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.

perivascular space (PVS).This hypothesis has been investigated in experimental animals using in vivo intracisternal administrations of two-photon imaging with small fluorescent CSF tracers 2 and dynamic contrast-enhanced magnetic resonance imaging (MRI) using gadolinium-based contrast agents (GBCAs). 3However, intracisternal or intrathecal administration in humans has rarely been performed because it is invasive and risky for health (i.e., it is painful, time-consuming, and causes gadolinium deposition).Therefore, to enable glymphatic research focusing on humans, which might facilitate clinical application, noninvasive procedures for monitoring the glymphatic system are needed in addition to the use of tracer studies.
This review focuses on noninvasive MRI methods for measuring glymphatic system activity and seeks to contribute to the development of noninvasive MRI methods for assessing ISF activity in the CNS.Moreover, this review discusses the weak and strong points and future directions in the use of noninvasive MRI methods for evaluating glymphatic system activity.

Hypothesis of the Glymphatic System
The "glymphatic system" is based on the hypothesis that the brain has a clearance system that eliminates some waste products, including Aβ and tau proteins, which could cause neurodegeneration in the cerebral parenchyma, through fluid circulation (Fig. 1). 2 First, the CSF moves to the cerebral parenchyma along the periarterial space.Second, the CSF moves out from the cerebral parenchyma via the perivenous space.Aquaporin 4 (AQP4) channels facilitate the exchange of CSF flow in the cerebral parenchyma from the periarterial and perivenous spaces.These components related to CSF flow in the cerebral parenchyma are collectively referred to as the "glymphatic system."The concept of the "glymphatic system" was created because the borders of this circulatory system are formed by glial endfeet, and the flow of this system depends on the characteristics of these endfeet.The roles of this system are similar to those of the lymphatic system, which are as follows: 1) delivery system of nutrients and drugs into the interstitial spaces 2 ; 2) mechanism for disposing of waste products such as Aβ and tau proteins, which could cause neurodegeneration, into the perivenous space 2 ; and 3) transportation system of metabolites and wastes that flow from the perivenous space. 2 The glymphatic paradigm, according to which the intrinsic parts of cerebral blood vessels contain an essential compartment for the exchange of paravascular ISF and CSF, 5 is developing.This consists of the following three crucial elements: an intact blood-brain barrier (BBB), which regulates the movement of ions and molecules between the blood and CNS and is essential for CSF/ISF flow and waste clearance in the glymphatic system 6 ; the PVS, which functions as a water reservoir for the interstitium and a pathway for the transport of solutes toward CSF efflux pathways, and the astrocytic endfeet of the glia limitans.Therefore, the BBB, which consists of non-fenestrated vessels that tightly regulate the movement of ions and molecules between the blood and CNS, is an essential component of the glymphatic system involved in clearing interstitial solutes, including Aβ and tau proteins, from the cerebral parenchyma through perivascular water channels (i.e., AQP4) at astrocytic endfeet. 2

Clinical Significance
The adult human brain constitutes approximately 2% of the average body mass and weighs approximately 1.4 kg.The cells of the organism in question are responsible for using a significant portion, specifically 20%-25%, of the organism's overall energy. 7During this process, substantial quantities of protein wastes and biological debris with potential toxic proteins, such as Aβ and tau proteins, are produced in the cerebral parenchyma (Fig. 2).To ensure its survival, the brain requires a mechanism for eliminating waste materials with toxic proteins.
Recently, researchers have investigated the degree to which neural cells engage in intracellular waste processing versus the possibility of extraneural elimination of such waste materials and the mechanisms by which the brain eliminates toxic proteins.For instance, studies have focused on the potential correlation between disruptions to this process and the deposition of abnormal proteins-including Aβ, tau, and α-synuclein proteins-in the brain observed in neurodegenerative disorders, such as Alzheimer's disease (AD) and Parkinson's disease (PD). 8Therefore, a study hypothesized that the occurrence of certain disorders may be attributed to disruptions in waste clearance mechanisms.Indeed, disruptions are anticipated to result in the build-up of proteinaceous waste materials. 2his concept has been garnering attention because protein aggregates such as Aβ, tau, and α-synuclein proteins, which are frequently correlated with neurodegenerative conditions, accumulate within the brain. 9Furthermore, the presence of aggregates has been reported to impede the propagation of both electrical and chemical signals within the neuron, causing irreversible damage. 9Therefore, to investigate waste clearance mechanisms, animal models have been developed and used to replicate the pathology of neurodegenerative diseases including AD and PD by inducing the overproduction of protein aggregates, and considerable evidence has demonstrated that the glymphatic system functions as a pathway for the peripheral clearance of brain solutes and proteins (including Aβ, tau, and α-synuclein proteins) in AD and PD. 2,8Glymphatic system activity has been evaluated using invasive measurement techniques, including fluorescence and contrast agent tracers, in the CSF space.
Following these experiments in animal models, the hypothesis of the glymphatic system in humans has been tested using noninvasive MRI methods, and studies have suggested that human patients with AD [10][11][12][13][14][15][16][17][18][19] and PD [20][21][22][23][24] have glymphatic system impairments.Furthermore, hypertension, which involves cerebral vessel disease and is a risk factor for AD, was reported to be significantly associated with the impairment of glymphatic system function because of the decreased function of the perivascular pump driven by arterial pulsation. 25Moreover, it has been reported that impairments in the waste clearance system are related to CSF absorption and ISF bulk-flow (i.e., glymphatic system; Fig. 1) in patients with idiopathic intracranial hypertension, 26 isolated rapid eye movement (REM) sleep FIGURE 1: The hypothesis of glymphatic system.The cerebrospinal fluid (CSF) periarterial inflow is seen on the left side, passing a layer of glial endfeet via aquaporin 4 (AQP4) water channels.The solutes are subsequently propelled toward the perivenous conduits, depicted on the right side, which lead to efflux from the cerebral parenchyma, while the fluid continues to flow through the interstitial gaps.At the level of intraparenchymal capillaries, capillary endothelial cells are held together by tight junctions forming the blood-brain barrier (BBB), which consists of nonfenestrated vessels that tightly regulate the movement of ions and molecules between the blood and central nervous system (CNS) and is involved in the clearance of interstitial solutes, including Aβ and tau proteins, from the cerebral parenchyma through perivascular water channels (i.e., AQP4) at astrocytic endfeet.The concept of "glymphatic system" was created because the borders of this circulatory system are formed by glial endfeet and because the flow of this system depends on the characteristics of these endfeet and plays roles similar to those of the lymphatic system.This "glymphatic system" plays the following roles: 1) delivery system of nutrients and drugs into the interstitial spaces 2 ; 2) mechanism for washing out wastes, including Aβ and tau protein that could cause neurodegeneration, into perivenous spaces 2,10 ; and 3) transportation system of metabolites and wastes flowed from the perivenous space. 2,11To evaluate the activity of the glymphatic system, the following noninvasive MRI methods have been proposed: the volume and count of perivascular spaces (PVS) and diffusion tensor image analysis along the perivascular space (DTI-ALPS), which evaluates ISF dynamics; arterial spin labeling (ASL), which monitors the permeability of water in the BBB; chemical exchange saturation transfer (CEST); and intravoxel incoherent motion (IVIM), which indirectly predicts CSF/ISF flow in the brain parenchyma.
behavior disorder, 27 obstructive sleep apnea, 28 and metabolic syndrome, 29 and in participants playing contact sports 30 and patients with AD and PD.Furthermore, some lifestyle activities, including sleep, 31,32 could have significant associations with the glymphatic system.Thus, it is anticipated that comprehending the consequences of a malfunctioning glymphatic system will allow for the development of novel diagnostic approaches and therapies for various neurological disorders.

