Baz Guy, Schmidt Rita
Weizmann Institute of Science, Department of Brain Sciences, Rehovot, Israel.
The Azrieli National Institute for Human Brain Imaging and Research, Weizmann Institute of Science, Rehovot, Israel.
Imaging Neurosci (Camb). 2024 Oct 10;2. doi: 10.1162/imag_a_00309. eCollection 2024.
An increasing number of human fMRI studies aim to discern the time delays between evoked responses under different stimuli conditions in different brain regions. To achieve that, a primary goal is to acquire fMRI data with high sampling rates. This task is now possible with ultra-high field (≥7 T) MRI and the advancement of imaging acceleration methods. Consequently, it becomes imperative to understand what is the actual or effective temporal resolution (ETR) that is realized in given settings of an fMRI experiment. In this study, we utilized a dynamic phantom to reliably repeat a set of scans, generating a "ground truth" signal with controllable onset delays mimicking fMRI responses in a task-based block-designed fMRI. Here, we define the ETR and quantify a scan's ETR using the dynamic phantom. The quantification was performed for various scanning parameters, including echo time (TE), repetition time (TR), voxel size, and contrast-to-noise ratio (CNR). We further show that combining data from multi-echo EPI can improve the ETR (i.e., reduce it). In addition, parameters of the fMRI paradigm were examined, including the blocks' length and density. As tissue properties (e.g., level of iron deposition) affect the CNR and thus change the ETR, we examined the signal rise mimicking not only the cortex, but also the basal ganglia (known for its high iron deposition). Combining multi-echo data, the estimated ETR for the examined scans was 151 ms for a cortex-mimicking setup and 248 ms for a basal ganglia-mimicking setup, when scanning with a sampling time (i.e., TR) of 600 ms. Yet, a substantial penalty was paid when the CNR was low, in which case the ETR was even larger than the TR. A feasibility set of experiments was also designed to evaluate how the ETR is affected by physiological signal fluctuations and the variability of the hemodynamic response. This study shows the viability of studying time responses with fMRI, by demonstrating that a very short ETR can be achieved. However, it also emphasizes the need to examine the attainable ETR for a particular experiment.
越来越多的人类功能磁共振成像（fMRI）研究旨在辨别不同脑区在不同刺激条件下诱发反应之间的时间延迟。为实现这一目标，首要任务是获取高采样率的fMRI数据。借助超高场（≥7T）磁共振成像（MRI）以及成像加速方法的进步，如今这一任务已成为可能。因此，了解在fMRI实验的特定设置中实际实现的或有效的时间分辨率（ETR）变得至关重要。在本研究中，我们使用动态体模可靠地重复一组扫描，生成具有可控起始延迟的“真实信号”，模拟基于任务的块设计fMRI中的fMRI反应。在此，我们定义ETR并使用动态体模对扫描的ETR进行量化。针对各种扫描参数进行了量化，包括回波时间（TE）、重复时间（TR）、体素大小和对比度噪声比（CNR）。我们进一步表明，组合多回波平面回波成像（EPI）的数据可以改善ETR（即降低ETR）。此外，还研究了fMRI范式的参数，包括块的长度和密度。由于组织特性（如铁沉积水平）会影响CNR，从而改变ETR，我们不仅模拟了皮层，还模拟了以高铁沉积著称的基底神经节的信号上升情况。当以600ms的采样时间（即TR）进行扫描时，对于模拟皮层的设置，组合多回波数据后，所检查扫描的估计ETR为151ms，对于模拟基底神经节的设置为248ms。然而，当CNR较低时会付出巨大代价，在这种情况下ETR甚至大于TR。还设计了一组可行性实验来评估ETR如何受到生理信号波动和血流动力学反应变异性的影响。本研究通过证明可以实现非常短的ETR，展示了用fMRI研究时间反应的可行性。然而，它也强调了针对特定实验检查可达到的ETR的必要性。
