Mapping water exchange across the blood-brain barrier using 3D diffusion-prepared arterial spin labeled perfusion MRI

Tissue-to-fluid water-exchange imaging using T2-selective saturation labeling

PURPOSE

To propose and implement a new method to study water exchange between brain tissue and fluids.

METHODS

An MLEV T2-preparation combined with a cerebrospinal fluid (CSF) nulling inversion recovery was implemented in combination with an ultralong-echo time (TE) three-dimensional fast spin-echo readout. To handle systematic imperfections and isolate the exchange signal, T2-prepared images were subtracted from one of two control images. The first control turned off the T2 preparation and adjusted inversion timing to correct for relaxation. The second control used the same T2 preparation but shifted in time. Preparations were implemented on a 3T scanner and tested in 14 healthy volunteers. We evaluated the exchange signal magnitude and distribution, as well as robustness against B1 imperfection and intrasession reproducibility. We also compared the signal to that measured with ultralong-TE arterial spin labeling, another suggested marker of water exchange.

RESULTS

Initial experiments using the T2-preparation off control demonstrated a detectable exchange signal especially in the choroid plexus, but with substantial residual signal in CSF spaces, suggesting imperfect subtraction of non-exchanging spins. When using the time shifted control, we greatly reduced subtraction errors. Signal was consistently measured in the choroid plexus and at the boundaries between cortex and CSF with much higher signal-to-noise ratio and spatial resolution than ultralong-TE arterial spin labeling.

CONCLUSIONS

The measured water exchange distribution appears consistent with the localization of aquaporin channels at the CSF boundary. Because aquaporin activity may reflect CSF production and glymphatic clearance, our method may provide a noninvasive marker of these functions.

Volume and Permeability of White Matter Hyperintensity on Cognition: A DCE Imaging Study of an Older Cohort With and Without Cognitive Impairment

BACKGROUND

The impact of blood-brain barrier (BBB) leakage on white matter hyperintensity (WMH) subtypes (location) and its association with clinical factors and cognition remains unclear.

PURPOSE

To investigate the relationship between WMH volume, permeability, clinical factors, and cognition in older individuals across the cognitive spectrum.

STUDY TYPE

Prospective, cross-sectional.

SUBJECTS

A total of 193 older adults with/without cognitive impairment; 128 females; mean age 70.1 years (standard deviation 6.8).

FIELD STRENGTH/SEQUENCE

3 T, GE Dynamic contrast-enhanced, three-dimensional (3D) Magnetization-prepared rapid gradient-echo (MPRAGE T1WI), 3D fluid-attenuated inversion recovery (FLAIR).

ASSESSMENT

Periventricular WMH (PWMH), deep WMH (DWMH), and normal-appearing white matter (NAWM) were segmented using FMRIB automatic segmentation tool algorithms on 3D FLAIR. Hippocampal volume and cortex volume were segmented on 3D T1WI. BBB permeability (Ktrans) and blood plasma volume (Vp) were determined using the Patlak model. Vascular risk factors and cognition were assessed.

STATISTICAL TESTS

Univariate and multivariate analyses were performed to identify factors associated with WMH permeability. Logistic regression analysis assessed the association between WMH imaging features and cognition, adjusting for age, sex, apolipoprotein E4 status, education, and brain volumes. A P-value <0.05 was considered significant.

RESULTS

PWMH exhibited higher Ktrans (0.598 ± 0.509 × 10-3 minute-1) compared to DWMH (0.496 ± 0.478 × 10-3 minute-1) and NAWM (0.476 ± 0.398 × 10-3 minute-1). Smaller PWMH volume and cardiovascular disease (CVD) history were significantly associated with higher Ktrans in PWMH. In DWMH, higher Ktrans were associated with CVD history and cortical volume. In NAWM, it was linked to CVD history and dyslipidemia. Larger PWMH volume (odds ratio [OR] 1.106, confidence interval [CI]: 1.021-1.197) and smaller hippocampal volume (OR 0.069; CI: 0.019-0.253) were independently linked to worse global cognition after covariate adjustment.

DATA CONCLUSION

Elevated BBB leakage in PWMH was associated with lower PWMH volume and prior CVD history. Notably, PWMH volume, rather than permeability, was correlated with cognitive decline, suggesting that BBB leakage in WMH may be a consequence of CVD rather than indicate disease progression.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 3.

Longitudinal changes of blood-brain barrier and transcytolemmal water exchange permeability in Alzheimer's disease mice: A non-contrast MRI study

Growing evidence suggests that Alzheimer's disease (AD) has been linked with the dysfunction of glymphatic system. Previous studies were primarily cross-sectional and focused on only one specific component, hindering the understanding of overall glymphatic function in AD. We evaluated the longitudinal changes in multiple components of glymphatic system (blood-brain barrier (BBB) and transcytolemmal water exchange (TWE) permeability) in AD mice. Five female wild-type and four 3 × Tg-AD mice from 5 to 13 months of age were scanned monthly using two non-contrast MRI techniques, water-extraction-with-phase-contrast-arterial-spin-tagging (WEPCAST) and diffusion-time-dependent kurtosis imaging (tDKI), yielding BBB and TWE permeability. Immunostaining was used to evaluate tight junction proteins associated with BBB structural integrity, aquaporin 4 (AQP4) related to TWE, and AQP4 perivascular space (PVS) polarization that might represent PVS-parenchyma water exchange. The relationship between glymphatic function and AD pathology, as measured by amyloid beta (Aβ) and tau deposition, was also explored. Our results revealed significantly increased BBB and hippocampal TWE permeability in AD mouse brains, consistent with the histological findings of reduced tight junction proteins and upregulated AQP4, which were correlated with each other and can be predictive of Aβ and tau deposition. Impaired AQP4 PVS polarization was also found in AD mice. In conclusion, water exchange in multiple components of glymphatic system altered in AD mice, and these in vivo MRI findings were validated pathologically, which might affect the waste clearance in the glymphatic neurofluid.

Diffusion-weighted arterial spin labeling MRI to investigate mannitol-induced blood brain barrier disruption

PURPOSE

Diffusion-weighted arterial spin labeling (DW-ASL) MRI has been proposed to determine the rate of water exchange (Kw) across the blood brain barrier (BBB). This study aims to further evaluate Kw MRI by comparing it with standard dynamic contrast-enhanced (DCE) MRI and histology in association with mannitol-induced disruption of the BBB.

METHODS

DW-ASL was measured using a multiple b-value MRI protocol in normal rats at three post-labeling delays (N = 19), before and after intra-carotid injection of mannitol to disrupt BBB in one hemisphere (N = 13). An approach using only two b-values to detect mannitol-induced changes was also tested. DCE MRI and Evans blue histology were performed on the same animals. Quantitative analysis and pixel-by-pixel correlation were performed amongst Kw, DCE MRI and Evans blue histology.

RESULTS

Kw in the grey matter in the normal rat brain was 252 ± 38 min-1 (±standard error of the mean). The two b-value approach provided reasonable approximation of multiple-b DW-ASL parameters, reducing acquisition time. Kw is sensitive to mannitol-induced changes in BBB permeability and was reduced to 89 ± 17 min-1 in the affected hemisphere compared to 191 ± 22 min-1 in the unaffected hemisphere (P < 0.05). Regions with abnormality in Kw maps were in general agreement with DCE and Evans blue maps, although there are some distinct differences in location and the change in values.

CONCLUSION

Kw is sensitive to mannitol-induced changes in the BBB, with BBB disruption confirmed by DCE MRI and Evans blue histology.

A New Multiple Imputation Method for High-Dimensional Neuroimaging Data

Missing data are a prevalent challenge in neuroimaging, with significant implications for downstream statistical analysis. Neglecting this issue can introduce bias and lead to erroneous inferential conclusions, making it crucial to employ appropriate statistical methods for handling missing data. Although the multiple imputation is a widely used technique, its application in neuroimaging is severely hindered by the high dimensionality of neuroimaging data, and the substantial computational demands. To tackle the critical computational challenges, we propose a novel approach, High dimensional Multiple Imputation (HIMA), based on Bayesian models specifically designed for large-scale neuroimaging datasets. HIMA introduces a new computational strategy to sample large covariance matrices based on a robustly estimated posterior mode, significantly improving both computational efficiency and numerical stability. To assess the effectiveness of HIMA, we conducted extensive simulation studies and real-data analysis from a Schizophrenia brain imaging dataset with around 1000 voxels. HIMA showcases a remarkable reduction of computational burden, for example, 1 hour by HIMA versus 800 hours by classic multiple imputation packages. HIMA also demonstrates improved precision and stability of imputed data.

MRI detects blood-brain barrier alterations in a rat model of Alzheimer's disease and lung infection

Pneumonia is a common infection in people suffering with Alzheimer's disease, leading to delirium, critical illness or severe neurological decline, which may be due to an amplified response of the blood-brain barrier (BBB) to peripheral insult. We assess the response of the BBB to repeated Streptococcus pneumoniae lung infection in rat model of Alzheimer's disease (TgF344-AD), at 13- and 18-months old, using dynamic contrast-enhanced (DCE) MRI and filter exchange imaging. Higher BBB water exchange rate is initially detected in infected TgF344-AD rats. BBB water exchange rates correlated with hippocampus aquaporin-4 water channel expression in infected animals. We detected no differences in BBB permeability to gadolinium contrast agent measured by DCE-MRI, confirmed by staining for tight junction proteins, occludin and claudin-5. These findings provide insight into the mechanisms of how peripheral inflammation impacts the BBB.

