Exploring the Link Between Intravoxel Incoherent Motion Measured Brain Diffusivity During Wakefulness and Sleep Macrostructure in the Elderly.

Intravoxel incoherent motion (IVIM) is a diffusion-weighted magnetic resonance imaging (MRI) method that models slow (D, tissue diffusivity) and fast (D*, microvascular perfusion) signal components (f, signal fraction). This observational study explores the relationship between IVIM metrics and sleep macrostructure and physical activity levels in nineteen healthy elderly individuals (mean age 72.2±5.0). Participants underwent two MRI sessions during wakefulness, four weeks apart, on a GE 3T Premier scanner. IVIM parameters (D, D*, f and their product fD*) were calculated using non-linear least-squares and a deep learning transformer-based estimator and analysed across seven regions of interest. Sleep was recorded at home on 10-14 consecutive nights, following each MRI scan, using a portable electroencephalography (EEG) system. Physical activity was recorded in parallel with actigraphy. Associations were tested using linear mixed effects models, with activity included in sensitivity analyses. Better EEG-derived sleep quality and higher physical activity were predominantly linked to lower estimated values for IVIM fD*. These findings may reflect differences in microvascular perfusion and/or the cerebrospinal fluid compartment associated with sleep and warrant further investigation of the mechanisms linking IVIM metrics to sleep quality. Clinicaltrials.gov: NCT05539378, trial registry name: Exploring the Link Between Sleep and Brain Clearance (BBCDS).