Physiologic variations in dural venous sinus flow on phase-contrast MR imaging

Flow Analysis of Central Venous Outflow Tract: A New Approach to Understanding Pulse-Synchronous Tinnitus

OBJECTIVE

Pulse-synchronous tinnitus (PST) has been linked to multiple anatomical variants of the central venous outflow tract (CVOT) including sigmoid sinus (SS) dehiscence and diverticulum. This study investigates flow turbulence, pressure, and wall shear stress along the CVOT and proposes a mechanism that results in SS dehiscence and PST.

STUDY DESIGN

Case series.

SETTING

Tertiary Academic Center.

METHODS

Venous models were reconstructed from computed tomography scans of 3 patients with unilateral PST. Two models for each patient are obtained: a symptomatic and contralateral asymptomatic side. A turbulent model-enabled commercial flow solver was used to simulate the pulsatile blood flow over the cardiac cycle through the models. Fluid flow through the transverse and SS junction was analyzed to observe the velocity, pressure, turbulent kinetic energy (TKE), and shear stress over a simulated cardiac cycle.

RESULTS

Fluid flow on the symptomatic side showed increased vorticity in the presence of an SS diverticulum. Higher TKE with periodicity following the cardiac cycle was observed on the symptomatic side, and a sharp increase was observed if SS diverticulum was present. Shear stress was highest near the narrowest segments of the vessel. Pressure was observed to be lower on the symptomatic side at the transverse-SS junction for all 3 patients.

CONCLUSION

Computational fluid dynamics modeling of blood flow through the CVOT in PST suggests that low pressure may be the cause of dehiscence, and tinnitus may result from periodic increases in TKE.

Venous 3D Phase Contrast Magnetic Resonance Angiography Increases Diagnostic Certainty in Children with Ventriculoperitoneal Shunt and Suspected Shunt Failure

BACKGROUND

Clinical symptoms in children with suspected malfunction of ventriculoperitoneal shunt may not be specific and difficult to interpret. The presence or absence of ventricular enlargement on magnetic resonance imaging (MRI) does not reliably predict raised intracranial pressure (ICP) in these patients. Therefore, the aim was to investigate the diagnostic utility of 3D venous phase-contrast MR angiography (vPCA) in these patients.

MATERIALS

The MR studies of two groups of patients at two different examination dates were retrospectively analyzed; one group without clinical symptoms on both examinations and one with symptoms of shunt dysfunction on one examination receiving surgery. Both MRI examinations had to have been performed including axial T2 weighted (T2-w) images and 3D vPCA. Two (neuro)radiologists evaluated T2-w images alone and in combination with 3D vPCA in terms of suspected elevated ICP. Interrater reliability, sensitivity and specificity were assessed.

RESULTS

Compression of venous sinuses was seen significantly more often in patients with shunt failure (p = 0.00003). Consequently, evaluation of 3D vPCA and T2-w images increases sensitivity to 0.92/1.0 compared to T2-w images alone with 0.69/0.77, the interrater agreement for the diagnosis of shunt failure rises from κ = 0.71 to κ = 0.837. Concerning imaging markers, three groups could be identified in children with shunt failure.

CONCLUSION

In accordance with the literature, the results show that ventricular morphology alone is an unreliable marker for elevated ICP in children with shunt malfunction. The findings confirmed 3D vPCA as a valuable supplemental diagnostic tool improving diagnostic certainty for children with unchanged ventricular size in cases of shunt failure.

A novel computational fluid dynamic method and validation for assessing distal cerebrovascular microcirculatory resistance

BACKGROUND AND OBJECTIVE

The non-invasive assessment of microcirculatory resistance could improve the treatment of cerebrovascular stenosis. This study aimed to validate a novel computational strategy for determining the reference value of microcirculatory resistance in patients with cerebrovascular stenosis.

METHODS

We reconstructed a patient-specific 3-dimensional model of the extracranial-intracranial arteries. A computational strategy incorporating patient-specific pressure-wire measurements was developed to estimate the blood flow rate and microcirculatory resistance. Throughout the computational fluid dynamics (CFD) simulation, the boundary conditions were adjusted according to the developed algorithm. Pearson correlation and Bland-Altman analyses were used to quantify the correlation and agreement between CFD calculations and transcranial Doppler (TCD) assessment.

RESULTS

A strong correlation was found between the CFD-based and invasive distal pressure measurements (P<0.0001). Meanwhile, the CFD and TCD-based flow measurements were highly correlated (r = 0.853; P = 0.001). Furthermore, there was a correlation between the mean velocity measured by CFD and the mean velocity measured by TCD (r = 0.777; P<0.001). Good agreement was observed between the mass flow by CFD simulation and volumetric flow by TCD (P = 0.0266, mean difference: -0.7814 mmHg, limits of agreement, -4.0905 - 2.5276). However, the mean velocities from CFD simulation were in less agreement with those from the TCD assessment (P = 0.3992, mean difference, -0.0485; limits of agreement, -0.6141 - 0.5170). Results of the CFD simulation indicate that the flow resistance varies greatly between individuals.

