Relationship Between Cerebrospinal Fluid and Blood Dynamics in Healthy Volunteers and Patients with Communicating Hydrocephalus

Assessment of craniospinal pressure-volume indices

BACKGROUND AND PURPOSE

The PVI(CC) of the craniospinal compartment defines the shape of the pressure-volume curve and determines the damping of cyclic arterial pulsations. Despite no reports of direct measurements of the PVI(CC) among healthy elderly, it is believed that a change away from adequate accommodation of cardiac-related pulsations may be a pathophysiologic mechanism seen in neurodegenerative disorders such as Alzheimer disease and idiopathic normal pressure hydrocephalus. In this study, blood and CSF flow measurements are combined with lumbar CSF infusion measurements to assess the craniospinal PVI(CC) and its distribution of cranial and spinal compartments in healthy elderly.

MATERIALS AND METHODS

Thirty-seven healthy elderly were included (60-82 years of age). The cyclic arterial volume change and the resulting shift of CSF to the spinal compartment were quantified by PC-MR imaging. In addition, each subject underwent a lumbar CSF infusion test in which the magnitude of cardiac-related pulsations in intracranial pressure was quantified. Finally, the PVI was calculated by using a mathematic model.

RESULTS

After excluding 2 extreme values, the craniospinal PVI(CC) was calculated to a mean of 9.8 ± 2.7 mL and the estimated average 95% confidence interval of individual measurements was ± 9%. The average intracranial and spinal contributions to the overall compliance were 65% and 35% respectively (n = 35).

CONCLUSIONS

Combining lumbar CSF infusion and PC-MR imaging proved feasible and robust for assessment of the craniospinal PVI(CC). This study produced normative values and showed that the major compensatory contribution was located intracranially.

[Dynamic magnetic resonance imaging of the cerebrospinal fluid flow within the cerebral aqueduct by different FISP 2D sequences]

BACKGROUND/AIM

A vast majority of current radiogical techniques, such as computerized tomography (CT) and magnetic resonance imaging (MRI) have great potencial of vizualization and delineation of cerebrospinal fuid spaces morphology within cerebral aqueduct. The aim of this study was to determine the possibilities of two differently acquired FISP (Fast Imaging with Steady State Precession) 2D MR sequences in the estimation of the pulsatile cerebrospinal fluid (CSF) flow intensity through the normal cerebral aqueduct.

METHODS

Sixty eight volunteers underwent brain MRI on 1.5T MR imager with additionally performed ECG retrospectively gated FISP 2D sequences (first one, as the part of the standard software package, with following technical parameters: TR 40, TE 12, FA 17, Matrix: 192 x 256, Acq 1, and the second one, experimentally developed by our investigation team: TR 30, TE 12, FA 70, Matrix: 192 x 256, Acq 1) respectively at two fixed slice positions--midsagittal and perpendicular to cerebral aqueduct, displayed and evaluated by multiplegated images in a closed-loop cinematographic (CINE) format.

RESULTS

Normal brain morphology with preserved patency of the cerebral aqueduct in all of 68 healthy volunteers was demonstrated on MRI examination. Cerebrospinal fluid flow within the cerebral aqueduct was distinguishable on both CINE MRI studies in midsagittal plane, but the estimation of intraaqueductal CSF flow in perpendicular plane was possible on CINE MRI studies acquired with experimentally improved FISP 2D (TR 30, FA 70) sequence only.

CONCLUSION

Due to the changes of technical parameters CINE MRI study acquired with FISP 2D (TR 30, FA 70) in perpendicular plane demonstrated significantly higher capability in the estimation of the CSF pulsation intensity within the cerebral aqueduct.

The efficiency of PC-MRI in diagnosis of normal pressure hydrocephalus and prediction of shunt response

RATIONALE AND OBJECTIVES

In this prospective study, we aimed to reveal the efficiency of phase-contrast magnetic resonance imaging (PC-MRI) in the diagnosis of idiopathic normal pressure hydrocephalus (INPH) and prediction of shunt response.

