 |
Description  |
|
|
FIELD OF THE INVENTION
The present invention relates to an apparatus for measuring the
intracranial pressure using the ultrasound technique (will be referred to
as "cephalohemometer" hereinafter) and more particularly to an ultrasonic
cephalohemometer suitably usable for measuring the intracranial pressure
noninvasively and safely from outside the cranium.
BACKGROUND ART
The human being in the normal condition, that is, in good health, has a
constant intracranial cavity volume peculiar to himself. However, if he
suffers from, for example, a lesion such as cerebral tumor, hematoma or
the like or any other intracranial disease, the volume of the intracranial
cavity is increased. It is said that when the increase reaches about 10%
of the normal cavity volume, the intracranial pressure rises and leads to
a sthenia of the intracranial pressure, whereby a variety of diseases will
be caused. To make clear what these diseases and determine appropriate
therapy for them, it is necessary to throw a pathologic light on the
sthenia of intracranial pressure. The means and methods for such
elucidation have been studied in various fields of medicine, but no
satisfactory means and methods have yet been proposed. However, one of the
most important means for the elucidation is the measurement of the
intracranial pressure Heretofore, various many methods of measuring the
intracranial pressure have been studied and tried. Such conventional
methods include, for example, the latex baloon method in which a latex
baloon charged with water is introduced above the dura mater or into the
cerebral ventricle inside the cranium to measure the water pressure in the
baloon, thereby measuring the intracranial pressure and the EDP measuring
method in which a part of the cranium is opened to make an osseous window
through which the dura mater is exposed, a strain gauge is put into
contact with the dura mater and a change of the intracranial pressure is
measured as a change of strain measured by a dynamic strain gauge
connected to the strain gauge. However, since the measurement of the
intracranial pressure by these methods is done with an invasion to the
cranium, the patient must be subject to a craniotomy and have a sensor
placed inside the cranium. Thus, the conventional methods for intracranial
pressure measurement have problems since the patient must bear a heavy
physical and socioeconomic burdens because he has the possibility of being
infected and must be in rest as hospitalized to maintain his health. In
addition to the above-mentioned methods, there has been proposed a
telemetry system, as a noninvasive and safe method, in which an electric
resonant circuit composed of an inductance and capacitance is used of
which the resonant frequency is changed by changing one of the values,
inductance or capacitance, with the dislocation of a bellows or diaphragm
due to the intracranial pressure and measured from above the scalp.
However, this method also have practical problems; since air is used as
compression medium, it, if any between the scalp and cranium , is easily
affected by the temperature, the measuring gradation is required for each
patient and the measuring accuracy is also far from the practical use.
Heretofore, there has not yet been provided any cephalohemometer which can
measure the intracranial pressure easily, noninvasively without any
malinfluence on the brain inside and with a high reliability, and those in
the clinical fields have long waited for such cephalohemometer.
As having been described in the foregoing, the cephalohemometry has been
done by the methods with an invasion to the cranium in almost all the
cases while being one of the most important means in the elucidation and
therapy of the intracranial pressure sthenia. There have not yet been
proposed any method and apparatus which satisfy the required safety and
reliability for the measurement and can make the measurement with a
reduced socioeconomic burden to the patient. On the other hand, the
cephalohemometry by the noninvasive method is just under study and
development, and the accuracy and costs thereof are not yet at the
practical stage.
The present invention has an object to overcome the above-mentioned
drawbacks of the conventional techniques by providing an ultrasonic
cephalohemometer which can measure the intracranial pressure from outside
the cranium easily, safely, noninvasively, highly reliably and without any
malinfluence on the brain inside.
