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. J. Magn. Reson. Imaging 2009;30:878–883. © 2009 Wiley‐Liss, Inc.     

Determination of cranio-spinal canal compliance distribution by MRI: Methodology and early application in idiopathic intracranial hypertension

Purpose:

To develop a method for derivation of the cranial‐spinal compliance distribution, assess its reliability, and apply to obese female patients with a diagnosis of idiopathic intracranial hypertension (IIH).

Materials and Methods:

Phase contrast‐based measurements of blood and cerebrospinal fluid (CSF) flows to, from, and between the cranial and spinal canal compartments were used with lumped‐parameter modeling to estimate systolic volume and pressure changes from which cranial and spinal compliance indices are obtained. The proposed MRI indices are analogous to pressure volume indices (PVI) currently being measured invasively with infusion‐based techniques. The consistency of the proposed method was assessed using MRI data from seven aged healthy subjects. Measurement reproducibility was assessed using five repeated MR scans from one subject. The method was then applied to compare spinal canal compliance contribution in seven IIH patients and six matched healthy controls.

Results:

In the healthy subjects, as expected, spinal canal contribution was consistently larger than the cranial contribution (average value of 69%). Measurement variability was 8%. In IIH, the spinal canal contribution is significantly smaller than normal controls (60 versus 78%, P < 0.03).

Conclusion:

An MRI‐based method for derivation of compliance indices analogous to PVI has been implemented and applied to healthy subjects. The application of the method to obese IIH patients suggests a spinal canal involvement in the pathophysiology of IIH. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Assessment of CSF dynamics and venous flow in the superior sagittal sinus by MRI in idiopathic intracranial hypertension: a preliminary study

A velocity-sensitive magnetic resonance imaging (MRI) phase-mapping method was used for noninvasive study of cerebrospinal fluid (CSF) flow in the cerebral aqueduct, for indirect calculation of supra-tentorial CSF production, and for measurement of blood flow in the superoor sagittal sinus (SSS). We examined 12 patients with idiopathic intracranial hypertension (HH; pseudotumour cerebri), and 10 healthy volunteers. The peak caudal and rostal CSF flow in the aqueduct during the cardiac cycle did not differ significantly between the patients and the volunteers. A significant correlation was found between the CSF volume flow amplitude and the resistance to cerebrospinal fluid out-flow in the patients (p<0.05). The calculated mean supratentorial CSF production rate was 0.79 ml/min in the patients and 0.70 ml/min in the controls, but this difference was not statistically significant. However, the MRI measurements suggested CSF hypersecretion in three patients, whereas increased transependymal passage of CSF could have been the cause of negative calculated CSF production rates in two others. A tendency towards lower mean blood flow in the SSS (mean 345 ml/min) in the patients than in the controls (mean 457 ml/min) was found, and in two patients showed very low values. We showed that MRI phase-mapping may be used to study the relative importance of the pathophysiological factors thought to play a role in the development of IIH.     

Murine orthostatic response during prolonged vertical studies: effect on cerebral blood flow measured by arterial spin-labeled MRI.

High-field MRI scanners are, in principle, well suited for mouse studies; however, many high-field magnets employ a vertical design that may influence the physiological state of the rodent. The purpose of this study was to investigate the orthostatic response of cerebral blood flow (CBF) in mice during a prolonged MR experiment in the vertical position. Arterial spin-labeled (ASL) MRI was performed at 4.7-Tesla with a 15-cm gradient insert that allowed horizontal and vertical CBF measurements to be obtained with the same scanner. For mice in the head-up (HU) vertical position, CBF decreased by approximately 40% compared to the horizontal position, although blood pressure did not differ. Furthermore, CBF values for vertically positioned mice treated with phenylephrine remained constant while blood pressure increased. These results support the conclusion that cerebral autoregulation was intact, albeit at a lower level. Since CBF recovers to near horizontal values by volume loading with saline, it appears that a decrease in central venous pressure (CVP) leading to an increase in sympathetic tone may be a contributing mechanism for lowered CBF. This suggests that using an HU vertical position for MRI in mice may have broader implications, especially for studies that rely on CBF (such as BOLD and fMRI).     

