Department of Diagnostic Physics, Division of Radiology and Nuclear Medicine, Oslo University Hospital, Oslo, Norway; Department of Psychology, University of Oslo, Oslo, Norway; Norwegian Centre for Mental Disorders Research (NORMENT), Oslo University Hospital, Oslo, Norway.
Department of Psychology, University of Oslo, Oslo, Norway; Norwegian Centre for Mental Disorders Research (NORMENT), Oslo University Hospital, Oslo, Norway.
Neuroimage. 2020 May 15;212:116682. doi: 10.1016/j.neuroimage.2020.116682. Epub 2020 Feb 28.
Recently, several magnetic resonance imaging (MRI) studies have reported time-of-day effects on brain structure and function. Due to the possibility that time-of-day effects reflect mechanisms of circadian regulation, the aim of this prospective study was to assess these effects while under strict experimental control of variables that might influence biological clocks, such as caffeine intake and exposure to blue-emitting light. In addition, the current study assessed whether time-of-day effects were driven by changes to extracellular space, by including estimations of non-Gaussian diffusion metrics obtained from diffusion kurtosis imaging, white matter tract integrity and the spherical mean technique, in addition to conventional diffusion tensor imaging -derived parameters. Participants were 47 healthy adults who underwent diffusion-weighted imaging in the morning and evening of the same day. Morning and evening scans were compared using voxel-wise tract based spatial statistics and permutation testing. A day of wakefulness was associated with widespread increases in fractional anisotropy, indices of kurtosis and indices of the axonal water fraction. In addition, wakefulness was associated with widespread decreases in radial diffusivity, both in the single compartment and in extra-axonal space. These results suggest that an increase in the intra-axonal space relative to the extra-axonal volume underlies time-of-day effects in human white matter, which is in line with activity-induced reductions to the extracellular volume. These findings provide important insight into possible mechanisms driving time-of-day effects in MRI.
最近，几项磁共振成像（MRI）研究报告了时间对大脑结构和功能的影响。由于时间效应可能反映了生物钟调节的机制，本前瞻性研究的目的是在严格控制可能影响生物钟的变量（如咖啡因摄入和暴露于蓝光）的实验条件下评估这些效应。此外，本研究还评估了时间效应是否是由细胞外空间的变化驱动的，包括从扩散峰度成像中获得的非高斯扩散指标的估计、白质束完整性和球平均技术，以及传统的扩散张量成像衍生参数。47 名健康成年人在同一天的上午和晚上接受了弥散加权成像。使用基于体素的束空间统计学和置换检验比较了早晨和晚上的扫描。一天的清醒状态与各向异性分数、峰度指数和轴突水分数指数的广泛增加有关。此外，清醒状态与径向扩散率的广泛降低有关，无论是在单室还是在轴外空间。这些结果表明，在人类白质中，与细胞外体积相比，轴内空间的增加是时间效应的基础，这与活动诱导的细胞外体积减少一致。这些发现为 MRI 中时间效应的可能机制提供了重要的见解。
