 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for determining the
concentration of a light-absorbing material in blood.
An oximeter is an example of an apparatus currently employed to determine
the concentration of light-absorbing materials in blood. In a known type
of oximeter, the oxygen saturation of blood in a living tissue sample is
computer based on the fact that the relative quantities of two light beams
having different wavelengths that pass through the tissue sample differ on
account of blood pulsation. The operating principle of this type of
oximeter is described hereinafter.
It is assumed first that a living tissue sample through which light is to
pass is composed of a blood containing tissue layer and non-blood tissue
layer as shown in FIG. 1A. In this case, the light attentuation by the
overall tissue sample is expressed by:
-log (I.sub.1 /I.sub.0)=A+B (1)
where
I.sub.0 : the quantity of incident light,
I.sub.1 : the quantity of transmitted light,
A: the amount of light attenuation by the non-blood containing tissue, and
B: the amount of light attenuation by the blood containing layer.
The light attenuation by the blood layer (B) is expressed by:
B=E.multidot.C.multidot.D (2)
where
E: the absorptivity coefficient of hemoglobin,
C: the concentration of hemoglobin in blood, and
D: the thickness of the blood layer.
Therefore, equation (1) can be rewritten as:
-log (I.sub.1 /I.sub.0)=A+E.multidot.C.multidot.D (3)
The thickness of the blood layer D is variable due the normal arteiral
blood pulsating. It is assumed that the thickness of the blood layer
changes by .DELTA.D as shown in FIG. 1B. If the quantity of light
transmitted through the blood layer changed in thickness by .DELTA.D is
written as I.sub.2, analogy with equation (3) gives:
-log (I.sub.2 /I.sub.0)=A+E.multidot.C.multidot.(D+.DELTA.D) (4)
Subtracting equation (4) from equation (3),
-{log (I.sub.1 /I.sub.0)-log (I.sub.2 /I.sub.0)}=-EC.DELTA.D,
or
-log (I.sub.2 /I.sub.1)=EC.DELTA.D (5)
As can be seen from equation (3), equation (5) is equivalent to the
expression of light attenuation for the case where incident light having
an intensity of I.sub.1 passes through a blood containing layer with a
thickness of .DELTA.D to produce light transmission in a quantity I.sub.2.
This relation is depicted in FIG. 1C.
Next will be considered the case where two light beams having different
wavelengthsare transmitted through a blood containing layer at the
measurement site. FIG. 2 shows the relationship between the thickness of
the blood containing layer D and each of I.sub.1 (the quantity of
transmitted light at a wavelength of .lambda..sub.1) and I.sub.2 (the
quantity of transmitted light having a wavelength of .lambda..sub.2). If
the change in the thickness of the blood layer that occurs between two
points in time t.sub.1 and t.sub.2 is written as .DELTA.D, and if the
values of I.sub.1 and I.sub.2 at time t.sub.1 are written as I.sub.11 and
I.sub.12, respectively, with the values of I1 and I2 at time t.sub.2 being
written as I.sub.21 and I.sub.22, respectively, the following relations
are established in consideration of equation (5):
For the first wavelength .lambda..sub.1 :
-log (I.sub.21 /I.sub.11)=E.sub.1 C.DELTA.D (6)
For the second wavelength .lambda..sub.2 :
-log (I.sub.22 /I.sub.12)=E.sub.2 C.DELTA.D (7)
where E.sub.1 is the absorptivity coefficient of the blood for light at the
wavelength .lambda..sub.1 and E.sub.2 is the absorptivity coefficient of
the blood for light at the wavelength .lambda..sub.2.
Equations (6) and (7) can be rewritten as follows:
log (I.sub.11 /I.sub.21)=E.sub.1 C.DELTA.D (8)
log (I.sub.12 /I.sub.22)=E.sub.2 C.DELTA.D (9)
Dividing equation (9) by equation (8) and writing the quotient as .phi.,
.phi.={log (I.sub.12 /I.sub.22)}/{log (I.sub.11 /I.sub.21)}=E.sub.1
/E.sub.2 ( 10)
Since equation (10) does not contain the term .DELTA.D, the times t.sub.1
and t.sub.2 may be any two values.
Equation (10) can be rewritten as;
E.sub.2 =.phi..multidot.E.sub.1 ( 11)
If the absorptivity coefficient E.sub.1 in equation (11) is known, E.sub.2
can be determined by calculating .phi.. As equation (10) shows, .phi.can
be determined by calculating log (I.sub.11 /I.sub.21) and log (I.sub.12
/I.sub.22), and as already mentioned, log (I.sub.11 /I.sub.21) can be
determined by measuring I.sub.11 and I.sub.21 (the quantities of
transmitted light at the wavelength .lambda..sub.1 at any two points in
time), while log (I.sub.12 /I.sub.22) can be determined by measuring
I.sub.12 and I.sub.22 (the quantities of transmitted light at the
wavelength .lambda..sub.2 at the aforementioned any two points in time).
