Accuracy of 4D Flow Measurement of Cerebrospinal Fluid Dynamics in the Cervical Spine: An In Vitro Verification Against Numerical Simulation

CSF dynamics throughout the ventricular system using 4D flow MRI: associations to arterial pulsatility, ventricular volumes, and age

BACKGROUND

Cerebrospinal fluid (CSF) dynamics are increasingly studied in aging and neurological disorders. Models of CSF-mediated waste clearance suggest that altered CSF dynamics could play a role in the accumulation of toxic waste in the CNS, with implications for Alzheimer's disease and other proteinopathies. Therefore, approaches that enable quantitative and volumetric assessment of CSF flow velocities could be of value. In this study we demonstrate the feasibility of 4D flow MRI for simultaneous assessment of CSF dynamics throughout the ventricular system, and evaluate associations to arterial pulsatility, ventricular volumes, and age.

METHODS

In a cognitively unimpaired cohort (N = 43; age 41-83 years), cardiac-resolved 4D flow MRI CSF velocities were obtained in the lateral ventricles (LV), foramens of Monro, third and fourth ventricles (V3 and V4), the cerebral aqueduct (CA) and the spinal canal (SC), using a velocity encoding (venc) of 5 cm/s. Cerebral blood flow pulsatility was also assessed with 4D flow (venc = 80 cm/s), and CSF volumes were obtained from T1- and T2-weighted MRI. Multiple linear regression was used to assess effects of age, ventricular volumes, and arterial pulsatility on CSF velocities.

RESULTS

Cardiac-driven CSF dynamics were observed in all CSF spaces, with region-averaged velocity range and root-mean-square (RMS) velocity encompassing from very low in the LVs (RMS 0.25 ± 0.08; range 0.85 ± 0.28 mm/s) to relatively high in the CA (RMS 6.29 ± 2.87; range 18.6 ± 15.2 mm/s). In the regression models, CSF velocity was significantly related to age in 5/6 regions, to CSF space volume in 2/3 regions, and to arterial pulsatility in 3/6 regions. Group-averaged waveforms indicated distinct CSF flow propagation delays throughout CSF spaces, particularly between the SC and LVs.

CONCLUSIONS

Our findings show that 4D flow MRI enables assessment of CSF dynamics throughout the ventricular system, and captures independent effects of age, CSF space morphology, and arterial pulsatility on CSF motion.

Cerebro-spinal flow pattern in the cervical subarachnoid space of healthy volunteers: Influence of the spinal cord morphology

INTRODUCTION

Toward further cerebro-spinal flow quantification in clinical practice, this study aims at assessing the variations in the cerebro spinal fluid flow pattern associated with change in the morphology of the subarachnoid space of the cervical canal of healthy humans by developing a computational fluid dynamics model.

METHODS

3D T2-space MRI sequence images of the cervical spine were used to segment 11 cervical subarachnoid space. Model validation (time-step, mesh size, size and number of boundary layers, influences of parted inflow and inflow continuous velocity) was performed a 40-year-old patient-specific model. Simulations were performed using computational fluid dynamics approach simulating transient flow (Sparlart-Almaras turbulence model) with a mesh size of 0.6, 6 boundary layers of 0.05 mm, a time step of 20 ms simulated on 15 cycles. Distributions of components velocity and WSS were respectively analyzed within the subarachnoid space (intervertebral et intravertebral levels) and on dura and pia maters.

RESULTS

Mean values cerebro spinal fluid velocity in specific local slices of the canal range between 0.07 and 0.17 m.s-1 and 0.1 and 0.3 m.s-1 for maximum values. Maximum wall shear stress values vary between 0.1 and 0.5 Pa with higher value at the middle of the cervical spine on pia mater and at the lower part of the cervical spine on dura mater. Intra and inter-individual variations of the wall shear stress were highlighted significant correlation gwith compression ratio (r = 0.76), occupation ratio and cross section area of the spinal cord.

CONCLUSION

The inter-individual variability in term of subarachnoid canal morphology and spinal cord position influence the cerebro-spinal flow pattern, highlighting the significance of canal morphology investigation before surgery.

Research Trends and Visualization of Cerebrospinal Fluid Dynamics (2013-2023)

OBJECTIVE

This study aims to analyze cerebrospinal fluid (CSF) dynamics using VOSviewer, CiteSpace, and the Bibliometrix R-package software to identify research hotspots and future directions.

METHODS

Search by Web of Science Core Collection Database for related literature on CSF dynamics from 2013 to 2023. Bibliometric and visual analysis of data on number of citations, number of publications, most productive countries and institutions, important authors and journals, time of publication, popular topics, and keywords were performed by CiteSpace and VOSviewer.

