Diffusion-weighted image analysis along the perivascular space (DWI-ALPS) for evaluating interstitial fluid status: age dependence in normal subjects

A "glympse" into neurodegeneration: Diffusion MRI and cerebrospinal fluid aquaporin-4 for the assessment of glymphatic system in Alzheimer's disease and other dementias

The glymphatic system (GS) is a whole-brain perivascular network, consisting of three compartments: the periarterial and perivenous spaces and the interposed brain parenchyma. GS dysfunction has been implicated in neurodegenerative diseases, particularly Alzheimer's disease (AD). So far, comprehensive research on GS in humans has been limited by the absence of easily accessible biomarkers. Recently, promising non-invasive methods based on magnetic resonance imaging (MRI) along with aquaporin-4 (AQP4) quantification in the cerebrospinal fluid (CSF) were introduced for an indirect assessment of each of the three GS compartments. We recruited 111 consecutive subjects presenting with symptoms suggestive of degenerative cognitive decline, who underwent 3 T MRI scanning including multi-shell diffusion-weighted images. Forty nine out of 111 also underwent CSF examination with quantification of CSF-AQP4. CSF-AQP4 levels and MRI measures-including perivascular spaces (PVS) counts and volume fraction (PVSVF), white matter free water fraction (FW-WM) and mean kurtosis (MK-WM), diffusion tensor imaging analysis along the perivascular spaces (DTI-ALPS) (mean, left and right)-were compared among patients with AD (n = 47) and other neurodegenerative diseases (nAD = 24), patients with stable mild cognitive impairment (MCI = 17) and cognitively unimpaired (CU = 23) elderly people. Two runs of analysis were conducted, the first including all patients; the second after dividing both nAD and AD patients into two subgroups based on gray matter atrophy as a proxy of disease stage. Age, sex, years of education, and scanning time were included as confounding factors in the analyses. Considering the whole cohort, patients with AD showed significantly higher levels of CSF-AQP4 (exp(b) = 2.05, p = .005) and FW-WM FW-WM (exp(b) = 1.06, p = .043) than CU. AQP4 levels were also significantly higher in nAD in respect to CU (exp(b) = 2.98, p < .001). CSF-AQP4 and FW-WM were significantly higher in both less atrophic AD (exp(b) = 2.20, p = .006; exp(b) = 1.08, p = .019, respectively) and nAD patients (exp(b) = 2.66, p = .002; exp(b) = 1.10, p = .019, respectively) compared to CU subjects. Higher total (exp(b) = 1.59, p = .013) and centrum semiovale PVS counts (exp(b) = 1.89, p = .016), total (exp(b) = 1.50, p = .036) and WM PVSVF (exp(b) = 1.89, p = .005) together with lower MK-WM (exp(b) = 0.94, p = .006), mean and left ALPS (exp(b) = 0.91, p = .043; exp(b) = 0.88, p = .010 respectively) were observed in more atrophic AD patients in respect to CU. In addition, more atrophic nAD patients exhibited higher levels of AQP4 (exp(b) = 3.39, p = .002) than CU. Our results indicate significant changes in putative MRI biomarkers of GS and CSF-AQP4 levels in AD and in other neurodegenerative dementias, suggesting a close interaction between glymphatic dysfunction and neurodegeneration, particularly in the case of AD. However, the usefulness of some of these biomarkers as indirect and standalone indices of glymphatic activity may be hindered by their dependence on disease stage and structural brain damage.

Automated diffusion-weighted image analysis along the perivascular space index reveals glymphatic dysfunction in association with brain parenchymal lesions

Brain glymphatic dysfunction is critical in neurodegenerative processes. While animal studies have provided substantial insights, understandings in humans remains limited. Recent attention has focused on the non-invasive evaluation of brain glymphatic function. However, its association with brain parenchymal lesions in large-scale population remains under-investigated. In this cross-sectional analysis of 1030 participants (57.14 ± 9.34 years, 37.18% males) from the Shunyi cohort, we developed an automated pipeline to calculate diffusion-weighted image analysis along the perivascular space (ALPS), with a lower ALPS value indicating worse glymphatic function. The automated ALPS showed high consistency with the manual calculation of this index (ICC = 0.81, 95% CI: 0.662-0.898). We found that those with older age and male sex had lower automated ALPS values (β = -0.051, SE = 0.004, p < .001, per 10 years, and β = -0.036, SE = 0.008, p < .001, respectively). White matter hyperintensity (β = -2.458, SE = 0.175, p < .001) and presence of lacunes (OR = 0.004, 95% CI < 0.002-0.016, p < .001) were significantly correlated with decreased ALPS. The brain parenchymal and hippocampal fractions were significantly associated with decreased ALPS (β = 0.067, SE = 0.007, p < .001 and β = 0.040, SE = 0.014, p = .006, respectively) independent of white matter hyperintensity. Our research implies that the automated ALPS index is potentially a valuable imaging marker for the glymphatic system, deepening our understanding of glymphatic dysfunction.

Diffusion Tensor Image Analysis ALong the Perivascular Space (DTI-ALPS): Revisiting the Meaning and Significance of the Method

More than 5 years have passed since the Diffusion Tensor Image Analysis ALong the Perivascular Space (DTI-ALPS) method was proposed with the intention of evaluating the glymphatic system. This method is handy due to its noninvasiveness, provision of a simple index in a straightforward formula, and the possibility of retrospective analysis. Therefore, the ALPS method was adopted to evaluate the glymphatic system for many disorders in many studies. The purpose of this review is to look back and discuss the ALPS method at this moment.The ALPS-index was found to be an indicator of a number of conditions related to the glymphatic system. Thus, although this was expected in the original report, the results of the ALPS method are often interpreted as uniquely corresponding to the function of the glymphatic system. However, a number of subsequent studies have pointed out the problems on the data interpretation. As they rightly point out, a higher ALPS-index indicates predominant Brownian motion of water molecules in the radial direction at the lateral ventricular body level, no more and no less. Fortunately, the term "ALPS-index" has become common and is now known as a common term by many researchers. Therefore, the ALPS-index should simply be expressed as high or low, and whether it reflects a glymphatic system is better to be discussed carefully. In other words, when a decreased ALPS-index is observed, it should be expressed as "decreased ALPS-index" and not directly as "glymphatic dysfunction". Recently, various methods have been proposed to evaluate the glymphatic system. It has become clear that these methods also do not seem to reflect the entirety of the extremely complex glymphatic system. This means that it would be desirable to use various methods in combination to evaluate the glymphatic system in a comprehensive manner.

