Water content probe

Abstract

A method and system to determine brain stiffness is disclosed. A probe to measure tissue water content is inserted through an aperture (burr hole) in the cranium into brain tissue. The probe has two electrically separated plate conductors with a dielectric which forms a capacitor plane. One conductor has a surface mount resistor to allow exact impedance matching to the core of a coaxial cable. The other conductor attaches electrically to the shield of the coaxial cable. The probe is stabilized in the brain tissue through a plastic ventriculostomy bolt which has been secured by screw tapping into the cranium. The coaxial cable connects to a spectrum analyzer. Brain water content and blood congestion alter the resonant frequency of the probe, allowing a realtime readout of apparent tissue water content. By monitoring the momentary shift in center resonant frequency or, alternatively, the standing wave ratio slightly off resonant frequency, a beat-to-beat pulsatile waveform is derived relating to the perfusion of the brain. A strain gauge intracranial pressure sensor (ICP) is separately affixed through the bolt and adjacent to the water content probe. By comparing the phase angle or lag time difference between the pressure tracing and the perfusion tracing, a realtime measurement of organ stiffness or compliance is derived.

1 - 15 . (canceled)  
   
   
       16 . A method of measuring tissue water content in a selected region of interest in the brain, the method comprising: 
 calibrating a capacitive sensor having two plates outside the selected region of interest and determining the resonant frequency of the sensor in air;    calibrating the capacitive sensor in a mixture of water and NaCl,    determining the resonant frequency of the sensor in the mixture;    establishing a linear baseline frequency in relation to water content based on the resonant frequencies of the sensor in air and the mixture;    implanting the capacitive probe through a skull aperture such that the capacitive plates are exposed to the brain cortex and subjacent white matter;    producing interrogatory frequency scanning by a spectrum analyzer coupled to the sensor to determine the center point of resonance by passage of the signal; and    approximating true tissue water content by curve-fitting the frequency of resonance with the baseline frequency.    
   
   
       17 . The method of  claim 16  further comprising: 
 measuring the pressure at the selected area; and    interposing the pressure signal to the signal from the spectrum analyzer representing the resonant frequency.    
   
   
       18 . The method of  claim 17  further comprising: 
 measuring the lag time in each pulse cycle between peak water content and peak pressure; and    correlating the lag time to brain stiffness.    
   
   
       19 . The method of  claim 17  further comprising: 
 deriving a phase angle relationship between peak pressure and water content; and    correlating the phase angle to brain stiffness.    
   
   
       20 . The method of  claim 16  wherein the two plates are coated with insulation material sufficient to provide DC isolation.  
   
   
       21 . The method of  claim 16  wherein the capacitative sensor includes a coaxial cable having a core conductor coupled to the resistor and a circumferential conductor coupled to the proximal end of the other plate, the coaxial cable being coupled to the spectrum analyzer.  
   
   
       22 . The method of  claim 16  wherein the plates have a series of transverse holes.  
   
   
       23 . The method of  claim 16  further comprising: 
 inserting a threaded, self-tapping bolt within the skull aperture; and    positioning the sensor within an aperture through the bolt.    
   
   
       24 . The method of  claim 17  further comprising: 
 converting the analog signal representing pressure to a digital signal; and    converting the analog signal from the capacitive sensor to a digital signal.    
   
   
       25 . The method of  claim 16  further comprising: 
 recording the instantaneous water content and producing interrogatory frequency scanning by a spectrum analyzer coupled to the sensor to determine the center point of resonance by passage of the signal; and    approximating true tissue water content by curve-fitting the frequency of resonance with the baseline frequency to track the water content readings during periodic time intervals.    
   
   
       26 . A method of deriving beat-to-beat perfusional and congestion changes in brain tissue, the method comprising: 
 inserting a water content probe having two conductive plates and a dielectric in the brain tissue;    sending signals at different frequencies on the water content probe; determining a standing wave ratio at different frequencies; and    determining a water content change tracing which fluctuates with cardiac output pulsatile perfusion of the tissue.    
   
   
       27 . The method of  claim 26  wherein determining a standing wave ratio is performed using a spectrum analyzer coupled to the water content probe.  
   
   
       28 . The method of  claim 27  wherein determining a tracing includes: 
 plotting the change in standing wave ratio to the side of the return loss curve on the spectrum analyzer;    determining where the standing wave ratio change is maximum; and correlating the standing wave ratio change to a water content change which fluctuates with cardiac output pulsatile perfusion of the tissue.    
   
   
       29 . The method of  claim 28  wherein the spectrometer has a standing wave ratio setting of about 1.15.  
   
   
       30 . The method of  claim 27  wherein determining a tracing includes: 
 plotting the center frequency resonance shift; and    deriving the water content change tracing which fluctuates with cardiac output pulsatile perfusion of the tissue.    
   
   
       31 . The method of  claim 26  further comprising: 
 determining the pressure of the area of the brain;    plotting a trace of the pressure which fluctuates with the cardiac output pulsatile perfusion of the tissue;    determining the phase lag between the pressure trace and the water content change tracing; and    determining the relative stiffness of the brain based on the phase lag.    
   
   
       32 . The method of  claim 26  further comprising: 
 determining the pressure of the area of the brain;    plotting a trace of the pressure which fluctuates with the cardiac output pulsatile perfusion of the tissue;    determining the time lag between the pressure trace and the water content change tracing; and    determining the relative stiffness of the brain based on the time lag.    
   
   
       33 . A method of deriving realtime compliance or stiffness of brain tissue comprising: 
 measuring the intracranial pressure of the brain tissue; plotting an intracranial waveform from the measurements of the intracranial pressure;    measuring the pulsatile congestion changes in water content of the brain tissue;    plotting a pulsatile congestion change waveform from the measurements of the pulsatile congestion change;    simultaneously plotting the waveforms of intracranial pressure and the pulsatile congestion change in water content on a computer; and    determining the stiffness of the brain from the simultaneous plotting.    
   
   
       34 . The method of  claim 33  wherein determining the stiffness includes measuring the lag time in each pulse cycle between peak water content and peak pressure wherein lower lag time indicates severe stiffness or abnormal compliance and widened lag time relates to a relaxed brain.  
   
   
       35 . The method of  claim 33  wherein determining the stiffness includes: 
 deriving a phase angle relationship between peak pressure and water content;    adjusting for heatbeat frequency; and    wherein a smaller phase angle indicates severe stiffness or abnormal compliance and larger phase angle relates to a relaxed brain.    
   
   
       36 . The method of  claim 33  further comprising converting the pressure and water content waveform from an analog to a digital waveform.  
   
   
       37 . The method of  claim 33  further comprising: 
 obtaining a derivation of an indicator of realtime compliance by utilizing a transducer to measure local tissue fluctuation; and    measuring a relationship to the intracranial pressure sensor waveform.    
   
   
       38 . The method of  claim 37  wherein the transducer is a heat clearance sensor.  
   
   
       39 . The method of  claim 37  wherein the transducer is a laser Doppler sensor.  
   
   
       40 . The method of  claim 33  wherein measuring the intracranial pressure of the brain tissue is performed by a tissue-implanted strain gauge.  
   
   
       41 . The method of  claim 33  wherein measuring the intracranial pressure of the brain tissue is performed by a tissue-implanted strain gauge fiberoptic sensor.  
   
   
       42 . The method of  claim 33  wherein measuring the intracranial pressure of the brain tissue is performed by an external strain gauge coupled via tubing to a ventriculostomy catheter.  
   
   
       43 - 44 . (canceled)