RESULTS

In the field of CSF dynamics, there is a clear upward trend in annual publications. The United States, Japan, and China are among the top three countries in publishing output. The University of Copenhagen, the University of Idaho, and the University of Zurich are leading institutions in research publications. The most prolific writers in this field are Bryn A. Martin, and Olivier Baledent. Active authors and institutions in the field form multiple structurally stable research teams with each other, but the collaboration between different authors and institutional teams needs to be further strengthened. The literature with the highest citation rates in the past decade is "Blood-Brain Barrier Breakdown in the Aging Human Hippocampus," "Blood-Brain Barrier Breakdown Is an Early Biomarker of Human Cognitive Dysfunction," "Serum Neurofilament Dynamics Predicts Neurodegeneration and Clinical Progression in Presymptomatic Alzheimer's Disease," and Coupled Electrophysiological, Hemodynamic, and Cerebrospinal Fluid Oscillations in Human Sleep." Key research keywords such as CSF, hydrocephalus, dynamics, brain, blood flow, CSF, pressure, CSF flow, and MRI highlight focal areas for CSF dynamics studies. These keywords represent current research priorities and research frontiers in this field.

CONCLUSIONS

This bibliometric analysis reveals hot and future research issues in the field of CSF fluid dynamics, demonstrating the need for enhanced international collaboration and interdisciplinary research to deepen the field. Keyword analysis further clarified the research focus and provided useful guidance for subsequent studies.

Metabolomics of cerebrospinal fluid reveals candidate diagnostic biomarkers to distinguish between spinal muscular atrophy type II and type III

AIMS

Classification of spinal muscular atrophy (SMA) is associated with the clinical prognosis; however, objective classification markers are scarce. This study aimed to identify metabolic markers in the cerebrospinal fluid (CSF) of children with SMA types II and III.

METHODS

CSF samples were collected from 40 patients with SMA (27 with type II and 13 with type III) and analyzed for metabolites.

RESULTS

We identified 135 metabolites associated with SMA types II and III. These were associated with lysine degradation and arginine, proline, and tyrosine metabolism. We identified seven metabolites associated with the Hammersmith Functional Motor Scale: 4-chlorophenylacetic acid, adb-chminaca,(+/-)-, dodecyl benzenesulfonic acid, norethindrone acetate, 4-(undecan-5-yl) benzene-1-sulfonic acid, dihydromaleimide beta-d-glucoside, and cinobufagin. Potential typing biomarkers, N-cyclohexylformamide, cinobufagin, cotinine glucuronide, N-myristoyl arginine, 4-chlorophenylacetic acid, geranic acid, 4-(undecan-5-yl) benzene, and 7,8-diamino pelargonate, showed good predictive performance. Among these, N-myristoyl arginine was unaffected by the gene phenotype.

CONCLUSION

This study identified metabolic markers are promising candidate prognostic factors for SMA. We also identified the metabolic pathways associated with the severity of SMA. These assessments can help predict the outcomes of screening SMA classification biomarkers.

Finite element modeling and analysis of effect of preexisting cervical degenerative disease on the spinal cord during flexion and extension

Recent studies have emphasized the importance of dynamic activity in the development of myelopathy. However, current knowledge of how degenerative factors affect the spinal cord during motion is still limited. This study aimed to investigate the effect of various types of preexisting herniated cervical disc and the ligamentum flavum ossification on the spinal cord during cervical flexion and extension. A detailed dynamic fluid-structure interaction finite element model of the cervical spine with the spinal cord was developed and validated. The changes of von Mises stress and maximum principal strain within the spinal cord in the period of normal, hyperflexion, and hyperextension were investigated, considering various types and grades of disc herniation and ossification of the ligamentum flavum. The flexion and extension of the cervical spine with spinal canal encroachment induced high stress and strain inside the spinal cord, and this effect was also amplified by increased canal encroachments and cervical hypermobility. The spinal cord might evade lateral encroachment, leading to a reduction in the maximum stress and principal strain within the spinal cord in local-type herniation. Although the impact was limited in the case of diffuse type, the maximum stress tended to appear in the white matter near the encroachment site while compression from both ventral and dorsal was essential to make maximum stress appear in the grey matter. The existence of canal encroachment can reduce the safe range for spinal cord activities, and hypermobility activities may induce spinal cord injury. Besides, the ligamentum flavum plays an important role in the development of central canal syndrome.Significance. This model will enable researchers to have a better understanding of the influence of cervical degenerative diseases on the spinal cord during extension and flexion.

