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BACKGROUND OF THE INVENTION
This invention relates to tonometry and, in particular, to a non-contact
tonometer for determining the health of living tissue.
Tonometry broadly relates to the measurement of tension in living tissue
and has special meaning in ophthalmology relating to the intraocular
pressure and health of the eye. Pressure in the eye is not measured
directly, but is typically inferred by measuring the eye's response to
pressure exerted upon the cornea.
One of the most widely employed eye tests is an applanation test devised by
H. Goldmann in which the cornea of the eye is flattened by a device having
a plane contact surface. The cornea is depressed until the depression
reaches a predetermined radius at which time the amount of force required
to produce the depression is noted. This force is related to intraocular
pressure. By taking pressure readings on a large number of patients,
standards have been established for identifying both healthy and
glaucomatic eyes. These standards have proven to be reasonably reliable.
The Goldmann applanation and other contact type tests require the test
instrument to physically deform the cornea and therefore should only be
performed by a physician or a qualified technician to avoid harming and/or
stressing of the patient. It is common practice while carrying out an
applanation test to anesthetize the patient's eye to minimize discomfort.
This, however, poses certain risks in that harmful pressures may be
developed in the anesthetized eye.
The accuracy of the applanation tests can be adversely affected by a number
of variables. These include the size and shape of the patient's eye, the
amount of aqueous humor escaping from the eye when the cornea is
depressed, variations in the response of the sclera to the applied
pressure, and uncontrolled movement of the patient's head during testing.
Attempts to correct or compensate for the effects of these unwanted
variables have met with limited success. Furthermore, because the
applanation test requires that the instrument physically contact the
cornea, potentially harmful micro-organisms can enter the patient's body
through the eye fluids if the contact surfaces of the instrument are not
carefully cleansed.
Lechtenstein, et al. in U.S. Pat. No. 3,545,260 discloses an eye test
wherein the compliance of the cornea is used to describe intraocular
pressure. In this test, the eye is immersed in a chamber containing a
pressurized gas which causes the cornea to become depressed. The depth of
the depressure produced by the gas at a given pressure is measured using
sound waves. The waves are directed at the deformed region of the eye and
the echo return is then time converted to a distance measurement from
which intraocular pressure is inferred.
Lechtenstein et al. in a later U.S. Pat. No. 3,690,158 discloses another
test in which acoustical energy is used to measure the impedance of the
eye. Intraocular pressure is again inferred from these measurements. Here,
both the eye under test and a target having a known impedance are immersed
in the same liquid media and acoustical waves are directed through the
media at both the eye and the target. The measured impedance of the eye is
compared to that of the target to determine the health of the eye. It
should be noted that a fluid tight seal must be maintained about the
patient's eye throughout this test. This type of seal is very difficult to
maintain. If the seal is broken or the liquid is disturbed the test
results will be erroneous.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of this invention to improve tonometry and, in
particular, tonometers used in measuring the compliance in living tissues.
A still further object of the present invention is to provide a non-contact
tonometer that will provide accurate information relating to intraocular
pressure.
Another object of the present invention is to provide an acoustical
technique for measuring tissue tension in a patient to detect the presence
of tumors and other abnormalities.
Yet another object of the present invention is to provide a small hand held
instrument, that can be used safely by a patient to take tension readings
of the eye and other body areas.
A further object of the present invention is to provide a non-contact
tonometer for determining intraocular pressure.
These and other objects of the present invention are attained by
dynamically sealing a target containing human tissue within an
unobstructive opening to a chamber contained within a housing,
acoustically exciting the target over a range of frequencies, measuring
the frequency response of the pressure within the chamber, and relating
the measured frequency response to the compliance of the target.
BRIEF DESCRIPTION OF THE DRAWING
For a better understanding of these and other objects of the present
invention reference is made to the following detailed description of the
invention which is to be read in conjunction with the accompanying
drawings, wherein:
FIG. 1 is a perspective view of an instrument embodying the teachings of
the present invention;
FIG. 2 is a schematic view of the present instrument with an accompanying
bond graph representing a model of the instrument wherein termination is
provided by a human eye, and
FIG. 3 is a response curve showing variations in the instrument chamber
pressure over a desired range of frequencies.
BRIEF DESCRIPTION OF THE INVENTION
Turning now to FIG. 1 there is shown an acoustical tonometer, generally
referenced 10, that embodies the teachings of the present invention. The
instrument includes a cylindrical housing 11 having an unrestricted target
opening 12 formed in one end of the housing. An acoustical seal 13 is
joined to the housing which is radially disposed about the target opening
and protrudes slightly forward from the housing. The seal is formed of a
pliable material that can be placed around a given target and pressed into
conformity with the surrounding tissue to provide an acoustical or dynamic
seal for preventing acoustical energy within a given frequency from
escaping from the chamber. The contour of the seal may be preformed to
conform to a specific part of the body which, in the case of this
preferred embodiment, is the bone structure 14 surrounding a human eye 15
(FIG. 2). FIG. 2 shows the instrument with the eye centered in the opening
and the seal in place.
