Biomedical MR Imaging and Spectroscopy Group, Image Sciences Institute, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands.
Neuroimage. 2012 May 15;61(1):106-14. doi: 10.1016/j.neuroimage.2012.02.050. Epub 2012 Feb 28.
In some recent studies, diffusion weighted functional MRI has been proposed to provide contrast immune to vascular changes. Increases in relative signal change during neuronal activation observed under increasing diffusion weighting support the possible diffusion based origin of this contrast. A recent diffusion tensor imaging (DTI) study has also reported the use of Fractional Anisotropy (FA) to track activation in white matter. In this study we aimed to establish if relatively high diffusion weighting (b=1200 and 1800 s/mm(2)) eliminates the strong vascular influences brought about by 100% O(2) and carbogen (95%O(2)+5% CO(2)) induced vascular challenges in gray matter (GM) and white matter (WM) of rat brain. We also aimed to characterize the influences of these vascular changes on FA, both in GM and in WM. Our study endorses previous reports that even relatively heavily diffusion weighted data can be significantly influenced by hemodynamic changes. However, this was not only observed in GM, but also in WM. Moreover, our study demonstrates that the estimator used to calculate the relative changes should be carefully chosen in order to avoid biases at low signal-to-noise ratios (SNRs) which accompany increasing diffusion weighting. With the use of robust estimators, we found no increases in relative change with increasing b-value during both vascular challenges. Our data also demonstrate that FA can be significantly influenced by hemodynamics, both in GM and in WM. The observed influence of diffusion weighting direction on relative signal change in GM was shown to be associated with structural differences among various regions. If diffusion based functional contrasts immune to hemodynamics do exist, our results highlight the difficulty in discerning those diffusion changes from accompanying vascular changes.
在一些最近的研究中，扩散加权功能磁共振成像（DWI-fMRI）已被提出用于提供对血管变化免疫的对比。在神经元激活期间观察到的相对信号变化增加，支持这种对比可能基于扩散的起源。最近的一项扩散张量成像（DTI）研究也报告了使用分数各向异性（FA）来跟踪白质中的激活。在本研究中，我们旨在确定相对较高的扩散加权（b=1200 和 1800 s/mm(2)) 是否可以消除 100% O(2)和碳化氧（95%O(2)+5% CO(2)) 诱导的血管挑战对大鼠脑灰质（GM）和白质（WM）带来的强烈血管影响。我们还旨在表征这些血管变化对 FA 的影响，包括 GM 和 WM 中的 FA。我们的研究证实了之前的报告，即即使是相对较重的扩散加权数据也可能受到血液动力学变化的显著影响。然而，这不仅在 GM 中观察到，在 WM 中也观察到。此外，我们的研究表明，为了避免在高信噪比（SNR）下出现偏倚，在计算相对变化时应谨慎选择所使用的估计器，SNR 伴随着扩散加权的增加而增加。通过使用稳健的估计器，我们发现在两种血管挑战期间，相对变化随 b 值的增加而增加。我们的数据还表明，FA 可受到血液动力学的显著影响，无论是在 GM 还是在 WM 中。在 GM 中观察到的扩散加权方向对相对信号变化的影响与各个区域之间的结构差异有关。如果存在对血液动力学免疫的基于扩散的功能对比，那么我们的结果强调了从伴随的血管变化中辨别这些扩散变化的困难。
