Aneurysm growth occurs at region of low wall shear stress: patient-specific correlation of hemodynamics and growth in a longitudinal study

Hemodynamic changes quantified in abdominal aortic aneurysms with increasing exercise intensity using mr exercise imaging and image-based computational fluid dynamics

Abdominal aortic aneurysm (AAA) is a vascular disease resulting in a permanent, localized enlargement of the abdominal aorta. We previously hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise [Dalman, R. L., M. M. Tedesco, J. Myers, and C. A. Taylor. Ann. N.Y. Acad. Sci. 1085:92-109, 2006]. To quantify the effect of exercise intensity on hemodynamic conditions in 10 AAA subjects at rest and during mild and moderate intensities of lower-limb exercise (defined as 33 ± 10% and 63 ± 18% increase above resting heart rate, respectively), we used magnetic resonance imaging and computational fluid dynamics techniques. Subject-specific models were constructed from magnetic resonance angiography data and physiologic boundary conditions were derived from measurements made during dynamic exercise. We measured the abdominal aortic blood flow at rest and during exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and particle residence time (PRT). We observed that an increase in the level of activity correlated with an increase of MWSS and a decrease of OSI at three locations in the abdominal aorta, and these changes were most significant below the renal arteries. As the level of activity increased, PRT in the aneurysm was significantly decreased: 50% of particles were cleared out of AAAs within 1.36 ± 0.43, 0.34 ± 0.10, and 0.22 ± 0.06 s at rest, mild exercise, and moderate exercise levels, respectively. Most of the reduction of PRT occurred from rest to the mild exercise level, suggesting that mild exercise may be sufficient to reduce flow stasis in AAAs.

Hemodynamics of cerebral aneurysm initiation: the role of wall shear stress and spatial wall shear stress gradient

BACKGROUND AND PURPOSE

Cerebral aneurysms are preferentially located at arterial curvatures and bifurcations that are exposed to major hemodynamic forces, increasingly implicated in the life cycle of aneurysms. By observing the natural history of aneurysm formation from its preaneurysm state, we aimed to examine the hemodynamic microenvironment related to aneurysm initiation at certain arterial segments later developing an aneurysm.

MATERIALS AND METHODS

The 3 patients included in the study underwent cerebral angiography with 3D reconstruction before a true aneurysm developed. The arterial geometries obtained from the 3D-DSA models were used for flow simulation by using finite-volume modeling. The WSS and SWSSG at the site of the future aneurysm and the flow characteristics of the developed aneurysms were analyzed.

RESULTS

The analyzed regions of interest demonstrated significantly increased WSS, accompanied by an increased positive SWSSG in the adjacent proximal region. The WSS reached values of >5 times the temporal average values of the parent vessel, whereas the SWSSG approximated or exceeded peaks of 40 Pa/mm in all 3 cases. All patients developed an aneurysm within 2 years, 1 of which ruptured.

CONCLUSIONS

The results of this hemodynamic study, in accordance with the clinical follow-up, suggest that the combination of high WSS and high positive SWSSG focused on a small segment of the arterial wall may have a role in the initiation process of aneurysm formation.

Aneurysm Volume-to-Ostium Area Ratio

BACKGROUND:

Slow or stagnant flow is a hemodynamic feature that has been linked to the risk of aneurysm rupture.

OBJECTIVE:

To assess the potential value of the ratio of the volume of an aneurysm to the area of its ostium (VOR) as an indicator of intra-aneurysmal slow flow and, thus, in turn, the risk of rupture.

METHODS:

Using a sample defined from internal databases, a retrospective analysis of aneurysm size, aspect ratio (AR), and VOR was performed on a series of 155 consecutive aneurysms having undergone 3-dimensional digital subtraction angiography as a part of their evaluation. Measurements were obtained from 3-dimensional digital subtraction angiography studies using commercial software. Aneurysm size, AR, and VOR were correlated with rupture status (ruptured or unruptured). A multiple logistic regression model that best correlated with rupture status was generated to evaluate which of these parameters was the most useful to discriminate rupture status. This model was validated using an independent database of 62 consecutive aneurysms acquired outside the retrospective study interval.

RESULTS:

VOR showed better discrimination for rupture status than did size and AR. The best logistic regression model, which included VOR rather than size or AR, determined rupture status correctly in 80.6% of subjects. The reproducibility calculating AR and VOR was excellent.