DISCLOSURE OF THE INVENTION
According to an aspect of the present invention, an ultrasonic
cephalohemometer is provided which has an electrocardiograph which detects
the heart beat of a living body or patient, a pulser which generates a
voltage pulse taking as trigger the heart beat detected by the
electrocardiograph, an ultrasonic probe which receives the pulse generated
from the pulser, transmits an ultrasonic pulse into the cranium of the
patient from the outside thereof and receives the echo of the incident
wave, a receiver which amplifies the received echo, and a processor which
processes the output from the receiver, comprising:
an A/D converter which digitizes the discrete values of the echo waveform
received by the probe; and
an arithmetic unit which extracts from the output from the A/D converter a
range including the reflection wave from the dura mater, determines a time
difference between element waves from a quefrency value obtained through
frequency analysis of the extracted range by the arithmetic algorithm of
cepstrum method, thereby calculating the thickness of the dura mater, and
compares the calculated dura mater thickness with a previously measured
reference value for thereby calculating a dura mater distortion factor in
a correlation with the intracranial pressure.
According to another aspect of the present invention, an ultrasonic
cephalohemometer is provided which has an electrocardiograph which detects
the heart beat of a living body or patient, a pulser which generates a
voltage pulse taking as trigger the heart beat detected by the
electrocardiograph, an ultrasonic probe which receives the pulse generated
from the pulser, transmits an ultrasonic pulse into the cranium of the
patient from the outside thereof and receives the echo of the incident
wave, a receiver which amplifies the received echo, and a processor which
processes the output from the receiver, comprising:
a gate circuit which gates the echo received by the probe to a range
including the reflection waves from the dura mater to deliver a waveform
within the gate;
an A/D converter which digitizes the discrete values of the output waveform
from the gate circuit; and
an arithmetic unit which determines a time difference between element waves
from a quefrency value obtained through frequency analysis of the output
from the A/D converter by the arithmetic algorithm of cepstrum method,
thereby calculating the thickness of the dura mater, and compares the
calculated dura mater thickness with a previously measured reference value
for thereby calculating a dura mater distortion factor in a correlation
with the intracranial pressure.
The above-mentioned first and second ultrasonic cephalohemometers utilize
the correlation between the intracranial pressure and dura mater
thickness, verified by the Inventor et al, based on which the intracranial
and its change can be measured by measuring the dura mater thickness and
its change (thickness strain).
The thickness of the dura mater and its change are measured in both the
first and second ultrasonic cephalohemometers by utilizing the output
derived from the calculation of the difference in propagation time between
the element waves of the interference wave as mentioned above. The output
of the difference in propagation time between the element waves is first
connected to the pulse taking an arbitrary trigger pulse as trigger
signal, and a transmission pulse is sent from the pulser to the probe
disposed outside the patient's cranium and which is connected to the
pulser. In the probe, the transmission pulse sent from the pulser is
converted to an ultrasonic pulse which is transmitted into the cranium.
Then the ultrasonic wave transmitted into the cranium is reflected at
various interstital boundaries being the acoustic boundaries such as
skull, dura mater, etc., and the reflection waves interfere with each
other while being subject to the transmission loss and reflection loss.
The echo of the interference reflection wave is received by the probe. The
reflection wave from each interstital boundary, which forms the
interference waveform of this received echo, that is, a wave having a same
waveform as the incident wave and of which the amplitude decreases along
the path, will be described as the element wave herein In the first
cephalohemometer, the echo received by the probe is sent to the receiver
in which it is amplified, and this amplified echo is sent to the A/D
converter where the discrete values of the received waveform are converted
from analog to digital The output from the A/D converter is sent to the
arithmetic unit. In the arithmetic unit, digital data including the
waveform discrete values of the above-mentioned interference reflection
waveform resulted from the reflections within the cranium is extracted and
this extracted data is analyzed for frequency to provide a difference in
propagation time between the element waves of the interference wave.
