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The objective of this study is to assess the sensitivity of intracranial aneurysm geometry to the modality of imaging. Four imaging modalities—3D rotational angiography (3DRA), computed tomography angiography (CTA), contrast enhanced magnetic resonance angiography (CE-MRA), and time-of-flight magnetic resonance angiography (TOF-MRA)—were assessed using data from a flow phantom and human subjects. A silicone flow phantom of the head and neck arteries with a 10 mm ACOM aneurysm was imaged using all four modalities under steady flow conditions. Three human subjects with mid to large sized intracranial aneurysm who had a 3DRA scan and one of CTA, CE-MRA, or TOF-MRA performed within a day were also studied. The aneurysm and contiguous vasculature were segmented for all available scans and geometric measures of their size (5 indices) and shape (6 indices) were estimated and compared. Visually, the size and shape of segmented 3D models of the aneurysms were similar across scan modalities for both the human subjects and the flow phantom. Consequently, the computed indices were consistent across modalities in the key morphometric indices. In conclusion, quantified indices of 3D geometry of the mid to large sized intracranial aneurysms investigated in this small study population are not sensitive to scanning modality.

OBJECT The goal of this prospective longitudinal study was to test whether image-derived metrics can differentiate unruptured aneurysms that will become unstable (grow and/or rupture) from those that will remain stable. METHODS One hundred seventy-eight patients harboring 198 unruptured cerebral aneurysms for whom clinical observation and follow-up with imaging surveillance was recommended at 4 clinical centers were prospectively recruited into this study. Imaging data (predominantly CT angiography) at initial presentation was recorded. Computational geometry was used to estimate numerous metrics of aneurysm morphology that described the size and shape of the aneurysm. The nonlinear, finite element method was used to estimate uniform pressure-induced peak wall tension. Computational fluid dynamics was used to estimate blood flow metrics. The median follow-up period was 645 days. Longitudinal outcome data on these aneurysm patients—whether their aneurysms grew or ruptured (the unstable group) or remained unchanged (the stable group)—was documented based on follow-up at 4 years after the beginning of recruitment. RESULTS Twenty aneurysms (10.1%) grew, but none ruptured. One hundred forty-nine aneurysms (75.3%) remained stable and 29 (14.6%) were lost to follow-up. None of the metrics—including aneurysm size, nonsphericity index, peak wall tension, and low shear stress area—differentiated the stable from unstable groups with statistical significance. CONCLUSIONS The findings in this highly selected group do not support the hypothesis that image-derived metrics can predict aneurysm growth in patients who have been selected for observation and imaging surveillance. If aneurysm shape is a significant determinant of invasive versus expectant management, selection bias is a key limitation of this study.

Indices of the intra-aneurysm hemodynamic environment have been proposed as potentially indicative of their longitudinal outcome. To be useful, the indices need to be used to stratify large study populations and tested against known outcomes. The first objective was to compile the diverse hemodynamic indices reported in the literature. Furthermore, as morphology is often the only patient-specific information available in large population studies, the second objective was to assess how the ranking of aneurysms in a population is affected by the use of steady flow simulation as an approximation to pulsatile flow simulation, even though the former is clearly non-physiological. Sixteen indices of aneurysmal hemodynamics reported in the literature were compiled and refined where needed. It was noted that, in the literature, these global indices of flow were always time-averaged over the cardiac cycle. Steady and pulsatile flow simulations were performed on a population of 198 patient-specific and 30 idealised aneurysm models. All proposed hemodynamic indices were estimated and compared between the two simulations. It was found that steady and pulsatile flow simulations had a strong linear dependence (r ≥ 0.99 for 14 indices; r ≥ 0.97 for 2 others) and rank the aneurysms in an almost identical fashion (ρ ≥ 0.99 for 14 indices; ρ ≥ 0.96 for other 2). When geometry is the only measured piece of information available, stratification of aneurysms based on hemodynamic indices reduces to being a physically grounded substitute for stratification of aneurysms based on morphology. Under such circumstances, steady flow simulations may be just as effective as pulsatile flow simulation for estimating most key indices currently reported in the literature.