MATERIALS AND METHODS

The study group consisted of 43 patients with INPH diagnosis and 15 asymptomatic age-matched controls. PC-MRI studies were applied on cerebral aqueduct and superior sagittal sinus (SSS) in all the cases.

RESULTS

The maximum and mean cerebrospinal fluid (CSF) flow velocities were significantly higher in the INPH patients compared with the controls (P < .05). CSF stroke volume (43.2 + or - 63.8 microL) and output/min (3921 + or - 5668 microL) were remarkably higher in the NPH group compared with the control group (3.9 + or - 3.9 microL, 439 + or - 487 microL, respectively) (P < .05). Maximum and mean venous velocity values of the INPH patients (maximum, 19.2 + or - 4.3 cm/s; mean, 16 + or - 3.7 cm/s), were lower than those of the control group (maximum, 21.8 + or - 4.6 cm/s; mean, 18.9 + or - 3.9 cm/s) (P < .05). Stroke volume and venous output/min values of INPH patients in SSS, were significantly lower than those of the control group (P < .001, P = .007, respectively). The response of INPH patients against shunt treatment showed no statistical correlation with any of the PC-MRI parameters (P > .05).

CONCLUSION

The measurement of CSF venous flow velocities with PC-MRI is a noninvasive test that benefits INPH diagnosis, but remains inadequate in prediction of response against shunt treatment.

[Contribution of phase-contrast MRI to the management of patients with normal pressure hydrocephalus: Can it predict response to shunting?]

The diagnosis and management of patients with idiopathic normal-pressure hydrocephalus (NPH) remain somewhat controversial and there is no clear guideline for assessing the post-shunt outcome. The objective of this study was to investigate whether cerebrospinal fluid (CSF) flow dynamics is linked to post-shunt improvement. Fourteen NPH patients (nine males and five females; mean age, 68 years) investigated by magnetic resonance imaging (MRI) before surgical diversion of CSF were retrospectively reviewed. Phase-contrast sequences were added to the morphological clinical protocol for quantification of CSF oscillations, which were recorded at the level of the cerebral aqueduct and the C2 and C3 subarachnoid spaces (SAS). The phase-contrast images were analysed with custom-designed dedicated flow segmentation software. The oscillations measured in this hydrocephalus population were compared to a previously studied healthy population. A difference of at least two standard deviations was used to define a hyperdynamic or hypodynamic state of CSF flow. The cervical CSF flow of the hydrocephalus patients was not significantly different from those of the volunteer population. Of the 14 hydrocephalus patients, 12 had a good response to the shunt. Of these, 10 presented an increased ventricular CSF flow, one a low ventricular CSF flow, and the last one had a normal ventricular CSF flow. Phase-contrast MRI can help develop guidelines for surgical management of NPH. The shunt responders appear to be the patients with hyperdynamic ventricular CSF flow and normal cervical CSF flow.

Influence of the compliance of the neck arteries and veins on the measurement of intracranial volume change by phase-contrast MRI

PURPOSE

To assess the influence of arterial and venous vascular compliances in the neck region on the measurement of the change in intracranial volume during the cardiac cycle.

MATERIALS AND METHODS

Arterial and venous blood flows were imaged by MRI phase contrast at two different locations, one close to the skull base (upper) and one 2-3 cm lower, around C3 level (lower). Maximal intracranial volume change (ICVC) measurements were derived from the momentary difference between the arterial inflow and venous outflow rates at the upper and lower locations separately to assess the influence of the compliances of the vessel segments bounded by the two different imaging locations. Imaging location for the craniospinal cerebrospinal fluid flow was a constant variable in this experiment.

RESULTS

The systolic ICVC obtained using the lower location was consistently larger than when using the upper location. Comparison between arterial and venous flow dynamics revealed a much larger changes in flow dynamic and lumen areas in the veins compared with the arteries, which explain the large venous influence on the intracranial volume change measurement.