Measurement Technique for Glymphatic System Activity
Intrathecal injection of GBCA can be considered the current gold standard for evaluating the glymphatic functions. 33lthough this evaluation has been mainly utilized in animal models, adopting this evaluation method in humans has numerous challenges.To overcome these problems, noninvasive MRI techniques for monitoring the glymphatic system have been introduced, including intravenous injection of GBCA, PVS, diffusion tensor image analysis along the PVS (DTI-ALPS), arterial spin labeling (ASL), chemical exchange saturation transfer (CEST), and intravoxel incoherent motion (IVIM).As shown in Fig. 1, the CSF/ISF flow in the brain parenchyma is predicted by the volume and count of PVS and DTI-ALPS evaluate ISF dynamics, ASL monitors the water permeability of the BBB, and CEST and IVIM indirectly predict.In this section, we introduce the intrathecal injection of GBCA in humans as well as animals, highlight the associated problems, and suggest the solutions with explanation of the technique with the concepts and clinical applications of these noninvasive MRI techniques.

Intrathecal Injection of GBCA
In the tracer studies using intrathecal GBCA and fluorescent materials in the live mouse brain, 3 the glymphatic pathway for waste clearance and exchange between CSF and ISF was demonstrated.Additionally, studies have evaluated the glymphatic system using intrathecal injections of GBCA in human brain MRI.Eide and Ringstad examined a 27-yearold woman with spontaneous CSF leakage and intracranial hypotension via MRI with intrathecal GBCA, which was distributed throughout her brain, including the cerebral cortex, white matter, and lateral ventricles, after 1 and 4.5 hours. 34GURE 2: The trajectories involved in the transition from healthy cognition to Alzheimer's disease.The graph demonstrates a model for the temporal changes in biomarkers along the cognitive continuum from healthy cognition to Alzheimer's disease.During the course of dementia, substantial quantities of protein wastes and biological debris with potential toxic proteins, such as Aβ and tau proteins, which could cause neurodegeneration, are produced in the cerebral parenchyma.To ensure its survival, the brain requires a mechanism for eliminating waste materials with toxic proteins.9][20] Several studies on the brain clearance system have been uncovering a previously unknown mechanism for removing neurotoxic proteins from the cerebral parenchyma.The mechanism is the "glymphatic system."It is anticipated that comprehending the consequences of a malfunctioning glymphatic system will allow for novel diagnostic approaches and therapies for various neurological disorders (Adapted and reproduced with permission by Dr. Jack and ADNI, http://adni.loni.usc.edu/study-design/).
Results of these studies on intrathecal GBCA distribution across the entire brain implied free transport of the contrast medium and CSF and CSF-ISF exchange.Another study with eight healthy human adult volunteers showed that GBCA flowed in the basal cisterns by the first imaging time point after 1.8 hours from the intrathecal injection and peaked in the intracranial CSF spaces within 1-3 hours and started to decrease by 7 hours. 35After 11 hours, the MR signal remained elevated in the cerebral cortex and white matter, suggesting the GBCA persisted in the tissues.Thus, the enhanced brain-wide MR signal due to intrathecal GBCA administration might indicate CSF/ISF exchange and brain parenchymal penetration throughout the brain, suggesting that MRI with intrathecal GBCA could be used to assess human brain glymphatic function.
However, intrathecal GBCA injections may elicit anaphylactic reactions including headache and severe nausea 36 and neurotoxic effects including speech issues, psychotic symptoms, lethargy, and visual impairment. 37Additionally, image acquisition is a time-consuming process.The gadolinium of GBCA enters the brain tissue through the glymphatic system according to CSF flow and can deposit in the parenchyma tissues, including the dentate nucleus and globus pallidus. 38,39Thus, it is unlikely that this examination of the glymphatic system will be widely used as a screening tool for asymptomatic patients.

Intravenous Injection of GBCA
To assess the kinetics of signal intensity changes in the brain parenchyma and CSF space, Taoka et al 40 performed MRI examinations on rats after administering intravenous GBCA.The fourth ventricle demonstrated an immediate increase in signal intensity after intravenous GBCA administration. 40eyond the fourth ventricle, albeit before the prepontine cistern, the signal curve of the cerebral cortex and deep cerebellar nuclei attained the highest signal strength.This time course indicated that the GBCA distribution in the cerebral cortex and deep cerebellar nuclei is reliant on both blood flow and CSF, indicating that CSF is one of the possible entry routes of GBCA into the cerebral parenchyma. 34,41nother intravenous GBCA injection experiment performed on rats revealed that the concentrations of GBCA in the brains of rats in the morning injection group were significantly lower than those of rats in the late afternoon injection group, indicating increased glymphatic clearance after the morning injection and decreased glymphatic activity after injection in the late afternoon. 40The glymphatic system should be more active throughout the day because rats are nocturnal creatures. 42imilarly, transitions into the cerebral parenchyma have been observed in human studies using intravenous GBCA injections. 43A histogram analysis was used in one investigation to assess the extent of BBB leaking in patients with AD.The findings indicated that the rate of BBB leakage was considerably greater in the cortex and total gray matter of AD patients than those of the control participants, indicating that a damaged BBB may influence the chain of pathologic events that finally culminates in cognitive decline and dementia. 43][46][47] Even in participants without renal impairment, Naganawa et al demonstrated a signal intensity in the PVS for 4 hours after intravenous administration of GBCA. 44,45dditionally, after injection, no discernible contrast increase was observed in the PVS in the white matter; however, an increase was noted after injection in the CSF in the Sylvian fissure, the ambient cistern, and the basal ganglia.This discrepancy, in contrast with the enhancement, may be considered an indication to assess the drainage function of waste products including neurotoxic proteins in these regions. 46n summary, research on rats and humans using intravenous GBCA administration revealed insights into the dynamics of signal intensity changes, GBCA distribution, glymphatic clearance, BBB leakage, and PVS enhancement.These findings contribute to our understanding of GBCA behavior following intravenous administration in the brain and its implications for various conditions and processes, including brain entry, cognitive decline, and glymphatic system involvement.