Since
log (I.sub.11 /I.sub.21)=log I.sub.11 -log I.sub.21 ( 12)
log (I.sub.12 /I.sub.22)=log I.sub.12 -log I.sub.22 ( 13)
the logarithm of I.sub.21 may be subtracted from the logarithm of I.sub.11
to obtain log (I.sub.11 /I.sub.21) while the logarithm of I.sub.22 is
subtracted from the logarithm of I.sub.12 to obtain log (I.sub.12
/I.sub.22).
Equation (12) can be rewritten as log (I.sub.11 /I.sub.21)=log {1+(I.sub.11
-I.sub.21)/I.sub.21 }. Since I.sub.11 -I.sub.21, the following
approximation is valid:
log (I.sub.11 /I.sub.21)=(I.sub.11 -I.sub.21)/I.sub.21 ( 14)
In like manner, the following approximation is valid:
log (I.sub.12 /I.sub.22)=(I.sub.22 -I.sub.22)/I.sub.22 ( 15)
Using E.sub.2, the oxygen saturation S of blood may be calculated by the
following procedures.
The absorptivity coefficient E of the blood versus the wavelength .lambda.
of light with which a living body is irradiated is shown in FIG. 3 for
S=0% and S=100%. The wavelength at which the curve for S=0% crosses the
curve for S=100% is selected as the first wavelength .lambda..sub.1, which
falls at 805 nm in FIG. 3. The absorptivity coefficient E.sub.1 for the
light beam having the wavelength .lambda..sub.1 is insensitive to changes
in the oxygen saturation of blood S. Accordingly, a wavelength different
from .lambda..sub.1 is selected as the second wavelength .lambda..sub.2,
which falls, for instance, at 660 nm in FIG. 3. At the wavelength
.lambda..sub.2, the absorptivity coefficient assumes the value E.sub.r
when S=0% and the value E.sub.0 if S=100%. E.sub.2 is a value between
E.sub.0 and E.sub.r. Using E.sub.r, E.sub.0 and E.sub.2, S can be
calculated by the following equation:
S=(E.sub.2 -E.sub.r)/(E.sub.0 -E.sub.r) (16)
An apparatus which determines the oxygen saturation S of blood using the
procedure described above is shown schematically in FIG. 4. Detectors 1
and 2 receive light beams that have passed through a living tissue sample
and which have wavelengths of .lambda..sub.1 and .lambda..sub.2,
respectively, and produce output signals indicative of the intensities of
the two beams. Variation computing circuits 3 and 4 compute the respective
amounts of light attenuation on the basis of the changes in the detection
signals produced by detectors 1 and 2 at two identical points in time. In
other words, using I.sub.11 and I.sub.12 representing the quantities of
transmitted light at time t.sub.1, as well as I.sub.21 and I.sub.22
representing the quantities of transmitted light at time t.sub.2 (see FIG.
2), the circuits 3 and 4 compute log (I.sub.11 /I.sub.21) and log
(I.sub.12 /I.sub.22), respectively, which are the left side of equations
(8) and (9). As a result, the variation in light attenuation due to the
change in blood thickness (.DELTA.D) on the right side of each of
equations (8) and (9) is determined. Using the calculation results
produced by the circuits 3 and 4, a divider circuit 5 determines .phi.
expressed by equation (10). In the next step, an oxygen saturation
computing circuit 6 computes S from equations (11) and (12) using the
value of .phi. calculated by the divider cirucit 5 and the preliminary
stored values of E.sub.1, E.sub.r and E.sub.0 as indicated in FIG. 3.
The apparatus described above has the disadvantage that noise is
unavoidably present in the signals produced by the detectors 1 and 2.
Therefore, a single sampling will not yield a reliable value and the
values obtained over several samplings must be averaged. However, the
sampling for a single measurement can only be performed a finite number of
times since the oxygen saturation of blood varies constantly. Furthermore,
the two sets of data I.sub.11 and I.sub.12 and data I.sub.21 and I.sub.22
employed for the calculation by the variation computing circuits 3 and 4
are values obtained at any two respective arbitrary points in time t.sub.1
and t.sub.2, as shown in FIG. 2, and hence it sometimes occurs that the
difference between I.sub.11 and I.sub.21 or between I.sub.12 and I.sub.22
is very small. If this happens, computation using the two sets of data
I.sub.11 and I.sub.12 and data I.sub.21 and I.sub.22 will not produce
highly precise results.
