U.S. patent application number 11/109370 was filed with the patent office on 2005-09-15 for water content probe.
This patent application is currently assigned to Brain Child Foundation. Invention is credited to Manwaring, Kim, Manwaring, Mark L..
Application Number | 20050203438 11/109370 |
Document ID | / |
Family ID | 21784709 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050203438 |
Kind Code |
A1 |
Manwaring, Kim ; et
al. |
September 15, 2005 |
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.
Inventors: |
Manwaring, Kim; (Provo,
UT) ; Manwaring, Mark L.; (Provo, UT) |
Correspondence
Address: |
Wayne L. Tang
MAYER, BROWN, ROWE & MAW LLP
P.O. Box 2828
Chicago
IL
60690-2828
US
|
Assignee: |
Brain Child Foundation
|
Family ID: |
21784709 |
Appl. No.: |
11/109370 |
Filed: |
April 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11109370 |
Apr 19, 2005 |
|
|
|
10017820 |
Dec 12, 2001 |
|
|
|
Current U.S.
Class: |
600/547 |
Current CPC
Class: |
A61B 5/031 20130101;
A61B 5/0538 20130101; A61B 5/6864 20130101; A61B 5/0537 20130101;
A61B 5/0031 20130101; A61B 5/4869 20130101 |
Class at
Publication: |
600/547 |
International
Class: |
A61B 008/00; A61B
005/05 |
Claims
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)
Description
FIELD OF INVENTION
[0001] The invention relates to an apparatus and method for
measuring local brain water content, perfusional pulsatile changes
and the real time derivation of brain stiffness by comparison of
perfusional and intracranial pressure tracings.
BACKGROUND OF INVENTION
[0002] Monitoring intracranial pressure (ICP) in real time in
intensive care units has become an established standard of care in
guiding physicians in the management of severe head injury.
Treatment of head trauma increases pressure on the brain requiring
monitoring intracranial pressure. This is particularly true in
complicated cases of hydrocephalus as a post-craniotomy adjunct to
detect brain swelling and in selected instances of brain infection
and stroke. As brain swelling worsens due to the disease process,
baseline pressure and waveform changes signal the need to
aggressively attempt to reverse the course of the swelling with
medications and pulmonary ventilation changes.
[0003] Intracranial pressure monitoring is normally performed by
inserting a shunt through a hole in the cranium. A ventriculostomy
catheter connected to an external pressure transducer is then
introduced via the shunt into the brain substance. The shunt may
also be used to drain excess fluid from the brain substance. An
external pressure transducer provides accurate pressure
measurements since a reliable baseline may be established. However,
an external pressure transducer requires invasive procedures,
risking a patient's health.
[0004] More recently, a miniaturized fiberoptic or strain gauge
pressure transducer is inserted into the brain substance. The
miniaturized transducer greatly reduces the invasiveness of the
insertion procedure, but no practical method exists to establish a
baseline measurement. This creates accuracy problems since many
factors over the course of treatment may shift baseline
measurements. Additionally, the ICP sensor and data from it alone
do not allow a direct measurement of how edematous or congested the
specific region of the brain is. Furthermore, swelling provides a
widely ranging pressure change related to age and causes of the
swelling. Finally, the ICP sensor alone does not provide a
measurement of real time brain stiffness or compliance, a helpful
indicator of imminent deterioration risk.
[0005] Static measurement may be achieved by magnetic resonance
imaging ("MRI"), but this does not provide real time data. Real
time information would greatly facilitate the detection of true
shunt failure in the management of hydrocephalus. However, since
real time measurement cannot be done with internal sensors, shunt
failure must be inferred from late presenting clinical
deterioration and anatomical changes as seen in imaging studies of
the MRI. Additionally, the transport of a critically ill patient to
an MRI facility is hazardous.
