Yacoub Essa, Uludağ Kâmil, Uğurbil Kâmil, Harel Noam
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, MN 55455, USA.
Magn Reson Imaging. 2008 Sep;26(7):889-96. doi: 10.1016/j.mri.2008.01.046. Epub 2008 May 16.
Recent studies in the human visual cortex using diffusion-weighted functional magnetic resonance imaging (fMRI) have suggested that the apparent diffusion coefficient (ADC) decreases, in contrast to earlier studies that consistently reported ADC increases during neuronal activation. The changes, in either case, are hypothesized to provide the ability to improve the spatial specificity of fMRI over conventional blood-oxygenation-level-dependent (BOLD) methods. Most recently, the ADC decreases have been suggested as originating from transient cell swelling caused by either shrinkage of the extracellular space or some intracellular neuronal process that precedes the hemodynamic response. All of these studies have been conducted in humans and at lower magnetic fields, which can be limited by the signal-to-noise ratio (SNR). The low SNR can lead to significant partial-volume effects because of the lower spatial resolutions required to attain sufficient SNR in diffusion-weighted images. Human studies also have the potential confound of motion. At high magnetic fields and in animal model studies, these limitations are alleviated. At high fields, SNR increases, tissue signals are enhanced and signal changes inside the blood are significantly reduced compared to lower fields. In this work, we were able to measure a small but significant ADC decrease in tissue areas, in conjunction with brain activation in the cat visual cortex at 9.4 T when using highly diffusion-weighted images (b>1200 s/mm2) where intravascular effects are minimal. When using low b-values, delayed increases in the tissue ADC during activation were observed. No significant changes in ADC were observed in surface vessels for any diffusion weighting. Furthermore, we did not observe any temporal differences in the highly diffusion-weighted data compared to BOLD; however, although the changes may likely be vascular in nature, they are highly localized to the tissue areas.
最近利用扩散加权功能磁共振成像（fMRI）对人类视觉皮层进行的研究表明，表观扩散系数（ADC）降低，这与早期一致报道神经元激活期间ADC增加的研究相反。在这两种情况下，这些变化都被假定为能够比传统的血氧水平依赖（BOLD）方法提高fMRI的空间特异性。最近，有人提出ADC降低是由于细胞外空间收缩或血流动力学反应之前的一些细胞内神经元过程导致的短暂细胞肿胀所致。所有这些研究都是在人类身上以及较低磁场下进行的，这可能受到信噪比（SNR）的限制。低信噪比可能导致显著的部分容积效应，因为在扩散加权图像中获得足够信噪比所需的空间分辨率较低。人体研究还存在运动这一潜在干扰因素。在高磁场和动物模型研究中，这些限制得到了缓解。在高磁场下，信噪比增加，组织信号增强，与低磁场相比，血液内部的信号变化显著减少。在这项工作中，当使用血管内效应最小的高扩散加权图像（b>1200 s/mm2）时，我们能够在9.4 T的猫视觉皮层中测量到组织区域内ADC有微小但显著的降低，同时伴有大脑激活。当使用低b值时，观察到激活期间组织ADC延迟增加。对于任何扩散加权，在表面血管中均未观察到ADC有显著变化。此外，与BOLD相比，我们在高扩散加权数据中未观察到任何时间差异；然而，尽管这些变化可能本质上是血管性的，但它们高度局限于组织区域。
