Patent No. 4819648 Non-invasive electromagnetic technique for monitoring time-trends of physiological changes at a particular location in the brain

 

Patent No. 4819648 

Non-invasive electromagnetic technique for monitoring time-trends of physiological changes at a particular location in the brain (Ko, Apr 11, 1989)

Abstract

An apparatus and method for non-invasively sensing physiological changes in the brain is disclosed. The apparatus and method uses an electromagnetic field to measure localized impedance changes in brain matter and fluid. The apparatus and method has particular application in providing time-trend measurements of the process of brain edema associated with head trauma.

Notes:  

Non-invasive electromagnetic technique for monitoring time-trends of physiological changes at a particular location in the brain. Filed September 1987 granted April 1989. Funded by the Navy. Uses electromagnetic fields to measure electrical resistivity changes in brain matter and fluid.

 

STATEMENT OF GOVERNMENTAL INTEREST

The Government has rights in this invention pursuant to Contract No. N00039-87-C-5301 (formerly N00024-85-C-5301), awarded by the Department of the Navy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for using an electromagnetic technique to monitor physiological changes in the brain. More particularly, the invention uses an electromagnetic field to non-invasively measure impedance changes at a localized point within an animal or human brain over time. For example, these localized impedance measurements can be used to detect and monitor the advent and growth of edematous tissue, or the process of hydrocephalus.

2. Description of the Prior art

It is important in diagnosing and treating various life-threatening conditions, such as brain edema and hydrocephalus, to monitor the time-trends of physiological changes in the brain. Brain edema, which is an increase in brain volume caused by grey and/or white brain tissue absorbing edematous fluid, can develop from general hypoxia; from cerebral hemorrhage, thrombosis, or embolus; from trauma (including post-surgical); from a tumor; or from inflammatory diseases of the brain. Brain edema can directly compromise vital functions, distort adjacent structures, or interfere with perfusion. It can produce injury indirectly by increasing intracranial pressure. In short, brain edema is often a life-threatening manifestation of a number of disease processes.

There are several effective therapeutic measures to treat brain edema. These include osmotic agents, corticosteroids, hyperventilation to produce hypocapnia, and surgical decompression. As with all potent therapy, it is important to have a continuous measure of its effect on the manifestation, in this case, the brain edema.

All current techniques for measuring physiological changes in the brain, including the manifestation of brain edema, have shortcomings in providing continuous or time-trend measurements. Intracranial pressure can be monitored continuously, but this is an invasive procedure. Furthermore, intracranial compliance is such that substantial edema must occur before there is any significant elevation in pressure. When the cranium is disrupted surgically or by trauma, or is compliant (as in infants), the pressure rise may be further delayed. These patients are often comatose, and localizing neurological signs are a late manifestation of edema. Impairment of respiration and circulation are grave late signs. Thus, clinical examination is not a sensitive indicator of the extent of edema. X-ray computed tomography (CT) scanning can produce valuable evidence of structural shifts produced by brain edema, and it is a non-invasive procedure. Structural shirts, however, may not correlate well with dysfunction, especially with diffuse edema. Furthermore, frequent repetition is not feasible, particularly with acutely ill patients. NMR proton imaging can reveal changes in brain water, it does not involve ionizing radiation, and it is non-invasive. However, it does not lend itself to frequent repetition in the acutely ill patient. PET scanning can reveal the metabolic disturbances associated with edema and will be invaluable in correlating edema with its metabolic consequences. However, it too is not suited to frequent repetition.

For these reasons it would be a significant advance to have a measurement which (1) gives reliable time-trend information continuously; (2) is non-invasive; (3) does not depend upon the appearance of increased intracranial pressure, and (4) can be performed at the bedside even in the presence of life-support systems.

As will be discussesd in detail subsequently in this application, Applicant has related localized impedance changes in the brain with physiological changes in the brain. Applicant was the first to identify that edematous tissue has a significantly different conductivity from healthy white or grey matter.

To non-invasively detect such an impedance change, Applicant has invented a method and apparatus which uses an electromagnetic field for sensing such an impedance change at localized portions of the brain. U.S. Pat. No. 3,735,245 entitled "Method and Apparatus for Measuring Fat Content in Animal Tissue Either in Vivo or in Slaughtered and Prepared Form", invented by Wesley H. Harker, teaches that the fat content in meat can be determined by measuring the impedance difference between fat and meat tissue. The Harker apparatus determines gross impedance change and does not provide adequate spatial resolution for the present use. As will be discussed in detail later, brain impedance measurements must be spatially localized to provide a useful measure of physiological changes. A general measurement of intracranial conductivity would not be revealing, since as in the case of brain edema, the edematous fluid would initially displace CSF fluid and blood from the cranium; and, since these fluids have similar conductivities, a condition of brain edema would be masked.

U.S. Pat. No. 4,240,445 invented by Iskander et al teaches the use of an electromagnetic field responsive to the dielectric impedance of water to detect the presence of water in a patient's lung. The Iskander et al apparatus generates an electromagnetic wave using a microwave strip line. Impedance changes within the skin depth of the signal will cause a mode change in the propagating wave which is detected by associated apparatus. Therefore, Iskander et al uses a different technique from the present invention and does not detect conductivity variations with the degree of localization required in the present invention. U.S. Pat. No. 3,789,834, invented by Duroux, relates to the measurement of body impedance by using a transmitter and receiver and computing transmitted wave impedance from a propagating electromagnetic field. The Duroux apparatus measures passive impedance along the path of the propagating wave, whereas the present invention measures localized impedance changes in brain matter and fluid by measuring the eddy currents generated in localized portions of the brain matter and fluid. None of the above-cited references 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.

SUMMARY OF THE INVENTION

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.