CONCLUSIONS

The computational strategy of incorporating patient-specific pressure-wire measurements may serve as an effective approach to evaluate the actual reference values of microcirculatory resistance. In addition, an individualized assessment of non-invasive flow resistance is necessary for the accurate determination of non-invasive cerebrovascular pressure.

Anatomy imaging and hemodynamics research on the cerebral vein and venous sinus among individuals without cranial sinus and jugular vein diseases

In recent years, imaging technology has allowed the visualization of intracranial and extracranial vascular systems. However, compared with the cerebral arterial system, the relative lack of image information, individual differences in the anatomy of the cerebral veins and venous sinuses, and several unique structures often cause neurologists and radiologists to miss or over-diagnose. This increases the difficulty of the clinical diagnosis and treatment of cerebral venous system diseases. This review focuses on applying different imaging methods to the normal anatomical morphology of the cerebral venous system and special structural and physiological parameters, such as hemodynamics, in people without cranial sinus and jugular vein diseases and explores its clinical significance. We hope this study will reinforce the importance of studying the cerebral venous system anatomy and imaging data and will help diagnose and treat systemic diseases.

Lead toxicity due to retained intracranial bullet fragments: illustrative case

BACKGROUND

Lead toxicity (plumbism) secondary to retained lead bullet fragments is a rare complication in patients with gunshot wounds. To the authors' knowledge, there has been no definitive case reported of lead toxicity due to retained intracranial bullet fragments.

OBSERVATIONS

The authors reported the case of a 23-year-old man who presented after being found down. Computed tomography scanning of the head revealed bullet fragments within the calvaria adjacent to the left transverse sinus. During follow-up, he developed symptoms of plumbism with paresthesias in his bilateral hands and thighs, abdominal cramping, labile mood, and intermittent psychosis. Plumbism was confirmed with sequentially elevated blood lead levels (BLLs). The patient opted for surgical removal of the bullet fragments, which led to reduction in BLLs and resolution of his symptoms.

LESSONS

Although rare, lead toxicity from retained intracranial bullet fragments should be considered in patients who have suffered a gunshot wound to the head and have symptoms of lead toxicity with elevated BLLs. For safe and accessible intracranial bullet fragments in patients with plumbism, surgical intervention may be indicated.

Idiopathic intracranial hypertension imaging approaches and the implications in patient management

Idiopathic intracranial hypertension (IIH) represents a clinical disease entity without a clear etiology, that if left untreated, can result in severe outcomes, including permanent vision loss. For this reason, early diagnosis and treatment is necessary. Historically, the role of cross-sectional imaging has been to rule out secondary or emergent causes of increased intracranial pressure, including tumor, infection, hydrocephalus, or venous thrombosis. MRI and MRV, however, can serve as valuable imaging tools to not only rule out causes for secondary intracranial hypertension but can also detect indirect signs of IIH resultant from increased intracranial pressure, and demonstrate potentially treatable sinus venous stenosis. Digital subtraction venographic imaging also plays a central role in both diagnosis and treatment, providing enhanced anatomic delineation and temporal flow evaluation, quantitative assessment of the pressure gradient across a venous stenosis, treatment guidance, and immediate opportunity for endovascular therapy. In this review, we discuss the multiple modalities for imaging IIH, their limitations, and their contributions to the management of IIH.

Computational fluid dynamics study of the effect of transverse sinus stenosis on the blood flow pattern in the ipsilateral superior curve of the sigmoid sinus

OBJECTIVE

To investigate the effect of different types of transverse sinus stenosis on blood flow patterns in the ipsilateral superior curve of the sigmoid sinus.

METHODS

According to the morphology of transverse and sigmoid sinus sections in pulsatile tinnitus patients, ten idealized models with different degrees and positions of transverse sinus stenosis were constructed. Computational fluid dynamics simulations were performed to compare the hemodynamic characteristics among these models. Follow-up images of previous cases were included, which preliminarily confirmed the hypothesis that bone plate erosion of the sigmoid sinus sulcus is related to blood flow impingement.

RESULTS

Blood flow impingement on the superior curve of the sigmoid sinus wall intensified with increasing degree of stenosis and decreased with increasing distance between the stenosis and the sigmoid sinus. The impact zone was generally confined to the anterior and lateral walls of the superior curve of the sigmoid sinus. When the stenosis was located far from the middle of the transverse sinus, the blood flow impingement on the sigmoid sinus wall was very weak.

CONCLUSIONS

When stenosis is located far from the sigmoid sinus, the causes of tinnitus should be comprehensively considered instead of assuming that stenosis is the main cause. Bone plate erosion of the sigmoid sinus sulcus was promoted by blood flow impingement.

KEY POINTS

• Ten idealized models with different degrees and positions of stenosis were constructed. • The causes of pulsatile tinnitus should be comprehensively considered. • Sigmoid sinus plate dehiscence was promoted by blood flow impingement.

Extraluminal Sigmoid Sinus Angioplasty: A Pertinent Reconstructive Surgical Method Targeting Dural Sinus Hemodynamics to Resolve Pulsatile Tinnitus

OBJECTIVES

1) To provide information on the treatment of pulsatile tinnitus (PT) with transtemporal extraluminal sigmoid sinus angioplasty (ESSA); and 2) to discuss the current clinical management of PT.