In the second cephalohemometer, the echo received by the probe is sent to
the receiver where it is amplified, and the amplified signal is sent to
the gate circuit. On the other hand, the gate circuit is supplied with the
above-mentioned trigger pulse which is used as trigger signal to provide
an arbitrary delay time from the rise of the pulse and gate, by an
arbitrary time duration, the received echo sent through the receiver so as
to include the interference reflection wave, thereby delivering an
gated-in waveform. This output waveform is sent to the A/D converter in
which it is converted from analog to digital and the output from the A/D
converter is sent to the arithmetic unit. In this arithmetic unit, a means
such as frequency analyzer in provided to generate a difference in
propagation time between the element waves of the interference reflection
wave.
As having been described in the foregoing, since both the first and second
cephalohemometers are so designed as to calculate and deliver a difference
in propagation time between the elements waves, it is possible to measure
the dura mater thickness and its change and also to measure the
intracranial pressure and its change easily, safely and noninvasively,
based on the correlation between the intracranial pressure and dura mater
thickness which will be further described later with reference to the
embodiments of the present invention.
Also, by connecting a display unit such as CRT monitor or a recorder like a
printer, floppy disk unit or the like to the above-mentioned arithmetic
unit, it is possible to display or record the above-mentioned measured
values which will thus be utilized more conveniently as diagnostic and
clinical information.
BRIEF DESCRIPTION OF THE DRAWINGS
All the drawings attached are to explain the present invention, of which,
FIGS. 1 and 2 relate to a first embodiment of the present invention, FIG. 1
showing the system configuration of the first embodiment of the
cephalohemometer according to the present invention while FIG. 2 is an
operation-explanatory drawing showing the output pulses at various
functional steps;
FIGS. 3 and 4 relate to a second embodiment of the present invention, FIG.
3 being the system configuration of the second embodiment of the
cephalohemometer according to the present invention while FIG. 4 is an
operation-explanatory drawing showing the output pulses at various
functional steps;
FIG. 5 is a drawing showing the algorithm for the cepstrum analysis;
FIG. 6 is a drawing showing the experimental apparatus used for
verification of the application of the cepstrum analysis;
FIG. 7 is a drawing showing the reflection waveform from the bottom face of
the acrylic plate in the experimental apparatus in FIG. 6;
FIG. 8 is a drawing showing the reflection waveform which is the same as in
FIG. 7 provided that the clearance (oil layer thickness) is 0.3 mm;
FIG. 9 is a drawing showing the result of a frequency analysis in the
experimental apparatus shown in FIG. 6;
FIG. 10 is a drawing showing the result of a frequency analysis having been
done using a different probe;
FIG. 11 is a drawing showing the time of a delay due to the oil layer when
the clearance is 0.1 mm in the experimental apparatus in FIG. 6;
FIG. 12 is a similar drawing to that in FIG. 11, showing the time of a
delay due to the oil layer when the clearance is 0.5 mm in the
experimental apparatus;
FIG. 13 is a drawing showing the comparison between the oil layer thickness
determined by the cepstrum analysis and the real value;
FIG. 14 is a block diagram of the experimental apparatus used in a test on
a dog;
FIG. 15 is an explanatory drawing on the contact on the dog head of the
probe in FIG. 14;
FIG. 16 is a drawing showing the reflection echo (S echo) from the surface
of the delaying member in the probe shown in FIG. 15;
FIG. 17 is a drawing showing the reflection echoes (S' echo and Bo echo)
from the boundaries shown in FIG. 15;
FIG. 18 is a drawing showing the result of a cepstrum analysis of the
interference reflection echoes shown in FIGS. 16 and 17 when the
intracranial pressure is 0 mmH.sub.2 O;
FIG. 19 is a similar drawing to that in FIG. 18 provided that the
intracranial pressure is 650 mmH.sub.2 O;
FIG. 20 is a drawing showing the S echo when the probe is placed in direct
contact with the skull, not with an acrylic plate placed between the probe
and skull;
FIG. 21 is a similar drawing to that in FIG. 20, showing the S' echo and Bo
echo;
FIG. 22 is a drawing showing the result of a cepstrum analysis of the
interference reflection echo as in FIGS. 20 and 21 when the intracranial
pressure is 0 mmH.sub.2 O;
FIG. 23 is a similar drawing to that in FIG. 22 provided that the
intracranial pressure is 300 mmH.sub.2 O; and
FIG. 24 is a drawing showing the relation between the intracranial pressure
and strain of the dura mater, determined by the experiment shown in FIG.
