血流動(dòng)力學(xué)因素在動(dòng)脈粥樣硬化的發(fā)生發(fā)展及血管重構(gòu)過程中起到關(guān)鍵性作用[1-3]。頸動(dòng)脈局部血流動(dòng)力學(xué)研究已經(jīng)成為此領(lǐng)域研究的熱點(diǎn)問題[4-6]。頸動(dòng)脈分叉部位斑塊形成及血管狹窄能夠通過影響顱內(nèi)血流狀態(tài)從而引起臨床缺血癥狀，反之，顱內(nèi)動(dòng)脈的血流狀態(tài)也會(huì)對(duì)頸動(dòng)脈分叉部血流動(dòng)力學(xué)狀態(tài)起到重要影響。
顱內(nèi)willis動(dòng)脈環(huán)（circle of willis，CoW）是由顱內(nèi)的幾大供血?jiǎng)用}共同構(gòu)成的環(huán)狀結(jié)構(gòu)。從血流動(dòng)力學(xué)角度看，CoW具有自動(dòng)調(diào)節(jié)功能，能夠分配含氧血流至顱內(nèi)的各大動(dòng)脈。在顱內(nèi)動(dòng)脈狹窄阻塞時(shí)，此結(jié)構(gòu)能夠提供必要的側(cè)支循環(huán)。但CoW存在明顯的個(gè)體差異，約有50%的正常人群存在變異[7]，表現(xiàn)為環(huán)狀結(jié)構(gòu)不完整，部分血管發(fā)育不良（圖1）。
血管的幾何結(jié)構(gòu)是血流動(dòng)力學(xué)的基礎(chǔ)[8-9]。顱內(nèi)血管的形態(tài)和結(jié)構(gòu)的差異必然導(dǎo)致局部血流動(dòng)力學(xué)狀態(tài)的差異，而遠(yuǎn)端血管的血流速度、血流阻力等血流動(dòng)力學(xué)因素必然對(duì)近端頸動(dòng)脈分叉部位血流動(dòng)力學(xué)狀態(tài)產(chǎn)生影響。雖然迄今為止，頸動(dòng)脈粥樣硬化病變被認(rèn)為主要與局部血流動(dòng)力學(xué)狀態(tài)改變有關(guān)，但由于顱外段頸動(dòng)脈與顱內(nèi)動(dòng)脈的一體性，僅僅對(duì)頸動(dòng)脈分叉局部血流動(dòng)力學(xué)相關(guān)參數(shù)進(jìn)行評(píng)估顯然是不全面的。
近年來，對(duì)頸動(dòng)脈等動(dòng)脈粥樣硬化好發(fā)部位的血流動(dòng)力學(xué)參數(shù)的評(píng)估成為相關(guān)領(lǐng)域研究的熱點(diǎn)。計(jì)算流體力學(xué)(computational fluid dynamic, CFD) 技術(shù)在醫(yī)學(xué)領(lǐng)域的應(yīng)用使得血管動(dòng)力學(xué)模擬方面的研究取得了一定進(jìn)展[11-14]。利用CT、MR等影像技術(shù)獲取血管形態(tài)和速度數(shù)據(jù)，應(yīng)用CFD方法可以重建局部流場(chǎng)，計(jì)算血流動(dòng)力學(xué)參數(shù)。基于計(jì)算流體力學(xué)方法的頸動(dòng)脈血流動(dòng)力學(xué)研究已經(jīng)取得了很多令人滿意的結(jié)果。目前，此領(lǐng)域的研究熱點(diǎn)集中于斑塊進(jìn)展與局部血流動(dòng)力學(xué)因素之間的相關(guān)性，研究發(fā)現(xiàn)局部血流動(dòng)力學(xué)因素在斑塊的發(fā)展以及表面破裂過程中起到重要作用[15-17]。
磁共振成像(magnetic resonanceimaging，MRI)具備無創(chuàng)、圖像對(duì)比分辨率較高、可以同時(shí)獲取血管的形態(tài)、血管壁的特征及血流速度向量值等優(yōu)勢(shì)，使得應(yīng)用MR技術(shù)進(jìn)行血流動(dòng)力學(xué)的在體評(píng)估具備可能性。應(yīng)用相位對(duì)比法(phase-contrast，PC)MR成像技術(shù)可準(zhǔn)確地獲取管腔內(nèi)各個(gè)像素點(diǎn)的流體速度，并反映速度在心動(dòng)周期內(nèi)的分布和變化特征，是一種成熟的血流定量測(cè)量技術(shù)。我們的前期研究顯示，應(yīng)用2D PC成像獲取的血流速度數(shù)據(jù)，結(jié)合圖像后處理技術(shù)，可以準(zhǔn)確地對(duì)正常人群頸動(dòng)脈的血流速度、血流率以及管壁切應(yīng)力進(jìn)行計(jì)算，并能夠?qū)植康难髂Ｊ竭M(jìn)行評(píng)估（圖2）。但二維成像局限于對(duì)軸位單一層面血流數(shù)據(jù)的測(cè)量，對(duì)一段范圍的血流進(jìn)行分析存在局限性。