Optical diffuse reflectance accessory for measurements of skin tissue by near-infrared spectroscopy

Optical parameters of healthy and tumor breast tissues in mice

Knowledge of the optical parameters of tumors is important for choosing the correct laser treatment parameters. In this paper, optical properties and refraction indices of breast tissue in healthy mice and a 4T1 model mimicking human breast cancer have been measured. A significant decrease in both the scattering and refractive index of tumor tissue has been observed. The change in tissue morphology has induced the change in the slope of the scattering spectrum. Thus, the light penetration depth into tumor has increased by almost 1.5-2 times in the near infrared "optical windows." Raman spectra have shown lower lipid content and higher protein content in tumor. The difference in the optical parameters of the tissues under study makes it possible to reliably differentiate them. The results may be useful for modeling the distribution of laser radiation in healthy tissues and cancers for deriving optimal irradiation conditions in photodynamic therapy.

Hyperspectral imaging of lipids in biological tissues using near-infrared and shortwave infrared transmission mode: A pilot study

Label-free hyperspectral imaging (HSI) of lipids was demonstrated in the near-infrared (NIR) and shortwave infrared (SWIR) regions (950-1800 nm) using porcine tissue. HSI was performed in the transmission light-pass configuration, using a NIR-SWIR camera coupled with a liquid crystal tunable filter. The transmittance spectra of the regions of interest (ROIs), which correspond to the lipid and muscle areas in the specimen, were utilized for the spectrum unmixing. The transmittance spectra in ROIs were compared with those recorded by a spectrophotometer using samples of adipose and muscle. The lipid optical absorption bands at 1210 and 1730 nm were first used for the unmixing and mapping. Then, we performed the continuous multiband unmixing over the entire available spectral range, thereby, considering a combination of characteristic absorption bands of lipids, proteins, and water. The enhanced protocol demonstrates the ability to visualize small adipose inclusions of 1-10 μm size.

Noninvasive Monitoring of Glucose Using Near-Infrared Reflection Spectroscopy of Skin-Constraints and Effective Novel Strategy in Multivariate Calibration

For many years, successful noninvasive blood glucose monitoring assays have been announced, among which near-infrared (NIR) spectroscopy of skin is a promising analytical method. Owing to the tiny absorption bands of the glucose buried among a dominating variable spectral background, multivariate calibration is required to achieve applicability for blood glucose self-monitoring. The most useful spectral range with important analyte fingerprint signatures is the NIR spectral interval containing combination and overtone vibration band regions. A strategy called science-based calibration (SBC) has been developed that relies on a priori information of the glucose signal ("response spectrum") and the spectral noise, i.e., estimates of the variance of a sample population with negligible glucose dynamics. For the SBC method using transcutaneous reflection skin spectra, the response spectrum requires scaling due to the wavelength-dependent photon penetration depth, as obtained by Monte Carlo simulations of photon migration based on estimates of optical tissue constants. Results for tissue glucose concentrations are presented using lip NIR-spectra of a type-1 diabetic subject recorded under modified oral glucose tolerance test (OGTT) conditions. The results from the SBC method are extremely promising, as statistical calibrations show limitations under the conditions of ill-posed equation systems as experienced for tissue measurements. The temporal profile differences between the glucose concentration in blood and skin tissue were discussed in detail but needed to be further evaluated.

Absorption and Remission Characterization of Pure, Dielectric (Nano-)Powders Using Diffuse Reflectance Spectroscopy: An End-To-End Instruction

This paper addresses the challenging task of optical characterization of pure, dielectric (nano-)powders with the aim to provide an end-to-end instruction from appropriate sample preparation up to the determination of material remission and absorption spectra. We succeeded in establishing an innovative preparation procedure to reproducibly obtain powder pellet samples with an ideal Lambertian scattering behavior. As a result, a procedure based on diffuse reflectance spectroscopy was developed that allows for (i) performing reproducible and artifact-free, high-quality measurements as well as (ii) a thorough optical analysis using Monte Carlo and Mie scattering simulations yielding the absorption spectrum in the visible spectral range. The procedure is valid for the particular case of powders that can be compressed into thick, non-translucent pellets and neither requires embedding of the dielectric (nano-)powders within an appropriate host matrix for measurements nor the use of integrating spheres. The reduced spectroscopic procedure minimizes the large number of sources for errors, enables an in-depth understanding of non-avoidable artifacts and is of particular advantage in the field of material sciences, i.e., for getting first insights to the optical features of a newly synthesized, pure dielectric powder, but also as an inline inspection tool for massively parallelised material characterization.