Bilateral glymphatic dysfunction and its association with disease duration in unilateral temporal lobe epilepsy patients with hippocampal sclerosis

OBJECTIVE

In this study, the diffusion tensor imaging along perivascular space analysis (DTI-ALPS) technique was utilized to evaluate the functional changes in the glymphatic system of the bilateral hemispheres in patients with unilateral temporal lobe epilepsy (TLE) accompanied by hippocampal sclerosis (HS). The aim was to gain insights into the alterations in the glymphatic system function in TLE patients.

METHODS

A total of 61 unilateral TLE patients with HS and 53 healthy controls (HCs) from the Department of Neurosurgery at Xiangya Hospital were included in the study. All subjects underwent DTI using the same 3 T MR Scanner, and the DTI-ALPS index was calculated. Differences in the DTI-ALPS index between TLE patients and HCs were evaluated, along with the correlation between the DTI-ALPS index of TLE and clinical features of epilepsy. These features included age, age of onset, seizure duration, and neuropsychological scores.

RESULTS

Compared to the bilateral means of the HCs, both the ipsilateral and contralateral DTI-ALPS index of the TLE patients were significantly decreased (TLE ipsilateral 1.41 ± 0.172 vs. HC bilateral mean: 1.49 ± 0.116, p = 0.006; TLE contralateral: 1.42 ± 0.158 vs. HC bilateral mean: 1.49 ± 0.116, p = 0.015). The ipsilateral DTI-ALPS index in TLE patients showed a significant negative correlation with disease duration (r = -0.352, p = 0.005).

CONCLUSIONS

The present study suggests the presence of bilateral dysfunctions in the glymphatic system and also highlight a laterality feature in these dysfunctions. Additionally, the study found a significant negative correlation between the ipsilateral DTI-ALPS index and disease duration, underscoring the significance of early effective interventions and indicating potential for the development of innovative treatments targeting the glymphatic system.

Glymphatic Imaging in Pediatrics

The glymphatic system, which facilitates cerebrospinal fluid (CSF) flow through the brain parenchyma, is important for brain development and waste clearance. Advances in imaging techniques, particularly magnetic resonance imaging, have make it possible to evaluate glymphatic structures and functions in vivo. Recently, several studies have focused on the development and alterations of the glymphatic system in pediatric disorders. This review discusses the development of the glymphatic system, advances of imaging techniques and their applications in pediatric disorders. First, the results of the reviewed studies indicate that the development of the glymphatic system is a long-lasting process that continues into adulthood. Second, there is a need for improved glymphatic imaging techniques that are non-invasive and fast to improve suitability for pediatric applications, as some of existing methods use contrast injection and are susceptible to motion artifacts from long scanning times. Several novel techniques are potentially feasible for pediatric patients and may be used in the future. Third, the glymphatic dysfunction is associated with a large number of pediatric disorders, although only a few have recently been investigated. In conclusion, research on the pediatric glymphatic system remains an emerging field. The preliminary applications of glymphatic imaging techniques have provided unique insight into the pathological mechanism of pediatric diseases, but mainly limited in visualization of enlarged perivascular spaces and morphological measurements on CSF volumes. More in-depth studies on glymphatic functions are required to improve our understanding of the mechanisms underlying brain development and pediatric diseases. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

Noninvasive Magnetic Resonance Imaging Measures of Glymphatic System Activity

The comprehension of the glymphatic system, a postulated mechanism responsible for the removal of interstitial solutes within the central nervous system (CNS), has witnessed substantial progress recently. While direct measurement techniques involving fluorescence and contrast agent tracers have demonstrated success in animal studies, their application in humans is invasive and presents challenges. Hence, exploring alternative noninvasive approaches that enable glymphatic research in humans is imperative. This review primarily focuses on several noninvasive magnetic resonance imaging (MRI) techniques, encompassing perivascular space (PVS) imaging, diffusion tensor image analysis along the PVS, arterial spin labeling, chemical exchange saturation transfer, and intravoxel incoherent motion. These methodologies provide valuable insights into the dynamics of interstitial fluid, water permeability across the blood-brain barrier, and cerebrospinal fluid flow within the cerebral parenchyma. Furthermore, the review elucidates the underlying concept and clinical applications of these noninvasive MRI techniques, highlighting their strengths and limitations. It addresses concerns about the relationship between glymphatic system activity and pathological alterations, emphasizing the necessity for further studies to establish correlations between noninvasive MRI measurements and pathological findings. Additionally, the challenges associated with conducting multisite studies, such as variability in MRI systems and acquisition parameters, are addressed, with a suggestion for the use of harmonization methods, such as the combined association test (COMBAT), to enhance standardization and statistical power. Current research gaps and future directions in noninvasive MRI techniques for assessing the glymphatic system are discussed, emphasizing the need for larger sample sizes, harmonization studies, and combined approaches. In conclusion, this review provides invaluable insights into the application of noninvasive MRI methods for monitoring glymphatic system activity in the CNS. It highlights their potential in advancing our understanding of the glymphatic system, facilitating clinical applications, and paving the way for future research endeavors in this field. EVIDENCE LEVEL: 3 TECHNICAL EFFICACY: Stage 5.