14.
BEST MODE FOR CARRYING OUT THE INVENTION
The first embodiment of the present invention will be explained with
reference to FIGS. 1 and 2. FIG. 1 shows the system configuration of the
cephalohemometer and FIG. 2 is an operation-explanatory drawing showing
the output pulses at various functional step. In Figures, the reference
numeral 1 indicates a pulser connected to a probe 2 placed on the skull of
the patient M and which transmits a transmission pulse V1 shown in FIG. 2
(b) to the probe 2 simultaneously with the rise of the internal trigger
pulse T shown in FIG. 2 (a).
The probe 2 converts the transmission pulse V1 to an ultrasonic pulse which
will be transmitted into the cranium.
The ultrasonic pulse transmitted into the cranium is subject to the
reflections at various interstital boundaries being the acoustic
boundaries such as skull, dura mater, etc., and the reflection waves
interfere with each other while being subject to the transmission loss and
reflection loss. The reflection waves include the reflection waves from
the inner and outer surfaces of the dura mater and the multiple-reflection
waves from the dura mater. The echo of the interference reflection wave
from these reflection waves is also received by the probe 2. The reference
numeral 3 indicates a receiver which amplifies the echo including the
interference reflection wave received by the probe 2 and delivers an echo
V2 shown in FIG. 2 (c).
The reference numeral 5 indicates an A/D converter which receives an
amplified echo including the interference wave delivered from the receiver
3, converts it from analog to digital taking the pulse T as trigger signal
and delivers a digital waveform signal D1 consisting of the discrete
values of the echo waveform shown in FIG. 2 (d). This digital waveform
signal D1 is sent to an arithmetic unit 6 which extracts data in a range
including the digital waveform of the interference wave and analyzes for
frequency the data to calculate the time difference between element waves
forming together the interference wave.
Next, a second embodiment of the cephalohemometer according to the present
invention will be described with reference to FIGS. 3 and 4. FIG. 3 is the
system configuration of the cephalohemometer and FIG. 4 is an
operation-explanatory drawing showing the output pulses at various
functional steps. In Figures, the same reference numerals as in FIGS. 1
and 2 indicate the same elements as in FIGS. 1 and 2. The explanation
having been made of the first embodiment with reference to FIGS. 1 and 2
up to the delivery of the echo V2 shown in FIG. 2 (c) is also true for
this second embodiment except that "FIG. 1" should be replaced with "FIG.
3" and "FIG. 2" with "FIG. 4". The reference numeral 4 indicate a gate
circuit which is supplied with the echo V2 from the receiver 3 and the
above-mentioned internal trigger pulse T to delay the echo V2 an arbitrary
time from the rise of the pulse T as trigger signal and gate, by an
arbitrary time duration, the echo V2 delivered from the receiver 3 so as
to include the interference reflection wave, thereby delivering an
gated-in waveform V3 as shown in FIG. 4 (d). The reference numeral 5'
indicates an A/D converter which is supplied with the gated-in waveform V3
delivered from the above-mentioned gate circuit 4 and converts the
gated-in waveform V3 from analog to digital taking the above-mentioned
pulse T as trigger signal, thereby generating a digital waveform signal D
shown in FIG. 2 (a). This digital waveform signal D is sent to an
arithmetic unit 6' in which a time difference between elements waves
forming together the above-mentioned interference reflection wave is
calculated by a frequency analysis.