Perivascular Space
GLYMPHATIC SYSTEM AND PVS.The PVS covers the walls of arteries, arterioles, capillaries, veins, and venules to ensure the flow from the subarachnoid space through the cerebral parenchyma.The outer limits of PVS are the glia limitans of the brain, and the inner limits including the capillary basement membranes of the vessel. 38As the arterioles branch into the capillaries, the basement membranes of the pial sheath and the astrocytes (i.e., glia limitans) connect with each other to form a perivascular space. 48The space (e.g., PVS) between the vessel and the brain tissue is filled with ISF.
The PVS has several functions as follows: 1) it represents the main conduit to the drainage of ISF from and to the cerebral parenchyma; 2) it serves as a conduit for the transport of various signaling molecules; 3) it is a component for the exchange between ISF and CSF; 4) it is a neurotoxic waste clearance system; 5) it functions in the control of fluid circulation homeostasis in the brain. 49PVS enlargement has a negative impact on its functions and deteriorates the function of the glymphatic system.PVS enlargement may occur because of CSF stagnation caused by the disruption of CSF flow; however, to the best of our knowledge, no study has reported the evidence related to the precise mechanisms and etiologies of PVS enlargement. 50eportedly, PVS enlargement may have different pathophysiological bases.PVS enlargement is often accompanied by lacunar infarction and becomes prominent in patients with basal ganglia lacunar stroke. 50The recurrence rate of stroke is higher in patients with ischemic stroke and PVS enlargement in the basal ganglia. 50Additionally, in the centrum semiovale in particular, the cortical accumulation of Aβ proteins is higher, and Aβ protein clearance is lower because of PVS enlargement in cerebral amyloid angiopathy (CAA). 51Spontaneous cerebral hemorrhage is associated with hypertensive arteriopathy caused by PVS enlargement in the basal ganglia, 51 whereas PVS enlargement in the centrum semiovale increases the risk of cerebral hemorrhage in CAA. 52ost-stroke depression is often accompanied by neuropsychiatric symptoms in approximately 30% of cases 53 and is associated with PVS enlargement, particularly in the centrum semiovale, with a negative correlation of antidepressant response scores.
MRI OF THE PVS.Visual rating scale.A common tool for evaluating PVS severity in human is a visual rating scale based on the Potter scoring with five grades according to the PVS count in the centrum semiovale and basal ganglia and grades 0-1 for the midbrain based on the presence of PVS; the grades 0, 1, 2, 3, 4, and 5 indicate PVS counts of 0, <1-10, 11-20, 21-40, and >40, respectively 54 (Fig. 3).To evaluate PVS in the hippocampus, the study group of the Chinese IntraCranial AtheroSclerosis defined PVS severity in the hippocampus according to the PVS count with three grades, i.e., grades 1, 2, and 3 indicating PVS counts of <5, 5-10, and >10, respectively. 55Although the abovementioned visual rating scales evaluate PVS severity based on the PVS count, Heier et al evaluated PVS severity based on the diameter of the PVS with three grades, i.e., grades 1, 2, and 3 indicating sizes of <2, 2-3, and >3 mm, respectively. 56hese PVS visual rating scales demonstrated that PVS severity has a significant positive correlation with age and a negative correlation with cognitive scores.Mini-Mental State Examination (MMSE) scores and PVS severity are higher in patients with AD and mild cognitive impairment (MCI) than in the participants in the normal cognition group. 12,13oreover, these visual rating scales are associated with microvascular abnormalities and showed significant differences between patients with ischemic vascular diseases and those with degenerative dementia, including AD, and frontotemporal dementia. 14Additionally, the number of PVS in the basal ganglia was significantly higher in patients with PD than in healthy controls and was significantly associated with cognitive function, especially that measured using the Montreal Cognitive Assessment. 22Furthermore, in a population-based study involving individuals without dementia (age ≥ 60 years; mean age 70.7 AE 9.1 years), the number of PVS globally, in the basal ganglia in particular, was reportedly correlated with the volume of white matter hyperintensity (WMH), implying that the PVS is a marker for cerebral vascular diseases. 57lthough these visual rating scales are useful and practical, their inter-and intra-reproducibility could change according to the experience of the observer and the coexistence of WMH with lacunae for basal ganglia PVS (ranges: 0.76-0.87 and 0.80-0.90,respectively) and centrum semiovale PVS (ranges: 0.68-0.75 and 0.61-0.8,respectively). 54oreover, these visual rating scales are influenced by the ceiling effect. 38To elaborate, either no PVS is visible or more than a specific number is visible in the lower and upper echelons for these categories.Therefore, assessors are encouraged to assign a scan to the lowest category if they are unable to observe any PVS or the highest category in the presence of several PVS.This may lead to floor and ceiling influences.Automated quantification can solve the problems of visual rating scales and allows for the measurement of increased PVS with a high degree of accuracy and may enable the use of the PVS as a biomarker for disease development.
Automatic computed system for PVS count and volumetry.Recently, an automatic computed system for PVS evaluation in human using MRI has been developed and is becoming promising in clinical settings.For instance, Dubost et al 58 have developed a 3D neural network to evaluate the degree of PVS enlargement in humans and showed high performance, which was as high as an expert's visual scaling scores (intraclass correlation coefficient [ICC], 0.74; ICC criteria, substantial). 59Moreover, the intra-rater consistency of their automatic system was higher than that of an expert (ICC: 0.93 vs. 0.80; ICC criteria, almost perfect). 59Thus, this automatic computed system has higher reproducibility than an expert in neuroradiology.Additionally, the computed PVS counts are significantly correlated with age 58 and can reflect biological information including age, sex, and diseases.Indeed, some computational PVS evaluation systems have been developed, including a mathematical method for evaluating PVS morphological characteristics-i.e., linearity, volume, width, length, and size 60,61 -and a novel fully convolutional neural network to automatically segment the PVS using MRI. 621][62] In particular, computed PVS features in the basal ganglia reflected WMH severity and lacunae. 61reprocessing for input MRI has also been proposed along with the development of automatic computed systems for PVS evaluation.For instance, Sepehrband et al 63 have used a combined image generated by dividing a T1-weighted image by a T2-weighted image of the human brain, resulting in an enhanced PVS contrast (EPC) image (Fig. 4).This technique effectively intensified the visibility of the PVS.EPC images were attained through the fusion of T1-and T2-weighted images that underwent adaptive filtration to eliminate nonstructured high-frequency spatial noises.EPC was assessed on a cohort of young and healthy individuals, where two proficient evaluators were engaged and automated quantification techniques were used.The implementation of EPC has been shown to enhance the visibility of the PVS and facilitate the resolution of greater quantity of such PVS.
The aforementioned developments of automatic computed systems for PVS evaluation have contributed to the elucidation of the glymphatic system.To elaborate, in a study involving 1789 healthy participants (mean age: 35 AE 22 years; age range: 8-100 years), 13 the degree of PVS enlargement was positively correlated with age.Additionally, the relationship between PVS volume and age changed in the patients' mid-30s and at the age of 55 years.Notably, PVS volume in the basal ganglia was negatively correlated with age until the patients' mid-30s; however, it was positively correlated with age until and over the age of 55 years. 13In a previous study, patients with AD had significantly higher PVS volumes in the basal ganglia than cognitively healthy individuals. 11Furthermore, Vilor et al reported that the levels of p-tau, t-tau, and neurogranin were positively correlated with PVS volume in Aβ-positive individuals with AD. 15 Patients with PD were reported to have larger PVS volumes than healthy controls, [20][21][22] and PVS enlargement was significantly correlated with the L-dopa equivalent dosage and PD disease severity, according to the Movement Disorder Society Unified Parkinson's Disease Rating Scale, Parts I-IV. 22,24imilar to patients with other diseases, PVS enlargement in the centrum semiovale in patients with neuromyelitis optica spectrum disorder was more severe than that in healthy controls, which was correlated with brain atrophy and poor cognitive performance according to the Paced Auditory Serial Addition Test 3. 64 Additionally, patients with late relapsingremitting multiple sclerosis (MS) had larger enlarged PVS in the basal ganglia than those with early relapsing-remitting MS and clinically isolated syndrome, and this finding was associated with lesion volume but not with brain atrophy. 65t has also been reported that arterial stiffness was positively and significantly correlated with PVS volume in the basal ganglia, associated with worse cognitive function, including information processing, episodic memory performances, visuospatial performances, and executive function. 