Non-invasive blood glucose sensing by near-infrared spectroscopy is easily interrupted
by the strong background variations compared to the weak glucose signals. In this work,
according to the distribution of diffuse reflectance intensity at different source-detector
separations, a method based on a reference point and a measuring point, where the diffuse
reflectance intensity is insensitive and most sensitive to the variation of glucose
concentration, respectively, is applied. And the data processing method based on the
information of two points is investigated to improve the precision of glucose sensing.
Based on the Monte Carlo simulation in 5% intralipid solution model, the corresponding
optical probe is designed which includes two detecting points: a reference point located at
1.3-1.7mm and a measuring point located at 1.7-2.1mm. Using the probe, the in vitro
experiment with different glucose concentrations in the intralipid solution is conducted at
1100-1600nm. As a result, compared to the PLS model built by the signal of the
measuring point, the root mean square error of prediction (RMSEP) and root mean
square error of cross calibration (RMSEC) of the corrected model built by reference point
and measuring point reduces by 45.10%, and 32.15% respectively.
Non-invasive blood glucose monitoring using NIR light has been suffered from the variety of optical background that is
mainly caused by the change of human body, such as the change of temperature, water concentration, and so on. In
order to eliminate these internal influence and external interference a so called floating-reference method has been
proposed to provide an internal reference. From the analysis of the diffuse reflectance spectrum, a position has been
found where diffuse reflection of light is not sensitive to the glucose concentrations. Our previous work has proved the
existence of reference position using diffusion equation. However, since glucose monitoring generally use the NIR light
in region of 1000-2000nm, diffusion equation is not valid because of the high absorption coefficient and small source-detector
separations. In this paper, steady-state high-order approximate model is used to further investigate the existence
of the floating reference position in semi-infinite medium. Based on the analysis of different optical parameters on the
impact of spatially resolved reflectance of light, we find that the existence of the floating-reference position is the result
of the interaction of optical parameters. Comparing to the results of Monte Carlo simulation, the applicable region of
diffusion approximation and higher-order approximation for the calculation of floating-reference position is discussed at
the wavelength of 1000nm-1800nm, using the intralipid solution of different concentrations. The results indicate that
when the reduced albedo is greater than 0.93, diffusion approximation results are more close to simulation results,
otherwise the high order approximation is more applicable.
KEYWORDS: Glucose, Monte Carlo methods, Diffuse reflectance spectroscopy, Absorption, Scattering, Water, Data corrections, In vitro testing, Single photon emission computed tomography, Interference (communication)
As an effective noninvasive method for glucose doesn't come into clinical realization due to the weakness of glucose
unique signal and complexity of background noise, a method based on a floating reference point and a measuring point,
where the diffuse reflectance intensity is insensitive and most sensitive to the variation of glucose concentration,
respectively, is applied. In this paper, the data processing method based on the information of reference point was
investigated to improve the precision of glucose sensing. The diffuse reflectance of intralipid solution with different
glucose concentration in different source-detector distances was obtained by Monte-Carlo simulation. And the radial
region selection of reference position and measuring position were discussed. Then in order to simulate the actual
measurement condition, the random noise and linear drift were added on the simulated spectra. And the spectra in the
proper measuring region corrected by that in the reference point were used to build the multivariate model. Further more,
the corresponding optical probe was designed according to the distribution of light intensity in the radial distance and an
in vitro experiment about intralipid solution with different glucose concentration was conducted to verify the effect of the
data correction based on the information from the reference point. Results showed that, three different measuring regions
should be determined in the wavelength of 1100nm-1700nm according to the wavelength characteristic of reference
point. And the measuring region should be about 0.2-0.3mm far away from the reference region. For the simulation and
in vitro experiment, after the correction by the information from the reference point, the prediction error for glucose was
reduced by 46.2% and 23.2%, respectively.
A reference position where the diffuse reflectance light intensity is insensitive to the variation of glucose concentration
exists in the radial detection space for glucose measurement in the scattering medium such as skin. The signal measured
in this position could be used as an inside reference to evaluate the influence on spectrum caused by other interferential
factors. The relationship between the position of radial reference point and the skin tissue property is studied in this paper.
Three-layer skin models with different optical parameters are designed to get sample sets at 1200~1700nm. In these
sets, μa, μs and g of dermis varies respectively, so does the depth of epidermis or dermis. The distribution rule of
dispersion of diffuse reflectance light intensity in the radial space is confirmed with the glucose concentration changes.
And the distribution property of the radial reference position in every sample set is obtained through Monte Carlo
simulation. The result shows that the distance of radial reference position from light source is insensitive to the variation
of absorption coefficient or the depth of dermis, but an increased scattering coefficient will shorten the distance; an
increased anisotropy coefficient or depth of epidermis will lengthen it. On the basis of that, the optical probes with
different structures are designed according to the skin tissue properties. So they could be used for the measurement of
corresponding patients, which enhances the practicability of floating reference method greatly.
In the noninvasive blood glucose sensing using near-infrared spectroscopy, the physiological noise was one of the
biggest challenges. In order to efficiently reduce the influence of the physiological background variations on the diffuse
reflectance spectra, the floating-reference method was used by differentially processing two signals from reference point
and measuring point. In this paper, the wavelength-dependent characteristic of the floating-reference point was discussed
by simulation and primary experiment. First, the wavelength-dependent characteristic of intralipid- 5% solution was
investigated in the wavelength range of 1300-1600nm. And source-detector distance for reference point in the
wavelength of 1300nm was conducted by different concentration of scatter media including 2%, 5% and 10% intralipid
solution. Then the single-layer and three layers skin model were built to investigate the wavelength characteristic of
reference point. The water displacement coefficients and relative large change in glucose concentration were considered
in the simulation. Finally, the primary experiment of intralipid model was conducted to validate the wavelength
dependence of reference point. The result showed that, the floating reference will not exist in the strong absorption
region (near 1450nm) and the region where the change of absorption coefficient is positive (high than 1525nm) due to
the corporate influence of scattering and absorption coefficient. And the wavelength-dependent characteristic is
consistent for intralipid solution and the skin model.
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