An analytic model for the flow induced in syringomyelia cavities.

A simple two-dimensional fluid-structure-interaction problem, involving viscous oscillatory flow in a channel separated by an elastic membrane from a fluid-filled slender cavity, is analyzed to shed light on the flow dynamics pertaining to syringomyelia, a neurological disorder characterized by the appearance of a large tubular cavity (syrinx) within the spinal cord. The focus is on configurations in which the velocity induced in the cavity, representing the syrinx, is comparable to that found in the channel, representing the subarachnoid space surrounding the spinal cord, both flows being coupled through a linear elastic equation describing the membrane deformation. An asymptotic analysis for small stroke lengths leads to closed-form expressions for the leading-order oscillatory flow, and also for the stationary flow associated with the first-order corrections, the latter involving a steady distribution of transmembrane pressure. The magnitude of the induced flow is found to depend strongly on the frequency, with the result that for channel flow rates of non-sinusoidal waveform, as those found in the spinal canal, higher harmonics can dominate the sloshing motion in the cavity, in agreement with previous in vivo observations. Under some conditions, the cycle-averaged transmembrane pressure, also showing a marked dependence on the frequency, changes sign on increasing the cavity transverse dimension (i.e. orthogonal to the cord axis), underscoring the importance of cavity size in connection with the underlying hydrodynamics. The analytic results presented here can be instrumental in guiding future numerical investigations, needed to clarify the pathogenesis of syringomyelia cavities.

An in vitro experimental investigation of oscillatory flow in the cerebral aqueduct.

This in vitro study aims at clarifying the relation between the oscillatory flow of cerebrospinal fluid (CSF) in the cerebral aqueduct, a narrow conduit connecting the third and fourth ventricles, and the corresponding interventricular pressure difference. Dimensional analysis is used in designing an anatomically correct scaled model of the aqueduct flow, with physical similarity maintained by adjusting the flow frequency and the properties of the working fluid. The time-varying pressure difference across the aqueduct corresponding to a given oscillatory flow rate is measured in parametric ranges covering the range of flow conditions commonly encountered in healthy subjects. Parametric dependences are delineated for the time-averaged pressure fluctuations and for the phase lag between the transaqueductal pressure difference and the flow rate, both having clinical relevance. The results are validated through comparisons with predictions obtained with a previously derived computational model. The parametric quantification in this study enables the derivation of a simple formula for the relation between the transaqueductal pressure and the stroke volume. This relationship can be useful in the quantification of transmantle pressure differences based on non-invasive magnetic-resonance-velocimetry measurements of aqueduct flow for investigation of CSF-related disorders.

A one-dimensional model for the pulsating flow of cerebrospinal fluid in the spinal canal.

The monitoring of intracranial pressure (ICP) fluctuations, which is needed in the context of a number of neurological diseases, requires the insertion of pressure sensors, an invasive procedure with considerable risk factors. Intracranial pressure fluctuations drive the wave-like pulsatile motion of cerebrospinal fluid (CSF) along the compliant spinal canal. Systematically derived simplified models relating the ICP fluctuations with the resulting CSF flow rate can be useful in enabling indirect evaluations of the former from non-invasive magnetic resonance imaging (MRI) measurements of the latter. As a preliminary step in enabling these predictive efforts, a model is developed here for the pulsating viscous motion of CSF in the spinal canal, assumed to be a linearly elastic compliant tube of slowly varying section, with a Darcy pressure-loss term included to model the fluid resistance introduced by the trabeculae, which are thin collagen-reinforced columns that form a web-like structure stretching across the spinal canal. Use of Fourier-series expansions enables predictions of CSF flow rate for realistic anharmonic ICP fluctuations. The flow rate predicted using a representative ICP waveform together with a realistic canal anatomy is seen to compare favourably with in vivo phase-contrast MRI measurements at multiple sections along the spinal canal. The results indicate that the proposed model, involving a limited number of parameters, can serve as a basis for future quantitative analyses targeting predictions of ICP temporal fluctuations based on MRI measurements of spinal-canal anatomy and CSF flow rate.