CONCLUSION

Arterial inflow and venous outflow should be sampled at a level close to the skull base (C1-C2) to minimize the influence of the compliance of arteries and the collapsibility of veins for a reliable measurement of ICVC.

Noninvasive intracranial compliance from MRI-based measurements of transcranial blood and CSF flows: indirect versus direct approach

Intracranial compliance (ICC) determines the ability of the intracranial compartment to accommodate an increase in volume without a large increase in intracranial pressure (ICP). The clinical utilization of ICC is limited by the invasiveness of current measurement. Several investigators attempted to estimate ICC noninvasively, from magnetic resonance imaging (MRI) measurements of cerebral blood and cerebral spinal fluid flows, either using indirect measures of ICC or directly by measuring the ratio of the changes in intracranial volume and pressure during the cardiac cycle. The indirect measures include the phase lag between the cerebrospinal fluid (CSF) and its driving force, either arterial inflow or net transcranial blood flow. This study compares the sensitivity of phase-based and amplitude-based measures of ICC to changes in ICC. In vivo volumetric blood and CSF flows measured by MRI phase contrast from healthy volunteers and from patients with elevated ICP were used for the comparison. An RLC circuit model of the craniospinal system was utilized to simulate the effect of a change in ICC on the CSF flow waveform. The simulations demonstrated that amplitude-based measures of ICC are considerably more sensitive than phase-based measures, and among the amplitude-based measures, the ICC index provides the most reliable estimate of ICC.

Clinical evaluation of the safety and efficacy of lumbar cerebrospinal fluid drainage for the treatment of refractory increased intracranial pressure

OBJECT

Several approaches have been established for the treatment of intracranial hypertension; however, a considerable number of patients remain unresponsive to even aggressive therapeutic strategies. Lumbar CSF drainage has been contraindicated in the setting of increased intracranial pressure (ICP) because of possible cerebral herniation. The authors of this study investigated the efficacy and safety of controlled lumbar CSF drainage in patients suffering from intracranial hypertension following severe traumatic brain injury (TBI) or aneurysmal subarachnoid hemorrhage (SAH).

METHODS

The authors prospectively evaluated 100 patients-45 with TBI and 55 with SAH-having a mean age of 43.7 +/- 15.7 years (mean +/- SD) and suffering from refractory intracranial hypertension (ICP > 20 mm Hg). Intracranial pressure and cerebral perfusion pressure (CPP) before and after the initiation of lumbar CSF drainage as well as related complications were documented. Patient outcomes were assessed 6 months after injury.

RESULTS

The application of lumbar CSF drainage led to a significant reduction in ICP from 32.7 +/- 10.9 to 13.4 +/- 5.9 mm Hg (p < 0.05) and an increase in CPP from 70.6 +/- 18.2 to 86.2 +/- 15.4 mm Hg (p < 0.05). Cerebral herniation with a lethal outcome occurred in 6% of patients. Thirty-six patients had a favorable outcome, 12 were severely disabled, 7 remained in a persistent vegetative state, and 45 died.

CONCLUSIONS

Lumbar drainage of CSF led to a significant and clinically relevant reduction in ICP. The risk of cerebral herniation can be minimized by performing lumbar drainage only in cases with discernible basal cisterns.

