Vinitski S, Mitchell D G, Szumowski J, Burk D L, Rifkin M D
Thomas Jefferson University Hospital, Department of Radiology, Philadelphia, PA.
Magn Reson Imaging. 1990;8(2):131-9. doi: 10.1016/0730-725x(90)90246-x.
Conventional "proton density" and "T2-weighted" spin-echo images are susceptible to motion induced artifact, which is exacerbated by lipid signals. Gradient moment nulling can reduce motion artifact but lengthens the minimum TE, degrading the "proton density" contrast. We designed a pulse sequence capable of optimizing proton density and T2-weighted contrast while suppressing lipid signals and motion induced artifacts. Proton density weighting was obtained by rapid readout gradient reversal immediately after the excitation RF pulse, within a conventional spin-echo sequence. By analyzing the behavior of the macroscopic magnetization and optimizing excitation flip angle, we suppressed T1 contribution to the image, thereby enhancing proton density and T2-weighted contrast with a two- to four-fold reduction of repetition time. This permitted an increased number of averages to be used, reducing motion induced artifacts. Fat suppression in the presence of motion was investigated in two groups of 8 volunteers each by (i) modified Dixon technique, (ii) selective excitation, and (iii) hybrid of both. Elimination of fat signal by the first technique was relatively uniform across the field of view, but it did not fully suppress the ghosts originating from fat motion. Selective excitation, while sensitive to the main field inhomogeneity, largely eliminated the ghosts (0.21 +/- 0.05 vs. 0.29 +/- 0.06, p less than 0.01). The hybrid of both techniques combined with bandwidth optimization, however, showed the best results (0.17 +/- 0.04, p less than 0.001). Variable flip-angle imaging allows optimization of image contrast which, along with averaging and effective fat suppression, significantly improves gradient- and spin-echo imaging, particularly in the presence of motion.
传统的“质子密度”和“T2加权”自旋回波图像易受运动诱导伪影的影响，而脂质信号会加剧这种伪影。梯度矩归零可以减少运动伪影，但会延长最小回波时间（TE），从而降低“质子密度”对比度。我们设计了一种脉冲序列，能够在抑制脂质信号和运动诱导伪影的同时，优化质子密度和T2加权对比度。在传统的自旋回波序列中，通过在激发射频脉冲后立即进行快速读出梯度反转来获得质子密度加权。通过分析宏观磁化的行为并优化激发翻转角，我们抑制了T1对图像的贡献，从而在重复时间减少两到四倍的情况下增强了质子密度和T2加权对比度。这允许使用更多的平均次数，减少运动诱导伪影。通过以下三种方法对两组各8名志愿者在存在运动的情况下进行脂肪抑制研究：（i）改良的狄克逊技术，（ii）选择性激发，以及（iii）两者的混合方法。第一种技术消除脂肪信号在整个视野范围内相对均匀，但它没有完全抑制源自脂肪运动的重影。选择性激发虽然对主场不均匀性敏感，但在很大程度上消除了重影（0.21±0.05对0.29±0.06，p＜0.01）。然而，将两种技术的混合方法与带宽优化相结合，显示出最佳效果（0.17±0.04，p＜0.001）。可变翻转角成像允许优化图像对比度，这与平均和有效的脂肪抑制一起，显著改善了梯度回波和自旋回波成像，特别是在存在运动的情况下。