CONCLUSION:

Determination of VOR was easily done and reproducible using widely available commercial equipment. It may be a more robust parameter to discriminate rupture status than AR.

Timing and size of flow impingement in a giant intracranial aneurysm at the internal carotid artery

Flow impingement is regarded as a key factor for aneurysm formation and rupture. Wall shear stress (WSS) is often used to evaluate flow impingement even though WSS and impinging force are in two different directions; therefore, this raises an important question of whether using WSS for evaluation of flow impingement size is appropriate. Flow impinging behavior in a patient-specific model of a giant aneurysm (GA) at the internal carotid artery (ICA) was analyzed by computational fluid dynamics simulations. An Impingement Index (IMI) was used to evaluate the timing and size of flow impingement. In theory, the IMI is related to the WSS gradient, which is known to affect vascular biology of endothelial cells. Effect of non-Newtonian fluid, aneurysm size, and heart rate were also studied. Maximum WSS is found to be proportional to the IMI, but the area of high wall shear is not proportional to the size of impingement. A faster heart rate or larger aneurysm does not produce a larger impinging site, and the Newtonian assumption overestimates the size of impingement. Flow impingement at the dome occurs approximately 0.11 s after the peak of flow waveform is attained. This time delay also increases with aneurysm size and varies with heart rate and waveform.

Hemodynamic-morphologic discriminants for intracranial aneurysm rupture

BACKGROUND AND PURPOSE

the purpose of this study was to identify significant morphological and hemodynamic parameters that discriminate intracranial aneurysm rupture status using 3-dimensional angiography and computational fluid dynamics.

METHODS

one hundred nineteen intracranial aneurysms (38 ruptured, 81 unruptured) were analyzed from 3-dimensional angiographic images and computational fluid dynamics. Six morphological and 7 hemodynamic parameters were evaluated for significance with respect to rupture. Receiver operating characteristic analysis identified area under the curve (AUC) and optimal thresholds separating ruptured from unruptured aneurysms for each parameter. Significant parameters were examined by multivariate logistic regression analysis in 3 predictive models-morphology only, hemodynamics only, and combined-to identify independent discriminants, and the AUC receiver operating characteristic of the predicted probability of rupture status was compared among these models.

RESULTS

morphological parameters (size ratio, undulation index, ellipticity index, and nonsphericity index) and hemodynamic parameters (average wall shear stress [WSS], maximum intra-aneurysmal WSS, low WSS area, average oscillatory shear index, number of vortices, and relative resident time) achieved statistical significance (P<0.01). Multivariate logistic regression analysis demonstrated size ratio to be the only independently significant factor in the morphology model (AUC, 0.83; 95% CI, 0.75 to 0.91), whereas WSS and oscillatory shear index were the only independently significant variables in the hemodynamics model (AUC, 0.85; 95% CI, 0.78 to 0.93). The combined model retained all 3 variables, size ratio, WSS, and oscillatory shear index (AUC, 0.89; 95% CI, 0.82 to 0.96).

CONCLUSIONS

all 3 models-morphological (based on size ratio), hemodynamic (based on WSS and oscillatory shear index), and combined-discriminate intracranial aneurysm rupture status with high AUC values. Hemodynamics is as important as morphology in discriminating aneurysm rupture status.

Role of hemodynamic shear stress in cardiovascular disease

Atherosclerosis is the main cause of morbidity and mortality in the Western world. Inflammation and blood flow alterations are new markers emerging as possible determinants for the development of atherosclerotic lesions. In particular, blood flow exerts a shear stress on vessel walls that alters cell physiology. Shear stress arises from the friction between two virtual layers of a fluid and is induced by the difference in motion and viscosity between these layers. Regions of the arterial tree with uniform geometry are exposed to a unidirectional and constant flow, which determines a physiologic shear stress, while arches and bifurcations are exposed to an oscillatory and disturbed flow, which determines a low shear stress. Atherosclerotic lesions develop mainly in areas of low shear stress, while those exposed to a physiologic shear stress are protected. The presence of areas of the arterial tree with different wall shear stress may explain, in part, the different localization of atherosclerotic lesions in both coronary and extracoronary arteries. The measurement of this parameter may help in identifying atherosclerotic plaques at higher risk as well as in evaluating the efficacy of different pharmacological interventions. Moreover, an altered shear stress is associated with the occurrence of both aortic and intracranial aneurysms, possibly leading to their growth and rupture. Finally, the evaluation of shear stress may be useful for predicting the risk of developing restenosis after coronary and peripheral angioplasty and for devising a coronary stent with a strut design less thrombogenic and more conducive to endothelization.

