U.S. patent number 4,446,871 [Application Number 06/216,526] was granted by the patent office on 1984-05-08 for optical analyzer for measuring a construction ratio between components in the living tissue.
This patent grant is currently assigned to Minolta Kabushiki Kaisha. Invention is credited to Kenji Imura.
United States Patent |
4,446,871 |
Imura |
May 8, 1984 |
Optical analyzer for measuring a construction ratio between
components in the living tissue
Abstract
An optical analyzer such as oximeter is provided including a
source of light having a plurality of different wavelengths. At
least two or more of the different wavelengths have a fixed
relationship of light absorption after coaction with hemoglobin
oxide. The light is directed at the subject tissue and received
after coaction by an optical probe. A first signal representative
of the degree of light absorption at a predetermined standard
wavelength is determined and then an attempt is made to match a
second wavelength having a fixed relationship of light absorption
to the predetermined standard wavelength, e.g., equal absorption,
to generate a second signal representative of the second
wavelength, whereby the amount of hemoglobin oxide can be
determined in the bloodstream.
Inventors: |
Imura; Kenji (Osaka,
JP) |
Assignee: |
Minolta Kabushiki Kaisha
(Osaka, JP)
|
Family
ID: |
11683074 |
Appl.
No.: |
06/216,526 |
Filed: |
December 15, 1980 |
Foreign Application Priority Data
|
|
|
|
|
Jan 25, 1980 [JP] |
|
|
55-8070 |
|
Current U.S.
Class: |
600/323;
356/41 |
Current CPC
Class: |
G01N
21/314 (20130101); A61B 5/1459 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); G01N 21/31 (20060101); A61B
005/00 () |
Field of
Search: |
;128/633,634,665,666,664
;356/39-42 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Merrick et al., Hewlett-Packard Journ., vol. 28, No. 2, pp. 2-9,
Oct. 1976..
|
Primary Examiner: Howell; Kyle L.
Assistant Examiner: Hanley; John E.
Attorney, Agent or Firm: Jackson, Jones & Price
Claims
What is claimed is:
1. An oximeter for obtaining information indicative of the level of
hemoglobin oxide in living tissue, comprising:
a source of light having a plurality of different wavelengths
susceptible of coaction with both hemoglobin and hemoglobin
oxide;
means for directing the light at the subject tissue;
means for measuring the degree of light absorption at a
predetermined standard wavelength;
means for finding a second wavelength in which the degree of
absorption is equal to that at the predetermined standard
wavelength; and means for determining the level of oxygen
saturation in the blood from the second wavelength.
2. The invention of claim 1 wherein said means for determining
includes means for storing a plurality of hemoglobin oxide values
corresponding to a plurality of respective wavelengths at which the
absorption is equal to that of the standard wavelength; means for
addressing the stored hemoglobin oxide values in response to the
second wavelength, and means for providing an indication of the
hemoglobin oxide value of the tissue corresponding to the matched
second wavelength.
3. The invention of claim 1 further including filter means for
removing any noise components from the signal.
4. The invention of claim 1 wherein said means for finding includes
a plurality of band pass filters for selective interaction with the
source of light to provide a plurality of wavelengths for finding
said second wavelength.
5. The invention of claim 4 further including a rotatable member
for supporting the band pass filters and encoding means to generate
a position signal indicative of each band pass filter when
interacting with the source of light whereby the means for finding
can determine a particular second wavelength.
6. An oximeter for measuring an oxygen saturation, which is defined
as a ratio of the hemoglobin oxide to the sum of the hemoglobin and
the hemoglobin oxide, in living tissue comprising:
means for providing a source light;
means for directing the source light at the living tissue;
means for receiving the source light after coaction with the living
tissue;
means for determining the intensity of the light received by the
receiving means at a predetermined standard wavelength;
means for searching for another wavelength at which the intensity
of light received by the receiving means is equal to that of the
standard wavelength of light determined by the determining means;
and
means for providing a record of values for the level of hemoglobin
oxide as a function of a plurality of wavelengths at which the
light absorption of said wavelengths is equal to the absorption at
a predetermined standard wavelength;
and means for finding the level of hemoglobin oxide from the
another wavelength in accordance with the preparatorily known
recorded valves of hemoglobin oxide.
