Comparative velocity investigations in cerebral arteries and aneurysms: 3D phase-contrast MR angiography, laser Doppler velocimetry and computational fluid dynamics

Assessment of Complex Flow Patterns in Patients With Carotid Webs, Patients With Carotid Atherosclerosis, and Healthy Subjects Using 4D Flow MRI

BACKGROUND

Carotid webs (CaWs) are fibromuscular projections in the internal carotid artery (ICA) that cause mild luminal narrowing (<50%), but may be causative in up to one-third of seemingly cryptogenic strokes. Understanding hemodynamic alterations caused by CaWs is imperative to assessing stroke risk. Time-Average Wall Shear Stress (TAWSS) and Oscillatory Shear Index (OSI) are hemodynamic parameters linked to vascular dysfunction and thrombosis.

PURPOSE

To test the hypothesis: "CaWs are associated with lower TAWSS and higher OSI than mild atherosclerosis or healthy carotid bifurcation."

STUDY TYPE

Prospective study.

POPULATION

A total of 35 subjects (N = 14 bifurcations with CaW, 11F, age: 49 ± 10, 10 mild atherosclerosis 6F, age: 72 ± 9, 11 healthy 9F, age: 42 ± 13).

FIELD STRENGTH/SEQUENCE

4D flow/STAR-MATCH/3D TOF/3T MRI, CTA.

ASSESSMENT

4D Flow velocity data were analyzed in two ways: 1) 3D ROI in the ICA bulbar segment (complex flow patterns are expected) was used to quantify the regions with low TAWSS and high OSI. 2) 2D planes were placed perpendicular to the centerline of the carotid bifurcation for detailed analysis of TAWSS and OSI.

STATISTICAL TESTS

Independent-samples Kruskal-Wallis-H test with 0.05 used for statistical significance.

RESULTS

The percent surface area where low TAWSS was present in the ICA bulb was 12.3 ± 8.0% (95% CI: 7.6-16.9) in CaW subjects, 1.6 ± 1.9% (95% CI: 0.2-2.9) in atherosclerosis, and 8.5 ± 7.7% (95% CI: 3.6-13.4) in healthy subjects, all differences were statistically significant (ƞ2 = 0.3 [95% CI: 0.05-0.5], P-value CaW vs. healthy = 0.2). OSI had similar values in the CCA between groups (ƞ2 = 0.07 [95% CI: 0.0-0.2], P-value = 0.5), but OSI was significantly higher downstream of the bifurcation in CaW subjects compared to atherosclerosis and normal subjects. OSI returned to similar values between groups 1.5 diameters distal to the bifurcation (ƞ2 = 0.03 [95% CI: 0.0-0.2], P-value = 0.7).

CONCLUSION

Lower TAWSS and higher OSI are present in the ICA bulb in patients with CaW when compared to patients with atherosclerotic or healthy subjects.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Fluid dynamic analysis in predicting the recanalization of intracranial aneurysms after coil embolization - A study of spatiotemporal characteristics

Purpose

Hemodynamics play a key role in the management of cerebral aneurysm recanalization after coil embolization; however, the most reliable hemodynamic parameter remains unknown. Previous studies have explored the use of both spatiotemporally averaged and maximal definitions for hemodynamic parameters, based on computational fluid dynamics (CFD) analysis, to build predictive models for aneurysmal recanalization. In this study, we aimed to assess the influence of different spatiotemporal characteristics of hemodynamic parameters on predictive performance.

Methods

Hemodynamics were simulated using CFD for 66 cerebral aneurysms from 65 patients. We evaluated 14 types of spatiotemporal definitions for two hemodynamic parameters in the pre-coiling model and five in virtual post-coiling model (VM) created by cutting the aneurysm from the pre-coiling model. A total of 91 spatiotemporal hemodynamic features were derived and utilized to develop univariate predictor (UP) and multivariate logistic regression (LR) models. The model's performance was assessed using two metrics: the area under the receiver operating characteristic curve (AUROC) and the area under the precision-recall curve (AUPRC).

Results

Different spatiotemporal hemodynamic features exhibited a wide range of AUROC values ranging from 0.224 to 0.747, with 22 feature pairs showing a significant difference in AUROC value (P-value <0.05), despite being derived from the same hemodynamic parameter. PDave,q1 was identified as the strongest UP with AUROC/AUPRC values of 0.747/0.385, yielding sensitivity and specificity value of 0.889 and 0.614 at the optimal cut-off value, respectively. The LR model further improved the prediction performance, having AUROC/AUPRC values of 0.890/0.903. At the optimal cut-off value, the LR model achieved a specificity of 0.877, sensitivity of 0.719, outperforming the UP model.

Conclusion

Our research indicated that the characteristics of hemodynamic parameters in terms of space and time had a significant impact on the development of predictive model. Our findings suggest that LR model based on spatiotemporal hemodynamic features could be clinically useful in predicting recanalization after coil embolization in patients, without the need for invasive procedures.

On the identification of hypoxic regions in subject-specific cerebral vasculature by combined CFD/MRI

A long-time exposure to lack of oxygen (hypoxia) in some regions of the cerebrovascular system is believed to be one of the causes of cerebral neurological diseases. In the present study, we show how a combination of magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) can provide a non-invasive alternative for studying blood flow and transport of oxygen within the cerebral vasculature. We perform computer simulations of oxygen mass transfer in the subject-specific geometry of the circle of Willis. The computational domain and boundary conditions are based on four-dimensional (4D)-flow MRI measurements. Two different oxygen mass transfer models are considered: passive (where oxygen is treated as a dilute chemical species in plasma) and active (where oxygen is bonded to haemoglobin) models. We show that neglecting haemoglobin transport results in a significant underestimation of the arterial wall mass transfer of oxygen. We identified the hypoxic regions along the arterial walls by introducing the critical thresholds that are obtained by comparison of the estimated range of Damköhler number (Da ⊂ 〈9; 57〉) with the local Sherwood number. Finally, we recommend additional validations of the combined MRI/CFD approach proposed here for larger groups of subject- or patient-specific brain vasculature systems.

Quality Control for 4D Flow MR Imaging

In recent years, 4D flow MRI has become increasingly important in clinical applications for the blood vessels in the whole body, heart, and cerebrospinal fluid. 4D flow MRI has advantages over 2D cine phase-contrast (PC) MRI in that any targeted area of interest can be analyzed post-hoc, but there are some factors to be considered, such as ensuring measurement accuracy, a long imaging time and post-processing complexity, and interobserver variability.Due to the partial volume phenomenon caused by low spatial and temporal resolutions, the accuracy of flow measurement in 4D flow MRI is reduced. For spatial resolution, it is recommended to include at least four voxels in the vessel of interest, and if possible, six voxels. In large vessels such as the aorta, large voxels can be secured and SNR can be maintained, but in small cerebral vessels, SNR is reduced, resulting in reduced accuracy. A temporal resolution of less than 40 ms is recommended. The velocity-to-noise ratio (VNR) of low-velocity blood flow is low, resulting in poor measurement accuracy. The use of dual velocity encoding (VENC) or multi-VENC is recommended to avoid velocity wrap around and to increase VNR. In order to maintain sufficient spatio-temporal resolution, a longer imaging time is required, leading to potential patient movement during examination and a corresponding decrease in measurement accuracy.For the clinical application of new technologies, including various acceleration techniques, in vitro and in vivo accuracy verification based on existing accuracy-validated 2D cine PC MRI and 4D flow MRI, as well as accuracy verification on the conservation of mass' principle, should be performed, and intraobserver repeatability, interobserver reproducibility, and test-retest reproducibility should be checked.

Morphological factors affecting vortex core instability on 4D flow MRI of unruptured cerebral aneurysms

OBJECTIVE

The spatiotemporal instability of intra-aneurysmal vortex flow may be associated with unruptured cerebral aneurysm rupture. We identified morphological factors that affect intra-aneurysmal vortex core patterns classified based on the instability on four-dimensional (4D) flow magnetic resonance imaging (MRI) and determined cutoff values for the factors to discriminate unstable core patterns.

