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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a spectroscopic method for measuring the
concentration changes of sugar and its derivatives in body liquids, for
example in blood, using a non-invasive technique which does not require
taking a sugar medium as a sample from the body for examination, and more
particularly to a method and apparatus for detecting the polarization
ratio of native emission of luminescence centers in a visible and/or near
infrared region of spectrum from body liquid chromophores undergoing
interaction with an optically active medium such as sugar.
2. Related art
The current state of the art in measuring sugar levels in body liquids or
other objects such as foods, fruits and other agricultural products
requires taking a sample from the object during the examination process.
Special instruments are available for the purpose of determining blood
glucose levels in people with diabetes. The technology uses a small blood
sample obtained from a finger prick which is placed on chemically prepared
strips and inserted into a portable instrument. The instrument analyzes
the blood sample and provides a blood glucose level measurement. Diabetics
must prick their fingers, sometimes up to four times a day, to draw blood
for monitoring their glucose levels.
To eliminate the pain of drawing blood, as well as to eliminate a source of
potential infection, non-invasive optical methods for sugar determination
were invented and use absorption, transmission or reflection methods for
spectroscopically analyzing blood glucose concentration.
In U.S. Pat. Nos. 3,958,560 and U.S. Pat. No. 4,014,321 to W. F. March, a
unique glucose sensor to determine the glucose level in patients is
described. The patient's eye is automatically scanned using a dual source
of polarized radiation, each transmitting in different wavelengths at one
side of the cornea of the patient. A sensor located at the other side of
the cornea detects the optical rotation of the radiation that passed
through the cornea. Because the level of glucose in the bloodstream of the
patient is a function (not a simple one) of the glucose level in the
cornea, rotation of polarization can determine the level of glucose
concentration.
In U.S. Pat. No. 3,963,019 to R. S. Quandt there is described a method and
apparatus for detecting changes in body chemistry, for example, glycemia,
in which a beam of light is projected into and through the aqueous humor
of the patient's eye. An analyzer positioned to detect the beam on its
exit from the patient's eye compares the effect the aqueous humor has on
said beam against a norm. The change in the glucose concentration is
indicated and detected.
In U.S. Pat. No. 4,750,830 to A. St. J. Lee there is described a method of
measuring the optical power of the living subject's eye and comparing it
with a calibration value that corresponds to a reference blood glucose
level. Optical power of the eye increases with blood glucose levels.
In U.S. Pat. No. 4,805,623 to F. Jobsis there is described a
spectrophotometric method of qualitatively determining the concentration
of a dilute component with a reference component of known concentration by
a series of contemporaneous radiation-directing and measurements steps of
radiation of selected varying wavelengths.
In U.S. Pat. No. 4,882,492 to K. J. Schlager there is described a
non-invasive apparatus and related method for measuring the concentration
of glucose or other blood analytes. It utilizes both diffuse reflected and
transmissive infrared absorption measurements. The apparatus and method
utilize non-dispersive correlation spectrometry. Differencing the light
intensity between the two lights paths, one with a negative correlation
filter and the other without, the apparatus provides a measure
proportional to analyte concentration.
In U.S. Pat. No. 4,883,953 to K. Koashi and H. Yokota there is disclosed a
method for measuring the concentration of sugar in liquids by use of near
infrared light. The concentration of the sugar in the sample is determined
by computing the absorption spectrum of the sugar at a different depth in
the sample measured by a relatively weak power of infrared light,
penetrating close to the surface in a sample, and a relatively strong
power of infrared light penetrating relatively deeply in the sample.
In U.S. Pat. No. 5,009,230 to D. P. Hutchinson there is disclosed a device
for the non-invasive determination of blood glucose in a patient. This
glucose monitor is based upon the effect of glucose in rotating polarized
infrared light. More specifically, two orthogonal and equally polarized
states of infrared light of minimal absorption are passed through a tissue
containing blood, and an accurate determination of change in signal
intensity is made due to the angle of rotation of these states. This
rotation depends upon the glucose level. This method uses transmission of
infrared light through the tissue at minimum absorption of the tissue.
In U.S. Pat. Nos. 5,028,787 and 5,068,536 to R. D. Rosenthal at al. there
is disclosed a near-infrared quantitative analysis instrument and method
of calibration for non-invasive measures of blood glucose by analyzing
near-infrared energy following interactance with venous or arterial blood,
or transmission through a blood contained in a body part.
In U.S. Pat. No. 5,054,487 to R. H. Clarke there is disclosed a methods for
non-invasive material analysis, in which a material is illuminated at a
plurality of discrete wavelengths. Measurements of the intensity of
reflected light at such wavelengths are taken, and an analysis of
reflection ratios for various wavelengths are correlated with specific
material properties such as concentration of analytes.
