 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an electrooptical analyzer, such
as an oximeter, for measuring a construction ratio between known
components in living tissue, and more particularly to a non-invasive
analyzer for optically analyzing living tissue without injuring it.
2. Description of the Prior Art
An optical oximeter for measuring the oxygen saturation of blood, which
includes a construction ratio between the hemoglobin oxide (HbO.sub.2) and
the hemoglobin (Hb), has been known in the prior art. In such an optical
oximeter, the oxygen saturation is determined by measuring the intensity
of light transmitted through a living tissue by at least a pair of
predetermined wavelengths and by subsequent processing of electrical
signals representing the intensities of the pair of wavelengths of light.
The measured intensities of light, however are influenced not only by the
absorption of the hemoglobin oxide and hemoglobin, but also by various
noise factors. Therefore, it is generally necessary to remove such noise
factors to provide a meaningful measurement.
Further, if a measurement is taken with reflected light, the light
reflected from the living tissue would also include an additional or
multiplying white noise factor due to surface reflection and/or light
scattering in the non-blood tissue. Such white noise factors are quite
difficult to be satisfactorily avoided or removed. In addition, the
measured intensity would also be influenced by the relative movement of
the probe to the living tissue.
Generally, in the prior art, the only optical oximeters that have been
practical use light transmitted through a limited portion of tissue, e.g.,
an earlobe or a finger tip attached to an optical probe positioned on the
opposite side from a light source.
An example of a prior art oximeter can be found in the Transactions on
Biomedical Engineering, Vol. BME-22, No. 3, p. 183, May 1975: "The
choroidal eye oximeter: an instrument for measuring oxygen saturation of
choroidal blood in vivo.:
Additional prior art references can be found in U.S. Pat. No. 4,086,915,
U.S. Pat. No. 3,825,324, U.S. Pat. No. 3,847,483, U.S. Pat. No. 3,787,124,
U.S. Pat. No. 3,998,550, and U.S. Pat. No. 4,157,708.
The prior art is still seeking a simplified but accurate oximeter.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical analyzer
utilizing a novel concept of operation.
Another object of the present invention is to provide an optical analyzer
capable of accurate measurement free from the influence of noise factors.
A further object of the present invention is to provide an optical analyzer
of a reflection light measurement type.
A still further object of the present invention is to provide an optical
analyzer widely applicable to various desired portions of living tissue.
An additional object of the present invention is to provide an optical
analyzer wherein it is not necessary to directly contact the living
tissue.
A still additional object of the present invention is to provide an optical
analyzer capable of measurement even when the living tissue moves relative
to the probe.
According to the present invention, the intensities of a source light,
after contact with living tissue, are measured at various wavelengths. A
wavelength .lambda. at which the intensity of light is equal to that of a
predetermined standard wavelength .lambda..sub.0 is searched, since the
searched wavelength .lambda. depends on a construction ratio to be
measured, such as hemoglobin oxide, the value of the construction ratio
can be ascertained.
The present invention can take the form of an oximeter having a source of
light with a plurality of different wavelengths, at least two or more of
the wavelengths have a fixed relative relationship of light absorption
after co-action with hemoglobin oxide, e.g., a predetermined standard
wavelength and a second wavelength can have equal light absorption
characteristics for a certain level of hemoglobin oxide. A fiber optical
probe can direct the light at the subject tissue and return it for
measurement after co-action and absorption by the tissue. A photodetector
and supplemental circuitry are capable of generating a noise-free first
signal representative of the degree of light absorption at the
predetermined standard wavelength. Correspondingly, a plurality of
electric signals representative of the degree of light absorption at the
other scanning wavelengths are also generated. Appropriate circuitry or a
microprocessor can select a second wavelength signal from the scanning
wavelengths having a fixed relationship to the light absorption of the
predetermined standard wavelength and generate a second signal
representative of the second wavelength whereby the amount of hemoglobin
oxide can be determined from a memory that stores the values of hemoglobin
oxide with subsequent appropriate display.
The many attendant advantages of the present invention may be best
understood by reference to the accompanying drawings in which like
reference symbols designate like parts throughout the figures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a partially cross sectional schematic elevation view of
the optical components of an embodiment of the present invention;
FIG. 2 represents a partially cross sectional schematic plane view of some
optical components of FIG. 1;
FIG. 3 represents a circuit diagram of an analog portion of the electric
circuit of the invention;
FIG. 4 represents a time chart showing the operation of the circuit of FIG.