Perivascular space diffusivity and brain microstructural measures are associated with circadian time and sleep quality

The glymphatic system is centred around brain cerebrospinal fluid flow and is enhanced during sleep, and the synaptic homeostasis hypothesis proposes that sleep acts on brain microstructure by selective synaptic downscaling. While so far primarily studied in animals, we here examine in humans if brain diffusivity and microstructure is related to time of day, sleep quality and cognitive performance. We use diffusion weighted images from 916 young healthy individuals, aged between 22 and 37 years, collected as part of the Human Connectome Project to assess diffusion tensor image analysis along the perivascular space index, white matter fractional anisotropy, intra-neurite volume fraction and extra-neurite mean diffusivity. Next, we examine if these measures are associated with circadian time of acquisition, the Pittsburgh Sleep Quality Index (high scores correspond to low sleep quality) and age-adjusted cognitive function total composite score. Consistent with expectations, we find that diffusion tensor image analysis along the perivascular space index and orbitofrontal grey matter extra-neurite mean diffusivity are negatively and white matter fractional anisotropy positively correlated with circadian time. Further, we find that grey matter intra-neurite volume fraction correlates positively with Pittsburgh Sleep Quality Index, and that this correlation is driven by sleep duration. Finally, we find positive correlations between grey matter intra-neurite volume fraction and cognitive function total composite score, as well as negative interaction effects between cognitive function total composite score and Pittsburgh Sleep Quality Index on grey matter intra-neurite volume fraction. Our findings propose that perivascular flow is under circadian control and that sleep downregulates the intra-neurite volume in healthy adults with positive impact on cognitive function.

Physical Exercise Alters Egress Pathways for Intrinsic CSF Outflow: An Investigation Performed with Spin-labeling MR Imaging

PURPOSE

Cerebrospinal fluid (CSF) clearance is essential for maintaining a healthy brain and cognition by removal of metabolic waste from the central nervous system. Physical exercise has been shown to improve human health; however, the effect of physical exercise on intrinsic CSF outflow in humans remains unexplored. The purpose of this study was to investigate intrinsic CSF outflow pathways and quantitative metrics of healthy individuals with active and sedentary lifestyles. In addition, the effect of exercise was investigated among the sedentary subjects before and after 3 weeks of physical activity.

METHODS

This study was performed on 18 healthy adults with informed consent, using a clinical 3-Tesla MRI scanner. We classified participants into two groups based on reported time spent sitting per day (active group: < 7 hours sitting per day and sedentary group: ≥ 7 hours sitting per day). To elucidate the effect of exercise, sedentary individuals increased their activity to 3.5 hours for 3 weeks.

RESULTS

We show that there are two intrinsic CSF egress pathways of the dura mater and lower parasagittal dura (PSD). The adults with an active lifestyle had greater intrinsic CSF outflow metrics than adults with a more sedentary lifestyle. However, after increased physical activity, the sedentary group showed improved CSF outflow metrics. This improvement was particularly notable at the lower PSD, where outflow metrics were highest among the active group.

CONCLUSION

Our findings describe the relationship between physical activity and intrinsic CSF outflow and show a potential selective outflow pathway with increasing physical activity in the lower PSD pathway, potentially from the perivascular space or cortical venous subpial space.

The Association of Metabolic Brain MRI, Amyloid PET, and Clinical Factors: A Study of Alzheimer's Disease and Normal Controls From the Open Access Series of Imaging Studies Dataset

BACKGROUND

Although brain activities in Alzheimer's disease (AD) might be evaluated MRI and PET, the relationships between brain temperature (BT), the index of diffusivity along the perivascular space (ALPS index), and amyloid deposition in the cerebral cortex are still unclear.

PURPOSE

To investigate the relationship between metabolic imaging measurements and clinical information in patients with AD and normal controls (NCs).

STUDY TYPE

Retrospective analysis of a prospective dataset.

POPULATION

58 participants (78.3 ± 6.8 years; 30 female): 29 AD patients and 29 age- and sex-matched NCs from the Open Access Series of Imaging Studies dataset.

FIELD STRENGTH/SEQUENCE

3T; T1-weighted magnetization-prepared rapid gradient-echo, diffusion tensor imaging with 64 directions, and dynamic 18 F-florbetapir PET.

ASSESSMENT

Imaging metrics were compared between AD and NCs. These included BT calculated by the diffusivity of the lateral ventricles, ALPS index that reflects the glymphatic system, the mean standardized uptake value ratio (SUVR) of amyloid PET in the cerebral cortex and clinical information, such as age, sex, and MMSE.

STATISTICAL TESTS

Pearson's or Spearman's correlation and multiple linear regression analyses. P values <0.05 were defined as statistically significant.

RESULTS

Significant positive correlations were found between BT and ALPS index (r = 0.44 for NCs), while significant negative correlations were found between age and ALPS index (rs  = -0.43 for AD and - 0.47 for NCs). The SUVR of amyloid PET was not significantly associated with BT (P = 0.81 for AD and 0.21 for NCs) or ALPS index (P = 0.10 for AD and 0.52 for NCs). In the multiple regression analysis, age was significantly associated with BT, while age, sex, and presence of AD were significantly associated with the ALPS index.

DATA CONCLUSION

Impairment of the glymphatic system measured using MRI was associated with lower BT and aging.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 1.

The glymphatic system in migraine and other headaches

Glymphatic system is an emerging pathway of removing metabolic waste products and toxic solutes from the brain tissue. It is made of a network of perivascular spaces, filled in cerebrospinal and interstitial fluid, encompassing penetrating and pial vessels and communicating with the subarachnoid space. It is separated from vessels by the blood brain barrier and from brain tissue by the endfeet of the astrocytes rich in aquaporin 4, a membrane protein which controls the water flow along the perivascular space. Animal models and magnetic resonance (MR) studies allowed to characterize the glymphatic system function and determine how its impairment could lead to numerous neurological disorders (e.g. Alzheimer's disease, stroke, sleep disturbances, migraine, idiopathic normal pressure hydrocephalus). This review aims to summarize the role of the glymphatic system in the pathophysiology of migraine in order to provide new ways of approaching to this disease and to its therapy.

