The glymphatic system is a physiological mechanism that removes neurotoxic proteins from the brain through cerebrospinal fluid [1]. It is primarily activated during sleep and crucial to prevent the neurological disorders [2]. The diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) index is a non-invasive method for assessing the glymphatic system [3]. Previous studies on the DTI-ALPS index reported that the reduction of DTI-ALPS index was related to the malfunction of the glymphatic system. However, none of them showed the activity of the glymphatic system during sleep through the DTI-ALPS index. Thus, this study aims to evaluate the glymphatic system from healthy participants during sleep using the DTI-ALPS index. This study was approved by the Institutional Review Board of Yonsei University MIRAE campus (1041849-202305-BM-089-100). This study excluded participants who had neurological disorders, worked night shifts, or smoked more than five cigarettes a day. All participants completed the Pittsburgh Sleep Quality Index, and those with consistent sleep habits and sleep efficiency over 85% were selected. Twelve participants underwent sleep deprivation for 24 hours and took 10 mg of Zolpidem 16.7 ± 6.99 minutes before the first diffusion tensor imaging (DTI) scans (male, age = 24.44 ± 2.5 years, sleep quality = 92.04%) [4]. The first DTI scan in the sleep state was acquired 13 minutes after the first DTI scan in the awake state. Electroencephalography (EEG) compatible with magnetic resonance scanner was used to identify the sleep states of participants. Susceptibility-weighted imaging was obtained to identify the perivascular space location in the medullary vein (three-dimensional gradient-recalled echo sequence, TR = 28 ms, TE = 20 ms, flip angle = 15°, matrix size = 269*269, and slice thickness=2 mm). All DTI scans were obtained once while participants were awake (21:57 ± 5:58 minutes) and sixteen times during their sleep (22:24 ± 00:13 to 00:09 ± 00:13 minutes). We had 90-seconds intervals between the DTI scans to obtain the EEG data without the gradient artifact to identify the sleep states. Sleep states of participants were identified by three EEG experts with over ten years of experience. The DTI-ALPS index was analyzed from seven participants who maintained stable sleep states for two hours during the DTI scans (age=24.57±3.15, sleep quality=90.96%). To investigate the differences between awake and sleep states, we compared these conditions using the Mann-Whitney U test. It showed a significant difference between awake and sleep states in the DTI-ALPS index ( p < 0.003). The Friedman test and Dunn-Bonferroni test were conducted to evaluate the significant differences between these states. Significant differences between the awake and sleep state were observed at 32, 52, and 104 minutes (adjust-p < 0.05). The fluctuation of the DTI-ALPS index during sleep has strong negative correlation with sleep quality (r = -0.77). This study is crucial for evaluating the glymphatic system during sleep using the DTI-ALPS index. Our results provide three major findings: (1) glymphatic system activity increases during sleep compared to the awake state, as indicated by the DTI-ALPS index; (2) the glymphatic system function exhibits time-dependent changes throughout sleep; and (3) It is a strong correlation between sleep quality and glymphatic activity. These findings support that the glymphatic system is primarily active during sleep and confirm the effectiveness of the DTI-ALPS index to evaluate glymphatic function during sleep.

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20214000000070).

This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2020R1A2C1015179)

Author