STUDY DESIGN

This was a retrospective study.

SETTINGS

Multi-institutional tertiary university medical centers.

PATIENTS

Fifty-four PT patients with transverse-sigmoid sinus enlargement and prominent transverse-sigmoid junction with or without sigmoid sinus wall anomalies or transverse sinus anomalies.

INTERVENTION

All patients underwent ESSA under local anesthesia.

MAIN OUTCOME MEASURES

Intraoperative discoveries and surgical resolution of PT, morphology, and computational fluid dynamics.

RESULTS

Fifty-three of the 54 (98%) patients experienced a significant reduction in, or complete resolution of, PT after ESSA. No major surgical complications occurred, except for one case where we observed a full collapse of the sinus wall. On average, this surgery reduced the cross-sectional area at the transverse-sigmoid junction by 61.5%. Our intraoperative discoveries suggest that sigmoid sinus wall anomalies may not be a definitive cause of PT. The transverse-sigmoid sinus system was significantly larger (in term of both cross-sectional area and volume) on the ipsilesional side compared with the contralesional side. Following ESSA, the vascular wall pressure and vortex flow at the transverse-sigmoid junction decreased considerably, and the flow velocity and wall shear stress increased significantly.

CONCLUSION

ESSA is a highly effective surgical technique for PT patients with transverse-sigmoid sinus enlargement and prominent transverse-sigmoid junction, regardless of whether they also have sigmoid sinus wall or transverse sinus anomalies. A large transverse-sigmoid system with prominent transverse-sigmoid junction is a predisposing factor for PT, and only by improving patients' intrasinus hemodynamics could PT be resolved efficiently. In cases without complete obstruction of venous return, ESSA is safe. No postoperative complications related to neurological disorders were observed.

Intracranial vascular flow oscillations in Alzheimer's disease from 4D flow MRI

Recent modeling and experimental evidence suggests clearance of soluble metabolites from the brain can be driven by low frequency flow oscillations (LFOs) through the intramural periarterial drainage (IPAD) pathway. This study investigates the use of 4D flow MRI to derive LFOs from arterial and venous measures of blood flow. 3D radial 4D flow MRI data were acquired on a 3.0 T scanner and reconstructed using a low-rank constraint to produce time resolved measurements of blood flow. Physical phantom experiments were performed to validate the time resolved 4D flow against a standard 2D phase contrast (PC) approach. To evaluate the ability of 4D flow to distinguish physiologic flow changes from noise, healthy volunteers were scanned during a breath-hold (BH) maneuver and compared against 2D PC measures. Finally, flow measures were performed in intracranial arteries and veins of 112 participants including subjects diagnosed with Alzheimer's disease (AD) clinical syndrome (n = 23), and healthy controls (n = 89) on whom apolipoprotein ɛ4 positivity (APOE4+) and parental history of AD dementia (FH+) was known. To assess LFOs, flow range, standard deviation, demeaned temporal flow changes, and power spectral density were quantified from the time series. Group differences were assessed using ANOVA followed by Tukey-Kramer method for pairwise comparison for adjusted means (P < 0.05). Significantly lower LFOs as measured from flow variation range and standard deviations were observed in the arteries of AD subjects when compared to age-matched controls (P = 0.005, P = 0.011). Results suggest altered vascular function in AD subjects. 4D flow based spontaneous LFO measures might hold potential for longitudinal studies aimed at predicting cognitive trajectories in AD and study disease mechanisms.

Superior Sagittal Venous Sinus Thrombosis in a Patient with Illicit Testosterone Use

Cerebral venous sinus thrombosis is a challenging diagnosis due in part to its variable clinical presentation and rarity. The annual incidence ranges from 0.22 to 1.57 per 100,000. The etiology of such disease is related to hypercoagulability states. Although illicit androgen use is a well-known cause of prothrombotic states, its risk of causing cerebral venous sinus thrombosis has been infrequently reported. We present the case of a 33-year-old male with no known past medical history who presented to the emergency department (ED) with persistent seizure activity, neurological deficits, and history of worsening headaches who was found to have an extensive superior sagittal sinus thrombosis on imaging. Radiologic findings demonstrated pathognomonic findings of cord sign and delta sign, the previous being highly specific but of low incidence. An inconclusive hypercoagulability workup prompted further questioning which revealed illicit androgenic anabolic steroid use. Prompt treatment with anticoagulation and anti-seizure medication was pursued with full resolution of his neurologic symptomatology.

Computational Fluid Dynamics Simulation of Hemodynamic Alterations in Sigmoid Sinus Diverticulum and Ipsilateral Upstream Sinus Stenosis After Stent Implantation in Patients with Pulsatile Tinnitus

OBJECTIVE

To investigate the relationships between upstream venous sinus stenosis and pulsatile tinnitus (PT), and to assess the correlation with diverticulum growth and the effectiveness of stent implantation.

