In Vivo Imaging of Glymphatic Circulation: Measurement of Brain Interstitial Fluid Flow During Wake and Sleep with Ultra-High Performance MRI

Topic Area:

This proposal is in response to FY21 PRMRP Topic Area 'Sleep Disorders and Restriction' and, in particular, the Area of Encouragement: 'Research on the effects of disrupted normal sleep and circadian rhythms on the physical and psychological health, safety, performance, and productivity, including sex differences.'

Project Overview:

Over the past 10 years, researchers have discovered a remarkable, albeit not yet fully understood, mechanism for waste removal from the brain, named the glymphatic pathway, due to its similarity with the lymphatic system present in the rest of the human body and involvement of glial cells in the brain. The glymphatic pathway for metabolic waste removal from the brain has been shown to be especially active during sleep and its disruption may be mechanistically linked to sleep deprivation. Furthermore, glymphatic dysfunction has been observed in animal models of traumatic brain injury (TBI) and Alzheimer's disease (AD). Recent animal studies have highlighted that the glymphatic pathway plays a substantial role in the clearance of amyloid plaques, which have linked to the cognitive decline seen in AD patients. Poor sleep quality is a known risk factor for cognitive decline in humans. It has been hypothesized that this risk may be due to reduced glymphatic function in the sleep deprived brain. To date, attempts to image glymphatic circulation have been largely limited to animal models.

Clinical imaging of the glymphatic pathway in humans is a growing field. Previous attempts to image the glymphatic pathway have been limited to either invasive procedures, such as magnetic resonance imaging (MRI) following injection of an off-label contrast agent via a spinal tap to observe flow and clearance of cerebrospinal fluid (CSF) through the brain, or indirect measurements of CSF flow via MRI techniques sensitive to soft tissue or blood oxygenation. No direct measurement of the very slow-flow (~0.01mm/s) of CSF through the brain has ever been attempted in vivo, especially during sleep due to limitations of clinically available MRI scanners, the loud environment in the MRI scanner, and acquisition sequences.

The proposed research seeks to develop a non-invasive quantitative MRI direct measurements of the interstitial slow-flow in the brain via the glymphatic pathway and test the technology in a sleep study of healthy Warfighters and sleepy watch standers or military shift workers. Our team has recently developed a novel flow-imaging technique named Simultaneous Coherent-Incoherent Motion Imaging (SCIMI), which allows for non-invasive measurement of extremely slow fluid flow in the brain. This technique and the proposed study leverage the utilization of a novel, ultra-high performance, prototype 3.0 T MRI brain dedicated scanner (Microstructure Anatomy Gradient for Neuroimaging with Ultrafast Scanning, MAGNUS), which was recently successfully installed at the Walter Reed National Military Medical Center (WRNMMC). In preliminary work using MAGNUS, the team reported bulk flow measurements in the brain as low as 0.02mm/s and have integrated cardiac-cycle synchronization to temporally resolve the pulsatile dynamics of fluid flow during the cardiac cycle.

The MRI environment is often quite loud and while many people fall asleep during an MRI scan, it is difficult to achieve full sleep cycles that include deep sleep. To address this problem, the team proposes to adapt a series of technologies previously developed to make the data acquisition sequences in this study essentially silent with minimal impact to the quality of the data.

Because glymphatic circulation has been shown to vary with sleep stages, being the most active during deep sleep, the team proposes to measure interstitial flow over at least a 3-hour sleep window. Electroencephalogram (EEG) is often used by sleep neurologists to monitor sleep stages. Due to excessive scalp heating concerns, the team has chosen not to use EEG and to develop an MRI-based technique to follow sleep stages in study subjects. In a recent study, functional MRI showed striking similar features to EEG during different sleep stages. The team will adopt this approach and improve on it by also collecting physiological data (respiratory and cardiac cycle).

The team hypothesizes that study subjects with disrupted sleep patterns will demonstrate reduced glymphatic flow especially during deep sleep, and the integration of silent imaging technologies to visualize this flow and correlate to the sleep stage will provide the United States military with a unique device to study effects of sleep disruption in the Warfighter.

Critical Problem Being Addressed:

The primary tools for assessment and management of sleep disorders have been limited to medical history and exam, and the use of decades-old technology such as accelerometry, EEG, and polysomnography. Unfortunately, these physiological biomarkers can be entirely normal despite highly unsatisfying sleep and unsafe daytime symptoms. There is a critical need for development of novel technologies to understand the mechanisms and physiology of restorative vs. non-restorative sleep. The link established in animal models between TBI, neurodegenerative diseases, and the disruption of the glymphatic pathway, as well as the interaction with sleep disorders, make the development of technologies necessary to improve our understanding of the mechanics of interstitial flow in the brain a critical priority for the long-term well-being of United States military personnel and Veterans.

Impact:

The demands of the military operational environment often impose insufficient or non-restorative sleep for the Warfighter, resulting in decreased vigilance and fatigue during daytime that can pose safety risks and threaten mission success. Studies estimate that, on average, over 70% of active-duty Warfighters sleep less than 6 hours per night, and even fewer hours while deployed. Furthermore, over 70% of Warfighters with TBI are reported to also present with sleep disorders, which possibly precipitate other TBI-related symptoms such as post-traumatic headache, mood disorder, and chronic pain. Due to the lack of targeted therapies, despite the known risks and side effects, a significant proportion of the Warfighter population becomes chronically dependent upon stimulants and sedatives to help achieve wakefulness or sleep, ultimately with minimal utility and benefit.

The successful completion of the proposed study will demonstrate for the first time in vivo a direct, non-invasive measurement of glymphatic function in relationship to sleep and compare interstitial flow in the brain between populations of good sleepers and sleep-deprived subjects. It will provide an invaluable set of technologies to address questions about properties of restorative sleep in military and civilian populations.

This research stands to answer important questions about the underlying reasons why sleep is necessary and may provide insight into mechanisms that can better stratify and treat the Warfighter who is not performing optimally. Upon successful completion of this project, the team will be well positioned to address critical, military-relevant sleep and human performance knowledge gaps, such as questions about improving sleep efficiency, targets for intervention for sleep disorders, and imaging-based readouts for therapy effectiveness in the Warfighter.