Decreased water exchange rate across the blood-brain barrier throughout the Alzheimer's disease continuum: Evidence from Chinese data

INTRODUCTION

Water exchange rate (Kw) across the blood-brain barrier (BBB) is used in magnetic resonance imaging (MRI) techniques to evaluate BBB functionality. Variations in BBB Kw across the Alzheimer's disease (AD) continuum remain uncertain.

METHODS

The study encompassed 38 cognitively normal individuals without AD biomarkers (CN_A-), 30 cognitively normal (CN_A+), and 31 cognitively impaired individuals (CI_A+) with positive AD biomarkers. Participants underwent clinical assessments, MRI/positron emission tomography scans, and assays of plasma biomarkers.

RESULTS

Significantly lower Kw was observed in multiple brain regions throughout the AD continuum. This alteration in Kw correlated with plasma biomarkers and neuropsychological performance. Elevated levels of phosphorylated tau 217 intensified the inverse relationship between Kw and neuropsychological performance. The integration of Kw, brain volume, and plasma biomarkers demonstrated potential in distinguishing stages within the AD continuum.

DISCUSSION

Consistently lower Kw was evident across the AD continuum and may act as a diagnostic tool for early AD screening.

HIGHLIGHTS

Observations revealed a decline in water exchange rate (Kw) across multiple brain regions within the Alzheimer's disease (AD) continuum, notably in the hippocampus, parahippocampal gyrus, and deep brain nuclei during the preclinical stage of AD. Strong correlations were established between Kw levels in various brain regions and plasma biomarkers, as well as neuropsychological performance in the AD continuum. Interaction between plasma phosphorylated tau (p-tau)217 and Kw in the hippocampus was linked to executive function, indicating a combined detrimental impact on cognitive abilities stemming from both blood-brain barrier Kw and p-tau 217. The combined use of Kw, brain volume, and plasma biomarkers-neurofilament light chain and glial fibrillary acidic protein-demonstrated potential for distinguishing individuals within the AD continuum.

MR Assessment of Acute Changes of Cerebral Perfusion, Metabolism, and Blood-Brain Barrier Permeability in Response to Aerobic Exercise

BACKGROUND

It remains unclear how a single bout of exercise affects brain perfusion, oxygen metabolism, and blood-brain barrier (BBB) permeability. Addressing this unresolved issue is essential to understand the acute changes in cerebral physiology induced by aerobic exercise.

PURPOSE

To dynamically monitor the acute changes in cerebral physiology subsequent to a single aerobic exercise training session using noninvasive MRI measurements.

STUDY TYPE

Prospective.

POPULATION

Twenty-three healthy participants (18-35 years, 10 females/13 males) were enrolled and divided into 10-minute exercising (N = 10) and 20-minute exercising (N = 13) groups.

FIELD STRENGTH/SEQUENCE

3.0 T/Phase Contrast (PC) MRI (gradient echo), T2-Relaxation-Under-Spin-Tagging (TRUST) MRI (gradient echo EPI), Water-Extraction-with-Phase-Contrast-Arterial-Spin-Tagging (WEPCAST) MRI (gradient echo EPI) and T1-weighted magnetization-prepared-rapid-acquisition-of-gradient-echo (MPRAGE) (gradient echo).

ASSESSMENT

A baseline MR measurement plus four repeated MR measurements immediately after 10 or 20 minutes moderate running exercise. MR measurements included cerebral blood flow (CBF) as measured by PC MRI, venous oxygenation (Yv) and cerebral metabolic rate of oxygen (CMRO2) as assessed by TRUST MRI, water extraction fraction (E), and BBB permeability-surface-area product (PS) as determined by WEPCAST MRI.

STATISTICAL TESTS

The time dependence of the physiological parameters was studied with a linear mixed-effect model. Additionally, pairwise t-tests comparison of the physiological parameters at each time point was conducted. A P-value of <0.05 was considered statistically significant.

RESULTS

There was an initial drop (8.22 ± 2.60%) followed by a recovery in CBF after exercise, while Yv revealed a significant decrease (6.37 ± 0.92%), i.e., an increased oxygen extraction, and returned to baseline at later time points. CMRO2 showed a trend of increase (5.68 ± 3.04%) and a significant interaction between time and group. In addition, E increased significantly (3.86% ± 0.89) and returned to baseline level at later time points, while PS remained elevated (13.33 ± 4.79%).

DATA CONCLUSION

A single bout of moderate aerobic exercise can induce acute alterations in cerebral perfusion, metabolism, and BBB permeability.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Liver as a new target organ in Alzheimer's disease: insight from cholesterol metabolism and its role in amyloid-beta clearance

Alzheimer's disease, the primary cause of dementia, is characterized by neuropathologies, such as amyloid plaques, synaptic and neuronal degeneration, and neurofibrillary tangles. Although amyloid plaques are the primary characteristic of Alzheimer's disease in the central nervous system and peripheral organs, targeting amyloid-beta clearance in the central nervous system has shown limited clinical efficacy in Alzheimer's disease treatment. Metabolic abnormalities are commonly observed in patients with Alzheimer's disease. The liver is the primary peripheral organ involved in amyloid-beta metabolism, playing a crucial role in the pathophysiology of Alzheimer's disease. Notably, impaired cholesterol metabolism in the liver may exacerbate the development of Alzheimer's disease. In this review, we explore the underlying causes of Alzheimer's disease and elucidate the role of the liver in amyloid-beta clearance and cholesterol metabolism. Furthermore, we propose that restoring normal cholesterol metabolism in the liver could represent a promising therapeutic strategy for addressing Alzheimer's disease.

A Test-Retest Study of Single- and Multi-Delay pCASL for Choroid Plexus Perfusion Imaging in Healthy Subjects Aged 19 to 87 Years

There is a growing interest in the choroid plexus (ChP) due to its critical role in cerebrospinal fluid (CSF) production and its involvement in neurodegenerative and cerebrovascular diseases. However, comprehensive studies comparing the accuracy and reliability of single- and multi-PLD (post-labeling delay) arterial spin labeling (ASL) techniques, specifically in relation to the ChP, remain limited. This study systematically evaluated the test-retest reliability and quantification accuracy of cerebral blood flow (CBF) measurements, focusing on the ChP, using single-delay and multi-delay 3D gradient-and-spin echo (GRASE) pseudo-continuous ASL (pCASL) on 28 subjects (aged 19 to 87 years, 14 males/14 females) at 3.0 tesla. Both single-delay (2 s) and 5-PLD (0.5 - 2.5 s) pCASL scans were repeated approximately one week apart with a spatial resolution of 2.5 × 2.5 × 3 mm³. Voxel-wise and regional CBF and arterial transit time (ATT) measurements were compared to assess test-retest reliability, with a particular focus on ChP perfusion changes with age. In this study, 12.15 % of ChP voxels exhibited ATTs longer than 2 s, potentially leading to a significant underestimation of CBF using single-delay ASL. Multi-delay ASL showed improved accuracy in estimating CBF values for the ChP compared to single-delay ASL when ATT > PLD. Additionally, ChP volume (mean ± std = 1.72± 0.85 ml) increased (p < 0.01) and ChP perfusion (43.07±14.18 mL/100 g/min) decreased (p = 0.04) with age. These findings underscore the robustness of multi-delay ASL with model-fitting quantification in assessing ChP perfusion, making it the preferred method for accurate CBF and ATT estimation, particularly in regions with prolonged transit time such as ChP.

Blood-brain barrier water permeability across the adult lifespan: A multi-echo ASL study

An emerging biomarker of blood-brain barrier (BBB) permeability is the time of exchange (Tex) of water from the blood to tissue, as measured by multi-echo arterial spin labeling (ASL) MRI. This new non-invasive sequence, already tested in mice, has recently been adapted to humans and optimized for clinical scanning time. In this study, we studied the normal variability of Tex over age and sex, which needs to be established as a reference for studying changes in neurological disease. We evaluated Tex, cerebral blood flow (CBF) and arterial transit time (ATT) in 209 healthy adults between 26 and 87 years, over age and sex, using general linear models in gray matter, white matter, and regionally in cerebral lobes. After QC, 194 participants were included in the main analysis, and the results demonstrated that both gray matter (GM) and white matter (WM) BBB permeability was higher with higher age (Tex lower by 0.47 ms per year in GM [p < 0.05], and by 0.49 ms in WM, for females; no significant for males), with the largest Tex difference in the frontal lobes (0.64 ms decrease per year, p = 0.011, population average). CBF was lower with higher age in the GM (-0.71 mL/min/100g per year, p < 0.001, for females; -0.31 mL/min/100g per year, p < 0.05, for males). When correcting Tex models for CBF and ATT, effect of age on Tex disappears in the GM, but not in the WM (β=-0.28, p = 0.08). The CBF findings of this study are in line with previous studies, demonstrating the validity of the new sequence. The BBB water permeability variation over age and sex described in this study provides a reference for future BBB research.

MRI-based surrogates of brain clearance in narcolepsy type 1

Brain clearance involves the drainage of waste molecules from the brain, a process that is suggested to be amplified during sleep. Recently proposed MRI-based methods attempt to approximate human brain clearance with surrogate measures. The current study aimed to explore whether two brain clearance surrogates are altered in narcolepsy. We processed diffusion-weighted and functional resting-state images to extract two surrogates: Diffusion Tensor Imaging Along the Perivascular Space (DTI-ALPS index), and dBOLD-CSF coupling. Both measures were analysed in 12 drug-free, awake people with narcolepsy type 1 and 11 age- and sex-matched controls, as well as in relation to clinical features. We also assessed the correlation between the DTI-ALPS index and dBOLD-CSF coupling. The DTI-ALPS index and dBOLD-CSF coupling amplitude did not show significant differences between narcolepsy and controls, nor significant relations with the severity of excessive daytime sleepiness. We found a significant correlation between dBOLD-CSF coupling and sleep efficiency, as well as a significant correlation between the DTI-ALPS index and dBOLD-CSF coupling. The hypothesis of altered brain clearance in narcolepsy type 1 is not supported by evidence from the current study. The two surrogates correlated with each other, suggesting that both offer different perspectives from the same underlying physiology. Yet, the suitability of the surrogates as brain clearance markers remains debatable. Whereas DTI is not exclusively sensitive to perivascular fluid, dBOLD-CSF coupling is reflecting large-scale CSF motion. Future work should explore other surrogate markers, preferably during sleep, to better understand the possible role of altered brain clearance in narcolepsy type 1 symptomatology.