There is increasing interest in assessing the role of hemodynamics in aneurysm growth and rupture mechanism. The ability to accurately predict the rupture risk of an aneurysm can help in providing immediate intervention to patients with aneurysms at high rupture risk Also, the small but significant risk associated with the treatment options can be avoided for patients with stable harmless aneurysms. Retrospective studies have been performed in the past to identify indices that differentiate ruptured aneurysms from unruptured aneurysms [1–3]. However, these differences may not necessarily translate to differences between aneurysms that present unruptured but over a period of time (months to years), fork towards growth/rupture and unruptured aneurysms that remain stable. In the present study, the hypothesis that hemodynamic indices of unruptured aneurysms when they first presented are predictive of their longitudinal outcome was tested.Copyright © 2013 by ASME

The development of intimal hyperplasia is the major cause of failure of both autologous saphenous vein and synthetic coronary artery bypass grafts. This is partially due to graft-host vessel compliance mismatch. Cellulose and chitosan (CELL:CHIT) are both biocompatible, nontoxic, and naturally occurring biopolymers that have been used extensively for biomedical applications. Elastic properties of membranes made of CELL:CHIT blends with different ratios between each polymer were determined using uniaxial tests and the ratio that yielded the less stiff membrane was chosen to prepare a small diameter hollow tube. The presence of chitosan had a favorable impact on the elasticity of the membranes, where the CELL:CHIT 5:5 ratio showed the lowest Young's modulus. Small diameter tubular constructs were fabricated using this optimal CELL:CHIT ratio and the compliance was determined on samples with different wall thickness and internal diameter. The compliance of the hollow tube with inner diameter of 4 mm and wall thickness of 1.2 mm was found to be 5.91%/mmHg×10(-2), which is higher than those of Dacron, expanded polytetrafluorethylene, and saphenous vein, but very close to that of human coronary artery. Burst strength tests revealed that the tubes can withstand at least 300 mmHg. Finally, the tubes showed satisfactory cell attachment property when myofibroblast cells adhered and proliferated on the lumen of the samples.

There is increasing interest in assessing metrics of blood flow characteristics as prognostic indicators of rupture risk in intracranial aneurysms. Aneurysmal hemodynamics may be and has been quantified in the literature by numerous indices (about 18, to the best of our knowledge) each capturing some characteristic of flow. But are all these indices capturing unique characteristics of flow or are many of them redundant? And are they mainly proxies for basic morphological characteristics? Answering this question and identifying the indices that capture unique aspects of flow has practical implications to prospective studies designed to test the hemodynamics-rupture correlation. Lesser the number of indices, lesser will be the required sample sizes for sufficient statistical power. Alternatively, greater the number of indices, greater the likelihood that such studies will descend into an exercise in data mining. Further, understanding how these indices relate among themselves and to morphology can help us better reconcile findings from independent studies. The objectives of this study were to first document and categorize all reported indices in the literature and use a population of unruptured intracranial aneurysms to understand correlations among them and with morphological indices.Copyright © 2012 by ASME

Hemodynamics is thought to play a role in the initiation and subsequent growth of intracranial aneurysms. Hemodynamics of an aneurysm is strongly related to its underlying morphology, specifically, the size and shape of the aneurysm sac and its contiguous vasculature [1]. Although the Computational Fluid Dynamic (CFD) simulations of transient blood flow seemingly provide in depth information of intra-aneurysmal hemodynamics, the simulations are often computationally expensive and demand additional measurements or assumptions such as the temporal variations in boundary conditions. When a population of aneurysms, with distinct sizes and shapes, are being studied in order to stratify them according to hemodynamics, it is conceivable that steady flow simulations are as effective as pulsatile flow simulations, especially when anatomical information is the only patient-specific information available. The objective of this study is to compare steady versus pulsatile flow simulations for a controlled hypothetical population of aneurysms with realistic variation in sac morphology and contiguous vasculature.Copyright © 2011 by ASME

Longitudinal studies of a large cohort of cerebral aneurysm patients are essential for assessing the ability of geometric and biomechanical indices in predicting their growth or rupture risk [1,2]. Patients with these lesions are routinely subjected to volumetric scans, but these can be one of four modalities — 3D rotational angiography (3DRA), Computed Tomography Angiography (CTA), Magnetic Resonance Angiography (MRA) and MR Time-of-flight (MR-TOF) — the choice of which is often dictated by clinical considerations. One of the challenges in geometric analysis of cerebral aneurysms is the consistency in the process of 3D reconstruction of aneurysm geometry when data from multiple imaging modalities are used. It is reasonable to wonder if the same aneurysm were imaged using different imaging modalities, whether the quantified measures of the aneurysm’s shape and size will be different. The objective of this study is to assess the sensitivity of geometric indices of cerebral aneurysms to the modality from which the image volume is obtained. A patient-specific silicone model of the head and neck vasculature with an aneurysm was imaged using all four modalities, geometric measures were estimated and compared.Copyright © 2010 by ASME