近幾年發(fā)展起來的四維血流磁共振成像技術(shù)（four dimensional flow MRI，4D flow MRI）可獲取三維方向的速度容積數(shù)據(jù)及一個(gè)時(shí)間維，又稱為時(shí)間分辨的3D PC技術(shù)。由于包含血流在三維方向上的信息，可以實(shí)時(shí)重建在采集范圍內(nèi)任何位置的血流分析信息。所獲取血流數(shù)據(jù)經(jīng)過后處理，可以經(jīng)專業(yè)血流軟件顯示三維血流分布，計(jì)算血流動(dòng)力學(xué)參數(shù)（圖3和圖4）。這種方法提供了一個(gè)無創(chuàng)性定性和定量評(píng)估心血管血流狀態(tài)的方法，給血流動(dòng)力學(xué)的在體評(píng)估提供了新的技術(shù)平臺(tái)。與CFD方法相比，這項(xiàng)技術(shù)最明顯的優(yōu)勢(shì)就是可以實(shí)現(xiàn)直接在體血流動(dòng)力學(xué)狀態(tài)顯示和分析，從而避免了由于建立假設(shè)引起的計(jì)算誤差。已經(jīng)有研究應(yīng)用這種技術(shù)對(duì)主動(dòng)脈[18]、頸動(dòng)脈[19]、顱內(nèi)動(dòng)脈[20]等血管進(jìn)行在體研究，取得了令人滿意的結(jié)果。不管是血流速度，還是壁切應(yīng)力數(shù)據(jù)均有較好的可重復(fù)性[21]。
由于CoW的結(jié)構(gòu)特點(diǎn)和其在側(cè)支循環(huán)中所起到的重要作用，越來越多的研究者開始關(guān)注CoW結(jié)構(gòu)相關(guān)的血流動(dòng)力學(xué)研究[22]。由于CoW結(jié)構(gòu)直接影響顱內(nèi)動(dòng)脈側(cè)支循環(huán)代償能力，頸動(dòng)脈狹窄后，顱內(nèi)結(jié)構(gòu)的差別可導(dǎo)致不同的血流模式分布[23-24]。一些研究應(yīng)用經(jīng)顱多普勒超聲方法，通過獲取血流頻譜、血流速度以及阻力指數(shù)等參數(shù)來間接判斷頸動(dòng)脈狹窄后顱內(nèi)血流動(dòng)力學(xué)和側(cè)支循環(huán)狀態(tài)[25-26]。也有研究[27-28]應(yīng)用動(dòng)脈自旋標(biāo)記動(dòng)態(tài)MR成像技術(shù)通過對(duì)達(dá)峰值時(shí)間和量值等參數(shù)分析頸動(dòng)脈狹窄及內(nèi)膜剝脫術(shù)后的腦血流狀態(tài)。這些研究的局限性在于無法直接顯示顱內(nèi)的血流模式以及血流動(dòng)力學(xué)特點(diǎn)。近年來，一些研究開始應(yīng)用計(jì)算流體力學(xué)方法重建頸動(dòng)脈狹窄狀態(tài)下顱內(nèi)的血流動(dòng)力學(xué)模型[29-31]，發(fā)現(xiàn)單側(cè)頸內(nèi)動(dòng)脈嚴(yán)重狹窄情況下，不同的CoW結(jié)構(gòu)可導(dǎo)致不同的側(cè)支循環(huán)代償狀態(tài)。
然而，以上這些研究都主要關(guān)注于頸動(dòng)脈狹窄對(duì)顱內(nèi)血流模式的影響。而由于頸動(dòng)脈與顱內(nèi)動(dòng)脈的一體性，這種影響作用必然是相互的。2006年，Tanaka H等的研究[32]就發(fā)現(xiàn)，變異的CoW結(jié)構(gòu)與基底動(dòng)脈以及顱內(nèi)頸內(nèi)動(dòng)脈血流率密切相關(guān)。說明CoW結(jié)構(gòu)對(duì)近心段血管的血流動(dòng)力學(xué)狀態(tài)會(huì)產(chǎn)生影響。
2011年的一項(xiàng)流體力學(xué)模擬研究[33]顯示，當(dāng)CoW結(jié)構(gòu)不完整，無法提供側(cè)支循環(huán)狀態(tài)下，頸動(dòng)脈狹窄使得遠(yuǎn)端血管供血下降。這時(shí)，為了盡量滿足局部腦組織的血供，頸動(dòng)脈斑塊管腔內(nèi)血流速度會(huì)明顯增高，額外增加了斑塊局部壓力，使斑塊易于出現(xiàn)破裂。這說明，顱內(nèi)動(dòng)脈CoW的結(jié)構(gòu)和血流動(dòng)力學(xué)狀態(tài)，對(duì)狹窄頸動(dòng)脈的血流動(dòng)力學(xué)狀態(tài)可產(chǎn)生重要的影響作用。隨著流體力學(xué)研究在此領(lǐng)域的發(fā)展，顱內(nèi)CoW結(jié)構(gòu)、血流動(dòng)力學(xué)狀態(tài)與頸動(dòng)脈血流動(dòng)力學(xué)狀態(tài)的綜合分析必然成為今后研究的發(fā)展方向。
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