A phase-contrast MRI study of physiologic cerebral venous flow

Although crucial in regulating intracranial hydrodynamics, the cerebral venous system has been rarely studied because of its structural complexity and individual variations. The purpose of our study was to evaluate the organization of cerebral venous system in healthy adults. Phase-contrast magnetic resonance imaging (PC-MRI) was performed in 18 healthy volunteers, in the supine position. Venous, arterial, and cerebrospinal fluid (CSF) flows were calculated. We found heterogeneous individual venous flows and variable side dominance in paired veins and sinuses. In some participants, the accessory epidural drainage preponderated over the habitually dominant jugular outflow. The PC-MRI enabled measurements of venous flows in superior sagittal (SSS), SRS (straight), and TS (transverse) sinuses with excellent detection rates. Pulsatility index for both intracranial (SSS) and cervical (mainly jugular) levels showed a significant increase in pulsatile blood flow in jugular veins as compared with that in SSS. Mean cervical and cerebral arterial blood flows were 714+/-124 and 649+/-178 mL/min, respectively. Cerebrospinal fluid aqueductal and cervical stroke volumes were 41+/-22 and 460+/-149 microL, respectively. Our results emphasize the variability of venous drainage for side dominance and jugular/epidural organization. The pulsatility of venous outflow and the role it plays in the regulation of intracranial pressure require further investigation.

[Acute CSF changes in the mesencephalon aqueduct after subarachnoid hemorrhage as measured by PC-MRI]

PURPOSE

Determining acute intracranial hydrodynamic changes after subarachnoid hemorrhage through an analysis of the CSF stroke volume (SV) as measured by phase-contrast MRI (PC-MRI) in the mesencephalon aqueduct.

METHOD

A prospective study was performed in 33 patients with subarachnoid hemorrhage. A PC-MRI imaging study was performed n the acute phase (< 48 hours). CSF flow was measured in the aqueduct. The appearance of acute hydrocephalus (HCA) was then compared with data on CSF flow, and the location of the intraventricular and perimesencephalic bleeding.

RESULTS

CSF analysis was performed on 27 patients, 11 of whom presented with an acute HCA. All 11 patients had an abnormal SV in the aqueduct: patients with a communicating HCA had an increased SV (n=8); and patients with a noncommunicating HCA had a nil SV (n=3). Patients with a normal SV in the aqueduct did not develop an acute HCA. Intraventricular bleeding significantly led to HCA (P=0.02), which was of the communicating type in 70% of cases.

CONCLUSION

Subarachnoid hemorrhage leads to intracranial CSF hydrodynamic modifications in the aqueduct in the majority of patients. CSF flow can help us to understand the mechanism of the appearance of acute HCA. Indeed, hydrocephalus occurred - of the communicating type in most cases - even in the presence of intraventricular bleeding.

Measurement of peak CSF flow velocity at cerebral aqueduct, before and after lumbar CSF drainage, by use of phase-contrast MRI: utility in the management of idiopathic normal pressure hydrocephalus

OBJECTIVE

Since it was first described, normal pressure hydrocephalus (NPH) and its treatment by means of cerebrospinal fluid (CSF) shunting have been the focus of much investigation. Whatever be the cause of NPH, it has been hypothesized that in this disease there occurs decreased arterial expansion and an increased brain expansion leading to increased transmantle pressure. We cannot measure the latter, but fortunately the effect of these changes (increased peak flow velocity through the aqueduct) can be quantified with cine phase-contrast magnetic resonance imaging (MRI). This investigation was thus undertaken to characterize and measure CSF peak flow velocity at the level of the aqueduct, before and after lumbar CSF drainage, by means of a phase-contrast cine MRI and determine its role in selecting cases for shunt surgery.

PATIENTS AND METHODS

37 patients with clinically suspected NPH were included in the study. Changes in the hyperdynamic peak CSF flow velocity with 50 ml lumbar CSF drainage (mimicking shunt) were evaluated in them for considering shunt surgery.

RESULTS

14 out of 15 patients who were recommended for shunt surgery, based on changes peak flow velocity after lumbar CSF drainage, improved after shunt surgery. None of the cases which were not recommended for shunt surgery, based on changes in CSF peak flow velocity after lumbar CSF drainage, improved after shunt surgery (2 out of 22 cases).

CONCLUSION

The study concluded that the phase-contrast MR imaging, done before and after CSF drainage, is a sensitive method to support the clinical diagnosis of normal pressure hydrocephalus, selecting patients of NPH who are likely to benefit from shunt surgery, and to select patients of NPH who are not likely to benefit from shunt surgery.