Effect of Normal Breathing on the Movement of CSF in the Spinal Subarachnoid Space.

BACKGROUND AND PURPOSE: Forced respirations reportedly have an effect on CSF movement in the spinal canal. We studied respiratory-related CSF motion during normal respiration. MATERIALS AND METHODS: Six healthy subjects breathed at their normal rate with a visual guide to ensure an unchanging rhythm. Respiratory-gated phase-contrast MR flow images were acquired at 5 selected axial planes along the spine. At each spinal level, we computed the flow rate voxelwise in the spinal canal, together with the associated stroke volume. From these data, we computed the periodic volume changes of spinal segments. A phantom was used to quantify the effect of respiration-related magnetic susceptibility changes on the velocity data measured. RESULTS: At each level, CSF moved cephalad during inhalation and caudad during expiration. While the general pattern of fluid movement was the same in the 6 subjects, the flow rates, stroke volumes, and spine segment volume changes varied among subjects. Peak flow rates ranged from 0.60 to 1.59 mL/s in the cervical region, 0.46 to 3.17 mL/s in the thoracic region, and 0.75 to 3.64 mL/s in the lumbar region. The differences in flow rates along the canal yielded cyclic volume variations of spine segments that were largest in the lumbar spine, ranging from 0.76 to 3.07 mL among subjects. In the phantom study, flow velocities oscillated periodically during the respiratory cycle by up to 0.02 cm/s or 0.5%. CONCLUSIONS: Respiratory-gated measurements of the CSF motion in the spinal canal showed cyclic oscillatory movements of spinal fluid correlated to the breathing pattern.

Transmantle Pressure Computed from MR Imaging Measurements of Aqueduct Flow and Dimensions.

BACKGROUND AND PURPOSE: Measuring transmantle pressure, the instantaneous pressure difference between the lateral ventricles and the cranial subarachnoid space, by intracranial pressure sensors has limitations. The aim of this study was to compute transmantle pressure noninvasively with a novel nondimensional fluid mechanics model in volunteers and to identify differences related to age and aqueductal dimensions. MATERIALS AND METHODS: Brain MR images including cardiac-gated 2D phase-contrast MR imaging and fast-spoiled gradient recalled imaging were obtained in 77 volunteers ranging in age from 25-92 years of age. Transmantle pressure was computed during the cardiac cycle with a fluid mechanics model from the measured aqueductal flow rate, stroke volume, aqueductal length and cross-sectional area, and heart rate. Peak pressures during caudal and rostral aqueductal flow were tabulated. The computed transmantle pressure, aqueductal dimensions, and stroke volume were estimated, and the differences due to sex and age were calculated and tested for significance. RESULTS: Peak transmantle pressure was calculated with the nondimensional averaged 14.4 (SD, 6.5) Pa during caudal flow and 6.9 (SD, 2.8) Pa during rostral flow. It did not differ significantly between men and women or correlate significantly with heart rate. Peak transmantle pressure increased with age and correlated with aqueductal dimensions and stroke volume. CONCLUSIONS: The nondimensional fluid mechanics model for computing transmantle pressure detected changes in pressure related to age and aqueductal dimensions. This novel methodology can be easily used to investigate the clinical relevance of the transmantle pressure in normal pressure hydrocephalus, pediatric communicating hydrocephalus, and other CSF disorders.

Subject-Specific Studies of CSF Bulk Flow Patterns in the Spinal Canal: Implications for the Dispersion of Solute Particles in Intrathecal Drug Delivery.