Although the present instrument is ideally well suited for determining
intraocular pressure, it can also be used with equal adaptability to
determine the compliance or stiffness of other types of tissue. The device
can thus be used as a diagnostic instrument for use in early detection of
abnormal tissue growth and/or tumors which exhibit a greater amount of
stiffness when compared to normal tissue. This type of non-contact
diagnosis can be accomplished without having to resort to surgical
procedures and therefore reduces the cost involved and eliminates to a
great extent patient anxiety.
As seen in FIG. 2, when the present device is used to determine intraocular
pressure, the instrument does not deform or in any way make physical
contact with the cornea and thus poses little or no risk to the patient.
As will be explained below in greater detail, to conduct a diagnostic
examination, the seal is simply placed about the target with the target
being centered in the opening 12. The target is then acoustically excited
over a selected range of frequencies. Pressure variations produced in the
chamber 16 are measured and recorded. From these dynamic measurements, the
response of the target over the selected frequency range is plotted to
provide a characteristic curve relating to the compliance or stiffness of
the target. In the case of the eye, intraocular pressure is directly
inferred from the dynamic response characteristics.
The term compliance and stiffness may be used synonymously hereinafter with
the understanding that compliance is the reciprocal of stiffness.
Similarly, the operation of the present invention will be explained with
particular reference to diagnosing the health of the human eye. It should
be understood, however, that the present instrument may be used to
determine the compliance or stiffness of all types of human tissue in the
target opening of the instrument without departing from the teachings of
the present invention.
As noted above, the present instrument develops valuable information from
which intraocular pressure and thus the health of the eye is accurately
and directly inferred without having to contact the eye with a probe or
the like. Accordingly, the use of local anesthesia is avoided which, in
turn, allows for simplified and safer testing when compared to known
applanation tests. This non-contact procedure can be repeated at short
intervals without danger to the patient and valuable information about
changes in the eye's response at various times of the day and night are
obtained within a relatively short period of time.
An acoustical driver general reference 17 is mounted in the back wall 18 of
the housing opposite the unrestricted opening 12. The driver includes a
piston 22 that communicates with a chamber 16 contained within the
housing. The piston is coupled to an electrical coil 23 surrounded by a
permanent magnet 24. The coil is excited by frequency generator 25 which
causes the piston to vibrate linearly through a predetermined range of
frequencies to produce pressure fluctuations within the chamber. When
testing a human eye, a frequency range of between 0 and 500 Hz has been
found to be preferred, however, this range may be varied depending on the
nature of the target undergoing diagnostic testing. The frequency
generator can be either a wide band noise generator or a sweep generator
that is selectively tuned to a desired frequency range. Both of these
devices can be purchased from a number of suppliers.
A pressure sensor 27 is mounted within the chamber 16 and is adapted to
sense changes in chamber pressure (P.sub.C) over the selected range of
input frequencies. Pressure related information from the sensor is applied
to a spectrum analyzer 30. The analyzer can be any one of many
commercially available instruments that are capable of accepting this type
of input and providing a visual presentation 31 of the input data over the
desired range of frequency. Alternatively, the analyzer may utilize custom
circuits that are dedicated to a specific diagnostic application.
A typical characteristic response curve 35 for a human eye undergoing
diagnostic testing is shown in FIG. 3. The curve is a plot of pressure
related measurements taken by sensor 27 over a frequency range of 0 to 500
Hz. In this case, a ratio of measured chamber pressure (P.sub.C) to the
exciting voltage (E.sub.C) applied to the driver is plotted against
frequency to develop the response curve 35. Accordingly, the effects of
fluctuations in the driver input voltage are minimized. This curve shows a
pronounced minimum peak value 36 at a first frequency and another
pronounced maximum peak value 37 at a second higher frequency. After
reaching the maximum peak value, the curve drops and becomes asymptotic at
a point 38 which represents some definable P.sub.C /E.sub.C value. As can
be seen, there are three clearly discernable points on the characteristic
curve which, as will be explained in greater detail below, can be used to
directly infer the stiffness or compliance of the target.
Eye tests using the present instruments were conducted on both live
subjects and enucleated eyes containing pressure taps. It was found in
both cases, that the frequency at which the recorded response curve
reached a minimum peak was directly related to the compliance of the eye
undergoing testing. This relationship was further verified in accordance
with the analytical model of the physical system illustrated by the bond
graph in FIG. 2. These results also compared favorably with those
developed using Goldmann's applanation testing methods.
The eye and other living tissues exhibit a dynamic response similar to a
mass-spring-damper system when exposed to excitation at different
frequencies. In the present instrument a target located in the target
opening 12 is acoustically excited within a range spanning the predictable
resonant frequency of the target. Accordingly, as the target oscillates
within this range it produces changes in the internal pressure of chamber
16. By measuring the internal pressure of the chamber over the input range
of the generator, the frequency at which the target reaches resonance can
be accurately and quickly identified. This, of course, occurs at the
minimum peak frequency 36. (FIG. 3). It is known that a healthy eye will
reach resonance within some definable range of frequencies while an
unhealthy eye exhibiting an elevated intraocular pressure will reach
resonance at higher frequencies outside of this normal range.
