van Horen T W P, Siero J C W, Bhogal A A, Petridou N, Báez-Yáñez M G
Department of Radiology, Centre for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.
Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.
bioRxiv. 2023 Sep 15:2023.09.15.557938. doi: 10.1101/2023.09.15.557938.
A spatially specific fMRI acquisition requires specificity to the microvasculature that serves active neuronal sites. Macrovascular contributions will reduce the microvascular specificity but can be reduced by using spin echo (SE) sequences that use a π pulse to refocus static field inhomogeneities near large veins. The microvascular specificity of a SE-echo planar imaging (SE-EPI) scan depends on the echo train length (ETL)-duration, but the dependence is not well-characterized in humans at 7T. To determine how microvascular-specific SE-EPI BOLD is in humans at 7T, we developed a Monte Carlo voxel model that computes the signal of a proton ensemble residing in a vasculature subjected to a SE-EPI pulse sequence. We characterized the ETL-duration dependence of the microvascular specificity by simulating the BOLD signal as a function of ETL, the range adhering to experimentally realistic readouts. We performed a validation experiment for our simulation observations, in which we acquired a set of SE-EPI BOLD time series with varying ETL during a hyperoxic gas challenge. Both our simulations and measurements show an increase in macrovascular contamination as a function of ETL, with an increase of 30% according to our simulation and 60% according to our validation experiment between the shortest and longest ETL durations (23.1 - 49.7 ms). We conclude that the microvascular specificity decreases heavily with increasing ETL-durations. We recommend reducing the ETL-duration as much as possible to minimize macrovascular contamination in SE-EPI BOLD experiments. We additionally recommend scanning at high resolutions to minimize partial volume effects with CSF. CSF voxels show a large BOLD response, which can be attributed to both the presence of large veins (high blood volume) and molecular oxygen-induced -shortening (significant in a hyperoxia experiment). The magnified BOLD signal in a GM-CSF partial volume voxel reduces the desired microvascular specificity and, therefore, will hinder the interpretation of functional MRI activation patterns.
空间特异性功能磁共振成像（fMRI）采集需要针对为活跃神经元位点供血的微血管具有特异性。大血管的影响会降低微血管特异性，但可通过使用自旋回波（SE）序列来减少，该序列利用π脉冲使大静脉附近的静态场不均匀性重新聚焦。SE - 回波平面成像（SE - EPI）扫描的微血管特异性取决于回波链长度（ETL）持续时间，但在7T场强下人体中的这种依赖性尚未得到充分表征。为了确定在7T场强下人体中微血管特异性SE - EPI血氧水平依赖性功能磁共振成像（BOLD）的情况，我们开发了一个蒙特卡罗体素模型，该模型可计算处于SE - EPI脉冲序列作用下的脉管系统中质子系综的信号。我们通过模拟作为ETL函数的BOLD信号来表征微血管特异性对ETL持续时间的依赖性，该范围符合实验实际读出情况。我们对模拟观察结果进行了验证实验，在该实验中，我们在高氧气体激发期间采集了一组具有不同ETL的SE - EPI BOLD时间序列。我们的模拟和测量结果均显示，大血管污染随ETL增加而增加，根据我们的模拟，在最短和最长ETL持续时间（23.1 - 49.7毫秒）之间增加了30%，根据我们的验证实验则增加了60%。我们得出结论，微血管特异性会随着ETL持续时间的增加而大幅降低。我们建议在SE - EPI BOLD实验中尽可能缩短ETL持续时间，以尽量减少大血管污染。我们还建议以高分辨率进行扫描，以尽量减少脑脊液（CSF）的部分容积效应。脑脊液体素显示出较大的BOLD反应，这可归因于大静脉的存在（高血容量）和分子氧诱导的缩短效应（在高氧实验中很显著）。灰质 - 脑脊液部分容积体素中放大的BOLD信号会降低所需的微血管特异性，因此会妨碍对功能磁共振成像激活模式的解读。