Impaired Glymphatic and Meningeal Lymphatic Functions in Patients with Chronic Migraine

OBJECTIVE

This study was undertaken to investigate migraine glymphatic and meningeal lymphatic vessel (mLV) functions.

METHODS

Migraine patients and healthy controls (HCs) were prospectively recruited between 2020 and 2023. Diffusion tensor image analysis along the perivascular space (DTI-ALPS) index for glymphatics and dynamic contrast-enhanced magnetic resonance imaging parameters (time to peak [TTP]/enhancement integral [EI]/mean time to enhance [MTE]) for para-superior sagittal (paraSSS)-mLV or paratransverse sinus (paraTS)-mLV in episodic migraine (EM), chronic migraine (CM), and CM with and without medication-overuse headache (MOH) were analyzed. DTI-ALPS correlations with clinical parameters (migraine severity [numeric rating scale]/disability [Migraine Disability Assessment (MIDAS)]/bodily pain [Widespread Pain Index]/sleep quality [Pittsburgh Sleep Quality Index (PSQI)]) were examined.

RESULTS

In total, 175 subjects (112 migraine + 63 HCs) were investigated. DTI-ALPS values were lower in CM (median [interquartile range] = 0.64 [0.12]) than in EM (0.71 [0.13], p = 0.005) and HCs (0.71 [0.09], p = 0.004). CM with MOH (0.63 [0.07]) had lower DTI-ALPS values than CM without MOH (0.73 [0.12], p < 0.001). Furthermore, CM had longer TTP (paraSSS-mLV: 55.8 [12.9] vs 40.0 [7.6], p < 0.001; paraTS-mLV: 51.2 [8.1] vs 44.0 [3.3], p = 0.002), EI (paraSSS-mLV: 45.5 [42.0] vs 16.1 [9.2], p < 0.001), and MTE (paraSSS-mLV: 253.7 [6.7] vs 248.4 [13.8], p < 0.001; paraTS-mLV: 252.0 [6.2] vs 249.7 [1.2], p < 0.001) than EM patients. The MIDAS (p = 0.002) and PSQI (p = 0.002) were negatively correlated with DTI-ALPS index after Bonferroni corrections (p < q = 0.01).

INTERPRETATION

CM patients, particularly those with MOH, have glymphatic and meningeal lymphatic dysfunctions, which are highly clinically relevant and may implicate pathogenesis for migraine chronification. ANN NEUROL 2024;95:583-595.

Glymphatic system dysfunction in middle-aged and elderly chronic insomnia patients with cognitive impairment evidenced by diffusion tensor imaging along the perivascular space (DTI-ALPS)

BACKGROUND

Chronic insomnia impairs the glymphatic system and may lead to cognitive impairment and dementia in elderly population. The diffusion tensor image analysis along the perivascular space (DTI-ALPS) has been proposed as a non-invasive method to measure the activity of human brain glymphatic. We aim to explore whether glymphatic function is impaired in middle-aged and elderly chronic insomnia individuals and to identify the relationships between glymphatic dysfunction and cognitive impairment.

METHODS

A total of 33 chronic insomnia patients (57.36 ± 5.44 years, 30 females) and 20 age- and sex-matched healthy controls (57.95 ± 5.78 years, 16 females) were prospectively enrolled between May 2022 and January 2023. All participants completed MRI screening, cognition and sleep assessments, and DTI-ALPS index analysis.

RESULTS

Our findings revealed that the DTI-ALPS index was significantly difference among the chronic insomnia patients with impaired cognition group (1.32 ± 0.14), with normal cognition group (1.46 ± 0.09), and healthy controls (1.61 ± 0.16) (p = 0.0012, p < 0.0001, p = 0.0008, respectively). Mini-Mental State Examination (MMSE) scores of chronic insomnia patients with cognitive impairment were positively correlated with the DTI-ALPS index (Partial correlation analyses after correction for age, sex, education level and duration of chronic insomnia: r = 0.78, p = 0.002). DTI-ALPS had moderate accuracy in distinguishing chronic insomnia patients with cognitive impairment from those with normal cognition.

DATA CONCLUSION

The glymphatic system dysfunction is involved in chronic insomnia among middle-aged and elderly individuals, and it has been found to be correlated with cognitive decline.

Perivascular spaces and where to find them - MR imaging and evaluation methods

BACKGROUND

 Perivascular spaces (synonym: Virchow-Robin spaces) were first described over 150 years ago. They are defined as the fluid-filled spaces surrounding the small penetrating cerebral vessels. They gained growing scientific interest especially with the postulation of the so-called glymphatic system and their possible role in neurodegenerative and neuroinflammatory diseases.

METHODS

 PubMed was used for a systematic search with a focus on literature regarding MRI imaging and evaluation methods of perivascular spaces. Studies on human in-vivo imaging were included with a focus on studies involving healthy populations. No time frame was set. The nomenclature in the literature is very heterogeneous with terms like "large", "dilated", "enlarged" perivascular spaces whereas borders and definitions often remain unclear. This work generally talks about perivascular spaces.

RESULTS

 This review article discusses the morphologic MRI characteristics in different sequences. With the continual improvement of image quality, more and tinier structures can be depicted in detail. Visual analysis and semi or fully automated segmentation methods are briefly discussed.

CONCLUSION

 If they are looked for, perivascular spaces are apparent in basically every cranial MRI examination. Their physiologic or pathologic value is still under debate.

