Stereotaxis System, will be used to deliver drugs and other therapies directly into deep brain tissues, under control of a computer and fluoroscopic imaging system. This paper considers only the force application aspects of the instrument. The primary design features of the helmet and power supply controls are presented, along with field plot data and single-axis motion results. The field plot data show that agreement with the finite-element iron-free field calculations is sufficiently high (> 1%) for the instrument. These preliminary motion data indicate accuracy better than 2 mm for the impulsive pellet motion, even though the visual position observations had significantly greater error than the completed imaging system will have. The companion paper will take up analysis of the control aspects of the motion, and our recent solutions to difficulties found in the experimental work described here.

Characteristics of an improved magnetic-implant guidance system.

The previous companion paper 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, we 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 Earnshaw's theorem governing stability in static magnetic fields. We also describe solutions that have been found to the primary difficulties limiting controlled pellet motion in the studies presented in the companion paper.

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 developmental 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 aluminum) to minimize distortion of the superconducting coil's field. In this paper, we present the design principles for the goniometer, describe the essential features of its construction, and discuss its performance characteristics and limitations. We also discuss a strategy for performing precision magnetic stereotaxis procedures with an arrangement of static superconducting coils.

Thermodynamics of movable inductively heated seeds for the treatment of brain tumors.

A thermodynamic study is presented of temperature distributions created by an inductively heated 6-mm-diam Ni sphere imbedded in vivo and in vitro into porcine brain tissue. This study was performed in support of the development of a system that creates localized heat-induced lesions in deep-seated brain tumors. In this system, a magnetic "seed" will be remotely repositioned within the brain by an externally produced magnetic field. Convective effects of a hot moving seed will produce a different thermodynamic situation than that arising from an array of static implants. In this work, a study is presented of part of the expected change, in which a static sphere is heated to high temperature. Measurements were made of the temporal and spatial dependence of the temperature rise in the vicinity of the heated sphere, in vivo in four animals and in one that was euthanized immediately prior to experimentation. These results are used for parameter estimation with a theoretical model based on a point source solution to a form of the thermal diffusion equation, i.e., the "bioheat transfer equation." With this model thermal distributions from a power source of arbitrary geometry can be found using appropriate integration methods, and the method has widespread applicability. Estimates of blood flow rates, tissue thermal conductivity, and seed power absorption were found using the parameter estimation algorithm. The estimated blood perfusion exhibits a step increase following the first heating in multiple heating experiments. Thermal conductivity estimated using data from the nonperfused (in vitro) animal is 0.6 W/m degrees C. Seed power absorption is estimated correspondingly to be 0.9 W, a result confirmed independently with calorimetry. Statistical uncertainty is established for the radial decrease of the tissue temperature rise created by this method. This result allows estimation of a cell death boundary uncertainty of 0.6 mm, caused by fluctuations in power delivered to the seed, uncertainty in the temperature probe placements, and thermal properties such as blood perfusion and tissue thermal conductivity.

Magnetic stereotaxis: a technique to deliver stereotactic hyperthermia.

Advances in imaging techniques and computer software over the past decade now define brain abnormalities such as tumors in precise, three-dimensional images. We have taken advantage of these technological improvements in designing a system capable of performing magnetic manipulation of an object in a nonlinear trajectory and able to deliver hyperthermia to highly specific targets within the brain. This device relies on external magnets to pull a small metal pellet (thermoceptor) through the brain, and on biplane fluoroscopy to localize the thermoceptor with respect to previously obtained magnetic resonance images. A radiofrequency tuned circuit serves as the hyperthermia applicator and selectively heats the thermoceptor. This paper describes experiments conducted in a series of dogs showing that all three components of the system (magnetic drive, stereotactic real time imaging, and hyperthermia) can be achieved. Integration of the system was accomplished in one animal. These encouraging results need further detailed substantiation in each of the components, yet demonstrate the feasibility of such a device.

Nonlinear magnetic stereotaxis: three-dimensional, in vivo remote magnetic manipulation of a small object in canine brain.