Toward integrated management of cerebral aneurysms

In the last few years, some of the visionary concepts behind the virtual physiological human began to be demonstrated on various clinical domains, showing great promise for improving healthcare management. In the current work, we provide an overview of image- and biomechanics-based techniques that, when put together, provide a patient-specific pipeline for the management of intracranial aneurysms. The derivation and subsequent integration of morphological, morphodynamic, haemodynamic and structural analyses allow us to extract patient-specific models and information from which diagnostic and prognostic descriptors can be obtained. Linking such new indices with relevant clinical events should bring new insights into the processes behind aneurysm genesis, growth and rupture. The development of techniques for modelling endovascular devices such as stents and coils allows the evaluation of alternative treatment scenarios before the intervention takes place and could also contribute to the understanding and improved design of more effective devices. A key element to facilitate the clinical take-up of all these developments is their comprehensive validation. Although a number of previously published results have shown the accuracy and robustness of individual components, further efforts should be directed to demonstrate the diagnostic and prognostic efficacy of these advanced tools through large-scale clinical trials.

The effect of stents on intra-aneurysmal hemodynamics: in vitro evaluation of a pulsatile sidewall aneurysm using laser Doppler anemometry

INTRODUCTION

Hemodynamic modification by means of flow diversion is increasingly used for treatment of intracranial aneurysms. Despite of promising results, there is still a paucity of methods to reliably predict long-term success of this technique. Laser Doppler anemometry (LDA) can be used to quantify the influence of stents on intra-aneurysmal flow in vitro.

METHODS

All experiments were performed with a pulsatile model of a sidewall aneurysm. A physiologic flow was created with a circulatory experimental setup, and a transparent non-Newtonian glycerol-water solution was used to substitute human blood. Flow velocity was measured with a one-component LDA system, recording flow components parallel and perpendicular to the parent vessel. Three different stents (Solitaire, Silk, Phenox flow diverter) were deployed over the aneurysm neck, respectively.

RESULTS

Flow reduction was 67.59% (inflow zone), 9.65% (dome) and 37.94% (outflow zone) by the Solitaire stent. The Silk stent reduced the flow by 58.15% (inflow zone), 89.06% (dome) and 90.06% (outflow zone). The Phenox flow diverter reduced the flow by 96.76% (inflow zone), 90% (dome) and 90.91% (outflow zone) when positioned with narrow stent struts but increased the velocity of up to seven times compared to the unstented model when placed with loose strut packing in the proximal part of the aneurysm.

CONCLUSION

LDA is a feasible method to quantify intra-aneurysmal flow and flow reduction efficacy of stents in vitro. Flow reduction was negligible with a standard self-expanding stent. For dedicated flow diverters, it depended both on stent design and on appropriate positioning.

The critical role of hemodynamics in the development of cerebral vascular disease

Atherosclerosis and intracranial saccular aneurysms predictably localize in areas with complex arterial geometries such as bifurcations and curvatures. These sites are characterized by unique hemodynamic conditions that possibly influence the risk for these disorders. One hemodynamic parameter in particular has emerged as a key regulator of vascular biology—wall shear stress (WSS). Variations in geometry can change the distribution and magnitude of WSS, thus influencing the risk for vascular disorders. Computer simulations conducted using patient-specific data have suggested that departures from normal levels of WSS lead to aneurysm formation and progression. In addition, multiple studies indicate that disturbed flow and low WSS predispose patients to extracranial atherosclerosis, and particularly to carotid artery disease. Conversely, in the case of intracranial atherosclerosis, more studies are needed to provide a firm link between hemodynamics and atherogenesis. The recognition of WSS as an important factor in cerebral vascular disease may help to identify individuals at risk and guide treatment options.

Aneurysm inflow-angle as a discriminant for rupture in sidewall cerebral aneurysms: morphometric and computational fluid dynamic analysis

BACKGROUND AND PURPOSE

The ability to discriminate between ruptured and unruptured cerebral aneurysms on a morphological basis may be useful in clinical risk stratification. The objective was to evaluate the importance of inflow-angle (IA), the angle separating parent vessel and aneurysm dome main axes.