KEY POINTS

  · Perivascular spaces can be seen in basically every cranial MRI examination.. · Primarily T2-weighend sequences are used for visual analysis. Additional sequences are helpful for distinction from their differential diagnoses.. · There are promising approaches for the semi or fully automated segmentation of perivascular spaces with the possibility to collect more quantitative parameters..

CITATION FORMAT

· Seehafer S, Larsen N, Aludin S et al. Perivascular spaces and where to find them - MRI imaging and evaluation methods. Fortschr Röntgenstr 2024; DOI: 10.1055/a-2254-5651.

Age-related Decline of Intrinsic Cerebrospinal Fluid Outflow in Healthy Humans Detected with Non-contrast Spin-labeling MR Imaging

PURPOSE

Clearance of cerebrospinal fluid (CSF) is important for the removal of toxins from the brain, with implications for neurodegenerative diseases. Imaging evaluation of CSF outflow in humans has been limited, relying on venous or invasive intrathecal injections of contrast agents. The objective of this study was to introduce a novel spin-labeling MRI technique to detect and quantify the movement of endogenously tagged CSF, and then apply it to evaluate CSF outflow in normal humans of varying ages.

METHODS

This study was performed on a clinical 3-Tesla MRI scanner in 16 healthy subjects with an age range of 19-71 years with informed consent. Our spin-labeling MRI technique applies a tag pulse on the brain hemisphere, and images the outflow of the tagged CSF into the superior sagittal sinus (SSS). We obtained 3D images in real time, which was analyzed to determine tagged-signal changes in different regions of the meninges involved in CSF outflow. Additionally, the signal changes over time were fit to a signal curve to determine quantitative flow metrics. These were correlated against subject age to determine aging effects.

RESULTS

We observed the signal of the tagged CSF moving from the dura mater and parasagittal dura, and finally draining into the SSS. In addition, we observed a possibility of another pathway which is seen in some young subjects. Furthermore, quantitative CSF outflow metrics were shown to decrease significantly with age.

CONCLUSION

We demonstrate a novel non-invasive MRI technique identifying two intrinsic CSF clearance pathways, and observe an age-related decline of CSF flow metrics in healthy subjects. Our work provides a new opportunity to better understand the relationships of these CSF clearance pathways during the aging process, which may ultimately provide insight into the age-related prevalence of neurodegenerative diseases.

Interstitial Fluidopathy of the Central Nervous System: An Umbrella Term for Disorders with Impaired Neurofluid Dynamics

Interest in interstitial fluid dynamics has increased since the proposal of the glymphatic system hypothesis. Abnormal dynamics of the interstitial fluid have been pointed out to be an important factor in various pathological statuses. In this article, we propose the concept of central nervous system interstitial fluidopathy as a disease or condition in which abnormal interstitial fluid dynamics is one of the important factors for the development of a pathological condition. We discuss the aspects of interstitial fluidopathy in various diseases, including Alzheimer's disease, Parkinson's disease, normal pressure hydrocephalus, and cerebral small vessel disease. We also discuss a method called "diffusion tensor image analysis along the perivascular space" using MR diffusion images, which is used to evaluate the degree of interstitial fluidopathy or the activity of the glymphatic system.

Association between the Putative Meningeal Lymphatics at the Posterior Wall of the Sigmoid Sinus and Delayed Contrast-agent Elimination from the Cerebrospinal Fluid

PURPOSE

To investigate the characteristics of the putative meningeal lymphatics located at the posterior wall of the sigmoid sinus (PML-PSS) in human subjects imaged before and after intravenous administration (IV) of a gadolinium-based contrast agent (GBCA). The appearance of the PML-PSS and the enhancement of the perivascular space of the basal ganglia (PVS-BG) were analyzed for an association with gender, age, and clearance of the GBCA from the cerebrospinal fluid (CSF).

METHODS

Forty-two patients with suspected endolymphatic hydrops were included. Heavily T2-weighted 3D-fluid attenuated inversion recovery (hT2w-3D-FLAIR) and 3D-real inversion recovery (IR) images were obtained at pre-administration, immediately post-administration, and at 4 and 24 hours after IV-GBCA. The appearance of the PML-PSS and the presence of enhancement in the PVS-BG were analyzed for a relationship with age, gender, contrast enhancement of the CSF at 4 hours after IV-GBCA, and the washout ratio of the GBCA in the CSF from 4 to 24 hours after IV-GBCA.

RESULTS

The PML-PSS and PVS-BG were seen in 23 of 42 and 21 of 42 cases, respectively, at 4 hours after IV-GBCA. In all PML-PSS positive cases, hT2w-3D-FLAIR signal enhancement was highest at 4 hours after IV-GBCA. A multivariate analysis between gender, age, CSF signal elevation at 4 hours, and washout ratio indicated that only the washout ratio was independently associated with the enhancement of the PML-PSS or PVS-BG. The odds ratios (95% CIs; P value) were 4.09 × 10-5 (2.39 × 10-8 - 0.07; 0.0078) for the PML-PSS and 1.7 × 10-4 (1.66 × 10-7 - 0.174; 0.014) for the PVS-BG.

CONCLUSION

The PML-PSS had the highest signal enhancement at 4 hours after IV-GBCA. When the PML-PSS was seen, there was also often enhancement of the PVS-BG at 4 hours after IV-GBCA. Both observed enhancements were associated with delayed GBCA excretion from the CSF.

Glymphatic System Dysfunction in Myelin Oligodendrocyte Glycoprotein Immunoglobulin G Antibody-Associated Disorders: Association with Clinical Disability

BACKGROUND AND PURPOSE

Impaired glymphatic function has been suggested to be implicated in the pathophysiology of MS and aquaporin-4 immunoglobulin G-positive neuromyelitis optica spectrum disorder. This study aimed to investigate the interstitial fluid dynamics in the brain in patients with myelin oligodendrocyte glycoprotein antibody disorders (MOGAD), another demyelinating disorder, using a noninvasive imaging technique called the diffusivity along the perivascular space (ALPS) index.

