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Claims  |
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I claim:
1. An eye fundus oximeter comprising: an optical system for separately and
sequentially illuminating the fundus of the eye with at least four
specific wavelengths of light and transmitting reflected light from the
fundus of the eye to a first light receiving element; a second light
receiving element for receiving the light in a condition prior to its
incidence upon said first light receiving element and the patient's eye;
means for illuminating the fundus of the eye with a light of high power
including at least the predetermined four wavelengths, prior to a separate
and sequential illumination by at least four specific wavelengths of
light, to make the photoreceptor cells in the eye fundus substantially
transparent with respect to the four specifice wavelengths of light; means
for deriving and discriminating the outputs of the two light receiving
elements at each of the at least four different wavelengths of light,
respectively, and an arithmetic operation circuit performing operations on
the two outputs of the at least four different wavelengths, respectively.
2. An eye fundus oximeter according to claim 1 wherein said operation
circuit comprises a divider for calculating I.sub.1 =I.sub.1 '/I.sub.1 ",
I.sub.2 =I.sub.2 '/I.sub.2 ", I.sub.3 =I.sub.3 '/I.sub.3 " and I.sub.4
=I.sub.4 '/I.sub.4 " wherein I.sub.1 ', I.sub.2 ', I.sub.3 ' and I.sub.4 '
are the intensities of the reflected light from the fundus of the
patient's eye at the four different wavelengths of light and I.sub.1 ",
I.sub.2 ", I.sub.3 " and I.sub.4 " are the intensities of the intensities
of light before impinging on the patient's eye, a subtractor for
calculating (I.sub.1 -I.sub.3), (I.sub.1 -I.sub.4), (I.sub.2 -I.sub.3) and
(I.sub.2 -I.sub.4) based upon the outputs I.sub.1 -I.sub.4 of said divider
and a divider for calculating A=(I.sub.1 -I.sub.3)/(I.sub.1 -I.sub.4) and
B=(I.sub.2 -I.sub.3 )/(I.sub.2 -I.sub.3) based upon the output of said
subtractor, thereby identifying one of the oxygen saturation values, which
are preparatory calculated, in response to the values A and B.
3. An eye fundus oximeter according to claim 1 or claim 2 further
comprising a computer wherein the values A and B are decoded into a column
address specifying signal and a line address specifying signal for a
memory from which an oxygen saturation is read out according to the values
A and B and placed on a display.
4. An eye fundus oximeter comprising:
means for directing at least four preselected wavelengths of source light
to an object eye;
means for receiving the source light reflected from the object eye;
means, responsive to said receiving means, for generating measurement
signals representative of the intensity of reflected light with respect to
the preselected wavelengths, respectively;
means for varying the wavelength transmission characteristic of the eye
fundus, before the direction to and receiving of said four preselected
wavelengths of source light from the object eye to generate measurement
signals, to make the photoreceptor cell in the eye fundus substantially
transparent with respect to said preselected wavelengths of light, and
means, connected to said generating means, for calculating an oxygen
saturation of the blood in the eye fundus from said measurement signals.
5. The invention of claim 4 further comprising means for detecting the
intensity of said source light with respect to said preselected
wavelengths, respectively, wherein said calculating means comprises means,
connected to said generating means and to said detecting means, for
obtaining information relating to the light absorption by the eye fundus
with respect to said preselected wavelengths, respectively.
6. The invention of claim 5, wherein the means for varying the wavelength
transmission characteristic includes means for illuminating the eye fundus
with a high power of light including all the preselected wavelengths prior
to a measurement sampling with the preselected wavelengths.
7. The invention of claim 6 further comprising means, responsive to said
directing means, for identifying the wavelength which is actually
directed, and for relating an identification of the wavelength of the
signals from the generating means and the intensity detection of the
detecting means.
8. The invention of claim 4, wherein said preselected wavelengths consist
of four different wavelengths.
9. The invention of claim 4 further comprising means for detecting the
intensity of said source light with respect to each of said preselected
wavelengths, and wherein said calculating means comprising means for
calculating the following values, A and B:
A=(I.sub.1 "/I.sub.1 '-I.sub.3 "/I.sub.3 ')/(I.sub.1 "/I.sub.1 '-I.sub.4
"/I.sub.4 ')
B=(I.sub.2 "/I.sub.2 '-I.sub.3 "/I.sub.3 ')/(I.sub.2 "/I.sub.2 '-I.sub.4
"/I.sub.4 ')
wherein: I.sub.1 ' to I.sub.4 ' represent the intensity of the source light
detected by said detecting means with respect to a first to fourth
wavelength, respectively; and
I.sub.1 " to I.sub.4 " represent intensity of the reflected light received
by said receiving means, which correspond to the signals of said
generating means with respect to said first to fourth wavelengths of
light, respectively.