Optical windows for head tissues in near-infrared and short-wave infrared regions: Approaching transcranial light applications

Optical properties of the rat head tissues (brain cortex, cranial bone and scalp skin) are assessed, aiming at transcranial light applications such as optical imaging and phototherapy. The spectral measurements are carried out over the wide spectral range of 350 to 2800 nm, involving visible, near-infrared (NIR) and short-wave infrared (SWIR) regions. Four tissue transparency windows are considered: ~700 to 1000 nm (NIR-I), ~1000 to 1350 nm (NIR-II), ~1550 to 1870 nm (NIR-III or SWIR) and ~2100 to 2300 nm (SWIR-II). The values of attenuation coefficient and total attenuation length are determined for all windows and tissue types. The spectra indicate transmittance peaks in NIR, NIR-II and SWIR-II, with maximum tissue permeability for SWIR light. The use of SWIR-II window for the transcranial light applications is substantiated. Furthermore, absorbance of the head tissues is investigated in details, by defining and describing the characteristic absorption peaks in NIR-SWIR.

Optical properties of animal tissues in the wavelength range from 350 to 2600 nm

The optical properties of different cow and pig biological tissues such as skeletal muscle, adipose, spinal cord, and dura mater of the spinal cord were investigated in the spectral range of 350 to 2600 nm. The measurements were carried out by a commercially available spectrophotometer SHIMADZU UV 3101PC. The wavelength dependence on the scattering coefficient has been observed to follow a power-law decay for skeletal muscle and dura mater of spinal cord. The influence of time delay between the sample preparation and measuring of transmittance spectra on the data reasonableness was reviewed. The conclusion about the benefits of 2 - ? m lasers application in surgery is given for the tissue types listed above.

Comparison of performance of partial least squares regression, secured principal component regression, and modified secured principal component regression for determination of human serum albumin, gamma-globulin and glucose in buffer solutions and in vivo blood glucose quantification by near-infrared spectroscopy

The performances of three multivariate analysis methods--partial least squares (PLS) regression, secured principal component regression (sPCR) and modified secured principal component regression (msPCR)--are compared and tested for the determination of human serum albumin (HSA), gamma-globulin, and glucose in phosphate buffer solutions and blood glucose quantification by near-infrared (NIR) spectroscopy. Results from the application of PLS, sPCR and msPCR are presented, showing that the three methods can determine the concentrations of HSA, gamma-globulin and glucose in phosphate buffer solutions almost equally well provided that the prediction samples contain the same spectral information as the calibration samples. On the other hand, when some potential spectral features appear in new measurements, sPCR and msPCR outperform PLS significantly. The reason for this is that such spectral features are not included during calibration, which leads to a degradation in PLS prediction performance, while sPCR and msPCR can improve their predictions for the concentrations of the analytes by removing the uncalibrated features from the original spectra. This point is demonstrated by successfully applying sPCR and msPCR to in vivo blood glucose measurements. This work therefore shows that sPCR and msPCR may provide possible alternatives to PLS in cases where some uncalibrated spectral features are present in measurements used for concentration prediction.

Fiber optic near-infrared Raman spectroscopy for clinical noninvasive determination of water content in diseased skin and assessment of cutaneous edema

Currently, measuring Raman spectra of tissues of living patients online and in real time, collecting the spectra in a very short measurement time, and allowing diagnosis immediately after the spectrum is recorded from any body region, are specific advantages that fiber optic near-infrared Raman spectroscopy (NIR RS) might represent for in vivo clinical applications in dermatology. We discuss various methodological aspects and state of the art of fiber optic NIR RS in clinical and experimental dermatology to outline its present advantages and disadvantages for measuring skin in vivo, particularly its water content. Fiber optic NIR Fourier transform (FT) RS has been introduced to dermatological diagnostics to obtain information regarding the molecular composition of the skin up to several hundred micrometers below the skin surface in a relatively fast nondestructive manner. This has been especially important for probing for in vivo assessment of cutaneous (intradermal) edema in patients patch test reactions. Fiber optic NIR FT Raman spectrometers still require further technological developments and optimization, extremely accurate water concentration determination and its intensity calculation in skin tissue, and for clinical applications, a reduction of measurement time and their size. Another promising option could be the possibility of applying mobile and compact fiber optic charge-coupled device (CCD)-based equipment in clinical dermatology.