METHODS

Patient-specific geometric models were constructed using computed tomography venography images from a patient with PT, with sigmoid sinus diverticulum, and with upstream transverse sinus stenosis, in whom stenting of the upstream sinus stenosis alone achieved complete remission of PT. Computational fluid dynamics simulation based on this patient-specific geometry was performed using commercially available finite element software (ANSYS-14) to qualitatively and quantitatively compare the flow velocity, flow rate, velocity vector, pressure, vorticity, and wall shear stress on the affected side transverse and sigmoid sinuses, before and after stent implantation.

RESULTS

Stenting improved the flow direction and magnitude. After stenting, the flow pattern became smoother and more regular. High-speed blood flow at the level of the diverticulum neck was confined to a smaller area, and its direction changed from approximately perpendicular to the diverticular dome to the distal side of the diverticular neck. The diverticulum showed obvious flow reduction, with decreases of 80.7%, 68.7%, 96.1%, and 91.3% in peak velocity, inflow rate, pressure gradient, and peak vorticity, respectively. The abnormally low wall shear stress at the dome of diverticulum was eliminated.

CONCLUSIONS

Our findings strongly support a major role of diverticulum stenosis before in PT development and suggest that such stenosis is a causative factor of diverticulum growth. They also confirm the effectiveness of stent implantation for the treatment of PT.

Analysis of Fluctuation in Cerebral Venous Oxygenation Using MR Imaging: Quantitative Evaluation of Vasomotor Function of Arterioles

Purpose:

Cerebral arteriolar vasomotor function plays an important role in brain health. Since respiratory changes in the partial arterial pressure of CO₂ (PaCO₂) cause arterioles to vasodilate or vasoconstrict, resting-state arteriolar vasomotion results in the fluctuation of venous blood oxygenation, which can be monitored by observing magnetic resonance (MR) signals. Focusing on the superior sagittal sinus as the largest cerebral vein, we developed a method to elucidate the respiratory fluctuation of cerebral venous oxygenation that may reflect the vasomotor function.

Methods:

Single slices of varying thickness (7–15 mm) taken perpendicular to the superior sagittal sinus of five volunteers were imaged by spin-echo echo-planar imaging using a 1.5-T MR system. The time series of the signal intensity at the superior sagittal sinus was Fourier-transformed, and the spectral fluctuation intensity (SFI) at respiratory frequency was obtained. The amplitude of the respiratory fluctuation in the cerebral venous oxygenation was calculated from the gradient of the relation between the SFI and the average signal intensity, which increased proportionally with an increase in slice thickness. The amplitude of the fluctuation in cerebral venous oxygenation at low (<0.1 Hz) and cardiac pulsation frequencies was also calculated for comparison with the respiratory fluctuation.

Results:

The amplitude of respiratory fluctuation in the cerebral venous oxygenation was quantified as 1.2%, demonstrating the validity of our method via the highest significant correlation (r = 0.82) in the plot of SFI and average signal intensities; the correlations at low and cardiac pulsation frequencies were 0.60 and 0.63, respectively.

Conclusion:

We have successfully demonstrated cerebral venous oxygenation fluctuation at respiratory frequencies in the resting state. This fluctuation was non-invasively evaluated as 1.2%, representing the control value for the arteriolar vasomotor function of a healthy human.

Multiple Sclerosis - A Vascular Etiology?

From the earliest pathological studies the perivenular localization of the demyelination in multiple sclerosis (MS) has been observed. It has recently been suggested that obstructions to venous flow or inadequate venous valves in the great veins in the neck, thorax and abdomen can cause damaging backflow into the cerebral and spinal cord circulations. Paolo Zamboni and colleagues have demonstrated abnormal venous circulation in some multiple sclerosis patients using non-invasive sonography and invasive venography. Furthermore, they have obtained apparent clinical improvement or stabilization by endovascular ballooning of points of obstruction in the great veins in some, at least temporarily. If non-invasive observations by others validate their initial observations of a significantly increased prevalence of venous obstructions in MS then trials of angioplasty/stenting would be justified in selected cases in view of the biological plausibility of the concept.

Advanced flow MRI: emerging techniques and applications

Magnetic resonance imaging (MRI) techniques provide non-invasive and non-ionising methods for the highly accurate anatomical depiction of the heart and vessels throughout the cardiac cycle. In addition, the intrinsic sensitivity of MRI to motion offers the unique ability to acquire spatially registered blood flow simultaneously with the morphological data, within a single measurement. In clinical routine, flow MRI is typically accomplished using methods that resolve two spatial dimensions in individual planes and encode the time-resolved velocity in one principal direction, typically oriented perpendicular to the two-dimensional (2D) section. This review describes recently developed advanced MRI flow techniques, which allow for more comprehensive evaluation of blood flow characteristics, such as real-time flow imaging, 2D multiple-venc phase contrast MRI, four-dimensional (4D) flow MRI, quantification of complex haemodynamic properties, and highly accelerated flow imaging. Emerging techniques and novel applications are explored. In addition, applications of these new techniques for the improved evaluation of cardiovascular (aorta, pulmonary arteries, congenital heart disease, atrial fibrillation, coronary arteries) as well as cerebrovascular disease (intra-cranial arteries and veins) are presented.

Amplified magnetic resonance imaging (aMRI)

PURPOSE

This work describes a new method called amplified MRI (aMRI), which uses Eulerian video magnification to amplify the subtle spatial variations in cardiac-gated brain MRI scans and enables better visualization of brain motion.