3;
FIG. 5 represents a block diagram of a digital portion of the electric
circuit of the invention;
FIGS. 6a and 6b represent cross sectional views of a conceptional
representation of living tissue for the cases of transmitted light
measurement and reflected light measurement, respectively, and
FIG. 7 represents a graph of wavelength versus the light absorption
coefficient of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description is provided to enable any person skilled in the
field of electro-optical medical instrumentation to make and use the
invention and sets forth the best mode contemplated by the inventor for
carrying out his invention. Various modifications, however, will remain
readily apparent to those skilled in the art, since the generic principles
of the present invention have been defined herein specifically to provide
a relatively simplified and easily manufactured electro-optical medical
instrumentation to determine the level of hemoglobin oxide in living
tissue.
The specific embodiment shown in the Figures is designed for use as an
optical oximeter. The object living tissue schematically depicted is a
human body containing known components of oxihemoglobin and
deoxihemoglobin. A resulting construction ratio between these components
can be obtained corresponding to the oxygen saturation of the human body.
FIGS. 1 and 2 represent a partially cross sectional schematic elevation
view and a partially cross sectional schematic plane view of an optical
system of an embodiment of the present invention, respectively. The light
emitted from lamp 1 is convergently directed towards an entrance port 5a
of an optical fiber guide 5 by means of a concave mirror 2 and a
collimator 3 and is further segmented or chopped by a chopper 4 driven by
a motor 4a. The timing of the chopping of the light entering into entrance
port 5a due to chopper 4 is detected by the combination of a light
emitting diode 4b and a phototransistor 4c. Other forms of monitors could
also be utilized to provide timing signals. The optical fiber guide 5
leads to exit 6, forming a part of a medical probe, from which the light
introduced from entrance port 5a emerges. A part of the light emerging
from exit 6 returns back to a return entrance portion 7, which is
surrounded by exit 6 after traveling through the living tissue, which is
the subject of the analysis. Entrance 7 leads to an exit 8a by way of a
second optical fiber 8. The light emerging from exit 8a is made parallel
by collimator 9 and is partially reflected by dichroic mirror 10, which is
slightly inclined with respect to the optical axis of collimator 9. The
reflected light is directed convergent toward photocell 15 by means of
collimator 9. Photocell 15 is located adjacent to exit 8a and is provided
with band-pass filter 14 in front of it for the purpose of detecting the
intensity of a standard wavelength of light.
The major portion of the parallel light passing through the collimator 9
towards dichroic mirror 10 is transmitted therethrough toward one of a
number of band-pass filters (interference filters) 11a to 11f. These
interference filters are mounted on the periphery of a rotary disc 11 for
selectively positioning one of the band-pass filters on the optical axis
of collimator 9 at various angles to the optical axis as seen in FIG. 2.
After passing through one of band-pass filters 11a to 11f located on the
optical axis, the light is made convergent toward photocell 13 by means of
collimator 12. Rotary disc 11 can be driven by motor 20 through pulleys 16
and 18 and belt 17. The angle of rotation of rotary disc 11 is monitored
or detected by a rotary encoder 19. Thus, one of band-pass filters 11a to
11f on the optical axis varies its angle with the optical axis in
accordance with the rotation of rotary disc 11 for continuously changing
the transmittable wavelength band with position being detected by rotary
encoder 19. A plurality of band-pass filters are used to continuously
change the wavelength to be detected by photocell 13 over a wide range
because the variable range of only one band-pass filter is limited.
A white reflector 21 of a standard reflectance is measured prior to the
measurement of an object for the purposes of calibrating the output of the
device with respect to the various wavelengths scanned by band-pass
filters 11a to 11f since the output of the device can be influenced by
variable factors in the device, such as the spectral characteristics of
the light source or the photocell.
By means of the above optical system, the light scattered through and
reflected from the living tissue is detected at a standard wavelength by
photocell 15 and at various other wavelengths by photocell 13 with the
specific wavelength of light received by photocell 13 encoded by means of
rotary encoder 19.
FIG. 3 represents a circuit diagram corresponding to an analog part of the
electric circuit of the invention for processing the output signal of the
above-described optical system, the operation thereof being shown in the
time chart of FIG. 4. The output current of photocell 13, which receives
the light of the variable scanning wavelength .lambda., is converted into
a corresponding voltage signal A by means of current-voltage converter 21.
