Radiofrequency saturation induced bias in aqueductal cerebrospinal fluid flow quantification obtained using two-dimensional cine phase contrast magnetic resonance imaging

A subject-specific assessment of measurement errors and their correction in cerebrospinal fluid velocity maps using 4D flow MRI

PURPOSE

Phase-contrast MRI (PC-MRI) of cerebrospinal fluid (CSF) velocity is used to evaluate the characteristics of intracranial diseases, such as normal-pressure hydrocephalus (NPH). Nevertheless, PC-MRI has several potential error sources, with eddy-current-based phase offset error being non-negligible in CSF measurement. In this study, we assess the measurement error of CSF velocity maps obtained using 4D flow MRI and evaluate correction methods.

METHODS

CSF velocity maps of 10 patients with NPH were acquired using 4D flow MRI (velocity-encoding = 5 cm/s). Distributed phase offset error was estimated for a whole 3D background field by polynomial fitting using robust regression analysis. This estimated phase offset error was then used to correct the CSF velocity maps. The estimated error profiles were compared with those obtained using an existing 2D correction approach involving local background information near the region of interest.

RESULTS

The residual standard error of the polynomial fitting against the phase offset error extracted from the measured velocities was within 0.2 cm/s. The spatial dependencies of the phase offset errors showed similar tendencies in all cases, but sufficient differences in these values were found to indicate requirement of velocity correction. Differences of the estimated errors among other correction approaches were in the order of 10^-2 cm/s, and the estimated errors were in good agreement with those obtained using existing approaches.

CONCLUSION

Our method is capable of estimating the measurement error of CSF velocity maps obtained from 4D flow MRI and provides quantitatively reasonable characteristics for the main CSF profile in the cerebral aqueduct in patients with NPH.

Can pulsatile CSF flow across the cerebral aqueduct cause ventriculomegaly? A prospective study of patients with communicating hydrocephalus

Background

Communicating hydrocephalus is a disease where the cerebral ventricles are enlarged. It is characterized by the absence of detectable cerebrospinal fluid (CSF) outflow obstructions and often with increased CSF pulsatility measured in the cerebral aqueduct (CA). We hypothesize that the cardiac-related pulsatile flow over the CA, with fast systolic outflow and slow diastolic inflow, can generate net pressure effects that could source the ventriculomegaly in these patients. This would require a non-zero cardiac cycle averaged net pressure difference (ΔP_net) over the CA, with higher average pressure in the lateral and third ventricles.

Methods

We tested the hypothesis by calculating ΔP_net across the CA using computational fluid dynamics based on prospectively collected high-resolution structural (FIESTA-C, resolution 0.39 × 0.39 × 0.3 mm³) and velocimetric (2D-PCMRI, in-plane resolution 0.35 × 0.35 mm²) MRI-data from 30 patients investigated for communicating hydrocephalus.

Results

The ΔP_net due to CSF pulsations was non-zero for the study group (p = 0.03) with a magnitude of 0.2 ± 0.4 Pa (0.001 ± 0.003 mmHg), with higher pressure in the third ventricle. The maximum pressure difference over the cardiac cycle ΔP_max was 20.3 ± 11.8 Pa and occurred during systole. A generalized linear model verified an association between ΔP_net and CA cross-sectional area (p = 0.01) and flow asymmetry, described by the ratio of maximum inflow/outflow (p = 0.04), but not for aqueductal stroke volume (p = 0.35).

Conclusions

The results supported the hypothesis with respect to the direction of ΔP_net, although the magnitude was low. Thus, although the pulsations may generate a pressure difference across the CA it is likely too small to explain the ventriculomegaly in communicating hydrocephalus.

Magnitude and direction of aqueductal cerebrospinal fluid flow: large variations in patients with intracranial aneurysms with or without a previous subarachnoid hemorrhage

BACKGROUND

Net cerebrospinal fluid (CSF) flow within the cerebral aqueduct is usually considered to be antegrade, i.e., from the third to the fourth ventricle with volumes ranging between 500 and 600 ml over 24 h. Knowledge of individual CSF flow dynamics, however, is hitherto scarcely investigated. In order to explore individual CSF flow rate and direction, we assessed net aqueductal CSF flow in individuals with intracranial aneurysms with or without a previous subarachnoid hemorrhage (SAH).

METHODS

A prospective observational study was performed utilizing phase-contrast magnetic resonance imaging (PC-MRI) to determine the magnitude and direction of aqueductal CSF flow with an in-depth, pixel-by-pixel approach. Estimation of net flow was used to calculate CSF flow volumes over 24 h. PC-MRI provides positive values when flow is retrograde.