METHODS

IA, maximal dimension, height-width ratio, and dome-neck aspect ratio were evaluated in sidewall-type aneurysms with respect to rupture status in a cohort of 116 aneurysms in 102 patients. Computational fluid dynamic analysis was performed in an idealized model with variational analysis of the effect of IA on intra-aneurysmal hemodynamics.

RESULTS

Univariate analysis identified IA as significantly more obtuse in the ruptured subset (124.9 degrees+/-26.5 degrees versus 105.8 degrees+/-18.5 degrees, P=0.0001); similarly, maximal dimension, height-width ratio, and dome-neck aspect ratio were significantly greater in the ruptured subset; multivariate logistic regression identified only IA (P=0.0158) and height-width ratio (P=0.0017), but not maximal dimension or dome-neck aspect ratio, as independent discriminants of rupture status. Computational fluid dynamic analysis showed increasing IA leading to deeper migration of the flow recirculation zone into the aneurysm with higher peak flow velocities and a greater transmission of kinetic energy into the distal portion of the dome. Increasing IA resulted in higher inflow velocity and greater wall shear stress magnitude and spatial gradients in both the inflow zone and dome.

CONCLUSIONS

Inflow-angle is a significant discriminant of rupture status in sidewall-type aneurysms and is associated with higher energy transmission to the dome. These results support inclusion of IA in future prospective aneurysm rupture risk assessment trials.

Flow residence time and regions of intraluminal thrombus deposition in intracranial aneurysms

Thrombus formation in intracranial aneurysms, while sometimes stabilizing lesion growth, can present additional risk of thrombo-embolism. The role of hemodynamics in the progression of aneurysmal disease can be elucidated by patient-specific computational modeling. In our previous work, patient-specific computational fluid dynamics (CFD) models were constructed from MRI data for three patients who had fusiform basilar aneurysms that were thrombus-free and then proceeded to develop intraluminal thrombus. In this study, we investigated the effect of increased flow residence time (RT) by modeling passive scalar advection in the same aneurysmal geometries. Non-Newtonian pulsatile flow simulations were carried out in base-line geometries and a new postprocessing technique, referred to as "virtual ink" and based on the passive scalar distribution maps, was used to visualize the flow and estimate the flow RT. The virtual ink technique clearly depicted regions of flow separation. The flow RT at different locations adjacent to aneurysmal walls was calculated as the time the virtual ink scalar remained above a threshold value. The RT values obtained in different areas were then correlated with the location of intra-aneurysmal thrombus observed at a follow-up MR study. For each patient, the wall shear stress (WSS) distribution was also obtained from CFD simulations and correlated with thrombus location. The correlation analysis determined a significant relationship between regions where CFD predicted either an increased RT or low WSS and the regions where thrombus deposition was observed to occur in vivo. A model including both low WSS and increased RT predicted thrombus-prone regions significantly better than the models with RT or WSS alone.

The effect of aneurysm geometry on the intra-aneurysmal flow condition

Introduction

Various anatomical parameters affect on intra-aneurysmal hemodynamics. Nevertheless, how the shapes of real patient aneurysms affect on their intra-aneurysmal hemodynamics remains unanswered.

Methods

Quantitative computational fluid dynamics simulation was conducted using eight patients’ angiograms of internal carotid artery–ophthalmic artery aneurysms. The mean size of the intracranial aneurysms was 11.5 mm (range 5.8 to 19.9 mm). Intra-aneurysmal blood flow velocity and wall shear stress (WSS) were collected from three measurement planes in each aneurysm dome. The correlation coefficients (r) were obtained between hemodynamic values (flow velocity and WSS) and the following anatomical parameters: averaged dimension of aneurysm dome, the largest aneurysm dome dimension, aspect ratio, and dome–neck ratio.

Results

Negative linear correlations were observed between the averaged dimension of aneurysm dome and intra-aneurysmal flow velocity (r = −0.735) and also WSS (r = −0.736). The largest dome diameter showed a negative correlation with intra-aneurysmal flow velocity (r = −0.731) and WSS (r = −0.496). The aspect ratio demonstrated a weak negative correlation with the intra-aneurysmal flow velocity (r = −0.381) and WSS (r = −0.501). A clear negative correlation was seen between the intra-aneurysmal flow velocity and the dome–neck ratio (r = −0.708). A weak negative correlation is observed between the intra-aneurysmal WSS and the dome–neck ratio (r = −0.392).