Synergistic effects of plasma S100B and MRI measures of cerebrovascular disease on cognition in older adults

There is growing interest in studying vascular contributions to cognitive impairment and dementia (VCID) and developing biomarkers to identify at-risk individuals. A combination of biofluid and neuroimaging markers may better profile early stage VCID than individual measures. Here, we tested this possibility focusing on plasma levels of S100 calcium-binding protein B (S100B), which has been linked with blood-brain-barrier (BBB) integrity, and neuroimaging measures assessing BBB function (water exchange rate across the BBB (kw)) and cerebral small vessel disease (white matter hyperintensities (WMHs)). A total of 74 older adults without dementia had plasma samples collected and underwent cognitive assessment. A subsample had neuroimaging data including diffusion prepared pseudo-continuous arterial spin labeling (DP-pCASL) for assessment of BBB kw and T2-weighted fluid-attenuated inversion recovery (FLAIR) for quantification of WMHs. Results indicated that higher plasma S100B levels were associated with poorer episodic memory performance (β = - .031, SE = .008, p < .001). Moreover, significant interactions were observed between plasma S100B levels and parietal lobe BBB kw (interaction β = .095, SE = .042, p = .028) and between plasma S100B levels and deep WMH volume (interaction β = - .025, SE = .009, p = .007) for episodic memory. Individuals with the poorest memory performance showed both high plasma S100B and either low BBB kw in the parietal lobe or increased deep WMH burden. Taken together, our results provide support for the combined use of biofluid and neuroimaging markers in the study of VCID.

Tracer kinetic model detecting heterogeneous blood-brain barrier permeability to water and contrast agent in Alzheimer's disease and dementia with Lewy bodies

INTRODUCTION

Blood-brain barrier (BBB) breakdown is essential in Alzheimer's disease (AD) and dementia with Lewy bodies (DLB), whereas the variability in BBB permeability to water and contrast agent is less clear.

METHODS

We investigated BBB permeability to water and contrast agent simultaneously using a novel tracer kinetic model for dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in 42 AD patients, 22 DLB patients, and 22 healthy controls. All participants underwent clinical, cognitive, and MRI assessments.

RESULTS

AD patients exhibited a significant reduction in the water exchange rates across the BBB, whereas DLB patients showed a significant increase in BBB permeability to contrast agent. Moreover, BBB permeability to both water and contrast agent in multiple brain regions demonstrated correlations with clinical severity.

DISCUSSION

The simultaneous evaluation of BBB permeability to water and contrast agent based on the proposed model highlights the heterogeneous patterns of BBB breakdown in AD and DLB.

HIGHLIGHTS

We measured blood-brain barrier (BBB) permeability to water and contrast agent based on dynamic contrast-enhanced magnetic resonance imaging. Alzheimer's disease (AD) is characterized by lower water exchange rates across the BBB. Dementia with Lewy bodies exhibits higher BBB permeability to contrast agent. BBB permeability is related to cognitive impairment and disease burden. BBB permeability to water was negatively associated with that to contrast agent.

White matter free water mediates the associations between placental growth factor, white matter hyperintensities, and cognitive status

INTRODUCTION

Placental growth factor (PlGF) may regulate cerebrovascular permeability. We hypothesized that white matter interstitial fluid accumulation, estimated via magnetic resonance imaging (MRI) free water (FW), would explain the associations between elevated PlGF, white matter hyperintensities (WMH), and cognitive impairment.

METHODS

MarkVCID consortium participants ≥55 years old with plasma PlGF and brain MRI were included. We tested cross-sectionally whether FW mediated the associations between PlGF and WMH, or PlGF and cognition, measured using the Clinical Dementia Rating (CDR) scale and an executive function (EF) composite (Uniform Data Set version 3 [UDS3]-EF).

RESULTS

For 370 participants (mean age 72), a higher PlGF was associated with higher FW, higher WMH, and higher CDR, but not UDS3-EF. Higher FW was associated with higher WMH, higher CDR, and lower UDS3-EF. FW explained 26% of the association between PlGF and CDR and 73% of the association between PlGF and WMH.

DISCUSSION

Elevated PlGF may contribute to WMH and cognitive impairment through white matter FW accumulation.

CLINICAL TRIAL REGISTRATION

NCT06284213 HIGHLIGHTS: PlGF is a promising blood-based biomarker for vascular cognitive impairment. In MarkVCID, higher PlGF was associated with accumulated white matter FW on MRI. FW mediated the association between higher PlGF and MRI-visible white matter injury. FW mediated the association between PlGF and worse CDR scale. PlGF may contribute to cognitive dysfunction via accumulated interstitial fluid.

The interaction between dysfunction of vasculature and tauopathy in Alzheimer's disease and related dementias

Tauopathy is one of the pathological features of Alzheimer's disease and related dementias (ADRD). At present, there have been many studies on the formation, deposition, and intercellular transmission of tau in neurons and immune cells. The vasculature is an important component of the central nervous system. This review discusses the interaction between vasculature and tau in detail from three aspects. (1) The vascular risk factors (VRFs) discussed in this review include diabetes mellitus (DM), abnormal blood pressure (BP), and hypercholesterolemia. (2) In ADRD pathology, the hyperphosphorylation and deposition of tau interact with disrupted vasculature, such as different cells (endothelial cells, smooth muscular cells, and pericytes), the blood-brain barrier (BBB), and the cerebral lymphatic system. (3) The functions of vasculature are regulated by various signaling transductions. Endothelial nitric oxide synthase/nitric oxide, calcium signaling, Rho/Rho-associated coiled-coil containing Kinase, and receptors for advanced glycation end products are discussed in this review. Our findings indicate that the prevention and treatment of vascular health may be a potential target for ADRD combination therapy. HIGHLIGHTS: Persistent VRFs increase early disruption of vascular mechanisms and are strongly associated with tau pathology in ADRD. Cell dysfunction in the vasculature causes BBB leakage and drainage incapacity of the cerebral lymphatic system, which interacts with tau pathology. Signaling molecules in the vasculature regulate vasodilation and contraction, angiogenesis, and CBF. Abnormal signaling transduction is related to tau hyperphosphorylation and deposition.

Use of multi-modal non-contrast MRI to predict functional outcomes after stroke: A study using DP-pCASL, DTI, NODDI, and MAP MRI

PURPOSE

This study aims to assess whether water exchange rate (kw), a surrogate for blood-brain barrier (BBB) permeability, is associated with functional outcomes in patients with acute ischemic stroke (AIS).

METHODS

We studied 22 AIS patients enrolled from 1/2022 to 4/2024 who underwent multi-modal non-contrast imaging on a 3.0-Tesla scanner, including DP-pCASL, DTI, NODDI and MAP imaging. For each parametric map, the intensity and standard deviation (SD) were calculated for the infarcted region. The diffusion maps included were b0, Fractional Anisotropy (FA), Mean Diffusivity (MD), Intra-cellular Volume Fraction (ICVF), Free Water Fraction (FWF), and Orientation Dispersion Index (ODI), q-Space Mean Square Displacement (QMSD), Return-to-Axis Probability (RTAP), Return-to-Plane Probability (RTPP), Return-to-Origin Probability (RTOP), Propagator Anisotropy (PA), and non-Gaussianity (NG). The perfusion-based maps were cerebral blood flow (CBF), arterial transit time (ATT), and kw. The outcome variable was modified Rankin Scale (mRS).

RESULTS

Twenty-two patients were included. The average age was 69.5 ± 13.5, the mean NIHSS of 12.4 ± 7.7, and the median infarct of 25.7 (8.4-98.8) ml. Multivariable linear regression identified lower kw (β = -0.029, p = 0.041), longer time to MRI (β = 0.012, p = 0.013) and larger stroke volume (β = 0.014, p = 0.006) as predictive of higher mRS. Higher CBF (β = 0.660, p = 0.003) and RTAP (β = 1.528, p = 0.010), and lower SD RTAP (β = -0.709, p = 0.016), RTPP (β = -2.132, p = 0.006), and NG (β = -1.036, p = 0.011) were identified as most predictive of kw through multivariable linear regression analysis.

CONCLUSIONS

Lower kw is predictive of higher mRS in patients with AIS. Increasing CBF and RTAP and lower SD RTAP, RTPP, and NG were correlated with higher kw.

Blood-Brain Barrier Disruption in Schizophrenia: Insights, Mechanisms, and Future Directions

The blood-brain barrier (BBB) plays a crucial role in maintaining the homeostasis of the central nervous system by regulating solute transport and preventing neurotoxic substances from infiltrating brain tissue. In schizophrenia, emerging evidence identifies BBB dysfunction as a key pathophysiological factor associated with neuroinflammation, tight junction abnormalities, and endothelial dysfunction. Recent advancements in neuroimaging techniques, such as arterial spin labeling (ASL), have provided valuable tools for investigating BBB permeability and its role in disease progression. This review synthesizes findings from postmortem studies, serum and cerebrospinal fluid biomarker analyses, and advanced neuroimaging research to elucidate BBB alterations in schizophrenia. It highlights the mechanistic roles of tight junction protein dysregulation, neurovascular unit dysfunction, and immune responses in disrupting BBB integrity. Furthermore, the review examines the bidirectional effects of antipsychotic medications on BBB, addressing both therapeutic opportunities and potential challenges. By emphasizing the pivotal role of BBB dysfunction in schizophrenia pathogenesis, this review underscores its translational potential. Through the integration of multidisciplinary evidence, it lays the foundation for innovative diagnostic approaches and therapeutic strategies, enhancing our understanding of schizophrenia's complex pathophysiology.