66In the study conducted by Berezuk et al, 32 sleep efficiency was negatively correlated with PVS volume in the basal ganglia and the entire brain, whereas awakening after sleep onset was positively correlated with PVS volume in the basal ganglia, implying that sleep plays an important role in the PVS in ischemic brain diseases and the function of the glymphatic system.Thus, automatic computed systems for evaluating PVS count and volumetry assess PVS burden in the brain and may indirectly predict the function of the glymphatic system and adverse cognitive and motor functions.DTI-ALPS.DTI-ALPS is a technique that uses diffusion MRI to evaluate glymphatic system the activity based on the dynamics of ISF in the human brain 67 (Fig. 5).The veins and arteries of the medulla run alongside the PVS, which serves as the primary drainage route for the glymphatic system within the brain parenchyma.At the level of the lateral ventricle, the orientation of the PVS is aligned with medullary  63 veins, which run perpendicular to the ventricular wall.Specifically, the PVS exhibits a right-left directionality along the x-axis.Within this particular area, there exists a distinct orientation of projection fibers that traverse in the headto-foot direction (z-axis), primarily in close proximity to the lateral ventricle.Additionally, superior longitudinal fascicles (SLFs), which serve as association fibers, exhibit a distinct orientation in the anterior-posterior direction (y-axis), separate from the aforementioned projection fibers.In subcortical regions, the predominant direction of subcortical fibers is oriented along the rightleft axis (x-axis), except for SLFs.Therefore, it can be observed that in this particular region, the PVS is oriented at a right angle to both the SLFs and projection fibers.The PVS configuration and primary fibers permit autonomous examination of diffusivity along the PVS because the major fiber tracts are not oriented parallel to such a direction.The occurrence of comparable modifications in diffusivity along the horizontal axis (x-axis) in the projection and association fiber regions is attributed to variations in diffusivity along the same axis that aligns with the PVS.To assess glymphatic system activity using diffusion MRI, the DTI-ALPS technique is employed, which involves the analysis of diffusion tensor images along the PVS.The resulting scores are referred to as ALPS scores.The metrics are ascertained by computing the ratio between two diffusivity sets oriented orthogonally to the principal fibers present within the biological tissue.Regarding projection fibers, it can be observed that the prevailing fibers exhibit a z-axis orientation, whereas the xand y-axes are orthogonal to the aforementioned fibers.Likewise, regarding association fibers, the preponderant fibers exhibit a y-axis orientation, whereas the xand z-axes maintain a perpendicular relationship with the aforementioned fibers.The ALPS index was calculated as a ratio of the mean diffusivity 67 : where D xx proj is the x-axis diffusivity in the projection area, D xx assoc is the association area, D yy proj is the y-axis diffusivity in the projection area, and D zz assoc is the z-axis diffusivity in the association area.When the ALPS index approaches unity, the impact of water diffusion in the PVS is minimal, whereas a higher ratio signifies an increased diffusivity of water in the PVS.
In the work by Zhang et al, 68 who attempted to validate whether the ALPS index exactly reflects the function of the glymphatic system, they showed that the ALPS index has a significant high correlation (r = $À0.844;P < 0.001) with the classical detection of glymphatic system function calculated on glymphatic MRI after the intrathecal administration of GBCA.This finding suggests that the ALPS index represents the activity of the glymphatic system, which could be applied in clinical settings in the future.Following this research, there have been reports on the application of DTI-ALPS for evaluating various diseases.
The first application of DTI-ALPS was in AD.DTI-ALPS was used to evaluate the glymphatic system. 67The result The relationship between the perivascular space and the directions of the association and projection fibers.Dominant fibers in the association area run in the y-axis direction, perpendicular to both the x-and z-axes, whereas dominant fibers in the projection area run in the z-axis direction, perpendicular to both the x-and y-axes.(b) On the color-coded FA map, sphere ROIs were placed in the association and projection areas at the level of the lateral ventricle.(c) Finally, the ALPS index was calculated using the tensor map (D xx , D yy , and D zz ) with the sphere ROIs.When the ALPS index approaches unity, the impact of water diffusion in the PVS is minimal, whereas a higher ratio signifies an increased diffusivity of water in the PVS.For instance, the ALPS index had a significant negative association with Mini-Mental State Examination (MMSE) scores, which is a cognitive function test, and AD severity. 67howed that the ALPS index had a significant negative association with the MMSE score, which is a cognitive function test.Another study including participants with AD, MCI, and subjective memory complaints showed a significantly negative correlation between DTI-ALPS and MMSE scores. 10Moreover, the ALPS index was reported to be positively correlated with the level of CSF Aβ and FDG-PET SUVR in patients with MCI and AD. 11This result suggested that the impairment of the glymphatic system causes Aβ accumulation in the brain.Additionally, studies have reported a significant decline of the ALPS index in patients with isolated REM sleep behavior disorder, 27 PD, 23 obstructive sleep apnea, 28 and metabolic syndrome 29 and participants with contact sports 30 compared with healthy controls.As described, the association between the glymphatic system and the ALPS index in multiple diseases, including AD, has been reported.Moreover, significant associations of the ALPS index with sleep duration 31 and hypertension 25 have been reported.A study of sleep duration has indicated that short non-REM sleep reduces glymphatic system activity and induces neurotoxicity in areas responsible for non-REM sleep.Furthermore, the results of a hypertension study suggested that decreased arterial pulsation caused by atherosclerosis was associated with glymphatic system dysfunction.Furthermore, the ALPS index was reportedly improved after shunt surgery in patients with idiopathic normal-pressure hydrocephalus (iNPH). 69The ALPS index may be effective for postoperative evaluation of iNPH.Thus, the ALPS index based on DTI-ALPS has a potential as a biomarker to evaluate the activity of the glymphatic system in neurological diseases.
The proposed method for calculating the ALPS index based on DTI-ALPS was influenced by head rotation and could lead to low reproducibility and reliability.1][72][73][74] This technique included a registration of individual brain MRIs to create brain templates to remove the variance of head rotation with individual differences and then the calculation of the ALPS index, called reoriented ALPS 70 or ALPS index maintaining tensor vector orientation information (vALPS index) 71 after manually placing the regions of interest (ROIs) on the DTI tensor map.The reports using this novel technique demonstrated that the variance of the reoriented ALPS index was significantly smaller than that of the original ALPS index tested using the F-test and that the absolute difference in ALPS index values was reduced by approximately 0.6% between scan and rescan.This result suggested that the additional technique of DTI reorientation was useful for calculating the ALPS index.
Taoka et al 75,76 also suggested a technique that entails the use of diffusion-weighted imaging (DWI) with a threeaxis diffusion gradient direction to calculate the ALPS index based on DWI-ALPS.Furthermore, they found the correlation between the ALPS index and age in healthy subjects (118 cases) and assessed the difference in the ALPS index based on DWI-ALPS of different pathologies, such as AD (17 cases), PD (11 cases), and subdural hemorrhage (16 cases).DWI was acquired according to the following clinical order: echoplanar imaging; diffusion gradient directions, orthogonal three axes (x-, y-, and z-axes); b-value, 0 and 1000 s/mm 2 ; acquisition duration, 1 minute and 11 s.Instead of using the diffusion tensor map to determine the ALPS index, apparent diffusion coefficient (ADC) pictures in the x-, y-, and z-axes were calculated, and color fractional anisotropy-like images were produced similarly to DTI.Then, using ADC pictures, similar to DTI-ALPS, the ALPS index was derived, as follows: Only secondary regression-not linear regressionshowed a significant association in the polynomial regression.Patients in the 40's generations had a peak in the polynomial regression line, which was corroborated by the analysis of variance results.This may imply that, compared with younger or older age groups, glymphatic system function is most effective in the 40's generations.The evaluation of the DWI-ALPS index under pathological conditions can be improved by the scatter plot between age and the ALPS index based on DWI-ALPS.Additionally, only the AD group demonstrated a statistically significant difference from healthy subjects, even though both the PD and AD groups displayed lower mean DWI-ALPS indices.Interestingly, the subdural hemorrhage group also displayed a considerably lower DWI-ALPS score, which is most likely due to the patients' reduced glymphatic system function.Because of this, the DWI-ALPS approach is more popular in clinical practice and requires less imaging time than DTI-ALPS.In several clinical investigations, assessing glymphatic system function appears to be helpful.Therefore, DWI-ALPS based on DWI used in clinical settings can be a promising biomarker for assessing the ISF dynamics or glymphatic system and would be fundamental in diagnosing diseases in the future.