7. The invention of claim 6 further comprising a second means for
determining an intensity of the ligth received by the receiving
means at a predetermined second standard wavelength, a second means
for searching for a second wavelength at which the intensity of
light received by the receiving means is equal to that of the
second standard wavelength of light determined by the second
determining means, means for examining whether or not the second
wavelength is within a predetermined range of wavelengths, and
means responsive to the examining means for selecting the second
wavelength when it is within the predetermined wavelength range and
for selecting said another wavelength when the second wavelength is
outside the predetermined wavelength range, and means for using
said selected wavelength in said means for determining.
8. An optical analyzer for measuring a construction ratio of the
hemoglobin oxide to the sum of the hemoglobin and the hemoglobin
oxide to determine oxygen saturation in tissue comprising:
means for providing a source light;
means for directing the source light at the tissue;
means for receiving the source light after contact with the
tissue;
means for determining the intensity of the light received by said
receiving means at a predetermined standard wavelength;
means for searching for another wavelength at which the intensity
of light received by said receiving means is equal to that of said
standard wavelength of light determined by said determining
means;
means for indicating an oxygen saturation level from said another
wavelength including means for storing a plurality of previously
known values of oxygen saturation corresponding to respective
different wavelengths, and means for taking one oxygen saturation
value from said storing means in response to said another
wavelength whereby the wavelength found by said searching means is
converted into a corresponding level of oxygen saturation for
indicating the oxygen saturation in the living tissue.
9. The invention of claim 8, wherein said light source means
includes a light exit for directing the source light to the living
tissue and said receiving means includes a light entrance for
receiving the source light reflected by the living tissue.
10. The invention of claim 8, wherein said light exit and light
entrance are positioned so as not to contact the surface of the
living tissue.
11. The invention of claim 10, wherein said searching means
includes means for sequentially scanning different wavelengths to
measure each intensity thereof.
12. A method of optically determining the level of hemoglobin oxide
in living tissue by directing predetermined wavelengths of light at
the tissue and photoelectrically measuring the light absorption,
comprising the steps of:
preparatorily knowing and recording values for a the level of
hemoglobin oxide as a function of a plurality of wavelengths at
which the light absorption of said wavelengths is equal to the
absorption at a predetermined standard wavelength;
measuring the amount of light absorption of the predetermined
standard wavelength;
exposing the living tissue to a plurality of differnt wavelengths
of light;
determining which of the different wavelength is absorbed to a
degree equal to that of the standard wavelength, and
finding the level of hemoglobin oxide from the determined
wavelength in accordance with the preparatorily known recorded
values of hemoglobin oxide.
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,
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):
(b) In case of reflection (See FIG. 6b):
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:
Here, if I.lambda.=I.lambda..sub.0 at a specific scanning
wavelength .lambda.,
Namely,
In the case of reflection from the tissue, the measured intensity
I.lambda..sub.0 is as follows:
So, if I.lambda.=I.lambda..sub.0,
Therefore,
This is identical with equation (3) and shows that the equation (3)
is good in either case of reflection or transmission.
From equation (3),
And, this equation can be further modified according to the
following definitions (4) and (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.
From equations (7) and (8), the following equations are obtainable,
respectively.
If the following definitions (9) to (14) are introduced:
the equations are simplified as follows:
If, Cx is eliminated by means of this pair of equations,
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
intensity.
It should be noted that the present invention is not only
applicable to an oximeter as in the preferred embodiment, but also
to other devices for optically analyzing a construction ratio of a
known component to another known component, including living tissue
of various animals or plants.
As can be readily appreciated, it is possible to deviate from the
above embodiments of the present invention, and as will be readily
understood by those skilled in the art, the invention is capable of
many modifications and improvements within the scope and spirit
thereof. Accordingly, it will be understood that the invention is
not limited by the specific disclosed embodiments, but only by the
scope and spirit of the appended claims.
* * * * *