Associations between potential risk factors and blood-brain barrier water permeability in middle-aged and older adults

Background: Blood-brain barrier (BBB) dysfunction is suggested to be a potential mediator between vascular risk factors and cognitive impairment, leading to vascular cognitive impairment. Objective: To investigate the relationships between age, sex, and vascular risk factors and BBB water permeability as well as their relationship with cognition. Methods: To measure BBB permeability, a novel arterial spin labelling MRI technique (ME-ASL) was applied to derive the time of exchange (Tex), arterial time transit (ATT), and cerebral blood flow (CBF). The association of potential risk factors, such as age, sex, body mass index (BMI), blood pressure (BP), and medical history, with these BBB parameters were assessed in 144 community-dwelling adults (median age 59 years, 57% females). The relationship between BBB permeability and cognitive performance measured by the Montreal Cognitive Assessment (MoCA) was also assessed. Results: We found that increased BMI was significantly associated with decreased CBF (β = -0.06). Systolic BP and diastolic BP showed significant associations with all ASL parameters; systolic BP was negatively correlated with Tex (β = -0.02) and CBF (β = -0.01) but positively with ATT (β = 0.02). Diastolic BP was negatively associated with Tex (β = -0.03) and CBF (β = -0.03) but positively with ATT (β = 0.03). MoCA scores had a borderline significant association with Tex (OR = 1.51) and a significant association with CBF (OR = 1.84), which became non-significant after adjusting for confounders. Conclusions: These outcomes underscore the potential of using ME-ASL, warranting further research to strengthen these findings.

Measurement of blood-brain barrier water exchange rate using diffusion-prepared and multi-echo arterial spin labelling: Comparison of quantitative values and age dependence

Water exchange rate (Kw) across the blood-brain barrier (BBB) is an important physiological parameter that may provide new insight into ageing and neurodegenerative disease. Recently, two non-invasive arterial spin labelling (ASL) MRI methods have been developed to measure Kw, but results from the different methods have not been directly compared. Furthermore, the association of Kw with age for each method has not been investigated in a single cohort. Thirty participants (70% female, 63.8 ± 10.4 years) were scanned at 3 T with Diffusion-Prepared ASL (DP-ASL) and Multi-Echo ASL (ME-ASL) using previously implemented acquisition and analysis protocols. Grey matter Kw, cerebral blood flow (CBF) and arterial transit time (ATT) were extracted. CBF values were consistent; approximately 50 ml/min/100 g for both methods, and a strong positive correlation in CBF from both methods across participants (r = 0.82, p < 0.001). ATT was significantly different between methods (on average 147.7 ms lower when measured with DP-ASL compared to ME-ASL) but was positively correlated across participants (r = 0.39, p < 0.05). Significantly different Kw values of 106.6 ± 19.7 min-1 and 306.8 ± 71.7 min-1 were measured using DP-ASL and ME-ASL, respectively, and DP-ASL Kw and ME-ASL Kw were negatively correlated across participants (r = -0.46, p < 0.01). Kw measured using ME-ASL had a significant linear relationship with age (p < 0.05). In conclusion, DP-ASL and ME-ASL provided estimates of Kw with significantly different quantitative values and inconsistent dependence with age. We propose future standardisation of modelling and fitting methods for DP-ASL and ME-ASL, to evaluate the effect on Kw quantification. Also, sensitivity and bias analyses should be performed for both approaches, to assess the effect of varying acquisition and fitting parameters. Lastly, comparison with independent measures of BBB water transport, and with physiological and clinical biomarkers known to be associated with changes in BBB permeability, are essential to validate the ASL methods, and to demonstrate their clinical utility.

The links between neuroinflammation, brain structure and depressive disorder: A cross-sectional study protocol

INTRODUCTION

It is known that symptoms of major depressive disorder (MDD) are associated with neurodegeneration, that lipopolysaccharide (LPS) can induce symptoms of MDD, and that blood LPS levels are elevated in neurodegeneration. However, it is not known whether blood LPS and cytokine levels correlate with MDD, cognition and brain structure, and this is tested in this study.

METHODS AND ANALYSIS

This cross-sectional study includes individuals with MDD (n = 100) and a control group of individuals with no one-year history of a mental disorder (n = 50). A comprehensive evaluation is performed, including the collection of basic sociodemographic information, data on smoking status, body mass index, course of MDD, past treatment, comorbid diseases, and current use of medications. Diagnosis of MDD is performed according to the WHO's [2019] International Classification of Diseases and related health problems by psychiatrist and severity of MDD is evaluated using the Montgomery-Åsberg Depression Scale. The Cambridge Neuropsychological Test Automated Battery is used to evaluate cognitive functioning. Venous blood samples are taken to measure genetic and inflammatory markers, and multiparametric brain magnetic resonance imaging is performed to evaluate for blood-brain barrier permeability, structural and neurometabolic brain changes. Descriptive and inferential statistics, including linear and logistic regression, will be used to analyse relationships between blood plasma LPS and inflammatory cytokine concentrations in MDD patients and controls. The proposed sample sizes are suitable for identifying significant differences between the groups, according to a power analysis.

ADMINISTRATIVE INFORMATION

Trial registration: Clinicaltrials.gov NCT06203015.

Age-related decline in blood-brain barrier function is more pronounced in males than females in parietal and temporal regions

The blood-brain barrier (BBB) plays a pivotal role in protecting the central nervous system (CNS), and shielding it from potential harmful entities. A natural decline of BBB function with aging has been reported in both animal and human studies, which may contribute to cognitive decline and neurodegenerative disorders. Limited data also suggest that being female may be associated with protective effects on BBB function. Here, we investigated age and sex-dependent trajectories of perfusion and BBB water exchange rate (kw) across the lifespan in 186 cognitively normal participants spanning the ages of 8-92 years old, using a non-invasive diffusion-prepared pseudo-continuous arterial spin labeling (DP-pCASL) MRI technique. We found that the pattern of BBB kw decline with aging varies across brain regions. Moreover, results from our DP-pCASL technique revealed a remarkable decline in BBB kw beginning in the early 60 s, which was more pronounced in males. In addition, we observed sex differences in parietal and temporal regions. Our findings provide in vivo results demonstrating sex differences in the decline of BBB function with aging, which may serve as a foundation for future investigations into perfusion and BBB function in neurodegenerative and other brain disorders.

Enhanced parameter estimation in multiparametric arterial spin labeling using artificial neural networks

PURPOSE

Multiparametric arterial spin labeling (MP-ASL) can quantify cerebral blood flow (CBF) and arterial cerebral blood volume (CBVa). However, its accuracy is compromised owing to its intrinsically low SNR, necessitating complex and time-consuming parameter estimation. Deep neural networks (DNNs) offer a solution to these limitations. Therefore, we aimed to develop simulation-based DNNs for MP-ASL and compared the performance of a supervised DNN (DNNSup), physics-informed unsupervised DNN (DNNUns), and the conventional lookup table method (LUT) using simulation and in vivo data.

METHODS

MP-ASL was performed twice during resting state and once during the breath-holding task. First, the accuracy and noise immunity were evaluated in the first resting state. Second, CBF and CBVa values were statistically compared between the first resting state and the breath-holding task using the Wilcoxon signed-rank test and Cliff's delta. Finally, reproducibility of the two resting states was assessed.

RESULTS

Simulation and first resting-state analyses demonstrated that DNNSup had higher accuracy, noise immunity, and a six-fold faster computation time than LUT. Furthermore, all methods detected task-induced CBF and CBVa elevations, with the effect size being larger with the DNNSup (CBF, p = 0.055, Δ = 0.286; CBVa, p = 0.008, Δ = 0.964) and DNNUns (CBF, p = 0.039, Δ = 0.286; CBVa, p = 0.008, Δ = 1.000) than that with LUT (CBF, p = 0.109, Δ = 0.214; CBVa, p = 0.008, Δ = 0.929). Moreover, all the methods exhibited comparable and satisfactory reproducibility.

CONCLUSION

DNNSup outperforms DNNUns and LUT with respect to estimation performance and computation time.

Radiofrequency Enhancer to Recover Signal Dropouts in 7 Tesla Diffusion MRI

Diffusion magnetic resonance imaging (dMRI) allows for a non-invasive visualization and quantitative assessment of white matter architecture in the brain by characterizing restrictions on the random motion of water molecules. Ultra-high field MRI scanners, such as those operating at 7 Tesla (7T) or higher, can boost the signal-to-noise ratio (SNR) to improve dMRI compared with what is attainable at conventional field strengths such as 3T or 1.5T. However, wavelength effects at 7T cause reduced transmit magnetic field efficiency in the human brain, mainly in the posterior fossa, manifesting as signal dropouts in this region. Recently, we reported a simple approach of using a wireless radiofrequency (RF) surface array to improve transmit efficiency and signal sensitivity at 7T. In this study, we demonstrate the feasibility and effectiveness of the RF enhancer in improving in vivo dMRI at 7T. The electromagnetic simulation results demonstrated a 2.1-fold increase in transmit efficiency with the use of the RF enhancer. The experimental results similarly showed a 1.9-fold improvement in transmit efficiency and a 1.4-fold increase in normalized SNR. These improvements effectively mitigated signal dropouts in regions with inherently lower SNR, such as the cerebellum, resulting in a better depiction of principal fiber orientations and an enhanced visualization of extended tracts.