[0006] There is therefore a need for an instrument which may be
inserted through a single aperture in the skull for simultaneous
and continuous monitoring of both intracranial pressure and
cerebral water content. There is another need for an instrument
which may continuously measure pulsatile changes, altering apparent
water content relating to beat-to-beat tissue perfusion due to
cardiac output of blood to the brain. There is a further need for
an instrument which provides the continuous measurement of tissue
congestion related to venous back pressure from mechanical
ventilation. There is another need for an instrument which derives
the percent water content of the brain for comparison against
normal values. There is yet another need for a system to monitor
the more gradual baseline changes in wetness or brain edema of
intracellular or extracellular origin related to the disease
process. There is another need for an instrument which can
simultaneously display the intracranial pressure (ICP) waveform and
the pulsatile perfusional or momentary congestion changes of the
brain. There is still another need for an apparatus and method for
comparing the differences in lagtime between the ICP and
perfusional waveforms, from which a realtime measurement of brain
stiffness or compliance is derived.
SUMMARY OF THE INVENTION
[0007] These needs may be addressed by the present invention which
is embodied in one aspect of the invention which is a probe for
measuring tissue water content in a region of interest in the
brain. The probe has an implantable tissue water content sensor
having two plates with a proximal and distal end. The two plates
are separated by a dielectric material and the distal end is
implantable in brain tissue. An impedance matching circuit is
coupled to the proximal end of one of the plates. A first output
terminal is coupled to the matching circuit resistor and a second
output terminal is coupled to one of the plates. A remotely
positioned frequency spectrum analyzer receives an output signal
from the first and second output terminals. A digital computer has
a display, the digital computer having an input coupled to the
output signal from the water content probe and the spectrum
analyzer, the computer programmed to display the resonant frequency
of the sensor indicative of water content in the brain tissue.
[0008] Another aspect of the present invention is a method of
measuring tissue water content in a selected region of interest in
the brain. A capacitive sensor having two plates outside the
selected region of interest is calibrated and the resonant
frequency of the sensor in air is determined. The capacitive sensor
is calibrated in a mixture of water and NaCl. The resonant
frequency of the sensor in the mixture is determined. A linear
baseline frequency in relation to water content based on the
resonant frequencies of the sensor in air and the mixture is
established. The capacitive probe is implanted through a skull
aperture such that the capacitive plates are exposed to the brain
cortex and subjacent white matter. Interrogatory frequency scanning
by a spectrum analyzer coupled to the sensor is produced to
determine the center point of resonance by passage of the signal.
True tissue water content is approximated by curve-fitting the
frequency of resonance with the baseline frequency.
[0009] Another aspect of the present invention is a method of
deriving beat-to-beat perfusional and congestion changes in brain
tissue. The method includes inserting a water content probe having
two conductive plates and a dielectric in the brain tissue. Signals
at different frequencies on the water content probe are sent. A
standing wave ratio at different frequencies is determined. A water
content change tracing which fluctuates with cardiac output
pulsatile perfusion of the tissue is then determined.
[0010] Another aspect of the present invention is a method of
deriving realtime compliance or stiffness of brain tissue. The
intracranial pressure of the brain tissue is measured. An
intracranial waveform from the measurements of the intracranial
pressure is then plotted. The pulsatile congestion changes in water
content of the brain tissue is measured. A pulsatile congestion
change waveform is plotted from the measurements of the pulsatile
congestion change. The waveforms of intracranial pressure and the
pulsatile congestion change in water content on a computer are
simultaneously plotted. The stiffness of the brain is then
determined from the simultaneous plotting.
[0011] Another aspect of the present invention is a probe for
measuring tissue water content in a region of interest in the
brain. The probe has an implantable tissue water content sensor
having two plates with a proximal and distal end. The two plates
are separated by a dielectric material and the distal end is
implantable in brain tissue. A signal transmitting circuit is
coupled to the proximal end of one of the plates. A signal receiver
is provided. A remotely positioned frequency spectrum analyzer is
coupled to the signal receiver. A digital computer is provided
having a display and an input which is coupled to the output signal
from the water content probe and the spectrum analyzer. The
computer is programmed to display the resonant frequency of the
sensor indicative of water content in the brain tissue
[0012] It is to be understood that both the foregoing general
description and the following detailed description are not limiting
but are intended to provide further explanation of the invention
claimed. The accompanying drawings, which are incorporated in and
constitute part of this specification, are included to illustrate
and provide a further understanding of the method and system of the
invention. Together with the description, the drawings serve to
explain the principles of the invention.