METHODS

We classified vortex core patterns of 40 unruptured aneurysms on 4D flow MRI into stable, stable with a flapping tip, continuously deforming wave-or-coil-like, and non-visualized. We statistically compared nine morphological parameters among aneurysm groups with individual patterns.

RESULTS

The vortex cores were stable (n = 16) (group A), stable with a flapping tip (n = 15) (group B), wave-or-coil-like (n = 7) (group C), and non-visualized (n = 2) (group D). Since there were no statistically significant differences between groups A and B, we compared the difference between the groups A and B and the other groups. Multivariate logistic regression analyses found that size ratio (SR) was an only independently significant parameter (p < 0.05). The receiver-operating characteristic analysis between groups A and B and group C and between groups A and B and groups C and D revealed that the area under the curve value for SR was the highest (0.829 [95% CI, 0.642-1.0]; 0.867 [95% CI, 0.715-1.0], respectively) among morphological factors; the cutoff value for SR was 1.72 (specificity 0.714, sensitivity 0.756; specificity 0.806, sensitivity 0.778, respectively).

CONCLUSION

SR was an independent morphological factor contributing to vortex core instability based on the vortex core patterns on 4D flow MRI.Abbreviations: CFD: computational fluid dynamics; 3D: three-dimensional; 4D: four-dimensional; MRI: magnetic resonance imaging; MRA: magnetic resonance angiography; ICA: internal carotid artery; AR: aspect ratio; SR: size ratio; CI: confidence interval; AUC: area under the curve; ROC: receiver-operating characteristic.

Improving Blood Flow Visualization of Recirculation Regions at Carotid Bulb in 4D Flow MRI Using Semi-Automatic Segmentation with ITK-SNAP

Assessment of carotid bulb hemodynamics using four-dimensional (4D) flow magnetic resonance imaging (MRI) requires accurate segmentation of recirculation regions that is frequently hampered by limited resolution. This study aims to improve the accuracy of 4D flow MRI carotid bulb segmentation and subsequent recirculation regions analysis. Time-of-flight (TOF) MRI and 4D flow MRI were performed on bilateral carotid artery bifurcations in seven healthy volunteers. TOF-MRI data was segmented into 3D geometry for computational fluid dynamics (CFD) simulations. ITK-SNAP segmentation software was included in the workflow for the semi-automatic generation of 4D flow MRI angiographic data. This study compared the velocities calculated at the carotid bifurcations and the 3D blood flow visualization at the carotid bulbs obtained by 4D flow MRI and CFD. By applying ITK-SNAP segmentation software, an obvious improvement in the 4D flow MRI visualization of the recirculation regions was observed. The 4D flow MRI images of the recirculation flow characteristics of the carotid artery bulbs coincided with the CFD. A reasonable agreement was found in terms of velocity calculated at the carotid bifurcation between CFD and 4D flow MRI. However, the dispersion of velocity data points relative to the local errors of measurement in 4D flow MRI remains. Our proposed strategy showed the feasibility of improving recirculation regions segmentation and the potential for reliable blood flow visualization in 4D flow MRI. However, quantitative analysis of recirculation regions in 4D flow MRI with ITK-SNAP should be enhanced for use in clinical situations.

Comparison of Hemodynamic Visualization in Cerebral Arteries: Can Magnetic Resonance Imaging Replace Computational Fluid Dynamics?

A multimodality approach was applied using four-dimensional flow magnetic resonance imaging (4D flow MRI), time-of-flight magnetic resonance angiography (TOF-MRA) signal intensity gradient (SIG), and computational fluid dynamics (CFD) to investigate the 3D blood flow characteristics and wall shear stress (WSS) of the cerebral arteries. TOF-MRA and 4D flow MRI were performed on the major cerebral arteries in 16 healthy volunteers (mean age 34.7 ± 7.6 years). The flow rate measured with 4D flow MRI in the internal carotid artery, middle cerebral artery, and anterior cerebral artery were 3.8, 2.5, and 1.2 mL/s, respectively. The 3D blood flow pattern obtained through CFD and 4D flow MRI on the cerebral arteries showed reasonable consensus. CFD delivered much greater resolution than 4D flow MRI. TOF-MRA SIG and CFD WSS of the major cerebral arteries showed reasonable consensus with the locations where the WSS was relatively high. However, the visualizations were very different between TOF-MRA SIG and CFD WSS at the internal carotid artery bifurcations, the anterior cerebral arteries, and the anterior communicating arteries. 4D flow MRI, TOF-MRA SIG, and CFD are complementary methods that can provide additional insight into the hemodynamics of the human cerebral artery.

Possible Association Between Rupture and Intranidal Microhemodynamics in Arteriovenous Malformations: Phase-Contrast Magnetic Resonance Angiography-Based Flow Quantification

OBJECTIVE

To examine a potential association between intranidal microhemodynamics and rupture using a phase-contrast magnetic resonance angiography (PCMRA)-based flow quantification technique in arteriovenous malformations (AVMs).

METHODS

We retrospectively collected data on 30 consecutive patients with AVMs (23 unruptured and 7 ruptured). Based on PCMRA data, maximal (V_max) and mean (V_mean) intranidal velocities were calculated. Logistic regression analysis was performed to assess factors associated with previous AVM rupture.

RESULTS

All ruptures occurred within 6 months before PCMRA. The mean nidus volume was 4.7 mL. Eleven patients (37%) had deep draining vein(s), and 6 patients (20%) had a deep-seated nidus. The mean ± standard deviation V_mean and V_max were 9.6 ± 2.8 cm/second and 66.7 ± 26.2 cm/second, respectively. The logistic regression analyses revealed that higher V_max (P = 0.075, unit odds ratio [OR] = 1.05, 95% confidence interval [95% CI] = 1.00-1.10) was significantly associated with prior hemorrhage. The receiver-operating curve analyses demonstrated that a V_mean of 10.8 cm/second (area under the curve = 0.671) and V_max of 90.2 cm/second (area under the curve = 0.764) maximized the Youden Index. A V_max > 90 cm/second was significantly associated with AVM rupture both in the univariate (P = 0.025, OR = 9.0, 95% CI = 1.3-61.1) and multivariate (P = 0.008, OR = 51.7, 95% CI = 2.8-968.3) analyses.

CONCLUSIONS

Presence of faster velocities in intranidal vessels may suggest aberrant microhemodynamics and thus be associated with AVM rupture. PCMRA-based velocimetry seems to be a promising tool to predict future AVM rupture.

A Transparent Vessel-on-a-Chip Device for Hemodynamic Analysis and Early Diagnosis of Intracranial Aneurysms by CFD and PC-MRI

Hemodynamics plays a critical role in early diagnosis and investigating the growth mechanism of intracranial aneurysms (IAs), which usually induce hemorrhagic stroke, serious neurological diseases, and even death. We developed a transparent blood vessel-on-a-chip (VOC) device for magnetic resonance imaging (MRI) to provide characteristic flow fields of early IAs as the reference for early diagnosis. This VOC device takes advantage of the transparent property to clearly exhibit the internal structure and identify the needless air bubbles in the biomimetic fluid experiment, which significantly affects the MRI image quality. Furthermore, the device was miniaturized and easily assembled with arbitrary direction using a 3D-printed scaffold in a radiofrequency coil. Computational fluid dynamics (CFD) simulations of the flow field were greatly consistent with those data from MRI. Both internal flow and wall shear stress (WSS) exhibited very low levels during the IA growth, thus leading to the growth and rupture of IAs. PC-MRI images can also provide a reasonable basis for the early diagnosis of IAs. Therefore, we believed that this proposed VOC-based MR imaging technique has great potential for early diagnostic of intracranial aneurysms.