Conclusion

The aneurysm dome size showed a negative linear correlation with intra-aneurysmal flow velocity and WSS. Wide-necked aneurysm geometry was associated with faster intra-aneurysmal flow velocity.

Validation of the murine aortic arch as a model to study human vascular diseases

Although the murine thoracic aorta and its main branches are widely studied to gain more insight into the pathogenesis of human vascular diseases, detailed anatomical data on the murine aorta are sparse. Moreover, comparative studies between mice and men focusing on the topography and geometry of the heart and aorta are lacking. As this hampers the validation of murine vascular models, the branching pattern of the murine thoracic aorta was examined in 30 vascular corrosion casts. On six casts the intrathoracic position of the heart was compared with that of six younger and six older men of whom contrast-enhanced computer tomography images of the thorax were three-dimensionally reconstructed. In addition, the geometry of the human thoracic aorta was compared with that of the mouse by reconstructing micro-computer tomography images of six murine casts. It was found that the right brachiocephalic trunk, left common carotid artery and left subclavian artery branched subsequently from the aortic arch in both mice and men. The geometry of the branches of the murine aortic arch was quite similar to that of men. In both species the initial segment of the aorta, comprising the ascending aorta, aortic arch and cranial/superior part of the descending aorta, was sigmoidally curved on a cranial/superior view. Although some analogy between the intrathoracic position of the murine and human heart was observed, the murine heart manifestly deviated more ventrally. The major conclusion of this study is that, in both mice and men, the ascending and descending aorta do not lie in a single vertical plane (non-planar aortic geometry). This contrasts clearly with most domestic mammals in which a planar aortic pattern is present. As the vascular branching pattern of the aortic arch is also similar in mice and men, the murine model seems valuable to study human vascular diseases.

Quantification of hemodynamics in abdominal aortic aneurysms during rest and exercise using magnetic resonance imaging and computational fluid dynamics

Abdominal aortic aneurysms (AAAs) affect 5-7% of older Americans. We hypothesize that exercise may slow AAA growth by decreasing inflammatory burden, peripheral resistance, and adverse hemodynamic conditions such as low, oscillatory shear stress. In this study, we use magnetic resonance imaging and computational fluid dynamics to describe hemodynamics in eight AAAs during rest and exercise using patient-specific geometric models, flow waveforms, and pressures as well as appropriately resolved finite-element meshes. We report mean wall shear stress (MWSS) and oscillatory shear index (OSI) at four aortic locations (supraceliac, infrarenal, mid-aneurysm, and suprabifurcation) and turbulent kinetic energy over the entire computational domain on meshes containing more than an order of magnitude more elements than previously reported results (mean: 9.0-million elements; SD: 2.3 M; range: 5.7-12.0 M). MWSS was lowest in the aneurysm during rest 2.5 dyn/cm(2) (SD: 2.1; range: 0.9-6.5), and MWSS increased and OSI decreased at all four locations during exercise. Mild turbulence existed at rest, while moderate aneurysmal turbulence was present during exercise. During both rest and exercise, aortic turbulence was virtually zero superior to the AAA for seven out of eight patients. We postulate that the increased MWSS, decreased OSI, and moderate turbulence present during exercise may attenuate AAA growth.

Gene Expression Profiles in Human Ruptured and Unruptured Intracranial Aneurysms

Background and Purpose— Mechanisms underlying development and rupture of intracranial aneurysms (IA) are poorly recognized. The majority of studies on human tissue have focused on predefined pathways. We sought to analyze global gene expression patterns of ruptured IA, unruptured IA, and control vessels.

Methods— Transcription profiles were studied in human ruptured (n=8) and unruptured (n=6) IA, as well as in control intracranial arteries (n=5), using oligonucleotide microarrays. Real-time reverse-transcription polymerase chain reaction was used for confirmation. Functional analysis for determination of over-represented ontological groups among gene expression profiles was also performed.

