拉伸指数模型DWI对大鼠急性脑缺血的研究

目的:探讨拉伸指数模型多b值扩散加权成像(DWI)对大鼠急性脑缺血的诊断价值。方法:SD大鼠实验组18只、对照组9只术前行3.0T MRI检查,包括T1WI和T2WI、单b值(1000s/mm2)DWI、动脉自旋标记灌注成像(ASL-PWI)和多b值(0～4000s/mm2共12个b值)DWI,其中ASL-PWI可计算相对脑血流量(rCBF),多b值DWI采用拉伸指数模型,一次成像可得到Mono-ADC、F、DDC和α参数值。MRI检查后对实验组大鼠行右侧大脑中动脉闭塞(MCAO)的急性脑缺血手术,对照组行假手术,分别于术后0.5、3、6、9、12、18和24h行上述MRI检查,观察各时间点各参数的动态变化规律。最后,对大鼠脑缺血区行病理TTC染色确定实际脑梗死情况。结果:术后0.5h实验组右侧脑缺血区的rCBF值(0.470±0.002)较术前(0.990±0.010)明显下降、3h以后逐渐升高(从术后3h的0.980±0.006升至24h的1.970±0.070);实验组右侧脑缺血区F值术后缓慢升高(从术后0.5h的0.58±0.74升至0.76±0.57),在术后12、18和24h实验组双侧脑F值的差异有统计学意义(P值分别为0.031、0.004和0.001)。实验组双侧脑术后0.5～24.0h的DDC值为右侧(0.40±0.02)×10-3～(0.58±0.03)×10-3 mm2/s、左侧(0.46±0.03)×10-3～(0.49±0.06)×10-3 mm2/s,均较术前值右侧(1.18±0.05)×10-3、左侧(1.01±0.05)×10-3 mm2/s]降低,且在各时间点均低于对照组,差异有统计学意义(P值0.0083～0.0163);术后各时间点实验组右侧的α值均小于对照组右侧(P值范围0.0205～0.0443),其DDC值和Mono-ADC值之间有强相关性(r=0.9051,P<0.0001),且α值随着DDC值的升高而减低。结论:多b值DWI的拉伸指数衰减模型的各参数值所表现出的特征性可能为大鼠急性缺血脑组织中水分子不均一的扩散状态提供了一定的参考价值。

Diffusion weighted imaging in acute brain ischemia of rats with a stretched-exponential model

Objective： To evaluate the value of stretched-exponential model of multi-b diffusion weighted imaging （DWI） in the diagnosis of acute brain ischemie rats. Methods： Eighteen SD rats were enrolled in study group, and nine in control group. T1WI, T2 WI, single b-factor （b= 1000s/mm^2 ） diffusion weighted imaging （DWI）, arterial spin labeling perfusion weighted imaging （ASL-PWI） and multi-b （0-4000s/mm^2 altogether 12 b factors） DWI were performed with α 3.0T MR scanner. Relative cerebral blood flow （rCBF） was calculated from ASL-PWI. Mono-ADC, F, DDC and α values were calculated from multi-b DWI. Unilateral cerebral ischemic model with middle cerebral artery occlusion （MCAO） was performed in study group,pseudo-operation in control group. MRI with above-mentioned sequences were performed at the following time points after operation：0.5h,3h,6h,9h,12h,18h and 24h. The dynamic changes in different time-points were observed. Lastly,rats were sacrificed for TTC staining to study the real condition of infarct. Results：The rCBFs of the right cerebral ischemic areas in study group decreased markedly at 0.5h after operation, from 0.99 ± 0.01 （before operation to 0.47±0. 002 （0.5h after operation） ,but increased after 3h,from 0. 98010. 006 （3h after） to 1. 970±0. 070 （24h after）. The F values of the right cerebral ischemic areas in study group increased slowly after operation, from 0.58 ± 0.74 （0.5h after operation） to 0.76± 0.57, the F values on both sides in the study group had significant statistical differences, the p value was 0. 031 at 12h,0. 004 at 18h and 0. 001 at 24h respectively after operation. The DDC values 0.5-24. Oh after operation decreased,which were （0.40±0.02）×10^-3 -（0.58±0.03）×10^-3mm2/s for right side and （0.46±0.03）×10^-3（0.49±0.06）×10^-3mm2/s for left side,all were lower than those before operation （1.18±0.05） X 10-3 mm2/s for right side and （1.01±0.05）×10^-3mm2/s for left side. In addition,the DDC values in every time-points were lower than that of the control group （P value from 0. 0083 to 0. 0163）. The α values of the right side of study group were less than those in control group at every time point after operation （P values from 0. 0205 to 0. 0443）. In addition, the DDC values showed significant positive correlation with that of Mono-ADC values （r= 0. 9051, P〈0. 0001）, in addition, the α value decreased according to the increase of DDC values. Conclusion：The characteristics of stretched-exponential model of multi-b DWI may provide valuable references to describe intravoxel heterogeneity of water diffusion rates of acute ischemic cerebral tissue in rats.