BRIEF DESCRIPTION OF DRAWINGS
[0013] This invention is pointed out with particularity in the
appended claims. However, other objects and advantages together
with the operation of the invention may be better understood by
reference to the following illustrations, wherein:
[0014] FIG. 1 is a perspective view of a brain stiffness probe
according to an embodiment of the present invention.
[0015] FIG. 2 is a partial cutaway view depicting the probe in FIG.
1 inserted through an aperture in the skull such that it is exposed
to direct contact with brain tissue.
[0016] FIG. 3 is a block diagram with the probe components and
remotely placed measuring equipment for both the water content
sensor component and intracranial pressure component according to
one embodiment of the present invention.
[0017] FIG. 4A-FIG. 4D are frequency resonance curves and
calibration and measurement of tissue water content taken using a
system according to the present invention.
[0018] FIG. 5 is a waveform diagram showing pulsatile changes in
microscopic center frequency shifts in the water content probe
according to the present invention due to perfusion of the brain by
cardiac pulsatile output.
[0019] FIG. 6 is a block diagram of a wireless implementation of a
water content probe according to the present invention.
[0020] FIG. 7A-7B are waveform diagrams which show the phase or
lagtime relationship between the pressure waveform and perfusional
waveform derived from the water content component of the combined
probe according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] While the present invention is capable of embodiment in
various forms, there is shown in the drawings and will hereinafter
be described a presently preferred embodiment with the
understanding that the present disclosure is to be considered as an
exemplification of the invention, and is not intended to limit the
invention to the specific embodiment illustrated.
[0022] In accord with one embodiment of the invention, a combined
probe 10 for measuring brain wetness and intracranial pressure is
shown in FIG. 1. The probe 10 has a water content sensor 11 which
has two conductive plates 12 and 14 on opposite sides of a printed
circuit board (PCB) substrate 16. The conductive plates 12 and 14
are silver in the preferred embodiment but any suitable conductor
material may be used. The substrate 16 in the preferred embodiment
measures 5 cm in length, 2 mm in width, and 0.5 mm in depth. The
probe 10 has a proximal end 18 and a distal end 20. Multiple holes
22 extend across the PCB substrate 16. The holes 22 increase
sensitivity to real time pulsatile perfusional changes in tissue as
they increase the surface area in contact with the brain tissue.
The proximal end 18 has a surface mount resistor 24 on one side. A
coaxial cable 26 has a core conductor member 28 and a shielding
conductor 30 which is circumferentially located around the core
member 28.
[0023] The surface mount resistor 24 is coupled between the
proximal end 18 and one end of the coaxial cable 26. The surface
mount resistor 24 provides impedance matching between the core 28
of the coaxial cable 26 and the plate 12. The impedance matching
provided by the surface mount resistor 24 and the cable 26 is
employed to achieve noise immunity in the cable 26 and allow the
analysis electronics to be located at a distance from the water
content sensor 11. Other types of impedance matching circuits such
as a balanced antenna approach may be used as well. The plate 14 is
connected directly to the shielding conductor 30 of the coaxial
cable 26. The other end of the coaxial cable 26 is connected via an
adapter 32 to a controller unit 34. In this sample, the adapter 32
is a PL250 type which minimizes signal loss to the cable 26.
[0024] The water content sensor 11 is inserted through a plastic
bolt 36 via an aperture 38. The plastic bolt 36 has a pair of hex
nuts 40 and 42 which are mounted on a main body section 44. The
main body 44 has an exterior surface with threads. A lug nut 46 is
coupled to the main body 44 and has corresponding interior threads.
The lug nut 46 may be rotated on the main body 44 and provides a
connection for the cable 26.