Other Imaging Techniques for the Glymphatic System
ARTERIAL SPIN LABELING.ASL is a novel MR perfusion imaging technique that enables the noninvasive evaluation of cerebral perfusion using blood as an endogenous tracer, rather than relying on an exogenous contrast agent. 77The purpose of this technique was to assess glymphatic system activity based on the BBB's water permeability, which might determine CSF/ISF flow and waste clearance in the glymphatic system. 6,77,78igure 6 shows the concept of ASL for glymphatic system evaluation.Considering three components-the capillaries, BBB, and parenchyma (Fig. 6a)-these relationships are modeled and shown in Fig. 6b, where M c and M b are the longitudinal magnetizations in the capillaries and parenchyma, respectively.After ASL, M c decreases as M b increases.The change is defined by water exchange (kw [min À1 ]).Thus, ASL predicts glymphatic system activity using water exchange (kw), and therefore, we hypothesize that glymphatic system activity increases with the increment of water permeability in the BBB.
In a recent investigation for human, to evaluate the fluid exchange across the BBB, a diffusion-prepared arterial spin labeling (DP-ASL) MRI technique was employed in older adults with normal cognitive function (Fig. 7).The results revealed a significant association between the fluid exchange across the BBB and the level of Aβ42 concentration in CSF in a broad brain area. 78Dickie et al 79 have provided a comprehensive overview of contemporary MRI methodologies used for detecting water exchange across the BBB.These techniques were classified into three distinct categories.This article discusses various methods for measuring cerebral blood flow.The first category of methods is based on ASL and includes phase-contrast ASL, multiecho time ASL, magnetization transfer-weighted ASL, contrast-enhanced ASL, and diffusion-weighted ASL.The second category is a contrastbased technique that includes dose ramping at a steady state with varied infusion rates, first-pass methods, multiple flip angle multiecho MRI, and water exchange index methods.The third category consists of methods that rely on the injection of MRI-detectable water tracers, such as indirect detection of 2 H-labeled water and 17 O-labeled water via its effect on 1 H T 2 by proton replacement.These methods have been previously discussed in the literature. 79The exploration of alternative approaches for investigating the function of aquaporins within capillaries may be a valuable pursuit.The structural foundations of the BBB and glymphatic system exhibit similarities at the capillary level.These systems may work in tandem and provide mutually beneficial functions in preserving neuronal equilibrium and intracerebral homeostasis.Furthermore, Joseph et al 16 have assessed the perfusion clearance in the brain parenchyma in patients with AD using 3D turbo gradient spin-echo pulsed ASL with long inversion time and showed that the rate of signal clearance was significantly lower in patients with AD than in cognitively healthy participants.Moreover, patients with AD had a significant association with T 1 value in CSF.This suggests that a ratio of ultra-filtered labeled blood within perivascular fluid is maintained and shows reduced paravascular clearance in AD.
CHEMICAL EXCHANGE SATURATION TRANSFER.The use of CEST in MR spectroscopic imaging is a wellestablished method for detecting compounds present at low concentrations.Compared with standard magnetic resonance spectroscopy imaging, CEST can measure compounds at two orders of magnitude lower concentrations. 80The introduction of a saturation radiofrequency pulse to compounds containing exchangeable hydrogen protons leads to the transfer of the saturated hydrogen protons to the free water pool through chemical exchange.This phenomenon reduces the signal of free water.The detection of alterations in the signal intensity caused by free water is an indirect means of obtaining information on compounds.
According to recent studies, [17][18][19] the use of this novel MRI technology has demonstrated potential in identifying bodily parameters, including enzyme activity, pH levels, temperature, and metabolites, such as inositol, glutamate, creatine, and glucose.Therefore, the purpose of CEST is to evaluate glymphatic system activity by estimating how the clearance of these compounds in the brain proceeds.A mouse study 19 has evaluated the glymphatic system based on the kinetics of CSF using the injection of biodegradable d-glucose in transgenic AD models and wild-type mice of different ages.These studies demonstrated that the older AD model mice had decreased glucose uptake and delayed clearance of d-glucose from the CSF.Additionally, a pig study experimented on a porcine model to investigate the impact of impaired glymphatic system function. 81They used CEST MRI at 7-Tesla to differentiate intrinsic compounds present in the CSF, lymph, and blood.The results indicated that the CEST effect of lymph was more significant than that of blood and CSF.The authors referred to this phenomenon as the "Lym-CEST" effect 81 (Fig. 8).Subsequently, the "Lym-CEST" phenomenon was employed to evaluate the compromised condition of the glymphatic system in a pig model featuring unilateral cervical lymph node ligation.The findings of the study revealed a notable disparity in signal intensity between the ipsilateral and contralateral hippocampi.The observed variation in signal intensity was found to be closely associated with behavioral scores. 81he findings of this study suggest that CEST MRI can serve as a valuable instrument for examining metabolites and identifying alterations in the glymphatic system in vivo.Presently, to the best of our knowledge, no study has documented the use of CEST to examine substances in the lymphatic fluid of the human brain.However, this lack of documentation does not negate the significant potential of CEST in investigating glymphatic system function and pathogenic mechanisms.