Inlet flow rate of perfusion bioreactors affects fluid flow dynamics, but not oxygen concentration in 3D-printed scaffolds for bone tissue engineering: Computational analysis and experimental validation

Fluid flow dynamics and oxygen-concentration in 3D-printed scaffolds within perfusion bioreactors are sensitive to controllable bioreactor parameters such as inlet flow rate. Here we aimed to determine fluid flow dynamics, oxygen-concentration, and cell proliferation and distribution in 3D-printed scaffolds as a result of different inlet flow rates of perfusion bioreactors using experiments and finite element modeling. Pre-osteoblasts were treated with 1 h pulsating fluid flow with low (0.8 Pa; PFF^low) or high peak shear stress (6.5 Pa; PFF^high), and nitric oxide (NO) production was measured to validate shear stress sensitivity. Computational analysis was performed to determine fluid flow between 3D-scaffold-strands at three inlet flow rates (0.02, 0.1, 0.5 ml/min) during 5 days. MC3T3-E1 pre-osteoblast proliferation, matrix production, and oxygen-consumption in response to fluid flow in 3D-printed scaffolds inside a perfusion bioreactor were experimentally assessed. PFF^high more strongly stimulated NO production by pre-osteoblasts than PFF^low. 3D-simulation demonstrated that dependent on inlet flow rate, fluid velocity reached a maximum (50-1200 μm/s) between scaffold-strands, and fluid shear stress (0.5-4 mPa) and wall shear stress (0.5-20 mPa) on scaffold-strands surfaces. At all inlet flow rates, gauge fluid pressure and oxygen-concentration were similar. The simulated cell proliferation and distribution, and oxygen-concentration data were in good agreement with the experimental results. In conclusion, varying a perfusion bioreactor's inlet flow rate locally affects fluid velocity, fluid shear stress, and wall shear stress inside 3D-printed scaffolds, but not gauge fluid pressure, and oxygen-concentration, which seems crucial for optimized bone tissue engineering strategies using bioreactors, scaffolds, and cells.

Comparison of blood velocity between Transcranial Doppler and numerical method in the patient-specific Circle of Willis with aneurysm

BACKGROUND

The development of any disturbance in the Circle of Willis (COW) can change the hemodynamics of blood flow and result in damage to the vascular system. Clinical methods such as TCD for diagnosing an aneurysm or growth factors and rupture can measure blood velocity. Several factors influence the accuracy of TCD that can lead to wrong evaluations and affect the treatment planning.

OBJECTIVE

In this study, the conformity between CFD and TCD was accomplished for investigating the accuracy of the clinical method (TCD) in different vessels of the Circle of Willis.

METHODS

The realistic three-dimensional models have been produced from angiography images. Considering fluid-structure interaction, a domain of the blood flow and vessel wall has been simulated by the ANSYS.CFX software. The velocity in the cerebral arteries has been calculated and compared with the velocity acquired from TCD.

RESULTS

According to the findings, there were significant differences between the results obtained from computational fluid dynamics and Doppler test in different vessels of the Circle of Willis. In some areas, differences close to 80 cm/s were also reported.

CONCLUSION

According to the results, there are possibilities of errors in carrying out a Doppler test in some arteries and can lead to wrong estimates and ultimately incorrect decisions.

Validation of Zero TE-MRA in the Characterization of Cerebrovascular Diseases: A Feasibility Study

BACKGROUND AND PURPOSE

Zero TE-MRA is less sensitive to field heterogeneity, complex flow, and acquisition noise. This study aimed to prospectively validate the feasibility of zero TE-MRA for cerebrovascular diseases assessment, compared with TOF-MRA.

MATERIALS AND METHODS

Seventy patients suspected of having cerebrovascular disorders were recruited. Sound levels were estimated for each MRA subjectively and objectively in different modes. MRA image quality was estimated by 2 neuroradiologists. The degree of stenosis (grades 0-4) and the z-diameter of aneurysms (tiny group ≤3 mm and large group >3 mm) were measured for further quantitative analysis. CTA was used as the criterion standard.

RESULTS

Zero TE-MRA achieved significantly lower subjective perception and objective noise reduction (37.53%). Zero TE-MRA images showed higher signal homogeneity (3.29 ± 0.59 versus 3.04 ± 0.43) and quality of venous signal suppression (3.67 ± 0.47 versus 2.75 ± 0.46). The intermodality agreement was higher for zero TE-MRA than for TOF-MRA (zero TE, 0.90; TOF, 0.81) in the grading of stenosis. Zero TE-MRA had a higher correlation than TOF-MRA (zero TE, 0.84; TOF, 0.74) in the tiny group and a higher consistency with CTA (intraclass correlation coefficient, 0.83; intercept, -0.5084-1.1794; slope -0.4952 to -0.2093) than TOF-MRA (intraclass correlation coefficient, 0.64; intercept, 0.7000-2.6133; slope -1.0344 to -0.1923). Zero TE-MRA and TOF-MRA were comparable in the large group. Zero TE-MRA had more accurate details than TOF-MRA of AVM and Moyamoya lesions.

CONCLUSIONS

Compared with TOF-MRA, zero TE-MRA achieved more robust performance in depicting cerebrovascular diseases. Therefore, zero TE-MRA was shown to be a promising MRA technique for further routine application in the clinic in patients with cerebrovascular diseases.

Phase-contrast magnetic resonance imaging to assess renal perfusion: a systematic review and statement paper

Objective

Phase-contrast magnetic resonance imaging (PC-MRI) is a non-invasive method used to compute blood flow velocity and volume. This systematic review aims to discuss the current status of renal PC-MRI and provide practical recommendations which could inform future clinical studies and its adoption in clinical practice.

Methodology

A comprehensive search of all the PC-MRI studies in human healthy subjects or patients related to the kidneys was performed.

Results

A total of 39 studies were included in which PC-MRI was used to measure renal blood flow (RBF) alongside other derivative hemodynamic parameters. PC-MRI generally showed good correlation with gold standard methods of RBF measurement, both in vitro and in vivo, and good reproducibility. Despite PC-MRI not being routinely used in clinical practice, there are several clinical studies showing its potential to support diagnosis and monitoring of renal diseases, in particular renovascular disease, chronic kidney disease and autosomal dominant polycystic kidney disease.

Discussion

Renal PC-MRI shows promise as a non-invasive technique to reliably measure RBF, both in healthy volunteers and in patients with renal disease. Future multicentric studies are needed to provide definitive normative ranges and to demonstrate the clinical potential of PC-MRI, likely as part of a multi-parametric renal MRI protocol.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00772-0) contains supplementary material, which is available to authorized users.

Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH)-Phase Ib: Effect of morphology on hemodynamics

Background

Image-based blood flow simulations have been increasingly applied to investigate intracranial aneurysm (IA) hemodynamics. However, the acceptance among physicians remains limited due to the high variability in the underlying assumptions and quality of results.

Methods

To evaluate the vessel segmentation as one of the most important sources of error, the international Multiple Aneurysms AnaTomy CHallenge 2018 (MATCH) was announced. 26 research groups from 13 different countries segmented three datasets, which contained five IAs in total. Based on these segmentations, 73 time-dependent blood flow simulations under consistent conditions were carried out. Afterwards, relevant flow and shear parameters (e.g., neck inflow rate, parent vessel flow rate, spatial mean velocity, and wall shear stress) were analyzed both qualitatively and quantitatively.

Results

Regarding the entire vasculature, the variability of the segmented vessel radius is 0.13 mm, consistent and independent of the local vessel radius. However, the centerline velocity shows increased variability in more distal vessels. Focusing on the aneurysms, clear differences in morphological and hemodynamic parameters were observed. The quantification of the segmentation-induced variability showed approximately a 14% difference among the groups for the parent vessel flow rate. Regarding the mean aneurysmal velocity and the neck inflow rate, a variation of 30% and 46% was observed, respectively. Finally, time-averaged wall shear stresses varied between 28% and 51%, depending on the aneurysm in question.

Conclusions

MATCH reveals the effect of state-of-the-art segmentation algorithms on subsequent hemodynamic simulations for IA research. The observed variations may lead to an inappropriate interpretation of the simulation results and thus, can lead to inappropriate conclusions by physicians. Therefore, accurate segmentation of the region of interest is necessary to obtain reliable and clinically helpful flow information.

In-vitro validation of 4D flow MRI measurements with an experimental pulsatile flow model

PURPOSE

The purpose of this study was to assess the precision of four-dimensional (4D) phase-contrast magnetic resonance imaging (PCMRI) to measure mean flow and peak velocity (V_max) in a pulsatile flow phantom and to test its sensitivity to spatial resolution and Venc.