Removal of Interference Signals Due to Water from in vivo Near-Infrared (NIR) Spectra of Blood Glucose by Region Orthogonal Signal Correction (ROSC)

A novel chemometric method, region orthogonal signal correction (ROSC), is proposed and applied to pretreat near-infrared (NIR) spectra of blood glucose measured in vivo. Water is the most serious interference component in such kinds of noninvasive measurements, because it shows very high absorbance in the spectra. In the present study, the spectra of blood glucose in the range of 1212 - 1889 nm are used, in which the absorption of water around 1440 nm is very high. ROSC aims at removing the interference signal due to water from the spectra by selecting a set of spectra with a special region of 1404 - 1454 nm that mainly contain information about the variation of the interference component, water, and calculating the orthogonal components to the concentrations of glucose that will be removed. The difference between ROSC and orthogonal signal correction (OSC) is that ROSC uses a special region of spectra for the estimation of scores and loading weights of orthogonal components to pretreat the spectra in other regions, while OSC only uses one fixed region of spectra to calculate loadings, scores and weights of OSC components and removes the OSC components in the same region. A clear advantage of ROSC is that it is more interpretable than OSC, because one can select a spectral region to remove the variation of a special component such as water. Another chemometric method, moving window partial least squares (MWPLSR), is also used to select informative regions of glucose from the NIR spectra of blood glucose measured in vivo, leading to improved PLS models. Results of the application of ROSC demonstrate that ROSC-pretreated spectra including the whole spectral region of 1212 - 1889 nm or an informative region of 1600- 1730 nm selected by MWPLSR provide very good performance of the PLS models. Especially, the later region yields a model with RMSECV of 15.8911 mg/dL for four PLS components. ROSC is a potential chemometric technique in the pretreatment of various spectra.

Contact-free spectroscopy of leg ulcers: principle, technique, and calculation of spectroscopic wound scores

Objective wound monitoring is an essential tool for evidence-based medicine in leg ulcers and other chronic wounds. Non-invasive and contact-free optical remittance spectroscopy seems to be a useful approach as it can provide additional information with respect to more traditional techniques of wound scoring. Twenty-three patients with chronic venous, arterial, and mixed leg ulcers were enrolled in this study. The clinical state of the ulcers was documented by a clinical wound score (quantity, color, and consistency of granulation tissue). The spectroscopic readings were performed with a novel diode-array spectrometer system in the visible and near-infrared range of the spectrum (400-1600 nm) with a resolution of 5 nm. The wound spectra mainly depend on the absorption of hemoglobin and water. The maximum correlation coefficients of mean remittance spectra with the clinical wound scores did not exceed +/- 0.5. Discriminant and cluster analysis were applied for spectral classification of wound scores. By using cross-validation the percentage of correct predicted wound scores was about 69%. Our results indicate that the application of optical visible and near-infrared spectroscopy could be a valuable remedy for the clinician.

Near-infrared reflectance spectroscopy for noninvasive monitoring of metabolites

An important class of substances in clinical chemistry are metabolites in body fluids, which are accessible by near-infrared spectroscopy without sample treatment using reagentless, fast and readily automated in vitro assays. Furthermore, noninvasive sensing systems are under development for the determination of blood glucose, especially for diabetic patients or for monitoring in intensive care and surgery. Near-infrared diffuse reflectance spectrometry of skin was employed allowing a certain tissue volume to be integrally probed. For calibration, the partial least-squares (PLS) algorithm was used either based on wide spectral intervals or using special spectral variable selection. Capillary blood glucose reference concentrations were obtained by finger pricking and an automated laboratory method (hexokinase/G6P-DH). Clear evidence is provided for the physical effect, as manifested by the spectral glucose absorptivities, underlying the individual single-person calibration models, which still require improvements in the methodology in the normo- and hypoglycemic concentration range. In extending the potential of noninvasive blood assays by infrared spectroscopy, a novel technique is presented for probing the intravascular fluid space by using fast spectral near-infrared measurements of skin tissue. The pulsatile blood spectrum can be derived from reflectance spectra of oral mucosa by Fourier analysis (near-infrared plethysmography). Future applications and prospects for noninvasive blood assays are discussed.

