层面选择方向扩散加权成像在中脑大脑脚间产生高信号的机制

The mechanism of hyperintense signal at interpeduncle region on diffusion weighted imaging

目的研究层面选择方向扩散加权成像（DWIs）在中脑大脑脚间产生高信号的机制。方法健康志愿者10名，以邻近中脑大脑脚间为中心进行MR扫描：（1）采用心电门控（ECG），测量收缩期与舒张期扩散敏感因子（b值）为0和1000mm^2／s的单独DWIs，比较两期测量中脑亮点区MR信号强度及表观扩散系数（ADC）值。（2）变换选层方向，分别采集平行于颅底、平行于系统Y轴，Y—Z坐标30°夹角的单独DWIs，观察中脑亮点形状的变化。（3）采集全脑6个方向DWI，利用扩散张量成像（DTI）白质纤维束追踪成像技术显示以亮点区为种子的神经纤维束走行起止，测量亮点区及其相连邻近纤维走行区的ADC值与各向异性分数（FA）值。将测量得到的亮点区ADC及FA值与相连邻近纤维走行区的ADC及FA值分别进行配对t检验，比较两者之间的差异。结果心脏收缩期采集的DWIs信号强度分别为296．28±38．19、153．81±37．91，舒张期分别为295．36±38．84、154．03±37．52，两者间差异无统计学意义（t值分别为1．34、0．62，P值均〉0．05）；心脏收缩与舒张期测量的ADC值分别为（6．07±2．20）×10^-4、（6．69±1．44）×10^-4mm^2／s，两者间差异无统计学意义（t=0．94，P〉0．05）。纤维束追踪显示中脑高信号亮点位于小脑上脚神经纤维束的中脑交叉区。常规扫描层面上，DWIs高信号亮点表现为狭长心形和狭长条形，其形状随扩散敏感梯度施加方向的变化而改变。多方向DWI测量亮点区与结合臂区的ADC值分别为（10．61±3．42）×10^-4与（9．24±2．21）×10^-4mm^2／s，两者间差异无统计学意义（t=0．61，P〉0．05）。亮点区FA值为0．43±0．13，结合臂FA值为0．61±0．08，两者间差异有统计学意义（t=8．32，P〈0．05）。结论中脑水平DWIs亮点征象由小脑上脚在中脑层面神经纤维及交叉区的各向异性造成，因此，其亮点信号的形状与DWIs的选层层面方向有关，与心动周期相关的脑搏动对亮点征象无影响。

Objective To study the mechanism of hyperintensed signal at interpeduncle region in mesencephalon on DWI with diffusion sensitive gradient （Gd） applied perpendicular to the slice direction （DWIs）. Methods MRI scanning was centered at interpeduncle region, and the protocols were as follows ： （1） With electrocardiograph （ECG） triggering, DWIs was acquired during systole or diastole periods respectively. （2） To observe the shape of hyperintense signal, DWIs with various slice directions was applied, which was set parallel to skull base, along Y axis, or along the oblique line between Y and Z axis with 30° to Z axis. （3） Fiber tracking was performed with the hyperintense signal as ＂seed＂ region, fractional anisotropy （FA） and ADC were derived from （multiple directions diffusion weighted imaging, MDDW）. FA and ADC of hyperintense region were compared with those of the nearby fiber by using pair t-test. Results Signal intensity of hyperintense region on DWIs were 2296. 28 ± 38. 19 and 153.81 ± 37.91 for b =0 and b = 1000 at systole period, while they were 295.36 ± 38.84 and 154. 03 ± 37. 52 at diastole period. No statistical difference was demonstrated between them （ t = 1.34, 0. 62, P 〉 0. 05 ）. The ADC values on DWIs acquired during systole and diastole periods were（6.07 ± 2. 20） × 10 ^-4 and（6. 69 ± 1.44） × 10^-4 mm^2/s respectively. There was no statistical difference between them （t =0. 94 ,P 〉 0.05 ）. DTI fiber tracking verified that the hyperintense region located at the decussation of superior cerebellum peduncle （SCP） in mesencephalon. It was in long and narrow heart shape or rectangular shape on DWIs. The shape depended on the direction of Gd. The ADC value derived from MDDW at hyperintense region and the fiber below were （ 10. 61±3.42） x 10 4 and （9.24 ± 2. 21 ） ×10^-4 mm^2/s respectively. No statistical difference was demonstrated between them （ t = 0. 61, P 〉 0. 05 ）. While, the FA at hyperintensed region 0.43 ± 0. 13 was less than that of the fiber below 0. 61 ± 0. 08 （ t = 8.32, P 〈 0. 05 ）. Conclusions The shape of the hyperintense region on DWIs depends on the direction of Gd. The cardiac period has no effects on the hyperintense signal at peduncles, which may be attributed to the anisotropy of SCP decussation in mesencephalon.