監測顱內壓對於創傷性腦損傷的醫療監測是非常必要的。在臨床上，通常是利用侵入式的壓力感測器來監測顱內壓，然而這會升高感染的機率。閉鎖臨界壓力是一種非侵入式監測顱內壓得的量測方法。閉鎖臨界壓力的定義為血流量降為零時相對應的動脈血壓。閉鎖臨界壓力即為血管張力以及此血液動力系統中另一端的壓力之總和，而此另一端的壓力亦即顱內壓。
根據定義，推算閉鎖臨界壓力需要血流以及動脈壓的量測。本研究應用擴散相關性光譜儀來量測腦血管中的血流。擴散相關性光譜儀是一個基於光學的、非侵入式的可連續長時間監測血流的儀器。而動脈壓方面，採用商業化的光纖壓力量測儀，將其插入股動脈對其做直接性高時間解析度的長時間量測。此壓力量測儀同時也被使用於直接量測顱內壓，用以與閉鎖臨界壓力相比較。然而傳統的擴散相關性光譜儀的時間解析度稍低，通常只被用於量測單一心跳內或數個心跳週期之平均血流。為了量測搏動的血液訊號，我們研發了一個快速擴散相關性光譜儀。此儀器是一個基於現場可程式化閘陣列之光子到達時間記錄系統，並以USB3.0作為訊號傳遞之媒介。雖然新儀器之時間解析度較傳統為高，其訊雜比卻也同時因光子收集時間較短而下降。針對此一問題，我們研發了一個新的演算法來提升訊雜比。
在驗證實驗上，我們將此量測應用於六隻大鼠，於不同程度之顱內壓狀況下以及不同吸入氣體下進行量測。不同程度之高顱內壓以緩慢注入之人工腦脊液來誘發。這些不同程度之顱內壓的量測被用於比較在不同範圍內顱內壓與閉鎖臨界壓力的關係。另一方面，過去的實驗指出血液中不同二氧化碳濃度實際上會影響血管擴張以及收縮，也就是會影響血管張力，導致閉鎖臨界壓力估算之非侵入性顱內壓的準確度降低。在這個研究中我們觀察到顱內壓以及閉鎖臨界壓力確實如閉鎖臨界壓力理論所述，有著正相關的關係，而不同吸入氣體確實會影響血管張力。我們利用吸入不同氣體的量測來調校閉鎖臨界壓力中血管張力項之影響。校正過之閉鎖臨界壓力和顱內壓之間確實有良好的正相關關係。我們相信這個方法經過更多的研究，在未來或可做為一個非侵入性的顱內壓量測方法。

Monitoring of Intracranial Pressure (ICP) is essential in traumatic brain injury. In clinical, monitoring ICP is usually performed with invasive insertion of a pressure sensor which increases the chance of infection. Critical closing pressure (CrCP) is one of the non-invasive approaches of monitoring ICP. CrCP is defined as the arterial blood pressure at which the blood flow decreases to zero. It is equivalent to the sum of the tension of the blood vessels and the pressure on the other side of the hemodynamic system which is ICP measured in the cerebral vessels.
According to its definition, blood flow and arterial blood pressure are required to estimate CrCP. This study utilized Diffusion Correlation Spectroscopy (DCS) to measure the blood flow in the cerebral vessels. DCS is an optical-based, non-invasive, long term measurement of blood flow. For arterial blood pressure, a commercialized fiber optic pressure sensor is inserted into the femoral artery and to measure the ICP invasively for comparison. Traditional DCS, however, has a rather low time resolution which was used to monitor the mean blood flow over one or several heart beats. In order to derive the pulsatile blood flow, we developed a fast DCS device using field-programmable gate array (FPGA) based photon-arrival-time-documenting board and a USB3.0 interface. Albeit the time resolution of the new device is much higher, the accuracy of DCS decreases when the integration time decreases. A new algorithm is developed to restore the signal to noise ration of the DCS signal.
We performed measurements of elevated ICP and the inhalation of different gas mixtures on 6 rats. Levels of elevated ICP are induced through the infusion of artificial cerebrospinal fluid. They are used to find the relationship between ICP and CrCP over a wider range. On the other hand, it is shown that different partial pressure of CO2 in the blood can cause the dilation and constriction of blood vessels, hence changing the vessel wall tension which influences the accuracy of CrCP. It is observed that CrCP is an increasing function of ICP as the theory predicted, and different gas mixtures do change the wall tension. We use the measurements of the inhalation of different gas mixtures to eliminate the term of vessel wall tension in the formula of CrCP. Good correlation was found between the invasive ICP and the adjusted non-invasive CrCP derived from DCS. We believe that, in the future, the non-invasive method can be introduced to human.

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