The algorithm used in the aforementioned first and second embodiments uses
the cepstrum method which will be described later, and the time difference
between the element waves is obtained from the quefrency value calculated
by the cepstrum method. The result of this calculation is sent to a
display unit 8 on which it will be displayed and also to a recorder 7 in
which it will be recorded. In the first embodiment having not the
above-mentioned gate circuit, generally calculation data are fetched from
the memory in CPU and displayed on the CRT for the above-mentioned
extraction. So the calculation takes a correspondingly longer time but the
waveform will have less strain since it is not passed through the gate
circuit. On the contrary, in the second embodiment having the gate
circuit, the output waveform sent through the gate circuit to the A/D
converter where it is converted from analog to digital to provide an
output value. This output value may be calculated as it is. Therefore, the
memory may be of a correspondingly smaller capacity though this depends
upon the magnitude of the gate width, and the operation is facilitated
correspondingly.
In the above-mentioned first and second embodiments, an electrocardiograph
trigger apparatus is used which provides, each time an R-wave of
electrocardiogram (ECG) is generated as obtained by on an
electrocardiograph connected to the patient M, a delay of an arbitrary
time (for example, at every 80 to 100 msec) from the peak position of the
R-wave to produce continuous pulses which can be used as the trigger
signal.
To verify the correlation that as the intracranial pressure increases, the
dura mater thickness changes correspondingly, on which the measurement by
the above-mentioned apparatus is based, the Inventor effected the
following experiments on a filial grown-up dog weighing about 10 kg. An
osseous window was made in the left parietal region of the dog, and a
colorless transparent PVC plate was fitted in the window. The change of
the state of the veins in the dura mater due to the intracranial pressure
was observed and photographed. The sthenia of intracranial pressure was
caused by injecting a saline into the cisterna magna to raise the
intracranial pressure from 0 mmH.sub.2 O (reference value before the rise
of intracranial pressure) up to 700 mmH.sub.2 O. The observation and
photography of the veins in the dura mater revealed that the veins in the
dura mater appeared rather definite and showed no disturbance of the blood
flow therein and that as the intracranial pressure rose, the veins in the
dura mater were gradually constricted and fully constricted with no blood
flow therein when the intracranial pressure reached about 600 mmH.sub.2 O.
On the other hand, even when the intracranial pressure was lowered to less
than 600 mmH.sub.2 O, the veins in the dura mater remained constricted.
When the pressure fell down to around 200 mmH.sub.2 O, the blood flow was
resumed in the veins. The occurrence of the constriction of the veins in
the dura mater reveals that the dura matter was made thinner as
compressed, and the dura matter became increasingly thinner as the
intracranial pressure rose until it reached about 600 mmH.sub.2 O. Then
the thickness of the dura mater showed no change even with the
intracranial pressure further increased. Namely, it was proved that there
is a correlation between the dura mater thickness and intracranial
pressure.
The measurement according to the present invention is to know the
intracranial pressure by measuring the thickness of the dura mater based
on the aforementioned correlation between the strain of the dura mater
thickness and the intracranial pressure. The measurement of the dura mater
thickness is done by applying to the analysis of the thin-layer
multiple-reflection wave the cepstrum analysis method worked out in the
field of seismic wave analysis for the purpose of separating, by
approximation, the direct wave and the waves traveling through the other
paths from each other. The algorithm of the cepstrum analysis used in the
embodiment of the present invention is shown in FIG. 5.