Visualization of hemodynamics in intracranial arteries using time-resolved three-dimensional phase-contrast MRI

PURPOSE: To visualize the hemodynamics of the intracranial arteries using time-resolved three-dimensional phase-contrast (PC)-MRI (4D-Flow). MATERIALS AND METHODS: MR examinations were performed with a 1.5T MR unit on six healthy volunteers (22-50 years old, average = 30 years). 4D-Flow was based on a radiofrequency (RF)-spoiled gradient-echo sequence, and velocity encoding (VENC) was performed along all three spatial directions. Measurements were retrospectively gated to the electrocardiogram (ECG), and cine series of three-dimensional (3D) data sets were generated. The voxel size was 1 x 1 x 1 mm, and acquisition time was 30-40 minutes. 4D data sets were calculated into time-resolved images of 3D streamlines, 3D particle traces, and 2D velocity vector fields by means of flow visualization software. RESULTS: We were able to see the 3D streamlines from the circle of Willis to the bilateral M2 segment of the middle cerebral arteries (MCAs). Time-resolved images of 3D particle traces also clearly demonstrated intracranial arterial flow dynamics. 2D velocity vector fields on the planes traversing the carotid siphon or the basilar tip were clearly visualized. These results were obtained in all six volunteers. CONCLUSION: 4D-Flow helped to elucidate the in vivo 3D hemodynamics of human intracranial arteries. This method may be a useful noninvasive means of analyzing the hemodynamics of intracranial arteries in vivo.     

急性颅内高压对犬呼吸系统顺应性的影响

目的探讨急性颅内高压对犬呼吸系统静态容积-压力(P-V)曲线的影响。方法健康成年杂种犬14只,全麻肌松,控制性机械通气,采用自制双水封瓶加U形管系统测量呼吸系统吸气相静态P-V曲线。8只犬额叶注入自体血凝块形成颅内高压,另6只不注血凝块作为对照。结果犬急性颅内高压后呼吸系统静态P-V曲线右移,顺应性下降(注血前后比较P<0.01);而对照组无明显改变。结论严重颅脑损伤急性颅内压升高后肺的顺应性下降,易发生呼吸功能障碍。     

Noninvasive quantification of whole‐brain cerebral metabolic rate of oxygen (CMRO2) by MRI

Cerebral metabolic rate of oxygen (CMRO₂) is an important marker for brain function and brain health. Existing techniques for quantification of CMRO₂ with positron emission tomography (PET) or MRI involve special equipment and/or exogenous agents, and may not be suitable for routine clinical studies. In the present study, a noninvasive method is developed to estimate whole‐brain CMRO₂ in humans. This method applies phase‐contrast MRI for quantitative blood flow measurement and T₂‐relaxation‐under‐spin‐tagging (TRUST) MRI for venous oxygenation estimation, and uses the Fick principle of arteriovenous difference for the calculation of CMRO₂. Whole‐brain averaged CMRO₂ values in young, healthy subjects were 132.1 ± 20.0 μmol/100 g/min, in good agreement with literature reports using PET. Various acquisition strategies for phase‐contrast and TRUST MRI were compared, and it was found that nongated phase‐contrast and sagittal sinus (SS) TRUST MRI were able to provide the most efficient and accurate estimation of CMRO₂. In addition, blood flow and venous oxygenation were found to be positively correlated across subjects. Owing to the noninvasive nature of this method, it may be a convenient and useful approach for assessment of brain metabolism in brain disorders as well as under various physiologic conditions. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Phase‐contrast magnetic resonance imaging measurements in intracranial aneurysms in vivo of flow patterns,velocity fields,and wall shear stress: Comparison with computational fluid dynamics

