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Perivascular spaces include a variety of passageways around arterioles, capillaries and venules in the brain, along which a range of substances can move. Although perivascular spaces were first identified over 150 years ago, they have come to prominence recently owing to advances in knowledge of their roles in clearance of interstitial fluid and waste from the brain, particularly during sleep, and in the pathogenesis of small vessel disease, Alzheimer disease and other neurodegenerative and inflammatory disorders. Experimental advances have facilitated in vivo studies of perivascular space function in intact rodent models during wakefulness and sleep, and MRI in humans has enabled perivascular space morphology to be related to cognitive function, vascular risk factors, vascular and neurodegenerative brain lesions, sleep patterns and cerebral haemodynamics. Many questions about perivascular spaces remain, but what is now clear is that normal perivascular space function is important for maintaining brain health. Here, we review perivascular space anatomy, physiology and pathology, particularly as seen with MRI in humans, and consider translation from models to humans to highlight knowns, unknowns, controversies and clinical relevance.In this Review, Wardlaw et al. discuss the anatomy, physiology and pathology of perivascular spaces, particularly as seen with MRI in humans, and consider translation from models to humans to highlight knowns, unknowns, controversies and clinical relevance.

Flow of cerebrospinal fluid (CSF) through perivascular spaces (PVSs) in the brain is important for clearance of metabolic waste. Arterial pulsations are thought to drive flow, but this has never been quantitatively shown. We used particle tracking to quantify CSF flow velocities in PVSs of live mice. CSF flow is pulsatile and driven primarily by the cardiac cycle. The speed of the arterial wall matches that of the CSF, suggesting arterial wall motion is the principal driving mechanism, via a process known as perivascular pumping. Increasing blood pressure leaves the artery diameter unchanged but changes the pulsations of the arterial wall, increasing backflow and thereby reducing net flow in the PVS. Perfusion-fixation alters the normal flow direction and causes a 10-fold reduction in PVS size. We conclude that particle tracking velocimetry enables the study of CSF flow in unprecedented detail and that studying the PVS in vivo avoids fixation artifacts. Arterial pulsations are thought to drive CSF flow through perivascular spaces (PVSs), but this has never been quantitatively shown. Using particle tracking to quantify CSF flow velocities in PVSs of live mice, the authors show that flow speeds match the instantaneous speeds of the pulsing artery walls that form the inner boundaries of the PVSs.

INTRODUCTION Although glymphatic function is involved in Alzheimer's disease (AD), its potential for predicting the pathological and clinical progression of AD and its sequential association with core AD biomarkers is poorly understood. METHODS Whole‐brain glymphatic activity was measured by diffusion tensor image analysis along the perivascular space (DTI‐ALPS) in participants with AD dementia (n = 47), mild cognitive impairment (MCI; n = 137), and normal controls (n = 235) from the Alzheimer's Disease Neuroimaging Initiative. RESULTS ALPS index was significantly lower in AD dementia than in MCI or controls. Lower ALPS index was significantly associated with faster changes in amyloid positron emission tomography (PET) burden and AD signature region of interest volume, higher risk of amyloid‐positive transition and clinical progression, and faster rates of amyloid‐ and neurodegeneration‐related cognitive decline. Furthermore, the associations of the ALPS index with cognitive decline were fully mediated by amyloid PET and brain atrophy. DISCUSSION Glymphatic failure may precede amyloid pathology, and predicts amyloid deposition, neurodegeneration, and clinical progression in AD. Highlights The analysis along the perivascular space (ALPS) index is reduced in patients with Alzheimer's disease (AD) dementia, prodromal AD, and preclinical AD. Lower ALPS index predicted accelerated amyloid beta (Aβ) positron emission tomography (PET) burden and Aβ‐positive transition. The decrease in the ALPS index occurs before cerebrospinal fluid Aβ42 reaches the positive threshold. ALPS index predicted brain atrophy, clinical progression, and cognitive decline. Aβ PET and brain atrophy mediated the link of ALPS index with cognitive decline.