Current Understanding of the Anatomy, Physiology, and Magnetic Resonance Imaging of Neurofluids: Update From the 2022 "ISMRM Imaging Neurofluids Study group" Workshop in Rome

Neurofluids is a term introduced to define all fluids in the brain and spine such as blood, cerebrospinal fluid, and interstitial fluid. Neuroscientists in the past millennium have steadily identified the several different fluid environments in the brain and spine that interact in a synchronized harmonious manner to assure a healthy microenvironment required for optimal neuroglial function. Neuroanatomists and biochemists have provided an incredible wealth of evidence revealing the anatomy of perivascular spaces, meninges and glia and their role in drainage of neuronal waste products. Human studies have been limited due to the restricted availability of noninvasive imaging modalities that can provide a high spatiotemporal depiction of the brain neurofluids. Therefore, animal studies have been key in advancing our knowledge of the temporal and spatial dynamics of fluids, for example, by injecting tracers with different molecular weights. Such studies have sparked interest to identify possible disruptions to neurofluids dynamics in human diseases such as small vessel disease, cerebral amyloid angiopathy, and dementia. However, key differences between rodent and human physiology should be considered when extrapolating these findings to understand the human brain. An increasing armamentarium of noninvasive MRI techniques is being built to identify markers of altered drainage pathways. During the three-day workshop organized by the International Society of Magnetic Resonance in Medicine that was held in Rome in September 2022, several of these concepts were discussed by a distinguished international faculty to lay the basis of what is known and where we still lack evidence. We envision that in the next decade, MRI will allow imaging of the physiology of neurofluid dynamics and drainage pathways in the human brain to identify true pathological processes underlying disease and to discover new avenues for early diagnoses and treatments including drug delivery. Evidence level: 1 Technical Efficacy: Stage 3.

Magnetic resonance imaging-based classification of the myodural bridge complex and its influencing factors

Cerebrospinal fluid (CSF) circulation is considered the third circulation of the human body. Recently, some scholars have proposed the myodural bridge (MDB) as a novel power source for CSF flow. Moreover, the suboccipital muscles can exert a driving force on the CSF via the MDB. This hypothesis is directly supported by head rotation and nodding movements, which can affect CSF circulation. The MDB has been validated as a normal structure in humans and mammals. In addition, the fusion of MDB fibers of different origins that act in concert with each other forms the MDB complex (MDBC). The MDBC may be associated with several CSF disorder-related neurological disorders in clinical practice. Therefore, the morphology of the MDBC and its influencing factors must be determined. In this study, T2-weighted imaging sagittal images of the cervical region were analyzed retrospectively in 1085 patients, and magnetic resonance imaging (MRI) typing of the MDBC was performed according to the imaging features of the MDBC in the posterior atlanto-occipital interspace (PAOiS) and posterior atlanto-axial interspace (PAAiS). The effects of age and age-related degenerative changes in the cervical spine on MRI staging of the MDBC were also determined. The results revealed four MRI types of the MDBC: type A (no MDBC hyposignal shadow connected to the dura mater in either the PAOiS or PAAiS), type B (MDBC hyposignal shadow connected to the dura mater in the PAOiS only), type C (MDBC hyposignal shadow connected to the dura mater in the PAAiS only), and type D (MDBC hyposignal shadow connected to the dura mater in both the PAOiS and PAAiS). The influencing factors for the MDBC typing were age (group), degree of intervertebral space stenosis, dorsal osteophytosis, and degenerative changes in the cervical spine (P < 0.05). With increasing age (10-year interval), the incidence of type B MDBC markedly decreased, whereas that of type A MDBC increased considerably. With the deepening of the degree of intervertebral space stenosis, the incidence of type C MDBC increased significantly, whereas that of type A MDBC decreased. In the presence of dorsal osteophytosis, the incidence of type C and D MDBCs significantly decreased, whereas that of type A increased. In the presence of protrusion of the intervertebral disc, the incidence of type B, C, and D MDBCs increased markedly, whereas that of type A MDBC decreased considerably, with cervical degenerative changes combined with spinal canal stenosis. Moreover, the incidence of both type C and D MDBCs increased, whereas that of type A MDBC decreased. Based on the MRI signal characteristics of the dural side of the MDBC, four types of the MDBC were identified. MDBC typing varies dynamically according to population distribution, depending on age and cervical degeneration (degree of intervertebral space stenosis, vertebral dorsal osteophytosis formation, simple protrusion of intervertebral disc, and cervical degeneration changes combined with spinal canal stenosis, except for the degree of protrusion of the intervertebral disc and the degree of spinal canal stenosis); however, it is not influenced by sex.

Dorsal Versus Ventral Intrathecal Catheter Tip Location and Effect on Dose Escalation and Opioid Use in Patients With Cancer Pain

OBJECTIVES

In the practice of intrathecal drug delivery, consensus exists regarding the cephalad to caudad location of the catheter tip relative to dermatomal distribution of pain. However, data are lacking on the importance of dorsal vs ventral tip location relative to the spinal cord. We hypothesize that a dorsally placed catheter tip improves efficacy because of closer proximity to nociceptive pathways.

MATERIALS AND METHODS

A retrospective review of 298 patients with cancer with intrathecal drug delivery systems implanted at the Huntsman Cancer Institute from May 2014 to June 2020 was performed. Patients were stratified by catheter tip location zones based on available radiographic studies. Patient-controlled intrathecal medication dose requirements and rate of change were compared with catheter zone and other variables, including the presence of adjuncts such as bupivacaine and ziconotide.