A new lumped-parameter model of cerebrospinal hydrodynamics during the cardiac cycle in healthy volunteers

Our knowledge of cerebrospinal fluid (CSF) hydrodynamics has been considerably improved with the recent introduction of phase-contrast magnetic resonance imaging (phase-contrast MRI), which can provide CSF and blood flow measurements throughout the cardiac cycle. Key temporal and amplitude parameters can be calculated at different sites to elucidate the role played by the various CSF compartments during vascular brain expansion. Most of the models reported in the literature do not take into account CSF oscillation during the cardiac cycle and its kinetic energy impact on the brain. We propose a new lumped-parameter compartmental model of CSF and blood flows in healthy subjects during the cardiac cycle. The system was divided into five submodels representing arterial blood, venous blood, ventricular CSF, cranial subarachnoid space, and spinal subarachnoid space. These submodels are connected by resistances and compliances. The model developed was used to reproduce certain functional characteristics observed in seven healthy volunteers, such as the distribution (amplitude and phase shift) of arterial, venous, and CSF flows. The results show a good agreement between measured and simulated intracranial CSF and blood flows.

Noninvasive MRI assessment of intracranial compliance in idiopathic normal pressure hydrocephalus

PURPOSE

To assess the state and dynamics of the intracranial system in idiopathic normal-pressure hydrocephalus (I-NPH), we determined intracranial compliance using magnetic resonance imaging (MRI).

MATERIALS AND METHODS

The intracranial compliance index (ICCI), which was defined as the ratio of the peak-to-peak intracranial volume change (ICVC(p-p)) to the peak-to-peak cerebrospinal fluid (CSF) pressure gradient (PG(p-p)) during the cardiac cycle, was obtained from the net transcranial blood and CSF flow measured with phase-contrast (PC) cine MRI. ICCI was determined in patients with I-NPH (N = 7), brain atrophy, or asymptomatic ventricular dilation (VD) (N = 6), and in healthy volunteers (control group; N = 11). The changes in ICCI indices were also analyzed after a CSF tap test (N = 2).

RESULTS

The ICCI in the I-NPH group was significantly lower than in the control and VD groups, whereas no difference was found between the control and VD groups. The ICVC(p-p) was also lower than in the control and VD groups. However, no significant difference was found in the PG(p-p) between groups. The ICCI increased after the tap test.

CONCLUSION

Intracranial compliance analysis with MRI makes it possible to noninvasively obtain more detailed information of intracranial biomechanics in the I-NPH and to assist in the diagnosis of I-NPH.

Aging effects on cerebral blood and cerebrospinal fluid flows

Phase-contrast magnetic resonance imaging (PC-MRI) is a noninvasive reliable technique, which enables quantification of cerebrospinal fluid (CSF) and total cerebral blood flows (tCBF). Although it is used to study hydrodynamic cerebral disorders in the elderly group (hydrocephalus), there is no published evaluation of aging effects on both tCBF and CSF flows, and on their mechanical coupling. Nineteen young (mean age 27+/-4 years) and 12 elderly (71+/-9 years) healthy volunteers underwent cerebral MRI using 1.5 T scanner. Phase-contrast magnetic resonance imaging pulse sequence was performed at the aqueductal and cervical levels. Cerebrospinal fluid and blood flow curves were then calculated over the cardiac cycle, to extract the characteristic parameters: mean and peak flows, their latencies, and stroke volumes for CSF (cervical and aqueductal) and vascular flows. Total cerebral blood flow was (P<0.01) decreased significantly in the elderly group when compared with the young subjects with a linear correlation with age observed only in the elderly group (R(2)=0.7; P=0.05). Arteriovenous delay was preserved with aging. The CSF stroke volumes were significantly reduced in the elderly, at both aqueductal (P<0.01) and cervical (P<0.05) levels, whereas aqueduct/cervical proportion (P=0.9) was preserved. This is the first work to study aging effects on both CSF and vascular cerebral flows. Data showed (1) tCBF decrease, (2) proportional aqueductal and cervical CSF pulsations reduction as a result of arterial loss of pulsatility, and (3) preserved intracerebral compliance with aging. These results should be used as reference values, to help understand the pathophysiology of degenerative dementia and cerebral hydrodynamic disorders as hydrocephalus.