SUMMARY OF THE INVENTION
The present invention has been accomplished in order to solve these
problems of the prior art. An object, therefore, of the present invention
is to provide an apparatus for determining the concentration of a
light-absorbing material in blood that minimizes the error caused by noise
and which ensures highly precise results of computation under all
conditions.
The stated object of the present invention can be attained by an apparatus
for determining the concentration of a light-absorbing material in blood
that comprises: a light detector for detecting the intensities of light
beams having different wavelengths which have passed through a living
tissue; time point detecting means for detecting, for each wavelength of
light, a plurality of points in time that fall in the vicinity of one peak
and one trough of the detection signal from said light intensity detector;
memory means which stores, for each wavelength of light, the values of
detection signals produced at each of said points in time as detected by
said detecting means; and concentration computing means which computes the
concentration of the light-absorbing material of interest in the blood
sample on the basis of the values of the detection signals stored by the
memory means.
In the apparatus of the present invention having the arrangement described
above, the memory means stores, for each wavelength of light, a plurality
of values that fall in the vicinity of the peak and trough of one cycle of
a signal detected by the light intensity detector. The concentration
computing means computes the concentration of the light-absorbing material
of interest on the basis of the stored plurality of values. Accordingly,
the apparatus of the present invention yields highly precise values of
measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1C illustrate schematically a cross section of a tissue
containing blood vessels;
FIG. 2 is a waveform diagram showing changes in the thickness of the tissue
of FIGS. 1A to 1C and intensities of two different light beams passing
through the tissue;
FIG. 3 is a graph showing the relationship between absorptivity and
wavelength;
FIG. 4 is a schematic block diagram of a prior art oximeter;
FIG. 5 is a schematic block diagram of an apparatus according to a
preferred embodiment of the present invention; and
FIG. 6 shows graphically the detection signals from the two light intensity
detectors shown in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention is hereunder described with
reference to the accompanying drawings.
FIG. 5 is a schematic block diagram of an oximeter constructed according to
a preferred embodiment of the present invention. Shown at 10 and 11 in
FIG. 5 are detectors that detect the intensities of light beams having
wavelengths of .lambda..sub.1 and .lambda..sub.2 that have passed through
part of a living tissue.
These detectors produce electrical signals that are indicative of the
intensities of light I.sub.1 and I.sub.2 (see FIG. 6) having wavelengths
.lambda..sub.1 and .lambda..sub.2, respectively. Shown at 12 is a
peak/trough detector 12 which receives the output signal from the light
intensity detector 11 and senses the peaks and troughs of that signal.
Shown at 13 and 14 are sub-peak/trough data storage circuits, and 15 is a
memory control circuit. In response to a signal a produced by the detector
12 when it senses the peaks and troughs of the signal output from the
detector 11, the memory control circuit 15 produces a selected signal b at
a predetermined timing relative to the signals a and supplies it to the
sub-peak/trough data storage circuits 13 and 14. The sub-peak/trough data
storage circuit 13 (14) stores the value of the signal produced by the
light intensity detector 10 (11) a given time before or after the time
when the circuit 13 (14) is supplied with the signal b from the memory
control circuit 15.
Shown at 16 is a .phi. computing circuit which calculates .phi. in equation
(10) using the data stored in the sub-peak/trough data storage circuits 13
and 14. Instead of calculating log (I.sub.12 /I.sub.22)/log (I.sub.11
/I.sub.21) according to equation (10), the computing circuit 16 in the
embodiment shown computes (I.sub.11 -I.sub.21)/I.sub.21 and (I.sub.12
-I.sub.22)/I.sub.22 using approximations (14) and (15) and determines the
ratio of the computed values. Shown at 17 is a mean .phi. computing
circuit which computes the mean average of the values of .phi. determined
by the circuit 16. Shown at 18 is an S computing circuit which computes
the oxygen saturation S on the basis of the mean .phi. calculated by the
circuit 17. In the apparatus shown in FIG. 5, the peak/trough detector 12
and the memory control circuit 15 constitute a time-point detecting unit,
and the .phi. computing circuit 16, mean .phi. computing circuit 17, and S
computing circuit 18 together form a concentration computing unit.
The apparatus having the composition described above operates in the
following manner.
The light intensity detectors 10 and 11 detect the intensities I.sub.1, and
I.sub.2 of received light beams having the wavelengths .lambda..sub.1 and
.lambda..sub.2, respectively, and output detection signals having
waveforms as shown in FIG. 6. The peak/trough detector 12 supplies the
signal a to the memory control circuit 15 when it detects the peaks and
troughs of the detection signal from the detector 11. Upon receiving the
signal a, the memory control circuit 15 supplies the signal b to each of
the sub-peak/trough data storage circuits 13 and 14 both at times T.sub.1
to T.sub.3 and at times T.sub.4 to T.sub.6 as counted from the reception
of the signal a. Upon receiving the signal b, the sub-peak trough data
storage circuits 13 and 14 store the values of the signals then produced
by the detectors 10 and 11, respectively. Therefore, the circuit 13 stores
values corresponding to the intensities of light of points G to L in FIG.