The voltage signal A is integrated for a predetermined period of time to
produce signal B by means of integrating circuit 22 which is periodically
switched on and off by a signal 22a synchronized with a signal 4d from
phototransistor 4c indicative of the timing of chopping by chopper 4. (The
actions of the signals will be better understood by reference to FIG. 4.)
In the signal B, a part B.sub.1 integrated within a time period, in which
chopper 4 allows a light passage, would include both a signal component
and a noise component, while a part B.sub.2 integrated within another time
period, in which light is blocked by chopper 4, would include only the
noise component. The part B.sub.1 is successively stored by sample hold
circuit 23 controlled by sampling signal 23a. On the other hand, the part
B.sub.2 is successively stored by sample hold circuit 24 under the control
of sampling signal 24a. Thus, the output C of sample hold circuit 23
includes both signal and noise components, and the output D of sample hold
circuit 24 a noise component only. Output D is subtracted from output C at
a succeeding subtraction circuit 25 to form signal E in which the noise
component is eliminated.
In a similar manner, the output current of photocell 15, which receives the
light of standard wavelength .lambda..sub.0, is processed through current
voltage converter 26, integrating circuit 27, sample hold circuits 28 and
29 and subtraction circuit 30 to obtain signal J.
Signals E and J are alternatively transmitted by AGC amplifier 33 under the
control of a multiplexer 31. The automatic gain control of the AGC
amplifier is accomplished in the following manner. Namely, sample hold
circuit 34 is provided to store a signal component, which corresponds only
to an amplified signal J, of output K of the AGC amplifier 33 (the
sampling wavelength voltage is not stored). Further provided is a
differential amplifier 35 for amplifying the difference between the output
voltage of the sample hold circuit 34 and a given constant voltage Vc. The
output of differential amplifier 35 controls photo-FET 32 to change its
resistance, the output of photo-FET 32 being connected to the input
terminal of AGC amplifier. Thus, the closed feedback loop composed of the
above elements, 34, 35 and 32, controls the gain of the AGC amplifier so
that the component of output K corresponding to signal J is held at a
nearly constant voltage Vc irrespective of the value of signal J to
establish a predetermined voltage level for subsequent conversion of a
digital format.
According to the above analog circuit in FIG. 3, therefore, the output K
alternatingly shows the light intensities at the standard wavelength
.lambda..sub.0 (corresponding to the J signal) and at the scanning
wavelength .lambda. (corresponding to the E signal) in response to a
predetermined time sequence from multiplexer 31.
FIG. 5 represents a block diagram of a microcomputer constituting a digital
part of the electric circuit for processing the output signal of the
optical system and connected to the above-described analog part. A-D
converter 36 is for converting the components in analog signal K
corresponding to standard wavelength .lambda..sub.0 and scanning
wavelength .lambda. into digital signals D.lambda..sub.0 and D.lambda.,
respectively. A-D converter 36 as well as multiplexer 31 in FIG. 3 is
controlled by CPU 38. The microcomputer in FIG. 5 processes as its input
data the digital signals D.lambda..sub.0 and D.lambda., and a digital
output of the rotary encoder 19 indicative of the scanning wavelength
.lambda.. Read Only Memory (ROM) 40 previously stores the coordinates of
the scanning wavelengths to be addressed by the output of rotary encoder
19 and the oxygen saturation values addressed by the number of a scanning
wavelength. In FIG. 5, the microcomputer further includes a Random Access
Memory (RAM) 39, Input and Output ports 37 and 41, and a display 42 for
indicating the output of the microcomputer.
The following description will be directed to the function of the invention
and the procedure of measurement.
(i) The probe formed by light exit 6 and entrance portion 7 is
preparatorily applied to the standard white reflector 21.
(ii) CPU 38 readw the output of the rotary encoder 19 indicative of the
rotation of disc 11 to successively address the numbers or coordinates of
the scanning wavelengths. With respect to every number of the scanning
wavelength, (D.lambda.)cal.k and (D.lambda..sub.0)cal.k are
correspondingly read and stored in predetermined areas of RAM 39 in the
order of the number of the scanning wavelength, wherein "cal." means
"calibrating" and k represents the number of the scanning wavelength
numbered from 1 to n. It is needless to say that (D.lambda..sub.0)cal.k's
are all equal for various k's.
(iii) After the above preparatory measurement with respect to the standard
white reflector, the probe is applied to an object to be measured, such as
living tissue.