Results— The expression of 159 genes differed among the studied groups. Compared to the controls, 131 genes showed common directions of change in both IA groups. The most impacted biological processes for IA are: (1) the muscle system; (2) cell adhesion (downregulation); and (3) the immune system and inflammatory response (upregulation). Ruptured and unruptured IA differed in genes involved in immune/inflammatory processes; expression was reduced in ruptured IA.

Conclusions— Decreased expression of genes related to muscle system and cell adhesion is important for the development of IA. The role of immune/inflammatory processes is unclear. Inflammation may participate in the healing process within IA while playing a protective role against IA rupture.

Effects of smoking and hypertension on wall shear stress and oscillatory shear index at the site of intracranial aneurysm formation

OBJECTIVE

The mechanisms by which smoking and hypertension lead to increased incidence of intracranial aneurysm (IA) formation remain poorly understood. The current study investigates the effects of these risk factors on wall shear stress (WSS) and oscillatory shear index (OSI) at the site of IA initiation.

METHODS

Two (n=2) IAs from two patients with history of smoking and hypertension were artificially removed with the help of software @neuFuse (Supercomputing Solutions, Bologna, Italy) and the vessel geometry reconstructed to mimic the condition prior to IA formation. Two computational fluid dynamics (CFD) analyses were performed on each data-set by using in turn the normal physiological values of blood viscosity (BV), and high BV values specific to smoking and hypertension, obtained from literature.

RESULTS

At normal BV, high WSS (>15 Pa) was observed at the site of IA initiation in both patients. When BV values specific to smoking and hypertension were used, both the areas affected by high WSS (>15 Pa) and the maximum WSS were increased whilst the magnitude and distribution of OSI showed no significant change.

CONCLUSIONS

Long-term exposure to high WSS may result in an increased risk of IA development. An incremental increase in areas of high WSS observed secondary to smoking and hypertension may indicate a further increase in the risk of IA initiation. Interestingly, the relationship between BV and the area of increased WSS was not linear, reflecting the need for patient-specific CFD analysis.

Flow instability and wall shear stress variation in intracranial aneurysms

We investigate the flow dynamics and oscillatory behaviour of wall shear stress (WSS) vectors in intracranial aneurysms using high resolution numerical simulations. We analyse three representative patient-specific internal carotid arteries laden with aneurysms of different characteristics: (i) a wide-necked saccular aneurysm, (ii) a narrower-necked saccular aneurysm, and (iii) a case with two adjacent saccular aneurysms. Our simulations show that the pulsatile flow in aneurysms can be subject to a hydrodynamic instability during the decelerating systolic phase resulting in a high-frequency oscillation in the range of 20-50 Hz, even when the blood flow rate in the parent vessel is as low as 150 and 250 ml min(-1) for cases (iii) and (i), respectively. The flow returns to its original laminar pulsatile state near the end of diastole. When the aneurysmal flow becomes unstable, both the magnitude and the directions of WSS vectors fluctuate at the aforementioned high frequencies. In particular, the WSS vectors around the flow impingement region exhibit significant spatio-temporal changes in direction as well as in magnitude.

ATP transport in saccular cerebral aneurysms at arterial bends

ATP acts as an extracellular signaling molecule in purinergic signaling that regulates vascular tone. ATP binds purinergic P2 nucleotide receptors on endothelial cells. Understanding the mass transport of ATP to endothelial cells by blood flow is thus important to predict functional changes in aneurysmal walls. While some clinical observations indicate a difference of wall pathology between ruptured and unruptured aneurysms, no study has focused on the mass transport in aneurysms. We investigated the characteristics of ATP concentration at aneurysmal wall using a numerical model of ATP transport in aneurysms formed at arterial bends. The magnitude of ATP concentration at the aneurysmal wall was significantly smaller than that at the arterial wall. In particular, significantly low concentration was predicted at the proximal side of the aneurysmal sac. A strong correlation was revealed between the inflow flux at the aneurysmal neck and the resultant concentration at the aneurysmal wall.

In vivo hemodynamic analysis of intracranial aneurysms obtained by magnetic resonance fluid dynamics (MRFD) based on time-resolved three-dimensional phase-contrast MRI

INTRODUCTION

Hemodynamics is thought to play a very important role in the initiation, growth, and rupture of intracranial aneurysms. The purpose of our study was to perform in vivo hemodynamic analysis of unruptured intracranial aneurysms of magnetic resonance fluid dynamics using time-resolved three-dimensional phase-contrast MRI (4D-Flow) at 1.5 T and to analyze relationships between hemodynamics and wall shear stress (WSS) and oscillatory shear index (OSI).