Other patents for non-invasively analyzing glucose levels in blood based on
different spectroscopic, electrochemical and acoustic velocity measurement
methods are as follows:
In U.S. Pat. Nos. 4,875,486 and 5,072,732 to U. Rapoport at al. there is
disclosed a nuclear magnetic resonance apparatus, where predetermined
water and glucose peaks are compared with the measured water and glucose
peaks for determining the measured concentration.
In U.S. Pat. No. 5,056,521 to J. S. Parsons at al. there is disclosed a
method in which a sample of specially collected oral fluid is placed into
a monitoring instrument which generates an electrical glucose
representative readout for oral fluid or whole blood.
In U.S. Pat. No. 5,119,819 to G. H. Thomas at al. there is disclosed
acoustic velocity measurements for monitoring the effect of glucose
concentration upon the density and adiabatic compressibility of serum.
In U.S. Pat. No. 5,139,023 to T. H. Stanley at al. there is disclosed a
method for non-invasive blood glucose monitoring by correlating the amount
of glucose which permeates an epithelial membrane, such as skin, with a
glucose receiving medium over a specified time period. The glucose
receiving medium is then removed and analyzed for the presence of glucose
using conventional analytical technique.
In U.S. Pat. No. 5,140,985 to J. M. Schroeder at al. there is disclosed a
measuring instrument and indicating device which gives an indication of
blood glucose by metering the glucose content in sweat, or other body
fluids, using a plurality of oxygen sensors covered by a semi-porous
membrane. The device can be directly attached to the arm and the measuring
device will react with localized sweating and indicate the wearer's blood
glucose level.
The above described state of the art in non-invasive blood glucose
measurements devices contains many approaches and indicates the importance
of the problem. But none of the described devices have yet been marketed.
Some inventors claim that instruments which are being developed give
accurate blood glucose level readings and can be used for home testing by
diabetics. They have limitations stemming from the use of near infrared
light for measurement of absorption, transmission or reflectance; in this
region of spectrum one can observe interference in absorption from other
chemical components. Analyses based on only one or two wavelengths can be
inaccurate if there is alcohol in the blood or any other substances that
absorb at the same frequencies. In addition, these analyses can be thrown
off by instrument errors, outlier samples (samples with spectra that
differ from the calibration set) physiological differences between people
(skin pigmentation, thickness of the finger). Methods of near infrared
spectroscopy must be coupled with sophisticated mathematical and
statistical techniques to distinguish between nonglucose sources and to
extract a faint glucose spectral signature. Another limitation of these
types of blood glucose testers is that they have to be custom calibrated
for each user. The need for individual calibration results from the
different combination of water levels, fat levels and protein levels in
various people which cause changes in the absorption of near infrared
light. Since the amount of glucose in the body is less than one thousandth
that of other chemicals (and all of them possess absorption in the near
infrared), variations of these constituents which exist among people may
make universal calibration unlikely.
Other, non-invasive but also non-direct methods and instruments attempt to
determine blood glucose content by measuring the glucose in sweat, saliva,
urine or tears. These measurements, which can be quite reliable from the
chemical analysis point of view, do not determine blood glucose levels
because of the complicated, and not always well-defined, relation between
blood glucose levels and glucose concentration in other body fluids. Other
invented methods like acoustic velocity measurements in blood, are not
very reliable because of the lack of well established and simple relations
with blood glucose levels.
SUMMARY OF THE INVENTION
It is a principal object of the present invention to provide an apparatus
and methods for glucose concentration measurements which can analyze the
natural fingerprints of glucose: the state of rotations of polarization of
native luminescence of the tissue.
It is the further object of the present invention to provide an instrument
for glucose concentration determination. The said instrument externally
measures the rotation of polarization of light emitted from luminescing
centers of blood which are dissolved and/or attached to optically active
molecules of sugar and, undergoing interaction, rotate the initial
polarization.
It is still the further object of the present invention to provide the
glucose level determination by detection method of emitted light from
human tissue using electronic detection discrimination technique which can
distinguish between blood and other tissues. It relies on the cardiac
cycle of blood motion in the body. Detection is synchronized with the
frequency of the patient's heart beat.
It is yet still further object of this invention to provide a technique for
detecting the presence of other optically active molecules, as well as, of
emitting molecules in the human body by means of measurements of
polarization rotation of the light emitted by naturally emitting centers
in blood or in other tissue, utilizing the synchronization to the heart
beat technique for discriminatory detection, and to differentiate between
blood molecules and other tissues as well as to subtract blood
interference in measurement of molecules from other tissue in vivo.