Evolution of intracranial aneurysms is known to be related to hemodynamic forces such as wall shear stress (WSS) and maximum shear stress (MSS). Estimation of these parameters can be performed using numerical simulations with computational fluid dynamics (CFD), but can also be directly measured with magnetic resonance imaging (MRI) using a time‐dependent 3D phase‐contrast sequence with encoding of each of the three components of the velocity vectors (7D‐MRV). To study the accuracy of 7D‐MRV in estimating these parameters in vivo, in comparison with CFD, 7D‐MRV and patient‐specific CFD modeling was performed for 3 patients who had intracranial aneurysms. Visual and quantitative analyses of the flow pattern and distribution of velocities, MSS, and WSS were performed using the two techniques. Spearman's coefficients of correlation between the two techniques were 0.56 for the velocity field, 0.48 for MSS, and 0.59 for WSS. Visual analysis and Bland–Altman plots showed good agreement for flow pattern and velocities but large discrepancies for MSS and WSS. These results indicate that 7D‐MRV can be used in vivo to measure velocity flow fields and for estimating MSS and WSS. Currently, however, this method cannot accurately quantify the latter two parameters. Magn Reson Med 61:409–417, 2009. © 2009 Wiley‐Liss, Inc.     

Correction of temporal misregistration artifacts in jet flow by conventional phase-contrast MRI

PURPOSE: To show that accuracy of jet flow representation by magnetic resonance (MR) phase-contrast (PC) velocity-encoded (VE) cine imaging is dominated by error terms resulting from the temporal distribution of data, and to present a generally applicable data interpolation-based approach to correct for this phenomenon. MATERIALS AND METHODS: Phase-contrast data were acquired in a stenotic orifice flow phantom using a physiologic pulsatile flow waveform. A temporally registered scan, acquired without data segmentation or interleaving was obtained (17 minutes) and taken as the reference (REF). Conventional PC data sets were acquired using segmentation and data interleaving. An enhanced temporal registration (ETR) algorithm was applied to the acquired data to temporally interpolate component sets and output data at matching time points, thereby reducing temporal dispersion. RESULTS: Compared to the REF data, conventionally processed PC data consistently overestimated peak velocities in laminar jet flow regions (127% +/- 28%) and exhibited relatively weak correlations (r = 0.67 +/- 0.23). The ETR-processed data better represented peak velocities (101% +/- 13%, P < 0.001) and correlated more closely with the REF data (r = 0.94 +/- 0.05, P < 0.001). CONCLUSION: The temporal distribution of PC data impacts the accuracy of velocity representation in pulsatile jet flow. A temporal registration postprocessing algorithm can minimize loss of accuracy.     

MRI evidence for preserved regulation of intracranial pressure in patients with cerebral arteriovenous malformations

Purpose

The purpose of this study was to investigate intracranial pressure and associated hemo- and hydrodynamic parameters in patients with cerebral arteriovenous malformations AVMs.

Methods

Thirty consecutive patients with arteriovenous malformations (median age 38.7 years, 27/30 previously treated with radiosurgery) and 30 age- and gender-matched healthy controls were investigated on a 3.0 T MR scanner. Nidus volume was quantified on dynamic MR angiography. Total arterial cerebral blood flow (tCBF), venous outflow as well as aqueductal and craniospinal stroke volumes were obtained using velocity-encoded cine-phase contrast MRI. Intracranial volume change during the cardiac cycle was calculated and intracranial pressure (ICP) was derived from systolic intracranial volume change (ICVC) and pulse pressure gradient.

Results

TCBF was significantly higher in AVM patients as compared to healthy controls (median 799 vs. 692 mL/min, p = 0.007). There was a trend for venous flow to be increased in both the ipsilateral internal jugular vein (IJV, 282 vs. 225 mL/min, p = 0.16), and in the contralateral IJV (322 vs. 285 mL/min, p = 0.09), but not in secondary veins. There was no significant difference in median ICP between AVM patients and control subjects (6.9 vs. 8.6 mmHg, p = 0.30) and ICP did not correlate with nidus volume in AVM patients (ρ = −0.06, p = 0.74). There was a significant positive correlation between tCBF and craniospinal CSF stroke volume (ρ = 0.69, p = 0.02).

Conclusions

The elevated cerebral blood flow in patients with AVMs is drained through an increased flow in IJVs but not secondary veins. ICP is maintained within ranges of normal and does not correlate with nidus volume.