BACKGROUND AND PURPOSE: Recent flow dynamics studies have shown that the eccentricity of the spinal cord affects the magnitude and characteristics of the slow bulk motion of CSF in the spinal subarachnoid space, which is an important variable in solute transport along the spinal canal. The goal of this study was to investigate how anatomic differences among subjects affect this bulk flow. MATERIALS AND METHODS: T2-weighted spinal images were obtained in 4 subjects and repeated in 1 subject after repositioning. CSF velocity was calculated from phase-contrast MR images for 7 equally spaced levels along the length of the spine. This information was input into a 2-time-scale asymptotic analysis of the Navier-Stokes and concentration equations to calculate the short- and long-term CSF flow in the spinal subarachnoid space. Bulk flow streamlines were shown for each subject and position and inspected for differences in patterns. RESULTS: The 4 subjects had variable degrees of lordosis and kyphosis. Repositioning in 1 subject changed the degree of cervical lordosis and thoracic kyphosis. The streamlines of bulk flow show the existence of distinct regions where the fluid particles flow in circular patterns. The location and interconnectivity of these recirculating regions varied among individuals and different positions. CONCLUSIONS: Lordosis, kyphosis, and spinal cord eccentricity in the healthy human spine result in subject-specific patterns of bulk flow recirculating regions. The extent of the interconnectivity of the streamlines among these recirculating regions is fundamental in determining the long-term transport of solute particles along the spinal canal.

Fluid dynamics in syringomyelia cavities: Effects of heart rate, CSF velocity, CSF velocity waveform and craniovertebral decompression.

Purpose How fluid moves during the cardiac cycle within a syrinx may affect its development. We measured syrinx fluid velocities before and after craniovertebral decompression in a patient and simulated syrinx fluid velocities for different heart rates, syrinx sizes and cerebrospinal fluid (CSF) flow velocities in a model of syringomyelia. Materials and methods With phase-contrast magnetic resonance we measured CSF and syrinx fluid velocities in a Chiari patient before and after craniovertebral decompression. With an idealized two-dimensional model of the subarachnoid space (SAS), cord and syrinx, we simulated fluid movement in the SAS and syrinx with the Navier-Stokes equations for different heart rates, inlet velocities and syrinx diameters. Results In the patient, fluid oscillated in the syrinx at 200 to 210 cycles per minute before and after craniovertebral decompression. Velocities peaked at 3.6 and 2.0 cm per second respectively in the SAS and the syrinx before surgery and at 2.7 and 1.5 cm per second after surgery. In the model, syrinx velocity varied between 0.91 and 12.70 cm per second. Increasing CSF inlet velocities from 1.56 to 4.69 cm per second increased peak syrinx fluid velocities in the syrinx by 151% to 299% for the three cycle rates. Increasing cycle rates from 60 to 120 cpm increased peak syrinx velocities by 160% to 312% for the three inlet velocities. Peak velocities changed inconsistently with syrinx size. Conclusions CSF velocity, heart rate and syrinx diameter affect syrinx fluid velocities, but not the frequency of syrinx fluid oscillation. Craniovertebral decompression decreases both CSF and syrinx fluid velocities.

The Cervical Spinal Canal Tapers Differently in Patients with Chiari I with and without Syringomyelia.

BACKGROUND AND PURPOSE: The cause of syringomyelia in patients with Chiari I remains uncertain. Cervical spine anatomy modifies CSF velocities, flow patterns, and pressure gradients, which may affect the spinal cord. We tested the hypothesis that cervical spinal anatomy differs between Chiari I patients with and without syringomyelia. MATERIALS AND METHODS: We identified consecutive patients with Chiari I at 3 institutions and divided them into groups with and without syringomyelia. Five readers measured anteroposterior cervical spinal diameters, tonsillar herniation, and syrinx dimensions on cervical MR images. Taper ratios for C1-C7, C1-C4, and C4-C7 spinal segments were calculated by linear least squares fitting to the appropriate spinal canal diameters. Mean taper ratios and tonsillar herniation for groups were compared and tested for statistical significance with a Kruskal-Wallis test. Inter- and intrareader agreement and correlations in the data were measured. RESULTS: One hundred fifty patients were included, of which 49 had syringomyelia. C1-C7 taper ratios were smaller and C4-C7 taper ratios greater for patients with syringomyelia than for those without it. C1-C4 taper ratios did not differ significantly between groups. Patients with syringomyelia had, on average, greater tonsillar herniation than those without a syrinx. However, C4-C7 taper ratios were steeper, for all degrees of tonsil herniation, in patients with syringomyelia. Differences among readers did not exceed differences among patient groups. CONCLUSIONS: The tapering of the lower cervical spine may contribute to the development of syringomyelia in patients with Chiari I.

