使用光聲效應量測葡萄糖濃度

Abstract

在本研究中我們探討利用光聲效應來量測葡萄糖溶液的濃度之可行性。利用光聲效應最大的優點是：一、非侵入式，二、同時利用物質的光學特性以及聲學特性來判斷葡萄糖的濃度。目前發展中的非侵入式量測方法主要是利用血糖對近紅外光的吸收會隨濃度變化之特性。由於在人體血糖濃度變化範圍內的吸收係數的變化程度僅在0.2％左右，使得純光學血糖量測易受光散射影響而無法縮小誤差。另一方面，光聲效應產生訊號的機制是物質吸收光學能量後產生熱膨脹效應。因此利用光聲波可以獲得聲學參數用來修正光散射所造成的干擾。由於聲速亦隨葡萄糖濃度變化故此若能有效運用將可進一步提升濃度量測之準確度。本論文主要研究統合聲速與振幅變化量測葡萄糖濃度之方法，實驗架構主要以Ti:sapphire作為雷射之光源，發出波長在800到1000nm的近紅外光照射葡萄糖溶液並且以正向、側向兩種不同模式進行光聲波測量。在正向測量實驗中，使用中心頻率為1MHz聚焦深度為1.8公分之點聚焦超音波探頭作正向量測。用中心頻率為1MHz聚焦深度為1.27公分之線聚焦超音波探頭作側向量測。將所得訊號用以計算光聲訊號的振幅以及聲速變化來分辨葡萄糖水溶液濃度。目前實驗結果濃度每上升0.1％訊號增加約2％，標準差大約是4.2％，可以分辨濃度差在0.3%，實驗主要的誤差來自於系統端的穩定性，包括了雷射能量的穩定度以及各系統間的同步。未來工作將著重於進行血漿以及全血的in vitro，以及最後in vivo的實驗，靈敏度的提升、血球引起散射的影響以及利用多波長判別血液中其他物質的影響，也將是未來工作的重點。

Feasibility of glucose concentration measurements utilizing photoacoustic (PA) approach is explored in this research. Estimating glucose concentration by means of PA measurements has the advantages of non-invasiveness and concentration level evaluation based on both optical and acoustic characteristics of glucose molecules. Most current measurements methods make use of the dependence of blood glucose molecules absorption in NIR range. However, the variation of the absorption within human body is merely 0.2%, which in turn causes measurement in optical manner vulnerable to scattering, an important phenomenon in skin tissues. Photoacoustic signal is induced through thermal expansion of the locally heated area by incident laser pulse. By means of PA measurements, not only scattering interference can be reduced, but also the acoustic parameters, such as sound velocity, are able to be used to enhance the accuracy. In this thesis, glucose concentration measurements via PA signal amplitude and acoustic velocity variations are combined. Experiments are set up with a Ti:sapphire laser, a single-crystal ultrasound transducer, and a 100MHz ADC card. The PA signals from the glucose solution are received sidewardly and forwardly. Transducers operating at 1MHz but at different focus depths are used for different receiving modes, where a 1.8cm focal depth, point-focusing transducer is aligned along the laser beam as the forward receiving mode, and a focal depth of 1.27cm, line-focusing one is arranged perpendicularly with the direction of the incident beam for the sideward receiving one. Two glucose-concentration measurement approaches are carried out, including variation evaluation of the PA signal amplitude and sound velocity. Our experiment results reveal that a 0.3% concentration variation is detectable. The error is mostly due to the instability of laser output energy and asynchronization between laser, photodiode detector, and ADC. Future work includes detection in plasma both in vitro and in vivo, improvement in system sensitivity, further understanding of light scattering by red blood cell and discrimination of other molecules in blood utilizing PA absorption spectra.