Priatna A, Paschal C B
Department of Biomedical Engineering, Vanderbilt University, Nashville, TN 37232, USA.
J Magn Reson Imaging. 1995 Jul-Aug;5(4):421-7. doi: 10.1002/jmri.1880050409.
A spatially asymmetric RF pulse that improves the uniformity of blood signal intensity and vascular contrast in three-dimensional (3D) MR angiography (MRA) is presented. The pulse, called variable-angle uniform signal excitation (VUSE), was designed to provide uniform signal response and improved contrast for blood flowing through a 3D imaging volume during a FLASH sequence. The VUSE excitation profile was optimized on the basis of the number of pulses seen by the blood, which varied with the velocity of through-plane flow, repetition time, and slab thickness with the maximum flip angle at the flow exit constrained at 90 degrees. The theoretical results show that the optimal RF pulse gives more uniformity for flow signal than does a linear ramp excitation profile or a Gaussian pulse combined with a presaturation pulse. After truncation and filtering of the VUSE pulse in the time domain, the general shape of the VUSE RF excitation profile is maintained, but the maximum flip angle is reduced. The arteries of the neck in a healthy volunteer were imaged with the VUSE pulse, a constant flip angle (flat) pulse, and a linear ramp pulse in flow-compensated 3D MRA sequences. The VUSE pulse produced the most uniform signal as evidenced by the lowest relative dispersion of signal along the left vertebral artery (18.0 versus 26.4 to 23.6 for the other studies). F-distribution tests also showed that the signal distribution obtained with the VUSE pulse in a 3D FLASH sequence was statistically different from that for the flat and the linear ramp pulses.
本文提出了一种空间不对称射频脉冲，该脉冲可提高三维（3D）磁共振血管造影（MRA）中血液信号强度的均匀性和血管对比度。这种被称为可变角度均匀信号激发（VUSE）的脉冲旨在在快速低角度激发（FLASH）序列期间，为流经3D成像容积的血液提供均匀的信号响应并改善对比度。VUSE激发轮廓是根据血液所经历的脉冲数进行优化的，该脉冲数随层面内血流速度、重复时间和平板厚度而变化，且在血流出口处的最大翻转角被限制为90度。理论结果表明，与线性斜坡激发轮廓或高斯脉冲与预饱和脉冲相结合的情况相比，最优射频脉冲能使血流信号具有更高的均匀性。在对VUSE脉冲进行时域截断和滤波后，VUSE射频激发轮廓的总体形状得以保留，但最大翻转角减小。在血流补偿的3D MRA序列中，使用VUSE脉冲、恒定翻转角（平坦）脉冲和线性斜坡脉冲对一名健康志愿者的颈部动脉进行成像。VUSE脉冲产生的信号最为均匀，这一点通过沿左椎动脉信号的最低相对离散度得到证明（其他研究的相对离散度为26.4至23.6，而VUSE脉冲为18.0）。F分布测试还表明，在3D FLASH序列中使用VUSE脉冲获得的信号分布与平坦脉冲和线性斜坡脉冲的信号分布在统计学上存在差异。