Spinal fluid biomechanics and imaging: an update for neuroradiologists.

Flow imaging with cardiac-gated phase-contrast MR has applications in the management of neurologic disorders. Together with computational fluid dynamics, phase-contrast MR has advanced our understanding of spinal CSF flow. Phase-contrast MR is used to evaluate patients with Chiari I malformation who are candidates for surgical treatment. In theory, abnormal CSF flow resulting from the abnormal tonsil position causes syringomyelia and other neurologic signs and symptoms in patients with Chiari I. CSF flow imaging also has research applications in syringomyelia and spinal stenosis. To optimize MR acquisition and interpretation, neuroradiologists must have familiarity with healthy and pathologic patterns of CSF flow. The purpose of this review is to update concepts of CSF flow that are important for the practice of flow imaging in the spine.

Estimation of CSF flow resistance in the upper cervical spine.

Chiari I patients have increased CSF velocities in the foramen magnum due hypothetically to increased pressure gradients or reduced flow resistance. We calculated flow resistance in the cervical spinal canal in a group of subjects with and without the Chiari malformation. Eight subjects including healthy volunteers and Chiari I patients were studied. From 3D high resolution MR images of the cervical spine mathematical models of the subarachnoid spaces were created by means of standard programs for segmentation and discretization. Oscillatory flow through the subarachnoid space was simulated. Cross-sectional area of the subarachnoid space was computed at each level from C1 through C4 and the length of this spinal canal segment was measured. Peak caudad CSF flow velocity at each level was plotted against cross-section area. CSF volumetric flux and resistance were calculated for each subject. The correlation between velocity and resistance was calculated. In all subjects, peak velocities increased progressively from C1 to C4 by 0.6 to 0.7 cm/s per level. Spinal canal areas diminished from C1 to C5 in each subject at a rate of -0.25 to -0.29 cm(2) per level. Resistance averaged 4.3 pascal/ml/s in the eight subjects; 3.8 pascal/ml/s in patients with tonsilar herniation and 6.0 pascal/ml/s in volunteers. Velocity correlated inversely with resistance (R(2) = 0.6). CSF velocities correlated inversely with the flow resistance in the upper cervical spinal canal. Resistance tends to be lower in Chiari I patients than in healthy volunteers.

CSF pressure and velocity in obstructions of the subarachnoid spaces.

According to some theories, obstruction of CSF flow produces a pressure drop in the subarachnoid space in accordance with the Bernoulli theorem that explains the development of syringomyelia below the obstruction. However, Bernoulli's principle applies to inviscid stationary flow unlike CSF flow. Therefore, we performed a series of computational experiments to investigate the relationship between pressure drop, flow velocities, and obstructions under physiologic conditions. We created geometric models with dimensions approximating the spinal subarachnoid space with varying degrees of obstruction. Pressures and velocities for constant and oscillatory flow of a viscid fluid were calculated with the Navier-Stokes equations. Pressure and velocity along the length of the models were also calculated by the Bernoulli equation and compared with the results from the Navier-Stokes equations. In the models, fluid velocities and pressure gradients were approximately inversely proportional to the percentage of the channel that remained open. Pressure gradients increased minimally with 35% obstruction and with factors 1.4, 2.2 and 5.0 respectively with 60, 75 and 85% obstruction. Bernoulli's law underestimated pressure changes by at least a factor 2 and predicted a pressure increase downstream of the obstruction, which does not occur. For oscillatory flow the phase difference between pressure maxima and velocity maxima changed with the degree of obstruction. Inertia and viscosity which are not factored into the Bernoulli equation affect CSF flow. Obstruction of CSF flow in the cervical spinal canal increases pressure gradients and velocities and decreases the phase lag between pressure and velocity.

