Ryan T. Ochsner and Dr. Mark Manwaring, Electrical and Computer Engineering
The Brain Instrumentation Laboratory (1220 SWKT) associated with the Department of Neuroscience and Electrical and Computer Engineering at Brigham Young University specializes in the development of new technologies which may facilitate the management of diseases of the brain. Of specific interest to the laboratory is the improved treatment of hydrocephalus. Hydrocephalus is a condition which results in progressive damage to the brain because of tremendous pressure when cerebral spinal fluid (CFS) is not absorbed as fast as it is produced. This condition results in increasing ventricle size within the brain and associated pressure and deformation.
A treatment has been devised for patients with hydrocephalus. A silicone catheter is placed freehand into the brain during surgery. The other end of the catheter is tunneled through the skin into the abdomen where the excessive spinal fluid is once again absorbed into the blood stream.
While the freehand placement of the shunt in the brain is a common practice, it can also be awkward and inaccurate and can result in less-than-optimal alignment of the shunt and eventual malfunction, which may require further surgeries.
In an effort to reduce cost and increase accuracy, a promising new technology has been developed in the Brain Instrumentation Laboratory. This method, named “Knickebein” (for its conceptual model used to guide German aircraft during World War II) relies on the detection and measurement of a magnetic field emitted from a magnet at the tip of a surgical instrument. The system is presently in clinical trials to demonstrate its efficacy versus the freehand method as well as the less practical methods of framed and frameless stereotaxy, which are generally used in more complicated situations.
One project currently being examined is a variation of Knickebein. This variation would reverse the placement and usage of a magnet and an oscillator-equipped catheter (used instead of a magnetometer for cost and size reductions) inside the head. Instead of placing a magnetometer on the forehead and determining location inside the head by locating the magnet enclosed in the tiny catheter, a simple magnetically sensitive oscillatory circuit would be installed inside the catheter while the magnet would be placed outside the skin on the forehead.
The principal task was to design a satisfactory oscillating circuit. A standard “Colpitts” oscillator was chosen. However, in order for the device to interface effectively with a magnet, it was necessary to custom build the inductor component. This was done by wrapping ferrite torroids of different sizes with a thin wire; the number of wraps corresponding to a change in inductance.
About this time, the mathematical feasibility of the project was brought into question. Could a magnet induce a significant enough inductance change in order to effectively simulate a magnetometer? The answer remains to be determined.
A second project was introduced. This project focused on the design of an Inner-Cranial Pressure (ICP) sensor. The goal of the project includes the creation of an implantable ICP sensor inside the head, which is powered externally, outside the head. The circuit would be equipped with a pressure sensor to collect data, components to process the data, and an attached LED to output the information through the skin. A unit outside the head would take readings from the LED. Any consistent changes in intercranial pressure would be detected by the circuit, resulting in a change in blink-rate of the LED.
During the course of the semester, an operating model was created which effectively simulated most of the circuit’s functions. Data was collected within the head via a commercially available pressure sensor. Changes in pressure (which were detected in terms of hundredths of ohms) were fed into a Wheatstone bridge resistive network which converted small changes in resistance to greater differences in voltage. This difference was amplified through a low power operational amplifier which was used to feed the control voltage on a 555 timer device (the reason for this being that small changes in control voltages create much more detectable differences in output frequency in the 555).
Many schemes have been proposed for powering the circuit including piezoelectric, photocells, and magnetic coupling. The idea of powering the unit with photocells was explored. It was determined that the circuit would require at least three volts and 100mA of current. Photocells with this kind of output exceed (in most cases) three inches by two inches dimensionally, which prevent them from being implanted in the human head because of size constraints. It was determined that transmission through the skin could occur with highly focused, ultra bright LED’s. Even at high frequencies (above 20 kHz) these signals are able to pass through the skin with minimal diffusion. Other methods of powering the device are currently under examination.
Both of these projects, the variation on Knickebein, and the Intracranial Pressure sensor, have great future potential. Both will provide surgeons with a new set of tools to perform complicated brain surgeries and to measure intracranial pressure, ultimately facilitating the effective treatment of Hydrocephalus and its associated symptoms.