RESULTS

A total of 158 patients were suitable for analysis demonstrating a dorsal tip in 63.9% (n = 101) and ventral tip in 36.1% (n = 57), with a median follow-up of 17 days (interquartile range [IQR], 10-24). There was no difference in daily dose change from implant to discharge between the dorsal group 8.2% (IQR, 0.0-41.5) and ventral group 20.8% (IQR, 0.0-66.7; p = 0.12). Daily dose change from discharge to follow-up was 2.6% (IQR, 0.0-7.1) in the dorsal group and 1.8% (IQR, 0.0-5.7) in the ventral group (p = 0.92). Catheter tip location had no impact on systemic opioid use.

CONCLUSIONS

We did not find significant associations between dorsal vs ventral catheter tip location and measures of pain relief, including change in intrathecal dose or systemic opioid use.

Computational fluid dynamics model to predict the dynamical behavior of the cerebrospinal fluid through implementation of physiological boundary conditions

Cerebrospinal fluid (CSF) dynamics play an important role in maintaining a stable central nervous system environment and are influenced by different physiological processes. Multiple studies have investigated these processes but the impact of each of them on CSF flow is not well understood. A deeper insight into the CSF dynamics and the processes impacting them is crucial to better understand neurological disorders such as hydrocephalus, Chiari malformation, and intracranial hypertension. This study presents a 3D computational fluid dynamics (CFD) model which incorporates physiological processes as boundary conditions. CSF production and pulsatile arterial and venous volume changes are implemented as inlet boundary conditions. At the outlets, 2-element windkessel models are imposed to simulate CSF compliance and absorption. The total compliance is first tuned using a 0D model to obtain physiological pressure pulsations. Then, simulation results are compared with in vivo flow measurements in the spinal subarachnoid space (SAS) and cerebral aqueduct, and intracranial pressure values reported in the literature. Finally, the impact of the distribution of and total compliance on CSF pressures and velocities is evaluated. Without respiration effects, compliance of 0.17 ml/mmHg yielded pressure pulsations with an amplitude of 5 mmHg and an average value within the physiological range of 7-15 mmHg. Also, model flow rates were found to be in good agreement with reported values. However, when adding respiration effects, similar pressure amplitudes required an increase of compliance value to 0.51 ml/mmHg, which is within the range of 0.4-1.2 ml/mmHg measured in vivo. Moreover, altering the distribution of compliance over the four different outlets impacted the local flow, including the flow through the foramen magnum. The contribution of compliance to each outlet was directly proportional to the outflow at that outlet. Meanwhile, the value of total compliance impacted intracranial pressure. In conclusion, a computational model of the CSF has been developed that can simulate CSF pressures and velocities by incorporating boundary conditions based on physiological processes. By tuning these boundary conditions, we were able to obtain CSF pressures and flows within the physiological range.

Effect of degenerative factors on cervical spinal cord during flexion and extension: a dynamic finite element analysis

Spinal cord injury (SCI) is a global problem that brings a heavy burden to both patients and society. Recent investigations indicated degenerative disease is taking an increasing part in SCI with the growth of the aging population. However, little insight has been gained about the effect of cervical degenerative disease on the spinal cord during dynamic activities. In this work, a dynamic fluid-structure interaction model was developed and validated to investigate the effect of anterior and posterior encroachment caused by degenerative disease on the spinal cord during normal extension and flexion. Maximum von-Mises stress and maximum principal strain were observed at the end of extension and flexion. The abnormal stress distribution caused by degenerative factors was concentrated in the descending tracts of the spinal cord. Our finding indicates that the excessive motion of the cervical spine could potentially exacerbate spinal cord injury and enlarge injury areas. Stress and strain remained low compared to extension during moderate flexion. This suggests that patients with cervical degenerative disease should avoid frequent or excessive flexion and extension which could result in motor function impairment, whereas moderate flexion is safe. Besides, encroachment caused by degenerative factors that are not significant in static imaging could also cause cord compression during normal activities.

Quality Control for 4D Flow MR Imaging

In recent years, 4D flow MRI has become increasingly important in clinical applications for the blood vessels in the whole body, heart, and cerebrospinal fluid. 4D flow MRI has advantages over 2D cine phase-contrast (PC) MRI in that any targeted area of interest can be analyzed post-hoc, but there are some factors to be considered, such as ensuring measurement accuracy, a long imaging time and post-processing complexity, and interobserver variability.Due to the partial volume phenomenon caused by low spatial and temporal resolutions, the accuracy of flow measurement in 4D flow MRI is reduced. For spatial resolution, it is recommended to include at least four voxels in the vessel of interest, and if possible, six voxels. In large vessels such as the aorta, large voxels can be secured and SNR can be maintained, but in small cerebral vessels, SNR is reduced, resulting in reduced accuracy. A temporal resolution of less than 40 ms is recommended. The velocity-to-noise ratio (VNR) of low-velocity blood flow is low, resulting in poor measurement accuracy. The use of dual velocity encoding (VENC) or multi-VENC is recommended to avoid velocity wrap around and to increase VNR. In order to maintain sufficient spatio-temporal resolution, a longer imaging time is required, leading to potential patient movement during examination and a corresponding decrease in measurement accuracy.For the clinical application of new technologies, including various acceleration techniques, in vitro and in vivo accuracy verification based on existing accuracy-validated 2D cine PC MRI and 4D flow MRI, as well as accuracy verification on the conservation of mass' principle, should be performed, and intraobserver repeatability, interobserver reproducibility, and test-retest reproducibility should be checked.