INTRAVOXEL INCOHERENT MOTION.
The concept of IVIM pertains to varied speed movements within a voxel, which can be in different orientations or amplitudes. 82Originating in 1986 with diffusion MRI, IVIM was introduced to describe how the blood flows in tiny vessels-capillariesimitating a diffusion process.The IVIM model considers both diffusion and perfusion effects using the following biexponential model: where S is the MR signal intensity relative to baseline S 0 without diffusion-weighted images.S=S 0 is signal attenuation.f is the perfusion fraction, indicating the volume  fraction of capillaries in a voxel.In contrast, 1 À f ð Þ reflects diffusion effects with ADC in the extravascular space.D * is the pseudo-diffusion coefficient and reflects dephasing due to perfusion in capillaries.The IVIM effect occurs at low b-values (i.e., ≤300-500 s/mm 2 ) where the signal attenuation is greater (e.g., ADC is higher) than expected (S=S 0 Þ due to increased signal loss from IVIM of microscopic perfusion (Fig. 9).However, IVIM is not exclusive to the microcirculation of blood and has also been suggested as a technique that quantifies the complicated tiny CSF movements for assessing glymphatic system function. 2,49,83Le Bihan et al 84 evaluated two diffusion components in human brain using IVIM, namely, microvascular and parenchymal components, which are composed of intravascular diffusivity of microvascular blood and water diffusion in the parenchyma, respectively.For instance, in the diffusion spectrum (Fig. 10a), parenchymal diffusivity, which corresponds to water diffusion in the parenchyma, is 0.1-1.5 Â 10 À3 mm 2 /s, and intravascular diffusivity, which corresponds to microvascular perfusion, is 4.0-1000 Â 10 À3 mm 2 /s.However, small vessel diseases, such as WMH and enlarged PVS, include abnormal amounts of ISF caused by an impaired BBB; it can lead the water of ISF to more freely diffuse and decrease its dynamics, causing the decline of glymphatic system function. 85Therefore, it has been hypothesized that the brain with cerebral small vessel diseases has a mixture of diffusion and perfusion components, called the intermediate diffusion component, which has similar CSF diffusivity (3.0 Â 10 À3 mm 2 /s), and this component with small vessel diseases might have an impact on the dynamics of ISF and the activity of the glymphatic system. 85ecently, to analyze this intermediate diffusion component, a non-negative least-squares (NNLS) method, which is a tool for inferring nonparametric T2 spectrum, 86 has been proposed for IVIM analysis and provides a diffusion spectrum of microvascular perfusion and parenchymal diffusion in human brain. 87In the human studies by Wong et al 88 and Yamada et al, 89 aiming to enhance IVIM analysis in the brain with cerebral small vessel diseases to investigate the presence of an additional intermediate diffusion component in the diffusion spectrum and its potential relation to the legion of cerebral small vessel diseases, such as WMH and visible enlarged PVS on MRI using NNLS, the presence of an additional intermediate diffusion component and intermediate diffusion volume fraction (f int ) that differs from the parenchymal and microvascular diffusion components in lesion-prone regions and between parenchymal diffusion and microvascular perfusion was observed (Fig. 10a).The study found a significantly positive correlation of f int with the volume of PVS and WMH in both the basal ganglia and centrum semiovale, respectively (Fig. 10b,c).The mean f int values were 4.2% AE 1.7%, 7.0% AE 4.1%, and 13.6 AE 7.7% in the basal ganglia and 3.9% AE 1.3%, 4.4% AE 1.4%, and 4.5% AE 1.2% in the centrum semiovale for the low, moderate, and high number of enlarged PVS groups, respectively.Furthermore, f int significantly increased with the extent of enlarged PVS in the basal ganglia (r = 0.49, P < 0.01) and centrum semiovale (r = 0.23, P < 0.05).The prevalence of f int in WMH was 27.1% AE 13.1% and exhibited a significantly positive correlation with the WMH volume (r = 0.57, P < 0.01).Therefore, this result suggests the existence of an intermediary diffusion component in cerebral small vessel diseases and its relationship with WMH and PVS.The presence of such lesions in the tissue may result in perivascular edema or tissue degeneration, which may consequently increase the aberrant amount of ISF, thereby contributing to the phenomenon of f int .Therefore, it is plausible that the intermediate diffusion volume fraction may serve as an indirectly measurable imaging biomarker for the impairment of the glymphatic system by evaluating the degeneration of the cerebral parenchyma in cerebral small vessel diseases.

