基于CTA的椎動脈瘤血流動力學分析

劉芳 趙思淇 盧立彬

摘 ?要： 分析椎動脈的動脈瘤血流動力學指標在動脈瘤發生、發展及治療后的作用，判斷引起動脈瘤發生與治療后復發的特定血流動力學因素，并為動脈瘤的預防、治療提供理論依據。選取一例顱內動脈瘤患者的CTA影像數據三維建模和仿真計算獲取血流動力學指標：time average wall shear stress、time average wall shear stress grade、oscillatory shear index 、aneurysm formation index、relative retention time等參數作為觀察指標分析。結果顯示：1. 動脈瘤TAWSS及TAWSSG的不穩定，栓塞手術能夠降低動脈瘤破裂的風險性，而在栓塞術后的血管交叉處，血管壁則較易受損;2. OSI值較高，改變瘤體內震蕩水平導致血流紊亂，OSI值減低，血流趨于穩定;3. 隨著動脈瘤AFI值逐漸升高，血液流動可逐漸平穩，可降低動脈瘤破裂危險性。

關鍵詞： 顱內動脈瘤;計算流體力學

中圖分類號： TP319 ? ?文獻標識碼： A ? ?DOI：10.3969/j.issn.1003-6970.2020.08.028

本文著錄格式：劉芳，趙思淇，盧立彬，等. 基于CTA的椎動脈瘤血流動力學分析[J]. 軟件，2020，41（08）：97-102

【Abstract】： To analyze the role of vertebral artery aneurysm parameters in the occurrence， development and post treatment of aneurysms， ? ?to determine the specific Hemodynamics that cause the occurrence and recurrence of aneurysms after treatment， and to provide a theoretical basis for the prevention and treatment of aneurysms. Three-dimensional modeling and simulation of CTA images of a patient with cerebral aneurysm were used to obtain Hemodynamics parameters： Time average wall shear stress， time average wall shear stress， Oscillatory Shear Index， Eurasian SM Formation Index and relative retention time. 1. The instability of TAWSS and Tawssg， embolization can reduce the risk of aneurysm rupture， but the vessel wall is more vulnerable at the cross-section after embolization. 2. Osi Value is higher， changes in the level of turbulence in the tumor lead to blood flow disorder， Osi value is reduced， blood flow tends to be stable; 3. With the gradual increase of AFI， the blood flow could be stabilized and the risk of aneurysm rupture could be reduced.

【Key words】： Cerebral aneurysm; Computational fluid dynamics

0 ?引言

顱內動脈瘤（Intracranial aneurysm，IA）是多種因素導致的動脈壁的異常瘤樣擴張，常發生于顱內大動脈的分叉及彎曲處，破裂會導致蛛網膜下腔出血，具有極高的致死率及致殘率。影響動脈瘤生長和破裂的因素主要包括先天生理性、病理性及血流動力學因素。對于IA的治療主要包括開顱夾閉術及血管內介入栓塞兩種方法[1-3]。無論是哪種方法，由于動脈瘤自身的復雜性及不完全閉塞的發生，即使是有經驗的臨床醫生，術后的復發率依舊很高[4-7]。栓塞手術由于創傷小，操作相對簡單等優勢和栓塞材料及技術的不斷發展進步逐漸被廣泛應用于臨床。但同時由于彈簧圈具有可壓縮性使得動脈瘤復發的可能性大大增加，有研究表明栓塞程度是動動脈瘤復發的重要影響因素，Brzegowy等人回顧性分析破裂與未破裂前交動脈瘤的栓塞治療，同樣得出影響顱內動脈瘤復發的最大因素就是栓塞密度，初始栓塞的不完全極易引起動脈瘤的復發[8-9]。栓塞程度低，彈簧圈會隨著血流的沖擊逐漸壓縮，進而向遠側移位、復發。趙慶平等提出瘤腔內的血流速度與瘤腔大小呈負相關，即栓塞程度越低，腔內血流速度加快時，血液對壁面產生的力就可能導致動脈瘤的復發[10]。近年來隨著計算機的發展及有限元軟件的開發，尤其是計算流體動力學數值模擬方法的應用，使得血流建模能更好解釋血流動力學在IA發病機制中的作用[11-16]

1 ?材料與方法

原始影像數據采集：采集解放軍第78集團軍醫院一椎動脈瘤患者CTA影像數據，男性患者，62歲，患者主訴頭部持續頭痛，臨床表現為：行走不穩三個月。經臨床診斷為頭部椎最動脈瘤。經患者本人知情同意并簽署意見書與醫院倫理委員會批準。

圖像后處理工作站：DELL圖像向工作站：DELL 7810/CPU E5/16G內存/英偉達K2200顯卡;

圖像后處理軟件：醫學交互式影像控制系統（Materialis Interative Medical Image Control System，MIMICS，比利時Materialise公司）、醫學建模軟件3-matic medical（比利時Materialis公司）;

計算機仿真軟件：ANSYS 19.2：流體仿真軟件CFX，網格劃分軟件FLUENT MESHING

計算結果分析軟件：ENSIGHT10.6。

椎動脈瘤三維重建：將頭部影像DICOM數據導入MIMICS軟件，使用MIMICS分割工具：閾值分割等算法，最后三維計算生成動脈瘤三維初步模型以stl格式導入3-matic medical軟件中，使用光順表面、去除細小分支、切好出、入口平面，最后形成動脈瘤三維模型，如圖1所示。

網格劃分：由于模型結構復雜，使用非結構化的四面體網格劃分，為保證計算精度，在動脈瘤管壁進行五層加密。

邊界條件：本計算不考慮能量的傳遞，不考慮重力。血液密度為1056 kg/m3，動力粘度為0.0035 。計算采用瞬態計算，兩個入口，兩個出口，壁面無滑移。入口采用速度入口，出口采用壓力出口。為保證盡快收斂，入口速度采用極小的速度差。出口采用壓力出口，壓力曲線如圖3所示。

