Morphology Parameters for Intracranial Aneurysm Rupture Risk Assessment

Intracranial aneurysms (IA) affect approximately 2 to 5% of the entire population (23, 25). Ruptured IAs typically cause subarachnoid hemorrhage (SAH) and its sequelae, resulting in significant morbidity and mortality. Among patients who have SAH, 50 to 60% will die from the initial hemorrhage and a further 20 to 25% will experience complications (30). However, despite their expected common occurrence, only 1% of all IAs actually rupture (25). Although the morbidity and mortality associated with rupture may suggest that an incidentally detected aneurysm should be treated to forestall the catastrophic event of SAH, the two current methods of treatment (open microsurgical aneurysm clip ligation or endovascular aneurysm coil embolization) are not without some risk of major morbidity and mortality (8, 31). Therefore, an accurate metric (or several metrics) to judge the risk of rupture of an aneurysm is critical to aid in generating the best possible treatment algorithm. Hemodynamics has been shown to play an important role in IA pathophysiology and rupture. Using computational fluid dynamics, Hassan et al. (11) suggested that high wall shear stress (WSS) may be responsible for IA growth and rupture in high-flow aneurysms, whereas the predominant factors causing rupture in low-flow aneurysms are high intra-aneurysmal pressure and flow stasis. Cebral et al. (6) demonstrated that ruptured IAs have unstable flow patterns, smaller impinging jet diameters, and smaller impingement zones. Shojima et al. (24) found that ruptured IAs have a higher average WSS in the aneurysm sac than unruptured IAs. They observed recirculation zones and blood stasis at the apex of ruptured IAs. It is important to realize that IA hemodynamics are strongly dependent on the geometry of the aneurysmal sac and its feeding vessel (11, 13, 26). For a given geometry, Cebral et al. (5) showed that hemodynamics do not vary significantly with physiological variations of flow rate, blood pressure, and waveform. Therefore, suitable parameters characterizing IA geometry can capture the characteristic hemodynamics and potentially predict rupture risk. Several past studies have investigated such parameters. The most ubiquitous parameter is IA size. Although aneurysms exceeding 10 mm in size are considered to be dangerous, several studies have shown that a large percentage of ruptured aneurysms are, in fact, smaller than 10 mm (2, 9, 22, 23, 26, 27, 30). The relationship between IA rupture risk and IA size has yet to be completely elucidated. Aneurysm shape has been studied as well, and certain shape parameters show stronger correlation with rupture than IA size. Aspect ratio (AR), defined as IA height divided by neck diameter, is the most commonly studied shape parameter. Although most findings affirm its importance, they do not converge on a common threshold value (2, 22, 26, 27, 29). Other, more sophisticated, shape parameters such as undulation index (UI), nonsphericity index (NSI), and ellipticity index (EI) have been proposed (22) in an attempt to account for the three-dimensional (3D) nature of IA. Such 3D parameters show promise to be better predictors than lower-dimensional parameters such as size or AR, and they are further examined in the current study. Previous studies have also investigated additional factors that correlate with IA rupture risk, such as familial preponderance, smoking, hypertension, female sex, connective tissue disorder, aneurysm growth rate, and presence of multiple IAs (15-17, 32). However, these studies have not yielded quantifiable metrics that can be readily integrated into the clinical decision-making process. Adding complexity from such diverse variables into our current study would make risk assessment analysis unwieldy. Currently, morphometric evaluation, typically using size alone, is the mainstay of applied aneurysm rupture risk assessment in day-to-day clinical practice. Our aim is to improve such morphological evaluation and better the accuracy of aneurysm rupture risk assessment, something that is fundamental to the current practice of cerebrovascular neurosurgery. A limitation of previous morphology-based rupture risk studies, including those investigating 3D parameters, is that the geometry of the parent artery is typically ignored. Parent artery geometry has a significant influence on the resultant IA hemodynamics and, consequently, the rupture risk. Castro et al. (4) have demonstrated that upstream vessel tortuosity can critically influence intra-aneurysmal hemodynamics. Hassan et al. (11) observed that a greater parent vessel incidence angle shifts the high WSS area toward the aneurysm dome, where rupture-prone blebs often are present, whereas Hoi et al. (13) noted that highly curved parent vessels subject IAs to higher hemodynamic stresses at the inflow zone that might promote growth or rupture. Thus, parent vessel geometry should be accounted for when defining morphological parameters for IA rupture risk prediction. Furthermore, numerous studies have observed a connection between IA rupture risk and vessel location (3, 4, 9, 21, 26, 30). Because vessel location is strongly related to vessel geometry, this finding affirms the importance of vessel geometry for IA rupture risk. Incorporating parent vessel geometry in morphology parameters can, at least to some extent, capture the influence of IA location as well. In the current study, we address the above-mentioned issues and define three new morphology parameters that incorporate IA parent vessel geometry. We analyze a group of 45 IAs (20 ruptured, 25 unruptured) to evaluate new IA rupture parameters, in comparison with five “traditional” parameters that have been described in earlier studies.