RESULTS

The study included eight patients with intracranial aneurysms. Four were examined within days after their SAH; three were studied in the chronic stage after SAH while one patient had an unruptured intracranial aneurysm. There was a vast variation in magnitude and direction of aqueductal CSF flow between individuals. Net aqueductal CSF flow was retrograde, i.e., directed towards the third ventricle in 5/8 individuals. For the entire patient cohort, the estimated net aqueductal CSF volumetric flow rate (independent of direction) was median 898 ml/24 h (ranges 69 ml/24 h to 12.9 l/24 h). One of the two individuals who had a very high estimated net aqueductal CSF volumetric flow rate, 8.7 l/24 h retrograde, later needed a permanent CSF shunt.

CONCLUSIONS

The magnitude and direction of net aqueductal CSF flow vary extensively in patients with intracranial aneurysms. Following SAH, PC-MRI may offer the possibility to perform individualized assessments of the CSF circulation.

Cerebrospinal fluid volumetric net flow rate and direction in idiopathic normal pressure hydrocephalus

The aim of the present study was to examine cerebrospinal fluid (CSF) volumetric net flow rate and direction at the cranio-cervical junction (CCJ) and cerebral aqueduct in individuals with idiopathic normal pressure hydrocephalus (iNPH) using cardiac-gated phase-contrast magnetic resonance imaging (PC-MRI). An in-depth, pixel-by-pixel analysis of regions of interest from the CCJ and cerebral aqueduct, respectively, was done in 26 iNPH individuals, and in 4 healthy subjects for validation purposes. Results from patients were compared with over-night measurements of static and pulsatile intracranial pressure (ICP). In iNPH, CSF net flow at CCJ was cranially directed in 17/22 as well as in 4/4 healthy subjects. Estimated daily CSF volumetric net flow rate at CCJ was 6.9 ± 9.9 L/24 h in iNPH patients and 4.5 ± 5.0 L/24 h in healthy individuals. Within the cerebral aqueduct, the CSF net flow was antegrade in 7/21 iNPH patients and in 4/4 healthy subjects, while it was retrograde (i.e. towards ventricles) in 14/21 iNPH patients. Estimated daily CSF volumetric net flow rate in cerebral aqueduct was 1.1 ± 2.2 L/24 h in iNPH while 295 ± 53 mL/24 h in healthy individuals. Magnitude of cranially directed CSF net flow in cerebral aqueduct was highest in iNPH individuals with signs of impaired intracranial compliance. The study results indicate CSF flow volumes and direction that are profoundly different from previously assumed. We hypothesize that spinal CSF formation may serve to buffer increased demand for CSF flow through the glymphatic system during sleep and during deep inspiration to compensate for venous outflow.

Phase contrast MRI measurements of net cerebrospinal fluid flow through the cerebral aqueduct are confounded by respiration

Background

Net cerebrospinal fluid (CSF) flow through the cerebral aqueduct may serve as a marker of CSF production in the lateral ventricles, and changes that occur with aging and in disease.

Purpose

To investigate the confounding influence of the respiratory cycle on net CSF flow and stroke volume measurements.

Study Type

Cross‐sectional study.

Subjects

Twelve young, healthy subjects (seven male, age range 19–39 years, average age 28.3 years).

Field Strength/Sequence

Phase contrast MRI (PC‐MRI) measurements were performed at 7T, with and without respiratory gating on expiration and on inspiration. All measurements were repeated.

Assessment

Net CSF flow and stroke volume in the aqueduct, over the cardiac cycle, was determined.

Statistical Tests

Repeatability was determined using the intraclass correlation coefficient (ICC) and linear regression analysis between the repeated measurements. Repeated measures analysis of variance (ANOVA) was performed to compare the measurements during inspiration/expiration/no gating. Linear regression analysis was performed between the net CSF flow difference (inspiration minus expiration) and stroke volume.

Results

Net CSF flow (average ± standard deviation) was 0.64 ± 0.32 mL/min (caudal) during expiration, 0.12 ± 0.49 mL/min (cranial) during inspiration, and 0.31 ± 0.18 mL/min (caudal) without respiratory gating. Respiratory gating did not affect stroke volume measurements (41 ± 18, 42 ± 19, 42 ± 19 μL/cycle for expiration, no respiratory gating and inspiration, respectively). Repeatability was best during inspiration (ICC = 0.88/0.56/–0.31 for gating on inspiration/expiration/no gating). A positive association was found between average stroke volume and net flow difference between inspiration and expiration (R = 0.678/0.605, P = 0.015/0.037 for the first/second repeated measurement).

Data Conclusion

Measured net CSF flow is confounded by respiration effects. Therefore, net CSF flow measurements with PC‐MRI cannot in isolation be directly linked to CSF production.

Level of Evidence: 1

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2019;49:433–444.