In vivo spectroscopy in dermatology: methods and new fields of application

BACKGROUND

The human skin is an easily accessible organ on which non-invasive examination methods can be applied. Remittance spectroscopy uses registration of the proportion of reflected light from the skin surface. Optical characteristics of skin are determined by scattering, reflection and absorption. The remittance is influenced by various test conditions and topography. Spectroscopic methods are applied for determination of microcirculation, measuring of erythema and pigmentation and in pharmacology and toxicology of applied drugs.

CONCLUSION

The standardization of spectroscopic measurement of human skin is essential to reduce error sources. In future a better investigation of skin structures and molecules, which are responsible for optical characteristics of skin and a more precise evaluation of spectroscopic signals are necessary to get more information about the state of skin and to develop new fields of applications, such as early diagnosis of skin diseases or monitoring of metabolic processes.

Noninvasive Prediction of Glucose by Near-Infrared Diffuse Reflectance Spectroscopy

Background: Self-monitoring of blood glucose by diabetics is crucial in the reduction of complications related to diabetes. Current monitoring techniques are invasive and painful, and discourage regular use. The aim of this study was to demonstrate the use of near-infrared (NIR) diffuse reflectance over the 1050–2450 nm wavelength range for noninvasive monitoring of blood glucose.

Methods: Two approaches were used to develop calibration models for predicting the concentration of blood glucose. In the first approach, seven diabetic subjects were studied over a 35-day period with random collection of NIR spectra. Corresponding blood samples were collected for analyte analysis during the collection of each NIR spectrum. The second approach involved three nondiabetic subjects and the use of oral glucose tolerance tests (OGTTs) over multiple days to cause fluctuations in blood glucose concentrations. Twenty NIR spectra were collected over the 3.5-h test, with 16 corresponding blood specimens taken for analyte analysis.

Results: Statistically valid calibration models were developed on three of the seven diabetic subjects. The mean standard error of prediction through cross-validation was 1.41 mmol/L (25 mg/dL). The results from the OGTT testing of three nondiabetic subjects yielded a mean standard error of calibration of 1.1 mmol/L (20 mg/dL). Validation of the calibration model with an independent test set produced a mean standard error of prediction equivalent to 1.03 mmol/L (19 mg/dL).

Conclusions: These data provide preliminary evidence and allow cautious optimism that NIR diffuse reflectance spectroscopy using the 1050–2450 nm wavelength range can be used to predict blood glucose concentrations noninvasively. Substantial research is still required to validate whether this technology is a viable tool for long-term home diagnostic use by diabetics.

Applicability of the Kubelka-Munk theory for the evaluation of reflectance spectra demonstrated for haemoglobin-free perfused heart tissue

Reflectance spectrometry is a useful tool for studying in vivo kinetic changes in the oxygen saturation of haemoglobin and myoglobin as well as the redox state of cytochromes. A method is given which allows the quantification of tissue reflectance spectra using multicomponent analysis. This method utilizes the Kubelka-Munk theory for modelling the measured tissue spectra. To test this approach, reflectance spectra of a haemoglobin-free perfused guinea pig heart were measured by a fast scanning spectrophotometer (100 spectra/s, spectral resolution 1.0 nm) and evaluated using the component absorbance spectra measured separately. A relative mean spectral residual error of 0.15% was achieved by least-squares fitting. Using statistical error propagation, oxygenation of myoglobin is obtained within a relative precision of 1%, and the redox state of cytochromes aa3 and c are determined simultaneously within a margin of 3%; the results for the redox-state of cytochrome b, however, are less precise. Special component error functions are presented to provide a reliability measure for the concentration prediction using this multicomponent assay. The consistency of the theory and the component absorptivity data is tested by regressing the actual concentrations obtained for each of the redox pair components during the various states of tissue oxygenation. A method is described for the recognition and reduction of systematic errors.