Non-Invasive Electromagnetic Technique for Monitoring Physiological Changes in the Brain
None of the previous technologies contemplate measuring localized impedance changes in the brain to evaluate physiological changes in the brain, such as the occurrence of edematous tissue, and none of the references teach an apparatus capable of such spatially localized impedance measurements.
Applicant was the first to discover that edematous tissue has a significantly different conductivity (or impedance) from normal white or grey brain matter. Applicant believes that edematous tissue is formed when white or grey matter in the brain becomes diffused or prefused with edematous fluid by an as yet unknown intercellular or extracellular process. As will be described later, the discovery that impedance changes can be used to identify edematous tissue was made using invasive probes. Applicant generally found that the conductivity change between normal and edematous grey tissue, for instance, would change by as much as 0.14 mho/meter, or equivalently by 100% of the normal value.
The present invention detects the increase in conductivity (or decrease in impedance) of brain tissue overtime to identify edematous tissue in an area of the brain. Edematous tissue may occur in localized areas near the surface of the cranium or may occur deeper in the brain. Since edematous tissue swells, blood and CSF fluid in the brain which may have the same conductivity as edema fluid, might be displaced. Therefore, localized spatially discrete changes in impedance over time must be measured to detect the physiological changes associated with brain edema at a particular location in the brain.
Further, monitoring localized impedance changes in the brain will allow one to measure and diagnose hydrocephalus since an increase in the ventricular volume will result in an increase in conductivity in certain localized areas of the brain. This is because CSF fluid which fills the expanded ventricle has a significantly greater conductivity (1.5-1.75 mho/meter) than white matter (0.10 to 0.15 mho/meter) or grey matter (0.12 to 0.23 mho/meter).
Applicant also realized that such localized impedance changes can be sensed non-invasively using a magnetic field and detecting the changes in mutual inductance between the brain and a sense coil. The apparatus described herein, and also described in part in a copending commonly assigned patent applications entitled "Electromagnetic Bone Healing Sensor" (U.S. Pat. No. 4,688,580), and "Non-invasive Electromagnetic Technique for Monitoring Physiological Changes in the Brain" (U.S. Pat. No. 4,690,149), generates a spatially discrete oscillating magnetic field which radiates a pre-selected location of the brain. The magnetic field induces eddy currents in brain tissue and fluid in the radiated area. When these eddy current are generated, they produce a secondary weak magnetic field which is detected by the apparatus. The magnitude of the eddy currents is proportional to the actual impedance of the tissue and fluid where the eddy currents are generated. The magnitude of the eddy currents in turn directly affect the magnitude of the secondary weak magnetic field.
The invented apparatus is capable of detecting small variations in impedance changes and quantitatively measuring such changes. A magnetic drive/sensor means is designed to concentrate the magnetic field in spatially localized areas within the brain. The invention also teaches various techniques for monitoring a pre-selected and localized area in the brain over time to generate a time-trend view of brain impedance. An oscillator detector in combination with the magnetic drive/sensor means is specially designed to be sensitive to small impedance changes and to reduce polarization effects and background noise which could render such monitoring impossible.
It is hoped that continuous monitoring of a patient at his bedside would enable physicians to treat the first sign of swelling and also to measure any therapy's effectiveness. The invented device may prevent much of the brain damage that results from head injuries, stroke, brain tumors or drug abuse when injured brain tissue swells and presses against the inside of the skull.
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