(iv) CPU 38 reads (D.lambda.) mes.k and (D.lambda..sub.0)mes.k in the
similar manner as in step (ii), wherein the "mes." means "measuring" and k
represents the number of scanning wavelength. CPU 38 further obtains
D.sub.k according to a process expressed by the following formula, which
calibrates (D.lambda.)mes.k and (D.lambda..sub.0)mes.k with
(D.lambda.)cal.k and (D.lambda..sub.0)cal.k and obtains a ratio between
the values relating to wavelengths .lambda. and .lambda..sub.0 as follows:
##EQU1##
(v) CPU 38 stores every D.sub.k corresponding to each number of a scanning
wavelength in predetermined storage areas of RAM 39, respectively.
(vi) CPU 38 further operates to search a number of the scanning wavelength
at which D.sub.k =1, and addresses ROM 40 by the searched number of the
sanning wavelength to read out a corresponding oxygen saturation value.
(vii) Display 42 indicates the read out oxygen saturation value by means of
digital display elements, and all D.sub.k 's (k=1 to n) sotred in RAM 39
by means of a graphic display device.
As is easily recognizable from the above explanation, the above embodiment
searches a wavelength .lambda. having an intensity equal to that of the
standard wavelength .lambda..sub.0 with respect to the light contacting
the living tissue to determine an oxygen saturation value from the
searched wavelength .lambda. by way of a previously calculated and stored
relationship between the oxygen saturation value vs. a wavelength .lambda.
having an intensity equal to that of the standard wavelength
.lambda..sub.0.
In the above embodiment, the interference filters 10 and 11a to 11f are
used for the purpose of introducing into the optical system a sufficient
light flux with a wide range of wavelength values. Further, the light of
standard wavelength .lambda..sub.0 is measured in synchronization with the
measurement of every scanning wavelength .lambda. for the purpose of
cancelling all possible noise factors, which could be caused in cause of a
measurement wherein the probe does not directly contact the skin surface
of the living tissue. In other words, the procedure of measuring the light
of standard wavelength .lambda..sub.0 with respect to every scanning
wavelength .lambda. is not necessary from a theoretical view of obtaining
the necessary information of light intensity of standard wavelength
.lambda..sub.0 since it is theoretically equal with respect to all
scanning wavelengths.
Therefore, as an alternative structure to the above embodiment, the
standard wavelength may be selected fron the scanning wavelengths. Namely,
(D.lambda.)mes k/(D.lambda.)cal k=D'.sub.k
may be stored in RAM (39) with respect to all k's (k=1 to n). And, if a
wavelength, the number k of which is j. is selected, a D'.sub.k having a
value equal to D'.sub.j is searched to address ROM 40 from the number k of
the searched D'.sub.k. Even in this case, however, the synchronized
measurement of an identical wavelength of light with respect to every
scanning wavelength is still recommended for cancelling any possible
noise.
The above alternative structure discloses that a standard wavelength is not
necessarily selected from wavelengths other than the scanning wavelengths,
but can be from the scanning wavelengths. This means that two or more
standard wavelengths can be selected (as described later) simply by
modifying the software of the microcomputer without further complicating
the optical system to obtain two or more standard wavelengths. In other
words, a pair of standard wavelengths, the number k of which are
respectively l and m, can be selected from the scanning wavelengths
numbered k=1 to n without modifying the optical system.
The description of the invention will now be further advanced by an
explanation of the theoretical analysis of why oxygen saturation can be
obtained by the above measurements in connection with FIGS. 6 and 7.
The measured intensity I.lambda. of light of a scanning wavelength .lambda.
which is incident on the living tissue and reflected by or transmitted
through the same is expressed as follows:
(a) In case of transmission (See FIG. 6a):
I.lambda.=I.sub.0 (1-r)e.sup.-.epsilon..omega..sbsp.1.sup..d.sbsp.1.e.sup.
-.epsilon..omega..sbsp.2.sup..d.e.sup.-(.epsilon.HbO.sbsp.2.sup..lambda..CH
bO.sbsp.2.sup.+.epsilon.Hb.lambda..CHb+.epsilon..omega..c.omega.)d (1)
(b) In case of reflection (See FIG. 6b):
I.lambda.=I.sub.0
ar+.sbsp.0(I-r)e.sup.-.epsilon..omega..spsb.1.sup..2d.sbsp.1.e.sup.
