Eye fundus oximeter

An eye fundus oximeter adapted to measure spectrally the oxygen saturation of the blood in the fundus of the human eye. The oximeter includes an illuminating optical system (8, 21, 10) for first illuminating the fundus of the patient's eye (11) with light of a wide wavelength range and then illuminating the same with light of four different spectra, a photoelectric element (13) for receiving the four kinds of light directly and not via the patient's eye and a photoelectric element (16) for receiving the four kinds of light impringing on the fundus of the eye and then reflected therefrom. The outputs of these photoelectric elements (13, 16) are compared at each of the same spectra, the results of such comparison being subject to given arithmetic operations to evaluate the oxygen saturation. The purpose of the pre-illumination of the eye fundus with light of a wide wavelength range is to discolor visual pigments in the photoreceptor cell layer in the fundus and aid in deriving information about the blood in the fundus.

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.

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; 
EQU 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.