It is another object of this invention to provide calibration of the ratio
of light polarization changes or light polarization changes themselves to
blood glucose concentration during the Glucose Tolerance Test performed,
under medical supervision, where the whole range of glucose concentration
levels are obtained and are linearly interpolated with the value of
glucose measurement by conventional means.
The present inventions are based on the discovery that native visible and
near infrared luminescence spectra from blood chromophores are
substantially changed by means of light polarization rotation depending on
the concentration of sugar in the blood; the luminescence signal is
modulated by the frequency of the heart rate.
Due to its chemical structure glucose rotates the angle of transmitted
polarized light. For example, D(+) Glucose (dextrose) (M. W. 180.16)
solution in water with concentration c=10 [g/100 cm.sup.3 ] has the
specific rotation of .alpha..sub.D =+53.degree. (rotating clockwise). This
feature is one of the most dramatic and differentiates this component from
any others in blood. Some other organic components of blood or human
tissue also possess optical activity which cannot be excluded. It will be
measured as a constant background in comparison to the varying level of
glucose in individual diabetic patients.
The light emitted from chromophores undergoing excitation by linearly or
circularly polarized light has a polarization ratio corresponding to the
optical activity of the chromophores themselves because other components
of the tissue, except for the ones that are optically active, are
interacting as isotropic mediums do. Optically active molecules in blood
except sugar will create in measurements a constant background assuming a
constant number of emitting centers. This constant background ratio of
polarization rotation will be additionally modulated by optically active
sugar molecules the concentrations of which are changing during the
metabolic processes. These are the changes that interest us and are
measured by our apparatus.
Our apparatus will measure not only the concentration of sugar molecules
attached to the red blood cells but also concentration of sugar molecules
which are not attached to red blood cells. In a normal person, blood
contains about 5 to 6 percent of red cells attached to sugar molecules;
the amount shifts very little. The percentage of red blood cells of
diabetics attached to sugar molecules ranges from 7 to 13 percent or even
higher. The emitted photons from chromophores of, for example, red blood
cells will additionally pass through the optically active medium due to
sugar content. Detecting only native luminescence light, emitted from the
blood undergoing excitation in the main blood absorption band centered at
about 420 nm, our instrument has an increased signal to noise ratio for
the polarization rotation ratio measurements in comparison to polarization
rotation measurements of transmitted or reflected light through the
tissue. In the transmission and reflection of the light, light interacts
and is scattered by all the molecules along the optical path of
interaction in tissue. In our approach photons are emitted from, for
example, red blood cell molecules (discriminated by detection wavelength)
and the signal is proportional to the ratio of the red blood cell
molecules to the optically active molecules. Further changes in the signal
are proportional to the ratio of the red blood cells to the changes in the
number of sugar molecules due to their concentration changes. This signal
is directly proportional to the changes of blood glucose level.
To further increase the signal to noise ratio in the polarization ratio of
native luminescence measurements our apparatus is using a discriminatory
detection technique which can distinguish between blood and other tissue
in the human body. It is based on the discovery that a luminescence signal
from blood tissue is modulated by the frequency of the heart rate. The
technique relies on the synchronous amplification of the electrical signal
in phase with the cardiac cycle of blood motion in the body. It detects at
the frequency equal to the heart beat. The luminescence signal is detected
by a photomultiplier or a photodiode and amplified by an integrated
circuits amplifier with a band pass filter allowing amplification only of
AC (alternating current) band frequency signal in the range of heart rate.
An AC signal of luminescence with frequency of heart rate is emitted only
by tissue under constant motion in the body - the blood. The signal
contains spectral information only about the chemical substances in the
blood and not in bloodless tissue. This technique allows our instrument to
extract and discriminate spectral information from tissue in motion which,
in this case, is blood. This modulation, with frequency in the range of
heart rate will be discriminated by a band pass electronic filter and
amplified by a lock-in amplifier for the detection of the polarization
ratio.
Changes of luminescence light rotation of polarization in the value of, for
example, one-tenth part of degree will be measured using Glan-Thompson
prism Polarizers made of two cemented calcite elements whose optical axis
are parallel to the hypotenuse as well as to the entrance and exit faces.
A characteristic parameter which allows measurements of such small angle
rotations is the extinction ratio. The extinction ratio of this type of
polarizer is in the range of 10.sup.-6 and for special quality prisms can
reach the level of 10.sup.-8. However, by carefully selecting particular
crystals and localized regions therein which are free from imperfections,
an extinction ratio as high as 10.sup.-10 can be readily achieved.
