19 research outputs found
Quantification of hemodynamic changes induced by virtual placement of multiple stents across a wide -necked Basilar trunk aneurysm
OBJECTIVE: The porous intravascular stents that are currently available may not cause
complete aneurysm thrombosis and may therefore fail to provide durable protection
against aneurysm rupture when used as a sole treatment modality. The goal of this study
was to quantify the effects of porous stents on aneurysm hemodynamics using computational fluid dynamics.
METHODS: The geometry of a wide-necked saccular basilar trunk aneurysm was reconstructed from a patient’s computed tomographic angiography images. Three commercial stents (Neuroform2; Boston Scientific/Target, San Leandro, CA; Wingspan; Boston
Scientific, Fremont, CA; and Vision; Guidant Corp., Santa Clara, CA) were modeled.
Various combinations of one to three stents were virtually conformed to fit into the vessel lumen and placed across the aneurysm orifice. An unstented aneurysm served as a
control. Computational fluid dynamics analysis was performed to calculate the hemodynamic parameters considered important in aneurysm pathogenesis and thrombosis
for each of the models.
RESULTS: The complex flow pattern observed in the unstented aneurysm was suppressed by stenting. Stent placement lowered the wall shear stress in the aneurysm,
and this effect was increased by additional stent deployment. Turnover time was moderately increased after single- and double-stent placement and markedly increased after
three stents were placed. The influence of stent design on hemodynamic parameters
was more significant in double-stented models than in other models.
CONCLUSION: Aneurysm hemodynamic parameters were significantly modified
by placement of multiple stents. Because the associated modifications may be helpful as well as harmful in terms of rupture risk, use of this technique requires careful
consideration
Comparison of two stents in modifying cerebral aneurysm hemodynamics
There is a general lack of quantitative understanding about how specific design features of
endovascular stents (struts and mesh design, porosity) affect the hemodynamics in intracranial
aneurysms. To shed light on this issue, we studied two commercial high-porosity stents (Tristar
stent™ and Wallstent®) in aneurysm models of varying vessel curvature as well as in a patientspecific model using Computational Fluid Dynamics. We investigated how these stents modify
hemodynamic parameters such as aneurysmal inflow rate, stasis, and wall shear stress, and how such
changes are related to the specific designs. We found that the flow damping effect of stents and
resulting aneurysmal stasis and wall shear stress are strongly influenced by stent porosity, strut
design, and mesh hole shape. We also confirmed that the damping effect is significantly reduced at
higher vessel curvatures, which indicates limited usefulness of high-porosity stents as a stand-alone
treatment. Finally, we showed that the stasis-inducing performance of stents in 3D geometries can
be predicted from the hydraulic resistance of their flat mesh screens. From this, we propose a
methodology to cost-effectively compare different stent designs before running a full 3D simulation
Saccular aneurysms on straight and curved vessels are subject to different hemodynamics: Implications of intravascular stenting
Our aim was to examine hemodynamic implications of intravascular stenting in the canine venous pouch (sidewall or straight-vessel) and rabbit elastase (curved-vessel) aneurysm models. Flow dynamics in stented (Wallstent) and nonstented versions were studied by using computational fluid dynamics simulations and in vitro flow visualization, with a focus on stent placement effects on aneurysmal flow stagnancy and flow impingement. Results show that sidewall and curved aneurysm models have fundamentally different hemodynamics (shear-driven versus inertia-driven) and thus stent placement outcomes
Validation of CFD simulations of cerebral aneurysms with implication of geometric variations
Background. Computational fluid dynamics (CFD) simulations using medical-image-based anatomical vascular geometry are now gaining clinical relevance. This study aimed at validating the CFD methodology for studying cerebral aneurysms by using particle image velocimetry (PIV) measurements, with a focus on the effects of small geometric variations in aneurysm models on the flow dynamics obtained with CFD. Method of Approach. An experimental phantom was fabricated out of silicone elastomer to best mimic a spherical aneurysm model. PIV measurements were obtained from the phantom and compared with the CFD results from an ideal spherical aneurysm model (S1). These measurements were also compared with CFD results, based on the geometry reconstructed from three-dimensional images of the experimental phantom. We further performed CFD analysis on two geometric variations, S2 and S3, of the phantom to investigate the effects of small geometric variations on the aneurysmal flow field. Results. We found poor agreement between the CFD results from the ideal spherical aneurysm model and the PIV measurements from the phantom, including inconsistent secondary flow patterns. The CFD results based on the actual phantom geometry, however, matched well with the PIV measurements. CFD of models S2 and S3 produced qualitatively similar flow fields to that of the phantom but quantitatively significant changes in key hemodynamic parameters such as vorticity, positive circulation, and wall shear stress. Conclusion. CFD simulation results can closely match experimental measurements as long as both are performed on the same model geometry. Small geometric variations on the aneurysm model can significantly alter the flow-field and key hemodynamic parameters. Since medical images are subjected to geometric uncertainties, image-based patient-specific CFD results must be carefully scrutinized before providing clinical feedback
Morphology parameters for intracranial aneurysm rupture risk assessment
OBJECTIVE—The aim of this study is to identify image-based morphological parameters that
correlate with human intracranial aneurysm (IA) rupture.