[0025] The probe 10 is inserted to a depth in brain tissue up to
the plastic bolt 36 via the aperture. The hex nuts 40 and 42 and
the lug nut 46 are tightened on the main body 44 of the bolt 36 to
provide a seal and to allow the plastic bolt 36 to be positioned
and held in the aperture 38. The bolt 36 is designed such that the
surface mount resistor 24 lies about 1 mm above the surface of the
brain, placing nearly the full length of the plates 12 and 14 in
the brain tissue. Since the water of the brain bears a moderate
salinity (typically 130-150 mEq Na+ per 1000 ml), an extremely
thin-sputtered layer of insulation 50 insulates the electrical
plates 12 and 14 from direct tissue contact. The insulation layer
50 is Teflon in the preferred embodiment, but any type of
insulation may be used. The insulation layer 50 allows the point of
resonance of the water content sensor 11 to be precisely
measurable. The configuration of the capacitive plates 12 and 14
may be used in a tubular configuration to allow a silicone external
ventricular drain through the lumen. In such a configuration, the
electrically conductive plate surfaces are located on the length of
the tube on opposite hemispheres to create a similar capacitive
effect.
[0026] FIG. 2 shows a cutaway view of a head 60 with a brain 62
shown through the frontal lobes as seen by a typical MRI. The brain
62 is encased by a cranium 64. The containment of the cranium 64
creates pressure on the brain 62 which may be excessive due to
fluid buildup. A skull aperture 66 (or burr hole) is created in the
cranium 64 after a scalp incision. This routine procedure in the
intensive care unit would normally be followed by the introduction
of an ICP sensor or ventriculostomy catheter as is presently
known.
[0027] The plastic ventriculostomy bolt 36 in the preferred
embodiment is commercially available through Codman and Shurtleff
Incorporated, Raynham, Mass. The plastic bolt 36 is tapped and
threaded snugly into the cranium 64. The water content sensor 11 is
passed through the bolt 36 to a depth such that the sensing
capacitive plates 12 and 14 are exposed to cortex and white matter
of the brain 62. The plastic bolt 36 provides stable fixation of
electrical connections and prevents movement of the sensor 11 in
the brain 62 by secure fixation at the skull aperture 66 (burr
hole).
[0028] An intra cranial pressure ("ICP") sensor 70 passes through
the bolt 36 into the subjacent cortical tissue of the brain 62. The
ICP sensor 70 is an electrical strain gauge type and measures
changes in resistance due to pressure. Alternatively, any
implantable pressure sensor such as a fiber optic sensor may be
used. A fiber optic sensor has lasers coupled to dual fiber optic
cables. A diaphragm is coupled to the end of the fiber optic cables
and distorts light in reaction to pressure, producing changes in
either light amplitude or frequency. In other cases, an external
strain gauge which is coupled via tubing to a ventriculostomy
catheter or a cranial bolt may be used to measure pressure.
[0029] The output voltage of the ICP sensor 70 is carried by a
cable 72. The strain gauge ICP sensor 70 in this example is
commercially available from Codman and Shurtleff Incorporated,
Raynham, Mass. but any appropriate pressure sensor may be used. The
ICP sensor 70 may be inserted separately from the bolt 36 and/or
inserted at a separate site on the cranium if desired. This is to
be avoided in most cases, but certain circumstances may require the
separate insertion of the ICP sensor 70 and the water content
sensor 11.
[0030] The respective wiring connections to and from the water
content sensor 11 and the ICP sensor 70 are coupled to the
controller unit 34 which is at a remote location. Alternatively,
the cables may be connected to a signal transmitter if it is
desired to eliminate the cables. The technique of positioning the
combined sensors is identical to the routine insertion of a
ventriculostomy catheter for monitoring and carries with it the
same acceptably low risks.
