| 摘要: | 近紅外光擴散光學斷層掃描(near-infrared diffuse optical tomography, NIR-DOT)系統乃一新穎之醫學造影方式，其架構可區分為光電量測與影像重建兩部分，本研究改善頻域式近紅外光擴散光學量測系統，以獲得有效的光強度與相位差資訊。在雷射穿透待測物後，以中性密度及紅外干涉濾光片過濾多餘之光功率，透過光電倍增管(Photomultiplier tube, PMT)接收並經光電轉換至為電訊號，後續藉由混波器解調變得到光功率大小及相位落後訊息。
為了善用PMT不同控制電壓的有效動態量測範圍，研究中以LabVIEW®軟體即時調整PMT之控制電壓，將其由先前固定電壓改為因應不同PMT接收光功率而自動調整控制電壓；此外，另進行不同控制電壓之光功率大小及相位校正，如此可大幅提高量測範圍並增加量測資訊有效性。
設計液態假體作有效性驗證，包含不同假體大小、吸收及散射係數等，模擬實際生理組織狀態，根據進入PMT光功率大小使用固定或不同ND減光片，並以20及60 MHz之調變光源進行實驗。經由測試，80 mm之假體以固定控制電壓量測時，當光纖與液態光導管夾角為67.5°即飽和；以自動調整控制電壓量測時，可在夾角小至22.5°時飽和。
本研究所提出之PMT自動調整控制電壓方法，可增加PMT有效量測範圍，藉由不同吸收及散射係數之假體和調變頻率等實驗參數設定，建立輸出訊號之幅值與相位校正式，以使PMT之光電檢測應用更為良善。
;Near-infrared diffuse optical tomography (NIR DOT) is a new non-invasive and non-radiation biomedical imaging technique, and can be divided into two parts including opto-electrical measurement system and image reconstruction scheme. In this study, we improved frequency-domain (FD) opto-electrical measurement system to acquire both amplitude and phase information that are essential in image reconstruction computation. During the imaging procedure, NIR light through fibers illuminates a phantom, and neutral density (ND) filter, infrared band-pass interference (IR) filter are used to attenuate and filter out the unwanted light. Then the filtered light is detected and transformed into electrical signal by a photomultiplier tube (PMT). Eventually, the amplitude and phase information are computed through the demodulation of mixers and spectral analysis.
In order to effectively use the dynamic measurement range of a PMT with different control voltage, we improved the FD measurement system, previously using constant PMT control voltage, by auto-adjusting control voltage according to the received light power. Meanwhile, the correction of power amplitude and phase delay corresponding to variant PMT control voltage was performed to ensure the acquired data in an effective range.
Designated liquid phantoms that mimic the bio-tissue were used for justifying the proposed method. The design parameters of phantoms include size, optical absorption and scattering coefficients. Moreover, measurement was performed with using fixed or varying ND filter, and at different modulation frequency (20 and 60 MHz). Results show that the saturation of PMT occurs at source-detection angle below 67.5° when constant control voltage was used to measure Φ-80mm phantom, and limits to 22.5° while even using auto-adjusting control voltage.
The proposed method of auto-adjusting PMT control voltage in the study can fully utilizes the dynamic range of the PMT. According to the designated phantoms (different optical coefficients and size of the phantom), and the modulation frequency adopted, amplitude- and phase- correction functions can be established and employed for the improvement of NIR FD opto-electrical measurement system. |