METHODS—For 45 patients with terminal or sidewall saccular IAs (25 unruptured, 20 ruptured),
three-dimensional geometries were evaluated for a range of morphological parameters. In addition
to five previously studied parameters (aspect ratio, aneurysm size, ellipticity index, nonsphericity
index, and undulation index), we defined three novel parameters incorporating the parent vessel
geometry (vessel angle, aneurysm [inclination] angle, and [aneurysm-to-vessel] size ratio) and
explored their correlation with aneurysm rupture. Parameters were analyzed with a two-tailed
independent Student's t test for significance; significant parameters (P < 0.05) were further examined
by multivariate logistic regression analysis. Additionally, receiver operating characteristic analyses
were performed on each parameter.
RESULTS—Statistically significant differences were found between mean values in ruptured and
unruptured groups for size ratio, undulation index, nonsphericity index, ellipticity index, aneurysm
angle, and aspect ratio. Logistic regression analysis further revealed that size ratio (odds ratio, 1.41;
95% confidence interval, 1.03−1.92) and undulation index (odds ratio, 1.51; 95% confidence interval,
1.08−2.11) had the strongest independent correlation with ruptured IA. From the receiver operating
characteristic analysis, size ratio and aneurysm angle had the highest area under the curve values of
0.83 and 0.85, respectively.
CONCLUSION—Size ratio and aneurysm angle are promising new morphological metrics for IA
rupture risk assessment. Because these parameters account for vessel geometry, they may bridge the
gap between morphological studies and more qualitative location-based studies
Time-series hyperpolarized xenon-129 MRI of lobar lung ventilation of COPD in comparison to V/Q-SPECT/CT and CT
Purpose To derive lobar ventilation in patients with chronic obstructive pulmonary disease (COPD) using a rapid time-series hyperpolarized xenon-129 (HPX) magnetic resonance imaging (MRI) technique and compare this to ventilation/perfusion singlephoton emission computed tomography (V/Q-SPECT), correlating the results with high-resolution computed tomography (CT) and pulmonary function tests (PFTs).Materials and methods Twelve COPD subjects (GOLD stages I–IV) participated in this study and underwent HPX-MRI, V/QSPECT/CT, high-resolution CT, and PFTs. HPX-MRI was performed using a novel time-series spiral k-space sampling approach. Relative percentage ventilations were calculated for individual lobe for comparison to the relative SPECT lobar ventilation and perfusion. The absolute HPX-MRI percentage ventilation in each lobe was compared to the absolute CT percentage emphysema score calculated using a signal threshold method. Pearson’s correlation and linear regression tests were performed to compare each imaging modality.Results Strong correlations were found between the relative lobar percentage ventilation with HPX-MRI and percentage ventilation SPECT (r = 0.644; p Conclusion Lobar ventilation with HPX-MRI showed a strong correlation with lobar ventilation and perfusion measurements derived from SPECT/CT, and is better than the emphysema score obtained with high-resolution CT.</div
A computational model of the cerebrospinal fluid system incorporating lumped-parameter cranial compartment and one-dimensional distributed spinal compartment
The dynamic transmission of pressure through
the cerebro-circulatory system may play a role in the
genesis of pathological conditions of the brain and spinal
cord. This study aims to lay down the foundations for
computer modelling of the cerebrospinal (CSF) pressure
dynamics in the cranio-spinal cavity as a single entity. The
cerebro-vascular system was modelled as a set of resistors
and capacitors. The model of the CSF space comprised a
lumped cranial compartment and a distributed spinal
compartment. Apart from simulating normal (baseline)
conditions, the effects of jugular vein compression, and
thoracic pressure elevation by coughing were investigated
by applying pressure waveforms at the appropriate points
in the model. The Chiari malformation was simulated by
assigning high resistance to the circulation of the CSF
between the cranium and the spine. The model was capable
of reproducing physiologically plausible results for all
forms of excitation. The spinal cavity behaved effectively
as a lumped compartment, except for the cough excitation
where wave-type behaviour was evident. In that case, the
Chiari obstruction resulted in prolonged periodic straining
of the spinal cord. This result can be of significance for
