4D血流MR成像评估颅内动脉血流动力学状态的实验研究

In vivo visualization and assessment of hemodynamics of intracranial arteries using 4D flow imaging

目的旨在应用4D血流MR成像研究正常志愿者及大脑中动脉(MCA)狭窄病人的颅内动脉血流动力学状态,并比较正常和狭窄颅内动脉的双侧血流动力学参数差异。方法纳入正常志愿者5名和MCA狭窄病人3例。应用3 T MR设备进行检查,采用3D时间飞跃法(TOF)MRA及高分辨3D-T1WI-SPACE序列显示及分析Willis环结构特点。应用4D血流MR成像获取血流数据,分析双侧颈内动脉(ICA)颅内段、MCA及大脑前动脉(ACA)的血流动力学状态,计算心动周期不同时相的平均血管面积、平均及最大血流速度、平均瞬时血流率,生成血流矢量图、流线图及粒子追踪图。分析颅内Willis环的结构特征及血流动力学状态。采用独立样本t检验比较正常志愿者及MCA狭窄病人双侧动脉血流动力学参数的差异。结果正常志愿者中,4名显示对称Willis环结构,1名右侧胚胎型大脑后动脉。双侧ICA、MCA和ACA的血流动力学参数包括平均血管面积、平均血流速度、最大血流速度和平均瞬时血流率之间差异均无统计学意义(均P>0.05)。血流矢量图、流线图及粒子追踪图显示,ICA可见高速血流,收缩晚期及舒张期显示血流速度下降,血流分布欠均匀。1名右侧胚胎型大脑后动脉志愿者双侧血流动力学参数明显不对称,除右侧ACA最大血流速度低于左侧,其余颅内血管段的平均血管面积、平均和最大血流速度以及平均瞬时血流率右侧均明显高于左侧。3例MCA狭窄病人的ICA、MCA及ACA双侧血流动力学参数之间的差异未见统计学意义(P>0.05)。3例狭窄侧平均瞬时血流率均低于对侧。2例局部平均血流速度及相对压力梯度较对侧降低;另1例则明显较对侧增大。血流矢量图、流线图及粒子追踪图显示狭窄局部血流矢量明显偏离血管主轴方向,血流分布明显不规则。结论应用4D血流MR成像可以提供对颅内动脉复杂血流动力学状态的综合评估,包括对血流参数进行定量评价。颅内Willis环的结构与其血流动力学状态及分布密切相关。

Objective The aim of this study were to investigate the hemodynamic status of intracranial artery of healthy subjects and patients with middle cerebral artery（MCA） stenosis using 4 D flow MR imaging, and to compare the bilateral hemodynamic parameters of stenosed and normal intracranial arteries. Methods Five healthy volunteers and three patients with MCA stenosis were enrolled in this study. MR images were acquired on a 3 T MR scanner. Three-dimensional TOF MRA and high-resolution 3 D T1-weighted SPACE sequences were used to visualize and analyse the structural features of the circle of Willis. The flow data were acquired using 4 D MR flow imaging. The hemodynamic status was assessed at the intracranial portion of the internal carotid artery（ICA）, the middle cerebral artery（MCA）, and the anterior cerebral artery（ACA）. The average vessel area, average and maximum flow velocity, and average net flow rate at different phases of the cardiac cycle were calculated. Flow velocity vectors, streamlines, and particle traces maps were generated. The structural features and hemodynamic status of intracranial circle of willis were analyzed. Independent t-test was applied to test the differ-ence in hemodynamic parameters between bilateral sides in healthy subjects and patients with stenosed MCAs. Results For the normal volunteers, symmetrical circle of Willis was found in 4 cases, and asymmetric circle of Willis with an unilateral right fetal PCA（f PCA） was found in one case. No statistical difference was found in hemodynamic parameters including average vessel area, average flow velocity, maximum flow velocity, and average net flow rate between the left and right side of the ICA, MCA, and ACA（P〉0.05）. Flow velocity vectors, streamlines, and particle traces maps showed flow with high velocity at ICA. The flow velocity was decreased at late systolic and diastolic phases with irregular flow distribution. For one volunteer with an unilateral right f PCA, the bilateral hemodynamic parameters demonstrated asymmetry. It showed that only the maximum flow velocity of right ACA was lower than the contralateral side. Other parameters including average area, average flow velocity, maximum flow velocity, and average net flow rate of the rest cerebral arteries were all higher at the right side than the contralateral side. For the three patients with MCA stenosis, no statistical difference in hemodynamic parameters was found between bilateral ICA, MCA, and ACA（P〉0.05）. The local average net flow rate was lower than the contralateral side in all three cases. The local average flow velocity and relative pressure gradient were decreased than the contralateral side in two cases, while increased intensely in one case. Flow velocity vectors, streamlines, and particle traces maps showed the directions of flow velocity vectors obviously diverged from the main axis of the vessel at stenosed site, with obviously irregular flow distribution. Conclusion Four-dimensional flow MRI can provide comprehensive evaluation of complex hemodynamics of intracranial arteries, including quantitative assessment of flow parameters. The structures of the circle of Willis are associated with hemodynamic status of intracranial arteries.