Translational approaches for brain delivery of biologics via cerebrospinal fluid

Delivery of biologics via cerebrospinal fluid (CSF) has demonstrated potential to access the tissues of the central nervous system (CNS) by circumventing the blood‐brain barrier and blood‐CSF barrier. Developing an effective CSF drug delivery strategy requires optimization of multiple parameters, including choice of CSF access point, delivery device technology, and delivery kinetics to achieve effective therapeutic concentrations in the target brain region, whereas also considering the biologic modality, mechanism of action, disease indication, and patient population. This review discusses key preclinical and clinical examples of CSF delivery for different biologic modalities (antibodies, nucleic acid‐based therapeutics, and gene therapy) to the brain via CSF or CNS access routes (intracerebroventricular, intrathecal‐cisterna magna, intrathecal‐lumbar, intraparenchymal, and intranasal), including the use of novel device technologies. This review also discusses quantitative models of CSF flow that provide insight into the effect of fluid dynamics in CSF on drug delivery and CNS distribution. Such models can facilitate delivery device design and pharmacokinetic/pharmacodynamic translation from preclinical species to humans in order to optimize CSF drug delivery to brain regions of interest.

A phantom study comparing radial trajectories for accelerated cardiac 4D flow MRI against a particle imaging velocimetry reference

PURPOSE

Radial sampling is one method to accelerate 4D flow MRI acquisition, making feasible dual-velocity encoding (Venc) assessment of slow flow in the left ventricle (LV). Here, two radial trajectories are compared in vitro for this application: 3D radial (phase-contrast vastly undersampled isotropic projection, PC-VIPR) versus stack of stars (phase-contrast stack of stars, PC-SOS), with benchtop particle imaging velocimetry (PIV) serving as a reference standard.

METHODS

The study contained three steps: (1) Construction of an MRI- and PIV-compatible LV model from a healthy adult's CT images. (2) In vitro PIV using a pulsatile flow pump. (3) In vitro dual-Venc 4D flow MRI using PC-VIPR and PC-SOS (two repeat experiments). Each MR image set was retrospectively undersampled to five effective scan durations and compared with the PIV reference. The root-mean-square velocity vector difference (RMSE) between MRI and PIV images was compared, along with kinetic energy (KE) and wall shear stress (WSS).

RESULTS

RMSE increased as scan time decreased for both MR acquisitions. RMSE was 3% lower in PC-SOS images than PC-VIPR images in 30-min scans (3.8 vs. 3.9 cm/s) but 98% higher in 2.5-min scans (9.5 vs. 4.8 cm/s). PIV intrasession repeatability showed a RMSE of 4.4 cm/s, reflecting beat-to-beat flow variation, while MRI had intersession RMSEs of 3.8/3.5 cm/s for VIPR/SOS, respectively. Speed, KE, and WSS were overestimated voxel-wise in 30-min MRI scans relative to PIV by 0.4/0.3 cm/s, 0.2/0.1 μJ/mL, and 36/43 mPa, respectively, for VIPR/SOS.

CONCLUSIONS

PIV is feasible for application-specific 4D flow MRI protocol optimization. PC-VIPR is better-suited to dual-Venc LV imaging with short scan times.

In vitro evaluation of cerebrospinal fluid velocity measurement in type I Chiari malformation: repeatability, reproducibility, and agreement using 2D phase contrast and 4D flow MRI

BACKGROUND

Phase contrast magnetic resonance imaging, PC MRI, is a valuable tool allowing for non-invasive quantification of CSF dynamics, but has lacked adoption in clinical practice for Chiari malformation diagnostics. To improve these diagnostic practices, a better understanding of PC MRI based measurement agreement, repeatability, and reproducibility of CSF dynamics is needed.

METHODS

An anatomically realistic in vitro subject specific model of a Chiari malformation patient was scanned three times at five different scanning centers using 2D PC MRI and 4D Flow techniques to quantify intra-scanner repeatability, inter-scanner reproducibility, and agreement between imaging modalities. Peak systolic CSF velocities were measured at nine axial planes using 2D PC MRI, which were then compared to 4D Flow peak systolic velocity measurements extracted at those exact axial positions along the model.