Conclusion and Future Directions
Noninvasive MRI measures of glymphatic system activity have provided new evidence and findings of the glymphatic system in the human brain, contributing to the elucidation of the mechanism of the glymphatic system.Nonetheless, the clinical evidence of glymphatic system activity using noninvasive MRI remains insufficient to introduce these techniques in clinical settings, such as the clinical use of DWI to interpret acute infarction, unlike the voxel-based specific regional analysis system for AD that evaluates the hippocampus volume and one of the AD biomarkers to decrease the cost of clinical trials (by approximately 30%-40%) and sample size (by approximately 40%-60%). 90To achieve the clinical application of noninvasive MRI methods for evaluating the glymphatic system, there are several concerns to overcome.First, as Piantino et al mentioned the limitation of DTI-ALPS, 91 the relationship between the activity of the glymphatic system evaluated using noninvasive MRI measurements and pathological changes caused by the absence of pathological studies on the glymphatic system remain unknown.Recently, the ALPS index based on DTI-ALPS was reported to show a strong association with the function of the glymphatic system, as calculated via glymphatic MRI with intrathecal GBCA, which can be considered the current gold standard for evaluating glymphatic system functions.However, except for DTI-ALPS, whether noninvasive MRI measures can exactly reflect pathological changes in the glymphatic system remains unclear.To overcome this problem, further studies on the glymphatic system in autopsy animal models or humans are necessary to elucidate how these noninvasive MRI methods for evaluating glymphatic system activity reflect pathological findings.
Second, to ensure high statistical power and reliability, recent multisite studies with large sample sizes have increased.Some studies on mental health have reported that reproducible results require a sample size of at least a few thousand persons. 92However, using data from various locations inevitably introduces bias because of variations in MRI systems, including hardware (e.g., inhomogeneities of B0 field, B1 transmitter, and receive fields), acquisition parameters (e.g., repetition time, echo time, and voxel size), and reconstruction algorithms that can affect the signal-to-noise ratio, 93 reproducibility, and bias to identify disease-relative signals. 94,95Therefore, a multisite study contains sampling bias (e.g., sex, age, and disease) and measurement bias.The combined association test (COMBAT), a development of GLM with an empirical Bayesian-based harmonization, is one of the most widely used harmonization approaches in neuroimaging. 96COMBAT reduces measurement bias and improves statistical power, and was reported to be helpful for standardizing measurements, such as cortical thickness and volume for MR volumetry. 97This suggests that the COMBAT is a promising harmonization method for PVS volumetry and other measurements using multisite studies.According to the study conducted by Taoka et al, 98 the ALPS index of DTI-ALPS may change depending on the echo time and diffusion gradient direction.To enable the multisite study using DTI-ALPS, three 3-Tesla MRI scanners with various protocols were used for AD and cognitively normal populations to apply the COMBAT to the ALPS index of DTI-ALPS. 99he COMBAT consequently harmonized scanner differences and increased the effect size of the ALPS index between normal cognition and AD by approximately 1.5 times and improved the correlation of the ALPS index with cognitive scores, the standard uptake values of positron emission tomography (Fig. 11).Therefore, to elucidate the mechanism of the glymphatic system with high statistical power and reliability, a further harmonization study using a large sample size of noninvasive MRI measures of glymphatic system activity (similar to this one) is required. 99inally, the number of studies using combined noninvasive MRI measures of glymphatic system activity are lacking. 11,23,100Each noninvasive MRI measure of glymphatic system activity might reflect the different properties of the glymphatic system.fter harmonization, the distribution of the ALPS index was closer to each other.Estimating kernel density is shown by a solid curve (red, Prisma Fit; blue, Signa HDxt; green, Discovery MR750).(b) The difference between the AD and healthy cognition groups. 99Blue bar (before harmonization) and red bar (after harmonization) indicate the group differences (Cohen's d) of the ALPS indices between AD and healthy cognition.The distribution of the ALPS index was organized using COMBAT harmonization, which also reduced scanner differences from multiscanner data.As a result, while Cohen's d and the P-value declined, the group difference after harmonization increased to 1.5 times the original value before harmonization.This demonstrates an increase in statistical power.As a result, the COMBAT almost doubled the multisite ALPS index's statistical power between healthy cognition and AD.Using COMBAT may make it easier to apply the ALPS index in multisite investigations. 99