MATERIAL AND METHODS

The pulsatile flow phantom consisted of a straight tube connected to the systemic circulation of an experimental mock circulatory system. Four-dimensional-PCMR images were acquired using different spatial resolutions (minimum pixel size: 1.5×1.5×1.5mm³) and velocity encoding sensitivities (up to three times V_max). Mean flow and V_max calculated from 4D-PCMRI were compared respectively to the reference phantom flow parameters and to V_max obtained from two-dimensional (2D)-PCMRI.

RESULTS

4D-PCI measured mean flow with a precision of -0.04% to+5.46%, but slightly underestimated V_max when compared to 2D-PCMRI (differences ranging from -1.71% to -3.85%). 4D PCMRI mean flow measurement was influenced by spatial resolution (P<0.001) with better results obtained with smaller voxel size. There was no effect of Venc on mean flow measurement. Regarding V_max, neither spatial resolution nor Venc did influence the precision of the measurement.

CONCLUSION

Using an experimental pulsatile flow model 4D-PCMRI is accurate to measure mean flow and V_max with better results obtained with higher spatial resolution. We also show that Venc up to 3 times higher than V_max may be used with no effect on these measurements.

Fluid-structure interaction of patient-specific Circle of Willis with aneurysm: Investigation of hemodynamic parameters

BACKGROUND

Circle of Willis (COW) is a network of cerebral artery which continually supplies the brain with blood. Any disturbance in this supply will result in trauma or even death. One of these damages is known as brain Aneurysm. Clinical methods for diagnosing aneurysm can only measure blood velocity; while, in order to understand the causes of these occurrences it is necessary to have information about the amount of pressure and wall shear stress, which is possible through computational models.

OBJECTIVE

In this study purpose is achieving exact information of hemodynamic blood flow in COW with an aneurysm and investigation of effective factors on growth and rupture of aneurysm.

METHODS

Here, realistic three-dimensional models have been produced from angiography images. Considering fluid-structure interaction have been simulated by the ANSYS.CFX software.

RESULTS

Hemodynamic Studying of the COW and intra-aneurysm showed that the WSS and wall tension in the neck of aneurysms for case A are 129.5 Pa, and 12.2 kPa and for case B they are 53.3 Pa and 56.2 kPa, and more than their fundus, thus neck of aneurysm is prone to rupture.

CONCLUSION

This study showed that the distribution of parameters was dependent on the geometry of the COW, and maximum values are seen in areas prone to aneurysm formation.

Hemodynamics in a giant intracranial aneurysm characterized by in vitro 4D flow MRI

Experimental and computational data suggest that hemodynamics play a critical role in the development, growth, and rupture of cerebral aneurysms. The flow structure, especially in aneurysms with a large sac, is highly complex and three-dimensional. Therefore, volumetric and time-resolved measurements of the flow properties are crucial to fully characterize the hemodynamics. In this study, phase-contrast Magnetic Resonance Imaging is used to assess the fluid dynamics inside a 3D-printed replica of a giant intracranial aneurysm, whose hemodynamics was previously simulated by multiple research groups. The physiological inflow waveform is imposed in a flow circuit with realistic cardiovascular impedance. Measurements are acquired with sub-millimeter spatial resolution for 16 time steps over a cardiac cycle, allowing for the detailed reconstruction of the flow evolution. Moreover, the three-dimensional and time-resolved pressure distribution is calculated from the velocity field by integrating the fluid dynamics equations, and is validated against differential pressure measurements using precision transducers. The flow structure is characterized by vortical motions that persist within the aneurysm sac for most of the cardiac cycle. All the main flow statistics including velocity, vorticity, pressure, and wall shear stress suggest that the flow pattern is dictated by the aneurysm morphology and is largely independent of the pulsatility of the inflow, at least for the flow regimes investigated here. Comparisons are carried out with previous computational simulations that used the same geometry and inflow conditions, both in terms of cycle-averaged and systolic quantities.

Noncontrast-enhanced time-resolved 4D dynamic intracranial MR angiography at 7T: A feasibility study

BACKGROUND

Arterial spin labeling (ASL) based-noncontrast-enhanced 4D MR angiography (NCE 4D MRA) shows potential in characterizing cerebrovascular hemodynamics in cerebrovascular disorders. Ultrahigh-field theoretically benefits ASL signal with increased inherent signal-to-noise ratio (SNR) and prolonged blood T₁ , which may provide improved delineation of vasculature in 4D MRA.

PURPOSE

To investigate the feasibility of NCE 4D MRA using 3D Cartesian trajectory and stack-of-stars (SOS) golden angle radial trajectory at 7T.

STUDY TYPE

A prospective study.

SUBJECTS

Six normal volunteers and eight patients with arteriovenous malformation (AVM).

FIELD STRENGTH/SEQUENCE

NCE 4D MRA with Cartesian and radial trajectories were performed at 3T and 7T.

ASSESSMENT

Subjective image quality of 4D MRA was evaluated using a 4-point scale by two experienced neuroradiologists. The characterization of AVM components with 4D MRA and DSA was also graded using the Spetzler-Martin grading scale.

STATISTICAL TESTS

Cohen's kappa coefficient was calculated to evaluate the agreement between two readers within each 4D MRA technique (Cartesian and Radial). A Wilcoxon signed-rank test was performed to compare the subjective image quality scores of 4D MRA between Cartesian and radial trajectories, and between 7T and 3T, respectively.

RESULTS

Good-to-excellent image quality was achieved in 4D MRA with both Cartesian (3.83 ± 0.41) and radial (3.42 ± 0.49) acquisitions in healthy volunteers at 7T. However, markedly reduced scan time was needed with radial acquisition. 4D MRA at 7T (3.31 ± 0.59) shows better delineation of AVM lesion features, especially the vein drainage, compared with that of 3T (2.83 ± 0.75), although no statistical significance was achieved (P = 0.180).

DATA CONCLUSION

The feasibility of ASL based 4D MRA at 7T with Cartesian and SOS golden angle radial acquisition was demonstrated. The clinical evaluation of 4D MRA in AVMs between 3T and 7T suggested 7T 4D MRA images acquired with radial acquisition demonstrate excellent delineation of AVM features, especially the draining veins.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage 2 J. Magn. Reson. Imaging 2017.

Transitional hemodynamics in intracranial aneurysms - Comparative velocity investigations with high resolution lattice Boltzmann simulations, normal resolution ANSYS simulations, and MR imaging

PURPOSE

Blood flow in intracranial aneurysms has, until recently, been considered to be disturbed but still laminar. Recent high resolution computational studies have demonstrated, in some situations, however, that the flow may exhibit high frequency fluctuations that resemble weakly turbulent or transitional flow. Due to numerous assumptions required for simplification in computational fluid dynamics (CFD) studies, the occurrence of these events, in vivo, remains unsettled. The detection of these fluctuations in aneurysmal blood flow, i.e., hemodynamics by CFD, poses additional challenges as such phenomena cannot be captured in clinical data acquisition with magnetic resonance (MR) due to inadequate temporal and spatial resolutions. The authors' purpose was to address this issue by comparing results from highly resolved simulations, conventional resolution laminar simulations, and MR measurements, identify the differences, and identify their causes.

METHODS

Two aneurysms in the basilar artery, one with disturbed yet laminar flow and the other with transitional flow, were chosen. One set of highly resolved direct numerical simulations using the lattice Boltzmann method (LBM) and another with adequate resolutions under laminar flow assumption were conducted using a commercially available ANSYS Fluent solver. The velocity fields obtained from simulation results were qualitatively and statistically compared against each other and with MR acquisition.

RESULTS

Results from LBM, ANSYS Fluent, and MR agree well qualitatively and quantitatively for one of the aneurysms with laminar flow in which fluctuations were <80 Hz. The comparisons for the second aneurysm with high fluctuations of > ∼ 600 Hz showed vivid differences between LBM, ANSYS Fluent, and magnetic resonance imaging. After ensemble averaging and down-sampling to coarser space and time scales, these differences became minimal.

CONCLUSIONS

A combination of MR derived data and CFD can be helpful in estimating the hemodynamic environment of intracranial aneurysms. Adequately resolved CFD would suffice gross assessment of hemodynamics, potentially in a clinical setting, and highly resolved CFD could be helpful in a detailed and retrospective understanding of the physiological mechanisms.