-(.epsilon.HbO.sbsp.2.sup..lambda..CHbO.sbsp.2.sup.+.epsilon.Hb.lambda..CHb
+.epsilon..omega.C.omega.)d (2)
wherein:
I.sub.0 represents the intensity of the incident light (made identical
irrespective of the wavelength;
r represents the reflectance at the surface of the living tissue (which is
regarded as constant irregardless of the wavelength);
a represents the ratio of the light intensity I.sub.0.r to the measured
intensity;
.epsilon..omega..sub.1 and .epsilon..omega..sub.2 represent light
absorption coefficients of the cortical tissues E.sub.1 and E.sub.2 at the
light entering side and the light exiting side, respectively, (which are
regarded as independent of wavelength and include the attenuation factor
by the scattering), .epsilon..omega..sub.2 =.epsilon..omega..sub.1 in case
of reflection;
d.sub.1 and d.sub.2 represents the optical path length of E.sub.1 and
E.sub.2, respectively, d.sub.2 =d.sub.1 in case of reflection;
.epsilon.HbO.sub.2 .lambda. and .epsilon.Hb.lambda. represent the light
absorption coefficients of hemoglobin oxide and hemoglobin at wavelength
.lambda., respectively;
CHbO.sub.2 and CHb represent the densities of hemoglobin oxide and
hemoglobin, respectively;
.epsilon..omega. and C.omega. represent the light absorption coefficient of
a tissue other than hemoglobin oxide and hemoglobin in the blood layer
(which is regarded as independent of the wavelength and includes the
attenuation factor by scattering) and its density, respectively; and
d represents the optical path length of the blood layer B.
In case of transmission through the tissue, the measured intensity
I.lambda..sub.0 of light of a standard wavelength .lambda..sub.0 is
expressed as follows:
I.lambda..sub.0 =I.sub.0
(1-r)e.sup.-.epsilon..omega..sbsp.1.sup..d.sbsp.1.e.sup.
-.epsilon..omega..sbsp.2.sup..d.sbsp.2.e.sup.-(.epsilon.HbO.sbsp.2.sup..lam
bda..sbsp.0.sup..CHbO.sbsp.2.sup.+.epsilon.Hb.lambda..sbsp.0.sup.CHB+.epsil
on..omega..d.omega.)d
Here, if I.lambda.=I.lambda..sub.0 at a specific scanning wavelength
.lambda.,
I.sub.0
(1-r)e.sup.-.epsilon..omega..spsb.1.sup..d.sbsp.1.e.sup.-.epsilon..omega..
sbsp.2.sup..d.sbsp.2.e.sup.
-(.epsilon.HbO.sbsp.2.sup..lambda..CHbO.sbsp.2.sup.+.epsilon.Hb.lambda.CHb+
.epsilon..omega.d.omega.)d =I.sub.0 (1-r)e.sup.
-.epsilon..omega..sbsp.1.sup..d.sbsp.1.e.sup.-.epsilon..omega..sbsp.2.sup..
d.sbsp.2.e.sup.-(.epsilon.HbO.sbsp.2.sup..lambda..sbsp.0.sup..CHbO.sbsp.2.s
up.+.epsilon.Hb.lambda..sbsp.0.sup.CHb+.epsilon..omega.d.omega.)d
Namely,
.epsilon.HbO.sub.2 .lambda..CHbO.sub.2
+.epsilon.Hb.lambda..CHb=.epsilon.HbO.sub.2 .lambda..sub.0.CHbO.sub.2
+.epsilon.Hb.lambda..sub.0 CHb (3)
In the case of reflection from the tissue, the measured intensity
I.lambda..sub.0 is as follows:
I.lambda..sub.0 =I.sub.0 ar+I.sub.0
(1-r)e.sup.-.epsilon..omega..spsb.1.sup..2d.sbsp.1.e.sup.
-(.epsilon.HbO.sbsp.2.sup..lambda..sbsp.0.sup..CHbO.sbsp.2.sup.+.epsilon.Hb
.lambda..sbsp.0.sup..CHb+.xi..omega..C.omega.)d
So, if I.lambda.=I.lambda..sub.0,
I.sub.0 ar+I.sub.0 (I-r)e.sup.-.epsilon..omega..sbsp.1.sup..2d.sbsp.1.e.sup
.