RESULTS

Comparison of measurement results showed good overall agreement of CSF velocity detection between 2D PC MRI and 4D Flow (p = 0.86), fair intra-scanner repeatability (confidence intervals ± 1.5 cm/s), and poor inter-scanner reproducibility. On average, 4D Flow measurements had a larger variability than 2D PC MRI measurements (standard deviations 1.83 and 1.04 cm/s, respectively).

CONCLUSION

Agreement, repeatability, and reproducibility of 2D PC MRI and 4D Flow detection of peak CSF velocities was quantified using a patient-specific in vitro model of Chiari malformation. In combination, the greatest factor leading to measurement inconsistency was determined to be a lack of reproducibility between different MRI centers. Overall, these findings may help lead to better understanding for application of 2D PC MRI and 4D Flow techniques as diagnostic tools for CSF dynamics quantification in Chiari malformation and related diseases.

Quantification of arterial, venous, and cerebrospinal fluid flow dynamics by magnetic resonance imaging under simulated micro-gravity conditions: a prospective cohort study

BACKGROUND

Astronauts undergoing long-duration spaceflight are exposed to numerous health risks, including Spaceflight-Associated Neuro-Ocular Syndrome (SANS), a spectrum of ophthalmic changes that can result in permanent loss of visual acuity. The etiology of SANS is not well understood but is thought to involve changes in cerebrovascular flow dynamics in response to microgravity. There is a paucity of knowledge in this area; in particular, cerebrospinal fluid (CSF) flow dynamics have not been well characterized under microgravity conditions. Our study was designed to determine the effect of simulated microgravity (head-down tilt [HDT]) on cerebrovascular flow dynamics. We hypothesized that microgravity conditions simulated by acute HDT would result in increases in CSF pulsatile flow.

METHODS

In a prospective cohort study, we measured flow in major cerebral arteries, veins, and CSF spaces in fifteen healthy volunteers using phase contrast magnetic resonance (PCMR) before and during 15° HDT.

RESULTS

We found a decrease in all CSF flow variables [systolic peak flow (p = 0.009), and peak-to-peak pulse amplitude (p = 0.001)]. Cerebral arterial average flow (p = 0.04), systolic peak flow (p = 0.04), and peak-to-peak pulse amplitude (p = 0.02) all also significantly decreased. We additionally found a decrease in average cerebral arterial flow (p = 0.040). Finally, a significant increase in cerebral venous cross-sectional area under HDT (p = 0.005) was also observed.

CONCLUSIONS

These results collectively demonstrate that acute application of -15° HDT caused a reduction in CSF flow variables (systolic peak flow and peak-to-peak pulse amplitude) which, when coupled with a decrease in average cerebral arterial flow, systolic peak flow, and peak-to-peak pulse amplitude, is consistent with a decrease in cardiac-related pulsatile CSF flow. These results suggest that decreases in cerebral arterial inflow were the principal drivers of decreases in CSF pulsatile flow.

In vitro and numerical simulation of blood removal from cerebrospinal fluid: comparison of lumbar drain to Neurapheresis therapy

BACKGROUND

Blood removal from cerebrospinal fluid (CSF) in post-subarachnoid hemorrhage patients may reduce the risk of related secondary brain injury. We formulated a computational fluid dynamics (CFD) model to investigate the impact of a dual-lumen catheter-based CSF filtration system, called Neurapheresis™ therapy, on blood removal from CSF compared to lumbar drain.

METHODS

A subject-specific multiphase CFD model of CSF system-wide solute transport was constructed based on MRI measurements. The Neurapheresis catheter geometry was added to the model within the spinal subarachnoid space (SAS). Neurapheresis flow aspiration and return rate was 2.0 and 1.8 mL/min, versus 0.2 mL/min drainage for lumbar drain. Blood was modeled as a bulk fluid phase within CSF with a 10% initial tracer concentration and identical viscosity and density as CSF. Subject-specific oscillatory CSF flow was applied at the model inlet. The dura and spinal cord geometry were considered to be stationary. Spatial-temporal tracer concentration was quantified based on time-average steady-streaming velocities throughout the domain under Neurapheresis therapy and lumbar drain. To help verify CFD results, an optically clear in vitro CSF model was constructed with fluorescein used as a blood surrogate. Quantitative comparison of numerical and in vitro results was performed by linear regression of spatial-temporal tracer concentration over 24-h.

RESULTS

After 24-h, tracer concentration was reduced to 4.9% under Neurapheresis therapy compared to 6.5% under lumbar drain. Tracer clearance was most rapid between the catheter aspiration and return ports. Neurapheresis therapy was found to have a greater impact on steady-streaming compared to lumbar drain. Steady-streaming in the cranial SAS was ~ 50× smaller than in the spinal SAS for both cases. CFD results were strongly correlated with the in vitro spatial-temporal tracer concentration under Neurapheresis therapy (R² = 0.89 with + 2.13% and - 1.93% tracer concentration confidence interval).

