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PURPOSE: Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. However the physiologic impact of CPAP on cerebral blood flow (CBF) is not well established. Ultrasound can be used to estimate CBF but there is no widespread accepted protocol. We studied the physiologic influence of CPAP on CBF using a method integrating arterial diameter and flow velocity (FV) measurements obtained for each vessel supplying blood to the brain. METHODS: FV and lumen diameter of the left and right internal carotid vertebral and middle cerebral arteries were measured using duplex Doppler ultrasound with and without CPAP at 15 cm H(2)O applied in a random order. Transcutaneous carbon dioxide (PtcCO(2)) heart rate (HR) blood pressure (BP) and oxygen saturation were monitored. Results were compared with a theoretical prediction of CBF change based on the effect of partial pressure of carbon dioxide on CBF. RESULTS: Data were obtained from 23 healthy volunteers (mean?±?SD; 12 male age 25.1?±?2.6 years body mass index 21.8?±?2.0 kg/m(2)). The mean experimental and theoretical CBF decrease under CPAP was 12.5 (p?0.001) and 11.9 (p?

Interdisciplinary Neurosurgery, 2 (3), ISSN:2214-7519

The purpose of the present study was to compare subject-specific magnetic resonance imaging (MRI)-based computational fluid dynamics (CFD) simulations with time-resolved three-directional (3D) velocity-encoded phase-contrast MRI (4D PCMRI) measurements of the cerebrospinal fluid (CSF) velocity field in the cervical spinal subarachnoid space (SSS). Three-dimensional models of the cervical SSS were constructed based on MRI image segmentation and anatomical measurements for a healthy subject and patient with Chiari I malformation. CFD was used to simulate the CSF motion and compared to the 4D PCMRI measurements. Four-dimensional PCMRI measurements had much greater CSF velocities compared to CFD simulations (1.4 to 5.6× greater). Four-dimensional PCMRI and CFD both showed anterior and anterolateral dominance of CSF velocities, although this flow feature was more pronounced in 4D PCMRI measurements compared to CFD. CSF flow jets were present near the nerve rootlets and denticulate ligaments (NRDL) in the CFD simulation. Flow jets were visible in the 4D PCMRI measurements, although they were not clearly attributable to nerve rootlets. Inclusion of spinal cord NRDL in the cervical SSS does not fully explain the differences between velocities obtained from 4D PCMRI measurements and CFD simulations.

For the first time, inter-operator dependence of MRI based computational fluid dynamics (CFD) modeling of cerebrospinal fluid (CSF) in the cervical spinal subarachnoid space (SSS) is evaluated. In vivo MRI flow measurements and anatomy MRI images were obtained at the cervico-medullary junction of a healthy subject and a Chiari I malformation patient. 3D anatomies of the SSS were reconstructed by manual segmentation by four independent operators for both cases. CFD results were compared at nine axial locations along the SSS in terms of hydrodynamic and geometric parameters. Intraclass correlation (ICC) assessed the inter-operator agreement for each parameter over the axial locations and coefficient of variance (CV) compared the percentage of variance for each parameter between the operators. Greater operator dependence was found for the patient (0.19 < ICC < 0.99) near the craniovertebral junction compared to the healthy subject (ICC > 0.78). For the healthy subject, hydraulic diameter and Womersley number had the least variance (CV = similar to 2%). For the patient, peak diastolic velocity and Reynolds number had the smallest variance (CV = similar to 3%). These results show a high degree of inter-operator reliability for MRI-based CFD simulations of CSF flow in the cervical spine for healthy subjects and a lower degree of reliability for patients with Type I Chiari malformation.

Fine anatomical structures, including nerve roots and denticulate ligaments, can significantly influence cerebrospinal fluid (CSF) dynamics inside the spinal subarachnoid space (SSS). In this study, we completed computational fluid dynamics (CFD) simulations based on subject specific geometries of the cervical spine with and without idealized fine structures (nerve roots and denticulate ligaments, NRDL). The results show that NRDL had a significant impact on CSF dynamics in terms of velocity distribution, flow streamlines, bidirectional nature of the flow and the pressure drop.

The physiological impacts of continuous positive airway pressure (CPAP) are not yet fully understood. In this study we developed an MRI protocol to assess the impact of CPAP on cerebral hemodynamics and cerebrospinal fluid (CSF) hydrodynamics in the upper cervical spine. MRI measurements were obtained on 14 healthy male subjects. The preliminary results indicated that the CSF pulsation decreased and venous outflow was altered with CPAP usage in comparison to the baseline values. Cerebral arterial flow was not impacted by CPAP usage. These findings support the hypothesis that the CSF system can act to dampen cerebral blood flow (CBF) pulsations.