Dimensional Neuroimaging Endophenotypes: Neurobiological Representations of Disease Heterogeneity Through Machine Learning

Machine learning has been increasingly used to obtain individualized neuroimaging signatures for disease diagnosis, prognosis, and response to treatment in neuropsychiatric and neurodegenerative disorders. Therefore, it has contributed to a better understanding of disease heterogeneity by identifying disease subtypes with different brain phenotypic measures. In this review, we first present a systematic literature overview of studies using machine learning and multimodal magnetic resonance imaging to unravel disease heterogeneity in various neuropsychiatric and neurodegenerative disorders, including Alzheimer's disease, schizophrenia, major depressive disorder, autism spectrum disorder, and multiple sclerosis, as well as their potential in a transdiagnostic framework, where neuroanatomical and neurobiological commonalities were assessed across diagnostic boundaries. Subsequently, we summarize relevant machine learning methodologies and their clinical interpretability. We discuss the potential clinical implications of the current findings and envision future research avenues. Finally, we discuss an emerging paradigm called dimensional neuroimaging endophenotypes. Dimensional neuroimaging endophenotypes dissects the neurobiological heterogeneity of neuropsychiatric and neurodegenerative disorders into low-dimensional yet informative, quantitative brain phenotypic representations, serving as robust intermediate phenotypes (i.e., endophenotypes), presumably reflecting the interplay of underlying genetic, lifestyle, and environmental processes associated with disease etiology.

Blood-Brain Barrier Disruption and Imaging Assessment in Stroke

Disruption of the blood-brain barrier (BBB) is an important pathological hallmark of ischemic stroke. Blood-brain barrier disruption (BBBD) is a consequence of ischemia and may also exacerbate damage to brain parenchyma. Therefore, maintaining BBB integrity is critical for the central nervous system (CNS) homeostasis. This review offers a concise overview of BBB structure and function, along with the mechanisms underlying its impairment following a stroke. In addition, we review the recent imaging techniques employed to study blood-brain barrier permeability (BBBP) in the context of ischemic brain injury with the goal of providing imaging guidance for stroke diagnosis and treatment from the perspective of the BBBD. This knowledge is vital for developing strategies to safeguard the BBB during cerebral ischemia.

Cerebral blood flow and cognition after 3 months tadalafil treatment in small vessel disease (ETLAS-2): study protocol for a randomized controlled trial

BACKGROUND

Targeted treatment is highly warranted for cerebral small vessel disease, a causal factor of one in four strokes and a major contributor to vascular dementia. Patients with cerebral small vessel disease have impaired cerebral blood flow and vessel reactivity. Tadalafil is a specific phosphodiesterase 5 inhibitor shown to improve vascular reactivity in the brain.

METHODS

The ETLAS-2 trial is a phase 2 double-blind, randomized placebo-controlled, parallel trial with the feasibility of tadalafil as the primary outcome. The trial aims to include 100 patients with small vessel occlusion stroke or transitory ischemic attacks and signs of cerebral small vessel disease more than 6 months before administration of study medication. Patients are treated for 3 months with tadalafil 20 mg or placebo daily and undergo magnetic resonance imaging (MRI) to evaluate changes in small vessel disease according to the STandards for ReportIng Vascular changes on nEuroimaging (STRIVE) criteria as well as cerebral blood flow, cerebrovascular reactivity, and neurovascular coupling in a functional MRI sub-study. The investigation includes comprehensive cognitive testing using paper-pencil tests and Cambridge Neuropsychological Test Automated Battery (CANTAB) tests in a cognitive sub-study.

DISCUSSION

The ETLAS-2 trial tests the feasibility of long-term treatment with tadalafil and explores vascular and cognitive effects in cerebral small vessel disease in trial sub-studies. The study aims to propose a new treatment target and improve the understanding of small vessel disease. Currently, 64 patients have been included and the trial is estimated to be completed in the year 2024.

TRIAL REGISTRATION

Clinicaltrials.gov, NCT05173896. Registered on 30 December 2021.

Exercise and brain health in patients with coronary artery disease: study protocol for the HEART-BRAIN randomized controlled trial

Introduction

Patients with coronary artery disease (CAD) have a higher risk of developing cognitive impairment and mental health disorders compared to the general population. Physical exercise might improve their brain health. The overall goal of the HEART-BRAIN randomized controlled trial (RCT) is to investigate the effects of different types of exercise on brain health outcomes in patients with CAD, and the underlying mechanisms.

Methods

This three-arm, single-blinded RCT will include 90 patients with CAD (50-75 years). Participants will be randomized into: (1) control group-usual care (n = 30), (2) aerobic high-intensity interval training (HIIT) (n = 30), or (3) HIIT combined with resistance exercise training (n = 30). The 12-week intervention includes 3 supervised sessions (45-min each) per week for the exercise groups. Outcomes will be assessed at baseline and post-intervention. The primary outcome is to determine changes in cerebral blood flow assessed by magnetic resonance imaging. Secondary outcomes include changes in brain vascularization, cognitive measures (i.e., general cognition, executive function and episodic memory), and cardiorespiratory fitness. Additional health-related outcomes, and several potential mediators and moderators will be investigated (i.e., brain structure and function, cardiovascular and brain-based biomarkers, hemodynamics, physical function, body composition, mental health, and lifestyle behavior).

Conclusion

The HEART-BRAIN RCT will provide novel insights on how exercise can impact brain health in patients with CAD and the potential mechanisms explaining the heart-brain connection, such as changes in cerebral blood flow. The results may have important clinical implications by increasing the evidence on the effectiveness of exercise-based strategies to delay cognitive decline in this high-risk population.

Clinical trial registration

ClinicalTrials.gov, identifier [NCT06214624].

Quantitative non-contrast perfusion MRI in the body using arterial spin labeling

Arterial spin labeling (ASL) is a non-invasive magnetic resonance imaging (MRI) method that enables the assessment and the quantification of perfusion without the need for an exogenous contrast agent. ASL was originally developed in the early 1990s to measure cerebral blood flow. The utility of ASL has since then broadened to encompass various organ systems, offering insights into physiological and pathological states. In this review article, we present a synopsis of ASL for quantitative non-contrast perfusion MRI, as a contribution to the special issue titled "Quantitative MRI-how to make it work in the body?" The article begins with an introduction to ASL principles, followed by different labeling strategies, such as pulsed, continuous, pseudo-continuous, and velocity-selective approaches, and their role in perfusion quantification. We proceed to address the technical challenges associated with ASL in the body and outline some of the innovative approaches devised to surmount these issues. Subsequently, we summarize potential clinical applications, challenges, and state-of-the-art ASL methods to quantify perfusion in some of the highly perfused organs in the thorax (lungs), abdomen (kidneys, liver, pancreas), and pelvis (placenta) of the human body. The article concludes by discussing future directions for successful translation of quantitative ASL in body imaging.

Water exchange across the blood-brain barrier and epilepsy: Review on pathophysiology and neuroimaging

The blood-brain barrier (BBB) is a barrier protecting the brain and a milieu of continuous exchanges between blood and brain. There is emerging evidence that the BBB plays a major role in epileptogenesis and drug-resistant epilepsy, through several mechanisms, such as water homeostasis dysregulation, overexpression of drug transporters, and inflammation. Studies have shown abnormal water homeostasis in epileptic tissue and altered aquaporin-4 water channel expression in animal epilepsy models. This review focuses on abnormal water exchange in epilepsy and describes recent non-invasive MRI methods of quantifying water exchange. PLAIN LANGUAGE SUMMARY: Abnormal exchange between blood and brain contribute to seizures and epilepsy. The authors describe why correct water balance is necessary for healthy brain functioning and how it is impacted in epilepsy. This review also presents recent MRI methods to measure water exchange in human brain. These measures would improve our understanding of factors leading to seizures.

Clinical magnetic resonance imaging evaluation of glymphatic function

The glymphatic system is a system of specialized perivascular spaces in the brain that facilitates removal of toxic waste solutes from the brain. Evaluation of glymphatic system function by means of magnetic resonance imaging (MRI) has thus far been largely focused on rodents because of the limitations of intrathecal delivery of gadolinium-based contrast agents to humans. This review discusses MRI methods that can be employed clinically for glymphatic-related measurements intended for early diagnosis, prevention, and the treatment of various neurological conditions. Although glymphatic system-based MRI research is in its early stages, recent studies have identified promising noninvasive MRI markers associated with glymphatic system alterations in neurological diseases. However, further optimization in data acquisition, validation, and modeling are needed to investigate the glymphatic system within the clinical setting.