血流作用在內皮細胞上的力的血流動力學參數GON，壁面切向和正交向上的向量，如果空間梯度G數值變化，代表對內皮細胞產生震蕩張力和壓縮力，在一個心動周期內，如果某個點發生較大的梯度變化，單位面積內發生強烈的震蕩張力或者壓縮力作用于內皮細胞上，GON是用來量化震蕩張力和壓縮力的程度。

CFX無法直接實現上述參數指標，使用CFX ccl語言編程，如下為子程序的部分內容：

IBRARY：

CEL：

EXPRESSIONS：

DOMAIN： FLUIDdom

Coord Frame = Coord 0

Domain Type = Fluid

Location = Assembly

BOUNDARY： INLET1

Boundary Type = INLET

Location = INLET1

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Normal Speed = invel1

Option = Normal Speed

END

END

END

BOUNDARY： INLET2

Boundary Type = INLET

Location = INLET2

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Normal Speed = invel2

Option = Normal Speed

END

END

END

BOUNDARY： OUTLET1

Boundary Type = OPENING

Location = OUTLET1

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW DIRECTION：

Option = Normal to Boundary Condition

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Option = Opening Pressure and Direction

Relative Pressure = OUTLET1f

END

END

END

BOUNDARY： OUTLET2

Boundary Type = OPENING

Location = OUTLET2

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW DIRECTION：

Option = Normal to Boundary Condition

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Option = Opening Pressure and Direction

Relative Pressure = OUTLET2f

END

END

END

BOUNDARY： OUTLET3

Boundary Type = OPENING

Location = OUTLET3

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW DIRECTION：

Option = Normal to Boundary Condition

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Option = Opening Pressure and Direction

Relative Pressure = OUTLET3f

END

END

END

BOUNDARY： OUTLET4

Boundary Type = OPENING

Location = OUTLET4

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Option = Zero Flux

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = 0 [kg m^-1 s^-2]

Option = Value

END

FLOW DIRECTION：

Option = Normal to Boundary Condition

END

FLOW REGIME：

Option = Subsonic

END

MASS AND MOMENTUM：

Option = Opening Pressure and Direction

Relative Pressure = OUTLET4f

END

END

END

BOUNDARY： WALL_VESSEL

Boundary Type = WALL

Location = WALL_PARENT_VESSEL

BOUNDARY CONDITIONS：

ADDITIONAL VARIABLE： WSSField

Additional Variable Value = WallShearMag

Option = Value

END

ADDITIONAL VARIABLE： WSSxF

Additional Variable Value = Wall Shear X

Option = Value

END

ADDITIONAL VARIABLE： WSSyF

Additional Variable Value = Wall Shear Y

Option = Value

END

ADDITIONAL VARIABLE： WSSzF

Additional Variable Value = Wall Shear Z

Option = Value

END

MASS AND MOMENTUM：

Option = No Slip Wall

END

END

END

DOMAIN MODELS：

BUOYANCY MODEL：

Option = Non Buoyant

END

2.2 ?TAWSS云圖分析

從平均壁面切應力（TAWSS）的云圖（圖4）上來看，云圖顏色越偏紅，代表平均壁面切應力越大，越接近藍色則平均壁面切應力越小。載瘤動脈由于其具有較高的流速，因此載瘤動脈的平均壁面切應力要大于動脈瘤。

2.3 ?動脈瘤的TAWSSG云圖分析

從TAWSSG云圖中（圖5）可以看出，瘤體的TAWSSG開始時低于載瘤動脈，而后逐漸接近。可能是由于開始時動脈瘤體積較大[18-20]，血液流速較慢，壁面切應力數值變化不變明顯，所示動脈瘤偏藍色，TAWSSG值較低。

2.4 ?動脈瘤的OSI云圖分析

圖6為動脈瘤OSI云圖，在動脈瘤頂端存在小部分高OSI區域。動脈瘤頂端的高OSI區域較之前擴大，振蕩剪切系數代表的是整個心動周期內壁面切應力方向變化快慢的量，OSI不同是反應震蕩水平，即流動的強度和方向的改變，越大表示震蕩越強，流體在周期內流動的方向不穩定，導致動脈瘤內的血流運動趨于紊亂。

2.5 ?動脈瘤AFI云圖分析

圖7為動脈瘤AFI云圖，瘤體側壁上存在部分AFI低區域，即偏藍色區域。流增多形成渦流并不斷的沖擊著動脈瘤管壁，壁面切應力的方向變化明顯，血液流動不穩定。

2.6 ?動脈瘤GON云圖分析

動脈瘤GON云圖（圖8）表明在動脈瘤表面存在強烈的震蕩力和壓縮力，原因是血液在進入瘤腔后形成渦流，導致動脈瘤壁震蕩，這種沖擊對瘤壁造成膨脹或者擴張。

3 ?討論與結論

通過血流動力學計算分析，發現完全栓塞手術可以阻斷進入動脈瘤內的血液[21-24]，提高TAWSS及降低OSI等，降低了破裂出血的風險。通過本實驗對最動脈瘤血流動力學的參數的變化分析可得出以下結論：

（1）動脈瘤TAWSS及TAWSSG的不穩定，栓塞手術能夠降低動脈瘤破裂的風險性[25-30]，而在栓塞術后的血管交叉處，血管壁則較易受損;

（2）OSI值較高，改變瘤體內震蕩水平導致血流紊亂，OSI值減低，血流趨于穩定;

（3）隨著動脈瘤AFI值逐漸升高，血液流動可逐漸平穩，可降低動脈瘤破裂危險性。

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