Measurements of the emitted light intensity differences between parallel
and perpendicularly polarized luminescence with the above resolution
allowed our instrument to detect the angle of polarization changes which
are even a few orders of magnitude lower than required.
In this instrument we also use a lock-in amplifier synchronized detection
for amplification of small electrical signal coming from detectors such as
photomultiplieres or photodiodes. These approaches guarantee that small
signals of native luminescence from blood can be detected.
It is also our aim to provide a reliable calibration technique for the
instrument. Calibration which aims at distinguishing between changes of
emitted light polarization due to glucose blood level changes and constant
background due to optical activity of other centers in the blood is based
on a Glucose Tolerance Test. The test should be performed for every
individual patient under medical supervision. During the test the patient
orally intakes a certain amount of glucose. Measurements of the patient's
blood glucose levels are performed for the period of, for example, two
hours after glucose intake at certain intervals by analytical methods and
by our instrument. Intervals for analytical methods can be, for example,
every 15 minutes and measurements by the described instrument can be
performed, for example, every five minutes. Data are collected by an
instrument microprocessor. A computer program correlates the calibration
value of blood glucose levels with arbitrary units of changes of the ratio
of polarization rotation by linear or nonlinear approximations. The
advantage of this method of calibration is based on the wide range of
blood glucose levels during the Glucose Tolerance Test which can vary
between 70 mg/dL up to 400 mg/dL. Such a calibrated instrument can be
readily used by a patient.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an experimental setup used to measure
luminescence polarization ratio from human tissue.
FIG. 2 is a luminescence spectrum of the blood excited by krypton laser at
647.1 nm wavelength.
FIG. 3 is a diagram showing the modulation of the luminescence signal in
conjunction with the heart rate.
FIG. 4 is a simplified diagram of an embodiment of an apparatus of the
invention
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed at a method, apparatus and procedure for
non-invasive detection of the concentration changes of sugar in body
liquids, such as blood, using native visible and/or near infrared
luminescence.
An experimental arrangement used to measure the polarized luminescence
spectra from human tissue is shown in FIG. 1. A Krypton ion laser 1
operating at 647.1 nm was linearly polarized by polarizer 2, and was then
directed to the human tissue 3 which in our case was a part of the
finger-tip. The native luminescence from the tissue was collected into the
double grating monochromator 5 passing first through the polarization
analyzer 4 which had its optical axis set parallel or perpendicular to the
incident linearly polarized laser light. The photomultiplier tube 6,
located at the exit slit of the tunable monochromator 5 measured the
intensity at different wavelengths. The output of the PMT 6 was connected
to an electronic recording device 7 which included a lock-in amplifier
with band-pass filter and X-Y recorder to display each spectrum.
The luminescence spectra emitted from the blood were investigated. The
spectral curves for parallel and perpendicular polarization in respect to
the excitation polarization are displayed in FIG. 2. One can readily
observe the difference in intensity for parallel and perpendicular
polarization of emission from the tissue. Light is preferably absorbed by
molecules whose transition moment is parallel to the electric vector of
light. Light is also absorbed by molecules whose transition moment is not
parallel to the electric vector of light; however, absorption occurs with
a reduced probability. The electric vector of the luminescent emission
will be polarized in the same plane as the exiting beam if the chromphar
is held stationary and is not undergoing other changes. However, molecular
motion, energy transfer, the different direction of the emitting dipole as
well as the medium through which the emitted photons have to pass through
before they exit from tissue will depolarize the emitted beam. The
excitation wavelength of krypton laser at 647.1 nm chosen in our
experiment does not excite blood in its characteristic absorption band
centered at 420 nm. The main reason that this wavelength was selected is
that emission from the blood with this excitation centered at about 700 nm
which fits to the transparency window of the tissue constituents. It can
give a larger number of emitted photons for detection. Other reasons are
based on the observation that with a larger gap between excitation and
emission wavelengths one can have very small differences between parallel
and perpendicular emission from the tissue due to energy transfer
processes and other depolarization processes in the chemical constituents
in the tissue.
The signal from the lock-in amplifier was passed through a band-pass filter
which allowed only frequencies in the range of the heart rate to pass
through. FIG.3 shows a typical luminescence signal modulated by motion of
the blood in accordance with the heart rate. This signal was then
integrated with a time constant much longer than the heart frequency and
sent to the X-Y recorder to display each spectrum. The salient feature of
the recorded spectra is that the ratio of polarization can be calculated
according to the following formula:
P(.lambda.)=[I.sup..parallel. (.lambda.)-I.sub..perp.