Compromised Cerebral Arterial Perfusion, Altered Brain Tissue Integrity, and Cognitive Impairment in Adolescents with Complex Congenital Heart Disease

(1) Introduction: Adolescents with complex congenital heart disease (CCHD) show brain tissue injuries in regions associated with cognitive deficits. Alteration in cerebral arterial perfusion (CAP), as measured by arterial transit time (ATT), may lead to perfusion deficits and potential injury. Our study aims to compare ATT values between CCHD patients and controls and assess the associations between ATT values, MD values, and cognitive scores in adolescents with CCHD. (2) Methods: 37 CCHD subjects, 14-18 years of age, who had undergone surgical palliation and 30 healthy controls completed cognitive testing and brain MRI assessments using a 3.0-Tesla scanner. ATT values and regional brain mean diffusivity [MD] were assessed for the whole brain using diffusion tensor imaging. (3) Results: The mean MoCA values [23.1 ± 4.1 vs. 28.1 ± 2.3; p < 0.001] and General Memory Index, with a subscore of WRAML2 [86.8 ± 15.4 vs. 110.3 ± 14.5; p < 0.001], showed significant cognitive deficits in CCHD patients compared to controls. The mean global ATT was significantly higher in CCHD patients versus controls (mean ± SD, s, 1.26 ± 0.11 vs. 1.19 ± 0.11, p = 0.03), respectively. The partial correlations between ATT values, MD values, and cognitive scores (p < 0.005) showed significant associations in areas including the hippocampus, prefrontal cortices, cerebellum, caudate, anterior and mid cingulate, insula, thalamus, and lingual gyrus. (4) Conclusions: Adolescents with CCHD had prolonged ATTs and showed correlation with clinical measurements of cognitive impairment and MRI measurements of brain tissue integrity. This suggests that altered CAP may play a role in brain tissue injury and cognitive impairment after surgical palliation.

Blood-brain barrier integrity is linked to cognitive function, but not to cerebral arterial pulsatility, among elderly

Blood-brain barrier (BBB) disruption may contribute to cognitive decline, but questions remain whether this association is more pronounced for certain brain regions, such as the hippocampus, or represents a whole-brain mechanism. Further, whether human BBB leakage is triggered by excessive vascular pulsatility, as suggested by animal studies, remains unknown. In a prospective cohort (N = 50; 68-84 years), we used contrast-enhanced MRI to estimate the permeability-surface area product (PS) and fractional plasma volume ( v p ), and 4D flow MRI to assess cerebral arterial pulsatility. Cognition was assessed by the Montreal Cognitive Assessment (MoCA) score. We hypothesized that high PS would be associated with high arterial pulsatility, and that links to cognition would be specific to hippocampal PS. For 15 brain regions, PS ranged from 0.38 to 0.85 (·10-3 min-1) and v p from 0.79 to 1.78%. Cognition was related to PS (·10-3 min-1) in hippocampus (β = - 2.9; p = 0.006), basal ganglia (β = - 2.3; p = 0.04), white matter (β = - 2.6; p = 0.04), whole-brain (β = - 2.7; p = 0.04) and borderline-related for cortex (β = - 2.7; p = 0.076). Pulsatility was unrelated to PS for all regions (p > 0.19). Our findings suggest PS-cognition links mainly reflect a whole-brain phenomenon with only slightly more pronounced links for the hippocampus, and provide no evidence of excessive pulsatility as a trigger of BBB disruption.

Age-Related Decline in Blood-Brain Barrier Function is More Pronounced in Males than Females in Parietal and Temporal Regions

The blood-brain barrier (BBB) plays a pivotal role in protecting the central nervous system (CNS), shielding it from potential harmful entities. A natural decline of BBB function with aging has been reported in both animal and human studies, which may contribute to cognitive decline and neurodegenerative disorders. Limited data also suggest that being female may be associated with protective effects on BBB function. Here we investigated age and sex-dependent trajectories of perfusion and BBB water exchange rate (kw) across the lifespan in 186 cognitively normal participants spanning the ages of 8 to 92 years old, using a non-invasive diffusion prepared pseudo-continuous arterial spin labeling (DP-pCASL) MRI technique. We found that the pattern of BBB kw decline with aging varies across brain regions. Moreover, results from our DP-pCASL technique revealed a remarkable decline in BBB kw beginning in the early 60s, which was more pronounced in males. In addition, we observed sex differences in parietal and temporal regions. Our findings provide in vivo results demonstrating sex differences in the decline of BBB function with aging, which may serve as a foundation for future investigations into perfusion and BBB function in neurodegenerative and other brain disorders.

The impact of mechanical thrombectomy on the blood-brain barrier in patients with acute ischemic stroke: A non-contrast MR imaging study using DP-pCASL and NODDI

BACKGROUND AND PURPOSE

While mechanical thrombectomy (MT) achieves restoration of cerebral blood flow to the area at risk in patients with acute ischemic stroke (AIS), the influx of blood flow may exacerbate the blood-brain barrier (BBB) disruption and extravasation across the BBB, and it therefore remains unclear how reperfusion impacts the blood-brain barrier integrity. In this study, we use diffusion-prepared pseudocontinuous ASL (DP-pCASL) and Neurite Orientation Dispersion and Density Imaging (NODDI) sequence to measure the water exchange rate (kw) in patients who underwent either MT or medical management and determine its impact on the brain tissue microstructure in order to elucidate the impact of MT on BBB complex integrity.

MATERIALS AND METHODS

We prospectively enrolled 21 patients with AIS treated at our institution from 10/2021 to 6/2023 who underwent MR imaging at a 3.0-Tesla scanner. Patients underwent DP-pCASl and NODDI imaging in addition to the standard stroke protocol which generated cerebral blood flow (CBF), arterial transit time (ATT), water exchange rate (kw), orientation dispersion index (ODI), intracellular volume fraction (ICVF), and free water fraction (FWF) parametric maps.

RESULTS

Of the 21 patients, 11 underwent MT and 10 were treated non-operatively. The average age and NIHSS for the MT cohort and non-MT cohorts were 69.3 ± 16.6 years old and 15.0 (12.0-20.0), and 70.2 ± 10.7 (p = 0.882) and 6.0 (3.8-9.0, p = 0.003) respectively. The average CBF, ATT, and kw in the infarcted territory of the MT cohort were 38.2 (18.4-59.6), 1347.6 (1182.5-1842.3), and 107.8 (79.2-140.1) respectively. The average CBF, ATT, and kw in the stroke ROI were 16.0 (8.8-36.6, p = 0.036), 1090.8 (937.1-1258.9, p = 0.013), 89.7 (68.0-122.7, p = 0.314) respectively. Linear regression analysis showed increasing CBF (p = 0.008) and undergoing mechanical thrombectomy (p = 0.048) were significant predictors of increased kw.

CONCLUSION

Using our multimodal non-contrast MRI protocol, we demonstrate that increased CBF and mechanical thrombectomy increased kw, suggesting a better functioning BBB complex. Higher kw suggests less disruption of the BBB complex in the MT cohort.

MRI free water mediates the association between water exchange rate across the blood brain barrier and executive function among older adults

Vascular risk factors contribute to cognitive aging, with one such risk factor being dysfunction of the blood brain barrier (BBB). Studies using non-invasive magnetic resonance imaging (MRI) techniques, such as diffusion prepared arterial spin labeling (DP-ASL), can estimate BBB function by measuring water exchange rate (kw). DP-ASL kw has been associated with cognition, but the directionality and strength of the relationship is still under investigation. An additional variable that measures water in extracellular space and impacts cognition, MRI free water (FW), may help explain prior findings. A total of 94 older adults without dementia (Mean age = 74.17 years, 59.6% female) underwent MRI (DP-ASL, diffusion weighted imaging (DWI)) and cognitive assessment. Mean kw was computed across the whole brain (WB), and mean white matter FW was computed across all white matter. The relationship between kw and three cognitive domains (executive function, processing speed, memory) was tested using multiple linear regression. FW was tested as a mediator of the kw-cognitive relationship using the PROCESS macro. A positive association was found between WB kw and executive function [F(4,85) = 7.81, p < .001, R2= 0.269; β = .245, p = .014]. Further, this effect was qualified by subsequent results showing that FW was a mediator of the WB kw-executive function relationship (indirect effect results: standardized effect = .060, bootstrap confidence interval = .0006 to .1411). Results suggest that lower water exchange rate (kw) may contribute to greater total white matter (WM) FW which, in turn, may disrupt executive function. Taken together, proper fluid clearance at the BBB contributes to higher-order cognitive abilities.

Quantifying blood-brain barrier permeability in patients with ischemic stroke using non-contrast MRI

Increased blood-brain barrier permeability (BBBP) after ischemic stroke predisposes patients to hemorrhagic conversion. While altered BBBP can impact patient recovery, it is not routinely assessed during the workup of acute ischemic stroke (AIS). We study the effectiveness of the non-contrast MRI sequences diffusion-prepared pseudocontinuous arterial spin labeling (DP-pCASL) and Neurite Orientation Dispersion and Density Imaging (NODDI) in assessing BBBP and correlating to tissue microstructure after ischemic insult. Twelve patients with AIS were prospectively enrolled to undergo our multimodal MR imaging, which generated the DP-pCASL-derived cerebral blood flow (CBF), arterial transit time (ATT), and water exchange rate (kw) and the NODDI-derived b0, mean diffusivity (MD), orientation dispersion index (ODI), intracellular volume fraction (ICVF), and isotropic volume fraction (ISO) parametric maps. The mean age of the patients was 70.2 ± 14.8 with an average NIHSS of 13.0 (7.3-19.8). MR imaging was performed on average at 53.7 (27.8-93.3) hours from stroke symptom onset. The water exchange rate (kw) of the infarcted area and its contralateral territory were 89.7 min-1 (66.7-121.9) and 89.9 min-1 (65.9-106.0) respectively (p = 0.887). Multivariable linear regression analysis showed that b0, ODI, ISO and mechanical thrombectomy were significant predictors of kw. DP-pCASL and NODDI are promising non-contrast sequences for the routine assessment of BBBP.

Effects of the Training Data Condition on Arterial Spin Labeling Parameter Estimation Using a Simulation-Based Supervised Deep Neural Network

OBJECTIVE

A simulation-based supervised deep neural network (DNN) can accurately estimate cerebral blood flow (CBF) and arterial transit time (ATT) from multidelay arterial spin labeling signals. However, the performance of deep learning depends on the characteristics of the training data set. We aimed to investigate the effects of the ground truth (GT) ranges of CBF and ATT on the performance of the DNN when training data were prepared using arterial spin labeling signal simulation.

