以超音速剪力影像測量物質之黏彈性質及其於頸動脈血流動力學之應用

Abstract

臨床上，腦血流量被視為是在腦血管意外預防和治療上的重要指標。由於大腦血流是由頸動脈攜帶至腦部，我們有機會藉由量測頸部表面的壓力值來監測腦血流量。由理論觀點出發，腦血流量與表面壓力之間的關係可由納維斯托克斯方程式(Navier-Stokes equation)和拉梅問題(Lame problem)推導之壓力轉換函數描述。但在轉換函數中並未考慮到人體頸部組織為具有黏彈性質的材料，而是將組織假設為完全彈性體。因此為能夠提升此轉換函數的正確性，我們期待藉由測量材料的黏彈性質資訊進而修正此函數。 近年來黏彈性質的測量常使用於對病變組織如乳癌和肝纖維化的診斷，在此研究中我們使用超音速剪力影像(Supersonic shear imaging)作為測量黏彈性質的方法。選用此方法的原因在於其能夠即時以非侵入性的方式提供定量的資訊。研究中使用了不同材料的仿體進行測量。結果顯示我們能夠得到定量的彈性資訊以及定性的黏滯性資訊。而在非均勻仿體的實驗中，二維的黏彈性質資訊能分辨出硬塊與背景仿體的差別性。 在驗證血流監測方法的理論上，因從拉梅問題的推導中得知壓力轉換函數可由位移轉換函數驗證。我們利用都普勒超音波的方法測量血管仿體的液體流速及仿體位移。由都普勒超音波測量的液體流速此處視為是實際流速，並將其與由納維斯托克斯方程式得到之流速訊號作比較。位移訊號之實驗結果與理論之位移轉換函數呈現高度匹配性。而流速部分在值之大小與波形部分存在因實作方法及參數的近似而造成的差異。實驗之結果顯示了此種測量血流方法的可能性。

Clinically, cerebral blood flow(CBF) is an important indicator for the prevention and curation of cerebral vascular accident(CVA) which is a major cause of death worldwide. Since cerebral blood flow is provided by carotid artery, one possible monitoring method of CBF is to measure the pressure on the neck surface. Theoretically, blood flow can be estimated from the surface pressure using Navier-Stokes equation and pressure transfer function derived by Lame problem. The derived transfer function considers only purely elastic material which is different from the viscoelastic human tissues. Therefore viscoelasticity estimation might help us correct and improve the accuracy of this mechanical model. Estimation of viscoelasticity is used in diagnosis of pathologic tissues like breast cancer or liver fibrosis recently. In this thesis, Superesonic shear imaging(SSI) method is used to estimate viscoelasticity because of its advantages of real-time, noninvasive and quantitative mapping. Phantoms with different materials are prepared for estimation. Our results have obtained quantitative elasticity mapping and qualitative viscosity information. Two dimensional viscoelasticity mapping for inhomogeneous phantom is reconstructed and can show the difference between inclusion and background phantoms. As for validation of new monitoring approach's theory, because it gives access to verify pressure transfer function by displacement transfer function from the derivation of pressure transfer function. We measure flow velocity and material displacements of a vessel wall phantom based on Doppler ultrasound. The flow velocity computed by Doppler is considered to be the real velocity value and compared with the velocity evaluated by Navier-Stokes equation. Experimental data of displacements fit well to theoretic transfer function curve. Waveforms between true and estimated flow velocity show similarity except some distortion due to approximation of implementation method and parameters. These validation results show the potential to let us monitor the carotid flow by this approach.