-(.epsilon.HbO.sbsp.2.sup..lambda..sbsp.0.sup..CHbO.sbsp.2.sup.+.epsilon.Hb
.lambda..sbsp.0.sup..CHb+.epsilon..omega.C.omega.)d =I.sub.0 ar+I.sub.0
(1-r)e.sup.
-.epsilon..omega..sbsp.1.sup..2d.sbsp.1.e.sup.-(.epsilon.HbO.sbsp.2.sup..la
mbda..sbsp.0.sup.-CHbO.sbsp.2.sup.+.epsilon.Hb.lambda..sbsp.0.sup..CHb+.eps
ilon..omega.C.omega. )d
Therefore,
.epsilon.HbO.sub.2 .lambda..CHbO.sub.2
+.epsilon.Hb.lambda..CHb=.epsilon.HbO.sub.2 .lambda..sub.0.CHbO.sub.2
+.epsilon.Hb.lambda..sub.0 CHb
This is identical with equation (3) and shows that the equation (3) is good
in either case of reflection or transmission.
From equation (3),
CHbO.sub.2 (.epsilon.HbO.sub.2 .lambda.-.xi.HbO.sub.2
.lambda..sub.0)=CHb(.epsilon.Hb.lambda..sub.0 -.epsilon.Hb.lambda.)
And, this equation can be further modified according to the following
definitions (4) and (5)
.epsilon.HbO.sub.2 .lambda.-.epsilon.HbO.sub.2 .lambda..sub.0 =kHbO.sub.2
.lambda. (4)
.epsilon.Hb.lambda.-.epsilon.Hb.lambda..sub.0 =kHb.lambda. (5)
Thus, equation (3) can be modified as follows:
##EQU2##
If 1is added to both terms of this equation,
##EQU3##
which is identical with
##EQU4##
From this equation,
##EQU5##
The lefthand term of equation (6) is identical with the definition of the
oxygen saturation. This means that the oxygen saturation can be expressed
by values kHb.lambda. and kHbO.sub.2 .lambda. which are defined in formula
(4) and (5). In other words, the oxygen saturation can be exclusively
expressed by the combination of the known values .epsilon.HbO.sub.2
.lambda., .epsilon.HbO.sub.2 .epsilon..sub.0, .epsilon.Hb.lambda. and
.epsilon.Hb.lambda..sub.0, wherein .epsilon. is determined by the
condition I.lambda.=I.lambda..sub.0 and is free from the influence of
troublesome factors such as I.sub.0, r, a, .epsilon..omega..sub.1,
.epsilon..omega..sub.2, d.sub.1, d.sub.2, .epsilon..omega., C.omega. and
d.
In practice, the oxygen saturation is generally measured in accordance with
a procedure comprising the steps of:
(a) measuring the intensity of light at a predetermined standard
wavelength;
(b) searching for a wavelength at which the light intensity is equal to
that of the standard wavelength; and
(c) obtaining oxygen saturation from the values .epsilon.HbO.sub.2 .lambda.
and .epsilon.Hb.lambda. at the searched wavelength and the values
.epsilon.HbO.sub.2 .lambda..sub.0 and .epsilon.Hb.lambda..sub.0 at the
standard wavelength in accordance with equations (4), (5) and (6).
The above theory will be further explained in connection with FIG. 7, in
which curves .alpha., .beta. and .gamma. represent three kinds of
different spectral absorption characteristics of hemoglobin with different
oxygen saturations, 100 percent, 50 percent and 0 percent, respectively.
As is apparent from FIG. 7, the spectral absorption characteristics
specifically differ depending on the amount of oxygen saturation of
hemoglobin. Additionally, the present invention recognizes the cyclic
response of light absorption by various wavelengths and particularly the
fact that two or more wavelengths will experience equal absorption for the
same level of hemoglobin oxide and hemoglobin. Thus, if 757 nm is selected
as the standard wavelength .lambda..sub.0, the wavelength .lambda. at
which the absorption coefficient is equal to that of the standard
wavelength 757 nm is 664 nm in case of curve .alpha. for oxygen
saturation, 100%. Similarly, 700 nm and 708 nm are the wavelengths showing
equal absorption coefficient to that of standard wavelength in cases of
curves .beta. (for oxygen saturation, 50%) and .gamma. (for oxygen
saturation 0%), respectively. Thus, the wavelength at which the light
absorption coefficient is equal to that of the standard wavelength differs
in accordance with the oxygen saturation. The wavelength further depends
only on the oxygen saturation irrespective of any additional or
multiplying noise factors which could be included in the measured light.