MATERIALS AND METHODS

A prospective study was conducted on 16 patients with MOGAD in remission and 22 age- and sex-matched healthy control subjects. MR imaging was performed using a 3T scanner, and the ALPS index was calculated using diffusion MR imaging data with a b-value of 1000 s/mm2. The ALPS index and gray matter volumes were compared between the 2 groups, and these parameters were correlated with the Expanded Disability Status Scale.

RESULTS

The mean ALPS index of patients with MOGAD was significantly lower than that of healthy controls (Cohen d = 0.93, false discovery rate-corrected P = .02). The lower mean ALPS index was significantly associated with a worse Expanded Disability Status Scale score (Spearman ρ = -0.51; 95% CI, -0.85 to -0.02; P = .03). However, cortical volume and deep gray matter volume were not significantly different between the 2 groups, and they were not correlated with the Expanded Disability Status Scale.

CONCLUSIONS

This study suggests that patients with MOGAD may have impaired glymphatic function, as measured by the ALPS index, which is associated with patient disability. Further study is warranted with a larger sample size.

Impaired cerebral interstitial fluid dynamics in cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy, caused by cysteine-altering variants in NOTCH3, is the most prevalent inherited cerebral small vessel disease. Impaired cerebral interstitial fluid dynamics has been proposed as one of the potential culprits of neurodegeneration and may play a critical role in the initiation and progression of cerebral small vessel disease. In the present study, we aimed to explore the cerebral interstitial fluid dynamics in patients with cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy and to evaluate its association with clinical features, imaging biomarkers and disease severity of cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy. Eighty-one participants carrying a cysteine-altering variant in NOTCH3, including 44 symptomatic cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy patients and 37 preclinical carriers, and 21 age- and sex-matched healthy control individuals were recruited. All participants underwent brain MRI studies and neuropsychological evaluations. Cerebral interstitial fluid dynamics was investigated by using the non-invasive diffusion tensor image analysis along the perivascular space method. We found that cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy patients exhibited significantly lower values of diffusion tensor image analysis along the perivascular space index comparing to preclinical carriers and healthy controls. For the 81 subjects carrying NOTCH3 variants, older age and presence of hypertension were independently associated with decreased diffusion tensor image analysis along the perivascular space index. The degree of cerebral interstitial fluid dynamics was strongly related to the severity of cerebral small vessel disease imaging markers, with a positive correlation between diffusion tensor image analysis along the perivascular space index and brain parenchymal fraction and negative correlations between diffusion tensor image analysis along the perivascular space index and total volume of white matter hyperintensity, peak width of skeletonized mean diffusivity, lacune numbers and cerebral microbleed counts. In addition, diffusion tensor image analysis along the perivascular space index was a significant risk factor associated with the development of clinical symptoms of stroke or cognitive dysfunction in individuals carrying NOTCH3 variants. In cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy patients, diffusion tensor image analysis along the perivascular space index was significantly associated with Mini-Mental State Examination scores. Mediation analysis showed that compromised cerebral interstitial fluid dynamics was not only directly associated with cognitive dysfunction but also had an indirect effect on cognition by influencing brain atrophy, white matter disruption, lacunar lesions and cerebral microbleeds. In conclusion, cerebral interstitial fluid dynamics is impaired in cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy and its disruption may play an important role in the pathogenesis of cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy. Diffusion tensor image analysis along the perivascular space index may serve as a biomarker of disease severity for cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy.

Glymphatic system impairment in sleep disruption: diffusion tensor image analysis along the perivascular space (DTI-ALPS)

PURPOSE

This study aimed to evaluate the relationship between sleep quality as assessed using the Pittsburgh Sleep Quality Index (PSQI) and the index of diffusivity along the perivascular space (ALPS index), a possible indirect indicator of glymphatic system activity.

MATERIALS AND METHODS

This study included the diffusion magnetic resonance imaging (MRI) data of 317 people with sleep disruption and 515 healthy controls (HCs) from the Human Connectome Project (WU-MINN HCP 1200). The ALPS index was calculated automatically based on diffusion tensor image analysis (DTI)-ALPS of diffusion MRI. The ALPS index of the sleep disruption and HC groups was compared using general linear model (GLM) analysis with covariates, such as age, sex, level of education, and intracranial volume. In addition, to confirm the relationship between sleep quality and the ALPS index in the sleep disruption group as well as evaluate the effect of each PSQI component on the ALPS index, correlation analyses between the ALPS indices and PSQI scores of all the components and between the ALPS index and each PSQI component was performed using GLM analysis with the abovementioned covariates, respectively.

RESULTS

The ALPS index was significantly lower in the sleep disruption group than in the HC group (p = 0.001). Moreover, the ALPS indices showed significant negative correlations with the PSQI scores of all the components (false discovery rate [FDR]-corrected p < 0.001). Two significant negative correlations were also found between the ALPS index and PSQI component 2 (sleep latency, FDR-corrected p < 0.001) and 6 (the use of sleep medication, FDR-corrected p < 0.001).

CONCLUSION

Our findings suggest that glymphatic system impairment contributes to sleep disruption in young adults.

The asymmetry of glymphatic system dysfunction in patients with temporal lobe epilepsy: A DTI-ALPS study

BACKGROUND AND PURPOSE

While the occurrence of glymphatic system dysfunction has been observed in temporal lobe epilepsy (TLE), the potential asymmetry of this system has yet to be investigated in the TLE context. We aimed to investigate the glymphatic system function in both hemispheres and to analyze asymmetric features of the glymphatic system in TLE patients using diffusion tensor image analysis along the perivascular space (DTI-ALPS) method.