The glymphatic system is a fluid transport network of cerebrospinal fluid (CSF) entering the brain along arterial perivascular spaces, exchanging with interstitial fluid (ISF), ultimately establishing directional clearance of interstitial solutes. CSF transport is facilitated by the expression of aquaporin-4 (AQP4) water channels on the perivascular endfeet of astrocytes. Mice with genetic deletion of AQP4 (AQP4 KO) exhibit abnormalities in the brain structure and molecular water transport. Yet, no studies have systematically examined how these abnormalities in structure and water transport correlate with glymphatic function. Here, we used high-resolution 3D magnetic resonance (MR) non-contrast cisternography, diffusion-weighted MR imaging (MR-DWI) along with intravoxel-incoherent motion (IVIM) DWI, while evaluating glymphatic function using a standard dynamic contrast-enhanced MR imaging to better understand how water transport and glymphatic function is disrupted after genetic deletion of AQP4. AQP4 KO mice had larger interstitial spaces and total brain volumes resulting in higher water content and reduced CSF space volumes, despite similar CSF production rates and vascular density compared to wildtype mice. The larger interstitial fluid volume likely resulted in increased slow but not fast MR diffusion measures and coincided with reduced glymphatic influx. This markedly altered brain fluid transport in AQP4 KO mice may result from a reduction in glymphatic clearance, leading to enlargement and stagnation of fluid in the interstitial space. Overall, diffusion MR is a useful tool to evaluate glymphatic function and may serve as valuable translational biomarker to study glymphatics in human disease.

Meningeal lymphatic vessels have been described in animal studies, but limited comparable data is available in human studies. Here we show dural lymphatic structures along the dural venous sinuses in dorsal regions and along cranial nerves in the ventral regions in the human brain. 3D T2-Fluid Attenuated Inversion Recovery magnetic resonance imaging relies on internal signals of protein rich lymphatic fluid rather than contrast media and is used in the present study to visualize the major human dural lymphatic structures. Moreover we detect direct connections between lymphatic fluid channels along the cranial nerves and vascular structures and the cervical lymph nodes. We also identify age-related cervical lymph node atrophy and thickening of lymphatics channels in both dorsal and ventral regions, findings which reflect the reduced lymphatic output of the aged brain. Studies in animal models have visualized drainage of interstitial or cerebrospinal fluid via lymphatic vessels, but there is limited data on in humans. Here, the authors non-invasively visualize lymphatic structures in the human brain, including evidence of lymphatic flow from cranial nerves to cervical lymph nodes, and differences by age and sex, without use of contrast agents.

There is currently a lack of non-invasive tools to assess water transport in healthy and pathological brain tissue. Aquaporin-4 (AQP4) water channels are central to many water transport mechanisms, and emerging evidence also suggests that AQP4 plays a key role in amyloid-β (Aβ) clearance, possibly via the glymphatic system. Here, we present the first non-invasive technique sensitive to AQP4 channels polarised at the blood-brain interface (BBI). We apply a multiple echo time (multi-TE) arterial spin labelling (ASL) MRI technique to the mouse brain to assess BBI water permeability via calculation of the exchange time (Texw), the time for magnetically labelled intravascular water to exchange across the BBI. We observed a 31% increase in exchange time in AQP4-deficient (Aqp4−/−) mice (452 ± 90 ms) compared to their wild-type counterparts (343 ± 91 ms) (p = 0.01), demonstrating the sensitivity of the technique to the lack of AQP4 water channels. More established, quantitative MRI parameters: arterial transit time (δa), cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) detected no significant changes with the removal of AQP4. This clinically relevant tool may be crucial to better understand the role of AQP4 in water transport across the BBI, as well as clearance of proteins in neurodegenerative conditions such as Alzheimer's disease. [Display omitted]