Simulating CSF flow dynamics in the normal and the Chiari I subarachnoid space during rest and exertion.

BACKGROUND AND PURPOSE: CSF fluid dynamics in healthy subjects and patients with Chiari I have been characterized during rest with phase-contrast MR imaging and CFD. CSF flow velocities and pressures in the nonresting state have not been adequately characterized. We used computer simulations to study CSF dynamics during increased heart rates in the normal and Chiari I subarachnoid space. MATERIALS AND METHODS: Cyclic CSF flow was simulated for multiple cycles in idealized 3D models of the subarachnoid space for normal and Chiari I malformation subarachnoid spaces, with flow cycles corresponding to 80 or 120 heart beats per minute. Flow velocities and pressures were computed by the Navier-Stokes equations. Synchronous bidirectional flow and flow patterns were displayed in Star-CD and inspected visually. Peak velocities and pressure differences in the 2 models were compared for the 2-cycle frequencies. RESULTS: Elevating the cycle rate from 80 to 120 cpm increased peak superior-inferior pressure gradients (top-bottom) by just 0.01% in the normal model and 2% in the Chiari model. Corresponding average pressure gradients increased by 92% and 100%, respectively. In addition, in both models, the range of synchronous bidirectional flow velocities increased. Systolic velocities had smaller increases with faster cycling. For each cycle rate, peak and average pressure gradients in the Chiari model were greater than in the normal model by 11%-16%. CONCLUSIONS: Raising the cycle rate from 80 to 120 cpm increased superior-inferior average pressure gradients and the range of synchronous bidirectional flow velocities in the normal and Chiari I models.

Tapering of the cervical spinal canal in patients with scoliosis with and without the Chiari I malformation.

BACKGROUND: Cervical spinal canal tapering may increase CSF velocities and pressures. One report suggests that the cervical spinal canal tapers more steeply in patients with Chiari I than in healthy subjects. The goal of this study was to test the conclusion by measuring spinal canal tapering in another cohort of patients. MATERIALS AND METHODS: Consecutive patients with scoliosis and MR imaging were selected. The MR images were evaluated for tonsillar herniation and syringomyelia. On a midline T2-weighted MR image, the anteroposterior diameter of the spinal canal was measured at each cervical level, and a linear trend line was fit by least-squares regression. The slope of this line was recorded as the taper ratio in millimeters/level. Patients with >5 mm of tonsillar herniation (with or without syrinx) were compared with those without tonsillar herniation (with or without syrinx). Differences in taper ratios for the 2 groups were tested for significance by the Kruskal-Willis test with significance set at .05. RESULTS: Fifty-four patients with scoliosis were identified; 22 had a Chiari malformation and 32 did not. Syringomyelia was identified in 20 of the patients with Chiari and in 8 of the others. The taper ratios averaged -0.9 mm/level for the patients with a Chiari malformation (with or without a syrinx) and -0.4 mm/level for those without it, significant at P = .035. Syringomyelia did not substantially alter the taper ratio in either group. CONCLUSIONS: Patients with scoliosis with a Chiari malformation have more steeply tapering cervical spinal canals than those without it.

Patient-specific 3D simulation of cyclic CSF flow at the craniocervical region.

BACKGROUND AND PURPOSE: Flow simulations in patient-specific models of the subarachnoid space characterize CSF flow in more detail than MR flow imaging. We extended previous simulation studies by including cyclic CSF flow and patient-specific models in multiple patients with Chiari I. We compared simulation results with MR flow measurements. MATERIALS AND METHODS: Volumetric high resolution image sets acquired in 7 patients with Chiari I, 3 patients who had previous craniovertebral decompression, and 3 controls were segmented and converted to mathematical models of the subarachnoid space. CSF flow velocities and pressures were calculated with high spatial and temporal resolution during simulated oscillatory flow in each model with the Navier-Stokes equations. Pressures, velocities, and bidirectional flow were compared in the groups (with Student t test). Peak velocities in the simulations were compared with peak velocities measured in vivo with PCMR. RESULTS: Flow visualization for patients and volunteers demonstrated nonuniform reversing patterns resembling those observed with PCMR. Velocities in the 13 subjects were greater between C2 and C5 than in the foramen magnum. Chiari patients had significantly greater peak systolic and diastolic velocities, synchronous bidirectional flow, and pressure gradients than controls. Peak velocities measured in PCMR correlated significantly (P = .003; regression analysis) despite differences between them. CONCLUSIONS: In simulations of CSF, patients with Chiari I had significantly greater peak systolic and diastolic velocities, synchronous bidirectional flow, and pressure gradients than controls.