Changing concepts of cerebrospinal fluid hydrodynamics: role of phase-contrast magnetic resonance imaging and implications for cerebral microvascular disease

Phase-contrast magnetic resonance imaging (PC-MRI) or flow-sensitive MRI can be used to noninvasively measure intracranial vascular and CSF flow. Monro-Kellie homeostasis is the complex compensatory mechanism for the increase in intracranial blood volume during systole. Through PC-MRI techniques, our understanding of Monro-Kellie homeostasis and the associated intracranial hydrodynamics has greatly improved. Failure of this homeostatic mechanism has been implicated in a wide range of cerebral disorders, including vascular and Alzheimer's dementia, late-onset depression, benign and secondary intracranial hypertension, communicating and normal pressure hydrocephalus, and age-related white matter changes. The most common mode of homeostatic failure is due to vascular disease with decreased cerebral arterial compliance. This has wide-reaching implications in the investigation of patients with cerebral vascular disease. Here we discuss the role of PC-MRI in the study of cerebral hydrodynamics and the current understanding of Monro-Kellie homeostasis in both healthy and disease states. Quantitative assessment of the changes in this homeostatic mechanism using PC-MRI has important implications in the development of biomarkers of vascular involvement in disease with application in diagnosis, treatment planning, phenotype identification, and outcome assessment in clinical trials.

Value of phase contrast magnetic resonance imaging for investigation of cerebral hydrodynamics

OBJECTIVE

Phase Contrast Magnetic Resonance Imaging (PCMRI) is a noninvasive technique that can be used to quantify variations of flow during the cardiac cycle. PCMRI allows investigations of blood flow dynamics in the main arteries and veins of the brain but also the dynamics of cerebrospinal fluid. These cerebral flow investigations provide a description of the regulation mechanisms of intracranial pressure during the cardiac cycle. The objective of this paper is to describe the contribution of this technique in diseases related to disorders of cerebral hydrodynamics in the light of 5 clinical cases.

METHOD

Flow measurements were performed using PCMRI sequences on a 1.5 Tesla MR imager in 4 patients with symptomatic ventricular dilation and 1 patient with a syringomyelic cavity.

RESULTS

Flow quantification in these 5 patients, representative of the diseases mainly concerned by cerebral hydrodynamics, is useful to guide the indication for ventricular shunting in patients with hydrocephalus, to demonstrate obstruction of the cerebral aqueduct, to demonstrate recirculation of ventricular CSF after ventriculostomy and to characterize the dynamic features of CSF inside a spinal cavity.

CONCLUSION

PCMRI, now available to neurosurgeons, is complementary to morphological MR and provides quantitative information on cerebral hydrodynamics. This information is mainly used to confirm alteration of CSF flow in the cerebral and spinal compartments. PCMRI is also a functional tool to better understand the pathophysiology of hydrocephalus and syringomyelia.

Brain hydrodynamics study by phase-contrast magnetic resonance imaging and transcranial color doppler

PURPOSE

To evaluate the contributions of phase-contrast magnetic resonance (PCMR) and transcranial color Doppler (TCCD) imaging in the investigation of cerebral hydrodynamics.

MATERIALS AND METHODS

A total of 13 healthy subjects were studied. Blood velocity measurements were performed with TCCD and gated PCMR imaging in major intracranial and extracranial arteries stages. Peak systolic velocity and end-diastolic velocity were extracted to establish correlations between TCCD and PCMR imaging. Cerebral blood flow (CBF) and intracranial volume change (IVC) during the cardiac cycle were calculated, taking into account cerebrospinal fluid (CSF) oscillations.