The frequency at which the maximum peak value upon the response curve
occurs can also be used as an indicator of target compliance. Here again,
an increase in target stiffness causes the maximum peak to move to the
right and thus occur at some higher frequency. Changes produced in the
maximum peak frequency value by a given target will be different from
those produced in the minimum peak frequency value because the frequency
at which the curve reaches a peak maximum is influenced by factors other
than the resonant behavior of the target. However, this value can be used
in the same manner as the minimum peak value to determine the compliance
of the target.
The asymptotic values at both the high and low frequency ends of the
resonant response curve can also be used as a measure of the targets'
resonant behavior. Less compliant or stiffer targets become asymptotic
with the abscissa at increasingly high Pc/Ec values. Again, by conducting
a sufficient number of tests on different targets, a relationship between
these values and target compliance can be established whereby the
compliance can be accurately determined.
As illustrated in FIG. 2 the analyzer 31 is equipped with a readout screen
that provides a visual presentation of the measured response
characteristic curve 35. From this visual presentation direct reading of
the key values of interest can be taken to provide the user with a direct
indication of the health of the eye. With very little training a patient,
or one attending the patient, can monitor the eye at relatively short
intervals and quickly note changes in the eye's condition. To further aid
the user and provide for ease of reading, the analyzer is equipped with a
digital readout 32 that is wired into appropriate analyzer circuits to
provide a numerical reading of the minimum peak curve value or any other
value of immediate interest. A plurality of readout devices might also be
utilized in a similar manner to provide a number of different key
readings.
An analytical description of the acoustical tonometer shown in FIG. 2 can
be made in association with the bond graph representation of the model
also depicted in FIG. 2. The terminator or eye 15 is considered to consist
of an inertial effect (I.sub.T), a resistive effect (R.sub.T) and a
compliance effect (C.sub.T). The compliance effect is the parameter of
greatest interest. The termination effect has been simplified for the
purpose of this description but is considered reliable for the model
within the noted range of frequencies. It is also assumed for purposes of
explanation that both P.sub.C and E.sub.C are measurable values and are
thus known variables over the frequency range.
The bond graph variables are represented in a form where the general
variable is given first followed by the specific variable, (for example
R:R.sub.T,) relating to the modeled system, wherein:
R is the generalized variable representing system resistance, R.sub.S is
the specific resistance of the coil winding, and R.sub.T is the overall
damping resistance associated with the eye.
C is the general variable representing compliance, C.sub.C is the specific
variable relating to the compliance of the chamber and C.sub.T is the
specific compliance of the eye.
I is the generalized variable relating to inertia and I.sub.T is the
effective inertia of the eye.
GY is the generalized system function relating to the gyrator (driver) and
.tau. is the gyrator modulus relating to the velocity of the voice coil at
a specific appllied voltage.
TF is the generalized variable relating to the transformer function and
A.sub.P is the transformer modulus relating to the area of the piston.
SE is the generalized variable representing the effort source and E.sub.C
is the specific effort source.
Laplace transformed state equations are derived directly from the bond
graph representation and yield the following transfer function which
relates to the chamber pressure (P.sub.C) to the driving voltage
(E.sub.C).
##EQU1##
where:
s is the Laplace operator.
From this transfer function a frequency response curve similar to that
shown in FIG. 3 can be derived. If R.sub.T, the damping associated with
the terminator (eye), is small then the minimum frequency peak exists in
the frequency response,
##EQU2##
at:
##EQU3##
where:
.omega..sub.0 is the resonant frequency of the eye.
This solution to the numerator of the transfer function for the system
demonstrates that the frequency at which the minimum occurs, in the
frequency response curve, is directly proportional to the square root of
the termination compliance (i.e. stiffness). As can be seen, the frequency
at which the minimum occurs changes in response to changes in the
intraocular pressure.
Based on the same assumptions, it can be shown that a maximum will occur in
the frequency response at
##EQU4##
where: .omega..sub.n is the natural frequency of the combined termination
and instrument system.
The natural frequency (.omega..sub.n) of the eye/instrument system is
dependent on the compliance of both the terminator (eye) and the chamber
surrounding the termination. With realistic assumptions as to the values
of the stated parameters in this eye/instrument system the value of the
low frequency asymptote and that at the high frequency asymptote can be
used for additional information about the compliance at the termination.
If the resistance of the driving voice coil is small, R.sub.s .fwdarw.0,
then
For low frequencies, s.fwdarw.0
##EQU5##
For high frequencies, s.fwdarw..infin.
##EQU6##
From this relationship it is now evident that it is possible to determine
the termination compliance, C.sub.T, by measuring the high and low
frequency asymptotic behavior of the chamber pressure.
While this invention has been explained with reference to the structure
disclosed herein, it is not confined to the details set forth and this
application is intended to cover any modifications and changes as may come
within the scope of the following claims.
* * * * *
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