The cepstrum analysis method is such that the lengths of delay time of
random waves derived from the superposition of delayed echoes are
analyzed. A spectrum of the time series of the random waves is calculated,
and a logarithm of this spectrum is also calculated. This logarithmic
spectrum can be presented in the form of a diagram showing the logarithmic
spectrum along the vertical axis and the frequency along the horizontal
axis. Since the horizontal axis showing the frequency can be regarded as
time base, the logarithmic spectrum can be presented as a frequency
series. Since the logarithmic spectrum developed as this frequency series
appears as a waving curve, the logarithmic spectrum can be subjected to
the Fourier analysis to analyze what lengths of time delay the echo
components of the frequency series have. This method is called the
"cepstrum analysis". Namely, the frequency and spectrum when the time
series is subjected to the Fourier transformation and which correspond to
those when the frequency series is subjected to the Fourier transformation
are called "Quefrency" and "Cepstrum", respectively. Therefore, when the
frequency series is subjected to the Fourier transformation, a curve can
be present which shows the cepstrum along the vertical axis while showing
the quefrency along the horizontal axis. The value of this cepstrum
corresponds to the magnitude of the echo and the value of the quefrency
along the horizontal axis corresponds to the delay time of the echo.
The cepstrum analysis method is well known from, for example, "Spectrum
Analysis" by Mikio Hino, Sept. 15, 1987, 15th printing, published by
Asakura Shoten, pp. 280 to 283, and so it will not be explained in further
detail.
Next, the application of the cepstrum analysis method to the intracranial
echo will be explained below.
As seen in FIG. 5, the reflection wave from the boundary between the skull
and dura mater is an interference wave resulted from the reflection wave
(fundamental wave) from the boundary between the skull and dura mater and
the multiple-reflection wave. Assume that the interference reflection wave
from the boundary is x(t), fundamental wave is b(t), reflection intensity
is ai and delay time is .tau.i, and the interference reflection wave x(t)
is expressed as follows:
##EQU1##
the result of the Fourier transformation of both sides of the equation (1)
is shown below:
##EQU2##
where x(f) and B(f) are the results of Fourier transformation of x(t) and
b(t), respectively. From the equation (2), the power spectrum
.vertline.X(f).vertline. of X(f) is expressed as follows using the power
spectrum .vertline.B(f).vertline. of B(f):
##EQU3##
The logarithm of the equation (3) is the following equation (4), and the
interference reflection wave is separated into a frequency series being
the power spectrum of the fundamental wave and a frequency series having a
waving frequency on the frequency axis corresponding to the delay time
difference.
##EQU4##
Therefore, by determining a cepstrum through a further Fourier
transformation of the equation (4), a quefrency series of the delay time
on the quefrency axis can be determined, and so, if the acoustic velocity
of a thin layer is known, the thickness of the layer can also be known.
In fact, the received wave resulted from an ultrasonic wave transmitted
into the cranium is a complicate interference wave resulted from mutual
interference between the reflected waves from the tissues such as skull,
dura mater, etc. and the reflected waves derived from the multiple
reflections within such tissues. Therefore, the aforementioned quefrency
series represents the difference in time delay between the reflection
waves from these intracranial tissues, and so it is necessary to
discriminate the quefrency values corresponding to the reflection waves
from inside and outside the dura mater from the above-mentioned reflection
waves. The intracranial pressure varies as the heart beat changes, and as
described in the above, the dura mater thickness changes as the
intracranial pressure changes. On the other hand, since the skull will not
be affected by the intracranial pressure, the above-mentioned measurement
is repeated several times to discriminates the reflection waves from the
dura mater from among those of which the quefrency values vary. Not only
the dura mater changes in thickness as the intracranial pressure changes.
However, since the dura mater exists just below the skull, the reflection
waves of which the quefrency values do not vary just after the ultrasonic
wave is transmitted into the cranium are the waves reflected at the skull,
and the reflection wave next received and of which the quefrency value
vary, that is, the first reflection wave of which the quefrency value
vary, can be judged to be the reflection wave from the dura mater. After
the reflection wave from the dura mater can be discriminated by the above
method, it is necessary to measure the intracranial pressure in a certain
phase during pulsation of the intracranial pressure which varies as the
heart beat changes in order to eliminate the variation due to the heart
beat. For this reason, the output of the electrocardiograph is used as
trigger pulse to the pulser 1 as having been previously described.
Thus, this method can be used to measure the thickness of the dura mater.