METHODS

The aMRI method takes retrospective cardiac-gated cine MRI data as input, applies a spatial decomposition, followed by temporal filtering and frequency-selective amplification of the MRI cardiac-gated frames before synthesizing a motion-amplified cine data set.

RESULTS

This approach reveals deformations of the brain parenchyma and displacements of arteries due to cardiac pulsatility, especially in the brainstem, cerebellum, and spinal cord.

CONCLUSION

aMRI has the potential for widespread neuro- and non-neuro clinical use because it can amplify and characterize small, often barely perceptible motion and can visualize the biomechanical response of tissues using the heartbeat as an endogenous mechanical driver. Magn Reson Med 75:2245-2254, 2016. © 2016 Wiley Periodicals, Inc.

Chronic Influences of Obstructive Sleep Apnea on Cerebral Venous Flow

OBJECTIVES

Evidence reveals a pathophysiologic link between sleep apnea syndrome and cerebrovascular diseases. It is known that obstructive sleep apnea (OSA) may cause serial hemodynamic changes and structural abnormalities in the cerebral and cardiac arterial systems, but its effect on the cerebral venous system has remained unclear. The purpose of this study was to compare internal jugular vein hemodynamics between patients with OSA and healthy individuals.

METHODS

Patients with OSA and age-, body mass index-, and sex-matched healthy control participants were recruited for a jugular venous duplex study and neurologic examination. The luminal area of the internal jugular vein, jugular venous flow volume, time-averaged mean velocity, and presence of jugular venous reflux were recorded. These flow characteristics were obtained at different respiratory statuses, and we analyzed the differences between patients and controls.

RESULTS

In the OSA group, there was an increasing flow volume in total internal jugular veins at rest. The frequency of venous reflux in patients compared with controls was significantly decreased (26.7% versus 53.3%, respectively; P < .05). The internal jugular vein drainage dominance was greater on the left side in the OSA group (right versus left: 48.8% versus 51.2%), whereas it was greater on the right side in the control group (right versus left: 61.7% versus 38.3%).

CONCLUSIONS

Our data showed peculiar internal jugular vein hemodynamics at baseline and different respiratory statuses in patients with OSA. These characteristics imply that cerebral venous drainage conditions might be involved in the pathophysiologic mechanisms of OSA syndrome.

An anatomy-based lumped parameter model of cerebrospinal venous circulation: can an extracranial anatomical change impact intracranial hemodynamics?

BACKGROUND

The relationship between extracranial venous system abnormalities and central nervous system disorders has been recently theorized. In this paper we delve into this hypothesis by modeling the venous drainage in brain and spinal column areas and simulating the intracranial flow changes due to extracranial morphological stenoses.

METHODS

A lumped parameter model of the cerebro-spinal venous drainage was created based on anatomical knowledge and vessels diameters and lengths taken from literature. Each vein was modeled as a hydraulic resistance, calculated through Poiseuille's law. The inputs of the model were arterial flow rates of the intracranial, vertebral and lumbar districts. The effects of the obstruction of the main venous outflows were simulated. A database comprising 112 Multiple Sclerosis patients (Male/Female = 42/70; median age ± standard deviation = 43.7 ± 10.5 years) was retrospectively analyzed.

RESULTS

The flow rate of the main veins estimated with the model was similar to the measures of 21 healthy controls (Male/Female = 10/11; mean age ± standard deviation = 31 ± 11 years), obtained with a 1.5 T Magnetic Resonance scanner. The intracranial reflux topography predicted with the model in cases of internal jugular vein diameter reduction was similar to those observed in the patients with internal jugular vein obstacles.

CONCLUSIONS

The proposed model can predict physiological and pathological behaviors with good fidelity. Despite the simplifications introduced in cerebrospinal venous circulation modeling, the key anatomical feature of the lumped parameter model allowed for a detailed analysis of the consequences of extracranial venous impairments on intracranial pressure and hemodynamics.

High temporal resolution MRI quantification of global cerebral metabolic rate of oxygen consumption in response to apneic challenge

We present a technique for quantifying global cerebral metabolic rate of oxygen consumption (CMRO2) in absolute physiologic units at 3-second temporal resolution and apply the technique to quantify the dynamic CMRO2 response to volitional apnea. Temporal resolution of 3 seconds was achieved via a combination of view sharing and superior sagittal sinus-based estimation of total cerebral blood flow (tCBF) rather than tCBF measurement in the neck arteries. These modifications were first validated in three healthy adults and demonstrated to produce minimal errors in image-derived blood flow and venous oxygen saturation (SvO2) values. The technique was then applied in 10 healthy adults during an apnea paradigm of three repeated 30-second breath-holds. Subject-averaged baseline tCBF, arteriovenous oxygen difference (AVO2D), and CMRO2 were 48.6 ± 7.0 mL/100 g per minute, 29.4 ± 3.4 %HbO2, and 125.1 ± 11.4 μmol/100 g per minute, respectively. Subject-averaged maximum changes in tCBF and AVO2D were 43.5 ± 9.4% and -32.1 ± 5.7%, respectively, resulting in a small (6.0 ± 3.5%) but statistically significant (P=0.00044, two-tailed t-test) increase in average end-apneic CMRO2. This method could be used to investigate neurometabolic-hemodynamic relationships in normal physiology, to better define the biophysical origins of the BOLD signal, and to quantify neurometabolic responsiveness in diseases of altered neurovascular reactivity.