[0031] FIG. 3 is a block diagram of the control unit 34 of the
combined ICP-water content probe 10. The ICP sensor 70 is a
strain-gauge type which has a wheatstone bridge 74 of standard
configuration having a pressure transducer 76 and three resistors
78, 80 and 82. The voltage of the bridge 74 changes in accordance
to pressure changes on the pressure transducer 76. The output
voltage of the bridge 74 represents the sensed pressure on
transducer 76 and is coupled to the input of an analog to digital
convertor 84 via the cable 72. The output of the analog to digital
convertor 84 is coupled to a digital computer 86.
[0032] The water content sensor 11 is coupled via the coaxial cable
26 to an input of a spectrum analyzer 88. The spectrum analyzer 88
in the preferred embodiment is an AEA-Tempo 150-525 Analyst
manufactured by Tempo Research of Vista, Calif. The spectrum
analyzer 88 sweeps an interrogating frequency from 150 MHZ to 550
MHZ every 2 seconds to the water content sensor 11 in the preferred
embodiment. The frequency spectrum for measuring brain water
content without interference from other sources is optimally
measured between 400 and 600 MHZ. However, other ranges may be
useful depending on the probe length.
[0033] The direct output from the spectrum analyzer 88 is coupled
to the digital computer 86 and a second output is coupled to an
analog to digital convertor 90. This allows display of the resonant
frequency of the water content sensor 11 determined from the direct
output, as well as heart beat to heart beat changes in frequency
and standing wave ratio (SWR) from the digital to analog converter
90. The outputs from the spectrum analyzer 88 and the digital to
analog convertor 90 are plotted on a display 92. The display 92 is
preferably a high resolution monitor but any display device may be
used.
[0034] The digital computer 86 contains software necessary to
simultaneously display the pulsatile waveform outputs from the ICP
sensor 70 and the water content probe 11 on the display 92. As will
be explained below, the brain water content and blood congestion
alter the resonant frequency of the water content probe 11 and
provides an indication of the real time read out of apparent tissue
water content and the stiffness of the brain 62 which is
independent of baseline water content or pressure.
[0035] FIGS. 4A-4D illustrates the process of probe calibration and
water content determination of brain tissue which is displayed
using the software on the digital computer 86 in conjunction with
the display 92. The water content sensed by the water content
sensor 11 of the probe 10 in FIGS. 1 and 2 is indicative of the
effect of the surrounding tissue dielectric on the speed of
transmission of the interrogating signal through the plates 12 and
14. Similar in concept to time domain reflectometry and familiar to
those skilled in the art, the spectrum analyzer 88 will display a
resonant frequency when the water content sensor 11 is placed in
tissue. This resonance is a function of plate capacitance of the
plates 12 and 14 (most strongly affected by probe length in this
configuration) and the adjacent dielectric of the material of the
substrate 16. The PCB dielectric material 16 between the plates 12
and 14 and the extremely thin-sputtered layer 50 have dielectric
constants near air (dielectric of 1). In contrast, the brain is
normally about 70% water. As the dielectric of H2O is 80, the
tissue water content overwhelmingly determines the resonant
frequency measured from the water content sensor 11.
[0036] FIG. 4A shows the output plot of the spectrum analyzer 88
displayed by the digital computer 86 when the water content sensor
11 is entirely exposed to air. Since no significant water content
related dielectric slows the signal in air, the resonant frequency
of the water content sensor 11 is 440 MHZ. FIG. 4B shows the output
plot when the water content sensor 11 is inserted in a 100% normal
saline and water compound (simulating brain water and salinity).
The resonant frequency of the water content sensor 11 has decreased
to 167 MHZ as shown in FIG. 4B. This reduction is due to the
overwhelming dielectric effect of the surrounding water with its
high dielectric constant.
[0037] FIG. 4C shows the sharp resonant curve of the output of the
water content sensor 11 when placed in the brain tissue 62 as shown
in FIG. 2. The resonant frequency is 307 MHZ in FIG. 4C. The water
content of the brain tissue 62 is proportional to the resonant
frequency. The different resonant frequencies sensed by the sensor
11 in differing conditions of water content may be plotted. FIG. 4D
shows the linearity of a typical output curve from the water
content sensor 11 from submersing the sensor 11 in water as in FIG.
