Study Group Clinical fMRI, Department of Neurology, Medical University of Vienna , Vienna , Austria ; High Field Magnetic Resonance Imaging Center of Excellence, Medical University of Vienna , Vienna , Austria.
Front Hum Neurosci. 2013 Aug 9;7:474. doi: 10.3389/fnhum.2013.00474. eCollection 2013.
In functional MRI it is desirable for the blood-oxygenation level dependent (BOLD) signal to be localized to the tissue containing activated neurons rather than the veins draining that tissue. This study addresses the dependence of the specificity of the BOLD signal - the relative contribution of the BOLD signal arising from tissue compared to venous vessels - on magnetic field strength. To date, studies of specificity have been based on models or indirect measures of BOLD sensitivity such as signal to noise ratio and relaxation rates, and assessment has been made in isolated vein and tissue voxels. The consensus has been that ultra-high field systems not only significantly increase BOLD sensitivity but also specificity, that is, there is a proportionately reduced signal contribution from draining veins. Specificity was not quantified in prior studies, however, due to the difficulty of establishing a reliable network of veins in the activated volume. In this study we use a map of venous vessel networks extracted from 7 T high resolution Susceptibility-Weighted Images to quantify the relative contributions of micro- and macro-vasculature to functional MRI results obtained at 3 and 7 T. High resolution measurements made here minimize the contribution of physiological noise and Independent Component Analysis (ICA) is used to separate activation from technical, physiological, and motion artifacts. ICA also avoids the possibility of timing-dependent bias from different micro- and macro-vasculature responses. We find a significant increase in the number of activated voxels at 7 T in both the veins and the microvasculature - a BOLD sensitivity increase - with the increase in the microvasculature being higher. However, the small increase in sensitivity at 7 T was not significant. For the experimental conditions of this study, our findings do not support the hypothesis of an increased specificity of the BOLD response at ultra-high field.
在功能磁共振成像（fMRI）中，希望血氧水平依赖（BOLD）信号定位于包含激活神经元的组织，而不是引流该组织的静脉。本研究探讨了 BOLD 信号特异性（来自组织的 BOLD 信号相对于静脉血管的相对贡献）与磁场强度的依赖性。迄今为止，特异性研究基于模型或 BOLD 灵敏度的间接测量，如信噪比和弛豫率，并且评估是在孤立的静脉和组织体素中进行的。共识是，超高场系统不仅显著提高了 BOLD 灵敏度，而且还提高了特异性，即引流静脉的信号贡献成比例降低。然而，由于在激活体积中建立可靠的静脉网络具有一定难度，因此在之前的研究中并未对特异性进行量化。在这项研究中，我们使用从 7T 高分辨率磁化率加权图像中提取的静脉网络图谱来量化微血管和大血管对在 3T 和 7T 获得的功能磁共振成像结果的相对贡献。这里进行的高分辨率测量最大限度地减少了生理噪声的贡献，并且使用独立成分分析（ICA）来将激活与技术、生理和运动伪影分开。ICA 还避免了来自不同微血管和大血管反应的定时相关偏差的可能性。我们发现，在 7T 下，静脉和微血管中的激活体素数量都显著增加 - 这是 BOLD 灵敏度的增加 - 微血管的增加更高。然而，7T 时灵敏度的微小增加并不显著。对于本研究的实验条件，我们的发现不支持超高场 BOLD 响应特异性增加的假设。
