Murphy Kevin, Bodurka Jerzy, Bandettini Peter A
Section on Functional Imaging Methods, National Institute of Mental Health, NIH, Bethesda, MD 20892-1148, USA.
Neuroimage. 2007 Jan 15;34(2):565-74. doi: 10.1016/j.neuroimage.2006.09.032. Epub 2006 Nov 22.
Recent advances in MRI receiver and coil technologies have significantly improved image signal-to-noise ratios (SNR) and thus temporal SNR (TSNR). These gains in SNR and TSNR have allowed the detection of fMRI signal changes at higher spatial resolution and therefore have increased the potential to localize small brain structures such as cortical layers and columns. The majority of current fMRI processing strategies employ multi-subject averaging and therefore require spatial smoothing and normalization, effectively negating these gains in spatial resolution higher than about 10 mm3. Reliable detection of activation in single subjects at high resolution is becoming a more common desire among fMRI researchers who are interested in comparing individuals rather than populations. Since TSNR decreases with voxel volume, detection of activation at higher resolutions requires longer scan durations. The relationship between TSNR, voxel volume and detectability is highly non-linear. In this study, the relationship between TSNR and the necessary fMRI scan duration required to obtain significant results at varying P values is determined both experimentally and theoretically. The results demonstrate that, with a TSNR of 50, detection of activation of above 2% requires at most 350 scan volumes (when steps are taken to remove the influence of physiological noise from the data). Importantly, these results also demonstrate that, for activation magnitude on the order of 1%, the scan duration required is more sensitive to the TSNR level than at 2%. This study showed that with voxel volumes of approximately 10 mm3 at 3 T, and a corresponding TSNR of approximately 50, the required number of time points that guarantees detection of signal changes of 1% is about 860, but if TSNR increases by only 20%, the time for detection decreases by more than 30%. More than just being an exercise in numbers, these results imply that imaging of columnar resolution (effect size=1% and assuming a TR of 1 s) at 3 T will require either 10 min for a TSNR of 60 or 40 min for a TSNR of 30. The implication is that at these resolutions, TSNR is likely to be critical for determining success or failure of an experiment.
磁共振成像（MRI）接收器和线圈技术的最新进展显著提高了图像的信噪比（SNR），进而提高了时间信噪比（TSNR）。SNR和TSNR的这些提升使得能够在更高的空间分辨率下检测功能磁共振成像（fMRI）信号变化，因此增加了定位诸如皮质层和柱状结构等小脑结构的可能性。当前大多数fMRI处理策略采用多受试者平均法，因此需要进行空间平滑和归一化，这实际上抵消了高于约10立方毫米的空间分辨率提升。在高分辨率下可靠地检测单个受试者的激活情况，正成为对个体而非群体进行比较感兴趣的fMRI研究人员越来越普遍的愿望。由于TSNR随体素体积减小，在更高分辨率下检测激活需要更长的扫描持续时间。TSNR、体素体积和可检测性之间的关系高度非线性。在本研究中，通过实验和理论确定了TSNR与在不同P值下获得显著结果所需的fMRI扫描持续时间之间的关系。结果表明，当TSNR为50时，检测2%以上的激活最多需要350个扫描体积（当采取措施消除数据中生理噪声的影响时）。重要的是，这些结果还表明，对于量级为1%的激活，所需的扫描持续时间对TSNR水平比2%时更敏感。该研究表明，在3 T时，体素体积约为10立方毫米，相应的TSNR约为50，保证检测到1%信号变化所需的时间点数约为860，但如果TSNR仅增加20%，检测时间将减少超过30%。这些结果不仅仅是数字上的运算，它们意味着在3 T时进行柱状分辨率成像（效应大小 = 1%，假设重复时间为1 s），对于TSNR为60需要10分钟，对于TSNR为30则需要40分钟。这意味着在这些分辨率下，TSNR可能是决定实验成败的关键因素。