Tapering of the cervical spinal canal in patients with Chiari I malformations.

BACKGROUND AND PURPOSE: Upper cervical spinal canal dimension may have a role in abnormal CSF dynamics in patients with Chiari I malformation. We attempted to measure spinal canal tapering from anteroposterior spinal canal dimensions in patients with Chiari I. MATERIALS AND METHODS: Twenty-one patients with Chiari I malformation, including 12 with syringomyelia and 7 patients with IS were identified from a local registry. Age- and sex-matched control subjects with cervical spine MR imaging findings reported as normal were selected from the PACS. The anteroposterior diameter of the spinal canal was measured at C1-C7 on T2-weighted sagittal MR images. The taper ratio of the spinal canal was calculated with the regression line. Goodness of fit was calculated as R(2). Differences between patients with Chiari I and other patients were tested for significance with Kruskal-Wallis tests and multivariate analysis. RESULTS: Taper ratios averaged -0.6 ± 0.3 mm/level in the patients with Chiari and syrinx, -0.4 ± 0.2 mm/level (mean ± 1 SD) in the patients with Chiari without syrinx, and -0.3 ± 0.5 mm/level in the patients with IS; control groups had average taper ratios of -0.3 ± 0.2 mm/level. Mean R(2) equaled 0.43. Taper ratios in patients with Chiari and syringomyelia differed significantly from those in the control group (P = .003). Taper ratios in the patients with Chiari without syrinx and in patients with IS did not differ significantly from their matched control groups (P = .60 and 0.76, respectively). CONCLUSIONS: Patients with Chiari I and a syrinx have steeper tapering of the upper cervical spinal canal than matched controls.

Effect of tonsillar herniation on cyclic CSF flow studied with computational flow analysis.

BACKGROUND AND PURPOSE: The Chiari I malformation, characterized by tonsils extending below the foramen magnum, has increased CSF velocities compared with those in healthy subjects. Measuring the effect of tonsillar herniation on CSF flow in humans is confounded by interindividual variation. The goal of this study was to determine the effect of herniated tonsils on flow velocity and pressure dynamics by using 3D computational models. MATERIALS AND METHODS: A previously described 3D mathematic model of the normal subarachnoid space was modified by extending the tonsils inferiorly. The chamber created was compared with the anatomy of the subarachnoid space. Pressures and velocities were calculated by CFA methods for sinusoidal flow of a Newtonian fluid. Results were displayed as 2D color-coded plots and 3D animations. Pressure gradients and flow velocities were compared with those in the normal model. Velocity distributions were also compared with those in clinical images of CSF flow. RESULTS: The model represented grossly the subarachnoid space of a patient with Chiari I malformation. Fluid flow patterns in the Chiari model were complex, with jets in some locations and stagnant flow in others. Flow jets, synchronous bidirectional flow, and pressure gradients were greater in the Chiari model than in the normal model. The distribution of flow velocities in the model corresponded well with those observed in clinical images of CSF flow in patients with Chiari I. CONCLUSIONS: Tonsillar herniation per se increases the pressure gradients and the complexity of flow patterns associated with oscillatory CSF flow.

CSF flow through the upper cervical spinal canal in Chiari I malformation.