A modeling study of idiopathic intracranial hypertension: etiology and diagnosis

OBJECTIVE

To investigate the relationship between idiopathic intracranial hypertension (IIH) and transverse sinus stenosis through experiments performed on a validated mathematical model.

METHODS

A mathematical model of intracranial pressure (ICP) dynamics has been extended to accommodate venous sinus compression through the introduction of a Starling-like resistor between the sagittal and transverse sinuses.

RESULTS

In the absence of this type of resistor, the sinuses are rigid, and the model has only a unique, stable steady state with normal pressures. With resistance a function of the external pressure on the sinus, a second stable steady state may exist. This state is characterized by elevated ICP concurrent with a compressed transverse sinus. Simulations predict that a temporary perturbation that causes a transient elevation of ICP can induce a permanent transition from the normal to the higher steady state. Comparisons to clinical data from IIH patients provide supporting evidence for the validity of the model's predictions. Simulations suggest a possible clinical diagnostic technique to determine if an individual has a compressible transverse sinus and is at risk for developing IIH.

CONCLUSIONS

Results of the model experiments suggest that the primary cause of IIH may be a compressible, as opposed to rigid, transverse sinus, and that the observed stenosis is a necessary characteristic of the elevated pressure state.

Haemodynamic alterations in cerebral blood vessels after carotid artery revascularisation: quantitative analysis using 2D phase-contrast MRI

OBJECTIVES

This study was conducted to evaluate the effect of revascularisation, whether revascularisation improves total cerebral blood flow volume (FVTCBF), and how cerebral veins would respond to altered FVTCBF.

METHODS

The 39 carotid artery stenoses in 37 patients who underwent revascularisation including 32 stentings and 7 endarterectomies were included in this prospective study. From the two-dimensional phase-contrast (2D-PC) MRI acquired before and after revascularisation, the flow volumes (FVs) of the arteries and veins were compared using paired t-test. The relationships between these parameters were correlated using Pearson's correlation coefficient.

RESULTS

The mean FV in the treated carotid artery (proportion of treated artery among total FV) increased from 162.06 ml/min (25.80 %) to 267.71 ml/min (37.21 %; P < 0.001). Revascularisation increased the FVTCBF of patients from 638.66 ml/min to 716.72 ml/min (P < 0.001). The FV of the internal jugular veins, superior sagittal and straight sinuses (FVSS + SSS), and transverse sinuses increased after revascularisation (P < 0.05). Positive relationships were shown between the FVTCBF and the FVSS + SSS (r = 0.584-0.741, P < 0.001).

CONCLUSIONS

Revascularisation improves the FVTCBF by increasing the FV in the treated carotid artery. The venous drainages are closely linked to FVTCBF. 2D-PC-MRI is a feasible method for evaluating comprehensively the haemodynamic improvement after revascularisation.

KEY POINTS

• Revascularisation may be beneficial in ischaemic strokes due to carotid artery stenosis. • Revascularisation of the affected artery increases total cerebral blood flow volume ( FV TCBF). • Cerebral venous drainage, closely linked to FV TCBF, is also improved. • Two-dimensional phase-contrast MRI can comprehensively assess these haemodynamic improvements after carotid revascularisation.

Cerebral Blood and CSF Flow Patterns in Patients Diagnosed for Cerebral Venous Thrombosis - An Observational Study

Background and Purpose:

Recent studies of the organization of the cerebral venous system in healthy subjects using phase contrast magnetic resonance imaging (PC-MRI) show its structural complexity and inter-individual variations. Our objective was to study the venous blood and CSF flows in cerebral venous thrombosis (CVT).

Materials and Methods:

PC-MRI sequences were added to brain MRI conventional protocol in 19 patients suspected of CVT, among whom 6 patients had CVT diagnosis confirmed by MR venography. Results were compared with 18 healthy age-matched volunteers (HV).

Results:

In patients without CVT (NoCVT) confirmed by venography, we found heterogeneous individual venous flows, and variable side dominance in paired veins and sinuses, comparable to those in healthy volunteers. In CVT patients, PC-MRI detected no venous flow in the veins and/or sinuses with thrombosis. Arterial flows were preserved. CSF aqueductal and cervical stroke volumes were increased in a patient with secondary cerebral infarction, and decreased in 4 patients with extended thrombosis in the superior sagittal and transverse sinuses. These results suggest the main role of the venous system in the regulation of the dynamic intracranial equilibrium.

Conclusions:

CVT produces highly individualized pattern of disturbance in venous blood drainage. Complementary to MRI venography, PC-MRI provides non-invasive data about venous blockage consequences on CSF flow disturbances.