METHODS

Deep neural networks were individually trained using 36 patterns of the training data sets. Simulation test data (1,000,000 points), 17 healthy volunteers, and 1 patient with moyamoya disease were included. The simulation test data were used to evaluate accuracy, precision, and noise immunity of the DNN. The best-performing DNN was determined by the normalized mean absolute error (NMAE), normalized root mean squared error (NRMSE), and normalized coefficient of variation over repeated training (CV Net ). Cerebral blood flow and ATT values and their histograms were compared between the GT and predicted values. For the in vivo data, the dependency of the predicted values on the GT ranges was visually evaluated by comparing CBF and ATT maps between the best-performing DNN and the other DNNs. Moreover, using the synthesized noisy images, noise immunity was compared between the best-performing DNN based on the simulation study and a conventional method.

RESULTS

The simulation study showed that a network trained by the GT of CBF and ATT in the ranges of 0 to 120 mL/100 g/min and 0 to 4500 milliseconds, respectively, had the highest performance (NMAE CBF , 0.150; NRMSE CBF , 0.231; CV NET CBF , 0.028; NMAE ATT , 0.158; NRMSE ATT , 0.257; and CV NET ATT , 0.028). Although the predicted CBF and ATT varied with the GT range of the training data sets, the appropriate settings preserved the accuracy, precision, and noise immunity of the DNN. In addition, the same results were observed in in vivo studies.

CONCLUSIONS

The GT ranges to prepare the training data affected the performance of the simulation-based supervised DNNs. The predicted CBF and ATT values depended on the GT range; inappropriate settings degraded the accuracy, whereas appropriate settings of the GT range provided accurate and precise estimates.

Developing blood-brain barrier arterial spin labelling as a non-invasive early biomarker of Alzheimer's disease (DEBBIE-AD): a prospective observational multicohort study protocol

INTRODUCTION

Loss of blood-brain barrier (BBB) integrity is hypothesised to be one of the earliest microvascular signs of Alzheimer's disease (AD). Existing BBB integrity imaging methods involve contrast agents or ionising radiation, and pose limitations in terms of cost and logistics. Arterial spin labelling (ASL) perfusion MRI has been recently adapted to map the BBB permeability non-invasively. The DEveloping BBB-ASL as a non-Invasive Early biomarker (DEBBIE) consortium aims to develop this modified ASL-MRI technique for patient-specific and robust BBB permeability assessments. This article outlines the study design of the DEBBIE cohorts focused on investigating the potential of BBB-ASL as an early biomarker for AD (DEBBIE-AD).

METHODS AND ANALYSIS

DEBBIE-AD consists of a multicohort study enrolling participants with subjective cognitive decline, mild cognitive impairment and AD, as well as age-matched healthy controls, from 13 cohorts. The precision and accuracy of BBB-ASL will be evaluated in healthy participants. The clinical value of BBB-ASL will be evaluated by comparing results with both established and novel AD biomarkers. The DEBBIE-AD study aims to provide evidence of the ability of BBB-ASL to measure BBB permeability and demonstrate its utility in AD and AD-related pathologies.

ETHICS AND DISSEMINATION

Ethics approval was obtained for 10 cohorts, and is pending for 3 cohorts. The results of the main trial and each of the secondary endpoints will be submitted for publication in a peer-reviewed journal.

Vessel density mapping of small cerebral vessels on 3D high resolution black blood MRI

Small cerebral blood vessels are largely inaccessible to existing clinical in vivo imaging technologies. This study aims to present a novel analysis pipeline for vessel density mapping of small cerebral blood vessels from high-resolution 3D black-blood MRI at 3T. Twenty-eight subjects (10 under 35 years old, 18 over 60 years old) were imaged with the T1-weighted turbo spin-echo with variable flip angles (T1w TSE-VFA) sequence optimized for black-blood small vessel imaging with iso-0.5 mm spatial resolution (interpolated from 0.51×0.51×0.64 mm3) at 3T. Hessian-based vessel segmentation methods (Jerman, Frangi and Sato filter) were evaluated by vessel landmarks and manual annotation of lenticulostriate arteries (LSAs). Using optimized vessel segmentation, large vessel pruning and non-linear registration, a semiautomatic pipeline was proposed for quantification of small vessel density across brain regions and further for localized detection of small vessel changes across populations. Voxel-level statistics was performed to compare vessel density between two age groups. Additionally, local vessel density of aged subjects was correlated with their corresponding gross cognitive and executive function (EF) scores using Montreal Cognitive Assessment (MoCA) and EF composite scores compiled with Item Response Theory (IRT). Jerman filter showed better performance for vessel segmentation than Frangi and Sato filter which was employed in our pipeline. Small cerebral blood vessels including small artery, arterioles, small veins, and venules on the order of a few hundred microns can be delineated using the proposed analysis pipeline on 3D black-blood MRI at 3T. The mean vessel density across brain regions was significantly higher in young subjects compared to aged subjects. In the aged subjects, localized vessel density was positively correlated with MoCA and IRT EF scores. The proposed pipeline is able to segment, quantify, and detect localized differences in vessel density of small cerebral blood vessels based on 3D high-resolution black-blood MRI. This framework may serve as a tool for localized detection of small vessel density changes in normal aging and cerebral small vessel disease.

Regional differences in the link between water exchange rate across the blood-brain barrier and cognitive performance in normal aging

The blood-brain barrier (BBB) undergoes functional changes with aging which may contribute to cognitive decline. A novel, diffusion prepared arterial spin labeling-based MRI technique can measure the rate of water exchange across the BBB (kw) and may thus be sensitive to age-related alterations in water exchange at the BBB. However, studies investigating relationships between kw and cognition have reported different directions of association. Here, we begin to investigate the direction of associations between kw and cognition in different brain regions, and their possible underpinnings, by evaluating links between kw, cognitive performance, and MRI markers of cerebrovascular dysfunction and/or damage. Forty-seven healthy older adults (age range 61-84) underwent neuroimaging to obtain whole-brain measures of kw, cerebrovascular reactivity (CVR), and white matter hyperintensity (WMH) volumes. Additionally, participants completed uniform data set (Version 3) neuropsychological tests of executive function (EF) and episodic memory (MEM). Voxel-wise linear regressions were conducted to test associations between kw and cognitive performance, CVR, and WMH volumes. We found that kw in the frontoparietal brain regions was positively associated with cognitive performance but not with CVR or WMH volumes. Conversely, kw in the basal ganglia was negatively associated with cognitive performance and CVR and positively associated with regional, periventricular WMH volume. These regionally dependent associations may relate to different physiological underpinnings in the relationships between kw and cognition in neocortical versus subcortical brain regions in older adults.

Blood-Brain Barrier Disruption and Perivascular Spaces in Small Vessel Disease and Neurodegenerative Diseases: A Review on MRI Methods and Insights

Perivascular spaces (PVS) and blood-brain barrier (BBB) disruption are two key features of cerebral small vessel disease (cSVD) and neurodegenerative diseases that have been linked to cognitive impairment and are involved in the cerebral waste clearance system. Magnetic resonance imaging (MRI) offers the possibility to study these pathophysiological processes noninvasively in vivo. This educational review provides an overview of the MRI techniques used to assess PVS functionality and BBB disruption. MRI-visible PVS can be scored on structural images by either (subjectively) counting or (automatically) delineating the PVS. We highlight emerging (diffusion) techniques to measure proxies of perivascular fluid and its movement, which may provide a more comprehensive understanding of the role of PVS in diseases. For the measurement of BBB disruption, we explain the most commonly used MRI technique, dynamic contrast-enhanced (DCE) MRI, as well as a more recently developed technique based on arterial spin labeling (ASL). DCE MRI and ASL are thought to measure complementary characteristics of the BBB. Furthermore, we describe clinical studies that have utilized these MRI techniques in cSVD and neurodegenerative diseases, particularly Alzheimer's disease (AD). These studies demonstrate the role of PVS and BBB dysfunction in these diseases and provide insight into the large overlap, but also into the differences between cSVD and AD. Overall, MRI techniques may provide valuable insights into the pathophysiological mechanisms underlying these diseases and have the potential to be used as markers for disease progression and treatment response. LEVEL OF EVIDENCE: 3 TECHNICAL EFFICACY: Stage 2.

Imaging the neurovascular unit in health and neurodegeneration: a scoping review of interdependencies between MRI measures

The neurovascular unit (NVU) is a complex structure that facilitates nutrient delivery and metabolic waste clearance, forms the blood-brain barrier (BBB), and supports fluid homeostasis in the brain. The integrity of NVU subcomponents can be measured in vivo using magnetic resonance imaging (MRI), including quantification of enlarged perivascular spaces (ePVS), BBB permeability, cerebral perfusion and extracellular free water. The breakdown of NVU subparts is individually associated with aging, pathology, and cognition. However, how these subcomponents interact as a system, and how interdependencies are impacted by pathology remains unclear. This systematic scoping review identified 26 studies that investigated the inter-relationships between multiple subcomponents of the NVU in nonclinical and neurodegenerative populations using MRI. A further 112 studies investigated associations between the NVU and white matter hyperintensities (WMH). We identify two putative clusters of NVU interdependencies: a 'vascular' cluster comprising BBB permeability, perfusion and basal ganglia ePVS; and a 'fluid' cluster comprising ePVS, free water and WMH. Emerging evidence suggests that subcomponent coupling within these clusters may be differentially related to aging, neurovascular injury or neurodegenerative pathology.

