Nägele T, Klose U, Grodd W, Martos J
Abteilung für Neuroradiologie, Radiologische Klinik, Universität Tübingen.
Rofo. 1994 Nov;161(5):399-403. doi: 10.1055/s-2008-1032556.
Spin saturation effects in 3D time-of-flight (TOF) MR angiography (MRA) can be reduced by using radio-frequency (RF) pulses with linearly increasing flip angles (ramp pulses) in the main direction of flow. MR angiograms of the cervical arteries of five healthy volunteers and eleven patients suffering from stenosis of the internal carotid arteries were acquired using a standard flow-compensated 3D FISP sequence with a single volume technique. Compared with the conventional MR angiograms acquired with constant flip angles, the MR angiograms could be considerably improved using the ramp pulses. Especially the distal parts of the vessels were depicted more clearly due to significantly reduced saturation effects. Since the vessel signal largely depends on the shape of the ramp pulses, we developed a model for calculating the signal distribution of the flowing protons for different ramp shapes and a given excitation volume (slab). The calculated signals were largely consistent with the signals in the MR angiograms, thus permitting numerical optimization of the shape of the ramp pulses for the cervical arteries. This optimized shape of the ramp pulse provided a high homogeneous vessel signal across the slab.
通过在血流主方向上使用翻转角线性增加的射频（RF）脉冲（斜坡脉冲），可以减少三维时间飞跃（TOF）磁共振血管造影（MRA）中的自旋饱和效应。使用具有单个体积技术的标准血流补偿三维快速成像稳态进动序列（3D FISP），获取了5名健康志愿者和11名患有颈内动脉狭窄患者的颈动脉磁共振血管造影图像。与采用恒定翻转角获取的传统磁共振血管造影图像相比，使用斜坡脉冲可使磁共振血管造影图像得到显著改善。特别是由于饱和效应显著降低，血管的远端部分显示得更清晰。由于血管信号在很大程度上取决于斜坡脉冲的形状，我们开发了一个模型，用于计算不同斜坡形状和给定激发体积（层面）下流动质子的信号分布。计算得到的信号与磁共振血管造影图像中的信号基本一致，从而能够对颈动脉斜坡脉冲的形状进行数值优化。这种优化后的斜坡脉冲形状在整个层面上提供了高度均匀的血管信号。