METHODS

This study included nine subjects with 14 unruptured aneurysms. 4D-Flow was performed by a 1.5-T magnetic resonance scanner with a head coil. We calculated in vivo streamlines, WSS, and OSI of intracranial aneurysms based on 4D-Flow with our software. We evaluated the number of spiral flows in the aneurysms and compared the differences in WSS or OSI between the vessel and aneurysm and between whole aneurysm and the apex of the spiral flow.

RESULTS

3D streamlines, WSS, and OSI distribution maps in arbitrary direction during the cardiac phase were obtained for all intracranial aneurysms. Twelve aneurysms had one spiral flow each, and two aneurysms had two spiral flows each. The WSS was lower and the OSI was higher in the aneurysm compared to the vessel. The apex of the spiral flow had a lower WSS and higher OSI relative to the whole aneurysm.

CONCLUSION

Each intracranial aneurysm in this study had at least one spiral flow. The WSS was lower and OSI was higher at the apex of the spiral flow than the whole aneurysmal wall.

Comparison of hemodynamics of intracranial aneurysms between MR fluid dynamics using 3D cine phase-contrast MRI and MR-based computational fluid dynamics

INTRODUCTION

Hemodynamics is thought to play a very important role in the initiation, growth, and rupture of intracranial aneurysms. The purpose of our study was to compare hemodynamics of intracranial aneurysms of MR fluid dynamics (MRFD) using 3D cine PC MR imaging (4D-Flow) at 1.5 T and MR-based computational fluid dynamics (CFD).

METHODS

4D-Flow was performed for five intracranial aneurysms by a 1.5 T MR scanner. 3D TOF MR angiography was performed for geometric information. The blood flow in the aneurysms was modeled using CFD simulation based on the finite element method. We used MR angiographic data as the vascular models and MR flow information as boundary conditions in CFD. 3D velocity vector fields, 3D streamlines, shearing velocity maps, wall shear stress (WSS) distribution maps and oscillatory shear index (OSI) distribution maps were obtained by MRFD and CFD and were compared.

RESULTS

There was a moderate to high degree of correlation in 3D velocity vector fields and a low to moderate degree of correlation in WSS of aneurysms between MRFD and CFD using regression analysis. The patterns of 3D streamlines were similar between MRFD and CFD. The small and rotating shearing velocities and higher OSI were observed at the top of the spiral flow in the aneurysms. The pattern and location of shearing velocity in MRFD and CFD were similar. The location of high oscillatory shear index obtained by MRFD was near to that obtained by CFD.

CONCLUSION

MRFD and CFD of intracranial aneurysms correlated fairly well.

Three-dimensional analysis of segmental wall shear stress in the aorta by flow-sensitive four-dimensional-MRI

PURPOSE

To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta.

MATERIALS AND METHODS

Thirty-one healthy volunteers (mean age = 23.7 +/- 3.3 years) were examined by flow-sensitive four-dimensional (4D)-MRI at 3T. For eight retrospectively positioned 2D analysis planes distributed along the thoracic aorta, flow parameters and vectorial WSS and OSI were assessed in 12 segments along the vascular circumference.

RESULTS

Mean absolute time-averaged WSS ranged between 0.25 +/- 0.04 N/m(2) and 0.33 +/- 0.07 N/m(2) and incorporated a substantial circumferential component (-0.05 +/- 0.04 to 0.07 +/- 0.02 N/m(2)). For each analysis plane, a segment with lowest absolute WSS and highest OSI was identified which differed significantly from mean values within the plane (P < 0.05). The distribution of atherogenic low WSS and high OSI closely resembled typical locations of atherosclerotic lesions at the inner aortic curvature and supraaortic branches.

CONCLUSION

The normal distribution of vectorial WSS and OSI in the entire thoracic aorta derived from flow-sensitive 4D-MRI data provides a reference constituting an important perquisite for the examination of patients with aortic disease. Marked regional differences in absolute WSS and OSI may help explaining why atherosclerotic lesions predominantly develop and progress at specific locations in the aorta.