•An improved sub-voxel QSM separation method was applied to calculate WMH iron burden.•Multimodal neuroimaging was applied to explore the pathological mechanism of CSVD.•WMH iron overload mediates glymphatic dysfunction and cognitive impairment in CSVD. Glymphatic system function has been increasingly linked to cognition in cerebral small vessel disease (CSVD), although the underlying pathological mechanisms related to brain metabolism remain to be fully clarified. Iron overload within white matter hyperintensity (WMH), potentially reflecting metabolic abnormalities, may play a pivotal role in this process. This study investigated whether WMH iron burden mediates the association between glymphatic dysfunction and cognitive impairment in CSVD. A total of 102 patients with CSVD and 29 matched healthy controls (HCs) underwent brain MRI and cognitive assessments. WMH iron burden was quantified using a sub-voxel quantitative approach, while glymphatic function was assessed with the Diffusion Tensor Image Analysis aLong the Perivascular Space (DTI-ALPS) index. Correlation and mediation analyses were then conducted to evaluate relationships among WMH iron burden, DTI-ALPS index, and cognitive scores. Compared with HCs, CSVD patients exhibited significantly higher WMH iron burden, lower DTI-ALPS index, and poorer cognitive performances. Elevated WMH iron burden was associated with deficits in attention-executive (att-exe), memory, and visual-spatial domains, whereas reduced DTI-ALPS index correlated with impaired att-exe and memory function. Importantly, WMH iron burden fully mediated the link between DTI-ALPS index and both att-exe function (p < 0.001) and memory (p = 0.02) in the CSVD group. These findings noninvasively identify WMH iron overload, a probable representative of microglial activation, as a key mediator between glymphatic dysfunction and cognitive decline in CSVD, prompting a potential therapeutic target for disease management.

•Dynamic contrast-enhanced MRI (DCE-MRI) revealed glymphatic dysfunction in the hypoxic-ischemic encephalopathy (HIE) mouse model.•Fluorescent cerebrospinal fluid (CSF) tracer demonstrated that hypoxic-ischemic encephalopathy (HIE) caused glymphatic dysfunction and impacted glymphatic system development in mice.•Decreased polarization of Aquaporin-4 (AQP4) is closely associated with glymphatic dysfunction. Hypoxic-ischemic encephalopathy (HIE) is a major cause of neonatal brain injury. The glymphatic system aids in waste clearance via perivascular pathways and is crucial in maintaining brain functions. While studies have shown that diseases such as stroke and traumatic brain injury disrupt glymphatic function, the impact of HIE on this system remains largely unexplored. We utilized an HIE mouse model with dynamic contrast-enhanced MRI (DCE-MRI) to conduct both qualitative and quantitative assessment of glymphatic transports dysfunction in different brain regions. Fluorescent cerebrospinal fluid (CSF) tracers were used to investigate the effects of HIE on glymphatic system development. Mice brain sections were subjected to Aquaporin-4 (AQP4) immunohistochemical staining, allowing for detailed morphological assessment of AQP4 polarization in affected brain regions. HIE mice exhibited delayed glymphatic transport dynamics, with prolonged time-to-peak tracer enhancement and increased retention in olfactory bulb, basal forebrain, and hypothalamus regions. Quantitative kinetic analysis showed significant reductions in Kf (CSF-to-perivascular space transfer constants) and Ks (perivascular-to-parenchyma transfer constants), alongside elevated Vf (perivascular volume fractions) across cortical and subcortical structures. Fluorescent CSF tracer analysis indicates that HIE impaired glymphatic system maturation in neonatal mice. This impairment progressed to persistent glymphatic dysfunction. Histologically validated via immunofluorescence, HIE-induced astrocytic AQP4 mis-polarization directly correlates with glymphatic transport dysfunction, underscoring AQP4′s critical role in glymphatic system integrity. Our multimodal imaging study combining DCE-MRI and CSF tracer analysis indicates that HIE can cause regional impairments of glymphatic function and adversely affect brain development.