4A to full exposure in air as in FIG. 4B. By testing the water
content sensor 11 in tissue utilizing dry and wet weight water
content determinations, the linear range of clinical significance
from 65% (very dehydrated brain) to 80% (very edematous brain) may
be tested and provides a measurement standard for water content
determination.
[0038] The measurable accuracy of the water content sensor 11 is up
to 0.1% of water content change. In clinical use, however, the
absolute local water content determination is not as useful as the
trending of water content of the brain tissue over the course in
the intensive care unit against a baseline measurement. The long
term trends are more useful data since insertion of the water
content sensor 11, as any probe, into the brain 62, causes a
temporary injury edema which develops about the sensor 11 and
artificially increases the baseline water content in the region.
Additionally, effects of local minor accumulation of a non-flowing
blood clot against the sensor plates 12 and 14 or incomplete
passage to full depth of the plates 12 and 14 will offset the true
water content baseline. Despite these considerations, the baseline
measurement is used as a control against the course of illness and
therapeutic intervention with dehydrating drugs such as furosemide
and mannitol or ventilator changes provide a real time feedback of
impact of the physician's regimen on the patient.
[0039] When the baseline water content is plotted over hours of
time on a computer such as the computer 86, gradual shifts in the
water content may be analyzed. For example, the initial shift in
water content represents the initial placement edema and its
resolution. The longer term shift in water content may represent
the trend of brain swelling in the region of monitoring, edema due
to head injury, or the effects of therapy. Alternatively, the
changes in resonant frequency may also be logged using a
spectrum/frequency analyzer such as a Model HP8568A manufactured by
Hewlett-Packard. However, much smaller changes of significance to
the course of the illness may be measured from heart beat to heart
beat as will be explained below. Thus, the water content sensor 11
may be used in isolation without the associated intracranial
pressure sensor 70, yielding profitable data for the patient.
[0040] FIG. 5 shows a pulsatile baseline 500 obtained from minute
apparent water content change. Either one of two techniques may be
used to obtain the water content change on a heart beat to heart
beat basis. The first technique involves use of the frequencies
around the resonant frequency. When the spectrum analyzer 88 is
employed to identify the standing wave ratio ("SWR") at resonance,
a properly placed water content sensor 11 will show an SWR of 1.0.
The frequency of resonance relates to the water content which is
307 MHZ in FIG. 4D.
[0041] However, if the frequency just to the right of the resonant
point in FIG. 4D is selected where maximum change in SWR occurs per
unit frequency change, typically an SWR of about 1.15, the
beat-to-beat change of SWR may be plotted. The beat to beat SWR
changes thus correlates to the local increased water content sensed
by the water content sensor 11 which is due to transient increased
tissue congestion and arteriolar dilation due to blood flow. An
undulating waveform 502 as a function of time is shown in FIG. 5.
The undulating waveform 502 is measured from the water content
sensor 11 as a function of the change in SWR from heart beat to
heart beat. A slower baseline undulation relates to back pressure
on the venous side of the brain from positive pressure ventilation
of the patient or may be evoked by transient jugular vein
compression (termed the Queckenstedt maneuver).
[0042] Alternatively, the beat-to-beat effect may be measured by
tracking the center frequency of resonance deviation when the water
content sensor 11 in FIGS. 1 and 2 is viewed as the variable
component of a simple LC resonant circuit 100 as shown in FIG. 6.
The sensor 11 is coupled to an inductor 102. The sensor 11 and the
inductor 102 may thus be integrated in an implanted sensor unit
104. A second inductor 106 is coupled to the processing circuitry
which includes a signal generator and resonant frequency
measurement device as explained above. Since the value of the first
inductor 102 is fixed, the resonant frequency will shift as a
function of water content of the tissue surrounding the sensor unit
104. The resonant frequency is measured wirelessly by sensing
magnetic field energy from the second inductor 106 and the signal
generator.