understanding the mechanism of the formation of cysts in
the spinal cord
A One-Dimensional Model of the Spinal Cerebrospinal-Fluid Compartment
Modeling of the cerebrospinal fluid (CSF) system in the spine is strongly motivated by the need to understand the origins of pathological conditions such as the emergence and growth of fluid-filled cysts in the spinal cord. In this study, a one-dimensional (1D) approximation for the flow in elastic conduits was used to formulate a model of the spinal CSF compartment. The modeling was based around a coaxial geometry in which the inner elastic cylinder represented the spinal cord, middle elastic tube represented the dura, and the outermost tube represented the vertebral column. The fluid-filled annuli between the cord and dura, and the dura and vertebral column, represented the subarachnoid and epidural spaces, respectively. The system of governing equations was constructed by applying a 1D form of mass and momentum conservation to all segments of the model. The developed 1D model was used to simulate CSF pulse excited by pressure disturbances in the subarachnoid and epidural spaces. The results were compared to those obtained from an equivalent two-dimensional finite element (FE) model which was implemented using a commercial software package. The analysis of linearized governing equations revealed the existence of three types of waves, of which the two slower waves can be clearly related to the wave modes identified in previous similar studies. The third, much faster, wave emanates directly from the vertebral column and has little effect on the deformation of the spinal cord. The results obtained from the 1D model and its FE counterpart were found to be in good general agreement even when sharp spatial gradients of the spinal cord stiffness were included; both models predicted large radial displacements of the cord at the location of an initial cyst. This study suggests that 1D modeling, which is computationally inexpensive and amenable to coupling with the models of the cranial CSF system, should be a useful approach for the analysis of some aspects of the CSF dynamics in the spine. The simulation of the CSF pulse excited by a pressure disturbance in the epidural space, points to the possibility that regions of the spinal cord with abnormally low stiffness may be prone to experiencing large strains due to coughing and sneezing
Anterior pulmonary ventilation abnormalities in COVID-19
Anterior pulmonary ventilation abnormalities in COVID-1
Gas exchange and ventilation imaging of healthy and COPD subjects using hyperpolarized xenon-129 MRI and a 3D alveolar gas-exchange model
Objectives: To investigate the utility of hyperpolarized xenon-129 (HPX) gas-exchange magnetic resonance imaging (MRI) and modeling in a chronic obstructive pulmonary disease (COPD) cohort in comparison to a minimal CT–diagnosed emphysema (MCTE) cohort and a healthy cohort. Methods: A total of 25 subjects were involved in this study including COPD (n = 8), MCTE (n = 3), and healthy (n = 14) subjects. The COPD subjects were scanned using HPX ventilation, gas-exchange MRI, and volumetric CT. The healthy subjects were scanned using the same HPX gas-exchange MRI protocol with 9 of them scanned twice, 3 weeks apart. The coefficient of variation (CV) was used to quantify image heterogeneities. A three-dimensional computational fluid dynamic (CFD) model of gas exchange was used to derive functional volumes of pulmonary tissue, capillaries, and veins. Results: The CVs of gas distributions in the images showed that there was a statistically significant difference between the COPD and healthy subjects (p < 0.0001). The functional volumes of pulmonary tissue, capillaries, and veins were significantly lower in the subjects with COPD than in the healthy subjects (p < 0.001). The functional volume of pulmonary tissue was found to be (i) statistically different between the healthy and MCTE groups (p = 0.02) and (ii) dependent on the age of the subjects in the healthy group (p = 0.0008) while their CVs (p = 0.13) were not. Conclusion: The novel HPX gas-exchange MRI and CFD model distinguished the healthy cohort from the MCTE and COPD cohorts. The proposed technique also showed that the functional volume of pulmonary tissue decreases with aging in the healthy group. Key Points: • The ventilation and gas-exchange imaging with hyperpolarized xenon-129 MRI has enabled the identification of gas-exchange variation between COPD and healthy groups. • This novel technique was promising to be sensitive to minimal CT–diagnosed emphysema and age-related changes in gas-exchange parameter in a small pilot cohort