In a series of in vivo experiments on five adult canines, a small cylindrical permanent magnet (approximately 5-mm diameter x 5 mm long) was magnetically moved under fluoroscopic guidance from an occipital-lobe burr hole to a predetermined destination within the brain and then removed. On three of the animals, dorsal and temporal skull markers were used to establish a coordinate system against which the motions of the seed were referenced. These procedures were sufficiently accurate to permit the guided motion of the seed along nonlinear paths within the brain, including traversal of the midline through the corpus callosum. For removal, the seed could be steered either to a frontal lobe location for extraction through an auxiliary burr hole, or back to the same burr hole through which it had been inserted. This article discusses the way in which stereotactic motions were obtained, the performance limits of the instrumentation and the precision of motion achieved.

Experimental determination of the force required for insertion of a thermoseed into deep brain tissues.

Our laboratories are developing a new technique for delivering localized hyperthermia to deep-seated brain tumors. In this technique, a spherical thermoseed is stereotactically navigated through the brain and tumour tissues via the noncontact application of an external magnetic force. The force required to produce motion of a 3 mm diameter sphere through in vitro brain tissues was measured to be 0.07 +/- 0.03 N. This result was obtained from a series of experiments performed on whole brain specimens extracted from adult canines. Data were also taken with a 3 mm x 3 mm cylinder and a 5 mm sphere. An experimental procedure simulating physiological conditions was developed prior to testing. Evaluations of systematic effects included determinations of the calibration uncertainties, tests of the dependence of the measured force on temperature, and studies of the effects of method of storage of the tissue specimens. The results obtained are compared with (and confirmed by) two different series of experiments performed in vivo on adult canines and with another series of experiments using brain phantom gelatin.

Preliminary experimental investigation of in vivo magnetic manipulation: results and potential application in hyperthermia.

The first in vivo experiments in support of a new technique for delivering stereotaxic hyperthermia have been conducted at the Experimental Surgery Facility of the University of Virginia's Medical Center. We call this technique the "Video Tumor Fighter." In each of twelve trials a single, small permanent magnet or train of small permanent magnets was implanted on the brain surface of adult canine models. In three of the trials, this "seed" (typically 6-mm diameter X 6-mm long) was moved by magnetic manipulation to different locations within the brain. In two other trials, the seed moved along the interface between the brain and the inner vault of the skull. The noncontact magnetic manipulation was accomplished by coupling the permanently magnetized seed to the large dc magnetic field gradient created by a water-cooled coil surrounding the animal's head. The seed's motions were monitored with x-ray fluoroscopy; its rate of movement was found to be approximately 0.8 mm s-1. The forces required to produce these motions were on the order of 0.07 N. We document here the instrumentation used in these trials, describe the experimental procedures employed, and discuss the technical aspects of the results.

Magnetic movement of a brain thermoceptor.

Hyperthermia has significant potential as an adjuvant form of brain tumor therapy. Current intracranial hyperthermia methods, however, are limited in their ability to control spatiotemporal thermal distribution. A stereotaxic magnetic movement system that may be capable of heating discrete regions of brain to a preselected temperature is described. With this system, a ferromagnetic object (referred to as a thermoceptor) is directed through the brain by an external drive magnet. Real time thermoceptor position is monitored with biplanar fluoroscopy and superimposed on a preoperative magnetic resonance imaging scan using a computer. Once in position, the thermoceptor can be inductively heated by externally generated radiofrequency signals. Experiments on the magnetic drive and imaging aspects of this system have been conducted in vitro and in vivo. Mechanical studies of cadaver dog brains revealed that a mean force of 0.07 +/- 0.03 N was required to move a 3-mm diameter sphere through brain at a speed of less than 1 cm/15 s. A cranial phantom with mechanical properties similar to brain was constructed of gelatin and Plexiglas. With the use of a "neck loop" design drive magnet with a maximum magnetic field strength of 0.10 T, a 3 x 3 mm cylindrical neodymium iron boron thermoceptor was smoothly directed through the phantom in two dimensions. Additional experiments were conducted with a larger drive magnet in five anesthetized dogs. Neodymium iron boron and samarium cobalt thermoceptors of various shapes and sizes were placed into the cerebral cortex through a burr hole, then directed with the drive magnet. Fluoroscopy was used to follow the thermoceptor movements.(ABSTRACT TRUNCATED AT 250 WORDS)