[0043] A significant advantage of this approach is that
beat-to-beat pulsatile changes and baseline water content may be
measured wirelessly using a spectrum analyzer pick-up circuit
across the scalp from a wholly implanted resonant circuit. This
technique allows long term, wireless monitoring of a region of
interest over months to years for determining optimal compliance
and control of hydrocephalus in patients treated by a
ventriculoperitoneal shunting procedure.
[0044] With reference to FIGS. 1 and 2, when the intracranial
pressure (ICP) waveform is plotted simultaneously with the
pulsatile water content waveform derived from the two techniques
described above, a phase relationship between the waveforms is
seen. FIG. 7A shows a simultaneous plot of pressure 600 versus a
pulsatile water content plot 602. The pressure plot 600 precedes
pulsatile congestion as sensed by the water content probe plot 602.
This indicates that peak vascular congestion lags peak pressure.
FIG. 7A depicts the phase relationship plotted of a healthy, normal
brain. In FIG. 7A, brain stiffness is within acceptable levels and
thus the phase of beat to beat water content resonant frequency is
phase shifted from the pressure changes by 115 degrees.
[0045] In contrast, FIG. 7B shows the pressure and water content
plots 600 and 602 superimposed on each other in an example of
worsening brain compliance or stiffness. The beat to beat water
content resonant frequency is phase shifted from the pressure
changes by 68 degrees. This relationship is also demonstrated by a
combined ICP-blood flow probe such as when monitoring a patient
with a thermal probe as described in U.S. Pat. No. 4,739,771 to the
same inventors and incorporated by reference herein. In a normal,
relaxed brain, the peak flow or vascular congestion may lag
substantially, especially in a child with an open antereor
fontanel. As the brain becomes progressively swollen with brain
edema in head injury the lag narrows until the two waveforms are
essentially co-incidental. Similarly, poor compliance in a patient
with shunt failure will show the pattern of narrowing of lag time.
The relationship can also be measured in real time as a function of
phase lag adjusted for frequency (heart beat), akin to phase lag
plotting in current phase compared to voltage phase in inductive
circuits. Thus, the relationship by lag in seconds or phase angle
adjusted for frequency provides a measure of brain stiffness which
is independent of transducer amplitude, accuracy or stability,
allowing a frequency domain relationship applicable to long term
monitoring including implants.
[0046] It will be apparent to those skilled in the art that the
disclosed measurement method and apparatus described above may be
modified in numerous ways and assume many embodiments other than
the preferred forms specifically set out and described above.
Alternatives to the capacitive water content sensing technology
include time domain reflectometry and square-wave frequency based
sensors as well as fiberoptic sensors. The time domain
reflectometry views the sensing components as a model transmission
line. The reflection of a signal is measured as a function of water
content. The square wave frequency based sensor uses a broad range
of frequencies to determine water content as a function of the
frequencies observed. The proper interpretation of the square wave
frequency signals requires the appropriate circuitry. The
fiberoptic sensor uses a light signal of a certain wavelength which
is propagated down an implanted fiber. An optical grating is used
to determine reflection of the light signal which is a function of
the water content.
[0047] The pulsatile flow relationship to the ICP waveform can be
derived by use of transducers such as thermistors (as described in
the author's cited patent), or other heat clearance transducers as
well as by transcranial impedance measurement and local tissue
laser Doppler technique. The transcranial impedance measurement is
performed by placing an ohmmeter on the head and measuring the
signals at high frequency. An alternate impedance measurement may
be used using a four probe method. Two impedance probes measure the
output while two probes input the signal. The laser Doppler
technique uses a laser to send a signal to the tissue of interest.
The shift in Doppler frequency is measured to determine the water
content.
[0048] An antenna sensor may be used for the water content sensor
instead of the capacitive approach explained above. The entirety of
the circuitry which includes the implanted circuit with an antenna
to sense the water content in the tissue and a transmitter can be
reduced to an integrated circuit as part of an implant or
integrated onto the probe itself, allowing transcranial, wireless
interrogation. The present invention is not limited by the
foregoing descriptions but is intended to cover all modifications
and variations that come within the scope of the spirit of the
invention and the claims that follow.
* * * * *