FIGURE 3 :
FIGURE 3: MR images and PVS visual rating score.The MR image is a T2-weighted image, and the visual rating scale is the Potter scoring.The left column shows the PVS severity in the basal ganglia, and the bottom column shows the PVS severity in the centrum semiovale.PVS severity is commonly evaluated using a visual rating scale based on the Potter scoring with five grades (grade 0: PVS count = 0; grade 1: PVS count ≤ 1-10; grade 2: PVS count = 11-20; grade 3: PVS count = 21-40; grade 4: PVS count ≥ 40) according to the PVS count in the centrum semiovale and basal ganglia and 0-1 grades for the midbrain based on the presence of PVS.54

FIGURE 4 :
FIGURE 4: Enhanced PVS contrast and segmentation.A comparative the visual performance of PVS using T1-weighted (top row), T2-weighted (middle row), and enhanced PVS contrast (EPC) images (bottom row).The orange arrows indicate PVS.The implementation of EPC enhanced the PVS visibility and facilitated the resolution of a greater quantity of PVS.63

FIGURE 5 :
FIGURE 5: The flowchart of ALPS index calculation.(a)The relationship between the perivascular space and the directions of the association and projection fibers.Dominant fibers in the association area run in the y-axis direction, perpendicular to both the x-and z-axes, whereas dominant fibers in the projection area run in the z-axis direction, perpendicular to both the x-and y-axes.(b) On the color-coded FA map, sphere ROIs were placed in the association and projection areas at the level of the lateral ventricle.(c) Finally, the ALPS index was calculated using the tensor map (D xx , D yy , and D zz ) with the sphere ROIs.When the ALPS index approaches unity, the impact of water diffusion in the PVS is minimal, whereas a higher ratio signifies an increased diffusivity of water in the PVS.For instance, the ALPS index had a significant negative association with Mini-Mental State Examination (MMSE) scores, which is a cognitive function test, and AD severity.67

FIGURE 6 :
FIGURE 6: The concept of ASL for glymphatic system evaluation.Considering three components-the capillaries, BBB, and parenchyma (a)-these relationships are modeled by (b), where M c and M b are the longitudinal magnetizations in the capillaries and parenchyma, respectively.After ASL, M c decreases as M b increases.The change is defined by water exchange (kw [min À1 ]).

FIGURE 7 :
FIGURE 7: Diffusion-prepared, arterial spin labeling.This study showcases cerebral blood flow (CBF), arterial transit time (ATT), and blood-brain barrier water exchange (kw) maps of three selected individuals.The results indicate a positive correlation between elevated kernel width (kw) values and heightened cerebrospinal fluid (CSF) amyloid beta (Aβ)42 concentration, as depicted in the progression from left to right.The ATT, CBF, and kw values of the participants have been provided alongside their respective maps.The CSF Aβ42 concentration values of the participants are displayed in the lower section of the illustration, enclosed within a white box.The structural foundations of the BBB and glymphatic system exhibit similarities at the capillary level.These systems may work in tandem and provide mutually beneficial functions in preserving neuronal equilibrium and intracerebral homeostasis78 (Adapted and reproduced from Gold et al,78 Creative Commons Attribution 4.0 [CC BY-NC]).

FIGURE 8 :
FIGURE 8: Chemical exchange saturation transfer.(a) The impacts of Lym-CEST at various time intervals following the ligation of deep cervical lymph nodes.The CESTR is represented by the color bars located on the right-hand side of the images.(b) The Lym-CEST images of the representative samples captured at various time intervals following the sham surgery.(c) In the Dcln model group, the CESTRs demonstrated an upward trend in the correct quadrant as time progressed after the surgery.(d) The CESTR of the brain was measured at various time intervals following the sham surgery.The linear correlation coefficient R 2 is 0.5600, and the P-value is >0.05.(e)The study examined the Lym-CEST effects on the ipsilateral hippocampus of rats that underwent sham surgery (Sham) and compared them with those on the contralateral hippocampus (Con_hip) and the ipsilateral hippocampus (Ips_hip) of rats Dcln models at various time points after surgery.The findings of this study suggest that CEST MRI can serve as a valuable instrument for examining metabolites and identifying alterations in glymphatic system function in vivo.81(Adapted and reproduced from Chen et al81 ).

FIGURE 9 :
FIGURE 9: Effects of IVIM on diffusion MRI signal.The spontaneous movement of water molecules, either individually or as part of larger groupings at the voxel level, decrease signal intensity, which is proportional to the strength of the field gradient encoding or the b-value.This occurs in an exponential fashion.The diffusion in the tissue and the flow of blood contribute to the signal independently, which results in a twostage exponential form.However, because the pseudo-diffusion coefficient (D * ) is significantly greater than the actual diffusion coefficient (D), the effect of intravoxel incoherent motion (IVIM), representing blood flow, is noticeable as a departure from the regular signal decay of tissue diffusion at lower b-values.This is seen in the red area and is called the IVIM effect.

FIGURE 10 :
FIGURE 10: Spectral diffusion analysis of intravoxel incoherent motion MRI.(a) Diffusion spectrum of the diffusion coefficient D i and amplitude a i in the basal ganglia.Both spectra exhibit a distinct parenchymal and microvascular perfusion component.An intermediate diffusion component (f int ) is between the microvascular and parenchymal components in the diffusion spectrum.(b) T1-weighted (left) and T2-weighted images (middle) and maps of f int (right) in the basal ganglia.The participant exhibiting a greater quantity of visible perivascular spaces displays an increased number of voxels, characterized by elevated f int values.(c) FLAIR image (left) and f int (right) with white matter hyperintensity (WMH).A significant correlation is observed between elevated levels of f int and the co-occurrence of WMH.f int can be used as a measurable imaging biomarker for the degeneration of the cerebral parenchyma in cerebral small vessel diseases, which could be associated with impaired glymphatic system function88 (Adapted and reproduced from Wong et al,88 Creative Commons Attribution 4.0 [CC BY]).

FIGURE 11 :
FIGURE 11: Harmonization of the ALPS index using multisite diffusion MRI.(a) Comparison of the distribution of the ALPS index in healthy cognition before and after harmonization.99After harmonization, the distribution of the ALPS index was closer to each other.Estimating kernel density is shown by a solid curve (red, Prisma Fit; blue, Signa HDxt; green, Discovery MR750).(b) The difference between the AD and healthy cognition groups.99Blue bar (before harmonization) and red bar (after harmonization) indicate the group differences (Cohen's d) of the ALPS indices between AD and healthy cognition.The distribution of the ALPS index was organized using COMBAT harmonization, which also reduced scanner differences from multiscanner data.As a result, while Cohen's d and the P-value declined, the group difference after harmonization increased to 1.5 times the original value before harmonization.This demonstrates an increase in statistical power.As a result, the COMBAT almost doubled the multisite ALPS index's statistical power between healthy cognition and AD.Using COMBAT may make it easier to apply the ALPS index in multisite investigations.99