COMMENT RE: Ruptured status discrimination performance of aspect ratio, height/width, and bottleneck factor is highly dependent on aneurysm sizing methodology

This article sheds light on the variability inherent in defining and measuring morphological features of intracranial aneurysms. The authors are commended for addressing this important issue because variability in definition and measurement adversely affect the ability to interpret and compare results from multiple studies evaluating the morphological features of intracranial aneurysms. This is a significant problem from both a clinical and research perspective. We first consider the issue as it applies to research protocols. Within a given study, the use of consistent definitions and measurements of morphometric indexes is essential. Computational approaches that allow consistent measurement of aneurysm size and shape indexes must be developed and clearly stated. It would be ideal if all investigational reports could agree on standard definitions of morphometric features and a standard methodology of measurement; unfortunately, this is not the case. It is also important to recognize that the biggest source of uncertainty in the morphological description of intracranial aneurysms may lie in the steps undertaken before we begin to calculate indexes like aspect ratio and bottleneck factor. The segmentation of the aneurysm surface from clinical images needs to be rigorously defined and measured, which can be very difficult. For instance, even determining where the normal vessel ends and the aneurysm begins can be fraught with difficulty and can significantly affect our measurements. Recent advances in computational image analysis such as the Vascular Modeling ToolKit (http://www.VMTK.org) and ongoing studies by many research groups are likely to help resolve these issues and lead to more generally accepted definitions and methods of measurement. In the clinical setting, definitions and measurement of aneurysm size and shape are far less rigorously determined than they are in a research study. This variability has significant clinical implications in that there is a growing body of evidence that morphological features affect the likelihood of aneurysm rupture and that these features are routinely used to decide whether to recommend treatment for unruptured aneurysms. Variability in definition and measurement of size and shape indexes may also be affected by the clinician’s desire to reinforce a treatment recommendation on the basis of other factors. Our work in this area has demonstrated that aneurysm shape is an important feature differentiating ruptured and unruptured aneurysms and that the morphology of the aneurysm and adjacent vessels determines the wall stress, shear stress, and other hemodynamically important forces acting on the aneurysm wall.1–4 We believe that this area of inquiry will be important for our understanding of aneurysm growth and rupture. Such understanding will allow us to refine our treatment indications for patients with unruptured aneurysms. The use of consistent definitions and measurements of aneurysm size and shape will facilitate this area of inquiry.