Thus, in case of FIG. 7, if 644 nm provides the same absorption
coefficient, then oxygen saturation is known to be 100%. In other words,
the above wavelength and the oxygen saturation correspond to each other on
an equal level of signal, irregardless of any white noise factors.
As is apparent from FIG. 7, there is a relatively wide width (66 nm)
between the wavelength (644 nm) indicative of the oxygen saturation at 100
percent, and the wavelength (700 nm) indicative of the oxygen saturation
at 50 percent, if the standard wavelength is 757 nm. Therefore, various
values of oxygen saturation can accurately correspond to their specific
wavelengths distributed within a relatively wide width (66 nm) in case of
an oxygen saturation range between 50 to 100 percent. On the contrary,
there is only a narrow width (8 nm) between the wavelengths, 700 nm and
708 nm, in case of an oxygen saturation range between 50 to 0 percent, and
various values of oxygen saturation between 50 to 0 percent would have to
be correlated with wavelengths distributed only in the relatively narrow
width (8 nm). This means that the measurement of oxygen saturation through
the search of wavelength would not be as accurate in the range of oxygen
saturation between 50 to 0 percent as it would be in the range of oxygen
saturation between 100 to 50 percent when the standard wavelength is
selected at 757 nm. Therefore, another different standard wavelength
should be selected, in place of 757 nm, if an accurate measurement is
desired in the oxygen saturation range between 50 to 0 percent. Further,
if a relatively uniform accuracy in the measurement is required in the
entire range from 100 to 0 percent of oxygen saturation, then the standard
wavelength should be selected balancing the entire range to avoid a biased
accuracy.
There may be a case, however, when a desired uniform accuracy within a
desired wide range cannot be obtained by only a selection of a single
standard wavelength if the requirement is relatively high. Or, there may
be another case wherein the search for a wavelength having the same
intensity as that of the standard wavelength is relatively difficult in a
part of the desired wavelength range if the change in intensity of light
corresponding to the change in the wavelength is insufficient.
In the above cases, it is recommended to utilize a second standard
wavelength (or third, fourth and additional standard wavelengths, if
necessary), in a manner to supplement each other. For example, this can be
achieved in the following manner comprising the steps of:
(I) Selecting a first standard wavelength suitable for an accurate
measurement in an oxygen saturation range between 100 to 50 percent, and a
second standard wavelength suitable for another oxygen saturation range
between 50 to 0 percent;
(II) Searching for a first and second wavelength having the same intensity
as the first and second standard wavelengths, respectively;
(III) Determining whether or not the first searched wavelength is within a
range of wavelengths corresponding to the oxygen saturation range, 100 to
50 percent (or, alternatively, examining whether or not the second
searched wavelength is within a range of wavelength corresponding to the
oxygen saturation range 50 to 0 percent); and
(IV) Deriving an oxygen saturation value corresponding to the first
searched wavelength if the answer of the examination is "YES", and
deriving an oxygen saturation value corresponding to the second searched
wavelength if the answer of the examination is "NO" (or, vice versa in
case of the alternative examination).
In the above manner, a uniform high accuracy can be obtained across the
whole oxygen saturation range, 100 to 0 percent. Above method can be
carried out by the hardware apparatus of the embodiment shown in FIG. 1 to
5 if a software program and the data stored in the ROM of the
microcomputer in FIG. 5 is suitably changed.
The following description is directed to a more complex case wherein a
third substantial non-white component other than the hemoglobin oxide and
the hemoglobin is included in the living tissue. If X represents such a
third component, .epsilon.X.lambda. represents the light absorption
coefficient of the third component X at wavelength .lambda., and Cx
represents the density of the third component X, the following equations,
which correspond to equation (3), are obtainable for a pair of standard
wavelengths .lambda..sub.0 and .lambda.'.sub.0, respectively, in a similar
manner as used in obtaining equation (3) in both the transmission and
reflection cases.
.epsilon.HbO.sub.2 .lambda..CHbO.sub.2
+.epsilon.Hb.lambda.CHb+.epsilon.x.lambda.Cx=.epsilon.HbO.sub.2
.lambda..sub.0.CHb+.epsilon.x.lambda..sub.0.Cx (7)
.epsilon.HbO.sub.2 .lambda.'.CHbO.sub.2
+.epsilon.Hb.lambda.'.CHb+.epsilon.x.lambda.'.Cx=.epsilon.HbO.sub.2
.lambda..sub.0 '.CHbO.sub.2 +.epsilon.Hb.lambda..sub.0
'.CHb+.epsilon.x.lambda..sub.0 '.Cx (8)
From equations (7) and (8), the following equations are obtainable,
respectively.
