Division of Vascular Surgery, Georgetown University Hospital, Washington, DC 20007, USA.
J Surg Res. 2011 Aug;169(2):311-8. doi: 10.1016/j.jss.2009.12.008. Epub 2010 Jan 1.
Hemodynamic factors at the distal anastomosis play an important role in prosthetic graft performance. A new magnetic resonance imaging (MRI) technique was used to determine the effect of anastomotic geometry on hemodynamic flow patterns.
Four dimensional (4D) magnetic resonance velocimetry (4D-MRV) is a noninvasive method of analyzing pulsatile flow in three dimensions (3D). End-to-side anastomotic models were constructed by suturing 6 mm polytetrafluoroethylene (ePTFE) grafts to silicone tubing (4 mm i.d.). The models included straight ePTFE, precuffed ePTFE, and patched ePTFE configurations in a pulsatile system, which created flow consistent with physiologic flow rates and pressures. Blood was simulated by a solution of 40% glycerol in distilled water with trace gadolinium. The different models were imaged using MRV techniques in a three-dimensional (3D) coronal slab (0.5 mm thick coronal slices, in-plane field of view (FOV) 18 cm.) The data were reconstructed, resulting in an interpolated resolution of 0.35 mm in each coronal plane. The 3D flow fields were represented as isosurfaces, visualizing the internal geometry of the models with streamlines tangent to the velocity vectors identifying the path of the fluid. Volumetric flow rates for each time phase were calculated by integrating the flow through cross sections of each anastomotic model. Analysis of the flow patterns focused on the anastomotic regions prone to the development of intimal hyperplasia and graft failure as identified in the literature; the toe, floor, heel, and hood.
Conventional end-to-side geometry resulted in uniform flow with a low angle of impingement on the recipient vessel floor. A small vortex at the anastomotic heel created minimal recirculation. The precuffed geometry resulted in a large recirculation vortex of chaotic, low flow that increased throughout the pulsatile cycle. Regions of low flow velocity were noted in a substantial portion of the precuffed anastomotic configuration. Flow separation distal to the toe occurred in both geometries, but was more apparent in the precuffed configuration. The patch model had flow characteristics similar to the straight end-to-side geometry.
Magnetic resonance velocimetry produces 3D, time varying velocity measurements with sufficient accuracy and resolution to analyze hemodynamics in anastomotic geometries. Flow structures in different graft configurations were effectively captured with marked differences noted between standard and precuffed anastomotic geometries. The findings support a conventional end-to-side anastomosis with a low incidence angle using a straight graft as producing favorable hemodynamics as compared to a cuffed configuration. The vein patch configuration closely approximates the conventional, straight anastomotic pattern. We believe the MRV technique has been sufficiently developed to warrant additional in vitro and in vivo studies providing insight into hemodynamic implications for the development of optimal prosthetic graft performance.
在移植物性能方面，远端吻合处的血流动力学因素起着重要作用。一种新的磁共振成像（MRI）技术被用于确定吻合几何形状对血流模式的影响。
四维度（4D）磁共振流速测量（4D-MRV）是一种分析三维（3D）脉动流的非侵入性方法。端侧吻合模型通过将 6 毫米聚四氟乙烯（ePTFE）移植物缝合到硅酮管（4 毫米内径）来构建。模型包括直的 ePTFE、预充气球囊的 ePTFE 和补丁 ePTFE 配置，在脉动系统中产生与生理流速和压力一致的流动。血液由 40%甘油在蒸馏水中的溶液模拟，加入痕量钆。使用 MRV 技术在三维（3D）冠状片层（0.5 毫米厚冠状切片，平面视野（FOV）18 厘米）中对不同的模型进行成像。数据被重建，结果是每个冠状平面的插值分辨率为 0.35 毫米。3D 流场表示为等位面，流线与速度矢量相切，以可视化模型的内部几何形状，从而确定流体的路径。通过整合每个吻合模型的横截面的流量来计算每个时相的体积流量率。对血流模式的分析侧重于文献中确定的易发生内膜增生和移植物失效的吻合区域；趾部、底部、跟部和罩部。
常规的端侧几何形状导致在流入血管底部时产生均匀的流动和低撞击角度。吻合跟部的小漩涡产生最小的再循环。预充气球囊的几何形状导致混沌、低流量的大再循环涡流增加整个脉动周期。在预充气球囊吻合的大部分配置中，都观察到低流速区域。在两种几何形状中，都发生了在趾部远端的流动分离，但在预充气球囊吻合的几何形状中更为明显。补丁模型的流动特性与直端端侧几何形状相似。
磁共振流速测量产生具有足够精度和分辨率的 3D、时变速度测量值，可用于分析吻合几何形状中的血液动力学。在不同移植物配置中，有效的捕获了流动结构，在标准和预充气球囊吻合几何形状之间存在明显差异。研究结果支持使用直的移植物作为传统的低角度端端吻合，与使用充气球囊的配置相比，产生更好的血液动力学。静脉补丁配置非常接近传统的直端端吻合模式。我们相信，磁共振流速测量技术已经得到充分发展，可以进行额外的体外和体内研究，为优化移植物性能的发展提供血液动力学方面的见解。