6, whereas the circuit 14 stores values corresponding to the intensities
of light at points A to F in FIG. 6.
It should be noted that the peak/trough detector 12 used in the apparatus
shown in FIG. 5 need not sense the absolute peaks and troughs of the
signal from the detector 11; it suffices if the peak/trough detector 12
outputs the signal a when the detection signal from the detector 11 has
reached an area in the vicinity of the peak and trough points. Moreover,
in accordance with the present invention, the detector 12 performs sensing
of the peaks and troughs solely in terms of the detection signal from the
light intensity detector 11 because the apparatus shown in FIG. 5 is so
designed that the intensities of the light beams having wavelengths
.lambda..sub.1 and .lambda..sub.2 simultaneously reach their peaks and
troughs.
In the next step, the .phi. computing circuit 16 performs the following
sequence of calculations in accordance with equations (14) and (15) using
the data stored in the sub-peak/trough data storage circuits 13 and 14. In
the sequence of calculations given below, the intensities of light
detected at points A to F and G to K are designated by the same symbols. A
to F and G to K;
##EQU1##
First, the .phi. computing circuit 16 determines .phi..sub.11 from the
intensities of light A, D having the wavelength .lambda..sub.2 that are
detected at time T.sub.1 and T.sub.4, and from the intensities of light G,
J having the wavelength .lambda..sub.1 that are detected at times T.sub.1
and T.sub.4. Then, the circuit 16 determines .phi..sub.12 from the
intensities of light A, E having the wavelength .lambda..sub.2 that are
detected at times T.sub.1 and T.sub.5, and from the quantities of light G,
K having the wavelength .lambda..sub.1 that are detected at times T.sub.1
and T.sub.5. After repeating these procedures, the circuit 16 finally
determines .phi..sub.33 from the quantities of light C, F having the
wavelength .lambda..sub.2 that are detected at times T.sub.3 and T.sub.6,
and from the quantities of light I, L having the wavelength .lambda..sub.1
that are detected at times T.sub.3 and T.sub.6. The thus-determined nine
values of .phi., .phi..sub.11, .phi..sub.12 . . . .phi..sub.33 are then
averaged by the mean .phi. computing circuit 17 which calculates
(.phi..sub.11 +.phi..sub.12 + . . . +.phi..sub.33)/9.
On the basis of the thus-obtained average .phi..sub.A and data stored
preliminarily (e.g., E.sub.1, E.sub.r and E.sub.0 shown in FIG. 3), the S
computing circuit 18 computes the oxygen saturation S and outputs it to an
external circuit such as a display.
The apparatus described above has the advantage that it requires only a
simple circuit to determine the accurate value of the oxygen saturation.
The apparatus shown in FIG. 5 employs the first and second light quantity
detectors 10 and 11 for detecting the intensities of the light beams
having different wavelengths of .lambda..sub.1 and .lambda..sub.2.
However, the present invention can be practiced using a single light
intensity detector if the apparatus is designed to operate on a time
sharing basis in which light beams having wavelengths .lambda..sub.1 and
.lambda..sub.2 are received alternately rather than simultaneously.
The apparatus shown in FIG. 5 processes the output signals from the light
intensity detectors in analog form. If desired, the output signals from
the light intensity detectors can be processed after being converted to
digital signals by an A/D converter. The apparatus may be designed in such
a manner that the digital signals obtained by the A/D converter are
processed with a microcomputer.
In an illustrative method for computing .phi..sub.11, .phi..sub.12 . . .
with the aid of a microcomputer, at least one cycle of the output signals
from the light intensity detectors is stored, the times at which the peak
and trough occur in the waveform are detected, a plurality of points in
time that fall in the vicinity of each of the peak and trough are
detected, .phi..sub.11, .phi..sub.12 . . . are determined on the basis of
the values of the signals detected at these points in time.
In the embodiment described above, the computation of the oxygen saturation
S is preceded by the averaging of individually determined values of .phi..
Alternatively, a plurality of values of S may be determined and then
averaged.
As described in the foregoing, the apparatus of the present invention
determines the average concentration of a light-absorbing material in
blood on the basis of a plurality of values in the vicinity of the peak
and trough of a detection signal from a light intensity detector. This is
effective not only in minimizing the adverse effects of noise present in
the detection signal, but also in producing highly precise measurement
values under all conditions.
* * * * *
|
|
 |
|
 |
Description |
|