The role of computational fluid dynamics in the management of unruptured intracranial aneurysms: a clinicians' view

Objective. The importance of hemodynamics in the etiopathogenesis of intracranial aneurysms (IAs) is widely accepted. Computational fluid dynamics (CFD) is being used increasingly for hemodynamic predictions. However, alogn with the continuing development and validation of these tools, it is imperative to collect the opinion of the clinicians. Methods. A workshop on CFD was conducted during the European Society of Minimally Invasive Neurological Therapy (ESMINT) Teaching Course, Lisbon, Portugal. 36 delegates, mostly clinicians, performed supervised CFD analysis for an IA, using the @neuFuse software developed within the European project @neurIST. Feedback on the workshop was collected and analyzed. The performance was assessed on a scale of 1 to 4 and, compared with experts' performance. Results. Current dilemmas in the management of unruptured IAs remained the most important motivating factor to attend the workshop and majority of participants showed interest in participating in a multicentric trial. The participants achieved an average score of 2.52 (range 0–4) which was 63% (range 0–100%) of an expert user. Conclusions. Although participants showed a manifest interest in CFD, there was a clear lack of awareness concerning the role of hemodynamics in the etiopathogenesis of IAs and the use of CFD in this context. More efforts therefore are required to enhance understanding of the clinicians in the subject.

Advances in Interventional Neuroradiology

In 2008 we witnessed a rapid advancement in stent technology, which is reflected in the high number of case reports, publications of case series, and randomized trials. Stents not only served for a combined intrasaccular and extrasaccular treatment of challenging aneurysms but also assisted the revascularization in acute and chronic ischemic conditions of the neurovascular system. Although a self-expanding nitinol semiopen cell stent is currently used for intracranial occlusive disease, a new retrievable closed-cell designed stent is widely used for aneurysms because of its easy delivery through a microcatheter in frequently tortuous head and neck as well as cerebrovascular circulation ( Figure 1 ). However, despite numerous publications in the field, the widespread acceptance of the use of stents to routinely treat carotid stenosis awaits the results of the multicenter randomized clinical trials that should be available in 2009. The role of interventional neuroradiology in the treatment of acute ischemic stroke continues to expand and excite interest.

Quantitative hemodynamic analysis of brain aneurysms at different locations

BACKGROUND AND PURPOSE

Studies have shown that the occurrence of brain aneurysms and risk of rupture vary between locations. However, the reason that aneurysms at different branches of the cerebral arteries have different clinical presentations is not clear. Because research has indicated that aneurysm hemodynamics may be one of the important factors related to aneurysm growth and rupture, our aim was to analyze and compare the flow parameters in aneurysms at different locations.

MATERIALS AND METHODS

A total of 24 patient-specific aneurysm models were constructed by using 3D rotational angiographic data for the hemodynamic simulation. Previously developed computational fluid dynamics software was applied to each aneurysm to simulate the blood-flow properties. Hemodynamic data at peak pulsatile flow were recorded, and wall shear stress (WSS) and flow rate in the aneurysms and parent arteries were quantitatively compared. To validate our method, a comparison with a previously reported technique was also performed.

RESULTS

WSS and flow rate in the aneurysms at the peak of the cardiac cycle were found to differ in magnitude between different locations. Multiple comparisons among locations showed higher WSS and flow rate in middle cerebral artery aneurysms and lower WSS and flow rate in basilar artery and anterior communicating artery aneurysms.

CONCLUSIONS

We observed changes in hemodynamic values that may be related to aneurysm location. Further study of aneurysm locations with a large number of cases is needed to test this hypothesis.

Patient-specific modeling of cardiovascular mechanics

Advances in numerical methods and three-dimensional imaging techniques have enabled the quantification of cardiovascular mechanics in subject-specific anatomic and physiologic models. Patient-specific models are being used to guide cell culture and animal experiments and test hypotheses related to the role of biomechanical factors in vascular diseases. Furthermore, biomechanical models based on noninvasive medical imaging could provide invaluable data on the in vivo service environment where cardiovascular devices are employed and on the effect of the devices on physiologic function. Finally, patient-specific modeling has enabled an entirely new application of cardiovascular mechanics, namely predicting outcomes of alternate therapeutic interventions for individual patients. We review methods to create anatomic and physiologic models, obtain properties, assign boundary conditions, and solve the equations governing blood flow and vessel wall dynamics. Applications of patient-specific models of cardiovascular mechanics are presented, followed by a discussion of the challenges and opportunities that lie ahead.