RESULTS

Despite an underestimation of velocities with PCMR imaging, significant correlations were observed for velocity measurements between the two modalities in extracranial vessels, but were poorly correlated in intracranial vessels. PCMR data processing gave a mean CBF of 690+/-90 mL/minute.

CONCLUSION

PCMR imaging provides complementary information to TCCD to assess various intracranial parameters such as instantaneous velocities, blood and CSF flow distributions, volume variation, or pressure regulation mechanisms during cardiac cycles.

Abnormalities of CSF flow patterns in the cerebral aqueduct in treatment-resistant late-life depression: a potential biomarker of microvascular angiopathy

There is growing evidence that microvascular angiopathy (MVA) plays an important role in the development of dementia and affective disorders in older people. At currently available image resolutions it is not possible to image directly the vascular changes associated with MVA, but the effects on blood and cerebrospinal fluid (CSF) flow may be detectable. The aim of this study was to investigate a potential biomarker for MVA based on MRI of abnormalities in CSF flow. Since there is considerable indirect evidence that treatment resistance in late-onset depressive disorder is related to MVA, we assessed the method in a group of 22 normal volunteers and 29 patients with responsive (N=21) or treatment-resistant (N=8) late-onset depressive disorder. Single-slice quantified phase-contrast (PC) images of cerebral blood and CSF flow were collected at 15 points over a cardiac cycle, and the resulting flow curves were parameterized. Significant differences in the CSF flow (width of systolic flow peak and diastolic flow volume, both P<0.01) through the cerebral aqueduct were observed for the group of treatment-resistant patients when compared to age matched controls. No significant difference was observed for a group of 21 patients with treatment-responsive depression. The findings support the hypothesis that MR measurement of CSF flow abnormalities provides a biomarker of MVA, and thus could have application in a wide range of age-related diseases.

Improved cerebrospinal fluid flow measurements using phase contrast balanced steady-state free precession

We present a demonstration of phase contrast balanced steady-state free precession (PC-bSSFP) for measuring cerebrospinal fluid (CSF) flow in the brain and spine, and a comparison of measurements obtained with this technique to conventional phase contrast using incoherent gradient echoes (PC-GRE). With PC-GRE sequences, CSF images suffer from low signal-to-noise ratio (SNR), due to short repetition times required for adequate temporal resolution, and the long relaxation time of CSF. Furthermore, CSF flow is often nonlaminar, causing phase dispersion and signal loss in PC-GRE images. It is hypothesized that PC-bSSFP can improve CSF flow measurements with its high SNR and insensitivity to turbulent flow effects. CSF images acquired from the two techniques were compared in 13 healthy volunteers. Three measures were used to objectively evaluate the PC-bSSFP sequence: the CSF flow percentage, defined as the percentage of the total CSF region exhibiting pulsatile flow, net stroke volume and SNR. Images acquired with PC-bSSFP demonstrated pulsatile CSF flow in 35.8% (P<.005), 11.2% (P<.05) and 27.8% (P<.0005) more pixels than PC-GRE in the prepontine cistern, anterior and posterior cervical subarachnoid space (SAS), respectively. Likewise, measurements of stroke volume in these regions increased by 61.6% (P<.05), 16.8% (P<.001) and 48.3% (P<.0001), respectively. Similar comparisons in the aqueduct showed no statistical difference in stroke volumes between the two techniques (P=.5). The average gain in SNR was 3.3+/-1.7 (P<.001) in the prepontine cistern, 5.0+/-0.2 (P<.01) at the cervical level and 2.0+/-0.4 (P<.001) in the aqueduct in PC-bSSFP magnitude images over PC-GRE images. In addition to the obvious advantage of increased SNR, these results indicate that PC-bSSFP provides more complete measurements of CSF flow data than PC-GRE. PC-bSSFP can be used as a reliable technique for CSF flow quantification for the characterization of normal and altered intracranial CSF flow patterns.