(.lambda.)]/[I.sub..parallel. (.lambda.)+I.sub..perp. (.lambda.)],
where .parallel. and .perp. indicates parallel and perpendicular
polarization, respectively; and .lambda. indicates the luminescence
wavelength. This ratio P(.lambda.) which measures the degree of
polarization has values in the range -1 to +1. It is directly proportional
to the concentration of optically active constituents of the blood.
The most salient feature of the present invention is that from the
polarization ratio of at native luminescence different emission
wavelengths from the blood one can determine the concentration of blood
sugar. Blood can be excited by different wavelengths not limited to the
krypton laser wavelength. For example, as mentioned early, one can excite
blood in its characteristic absorption band centered at 420 nm. Emission
spectra can be picked at different wavelengths not limited t those
presented. The ratio of polarization between parallel and perpendicular
luminescence intensity can be measured all over the visible, near infrared
and infrared region of the spectrum.
It is another feature of this invention that not only the polarization
ratio between parallel and perpendicular polarization of the emitted
luminescence can be measured in comparison with a linearly polarized
excitation beam. It is another salient feature of this invention that
excitation light can be linearly or circularly polarized. Collected
emission can also be polarized circularly by applying quarter wave plates
in the path of collection optics. This feature is based on the property of
the asymmetric molecules like sugar to absorb right-handed circularly
polarized light to a different extent than left-handed polarized light. By
monitoring the asymmetry of the signal from the emitting molecules, the
ratio for circular polarization of luminescence can be calculated
according to the following formula:
P.sub.c (.lambda.)=[I.sub.+ (.lambda.)-I.sub.- (.lambda.)]/[I.sub.+
(.lambda.)+I.sub.- (.lambda.)],
where- and+ indicates left- and right-handed polarization of light at
luminescence wavelength .lambda.. The ratio P.sub.c (.lambda.) provides
another coefficient directly proportional to the concentration of the
sugar in body liquids.
It is still another salient feature of the presented invention that changes
of blood sugar concentration are proportional to changes of the
polarization ratio for orthogonally or circularly polarized emission from
the chromophores in the blood undergoing interaction with attached to
and/or dissolved sugar molecules.
In FIG. 4 an embodiment of an apparatus for measuring changes in blood
glucose concentration according to the teaching of this invention is
illustrated. The apparatus includes a source 11 of light, such as laser,
laser diodes, Light Emitting Diodes, or tungsten-halogen filament lamp
with a narrow band filter 10 but not limited to it. The light source 11
has power attached to it from a power supply (not shown). Narrow band
filter 10 has a bandwidth of less than about 30 nm and preferably less
than about 10 nm and is designed to pass light at an excitation wavelength
of absorption band of chromophores the blood. Light from source 11 is
passed through filter 10 and polarizer 9 which can be any crystal or
Polaroid polarizer with a high extinction coefficient. Then optionally for
excitation by circular polarization, light is passed through the quarter
wave plate 8. Optionally light from source 11 can be focused on the sample
tissue but a lens or telescopic system must be positioned before polarizer
9 at the location corresponding to the position of the filter 10. This
positioning prevents any depolarization due to optical imperfection in the
lenses. Between the sample and polarizer only a quarter wave plate 8 can
be introduced. Emitted light from the tissue is collected by another
optical system. The main element includes analyzer 15. Between it and the
sample only a quarter wave plate 14 can be introduced to prevent any
distortion of polarization due to optical imperfection. Polarizer 15
working as an analyzer also has a high extinction coefficient. Collecting
lenses 16 and 18 are located behind the analyzer. They collect
luminescence light from the sample and focus it on the photocatode of
photomultiplieres 12 and 13 or photodiodes 12 and 13 depending on the
amount of the emitted luminescence photons from the tissue. In front of
both light detectors 12 and 13 narrow band or color filters are placed to
select emitted wavelength of luminescence. Analyzer 15 which can be a
calcite Clan Prism is dividing emitted luminescence into two paths. Each
path contains perpendicularly polarized or left- and right-handed
polarized luminescence from the tissue. Polarizer 9 can be oriented along
one of the polarization axis of analyzer 15 or can be oriented at
45.degree. or another angle in comparison with analyzer 15. The electrical
signals from the photodetectors 12 and 13 are passed to amplifier 20 which
can be a lock-in amplifier with filter for the heart beat frequencies or
can be an amplifier integrated circuit with said filter. An amplified
electrical signal is then passed to the computer system 21 which can
include a microprocessor and a software and display system but is not only
limited to it. This unit 21 also has an important role during the
calibration procedure for calculating and storing information. It will
process data during the | | |