10. The invention of claim 9, wherein said calculating means further
comprises means for preparatory storing a variety of predetermined values
of oxygen saturation and means for identifying one of said oxygen
saturation values in response to said values, A and B.
11. The invention of claim 10 wherein said storing means stores said
variety of oxygen saturation values at corresponding addresses to be
determined by a pair of address codes, respectively, and said identifying
means comprising means for decoding said values, A and B into a pair of
address codes to determine one of said addresses and to read out an oxygen
saturation value stored therein.
12. The invention of claim 4 further comprising a pair of visual targets of
collimation disposed along a line of sight with which the optical axis of
the object eye is to be coincided.
13. The invention of claim 4 wherein the means for varying the wavelength
transmission characteristic includes a flash of high intensity light for a
short period of time prior to measurement sampling with the preselected
wavelengths.
14. The invention of claim 4 wherein said means for directing source light
comprises means for successively and separately transmitting said
preselected wavelengths of light when the photoreceptor cells have been
made relatively transparent to said preselected wavelengths.
15. The invention of claim 14 further comprising means, responsive to said
transmitting means, for identifying the wavelength corresponding to the
measurement signals.
16. The invention of claim 4 further comprising means for controlling the
sequence of illuminating the eye fundus and directing the source light to
the eye so that said preselected wavelengths of light are successively and
separately transmitted to the eye following the rendering of the eye
fundus substantially transparent. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates to an eye fundus oximeter which measures
photoelectrically the oxygen saturation of the blood in the fundus of the
eye.
BACKGROUND
Measurements of the oxygen saturation of the blood in the fundus of the eye
are very instrumental for prevention and diagnosis of geriatric brain
diseases such as hypertension and arterial sclerosis and also for
premature infant monitoring.
In order to obtain information about the blood in the eye fundus, it is not
sufficient to merely know the status of blood vessels in the eye fundus
and thus necessary to carry on spectral analysis. In the case of this type
of analysis, however, great difficulties are expected in discriminating
the reflection or absorption of light by the eye fundus blood from the
reflection of light on the surfaces of the cornea and the crystalline lens
or absorption of light by various cell layers in the eye fundus.
Accordingly, there has been no prior art device available which could
measure the oxygen saturation of the blood in the eye fundus.
DISCLOSURE OF THE INVENTION
It is therefore an object of the present invention to obviate the above
discussed measurement difficulties and enable measurements of the
percentage oxygen saturation of the blood in the fundus of the eye.
Pursuant to the operating principle of the present invention, the
phenomenon in which visual pigments in a layer of photoreceptor cells may
discolor and become transparent upon illumination of light is utilized and
the influences of surfacial reflection about the cornea, the crystalline
lens and so forth and absorption of light in cell layers in the eyefundus
are removed by performing arithmetic operation on measurements utilizing
four different wavelengths of light.
As described briefly above, the present invention obviates optical
influences of tissues other than the blood to dispense with any
conpensation for personal differences, and makes it possible to trace
changes in the oxygen saturation over the progress of time by its
capability of prompt measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional schematic view of eye fundus cell layers for
illustration of the principle of the present invention;
FIG. 2 is an elevational cross sectional side view of an optical system
according to one embodiment of the present invention;
FIG. 3 is a plan view of a filter disc;
FIG. 4 is a plan view of apertured disc;
FIG. 5 is a circuit diagram of an arithmetic operation circuit according to
one embodiment of the present invention;
FIG. 6 is a basic block diagram of the interior of a computer;
FIG. 7 is a plan view of an example of a device for preventing the movement
of the eye to be examined.
PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 is a schematic representation of cell layers in the eye fundus for
explanation of the operating principle of the present invention. There are
illustrated blood vessel layers 1 and 2 containing oxided hemoglobin and
reduced hemoglobin, a layer 3 consisting of nerve fibers and ganglian
cells, a photoreceptor cell layer 4, a pigment epithelium layer 5, the
choroid 6 and the sclera 7. In FIG. 1, the left handed side thereof
corresponds to the front side of the eye. Light impinges on the
photoreceptor cell layer 4 through the blood vessel layers 1 and 2 and the
nerve fiber and ganglian cell layer 3. Visual pigments contained in the
photoreceptor cell layer 4 tend to discolor upon illumination of eye
intensity light, in other words, the absorption band thereof is shifted
from a visual range to an ultraviolet range so that the visual pigments
become transparent with respect to the visual range. The intensity of
light reflected from eye fundus is very small if the light is incident
upon the portion of the eye fundus where illuminating light has not been
previously applied and the photoreceptor cell layer does not discolor,
since a greater part of the incident light in this case is absorbed by the
photoreceptor cell layer 4 and the intensity of light returning from
reflection on the pigment epithelium layer 5, the choroid 6, etc., is very
small. This light is indicated by a reduced arrow schematic "a" in FIG. 1.
By contrast, measurement light incident upon the portion of the eye
wherein a quantity of illuminated light had been previously applied to
bring about that discoloring phenomenon in the photoreceptor cell layer 4
is not substantially absorbed by this layer 4, thus traveling through and
being reflected from the pigment epithelium layer 5, the choroid 6, etc.
as shown by schematic arrow "b" in FIG. 1. Since this light passes through
the blood vessel layers 1 and 2 twice, it is subject to absorption by
hemoglobin and comes to carry information about the blood. Accordingly,
the eye fundus is illuminated with four different wavelengths of light and
reflected lights therefrom are employed for spectral analysis under the
conditions where flashes of light with high intensity have been previously
applied to the eye fundus for a short period of time to cause the
discoloring phenomenon of the photoreceptor cell layer 4 and this
discoloring phenomenon remains.
The absorption coefficients of the respective layers 1 to 5 in FIG. 1 at a
specific wavelength of light are denoted as K.sub.1, K.sub.2, K.sub.3,
K.sub.4 and K.sub.5 and the thickness thereof as l.sub.1, l.sub.2,
l.sub.3, l.sub.4 and l.sub.5. The suffixes correspond to the reference
numbers of the respective layers of FIG. 1. The intensity I" of light
received by a measuring instrument can be written as follows;
I"=.alpha.I'+I' exp (-2K.sub.1 l.sub.1 -2K.sub.2 l.sub.2 -2K.sub.5 l.sub.5)
wherein I' represents the intensity of the incident light directed towards
the eye and .alpha. represents the efficiency of the incident light which
actually returns to the measuring instrument after scattering and
reflection from the eye, that is, the cornea and crystalline lens. Thus,
the first term of the righthand side of the above equation corresponds to
the intensity of the light reflected from the eye surface back to the
measuring instrument while the second term refers to that portion of the
intensity of light that has been transmitted into and returns from the
interior of the eye and has been affected by the abovementioned layers.
Since the absorption coefficient of the nerve fiber and ganglian cell
layer 3 is substantially zero and the counterpart of the photoreceptor
cell layer 4 is negligible when the same is subject to the previous light
illumination and manifests the discoloring phenomenon, the above formula
lacks items regarding K.sub.3 l.sub.3 and K.sub.4 l.sub.4. In the formula
defined above the respective absorption coefficients are known and four
factors .alpha., l.sub.1, l.sub.2 and l.sub.5 are unknown. The four
unknown factors can be evaluated by applying the above defined formula to
the intensity of the incident light and the intensity of the light
reaching the measuring instrument at the four different wavelengths of
light, respectively. The values necessary for evaluating the oxygen
saturation of the blood in the eye fundus are the thicknesses l.sub.1 and
l.sub.2 of the blood vessel layers 1 and 2. If the absorption coefficients
of the respective layers at the four wavelengths of light are labelled
K.sub.11, K.sub.21, K.sub.31, K.sub.41 and so forth (the first digit of
the suffixes identifies the wavelengths and the second identifies the
layers), then the following relationships will stand between the
intensities I.sub.1 ', I.sub.2 ', I.sub.3 ' and I.sub.4 ' of the incident
light and the intensities I.sub.1 ", I.sub.2 ", I.sub.3 " and I.sub.4 " of
the light entering the measuring instrument. It will be noted that .alpha.
and the absorption coefficient K.sub.5 of the pigment epithelium layer 5
are not dependent upon wavelength and constant for an overall range of
wavelength of light.