CONCLUSION

A subject-specific CFD model of CSF system-wide solute transport was used to investigate the impact of Neurapheresis therapy on tracer removal from CSF compared to lumbar drain over a 24-h period. Neurapheresis therapy was found to substantially increase tracer clearance compared to lumbar drain. The multiphase CFD results were verified by in vitro fluorescein tracer experiments.

Anthropomorphic Model of Intrathecal Cerebrospinal Fluid Dynamics Within the Spinal Subarachnoid Space: Spinal Cord Nerve Roots Increase Steady-Streaming

Cerebrospinal fluid (CSF) dynamics are thought to play a vital role in central nervous system (CNS) physiology. The objective of this study was to investigate the impact of spinal cord (SC) nerve roots (NR) on CSF dynamics. A subject-specific computational fluid dynamics (CFD) model of the complete spinal subarachnoid space (SSS) with and without anatomically realistic NR and nonuniform moving dura wall deformation was constructed. This CFD model allowed detailed investigation of the impact of NR on CSF velocities that is not possible in vivo using magnetic resonance imaging (MRI) or other noninvasive imaging methods. Results showed that NR altered CSF dynamics in terms of velocity field, steady-streaming, and vortical structures. Vortices occurred in the cervical spine around NR during CSF flow reversal. The magnitude of steady-streaming CSF flow increased with NR, in particular within the cervical spine. This increase was located axially upstream and downstream of NR due to the interface of adjacent vortices that formed around NR.

Hydraulic resistance of periarterial spaces in the brain

BACKGROUND

Periarterial spaces (PASs) are annular channels that surround arteries in the brain and contain cerebrospinal fluid (CSF): a flow of CSF in these channels is thought to be an important part of the brain's system for clearing metabolic wastes. In vivo observations reveal that they are not concentric, circular annuli, however: the outer boundaries are often oblate, and the arteries that form the inner boundaries are often offset from the central axis.

METHODS

We model PAS cross-sections as circles surrounded by ellipses and vary the radii of the circles, major and minor axes of the ellipses, and two-dimensional eccentricities of the circles with respect to the ellipses. For each shape, we solve the governing Navier-Stokes equation to determine the velocity profile for steady laminar flow and then compute the corresponding hydraulic resistance.

RESULTS

We find that the observed shapes of PASs have lower hydraulic resistance than concentric, circular annuli of the same size, and therefore allow faster, more efficient flow of cerebrospinal fluid. We find that the minimum hydraulic resistance (and therefore maximum flow rate) for a given PAS cross-sectional area occurs when the ellipse is elongated and intersects the circle, dividing the PAS into two lobes, as is common around pial arteries. We also find that if both the inner and outer boundaries are nearly circular, the minimum hydraulic resistance occurs when the eccentricity is large, as is common around penetrating arteries.

CONCLUSIONS

The concentric circular annulus assumed in recent studies is not a good model of the shape of actual PASs observed in vivo, and it greatly overestimates the hydraulic resistance of the PAS. Our parameterization can be used to incorporate more realistic resistances into hydraulic network models of flow of cerebrospinal fluid in the brain. Our results demonstrate that actual shapes observed in vivo are nearly optimal, in the sense of offering the least hydraulic resistance. This optimization may well represent an evolutionary adaptation that maximizes clearance of metabolic waste from the brain.

Nonuniform Moving Boundary Method for Computational Fluid Dynamics Simulation of Intrathecal Cerebrospinal Flow Distribution in a Cynomolgus Monkey

A detailed quantification and understanding of cerebrospinal fluid (CSF) dynamics may improve detection and treatment of central nervous system (CNS) diseases and help optimize CSF system-based delivery of CNS therapeutics. This study presents a computational fluid dynamics (CFD) model that utilizes a nonuniform moving boundary approach to accurately reproduce the nonuniform distribution of CSF flow along the spinal subarachnoid space (SAS) of a single cynomolgus monkey. A magnetic resonance imaging (MRI) protocol was developed and applied to quantify subject-specific CSF space geometry and flow and define the CFD domain and boundary conditions. An algorithm was implemented to reproduce the axial distribution of unsteady CSF flow by nonuniform deformation of the dura surface. Results showed that maximum difference between the MRI measurements and CFD simulation of CSF flow rates was <3.6%. CSF flow along the entire spine was laminar with a peak Reynolds number of ∼150 and average Womersley number of ∼5.4. Maximum CSF flow rate was present at the C4-C5 vertebral level. Deformation of the dura ranged up to a maximum of 134 μm. Geometric analysis indicated that total spinal CSF space volume was ∼8.7 ml. Average hydraulic diameter, wetted perimeter, and SAS area were 2.9 mm, 37.3 mm and 27.24 mm2, respectively. CSF pulse wave velocity (PWV) along the spine was quantified to be 1.2 m/s.