3D phase contrast MRI: Partial volume correction for robust blood flow quantification in small intracranial vessels

PURPOSE

Recent advances in 3D-PCMRI (phase contrast MRI) sequences allow for measuring the complex hemodynamics in cerebral arteries. However, the small size of these vessels vs spatial resolution can lead to non-negligible partial volume artifacts, which must be taken into account when computing blood flow rates. For this purpose, we combined the velocity information provided by 3D-PCMRI with vessel geometry measured with 3DTOF (time of flight MRI) or 3DRA (3D rotational angiography) to correct the partial volume effects in flow rate assessments.

METHODS

The proposed methodology was first tested in vitro on cylindrical and patient specific vessels subject to fully controlled pulsatile flows. Both 2D- and 3D-PCMRI measurements using various spatial resolutions ranging from 20 to 1.3 voxels per vessel diameter were analyzed and compared with flowmeter baseline. Second, 3DTOF, 2D- and 3D-PCMRI measurements were performed in vivo on 35 patients harboring internal carotid artery (ICA) aneurysms indicated for endovascular treatments requiring 3DRA imaging.

RESULTS

The in vitro 2D- and 3D-PCMRI mean flow rates assessed with partial volume correction showed very low sensitivity to the acquisition resolution above ≈2 voxels per vessel diameter while uncorrected flow rates deviated critically when decreasing the spatial resolution. 3D-PCMRI flow rates measured in vivo in ICA agreed very well with 2D-PCMRI data and a good flow conservation was observed at the C7 bifurcation. Globally, partial volume correction led to 10-15% lower flow rates than uncorrected values as those reported in most of the published studies on intracranial flows.

CONCLUSION

Partial volume correction may improve the accuracy of PCMRI flow rate measurements especially in small vessels such as intracranial arteries. Magn Reson Med 79:129-140, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Accelerated dual-venc 4D flow MRI for neurovascular applications

PURPOSE

To improve velocity-to-noise ratio (VNR) and dynamic velocity range of 4D flow magnetic resonance imaging (MRI) by using dual-velocity encoding (dual-venc) with k-t generalized autocalibrating partially parallel acquisition (GRAPPA) acceleration.

MATERIALS AND METHODS

A dual-venc 4D flow MRI sequence with k-t GRAPPA acceleration was developed using a shared reference scan followed by three-directional low- and high-venc scans (repetition time / echo time / flip angle = 6.1 msec / 3.4 msec / 15°, temporal/spatial resolution = 43.0 msec/1.2 × 1.2 × 1.2 mm³ ). The high-venc data were used to correct for aliasing in the low-venc data, resulting in a single dataset with the favorable VNR of the low-venc but without velocity aliasing. The sequence was validated with a 3T MRI scanner in phantom experiments and applied in 16 volunteers to investigate its feasibility for assessing intracranial hemodynamics (net flow and peak velocity) at the major intracranial vessels. In addition, image quality and image noise were assessed in the in vivo acquisitions.

RESULTS

All 4D flow MRI scans were acquired successfully with an acquisition time of 20 ± 4 minutes. The shared reference scan reduced the total acquisition time by 12.5% compared to two separate scans. Phantom experiments showed 51.4% reduced noise for dual-venc compared to high-venc and an excellent agreement of velocities (ρ = 0.8, P < 0.001). The volunteer data showed decreased noise in dual-venc data (54.6% lower) compared to high-venc, and improved image quality, as graded by two observers: fewer artifacts (P < 0.0001), improved vessel conspicuity (P < 0.0001), and reduced noise (P < 0.0001).

CONCLUSION

Dual-venc 4D flow MRI exhibits the superior VNR of the low-venc acquisition and reliably incorporates low- and high-velocity fields simultaneously. In vitro and in vivo data demonstrate improved flow visualization, image quality, and image noise.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:102-114.

Four-dimensional MRI flow examinations in cerebral and extracerebral vessels - ready for clinical routine?

PURPOSE OF REVIEW

To evaluate the feasibility of 4-dimensional (4D) flow MRI for the clinical assessment of cerebral and extracerebral vascular hemodynamics in patients with neurovascular disease.

RECENT FINDINGS

4D flow MRI has been applied in multiple studies to qualitatively and quantitatively study intracranial aneurysm blood flow for potential risk stratification and to assess treatment efficacy of various neurovascular lesions, including intraaneurysmal and parent artery blood flow after flow diverter stent placement and staged embolizations of arteriovenous malformations and vein of Galen aneurysmal malformations. Recently, the technique has been utilized to characterize age-related changes of normal cerebral hemodynamics in healthy individuals over a broad age range.

SUMMARY

4D flow MRI is a useful tool for the noninvasive, volumetric and quantitative hemodynamic assessment of neurovascular disease without the need for gadolinium contrast agents. Further improvements are warranted to overcome technical limitations before broader clinical implementation. Current developments, such as advanced acceleration techniques (parallel imaging and compressed sensing) for faster data acquisition, dual or multiple velocity encoding strategies for more accurate arterial and venous flow quantification, ultrahigh-field strengths to achieve higher spatial resolution and streamlined postprocessing workflow for more efficient and standardized flow analysis, are promising advancements in 4D flow MRI.

Inflow Jet Patterns of Unruptured Cerebral Aneurysms Based on the Flow Velocity in the Parent Artery: Evaluation Using 4D Flow MRI

BACKGROUND AND PURPOSE

Inflow jet characteristics may be related to aneurysmal bleb formation and rupture. We investigated the visualization threshold on the basis of the flow velocity in the parent artery to classify the inflow jet patterns observed on 4D flow MR imaging.

MATERIALS AND METHODS

Fifty-seven unruptured aneurysms (24 bifurcation and 33 sidewall aneurysms) were subjected to 4D flow MR imaging to visualize inflow streamline bundles whose velocity exceeded visualization thresholds corresponding to 60%, 75%, and 90% of the maximum flow velocity in the parent artery. The shape of the streamline bundle was determined visually, and the inflow jet patterns were classified as concentrated, diffuse, neck-limited, and unvisualized.

RESULTS

At the 75% threshold, bifurcation aneurysms exhibited a concentrated inflow jet pattern at the highest rate. At this threshold, the inflow jets were concentrated in 13 aneurysms (group C, 22.8%), diffuse in 18 (group D, 31.6%), neck-limited in 11 (group N, 19.3%), and unvisualized in 15 (group U, 26.3%). In 16 (28.1%) of the 57 aneurysms, the inflow jet pattern was different at various thresholds. Most inflow parameters, including the maximum inflow velocity and rate, the inflow velocity ratio, and the inflow rate ratio, were significantly higher in groups C and D than in groups N and U.

CONCLUSIONS

The inflow jet pattern may depend on the threshold applied to visualize the inflow streamlines on 4D flow MR imaging. For the classification of the inflow jet patterns on 4D flow MR imaging, the 75% threshold may be optimal among the 3 thresholds corresponding to 60%, 75%, and 90% of the maximum flow velocity in the parent artery.

MR derived volumetric flow rate waveforms of internal carotid artery in patients treated for unruptured intracranial aneurysms by flow diversion technique

Little is known about the hemodynamic disturbances induced by the cerebral aneurysms in the parent artery and the effect of flow diverter stents (FDS) on these latter. A better understanding of the aneurysm-parent vessel complex relationship may aid our understanding of this disease and to optimize its treatment. The ability of volumetric flow rate (VFR) waveform to reflect the arterial compliance modifications is well known. By analyzing the VFR waveform and the pulsatility in the parent vessel, this study aimed to test the hypotheses that (1) intracranial aneurysms might disrupt the blood flow of the parent vessel and (2) the treatment by FDS might have measurable corrective effect on these changes. Ten patients followed for unruptured intracranial aneurysms treated by FDS and ten healthy volunteers as control group were included in this study. Two-dimensional quantitative phase-contrast magnetic resonance imaging (MRI) was performed on each patient on the ICA artery upstream and downstream to the aneurysm, and on each volunteer at similar locations. The aneurysms altered significantly the parent vessel pulsatility and this effect was correlated to their volume. The aneurysms treatment by FDS allowed for the restoration of a normally modulated flow and pulsatility correction in the parent vessel.

