Yapo P, Sonier C B, Franconi F, Magni C, Cottier J P, Akoka S, Lafont J
Service de Neuroradiologie, CHU Bretonneau, Tours.
J Neuroradiol. 1996 Sep;23(2):56-61.
We describe in this paper how the STEAM sequence can be an efficient tool to obtain images free of flow artifacts in anatomical situation where the spin echo failed. The simplest way to eliminate flow artifacts is to exploit the dephasing induced by motion in magnetic field gradients and to reduce to zero the signal from moving tissues. This can be achieve by increasing the time elapsed between the spin excitation and the signal observed. Because of T2 relaxation, such an increase results in a signal decrease when the spin echo sequence is used. The STEAM sequence has the unique property that the time elapsed between observation and excitation can be increased without change in T2 value and so allows a good suppression of signals from the moving spins with short TE. Our results demonstrate that, although the stimulated echo intensity is only half that of a spin echo taken at the same read out time, the advantages of STEAM imaging can compensate for this partial loss in signal to noise in some particular clinical situations. The influence of mixing time on contrast has been evaluated using thoracic spine imaging and it has been shown that contrast between spine and CSF can be significantly improved (+ 60%) when TM is increased (from 17 ms to 50 ms). In the same time, the contrast between spine and fat issue decreases (40%). This last effect facilitates the adjustment of contrast window. Suppression of motion artifacts has first been evaluated with thoracic spine imaging, using a whole body coil. Suppression of artifacts was better than that obtained with a flow compensated spin echo sequence, especially in the case of kyphotic patients when a presaturation band was inefficient. In a second step suppression of motion artifacts has been evaluated from posterior fossa examination after injection of a paramagnetic contrast agent. The images obtained with the stimulated echo sequence show a dramatic reduction of signal from blood in the lateral sinus, and therefore an increase of quality by elimination of motion artifacts.
我们在本文中描述了STEAM序列如何能够成为一种有效的工具，在自旋回波失败的解剖情况下获取无流动伪影的图像。消除流动伪影的最简单方法是利用磁场梯度中运动引起的去相位，并将来自移动组织的信号降低到零。这可以通过增加自旋激发和观察到信号之间经过的时间来实现。由于T2弛豫，当使用自旋回波序列时，这种增加会导致信号降低。STEAM序列具有独特的特性，即观察和激发之间经过的时间可以增加而T2值不变，因此可以通过短TE很好地抑制来自移动自旋的信号。我们的结果表明，尽管受激回波强度仅为在相同读出时间获取的自旋回波强度的一半，但在某些特定临床情况下，STEAM成像的优势可以弥补这种部分的信噪比损失。使用胸椎成像评估了混合时间对对比度的影响，结果表明，当TM增加（从17毫秒增加到50毫秒）时，脊柱和脑脊液之间的对比度可显著提高（+60%）。同时，脊柱和脂肪组织之间的对比度降低（40%）。最后这个效果有助于调整对比度窗口。首先使用全身线圈通过胸椎成像评估运动伪影的抑制。伪影抑制比使用流动补偿自旋回波序列获得的更好，特别是在脊柱后凸患者中，当预饱和带无效时。第二步，在注射顺磁性造影剂后，通过后颅窝检查评估运动伪影的抑制。用受激回波序列获得的图像显示外侧窦中血液信号显著降低，因此通过消除运动伪影提高了图像质量。