Cardiac and vascular responses to thigh cuffs and respiratory maneuvers on crewmembers of the International Space Station

Background: the transition to microgravity eliminates the hydrostatic gradients in the vascular system. The resulting fluid redistribution commonly manifests as facial edema, engorgement of the external neck veins, nasal congestion, and headache. This experiment examined the responses to modified Valsalva and Mueller maneuvers measured by cardiac and vascular ultrasound (ECHO) in a baseline steady state and under the influence of thigh occlusion cuffs available as a countermeasure device (Braslet cuffs). Methods: nine International Space Station crewmember subjects (expeditions 16–20) were examined in 15 experiment sessions 101 ± 46 days after launch (mean ± SD; 33–185). Twenty-seven cardiac and vascular parameters were obtained with/without respiratory maneuvers before and after tightening of the Braslet cuffs (162 parameter states/session). Quality of cardiac and vascular ultrasound examinations was assured through remote monitoring and guidance by investigators from the NASA Telescience Center in Houston, TX, and the Mission Control Center in Korolyov, Moscow region, Russia. Results: 14 of 81 conditions (27 parameters measured at baseline, Valsalva, and Mueller maneuver) were significantly different when the Braslet was applied. Seven of 27 parameters were found to respond differently to respiratory maneuvers depending on the presence or absence of thigh compression. Conclusions: acute application of Braslet occlusion cuffs causes lower extremity fluid sequestration and exerts commensurate measurable effects on cardiac performance in microgravity. Ultrasound techniques to measure the hemodynamic effects of thigh cuffs in combination with respiratory maneuvers may serve as an effective tool in determining the volume status of a cardiac or hemodynamically compromised patient at the “microgravity bedside.”

Sympathetic influence on cerebral blood flow and metabolism during exercise in humans

This review focuses on the possibility that autonomic activity influences cerebral blood flow (CBF) and metabolism during exercise in humans. Apart from cerebral autoregulation, the arterial carbon dioxide tension, and neuronal activation, it may be that the autonomic nervous system influences CBF as evidenced by pharmacological manipulation of adrenergic and cholinergic receptors. Cholinergic blockade by glycopyrrolate blocks the exercise-induced increase in the transcranial Doppler determined mean flow velocity (MCA Vmean). Conversely, alpha-adrenergic activation increases that expression of cerebral perfusion and reduces the near-infrared determined cerebral oxygenation at rest, but not during exercise associated with an increased cerebral metabolic rate for oxygen (CMRO(2)), suggesting competition between CMRO(2) and sympathetic control of CBF. CMRO(2) does not change during even intense handgrip, but increases during cycling exercise. The increase in CMRO(2) is unaffected by beta-adrenergic blockade even though CBF is reduced suggesting that cerebral oxygenation becomes critical and a limited cerebral mitochondrial oxygen tension may induce fatigue. Also, sympathetic activity may drive cerebral non-oxidative carbohydrate uptake during exercise. Adrenaline appears to accelerate cerebral glycolysis through a beta2-adrenergic receptor mechanism since noradrenaline is without such an effect. In addition, the exercise-induced cerebral non-oxidative carbohydrate uptake is blocked by combined beta 1/2-adrenergic blockade, but not by beta1-adrenergic blockade. Furthermore, endurance training appears to lower the cerebral non-oxidative carbohydrate uptake and preserve cerebral oxygenation during submaximal exercise. This is possibly related to an attenuated catecholamine response. Finally, exercise promotes brain health as evidenced by increased release of brain-derived neurotrophic factor (BDNF) from the brain.

A method for rapid in vivo measurement of blood T1

We present a technique to measure the longitudinal relaxation time constant of venous blood (T(1b) ) in vivo in a few seconds. The MRI sequence consists of a thick-slab adiabatic inversion, followed by a series of slice-selective excitations and single-shot echo planar imaging readouts. The time intervals between excitations were chosen so that blood in macroscopic vessels is fully refreshed between excitations, making the blood signal follow an unperturbed inversion recovery curve. Static tissue, which experiences the inversion and all excitation pulses, quickly reaches a steady state at a low signal as a result of partial saturation. This allows blood-filled voxels to be discriminated from those containing static tissue, and to be fitted voxel-by-voxel to a simple inversion recovery model. The sequence was tested on a flow phantom with the proposed method, yielding T(1) values consistent to within 3% of those obtained using a conventional inversion recovery sequence with a spin-echo readout. The method was applied to seven adult volunteers and 18 neonates. The blood T(1) of the neonates (1799 ± 206 ms; range, 1393-2035 ms) was found to be more variable than that of adults (1717 ± 39 ms; range, 1662-1779 ms). A linear correlation between the inverse of T(1b) and the haematocrit was established in 12 neonates (R(2)  = 0.90).

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.

The Quantification of Cerebral Blood Flow by Phase Contrast MRA: Basics and Applications

Phase-contrast magnetic resonance (PCMRA) flow quantification can determine vascular velocities and volumetric flow rate (VFR) non-invasively for in vitro and in vivo studies. Recently, the increasing power of MR imaging units and the reduced time for data acquisition and post-processing have led to an increasing number of investigations on the use of phase-contrast flow measurements as an additional source of quantitative functional information in MR imaging. In addition, PCMRA can be added to morphologic MRI sequences, offering the option to correlate flow to morphology based on data generated during one examination. This review discusses the basics of phase-contrast imaging, describing the errors and avoiding methods associated with PCMRA, providing guidelines for flow measurement and data analysis, and presenting the current clinical cerebral applications.