BACKGROUND AND PURPOSE: Previous studies have quantified CSF flow in patients with Chiari I at the foramen magnum with single-axial or single-sagittal PCMR. The goal of this study was to measure CSF velocities at multiple cervical spinal levels in patients with Chiari I malformation. MATERIALS AND METHODS: In a patient registry, consecutive patients without surgery who had PCMR flow images in 5-8 axial planes between the foramen magnum and C4 were identified. Four contiguous regions were defined from the foramen magnum to C4. In each region, the fastest positive flow (PSV) and fastest negative flow (PDV) were tabulated. Changes in peak velocity by cervical spinal level and age and sex were tested for significance with linear mixed-effects models. RESULTS: In 17 patients studied, PSV increased progressively and significantly from the foramen magnum to C4. PDVs increased slightly from the foramen magnum to C3. The changes in velocity over the 4 regions tended to be smaller in the 13 patients with tonsilar ectopia than in the 4 patients without it. Age and sex had an effect on peak velocities. CONCLUSIONS: Peak diastolic and systolic CSF velocities are significantly greater below than at the foramen magnum.

CSF Flow in Chiari I and Syringomyelia from the Perspective of Computational Fluid Dynamics.

Phase contrast MR in patients with the Chiari I malformation demonstrates abnormal CSF flow in the foramen magnum and upper cervical spinal canal, related to abnormal pressure gradients. The purpose of this study was to analyze the role of CSF pressure in the pathogenesis of syringomyelia, with computational models. The spinal cord was modeled as a cylindrical poro-elastic structure with homogenous and isotropic permeability. The permeability was then made heterogeneous and anisotropic to represent the different properties of the central canal, gray and white matter. Fluid with a defined pressure, varying both in time and space, was prescribed in the SAS. Simulations were performed to quantify deformations and fluid movement within the cord. In the simulations with uniform permeability fluid moved into the cord in regions of higher pressure and out of the cord in regions of lower pressure. With permeability differences simulating gray and white matter the pattern was more complex, but similar. Adding the central spinal canal, fluid moved into the cord as in the previous case. However, preferential flow along the central canal hindered fluid from flowing back into the SAS. Pressure gradients in the SAS produce movement of fluid in the spinal cord. Assuming different relative permeability in gray matter, white matter and the central spinal canal, abnormal CSF gradients lead to accumulation of fluid within and adjacent to the spinal cord central canal.

New imaging strategies in degenerative disease of the intervertebral disks: functional spine imaging.

As it does for the brain, functional imaging provides additional clinically valuable information on the spine, especially in the problem of back and neck pain. While conventional anatomic spine imaging demonstrates many abnormalities, such as herniation of the intervertebral disk, with nearly perfect accuracy, it does not effectively distinguish incidental degenerative changes in the disk from those that results in pain production. Functional imaging of the spine, still under development and evaluation, will facilitate the identification of painful disks and the selection of patients for innovative treatments that are presently under development. Functional imaging of the spine includes: MR spectroscopy, fMRI of the spinal cord, diffusion imaging, T2 relaxation time, T1 rho measurement and dynamic imaging. The purpose of this presentation is to review the status of these functional MR techniques. MRS: MR spectroscopy demonstrates tissue constituents that have characteristic resonant frequencies. For the disk, the substances that can be recognized in MR spectra and quantified include lactic acid and glycosaminoglycans. Lactic acid has been documented by direct sampling of the disk in painful degenerating disks. With MRS, the concentration of lactic acid is measured non-invasively. In pilot studies, lactic acid concentration effectively distinguishes symptomatic from asymptomatic degenerating disks.

The "Dehydrated" Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics.

MR imaging of the lumbar spine often is requested to identify the cause of back or radicular pain. Official reports of lumbar spine images tend to focus on changes in the disk margin that may cause nerve root compression. The potential role of the dark disk, in back pain has not been adequately emphasized. The purpose of this review is to discuss the dark disk that has not produced nerve root compression. On T2-weighted images, a disk that has diminished signal intensity is called a dark disk or a dehydrated disk. It corresponds to a stage III disk in the Pfirrmann or the Thompson scale. Such a disk has specific morphologic, chemical and biomechanical properties, which will be reviewed in this presentation. The goal is to suggest the clinical significance of finding a dark disk on an MR image.