Spatial Projection Ratio: A Novel Shape Index to Evaluate Aneurysm Rupture Risk Using Three-dimensional Geometry

Shape indices such as size ratio are valuable for diagnosing aneurysm rupture status and may influence rupture risk. However, as these indices are calculated based on two-dimensional measurements, bias may arise from observation directions. To address this, we developed a novel parameter, spatial projection ratio, utilizing three-dimensional geometry. A retrospective analysis of 225 aneurysms diagnosed using three-dimensional computed tomography angiography was conducted to evaluate primary variables and spatial projection ratio. Spatial projection ratio is determined by defining the gravity point as the neck orifice center and identifying the furthest point from it using commercial software. The distance between these points, known as spatial projection length, is measured and divided by the equivalent neck diameter to calculate spatial projection ratio. Significant differences in morphological variables for rupture status were observed by Brunner-Munzel tests. Receiver-operating characteristic curve analysis was employed to assess diagnostic accuracy, with Spearman's rank correlation utilized to explore the potential for predicting rupture risk by correlating spatial projection ratio and size ratio. Ruptured aneurysms exhibited significantly higher primary variables and shape indices compared to unruptured ones. The area under receiver-operating characteristic curves of all shape indices surpassed that of primary variables, with spatial projection ratio demonstrating a particularly high area under receiver-operating characteristic curves of 0.791 (95% confidence interval 0.732-0.849; sensitivity, 0.770; specificity, 0.741; cut-off value, 1.047). Moreover, spatial projection ratio exhibited a significant correlation with size ratio (r = 0.575, p < 0.01). Thus, spatial projection ratio emerges as a robust morphological parameter for evaluating rupture status and may provide insights into aneurysm rupture risks.

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