MATERIALS AND METHODS

43 patients (left TLE (LTLE), n = 20; right TLE (RTLE), n = 23) and 39 healthy controls (HC) were enrolled in this study. The DTI-ALPS index was calculated for the left (left ALPS index) and right (right ALPS index) hemispheres respectively. An asymmetry index (AI) was calculated by AI = (Right - Left)/ [(Right + Left)/2] to represent the asymmetric pattern. Independent two sample t-test, two-sample paired t-test or one-way ANOVA with Bonferroni correction were conducted to compare the differences in ALPS indices and AI among the groups.

RESULTS

Both left ALPS index (p = 0.040) and right ALPS index (p = 0.001) of RTLE patients were significantly decreased, while only left ALPS index of LTLE patients (p = 0.005) was reduced. Compared to contralateral ALPS index, the ipsilateral ALPS index was significantly decreased in TLE (p = 0.008) and RTLE (p = 0.009) patients. Leftward asymmetry of the glymphatic system was found in HC (p = 0.045) and RTLE (p = 0.009) patients. The LTLE patients presented reduced asymmetric traits when compared to RTLE patients (p = 0.029).

CONCLUSION

The TLE patients exhibited altered ALPS indices, which could be triggered by glymphatic system dysfunction. Altered ALPS indices were more severe in ipsilateral than in the contralateral hemisphere. Moreover, LTLE and RTLE patients exhibited different change patterns of the glymphatic system. In addition, glymphatic system function presented asymmetric patterns in both normal adult brain and RTLE patients.

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.

Current State of Artificial Intelligence in Clinical Applications for Head and Neck MR Imaging

Due primarily to the excellent soft tissue contrast depictions provided by MRI, the widespread application of head and neck MRI in clinical practice serves to assess various diseases. Artificial intelligence (AI)-based methodologies, particularly deep learning analyses using convolutional neural networks, have recently gained global recognition and have been extensively investigated in clinical research for their applicability across a range of categories within medical imaging, including head and neck MRI. Analytical approaches using AI have shown potential for addressing the clinical limitations associated with head and neck MRI. In this review, we focus primarily on the technical advancements in deep-learning-based methodologies and their clinical utility within the field of head and neck MRI, encompassing aspects such as image acquisition and reconstruction, lesion segmentation, disease classification and diagnosis, and prognostic prediction for patients presenting with head and neck diseases. We then discuss the limitations of current deep-learning-based approaches and offer insights regarding future challenges in this field.

Altered Brain Glymphatic Flow at Diffusion-Tensor MRI in Rapid Eye Movement Sleep Behavior Disorder

Background Brain glymphatic dysfunction may contribute to the development of α-synucleinopathies. Yet, noninvasive imaging and quantification remain lacking. Purpose To examine glymphatic function of the brain in isolated rapid eye movement sleep behavior disorder (RBD) and its relevance to phenoconversion with use of diffusion-tensor imaging (DTI) analysis along the perivascular space (ALPS). Materials and Methods This prospective study included consecutive participants diagnosed with RBD, age- and sex-matched control participants, and participants with Parkinson disease (PD) who were enrolled and examined between May 2017 and April 2020. All study participants underwent 3.0-T brain MRI including DTI, susceptibility-weighted and susceptibility map-weighted imaging, and/or dopamine transporter imaging using iodine 123-2β-carbomethoxy-3β-(4-iodophenyl)-N-(3-fluoropropyl)-nortropane SPECT at the time of participation. Phenoconversion status to α-synucleinopathies was unknown at the time of MRI. Participants were regularly followed up and monitored for any signs of α-synucleinopathies. The ALPS index reflecting glymphatic activity was calculated by a ratio of the diffusivities along the x-axis in the projection and association neural fibers to the diffusivities perpendicular to them and compared according to the groups with use of the Kruskal-Wallis and Mann-Whitney U tests. The phenoconversion risk in participants with RBD was evaluated according to the ALPS index with use of a Cox proportional hazards model. Results Twenty participants diagnosed with RBD (12 men; median age, 73 years [IQR, 66-76 years]), 20 control participants, and 20 participants with PD were included. The median ALPS index was lower in the group with RBD versus controls (1.53 vs 1.72; P = .001) but showed no evidence of a difference compared with the group with PD (1.49; P = .68). The conversion risk decreased with an increasing ALPS index (hazard ratio, 0.57 per 0.1 increase in the ALPS index [95% CI: 0.35, 0.93]; P = .03). Conclusion DTI-ALPS in RBD demonstrated a more severe reduction of glymphatic activity in individuals with phenoconversion to α-synucleinopathies. © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Filippi and Balestrino in this issue.

Evaluation of glymphatic system activity by diffusion tensor image analysis along the perivascular space (DTI-ALPS) in dementia patients

OBJECTIVES

Dementia is a clinical syndrome caused by multiple etiologies, usually manifests with progressive and diffuse brain dysfunction. The activity of the human glymphatic system was evaluated in cases of dementia by the diffusion tensor image analysis along the perivascular space (DTI-ALPS).

METHODS

We recruited 28 healthy subjects and 77 patients, including 38 with Alzheimer's disease (AD),18 with mild cognitive impairment (MCI), 28 with normal controls (NC) and 21 with vascular cognitive impairment (VCI). All participants underwent DTI scanning. Diffusivities in the X, Y and Z axes were obtained in the lateral ventricle body plane of all subjects. We assessed the diffusivity along the perivascular spaces, as well as projection fibers and association fibers, respectively, in order to acquire an DTI-ALPS-index and correlated them with mini mental state examination (MMSE) and montreal cognitive assessment (MOCA) scores using partial correlation which the influence of age was controlled.

RESULTS

The AD, MCI, and VCI patients showed significantly lower DTI-ALPS-index (p < 0.001) compared to the NC. Besides, the VCI group had significantly higher DTI-ALPS-index than the AD group (p = 0.007). There was a significant positive correlation between DTI-ALPS-index and MMSE and MOCA scores (the effect of age was controlled), showing that lower water diffusivity along the perivascular spaces associated with dementia.The higher Dzassoc led to the reduced DTI-ALPS-index in VCI, while lower Dxassoc contributed to the decrease of DTI-ALPS-index in AD.