CHbO.sub.2 (.epsilon.HbO.sub.2 .lambda.-.epsilon.HbO.sub.2
.lambda..sub.0)+CHb(.epsilon.Hb.lambda.-.epsilon.Hb.lambda..sub.0)+Cx(.eps
ilon.x.lambda.-.epsilon.x.lambda..sub.0)=0
CHbO.sub.2 (.epsilon.HbO.sub.2 .lambda.'-.epsilon.HbO.sub.2 .lambda..sub.0
')+CHb(.epsilon.Hb.lambda.'-.epsilon.Hb.lambda..sub.0
')+Cx(.epsilon.x.lambda.'-.epsilon.x.lambda..sub.0 ')=0
If the following definitions (9) to (14) are introduced:
.epsilon.HbO.sub.2 .lambda.-.epsilon.HbO.sub.2 .lambda..sub.0 =kHbO.sub.2
.lambda. (9)
.epsilon.Hb.lambda.-.epsilon.Hb.lambda..sub.0 =kHb.lambda. (10)
.epsilon.x.lambda.-.epsilon.x.lambda..sub.0 =kx.lambda. (11)
.epsilon.HbO.sub.2 .lambda.'-.epsilon.HbO.sub.2 .lambda..sub.0 '=kHbO.sub.2
.lambda.' (12)
.epsilon.Hb.lambda.'-.epsilon.Hb.lambda..sub.0 '-kHb.lambda.'(13)
.epsilon.x.lambda.'-.epsilon.x.lambda..sub.0 '=kx.lambda.' (14)
the equations are simplified as follows:
kHbO.sub.2 .lambda..CHbO.sub.2 +kHb.lambda..CHb+kx.lambda..Cx=0
kHbO.sub.2 .lambda.'.CHbO.sub.2 +kHb.lambda.'.CHb+kx.lambda.'.Cx=0
If, Cx is eliminated by means of this pair of equations,
(kHbO.sub.2 .lambda..kx.lambda.'-kHbO.sub.2 .lambda.'.kx.lambda.)CHbO.sub.2
+(kHb.lambda..kx.lambda.'-kHb.lambda.'.kx.lambda.)CHb=0
Therefore,
##EQU6##
Similarly, if CHb is eliminated by means of the above pair of simplified
equations,
##EQU7##
In equations (9) to (14), kHbO.sub.2 .lambda., kHbO.sub.2 .lambda.',
kHb.lambda., kHb.lambda.', kx.lambda. and kx.lambda.' are all known.
Accordingly, the oxygen saturation can be calculated by equation (15), and
the construction ratio between the hemoglobin oxide, hemoglobin and the
third component X can be obtainable by equations (15) and (16).
As is apparent from the above description, one oxygen saturation value is
exclusively determined when at least one wavelength having an intensity
equal to that of at least one standard wavelength is found. Therefore,
various oxygen saturation values can be previously calculated in
accordance with equations (6) or (15) with respect to various wavelengths
and stored in ROM as in the prior embodiment. In this case, oxygen
saturation can be obtained by reading out the data stored in ROM by
addressing ROM with the searched wavelength.
The present invention is, however, not limited to such an embodiment, but
can be embodied by substituting, for the microcomputer having a ROM, a
calculation circuit, which actually carries out the calculation of
equation (6) or (15) with respect to each wavelength sequentially.
Alternatively, the results of the previously calculated oxygen saturation
values for various wavelengths can be printed as a sheet or table of
values and the person who is informed of the searched wavelength by the
device is capable of reading the oxygen saturation from the table by
himself. Further, the person who is informed of the searched wavelength by
the device can manually calculate the oxygen saturation in accordance with
equation (6) or (15) by the aid of a general calculator or computer. In
these cases, the device of the present invention is not for providing the
actual oxygen saturation, but rather for identifying a specific searched
wavelength which can indicate the actual oxygen saturation.
Finally, the optical system of the present invention is not limited to the
above-described embodiments, wherein the wavelengths to be measured are
sequentially scanned, but could incorporate an optical system wherein all
the necessary wavelengths are simultaneously separated into spectra and
simultaneously measured in inten | | |