Velocity Measurement in Carotid Artery: Quantitative Comparison of Time-Resolved 3D Phase-Contrast MRI and Image-based Computational Fluid Dynamics

Background:

Understanding hemodynamic environment in vessels is important for realizing the mechanisms leading to vascular pathologies.

Objectives:

Three-dimensional velocity vector field in carotid bifurcation is visualized using TR 3D phase-contrast magnetic resonance imaging (TR 3D PC MRI) and computational fluid dynamics (CFD). This study aimed to present a qualitative and quantitative comparison of the velocity vector field obtained by each technique.

Subjects and Methods:

MR imaging was performed on a 30-year old male normal subject. TR 3D PC MRI was performed on a 3 T scanner to measure velocity in carotid bifurcation. 3D anatomical model for CFD was created using images obtained from time-of-flight MR angiography. Velocity vector field in carotid bifurcation was predicted using CFD and PC MRI techniques. A statistical analysis was performed to assess the agreement between the two methods.

Results:

Although the main flow patterns were the same for the both techniques, CFD showed a greater resolution in mapping the secondary and circulating flows. Overall root mean square (RMS) errors for all the corresponding data points in PC MRI and CFD were 14.27% in peak systole and 12.91% in end diastole relative to maximum velocity measured at each cardiac phase. Bland-Altman plots showed a very good agreement between the two techniques. However, this study was not aimed to validate any of methods, instead, the consistency was assessed to accentuate the similarities and differences between Time-resolved PC MRI and CFD.

Conclusion:

Both techniques provided quantitatively consistent results of in vivo velocity vector fields in right internal carotid artery (RCA). PC MRI represented a good estimation of main flow patterns inside the vasculature, which seems to be acceptable for clinical use. However, limitations of each technique should be considered while interpreting results.

Quantitative Magnetic Resonance Angiography in Internal Carotid Artery Occlusion with Primary Collateral Pathway

Background and Purpose

Quantitative magnetic resonance angiography (Q-MRA) enables direct measurement of volume flow rate (VFR) of intracranial arteries. We aimed to evaluate the collateral flows in internal carotid artery (ICA) occlusion with primary collateral pathway via circle of Willis using Q-MRA, and to compare them between patients who recently developed ipsilateral symptomatic ischemia and those who did not.

Methods

Between 2012 and 2014, 505 patients underwent Q-MRA in our institution. Among these, 33 patients who had unilateral ICA occlusion with primary collateral pathway were identified, and grouped into asymptomatic patients, stable patients with chronic infarction, and symptomatic patients with acute/subacute infarction. Mean VFR (mVFR) in intracranial arteries was measured and compared between the patients’ groups. Kruskal-Wallis test was used for statistical analysis.

Results

Six patients were asymptomatic, fifteen with chronic infarction were stable, and twelve with acute/subacute infarction were symptomatic. The mVFR of ipsilateral middle cerebral artery in symptomatic patients was significantly lower than those in stable or asymptomatic patients (73.7±45.6 mL/min vs. 119.9±36.1 mL/min vs. 121.8±42.0 mL/min; P = 0.04). Total sum of the mVFR of ipsilateral anterior, middle, and posterior cerebral arteries was significantly lower in symptomatic patients than those in other groups (229.3 ± 51.3 mL/min vs. 282.0±68.6 mL/min vs. 314.0±44.4 mL/min; P = 0.02).

Conclusions

Q-MRA could be helpful to demonstrate the difference in the degree of primary collateral flow in ICA occlusion between the patients with recent symptomatic ischemia and those without.

Comparing velocity and fluid shear stress in a stenotic phantom with steady flow: phase-contrast MRI, particle image velocimetry and computational fluid dynamics

OBJECT

This study aims to validate phase-contrast magnetic resonance imaging (PC-MRI) measurements of a steady flow through a severe stenotic phantom using particle image velocimetry (PIV) and computational fluid dynamics (CFD).

MATERIALS AND METHODS

The study was performed in an axisymmetric 87 % area stenosis model using an inlet Reynolds number (Re) of 160, corresponding to a jet Re of 444. Velocity patterns and estimated fluid shear stresses from three modalities were analyzed and compared qualitatively and quantitatively.

RESULTS

Visual analysis via contour subtraction and Bland-Altman plots showed good agreement for flow velocities and less agreement for maximum shear stress (MSS). The Pearson's coefficients of correlation between PC-MRI and PIV were 0.97 for the velocity field and 0.82 for the MSS. The corresponding parameters between PC-MRI and CFD were 0.96 and 0.84, respectively.

CONCLUSION

Findings indicate that PC-MRI can be implemented to estimate velocity flow fields and MSS; however, this method is not sufficiently accurate to quantify the MSS at regions of high shear rate.

Parent artery curvature influences inflow zone location of unruptured sidewall internal carotid artery aneurysms

BACKGROUND AND PURPOSE

Future aneurysmal behaviors or treatment outcomes of cerebral aneurysms may be related to the hemodynamics around the inflow zone. Here we investigated the influence of parent artery curvature on the inflow zone location of unruptured sidewall internal carotid artery aneurysms.

MATERIALS AND METHODS

In 32 aneurysms, the inflow zone location was decided by 4D flow MR imaging, and the radius of the parent artery curvature was measured in 2D on an en face image of the section plane corresponding to the aneurysm orifice.

RESULTS

The inflow zone was on the distal neck in 10 (group 1, 31.3%), on the lateral side in 19 (group 2, 59.4%), and on the proximal neck in 3 (group 3, 9.4%) aneurysms. The radius in group 1 was significantly larger than that in group 2 (8.3 mm [4.5 mm] versus 4.5 mm [1.9 mm]; median [interquartile range]; P < .0001). All 7 aneurysms with a radius of >8.0 mm were in group 1. All 18 aneurysms with a radius of <6.0 mm were in group 2 or 3. In two group 3 aneurysms, the inflow zone was located in a part of the neck extending beyond the central axis of the parent artery.

CONCLUSIONS

The inflow zone locations of sidewall aneurysms can be influenced by the parent artery curvature evaluated in 2D on an en face image of the section plane corresponding to the aneurysm orifice.

Identification of the inflow zone of unruptured cerebral aneurysms: comparison of 4D flow MRI and 3D TOF MRA data

BACKGROUND AND PURPOSE

The hemodynamics of the inflow zone of cerebral aneurysms may be a key factor in coil compaction and recanalization after endovascular coil embolization. We performed 4D flow MR imaging in conjunction with 3D TOF MRA and compared their ability to identify the inflow zone of unruptured cerebral aneurysms.

MATERIALS AND METHODS

This series comprised 50 unruptured saccular cerebral aneurysms in 44 patients. Transluminal color-coded 3D MRA images were created by selecting the signal-intensity ranges on 3D TOF MRA images that corresponded with both the luminal margin and the putative inflow.

RESULTS

4D flow MR imaging demonstrated the inflow zone and yielded inflow velocity profiles for all 50 aneurysms. In 18 of 24 lateral-projection aneurysms (75%), the inflow zone was located distally on the aneurysmal neck. The maximum inflow velocity ranged from 285 to 922 mm/s. On 4D flow MR imaging and transluminal color-coded 3D MRA studies, the inflow zone of 32 aneurysms (64%) was at a similar location. In 91% of aneurysms whose neck section plane angle was <30° with respect to the imaging section direction on 3D TOF MRA, depiction of the inflow zone was similar on transluminal color-coded 3D MRA and 4D flow MR images.

CONCLUSIONS

4D flow MR imaging can demonstrate the inflow zone and provide inflow velocity profiles. In aneurysms whose angle of the neck-section plane is obtuse vis-a-vis the imaging section on 3D TOF MRA scans, transluminal color-coded 3D MRA may depict the inflow zone reliably.