Next, for verification of the aforementioned cepstrum analysis method, an
experiment as shown in FIG. 6 was done. Two thickness gauges 11 of a same
thickness l were interposed between an acrylic plate 9 of 5 mm in
thickness and a polystyrol block 10 of 50 mm in thickness, and a layer 12
of a machine oil was formed in the space defined by these elements,
thereby preparing an oil layer model.
A probe 13 of a 5 MHz split type was used and it was fixed to the acrylic
plate 9 using an instantaneous adhesive.
FIG. 7 shows the reflection waveform (fundamental waveform) from the bottom
of the acrylic plate 9, and FIG. 8 shows the reflection waveform when
l=0.3 mm. It is seen from these Figures that the mutual interference of
the multiple-reflection waves in the oil layer causes the waveform to be
deformed and the entire amplitude to be changed.
The reflection waves were sampled at intervals of 10 nsec with the
thickness l of the thickness gauge 11 changed within a range of 0.1 to 1.0
mm and with the thickness of the oil layer changed, and an FFT cepstrum
analysis was done using the algorithm shown in FIG. 3.
The results of frequency analysis of the 5-MHz split type probe used in the
verification test and the 5-MHz split type probe used in the animal test
which will be described later are shown in FIGS. 9 and 10. As seen from
these Figures, the center frequencies of these probe are 4.9 MHz and 5.4
MHz, respectively and the frequency bands are limited to the ranges of
about 2 to 7 MHz and 1 to 9 MHz, respectively.
On the assumption that the upper limit of the frequency band of a waveform
is W (MHz), the minimum necessary sampling time .DELTA.t (sec) for
sampling a waveform signal can be obtained from the following equation and
based on the sampling theorem:
##EQU5##
Since .DELTA.t=7.1.times.10.sup.-8 (sec), the sampling time of
1.0.times.10.sup.-8 (sec) used in this test can be said to be sufficient.
FIG. 11 shows the result of the FFT cepstrum analysis of the reflection
waveform when l=0.1 mm, and FIG. 12 shows the result of the FFT cepstrum
analysis when l=0.5 mm.
These results were obtained using as time window the hamming window shown
in the equation (6), and as cepstrum window the hanning digital window
shown in the equation (8). The hanning window serves to weight the result
of the cepstrum analysis and the hanning digital window serves to smooth
the result of the cepstrum analysis.
W.sub.ham (n)=0.54-0.46 cos (2n.pi./(N-1)). . . (6)
W.sub.han (n)=0.5-0.5 cos (2n.pi./(N-1)) . . . (7)
C'(n)=0.25C(n-1)+0.5C(n)+0.25C(n+1) . . . (8)
As seen from FIGS. 11 and 12, the quefrency values at the peaks of the
cepstrums are 150 nsec and 690 nsec, respectively. These quefrency values
correspond to the delay time .tau.i in the equation (1).
The quefrency value at the peak of the cepstrum in FIGS. 11 and 12, that
is, the time .tau.i of the delay due to the oil layer, correspond to the
difference in time (time for reciprocation of the ultrasound in the oil
layer) between the elements of the interference reflection wave.
Therefore, the multiplication of this value by the acoustic velocity in
the oil layer is the beam path. Hence, the thickness of the oil layer can
be calculated by dividing the beam path by 2.
The comparison between the oil layer thickness lcep calculated from the
delay time determined by the cepstrum analysis and the thickness of the
thickness gauge as true oil layer thickness ltrue is shown in FIG. 13. The
calculation was done taking the acoustic velocity of the machine oil as
1,400 m/sec. As seen in FIG. 13, the oil layer thickness values are very
similar to each other with the difference of about +/-10 .mu.m. Thus, the
cepstrum analysis is effectively usable in analysis of the delay time of
each of more than one waves which interfere with each other. So when the
acoustic velocity is already known, the use of this cepstrum analysis
makes it possible to measure the thickness of a thin layer which could not
otherwise be measured.
