Study On Equivalence And Calibration Model Of Different Pathlength NIR Spectra For Noninvasive Biochemical Sensing

Blood biochemical sensing has been a significant method in clinical diagnostic field. The effectiveness of conventional blood testing methods, which ensure a high accuracy, has been hindered by the following defects: (1) invasion; (2) long detection period; (3) requirement of reagents. As a promising noninvasive biochemical monitoring method, near infrared absorption spectroscopy analytic technique has many obvious advantages. The testing is painless, reagentless, and has no risk of infections. Besides, the method makes it convenient to process the real-time monitoring of blood biochemical indexes. However, the realization of NIR noninvasive biochemical sensing is rather complicated. Some major obstacles are: (1) weak signal of blood biochemical components; (2) strong interference of human tissue background.Theoretically, the spectral subtraction approach with different blood volume can eliminate human tissue background interference and obtain effective spectrum information of blood. This is because in an extremely short time period (seconds), human physiological states, including human tissue background and blood biochemical indexes, barely change. Yet as the result of pulsation or vascular occlusion, blood volume is all the time changing. Therefore, by subtracting two blood spectra measured in a row within this short period, information of certain-volume blood spectrum can be obtained, with human tissue background interference eliminated.In order to realize noninvasive biochemical sensing, the spectral subtraction approach with different blood volume aims at establishing calibration models to predict the biochemical indexes of unknown blood samples. However, the obtained spectra of different blood volumes actually reflect different blood pathlengths. So by subtracting the two spectra, we obtain a blood spectrum with the pathlength unknown. In this case, the model will be insufficiently effective if established directly with pathlength-unknown spectra, which is a big problem of subtracted blood volume spectrometry. Notably, the information of blood biochemical components remains constant despite of the changing pathlength. In this paper, whether the spectral subtraction approach with different blood volume can be practical applied is discussed. To look into this point, the paper involves several key problems—whether it’s possible to normalize spectra, same in concentration but different in pathlength, to an equivalent spectrum with no pathlength information; whether spectra with different pathlength information perform equivalently on reflecting information of blood biochemical components; how to eliminate the interference of the pathlength variations and thus establishing multivariate calibration models.The paper focuses on the key problems of the approach application of spectral subtraction with different blood volume. Particularly, it involves the research of the equivalence of blood spectra with different pathlengths. Besides, the paper discusses different pathlength calibration models and also investigates into the performance promotion of subtracted blood volume spectrometry. The details and main conclusions of the researches are as follows:1) The research applies tissue optics and chemometrics method to the analysis of different pathlength spectra equivalence. Firstly, the principle of spectral subtraction approach with different blood volume is derived. The origin of different pathlength blood spectra and the meaning ofacquire equivalentspectrum are analyzed. Then the spectral equivalence of different pathlength in non-scattering media is studied, and the normalized equivalent spectrum can be obtained using chemometric methods. After that, by analyzing the optical transmissionmodel in tissueoptics and the modifiedLambert-Beer’s law,the blood spectra with different pathlength is also equivalent. Finally, to prove the conclusion, experiments on glucose solution and intralipid were performed and equivalent spectra were obtained for each solution.2) To solve the problems occurred during the establishment of calibration models, a approach is proposed to improve the function of multivariate calibration model, which combines both orthogonal signal correction andnet analyte signal method. In this paper, analog spectrum is established with molar absorption coefficients of some biomolecules. The analyses of the analog spectra as well as various simulated solutions of glucose reveal that pathlength interference is restrained and the capability of multivariate calibration model is promoted.3) Analyze the practical performance of subtracted blood volume spectrometry through simulation solution experiment. To evaluate the effect of spectra subtraction, indicators such as mean Euclidean distance value and root-mean-square error are applied. To suppress the noise, algorithms of Savitzky-Galay smoothing, wavelet transform filtering and empirical mode decomposition filtering are adopted. Through these processes, the effect of spectra subtraction gets promoted. It is proves by related experiments that wavelet transform filtering and empirical mode decomposition filtering have an apparent advantage in the performance promotion.The researches of this paper provide both theoretical and experimental basis for the application of subtracted blood volume spectrometry in noninvasive biochemical sensing field.