Objectives To investigate glymphatic function in Alzheimer’s disease (AD) using the diffusion tensor image analysis along the perivascular space (DTI-ALPS) method and to explore the associations between DTI-ALPS index and perivascular space (PVS) volume, as well as between DTI-ALPS index and cognitive function. Methods Thirty patients with PET-CT-confirmed AD (15 AD dementia; 15 mild cognitive impairment due to AD) and 26 age- and sex-matched cognitively normal controls (NCs) were included in this study. All participants underwent neurological MRI and cognitive assessments. Bilateral DTI-ALPS indices were calculated. PVS volume fractions were quantitatively measured at three locations: basal ganglia (BG), centrum semiovale, and lateral ventricle body level. DTI-ALPS index and PVS volume fractions were compared among three groups; correlations among the DTI-ALPS index, PVS volume fraction, and cognitive scales were analyzed. Results Patients with AD dementia showed a significantly lower DTI-ALPS index in the whole brain ( p  = 0.009) and in the left hemisphere ( p  = 0.012) compared with NCs. The BG-PVS volume fraction in patients with AD was significantly larger than the fraction in NCs ( p  = 0.045); it was also negatively correlated with the DTI-ALPS index ( r  =  − 0.433, p  = 0.021). Lower DTI-ALPS index was correlated with worse performance in the Boston Naming Test ( β  = 0.515, p  = 0.008), Trail Making Test A ( β  =  − 0.391, p  = 0.048), and Digit Span Test ( β  = 0.408, p  = 0.038). Conclusions The lower DTI-ALPS index was found in patients with AD dementia, which may suggest impaired glymphatic system function. DTI-ALPS index was correlated with BG-PVS enlargement and worse cognitive performance in certain cognitive domains. Clinical relevance statement Diffusion tensor image analysis along the perivascular space index may be applied as a useful indicator to evaluate the glymphatic system function. The impaired glymphatic system in patients with Alzheimer’s disease (AD) dementia may provide a new perspective for understanding the pathophysiology of AD. Key Points • Patients with Alzheimer’s disease dementia displayed a lower diffusion tensor image analysis along the perivascular space (DTI-ALPS) index, possibly indicating glymphatic impairment. • A lower DTI-ALPS index was associated with the enlargement of perivascular space and cognitive impairment. • DTI-ALPS index could be a promising biomarker of the glymphatic system in Alzheimer’s disease dementia.

Abstract Background and Hypothesis The glymphatic system (GS), a brain waste clearance pathway, is disrupted in various neurodegenerative and vascular diseases. As schizophrenia shares clinical characteristics with these conditions, we hypothesized GS disruptions in patients with schizophrenia spectrum disorder (SCZ-SD), reflected in increased brain macromolecule (MM) and decreased diffusion-tensor-image-analysis along the perivascular space (DTI-ALPS) index. Study Design Forty-seven healthy controls (HCs) and 103 patients with SCZ-SD were studied. Data included 135 proton magnetic resonance spectroscopy (1H-MRS) sets, 96 DTI sets, with 79 participants contributing both. MM levels were quantified in the dorsal-anterior cingulate cortex (dACC), dorsolateral prefrontal cortex, and dorsal caudate (point resolved spectroscopy, echo-time = 35ms). Diffusivities in the projection and association fibers near the lateral ventricle were measured to calculate DTI-ALPS indices. General linear models were performed, adjusting for age, sex, and smoking. Correlation analyses examined relationships with age, illness duration, and symptoms severity. Study Results MM levels were not different between patients and HCs. However, left, right, and bilateral DTI-ALPS indices were lower in patients compared with HCs (P < .001). In HCs, age was positively correlated with dACC MM and negatively correlated with left, right, and bilateral DTI-ALPS indices (P < .001). In patients, illness duration was positively correlated with dACC MM and negatively correlated with the right DTI-ALPS index (P < .05). In the entire population, dACC MM and DTI-ALPS indices showed an inverse correlation (P < .01). Conclusions Our results suggest potential disruptions in the GS of patients with SCZ-SD. Improving brain’s waste clearance may offer a potential therapeutic approach for patients with SCZ-SD.