The Consistence of Dynamic Contrast-Enhanced MRI and Filter-Exchange Imaging in Measuring Water Exchange Across the Blood-Brain Barrier in High-Grade Glioma

BACKGROUND

Water exchange across blood-brain barrier (BBB) (WEXBBB ) is an emerging biomarker of BBB dysfunction with potential applications in many brain diseases. Several MRI methods have been proposed to measure WEXBBB , but evidence remains scarce whether different methods can produce comparable WEXBBB .

PURPOSE

To explore whether dynamic contrast-enhanced (DCE)-MRI and vascular water exchange imaging (VEXI) could produce comparable WEXBBB in high-grade glioma (HGG) patients.

STUDY TYPE

Prospective cross-sectional.

SUBJECTS

13 HGG patients (58.4 ± 9.4 years, 9 females, 4 WHO III and 9 WHO IV).

FIELD STRENGTH/SEQUENCE

A 3 T, spoiled gradient-recalled-echo DCE-MRI and VEXI containing two pulsed-gradient spin-echo blocks separated by a mixing block.

ASSESSMENTS

The enhanced tumor and contralateral normal-appearing white matter (cNAWM) volume-of-interests (VOIs) were drew by two neuroradiologists. And whole-brain NAWM and normal-appearing gray matter (NAGM) without tumor-affected regions were segmented by automated segmentation algorithm in FSL.

STATISTICAL TESTS

Student's t-test was used to evaluate parameters difference between cNAWM and tumor, NAGM and NAWM, respectively. The correlation between vascular water efflux rate constant (kbo ) from DCE-MRI and apparent exchange rate across BBB (AXRBBB ) from VEXI was evaluated by Pearson correlation. P < 0.05 was considered statistically significant.

RESULTS

Compared with cNAWM, both kbo and AXRBBB were significantly reduced in tumor (kbo  = 3.50 ± 1.18 sec-1 vs. 1.03 ± 0.75 sec-1 ; AXRBBB  = 3.54 ± 1.11 sec-1 vs. 1.94 ± 1.04 sec-1 ). Both kbo and AXRBBB showed significantly higher values in NAWM than NAGM (kbo  = 3.50 ± 0.59 sec-1 vs. 2.10 ± 0.56 sec-1 ; AXRBBB  = 3.35 ± 0.77 sec-1 vs. 2.07 ± 0.52 sec-1 ). The VOI-averaged kbo and AXRBBB were also linearly correlated in tumor, NAWM, and NAGM (r = 0.59).

DATA CONCLUSION

DCE-MRI and VEXI showed comparable and correlated WEXBBB in HGG patients, suggesting that the consistence and reliability of these two MRI methods in measuring WEXBBB .

EVIDENCE LEVEL

2.

TECHNICAL EFFICACY

Stage 1.

Quantification of T1 and T2 of subarachnoid CSF: Implications for water exchange between CSF and brain tissues

PURPOSE

To quantify the T1 and T2 values of CSF in the subarachnoid space (SAS) at 3 T and interpret them in the context of water exchange between CSF and brain tissues.

METHODS

CSF T1 was measured using inversion recovery, and CSF T2 was assessed using T2 -preparation. T1 and T2 values in the SAS were compared with those in the frontal horns of lateral ventricles, which have less brain-CSF exchange. Phantom experiments were performed to examine whether there were spatial variations in T1 and T2 that were unrelated to brain-CSF exchange. Simulations were conducted to investigate the relationship between the brain-CSF exchange rate and the apparent T1 and T2 values of SAS CSF.

RESULTS

The CSF T1 and T2 values were 4308.7 ± 146.9 ms and 1885.5 ± 67.9 ms, respectively, in the SAS and were 4454.0 ± 187.9 ms and 2372.9 ± 72.0 ms in the frontal horns. The SAS CSF had shorter T1 (p = 0.006) and T2 (p < 0.0001) than CSF in the frontal horns. Phantom experiments showed negligible (< 6 ms for T1 ; < 1 ms for T2 ) spatial variations in T1 and T2 , suggesting that the T1 and T2 differences between SAS and frontal horns were largely attributed to physiological reasons. Simulations revealed that faster brain-CSF exchange rates lead to shorter apparent T1 and T2 of SAS CSF. However, the experimentally observed T2 difference between SAS and frontal horns was greater than that attributable to typical exchange effect, suggesting that the T2 shortening in SAS may reflect a combined effect of exchange and deoxyhemoglobin susceptibility.

CONCLUSION

Quantification of SAS CSF relaxation times may be useful to assess the brain-CSF exchange.

Blood-brain barrier breakdown in COVID-19 ICU survivors: an MRI pilot study

Objectives

Coronavirus disease 2019 (COVID-19) results in severe inflammation at the acute stage. Chronic neuroinflammation and abnormal immunological response have been suggested to be the contributors to neuro-long-COVID, but direct evidence has been scarce. This study aims to determine the integrity of the blood-brain barrier (BBB) in COVID-19 intensive care unit (ICU) survivors using a novel MRI technique.

Methods

COVID-19 ICU survivors (n=7) and age and sex-matched control participants (n=17) were recruited from June 2021 to March 2023. None of the control participants were hospitalized due to COVID-19 infection. The COVID-19 ICU survivors were studied at 98.6 ± 14.9 days after their discharge from ICU. A non-invasive MRI technique was used to assess the BBB permeability to water molecules, in terms of permeability surface area-product (PS) in the units of mL/100 g/min.

Results

PS was significantly higher in COVID-19 ICU survivors (p=0.038) when compared to the controls, with values of 153.1 ± 20.9 mL/100 g/min and 132.5 ± 20.7 mL/100 g/min, respectively. In contrast, there were no significant differences in whole-brain cerebral blood flow (p=0.649) or brain volume (p=0.471) between the groups.

Conclusions

There is preliminary evidence of a chronic BBB breakdown in COVID-19 survivors who had a severe acute infection, suggesting a plausible contributor to neurological long-COVID symptoms.

Visualizing Cerebral Small Vessel Degeneration During Aging and Diseases Using Magnetic Resonance Imaging

Cerebral small vessel disease is a major contributor to brain disorders in older adults. It is associated with a much higher risk of stroke and dementia. Due to a lack of clinical and fluid biomarkers, diagnosing and grading small vessel disease are highly dependent on magnetic resonance imaging. In the past, researchers mostly used brain parenchymal imaging markers to represent small vessel damage, but the relationships between these surrogate markers and small vessel pathologies are complex. Recent progress in high-resolution magnetic resonance imaging methods, including time-of-flight MR angiography, phase-contrast MR angiography, black blood vessel wall imaging, susceptibility-weighted imaging, and contrast-enhanced methods, allow for direct visualization of cerebral small vessel structures. They could be powerful tools for understanding aging-related small vessel degeneration and improving disease diagnosis and treatment. This article will review progress in these imaging techniques and their application in aging and disease studies. Some challenges and future directions are also discussed. EVIDENCE LEVEL: 4. TECHNICAL EFFICACY: 3.

Blood-brain barrier water exchange measurements using contrast-enhanced ASL

A technique for quantifying regional blood-brain barrier (BBB) water exchange rates using contrast-enhanced arterial spin labelling (CE-ASL) is presented and evaluated in simulations and in vivo. The two-compartment ASL model describes the water exchange rate from blood to tissue,k b , but to estimatek b in practice it is necessary to separate the intra- and extravascular signals. This is challenging in standard ASL data owing to the small difference inT 1 values. Here, a gadolinium-based contrast agent is used to increase thisT 1 difference and enable the signal components to be disentangled. The optimal post-contrast bloodT 1 (T 1 , b post ) at 3 T was determined in a sensitivity analysis, and the accuracy and precision of the method quantified using Monte Carlo simulations. Proof-of-concept data were acquired in six healthy volunteers (five female, age range 24-46 years). The sensitivity analysis identified the optimalT 1 , b post at 3 T as 0.8 s. Simulations showed thatk b could be estimated in individual cortical regions with a relative error ϵ < 1 % and coefficient of variation CoV = 30 %; however, a high dependence on bloodT 1 was also observed. In volunteer data, mean parameter values in grey matter were: arterial transit timet A = 1 . 15 ± 0 . 49  s, cerebral blood flow f = 58 . 0 ± 14 . 3  mL blood/min/100 mL tissue and water exchange ratek b = 2 . 32 ± 2 . 49 s-1 . CE-ASL can provide regional BBB water exchange rate estimates; however, the clinical utility of the technique is dependent on the achievable accuracy of measuredT 1 values.

Neurofluids and the glymphatic system: anatomy, physiology, and imaging

First described in 2012, the glymphatic system is responsible for maintaining homeostasis within the central nervous system, including nutrient delivery, waste clearance, and consistency of the ionic microenvironment. It is comprised of glial cells and barrier systems that modulate neurofluid production, circulation, and exchange. Experimental interrogation of neurofluid dynamics is restricted to ex vivo and in vitro studies in animals and humans, therefore diagnostic imaging plays an important role in minimally invasive evaluation. This review article will synthesize current knowledge and theories regarding neurofluid circulation and implications for neuroimaging. First, we will discuss the anatomy of the neurogliovascular unit, including paravascular and perivascular pathways of fluid exchange. In addition, we will summarize the structure and function of barrier systems including the blood-brain, blood-cerebrospinal fluid, and brain-cerebrospinal fluid barriers. Next, we will mention physiologic factors that yield normal variations in neurofluid circulation, and how various disease pathologies can disrupt glymphatic drainage pathways. Lastly, we will cover the spectrum of diagnostic imaging and interventional techniques with relevance to glymphatic structure, flow, and function. We conclude by highlighting current barriers and future directions for translational imaging and applications to neurologic disorders.