■我们探讨了使用时域(TD)和连续波(CW)功能近红外光谱(fNIRS)监测人类静息状态活动期间脑血流动力学振荡的可行性,这种现象在科学和医学界引起了越来越多的兴趣,并且似乎对于促进对健康和病理性大脑功能的理解至关重要。
■我们的总目标是最大程度地提高fNIRS对大脑静息状态振荡的敏感性。更具体地说,我们的目标是定义优化fNIRS测量[平均光子计数率,测量长度,采样频率,和源探测器距离(SSD)]。此外,我们比较了TD和CWfNIRS在振荡检测和定位方面的性能。
■通过利用被探测介质的两种不同几何形状的扩散方程的解来产生一系列合成的TD和CWfNIRS信号:均匀介质和双层介质。在培养基中施加了氧合血红蛋白和脱氧血红蛋白浓度的已知和周期性扰动,确定其光学性质的变化。使用均匀平板模型来确定多个测量参数对fNIRS对振荡现象的灵敏度的影响,并使用双层模型来评估和比较TD和CWfNIRS在检测和隔离不同深度发生的振荡方面的能力。对于TDfNIRS,评估了两种增强深度选择性的方法:第一,执行了飞行时间的光子分布的时间窗口,然后,使用时间依赖性平均部分路径长度(TMPP)方法检索培养基中的血红蛋白浓度.
■在均匀介质情况下,TD和CWfNIRS对光学特性周期性扰动的灵敏度与平均光子计数率成比例地增加,测量长度,和采样频率,并与SSD的平方近似。在双层介质情况下,时间窗方法可以检测并正确定位TDfNIRS信号中振荡分量的存在,即使存在非常低的光子计数率。TMPP方法演示了如何从单个SSD处获取的TDfNIRS信号中正确检索不同深度处血红蛋白的周期性变化。对于CWfNIRS,在用于短分离通道回归的典型SSD上进行的测量显示出对深层中发生的振荡的显着敏感性,当重点是分析振荡现象时,挑战了这种校正方法的基本假设。
■我们证明了TDfNIRS技术允许使用单个SSD采集来检测和深度定位探测介质中血红蛋白浓度的周期性波动,提供多距离CWfNIRS设置的替代方案。此外,我们提供了一些有价值的指南,可以帮助研究者确定fNIRS研究的最佳实验方案.
UNASSIGNED: We explore the feasibility of using time-domain (TD) and continuous-wave (CW) functional near-infrared spectroscopy (fNIRS) to monitor brain hemodynamic oscillations during resting-state activity in humans, a phenomenon that is of increasing interest in the scientific and medical community and appears to be crucial to advancing the understanding of both healthy and pathological brain functioning.
UNASSIGNED: Our general object is to maximize fNIRS sensitivity to brain resting-state oscillations. More specifically, we aim to define comprehensive guidelines for optimizing main operational parameters in fNIRS measurements [average photon count rate, measurement length, sampling frequency, and source-detector distance (SSD)]. In addition, we compare TD and CW fNIRS performance for the detection and localization of oscillations.
UNASSIGNED: A series of synthetic TD and CW fNIRS signals were generated by exploiting the solution of the diffusion equation for two different geometries of the probed medium: a homogeneous medium and a bilayer medium. Known and periodical perturbations of the concentrations of oxy- and deoxy-hemoglobin were imposed in the medium, determining changes in its optical properties. The homogeneous slab model was used to determine the effect of multiple measurement parameters on fNIRS sensitivity to oscillatory phenomena, and the bilayer model was used to evaluate and compare the abilities of TD and CW fNIRS in detecting and isolating oscillations occurring at different depths. For TD fNIRS, two approaches to enhance depth-selectivity were evaluated: first, a time-windowing of the photon distribution of time-of-flight was performed, and then, the time-dependent mean partial pathlength (TMPP) method was used to retrieve the hemoglobin concentrations in the medium.
UNASSIGNED: In the homogeneous medium case, the sensitivity of TD and CW fNIRS to periodical perturbations of the optical properties increases proportionally with the average photon count rate, the measurement length, and the sampling frequency and approximatively with the square of the SSD. In the bilayer medium case, the time-windowing method can detect and correctly localize the presence of oscillatory components in the TD fNIRS signal, even in the presence of very low photon count rates. The TMPP method demonstrates how to correctly retrieve the periodical variation of hemoglobin at different depths from the TD fNIRS signal acquired at a single SSD. For CW fNIRS, measurements taken at typical SSDs used for short-separation channel regression show notable sensitivity to oscillations occurring in the deep layer, challenging the assumptions underlying this correction method when the focus is on analyzing oscillatory phenomena.
UNASSIGNED: We demonstrated that the TD fNIRS technique allows for the detection and depth-localization of periodical fluctuations of the hemoglobin concentrations within the probed medium using an acquisition at a single SSD, offering an alternative to multi-distance CW fNIRS setups. Moreover, we offered some valuable guidelines that can assist researchers in defining optimal experimental protocols for fNIRS studies.