CONCLUSION

The evaluation of DTI-ALPS demonstrates impairment of the glymphatic system in dementia patients by decreased DTI-ALPS-index. Different from AD, the VCI patients show glymphatic drainage disorder rather than glymphatic system impairment.

ADVANCES IN KNOWLEDGE

This article comprehensively covers several types of dementia and performs the comparison of VCI, AD and MCI in glymphatic system dysfunction.

Association of cognition and brain reserve in aging and glymphatic function using diffusion tensor image-along the perivascular space (DTI-ALPS)

The glymphatic system is a fluid-clearance pathway that clears cerebral waste products, and its dysfunction has been associated with protein aggregation diseases such as Alzheimer's disease. To understand how the glymphatic system changes with aging, we enrolled 433 cognitive unimpaired participants (236 women and 197 men, 13 to 88 years) and evaluated the glymphatic function by calculating diffusion tensor imaging analysis along the perivascular space (ALPS) index and explored how the ALPS index is associated with cortical atrophy and cognitive decline in older people. We found a significant inverse correlation between ALPS index and age (ρ=-0.45, p<0.001), with a peak value in people in their thirties. A higher ALPS index indicated a better cortical reserve in regions coincided with the default mode network. Declines in mental manipulation and short-term memory performance in the older participants were associated with a lower ALPS index and cortical atrophy in the amygdala, anterior and posterior cingulate, thalamus and middle frontal regions. Our findings highlight that the ALPS index could be used to evaluate brain reserve and cognitive reserve in older people.

Improved reproducibility of diffusion tensor image analysis along the perivascular space (DTI-ALPS) index: an analysis of reorientation technique of the OASIS-3 dataset

PURPOSE

Diffusion tensor image analysis along the perivascular space (DTI-ALPS) index is intended to reflect the glymphatic function of the brain; however, head rotation may reduce reproducibility and reliability. This study aimed to evaluate whether reorientation of DTI data improves the reproducibility of the ALPS index using the OASIS-3 dataset.

MATERIALS AND METHODS

234 cognitively normal subjects from the OASIS-3 dataset were included. Original and reoriented ALPS indices were calculated using a technique that registered vector information of DTI to another space and created reoriented diffusivity maps. The F test was used to compare variances of the original and reoriented ALPS indices. Subsequently, subjects with head rotation around the z- (inferior-superior; n = 43) or x axis (right-left; n = 25) and matched subjects with neutral head position were selected for evaluation of intra- and inter-rater reliability. Intraclass correlation coefficients (ICCs) of the original and reoriented ALPS indices for participants with head rotation and neutral head position were calculated separately. The Bland-Altman plot comparing the original and reoriented ALPS indices was also evaluated.

RESULTS

The reoriented ALPS index exhibited a significantly smaller variance than the original ALPS index (p < 0.001). For intra- and inter-reliability, the reorientation technique showed good-to-excellent reproducibility in calculating the ALPS index even in subjects with head rotation (ICCs of original ALPS index: 0.52-0.81; ICCs of reoriented ALPS index: > 0.85). A wider range of the 95% limit of agreement of the Bland-Altman plot for subjects with x axis rotation was identified, indicating that x axis rotation may remarkably affect calculation of the ALPS index.

CONCLUSION

The technique used in this study enabled the creation of reoriented diffusivity maps and improved reproducibility in calculating the ALPS index.

Glymphatic system dysfunction in pediatric acute lymphoblastic leukemia without clinically diagnosed central nervous system infiltration: a novel DTI-ALPS method

BACKGROUND AND OBJECTIVE

Central nervous system (CNS) infiltration commonly occurs in children with acute lymphoblastic leukemia (ALL). Nevertheless, CNS infiltration is rarely detected at the initial diagnosis. The glymphatic system, which regulates cerebrospinal fluid (CSF) and interstitial fluid transport, is considered one of the possible routes of CNS infiltration by leukemia cells. In this study, we used diffusion tensor image analysis along the perivascular space (DTI-ALPS) method to investigate glymphatic system function and obtained CSF volume using synthetic magnetic resonance imaging (SyMRI) in pediatric ALL without clinically diagnosed CNS infiltration.

MATERIALS AND METHODS

Twenty-nine ALL and 29 typically developing (TD) children were prospectively recruited (age 4-16 years) in the present study. Group differences in brain volumetric parameters, brain water diffusivities, and the ALPS index were evaluated after controlling for age, gender, and handedness. Furthermore, significant group-different parameters were correlated with clinical information using partial correlations analysis.

RESULTS

Lower Dxassoc and ALPS index, and increased CSF volume were found in pediatric ALL (all p_{FDR-corrected} < 0.05). Moreover, the ALPS index was negatively associated with the risk classification (r =  - 0.59, p_{FDR-corrected} = 0.04) in pediatric ALL.

CONCLUSIONS

Dysfunction of the glymphatic system and accumulation of CSF were presented in pediatric ALL without clinically diagnosed CNS infiltration. These novel findings suggested that the glymphatic system might be essential in the early-stage process of ALL CNS infiltration, which provides a new direction for exploring underlying mechanisms and early detection of pediatric ALL CNS infiltration.

KEY POINTS

• Lower Dxassoc and ALPS index, and increased CSF volume were found in pediatric ALL (all p_{FDR-corrected} < 0.05). • The ALPS index was negatively associated with the risk classification (r =  -0.59, p_{FDR-corrected} = 0.04) in pediatric ALL. • Dysfunction of the glymphatic system and accumulation of CSF were presented in pediatric ALL without clinically diagnosed CNS infiltration, which suggested that the ALPS index and CSF volume might be promising imaging markers for early detection of pediatric ALL CNS infiltration.