Cerebral Blood Flow in a Healthy Circle of Willis and Two Intracranial Aneurysms: Computational Fluid Dynamics Versus Four-Dimensional Phase-Contrast Magnetic Resonance Imaging

Computational fluid dynamics (CFD) opens up multiple opportunities to investigate the hemodynamics of the human vascular system. However, due to numerous assumptions the acceptance of CFD among physicians is still limited in practice and validation through comparison is mandatory. Time-dependent quantitative phase-contrast magnetic resonance imaging PC-MRI measurements in a healthy volunteer and two intracranial aneurysms were carried out at 3 and 7 Tesla. Based on the acquired images, three-dimensional (3D) models of the aneurysms were reconstructed and used for the numerical simulations. Flow information from the MR measurements were applied as boundary conditions. The four-dimensional (4D) velocity fields obtained by CFD and MRI were qualitatively as well as quantitatively compared including cut planes and vector analyses. For all cases a high similarity of the velocity patterns was observed. Additionally, the quantitative analysis revealed a good agreement between CFD and MRI. Deviations were caused by minor differences between the reconstructed vessel models and the actual lumen. The comparisons between diastole and systole indicate that relative differences between MRI and CFD are intensified with increasing velocity. The findings of this study lead to the conclusion that CFD and MRI agree well in predicting intracranial velocities when realistic geometries and boundary conditions are provided. Due to the considerably higher temporal and spatial resolution of CFD compared to MRI, complex flow patterns can be further investigated in order to evaluate their role with respect to aneurysm formation or rupture. Nevertheless, special care is required regarding the vessel reconstruction since the geometry has a major impact on the subsequent numerical results.

Three-dimensional hemodynamics in intracranial aneurysms: influence of size and morphology

PURPOSE

To use four-dimensional (4D)-flow MRI for the comprehensive in vivo analysis of hemodynamics and its relationship to size and morphology of different intracranial aneurysms (IA). We hypothesize that different IA groups, defined by size and morphology, exhibit different velocity fields, wall shear stress, and vorticity.

MATERIALS AND METHODS

The 4D-flow MRI (spatial resolution = 0.99-1.8 × 0.78-1.46 × 1.2-1.4 mm(3) , temporal resolution = 44-48 ms) was performed in 19 IAs (18 patients, age = 55.4 ± 13.8 years) with saccular (n = 16) and fusiform (n = 3) morphology and different sizes ranging from small (n = 8; largest dimension = 6.2 ± 0.4 mm) to large and giant (n = 11; 25 ± 7 mm). Analysis included quantification of volumetric spatial-temporal velocity distribution, vorticity, and wall shear stress (WSS) along the aneurysm's 3D surface.

RESULTS

The 4D-flow MRI revealed distinct hemodynamic patterns for large/giant saccular aneurysms (Group 1), small saccular aneurysms (Group 2), and large/giant fusiform aneurysms (Group 3). Saccular IA (Groups 1, 2) demonstrated significantly higher peak velocities (P < 0.002) and WSS (P < 0.001) compared with fusiform aneurysms. Although intra-aneurysmal 3D velocity distributions were similar for Group 1 and 2, vorticity and WSS was significantly (P < 0.001) different (increased in Group 1 by 54%) indicating a relationship between IA size and hemodynamics. Group 3 showed reduced velocities (P < 0.001) and WSS (P < 0.001).

CONCLUSION

The 4D-flow MRI demonstrated the influence of lesion size and morphology on aneurysm hemodynamics suggesting the potential of 4D-flow MRI to assist in the classification of individual aneurysms.

In vitro validation of flow measurement with phase contrast MRI at 3 tesla using stereoscopic particle image velocimetry and stereoscopic particle image velocimetry-based computational fluid dynamics

PURPOSE

To validate conventional phase-contrast MRI (PC-MRI) measurements of steady and pulsatile flows through stenotic phantoms with various degrees of narrowing at Reynolds numbers mimicking flows in the human iliac artery using stereoscopic particle image velocimetry (SPIV) as gold standard.

MATERIALS AND METHODS

A series of detailed experiments are reported for validation of MR measurements of steady and pulsatile flows with SPIV and CFD on three different stenotic models with 50%, 74%, and 87% area occlusions at three sites: two diameters proximal to the stenosis, at the throat, and two diameters distal to the stenosis.

RESULTS

Agreement between conventional spin-warp PC-MRI with Cartesian read-out and SPIV was demonstrated for both steady and pulsatile flows with mean Reynolds numbers of 130, 160, and 190 at the inlet by evaluating the linear regression between the two methods. The analysis revealed a correlation coefficient of > 0.99 and > 0.96 for steady and pulsatile flows, respectively. Additionally, it was found that the most accurate measures of flow by the sequence were at the throat of the stenosis (error < 5% for both steady and pulsatile mean flows). The flow rate error distal to the stenosis was primarily found to be a function of narrowing severity including dependence on proper Venc selection.

CONCLUSION

SPIV and CFD provide excellent approaches to in vitro validation of new or existing PC-MRI flow measurement techniques.

Physical factors effecting cerebral aneurysm pathophysiology

Many factors that are either blood-, wall-, or hemodynamics-borne have been associated with the initiation, growth, and rupture of intracranial aneurysms. The distribution of cerebral aneurysms around the bifurcations of the circle of Willis has provided the impetus for numerous studies trying to link hemodynamic factors (flow impingement, pressure, and/or wall shear stress) to aneurysm pathophysiology. The focus of this review is to provide a broad overview of such hemodynamic associations as well as the subsumed aspects of vascular anatomy and wall structure. Hemodynamic factors seem to be correlated to the distribution of aneurysms on the intracranial arterial tree and complex, slow flow patterns seem to be associated with aneurysm growth and rupture. However, both the prevalence of aneurysms in the general population and the incidence of ruptures in the aneurysm population are extremely low. This suggests that hemodynamic factors and purely mechanical explanations by themselves may serve as necessary, but never as necessary and sufficient conditions of this disease's causation. The ultimate cause is not yet known, but it is likely an additive or multiplicative effect of a handful of biochemical and biomechanical factors.

Compound ex vivo and in silico method for hemodynamic analysis of stented arteries

Hemodynamic factors such as low wall shear stress have been shown to influence endothelial healing and atherogenesis in stent-free vessels. However, in stented vessels, a reliable quantitative analysis of such relations has not been possible due to the lack of a suitable method for the accurate acquisition of blood flow. The objective of this work was to develop a method for the precise reconstruction of hemodynamics and quantification of wall shear stress in stented vessels. We have developed such a method that can be applied to vessels stented in or ex vivo and processed ex vivo. Here we stented the coronary arteries of ex vivo porcine hearts, performed vascular corrosion casting, acquired the vessel geometry using micro-computed tomography and reconstructed blood flow and shear stress using computational fluid dynamics. The method yields accurate local flow information through anatomic fidelity, capturing in detail the stent geometry, arterial tissue prolapse, radial and axial arterial deformation as well as strut malapposition. This novel compound method may serve as a unique tool for spatially resolved analysis of the relationship between hemodynamic factors and vascular biology. It can further be employed to optimize stent design and stenting strategies.

Comparison of blood flow velocity quantification by 4D flow MR imaging with ultrasound at the carotid bifurcation

BACKGROUND AND PURPOSE

4D flow MR imaging is an emerging technique that allows visualization and quantification of 3D blood flow in vivo. However, representative studies evaluating its accuracy are lacking. Therefore, we compared blood flow quantification by using 4D flow MR imaging with US within the carotid bifurcation.

MATERIALS AND METHODS

Thirty-two healthy volunteers (age 25.3 ± 3.4 years) and 20 patients with ≥50% ICA stenosis (age 67.7 ± 7.4 years) were examined preoperatively and postoperatively by use of 4D flow MR imaging, with complete coverage of the left and right carotid bifurcation. Blood flow velocities were assessed with standardized 2D analysis planes distributed along the CCA and the ICA and were compared with US at baseline and postoperatively in patients. In addition, we tested reproducibility and interobserver agreement of 4D MR imaging in 10 volunteers.

RESULTS

Overall, 101 CCAs and 79 ICAs were available for comparison. MR imaging underestimated (P < .05) systolic CCA and ICA blood flow velocity by 26% (0.79 ± 0.29 m/s vs 1.06 ± 0.31 m/s) and 19% (0.72 ± 0.21 m/s vs 0.89 ± 0.27 m/s) compared with US. Diastolic blood flow velocities were similar for MR imaging and US (differences, 9% and 3%, respectively; not significant). Reproducibility and interobserver agreement of 4D flow MR imaging was excellent.