I.sub.1 "=.alpha.I.sub.1 '+I.sub.1' exp (- 2K.sub.11 l.sub.1 -2K.sub.12
l.sub.2 -2K.sub.5 l.sub.5)
I.sub.2 "=.alpha.I.sub.2 '+I.sub.2 ' exp (-2K.sub.21 l.sub.1 =2K.sub.22
l.sub.2 -2K.sub.5 l.sub.5)
I.sub.3 "=.alpha.I.sub.3 '+I.sub.3 ' exp (-2K.sub.31 l.sub.1 -2K.sub.32
l.sub.2 -2K.sub.5 l.sub.5)
I.sub.4 "=.alpha.I.sub.4 '+I.sub.4 ' exp (-2K.sub.41 l.sub.1 -2K.sub.42
l.sub.2 -2K.sub.5 l.sub.5)
If I.sub.1 "/I.sub.1 '=I.sub.1, I.sub.2 "I.sub.2 "/=I.sub.2, etc., then
##EQU1##
The above formulas are all defined as a function of l.sub.1 and l.sub.2.
In relation to the fact that oxidized hemoglobin and reduced hemoglobin
are practically mixed in the blood, the thicknesses l.sub.1 and l.sub.2 of
the layers 1 and 2 are defined by assuming that only oxidized hemoglobin
is gathered to form layer 1 separately from reduced hemoglobin which is by
itself gathered to form layer 2, and that the thickness of layers 1 and 2
are representative of the amount of only oxidized hemoglobin and that of
only reduced hemoglobin, respectively. Therefore, the oxygen saturation
SO.sub.2 of the blood in the eye fundus can be written as follows:
##EQU2##
Thus, the oxygen saturation can be evaluated by preparing various cases of
values of the above defined two formulas which can be calculated in
advance under various assumptions of the coefficients K.sub.11, etc., and
the oxygen saturation and by retroactively identifying a desired oxygen
saturation through a set of the prepared values which is equal to actually
measured values (I.sub.1 -I.sub.3)/(I.sub.1 -I.sub.4) and (I.sub.2
-I.sub.3)/(I.sub.2 -I.sub.4). This is the conceptional principle of the
present invention. In practice, the identification of the oxygen
saturation through the actual measurement results is dependent upon a
function of a computer.
The present invention will now be described in more detail in terms of its
embodiment.
FIG. 2 shows one embodiment of the present invention, in which the eye to
be examined is labeled 11 and a light source for illuminating the eye
fundus with light is labeled 8. A disc 20, as indicated in FIG. 3, has
five sector-shaped windows 31-35 one of which is merely an opening as
denoted as 31 and the other four windows 32-35 carry filters having
different wavelengths for transmission. The disc 20 is rotated by a motor
19. The window 31 in the disc enables an overall quantity of light from
the light source 8 to pass therethrough to previously illuminate the eye
fundus with light for the development of the discoloring phenomenon in the
photoreceptor cell layer, whereas the other four windows 32-35 aid in
illuminating the eye fundus with the four different wavelengths of light.
A lens 21 is used to focus an image of the light source 8 on the cornea 24
of the patient's eye 11, thus leading light to the eye fundus. A portion
of light emerged from the light source 8 traverses a translucent mirror 10
and impinges on a light receiving photodiode 13 which in turn provides
information of the above discussed incident light intensities I.sub.1 ",
I.sub.2 ", I.sub.3 " and I.sub.4 ". The light receiving element 13 is
located so as to be conjugate with the cornea 24 with respect to the
translucent mirror 10. A neutral density filter 12 is disposed in front of
the light receiving element 13 to keep a linear relationship between
current and illumination. An aperture 14 on an optical axis extending
toward the right side of the patient's eye 11 is located so as to be
conjugate with the retina of the patient's eye 11 with respect to a lens
25 to effectively allow the passage of reflected light from the retina but
prohibit the passage of reflected and scattering light from the cornea 24
wherever practicable. Thus an improved S/N ratio is ensured since the
reflected light from the retina is relatively weak. Thereafter, the
reflected light from the retina is focused via a couple of lenses 22 and
27 on a light receiving element which is practically a photodiode 16. The
output of the light receiving element 16 bears information indicative of
the above discussed values I.sub.1 ', I.sub.2 ', I.sub.3 ' and I.sub.4 '.