Next, the following experiments were done with the dog in order to review
whether or not the verification done with the aforementioned apparatus is
applicable to a living body or patient.
The experiments were done with a filial grown-up dog weighing about 10 kg.
First, 5 to 6 ml of 2% (weight per volume) hydrochloric acid morphine was
injected into th muscle of the dog for the basal anesthesia. Thereafter,
100 to 150 mg of thiamylal sodium was administered to the dog by
intravenous injection for insufflation anesthesia, and the dog was placed
in prone position under an endotracheal tube with the head immobilized on
the Tohdai-Nohken stereotaxic table for use with dogs (this table was
developed by the Tohdai-Nohken=the Brain Research Institute of Tokyo
University). Thereafter, 10 to 15 mg of thiamylal sodium was additionally
administered to the dog at every about 60 minutes for maintenance
anesthesia as necessary and thus the experiment was done while the dog was
keeping the spontaneous natural breathing.
The block diagram of the experimental system is shown in FIG. 14. In the
Figure, the reference numeral 15 indicates pulser, and 16 a receiver which
receives and sends an electric signal with respect to an ultrasonic probe
17 which transduces an electric signal sent from the pulser 15 into an
ultrasound or ultrasonic wave and also transduces a received ultrasonic
wave into an electric signal. The reference numeral 19 indicates a wave
memory to provide a high speed A/D conversion of a received waveform
(analog signal), and 20 generally indicates a system composed of a
personal computer, CRT, floppy disk unit and an X-Y plotter, that is, a
so-called data processing system to process, analyzes, monitors and
records a received waveform having been digitized by the wave memory 19.
The reference numeral 21 indicates an EDP sensor placed outside the dura
mater and which transduces an intracranial pressure into an electric
signal, 22 a respiration sensor which pick up the respiration as a change
of temperature and transduces a change of breathing into an electric
signal, 23 a blood pressure sensor which monitors the systemic blood
pressure and transduces a blood pressure into an electric signal, 24 an
electrocardiograph (ECG) which produces an ECG waveform and generates a
trigger signal as delayed an arbitrary time from the R-wave of the
electrocardiogram (ECG), 25 an amplifier which amplifies signals sent from
the EDP sensor 21, respiration sensor 22, blood pressure sensor 23 and ECG
24, and 26 a pen recorder which continuously records the ECG waveform,
respiration waveform, systemic blood pressure waveform and EDP
sensor-generated intracranial pressure waveform all amplified by the
amplifier 25.
In the experiments, the intracranial pressure detected by the EDP sensor,
systemic blood pressure, ECG and respiration were monitored besides the
intracranial pressure measured with the ultrasonic technique. For
measurement of the intracranial pressure by the EDP sensor 21, the EDP
sensor 21 was attached on the right side of the dog's head and the
extradural pressure was measured. The systemic blood pressure was measured
with a catheter of 2 mm in inside diameter and about 400 mm in length
inserted from the femoral artery and self-retained in the thoracic aorta
and by a catheter-tip type pressure gauge. The ECG was measured by the ECG
24 having the electrodes thereof attached on the four extremities,
respectively, of the dog. The respiration was measured using a thermistor
probe attached to the tip of the endotracheal tube.
The experiments using the ultrasonic technique were conducted as follows:
As shown in FIG. 14, the entire experimental system was triggered with an
ECG trigger with the trigger pulse from the ECG 24 so adjusted to be
generated as delayed 80 msec from the peak of the R-wave in the ECG
(electrocardiogram).
The experiment was done on two models with a saline injected into the
cisterna magna to cause a sthenia of the intracranial pressure within a
range of 0 to 650 mmH.sub.2 O. One of the models is an acrylic plate model
in which an acrylic plate 28 of 5 mm in thickness (t) was introduced
through an opening formed in the skull 27 and fixed as closely attached on
the dura mater 29 as shown in | | |