Amplitude and phase of cerebrospinal fluid pulsations: experimental studies and review of the literature

OBJECT

A recently developed model of communicating hydrocephalus suggests that ventricular dilation may be related to the redistribution of pulsations in the cranium from the subarachnoid spaces (SASs) into the ventricles. Based on this model, the authors have developed a method for analyzing flow pulsatility in the brain by using the ratio of aqueductal to cervical subarachnoid stroke volume and the phase of cerebrospinal fluid (CSF) flow, which is obtained at multiple locations throughout the cranium, relative to the phase of arterial flow.

METHODS

Flow data were collected in a group of 15 healthy volunteers by using a series of images acquired with cardiac-gated, phase-contrast magnetic resonance imaging. The stroke volume ratio was 5.1 +/- 1.8% (mean +/- standard deviation). The phase lag in the aqueduct was -52.5 +/-16.5 degrees and the phase lag in the prepontine cistern was -22.1 +/- 8.2 degrees. The flow phase at the level of C-2 was -5.1 +/- 10.5 degrees, which was consistent with flow synchronous with the arterial pulse. The subarachnoid phase lag ventral to the pons was shown to decrease progressively to zero at the craniocervical junction. Flow in the posterior cervical SAS preceded the anterior space flow.

CONCLUSIONS

Under normal conditions, pulsatile ventricular CSF flow is a small fraction of the net pulsatile CSF flow in the cranium. A thorough review of the literature supports the view that modified intracranial compliance can lead to redistribution of pulsations and increased intraventricular pulsations. The phase of CSF flow may also reflect the local and global compliance of the brain.

Ventricular dilation as an instability of intracranial dynamics

We address the question of the ventricles' dilation as a possible instability of the intracranial dynamics. The ventricular system is shown to be governed by a dynamical equation derived from first principles. This general nonlinear scheme is linearized around a well-defined steady state which is mapped onto a pressure-volume model with an algebraic effective compliance depending on the ventricles' geometry, the ependyma's elasticity, and the cerebrospinal fluid (CSF) surface tension. Instabilities of different natures are then evidenced. A first type of structural instability results from the compelling effects of the CSF surface tension and the elastic properties of the ependyma. A second type of dynamical instability occurs for low enough values of the aqueduct's conductance. This last case is then shown to be accompanied by a spontaneous ventricle's dilation. A strong correlation with some active hydrocephalus is evidenced and discussed. The transfer function of the ventricles, compared to a low-pass filter, are calculated in both the stable and unstable regimes and appear to be very different.

Incidence of lower thoracic ligamentum flavum midline gaps † †Presented in abstract form at the IARS 78th Clinical and Scientific Congress, Tampa Bay, Florida, USA, 2004

BACKGROUND

Lower thoracic epidural anaesthesia and analgesia (EDA) has gained increasing importance in perioperative pain therapy. The loss-of-resistance technique used to identify the epidural space is thought to rely on the penetration of the ligamentum flavum. Investigations at the cervical and lumbar regions have demonstrated that the ligamentum flavum frequently exhibits incomplete fusion at different vertebral levels. Therefore, the aim of this study was to directly investigate the incidence of lower thoracic ligamentum flavum midline gaps in embalmed cadavers.

METHODS

Vertebral column specimens were obtained from 47 human cadavers. Ligamentum flavum midline gaps were recorded between the vertebral levels T6 and L1.

RESULTS

The incidence of midline gaps/number of viable specimens at the following levels was: T6-7: 2/45 (4.4%), T7-8: 1/47 (2.1%), T8-9: 2/45 (4.4%), T9-10: 7/39 (17.9%), T10-11: 12/34 (35.2%), T11-12: 10/35 (28.5%), T12/L1: 6/38 (15.8%).

CONCLUSIONS

In the present study we have determined the frequency of lower thoracic ligamentum flavum midline gaps. Gaps are less frequent than at cervical levels, but more frequent than at lumbar levels. Peak incidence was found in the region between T10 and T12. Using a strict midline approach, one cannot therefore rely on the ligamentum flavum to impede entering the epidural space in all patients.