Influence of intracranial aneurysm-to-parent vessel size ratio on hemodynamics and implication for rupture: results from a virtual experimental study.

Most clinical studies attempting to stratify the rupture risk of an intracranial aneurysm (IA) have relied on absolute aneurysm size as the primary metric for IA morphology (18, 20, 31, 41). Recent large- scale studies, such as the International Study of Unruptured Intracranial Aneurysms (18, 41), have generated considerable controversy (2, 9, 10, 12, 21, 26) as a result of predicting extremely low rupture rates on the basis of IA size. Numerous studies have disputed the application of size as a predictor of IA rupture risk, pointing out the frequency of subarachnoid hemorrhage from small aneurysms (3, 6, 13, 28). Location-based studies have consistently shown that certain parts of the cerebral vasculature, such as the posterior communicating artery and the anterior communicating artery (AComA), have a higher incidence of ruptured aneurysms when compared with other locations, such as the cavernous segment of the internal carotid artery (ICA) (3, 6, 13, 28, 35, 39, 40). It has also been noted that specific locations on the cerebral vasculature (e.g., AComA) harbor a high percentage of all small ruptured IAs; whereas at other locations (such as the ICA), small, ruptured IAs are substantially less frequent (3, 6, 13, 28). These observations suggest that IA size loses relevance to rupture risk if location is not accounted for. Furthermore, it is likely that the variability of the rupture risk of different locations may be related to the caliber of the originating, or parent, vessel, resulting in higher risks for small- diameter vessels (e.g., AComA or posterior communicating artery ) and lower risks for larger- diameter vessels (e.g., ICA) (3, 6, 13, 28, 35, 39, 40). Evaluation of the contribution of parent vessel geometry to aneurysm rupture risk may help resolve the above-noted conflicts regarding the use of IA size alone as a predictor of rupture risk. In a recent study, we proposed a new parameter, size ratio (SR), defined as maximum aneurysm height/average parent vessel diameter, that incorporates the IA parent vessel geometry into a morphological index (11). That study compared SR with previously identified parameters, such as aneurysm size, aspect ratio, undulation index, ellipticity index, and non-sphericity index, and found SR to have the strongest correlation with IA rupture. It has been hypothesized that the mechanism by which IA morphology relates to IA rupture is via the intra- aneurysmal hemodynamic environment created by a particular geometry (36). Several studies have evaluated the relationship between intra- aneurysmal hemodynamics and IA growth rate and rupture risk (8, 15–17, 19, 23–25, 32, 33, 36). Computational fluid dynamics (CFD) studies of intra- aneurysmal hemodynamics have identified certain flow characteristics to be specific to ruptured aneurysms and, therefore, classified as potentially “dangerous” (8, 32, 36). Regions of markedly low (<0.5 Pa) wall shear stress (WSS) on the IA inner wall, focused flow impingement zones, and complex flow patterns with features such as multiple vortices and creation and destruction of vortices during the cardiac cycle have all been identified as being more prevalent in ruptured IAs (8, 32, 36). Because SR has been shown to correlate with IA rupture (11), we now investigate the effect of SR on intra-aneurysmal hemodynamics to elucidate the specific flow patterns that are prevalent in rupture- prone IAs. We performed pulsatile CFD simulations on 2 types of saccular IAs (sidewall and terminal) while virtually modifying the relationship between the IA and parent vessel, thereby allowing computational simulation of IA hemodynamics on preexisting aneurysms with constant morphology—except for a varying SR. Each original IA model was morphed to change only SR while keeping all other shape metrics constant. For the original IA models, we used anatomically accurate geometries reconstructed from patient 3-dimensional angiography rather than idealized models. Both original IAs were unruptured. The intent of this virtual analysis was to gain insight concerning the relationship between SR and hemodynamic environments associated with elevated IA rupture risk, thereby highlighting the importance of considering both aneurysm and parent vessel anatomy when assessing IA rupture risk.