The viewer's eye 18 is located to observe an image of the retina in the
eye fundus of the patient's eye through a translucent mirror 15 and an
eyepiece 17. An aperture 23 having various holes as in FIG. 4 is disposed
in front of the lens 21 to enable one of the various portions of the eye
fundus to be selectively illuminated with light. The disc 20 is further
provided with arc-shaped slits 36 which correspond to respective windows
31-35. A photoelectric device although not shown in the drawings is
adapted to sense the arrival of the slits and provide synchronizing
signals for discriminating the outputs of the light receiving elements 13
and 16 at each wavelength.
FIG. 5 illustrates a circuit structure for evaluating (I.sub.1
-I.sub.3)/(I.sub.1 -I.sub.4) and (I.sub.2 -I.sub.3)/(I.sub.2 -I.sub.4).
I.sub.1, I.sub.2, etc are ratios of the intensities I.sub.1 ", I.sub.2 ",
etc. of the light reflected from the retina to the intensities I.sub.1 ',
I.sub.2 ', etc. of the incident light on the eyeball, respectively, and
equal to the output of the light receiving element 16 divided by the
output of the light receiving element 13. In FIG. 5, both the elements 13
and 16 correspond to those in FIG. 2, of which the output currents are
converted into voltage signals via current to voltage converters P and P',
respectively. The voltage signals are applied to integrators S.sub.1
-S.sub.4 and S.sub.1 '-S.sub.4 ' via gates G.sub.1 -G.sub.4 and G.sub.1
'-G.sub.4 '.
The gates G.sub.1 and G.sub.1 ' are open while the filter secured on the
window 32 of FIG. 3 is in front of the light source 8, similarly the gates
G.sub.2 and G.sub.2 ' open for the filter on the window 33, the gates
G.sub.3 and G.sub.3 ' open for the filter on the window 34, and the gates
G.sub.4 and G.sub.4 ' for the filter on the window 35, respectively. The
outputs of the respective light receiving elements are amplified and then
held through the integration operation. E is a reset gate, a similar reset
gate being provided for each of the integrators although not shown because
of its space requirement in the drawing. The outputs of the respective
integrators are representative of I.sub.1 '-I.sub.4 ' and I.sub.1
"-I.sub.4 " and are supplied respectively to dividers R.sub.1 -R.sub.4 for
evaluating I.sub.1 '/I.sub.1 ' (=I.sub.1), etc. and thus I.sub.1 through
I.sub.4. The outputs of R.sub.1 -R.sub. 4 are fed to subtractors Su.sub.1
-Su.sub.4 to calculate (I.sub.1 -I.sub.3), (I.sub.1 -I.sub.4), (I.sub.2
-I.sub.3) and (I.sub.2 -I.sub.4), the resulting outputs being fed to
dividers R.sub.A and R.sub.B to calculate (I.sub.1 -I.sub.3)/(I.sub.1
-I.sub.4), etc. The calculation results are stored in sample hold circuits
HA and HB through the gate g which becomes open after a predetermined
number of revolutions of the disc 20, and converted into digital signals
via AD.sub.1 and AD.sub.2 and eventually sent to a computer C.
FIG. 6 shows the internal structure of the computer C in which the outputs
of AD.sub.1 and AD.sub.2 are applied to decoders DA and DB, the respective
values (I.sub.1 -I.sub.3)/(I.sub.1 -I.sub.4)=A and (I.sub.2
-I.sub.3)/(I.sub.2 -I.sub.4)=B specifying a column address and a line
address of an n.times.n memory M. The n.times.n memory contains a number
of pre-calculated oxygen saturation values according to various
combinations of A and B. The memory is read out as if a desired oxygen
saturation is evaluated from the actually measured values A and B while
the viewer consults with a lookup table plotted with various values A as
the axis of abscissa and various values B as the axis of ordinate. The
read out oxygen saturation is displayed on a display device L.
Movement of the eye or the head, which is reasonably expected, may be an
obstacle in steadily illuminating a specific area of the eye fundus with
light and measuring reflected light therefrom. For this reason, as shown
in FIG. 7, targets of collimation V.sub.1 and V.sub.2 are disposed along
the line of vision of the patient's eye so that the patient may fixed his
eye so as to observe V.sub.1 and V.sub.2 as if they were a single target.
Thus, the movement of eye or head can be substantially prevented to a
degree sufficient for the purpose of measurement and this condition is
also capable of being sufficiently maintained for a time period required
for the measurement.
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