In this study, we employed time-resolved three directional velocity encoded phase contrast MRI (4DMRI) to quantify cerebrospinal fluid (CSF) movement within the complete cervical spinal subarachnoid space (SSS). The 4DMRI measurements were compared to a 3D computational fluid dynamics simulation (3DCFD) of the same space in healthy subjects and patients with Chiari I malformation. The results were assessed in terms of peak CSF flow velocities in the foot and head direction and flow patterns at nine axial locations along the cervical spine. It was found that the peak flow velocities measured by 4DMRI were consistently greater than those quantified by 3DCFD. In healthy subjects the 4DMRI flow velocities and patterns compared better to the 3DCFD than in Chiari I patients. Peak velocities had the greatest differences in the upper cervical spine of Chiari patients, where the 4DMRI quantified greater fluid jets at the craniocervical junction than the 3DCFD. Overall, these results represent the first quantitative comparison of 4DMRI to 3DCFD of CSF motion.

Computational fluid dynamic (CFD) simulations have provided detailed information about the complex flow and pressure field within the cerebrospinal fluid (CSF) system. The solutions of these simulations are sensitive to boundary conditions and thus need validation with in vivo measurements. However, validation of CFD flow results and MRI CSF flow measurements have been limited to through-plane pcMR measurements. Recent advancements in MRI flow measurement technology has enabled measurement of the 4D flow field within the spinal subarachnoid space (SSS), superior sagittal sinus and ventricles of the brain. Alterations in cerebrospinal fluid (CSF) dynamics have been associated with neurological symptoms and formation of Syringomyelia in patients with Chiari malformation and spinal stenosis. In these patients, analysis of morphology alone has proved to be insufficient in explaining the absence or presence of symptoms. In the present study, we compare quantitatively 4D flow measurements to CFD analysis within the SSS (C1-T1, cervical spinal segment) of a healthy volunteer without any lesions at the craniocervical junction or the cervical spinal canal.

Continuous Positive Airway Pressure (CPAP) is used as the gold standard treatment for sleep disordered breathing, acting as a pneumatic splint to prevent collapse of the pharyngeal airway. However, the influence that CPAP has on Cerebral Blood Flow (CBF) dynamics is not well understood. This preliminary study investigates the influence of CPAP on total CBF in 23 healthy awake subjects by measuring flow velocity and lumen diameter of the left and right proximal Internal Carotid Arteries (ICA), Vertebral Arteries (VA), and Middle Cerebral Arteries (MCA) using Duplex Color Doppler Ultrasound (US) with and without CPAP at a level of 15 cm H2O. Transcutaneous Carbon Dioxide (PtcCO2) level, heart rate, Blood Pressure (BP), and oxygen saturation (SaO2) were monitored before and after each test. The preliminary measurements indicate that CPAP results in a decrease of CBF by 17% (p-value < 0.05). The theoretically predicted decrease in CBF from PtcCO2 variation was 6%. The study should be further explored in patients with sleep apnea and various types of cerebrovascular and craniospinal disorders.Copyright © 2011 by ASME

Cerebrospinal fluid (CSF) dynamics in the spinal subarachnoid space (SSS) have been thought to play an important pathophysiological role in syringomyelia, Chiari I malformation (CM), and a role in intrathecal drug delivery. Yet, the impact that fine anatomical structures, including nerve roots and denticulate ligaments (NRDL), have on SSS CSF dynamics is not clear. In the present study we assessed the impact of NRDL on CSF dynamics in the cervical SSS. The 3D geometry of the cervical SSS was reconstructed based on manual segmentation of MRI images of a healthy volunteer and a patient with CM. Idealized NRDL were designed and added to each of the geometries based on in vivo measurments in the literature and confirmation by a neuroanatomist. CFD simulations were performed for the healthy and patient case with and without NRDL included. Our results showed that the NRDL had an important impact on CSF dynamics in terms of velocity field and flow patterns. However, pressure distribution was not altered greatly although the NRDL cases required a larger pressure gradient to maintain the same flow. Also, the NRDL did not alter CSF dynamics to a great degree in the SSS from the foramen magnum to the C1 level for the healthy subject and CM patient with mild tonsillar herniation (similar to 6 mm). Overall, the NRDL increased fluid mixing phenomena and resulted in a more complex flow field. Comparison of the streamlines of CSF flow revealed that the presence of NRDL lead to the formation of vortical structures and remarkably increased the local mixing of the CSF throughout the SSS.