Fast patient workup in acute stroke using parallel imaging

Treatment of ischemic stroke is a very frustrating topic for neurologists. Presently, the most promising therapy seems to be thrombolysis of the clot. However, this intervention is associated with complication risks, most significantly the risk of post-treatment hemorrhage. This risk of bleeding increases not only with the size of the ischemic brain tissue but also with the time-to-treatment interval. Studies suggest a time window of 3 hours for most effective treatment. Hence, there is demand for a rapid imaging workup, which thus far has been accomplished with computed tomography. Because of the risks associated with thrombolytic therapy, more detailed information is desirable. The distinction between patients with viable ischemically challenged neural tissue and those with complete infarcts is of great importance, and computed tomography is insufficient for this task. This is also true for outlining the etiology of stroke, which may impact treatment. For these tasks, magnetic resonance imaging has been proposed. However, comprehensive imaging protocols take time, which is limited in stroke treatment. Therefore, new imaging techniques are required that provide both in-depth information and short scanning times. Parallel imaging is uniquely suited for this purpose.

Physiology of cerebral venous blood flow: from experimental data in animals to normal function in humans

In contrast to the cerebroarterial system, the cerebrovenous system is not well examined and only partly understood. The cerebrovenous system represents a complex three-dimensional structure that is often asymmetric and considerably represent more variable pattern than the arterial anatomy. Particular emphasis is devoted to the venous return to extracranial drainage routes. As the state-of-the-art-imaging methods are playing a greater role in visualizing the intracranial venous system at present, its clinically pertinent anatomy and physiology has gain increasing interest, even so only few data are available. For this reason, experimental research on specific biophysical (fluid dynamic, rheologic factors) and hemodynamic (venous pressure, cerebral venous blood flow) parameters of the cerebral venous system is more on the focus; especially as these parameters are different to the cerebral arterial system. Particular emphasis is devoted to the venous return to extracranial drainage routes. From the present point of view, it seems that the cerebrovenous system may be one of the most important factors that guarantee normal brain function. In the light of this increasing interest in the cerebral venous system, the authors have summarized the current knowledge of the physiology of the cerebrovenous system and discuss it is in the light of its clinical relevance.

Anatomical variations of occipital bone impressions for dural venous sinuses around the torcular Herophili, with special reference to the consideration of clinical significance

Venous blood flow through the cerebral dural sinus is variable and clinically significant. It has been investigated by cadaver dissection or radiology; however, we thought that osteology might be informative. A total of 160 dried skulls were macroscopically examined for impressions on the inner surface of the occipital bone in order to interpret the sinus flow around the torcular Herophili. The continuity between the grooves for the superior sagittal sinus (SSS) and the transverse sinuses was categorized into four types. Confluence type was noted in 56 specimens (35%), in which SSS drained into a common pool of venous sinuses. Bifurcation type was noted in 22 cases (14%), in which SSS was divided to drain into the bilateral transverse sinuses. Right dominant type was the most frequent one with 66 cases (41%), in which SSS drained only into the right transverse sinus. Left dominant type was the least frequent one with 16 cases (10%), in which SSS drained to the left, in a mirror image to the right dominant type. Clinical significance is discussed with our preliminary trial for the optimization of the inner skull surface and venous flow using computed tomography and magnetic resonance imaging, and demonstration of cerebrovascular disease.

Real-time interactive duplex MR measurements: application in neurovascular imaging

OBJECTIVE

Real-time interactive duplex MR imaging is a new phase-contrast MR imaging technique that enables the quantification and display of flow velocities in real time without the need for cardiac gating. We investigated the feasibility and reliability of the technique to assess hemodynamic information both in vitro and in vivo in the carotid arteries and in the venous sinuses.

SUBJECTS AND METHODS

Real-time interactive duplex MR measurements (TR/TE, 53/27; flip angle, 90 degrees; encoding velocity, 100 or 150 cm/sec) were performed in vitro with a steady-flow phantom and in 10 healthy volunteers in whom common and internal carotid artery velocities were measured. In eight volunteers, velocity measurements were also performed in the superior sagittal sinus during both normal breathing and hyperventilation. Time-velocity plots were analyzed qualitatively and quantitatively and compared with findings from conventional segmented k-space phase-contrast MR imaging and Doppler sonography.

RESULTS

Velocity determinations for real-time duplex MR and conventional phase-contrast MR imaging showed an in vitro correlation of 0.99 and an in vivo correlation of 0.83 (carotid arteries) and 0.76 (venous sinus). Velocity measurements in the carotid arteries with real-time MR imaging were significantly lower than those obtained with conventional phase-contrast MR (averaged, 7.8%; p = 0.003) or sonography (23.7%, p < 0.001), likely because of volume averaging. Small but significant velocity changes occurring in the venous sinus during hyperventilation were reliably identified with both MR techniques.

CONCLUSION

Real-time interactive duplex MR imaging can be effectively applied in neurovascular imaging to obtain hemodynamic information.