CONCLUSIONS

4D flow MR imaging allowed for an accurate measurement of blood flow velocities in the carotid bifurcation of both volunteers and patients with only moderate underestimation compared with US. Thus, 4D flow MR imaging seems promising for a future combination with MRA to comprehensively assess ICA stenosis and related hemodynamic changes.

Quantification of blood flow in internal cerebral artery by optical flow method on digital subtraction angiography in comparison with time-of-flight magnetic resonance angiography

Objective

This study compared data on the blood flow velocity in the internal carotid artery, which was obtained using the optical flow method (OFM) with digital subtraction angiography (DSA) and the time-of-flight (TOF) technique using magnetic resonance angiography (MRA).

Materials and Methods

Images were obtained from 12 cerebrovascular patients who underwent both brain DSA and MRA imaging. The OFM was applied on the DSA images to determine the average blood flow velocity. The calculated results were compared with the values obtained from the TOF-MRA data. A linear fit was performed on the data and Bland-Altman plots were analyzed.

Results

The blood flow velocity was closely associated with vascular diseases. Color-coding of the OFM measurements were superimposed on to the DSA images, which quantitatively illustrated the relative flow in the vessels. The average blood flow velocity was calculated using OFM and DSA, which demonstrated a high correlation with the MRA measurements in the anterior-posterior (AP) view (R = 0.71). In contrast, the average blood flow velocity was low in the lateral view (R = 0.28). The consistency between the high and low blood velocity in the AP view was better compared to the lateral view. The blood flow velocity distribution in the AP view was statistically closer to the MRA measurement compared to the lateral view.

Conclusions

This study evaluated the correlation of blood flow measured using DSA and TOF-MRA in a small heterogeneous group of patients with cerebrovascular lesions. OFM with DSA imaging reveals hemodynamic information and TOF-MRA.

Targeted particle tracking in computational models of human carotid bifurcations

A significant and largely unsolved problem of computational fluid dynamics (CFD) simulation of flow in anatomically relevant geometries is that very few calculated pathlines pass through regions of complex flow. This in turn limits the ability of CFD-based simulations of imaging techniques (such as MRI) to correctly predict in vivo performance. In this work, I present two methods designed to overcome this filling problem, firstly, by releasing additional particles from areas of the flow inlet that lead directly to the complex flow region ("preferential seeding") and, secondly, by tracking particles both "downstream" and "upstream" from seed points within the complex flow region itself. I use the human carotid bifurcation as an example of complex blood flow that is of great clinical interest. Both idealized and healthy volunteer geometries are investigated. With uniform seeding in the inlet plane (in the common carotid artery (CCA)) of an idealized bifurcation geometry, approximately half the particles passed through the internal carotid artery (ICA) and half through the external carotid artery. However, of those particles entering the ICA, only 16% passed directly through the carotid bulb region. Preferential seeding from selected regions of the CCA was able to increase this figure to 47%. In the second method, seeding of particles within the carotid bulb region itself led to a very high proportion (97%) of pathlines running from CCA to ICA. Seeding of particles in the bulb plane of three healthy volunteer carotid bifurcation geometries led to much better filling of the bulb regions than by particles seeded at the inlet alone. In all cases, visualization of the origin and behavior of recirculating particles led to useful insights into the complex flow patterns. Both seeding methods produced significant improvements in filling the carotid bulb region with particle tracks compared with uniform seeding at the inlet and led to an improved understanding of the complex flow patterns. The methods described may be combined and are generally applicable to CFD studies of fluid and gas flow and are, therefore, of relevance in hemodynamics, respiratory mechanics, and medical imaging science.

The Role of 3 Tesla MRA in the Detection of Intracranial Aneurysms

Intracranial aneurysms constitute a common pathological entity, affecting approximately 1–8% of the general population. Their early detection is essential for their prompt treatment. Digital subtraction angiography is considered the imaging method of choice. However, other noninvasive methodologies such as CTA and MRA have been employed in the investigation of patients with suspected aneurysms. MRA is a noninvasive angiographic modality requiring no radiation exposure. However, its sensitivity and diagnostic accuracy were initially inadequate. Several MRA techniques have been developed for overcoming all these drawbacks and for improving its sensitivity. 3D TOF MRA and contrast-enhanced MRA are the most commonly employed techniques. The introduction of 3 T magnetic field further increased MRA's sensitivity, allowing detection of aneurysms smaller than 3 mm. The development of newer MRA techniques may provide valuable information regarding the flow characteristics of an aneurysm. Meticulous knowledge of MRA's limitations and pitfalls is of paramount importance for avoiding any erroneous interpretation of its findings.

Complex flow patterns in a real-size intracranial aneurysm phantom: phase contrast MRI compared with particle image velocimetry and computational fluid dynamics

The aim of this study was to validate the flow patterns measured by high-resolution, time-resolved, three-dimensional phase contrast MRI in a real-size intracranial aneurysm phantom. Retrospectively gated three-dimensional phase contrast MRI was performed in an intracranial aneurysm phantom at a resolution of 0.2 × 0.2 × 0.3 mm(3) in a solenoid rat coil. Both steady and pulsatile flows were applied. The phase contrast MRI measurements were compared with particle image velocimetry measurements and computational fluid dynamics simulations. A quantitative comparison was performed by calculating the differences between the magnitude of the velocity vectors and angles between the velocity vectors in corresponding voxels. Qualitative analysis of the results was executed by visual inspection and comparison of the flow patterns. The root-mean-square errors of the velocity magnitude in the comparison between phase contrast MRI and computational fluid dynamics were 5% and 4% of the maximum phase contrast MRI velocity, and the medians of the angle distribution between corresponding velocity vectors were 16° and 14° for the steady and pulsatile measurements, respectively. In the phase contrast MRI and particle image velocimetry comparison, the root-mean-square errors were 12% and 10% of the maximum phase contrast MRI velocity, and the medians of the angle distribution between corresponding velocity vectors were 19° and 15° for the steady and pulsatile measurements, respectively. Good agreement was found in the qualitative comparison of flow patterns between the phase contrast MRI measurements and both particle image velocimetry measurements and computational fluid dynamics simulations. High-resolution, time-resolved, three-dimensional phase contrast MRI can accurately measure complex flow patterns in an intracranial aneurysm phantom.

Unenhanced dynamic MR angiography: high spatial and temporal resolution by using true FISP-based spin tagging with alternating radiofrequency

PURPOSE

To present an unenhanced four-dimensional time-resolved dynamic magnetic resonance (MR) angiography technique with true fast imaging with steady-state precession-based spin tagging with alternating radiofrequency (STAR), also called TrueSTAR.

MATERIALS AND METHODS

This study received Institutional Review Board approval and was HIPAA compliant. Informed consent was obtained from all study subjects. In eight healthy volunteers, the spatial and temporal resolution of the TrueSTAR technique were optimized. In another six healthy volunteers, the contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) of the TrueSTAR dynamic MR angiography images were compared with those acquired by using a standard Look-Locker echo-planar technique by using the Wilcoxon signed rank test. Finally, one patient with an arteriovenous malformation (AVM) was studied by using this technique.

RESULTS

The SNR and CNR of the TrueSTAR dynamic MR angiography images were 29% and 39% higher, respectively, compared with those acquired by using a standard Look-Locker echo-planar imaging sequence (both P = .028). In the AVM patient, TrueSTAR dynamic MR angiography delineated the dynamic course of labeled blood flowing through feeding arteries into the nidus and draining veins.

CONCLUSION

The results suggest that TrueSTAR is a promising unenhanced dynamic MR angiography technique for clinical evaluation of cerebrovascular disorders such as AVM, steno-occlusive disease, and aneurysm.

Current progress in patient-specific modeling

We present a survey of recent advancements in the emerging field of patient-specific modeling (PSM). Researchers in this field are currently simulating a wide variety of tissue and organ dynamics to address challenges in various clinical domains. The majority of this research employs three-dimensional, image-based modeling techniques. Recent PSM publications mostly represent feasibility or preliminary validation studies on modeling technologies, and these systems will require further clinical validation and usability testing before they can become a standard of care. We anticipate that with further testing and research, PSM-derived technologies will eventually become valuable, versatile clinical tools.