M.S. Grady

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>>

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 superconducting 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 the authors recent solutions to difficulties found in the experimental work described here.<<ETX>>

Preoperative superselective arteriolar embolization: A new approach to enhance resectability of spinal tumors

The extent of surgical resection of spinal tumors is frequently limited by blood loss and technical difficulty associated with the vascularity of the tumors. We report here the use of superselective percutaneous arterial embolization to reduce the rate of blood loss at the time of surgical resection and enhance resectability. The types of tumors treated were metastatic renal carcinoma, metastatic thyroid carcinoma, metastatic melanoma, and giant cell tumor of the sacrum. Two of the patients required repeated embolization and surgery for recurrent symptoms. The estimated blood loss in seven of nine procedures performed on the six patients ranged from 300 to 800 ml, after which no transfusion was required. In two procedures, extensive resection of very large tumors resulted in larger losses of blood, and postoperative transfusion was necessary. No significant complications of embolization or surgery occurred. A key factor in our embolization technique is the use of microfibrillar collagen, which allows occlusion of tumor vessels as small as 20 microns and may prevent reconstitution of the embolized vessels by collateral flow. We conclude that preoperative arterial embolization enhances the resectability of a variety of spinal tumors by reducing intraoperative blood loss. This may provide an additional benefit by reducing the risk related to postoperative transfusion. By permitting a more aggressive surgical approach, the use of preoperative embolization also has the potential to improve outcome in patients with spinal tumors.

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>>

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

As part of their studies on the magnetic stereotaxis system (MSS), a means of delivering therapies to the bulk brain, the authors 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 tubes 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/spl plusmn/0.0012) N cm/sup -1/ [(1.32/spl plusmn/0.12) g cm/sup -1/] of length in the gelatin phantom and (0.0079/spl plusmn/0.0008) N cm/sup -1/ [(0.79/spl plusmn/0.08) g cm/sup -1/] of length in brain. The kinetic friction coefficient, b, was found to be (8.4/spl plusmn/2.1) N s m/sup -1//cm length of catheter in brain and (16.3/spl plusmn/7.6) N s m/sup -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/sup -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.

Use of the Philadelphia Collar as an Alternative to the Halo Vest in Patients with C-2, C-3 Fractures

We analyzed retrospectively 27 individuals with C-2, C-3 fractures, 8 of whom were treated with a Philadelphia collar rather than the more commonly used halo vest. Successful fusion without neurological deficit occurred in all cases whether treatment was by Philadelphia collar (n = 8), halo apparatus (n = 16), or prolonged bed rest (n = 3). There was an increase in subluxation in 3 patients in both the Philadelphia collar group and the halo apparatus group. However, 1 of the 3 in the collar group was uncooperative and discarded his collar during treatment. If he is excluded, the subluxation rate in the Philadelphia collar group is 27%; the rate in the halo group is 19%. The Philadelphia collar seems to be an acceptable means of stabilizing the neck in C-2, C-3 fractures, but one must consider the degree of cervical movement in the collar (as detected by dynamic x-ray films), patient reliability, and patient age. The presence of subluxation did not preclude successful fusion in either the Philadelphia collar or the halo vest groups.

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.

One-stage reduction cranioplasty for macrocephaly associated with advanced hydrocephalus.

A reduction cranioplasty in one stage was performed on a child with massive enlargement of the head secondary to advanced hydrocephalus. The authors discuss the techniques and advantages of a modified head position and a cross bar craniotomy for reduction cranioplasty.

Ectopic pituitary gland simulating a suprasellar tumor

A case report of ectopic pituitary gland in the suprasellar region of a normal 39-year-old woman with persistent headaches is presented. The embryological development of the pituitary gland is briefly reviewed, with a discussion of the relevant literature. No previous report of normal pituitary tissue in a suprasellar location in the absence of tumor could be found.

Profound Cerebral Vasospasm without Radiological Evidence of Subarachnoid Hemorrhage: Case Report

Profound clinical and radiographic arterial vasospasm in all major intracerebral vessels was identified in a patient with an intraparenchymal hemorrhage that extended into the lateral, 3rd, and 4th ventricles. By computed tomographic criteria and on early cerebrospinal fluid examination, no subarachnoid blood was detected. The temporal appearance of vasospasm was consistent with that typically seen in subarachnoid hemorrhage. Despite hypervolemia and iatrogenic hypertension, cerebral infarction occurred contralateral to the hemorrhage. We conclude that patients without significant subarachnoid hemorrhage (as determined by computed tomographic scanning) may still be at risk for developing vasospasm.