Comparison of phase-contrast MR and flow simulations for the study of CSF dynamics in the cervical spine

Background Investigators use phase-contrast magnetic resonance (PC-MR) and computational fluid dynamics (CFD) to assess cerebrospinal fluid dynamics. We compared qualitative and quantitative results from the two methods. Methods Four volunteers were imaged with a heavily T2-weighted volume gradient echo scan of the brain and cervical spine at 3T and with PC-MR. Velocities were calculated from PC-MR for each phase in the cardiac cycle. Mean pressure gradients in the PC-MR acquisition through the cardiac cycle were calculated with the Navier-Stokes equations. Volumetric MR images of the brain and upper spine were segmented and converted to meshes. Models of the subarachnoid space were created from volume images with the Vascular Modeling Toolkit. CFD simulations were performed with a previously verified flow solver. The flow patterns, velocities and pressures were compared in PC-MR and CFD flow images. Results PC-MR images consistently revealed more inhomogeneous flow patterns than CFD, especially in the anterolateral subarachnoid space where spinal nerve roots are located. On average, peak systolic and diastolic velocities in PC-MR exceeded those in CFD by 31% and 41%, respectively. On average, systolic and diastolic pressure gradients calculated from PC-MR exceeded those of CFD by 11% and 39%, respectively. Conclusions PC-MR shows local flow disturbances that are not evident in typical CFD. The velocities and pressure gradients calculated from PC-MR are systematically larger than those calculated from CFD.

An MRI-Compatible Hydrodynamic Simulator of Cerebrospinal Fluid Motion in the Cervical Spine

GOAL

Develop and test an MRI-compatible hydrodynamic simulator of cerebrospinal fluid (CSF) motion in the cervical spinal subarachnoid space. Four anatomically realistic subject-specific models were created based on a 22-year-old healthy volunteer and a five-year-old patient diagnosed with Chiari I malformation.

METHODS

The in vitro models were based on manual segmentation of high-resolution T2-weighted MRI of the cervical spine. Anatomically realistic dorsal and ventral spinal cord nerve rootlets (NR) were added. Models were three dimensional (3-D) printed by stereolithography with 50-μm layer thickness. A computer controlled pump system was used to replicate the shape of the subject specific in vivo CSF flow measured by phase-contrast MRI. Each model was then scanned by T2-weighted and 4-D phase contrast MRI (4D flow).

RESULTS

Cross-sectional area, wetted perimeter, and hydraulic diameter were quantified for each model. The oscillatory CSF velocity field (flow jets near NR, velocity profile shape, and magnitude) had similar characteristics to previously reported studies in the literature measured by in vivo MRI.

CONCLUSION

This study describes the first MRI-compatible hydrodynamic simulator of CSF motion in the cervical spine with anatomically realistic NR. NR were found to impact CSF velocity profiles to a great degree.

SIGNIFICANCE

CSF hydrodynamics are thought to be altered in craniospinal disorders such as Chiari I malformation. MRI scanning techniques and protocols can be used to quantify CSF flow alterations in disease states. The provided in vitro models can be used to test the reliability of these protocols across MRI scanner manufacturers and machines.

Cerebrospinal fluid dynamics at the lumbosacral level in patients with spinal stenosis: A pilot study

Spinal stenosis is a common degenerative condition. However, how neurogenic claudication develops has not been clearly elucidated. Moreover, cerebrospinal fluid physiology at the lumbosacral level has not received adequate attention. This study was conducted to compare cerebrospinal fluid hydrodynamics at the lumbosacral spinal level between patients with spinal stenosis and healthy controls. Twelve subjects (four patients and eight healthy controls; 25-77 years old; seven males) underwent phase-contrast magnetic resonance imaging to quantify cerebrospinal fluid dynamics. The cerebrospinal fluid flow velocities were measured at the L2 and S1 levels. All subjects were evaluated at rest and after walking (to provoke neurogenic claudication in the patients). The caudal peak flow velocity in the sacral spine (-0.25 ± 0.28 cm/s) was attenuated compared to that in the lumbar spine (-0.93 ± 0.46 cm/s) in both patients and controls. The lumbar caudal peak flow velocity was slower in patients (-0.65 ± 0.22 cm/s) than controls (-1.07 ± 0.49 cm/s) and this difference became more pronounced after walking (-0.66 ± 0.37 cm/s in patients, -1.35 ± 0.52 cm/s in controls; p = 0.028). The sacral cerebrospinal fluid flow after walking was barely detectable in patients (caudal peak flow velocity: -0.09 ± 0.03 cm/s). Cerebrospinal fluid dynamics in the lumbosacral spine were more attenuated in patients with spinal stenosis than healthy controls. After walking, the patients experiencing claudication did not exhibit an increase in the cerebrospinal fluid flow rate as the controls did. Altered cerebrospinal fluid dynamics may partially explain the pathophysiology of spinal stenosis. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:104-112, 2017.