- 學位論文
利用數值方法探討內頸動脈狹窄程度與血液流場變化之相關性
Evaluation of the Correlation between the Degree of Stenosis and the Alteration of Blood Flow Field in Internal Carotid Artery by Using Numerical Methods
摘要
內頸動脈狹窄起因於頭頸部血管的動脈粥樣硬化斑塊沉積使得血管內膜增厚，進而阻塞血管導致流經內頸動脈至腦部的血流供應減少。因此內頸動脈狹窄的程度愈高，發生缺血性腦中風的危險性也愈大。而臨床上的診斷與治療方向注重於供應至腦部的血流量是否充足，對於狹窄的型態通常不列入考慮。此外過去學者的研究數據，大多數是針對嚴重狹窄程度( > 70% )做討論，輕度狹窄(< 30%)的研究相對較少。本論文使用數值方法針對頸動脈狹窄的例子進一步分析與討論。簡而言之，以臨床健康受試者的電腦斷層掃描影像為基礎建立一個數學理論模型，來模擬各種頸動脈狹窄情形：狹窄的型態分為同心與偏心；狹窄程度分別為10%、30%、50%、70%與90%。除此之外，大量蒐集臨床文獻血液流量統計數據，以病人的血液流量條件做為邊界條件，使得到的數值結果具有臨床意義與可靠性。除了數值模擬之外，同時建立體外頸動脈模型做比較試驗分析，以探討內頸動脈狹窄程度與血液流場變化之相關性。 利用臨床血液流量的統計數據做為邊界的數值分析結果顯示，血液流量變化隨著狹窄程度增高而呈現減少的趨勢，此處結果與文獻學者的臨床數值相符合。流線速度、剖面向量與壁面剪應力分佈變化的結果顯示，輕、中程度(30%-50%)的狹窄流場無論哪一型態其速度分佈皆有明顯變化，並形成顯著的迴流、軸向渦流與二次流等紊流現象，而且偏心狹窄與同心狹窄比起，在斑塊的周邊有多數的渦流現象形成，遠側端的紊流現象也會增加，以及產生更顯著的速度梯度與壁面剪應力分佈。體外染劑實驗的流場觀察結果顯示，對於狹窄程度30%的同心與偏心狹窄流場，分岔處與遠側內壁面分別皆有顯著的軸向渦流與迴流形成，導致染色劑出現滯留情形，這些實驗結果皆與數值分析方法的流動模式結果一致。另外，體外實驗的壓力變化顯示，隨著狹窄程度增高，偏心狹窄型態的壓力呈現升高的趨勢，而同心型態狹窄的壓力卻為下降的趨勢。 本論文的研究結果證明在同心與偏心狹窄型態之間，流場變化有明顯的差異性，並且對於輕微與中度狹窄的流場皆已存在顯著的紊流現象，以及在兩狹窄型態之間的內頸動脈壓力變化呈現相反的趨勢。因此，本論文可進一步提供醫學診斷及臨床上頸動脈內膜切除術與頸動脈支架置入術術前的評估參考。
並列摘要
It is well known that carotid artery stenosis is the major factor causing ischemic stroke. Clinically, diagnosis for carotid artery stenosis is determinated by the sufficiency of blood supply to the brain. There are two types of carotid artery stenosis, however, no investigations emphasized the impact between their differences. Previous studies have focused on the degree of stenosis over 70% and less on the mild one lower than 30%. Accordingly, a stenosis model we built with a simulation method which is based on a carotid artery computer tomography image from a healthy adult with age 50. Several degree of stenosis (e.g. 10%, 30%, 50%, 70% and 90%) are generated and categorized into concentric or eccentric type. The boundaries have introduced to the values collected from the published clinical references. In addition, to address the correlation with the information from simulation method and clinical references we have also developed in vitro carotid artery models of normal, concentric and eccentric stenosis conditions. The later two conditions contained 30% and 70% degree of stenosis respectively. Furthermore, the dynamics of flow field is observed by application of three different color dyes to the in vitro model. The results suggest that the blood flow decreases while the degree of stenosis increases in both concentric and eccentric conditions. Eccentric stenosis has more significant turbulence and wall shear stress. In mild and moderate (30% to 50%) degree of stenosis, recirculation, streamwise vortex and secondary flow are initially occurred especially under the view of hydromechanics. Similarly, in vitro model shows that recirculation and streamwise vortex have significantly enhanced in a stenosis degree dependent manner, which are consistent with the numerical results. Besides, the blood pressure of internal carotid artery is reduced with an increase in stenosis degree, which is more significant in concentric stenosis type. This study demonstrates that the variation of flow field differs between concentric and eccentric type of stenosis. Existence of turbulence appears in the mild and moderate degree of stenosis. Variation of internal carotid artery blood pressure is opposite between two types of stenosis. Hence, this study could be able to provide a powerful medical evaluation before carotid endarterectomy and carotid artery stenting.