Overview

An aneurysm is a bulge or dilation of a blood vessel. A cerebral aneurysm is an abnormal enlargement of any artery located in the brain. This pathology tends to occur at or near bifurcations of arteries in the Circle of Willis or at high-curvature locations of arteries. Intracranial aneurysms bear the medical risk of rupture leading to intracranial bleeding, which in some cases can cause irreparable neurological damage or death of the patient. An aneurysm may be detected in angiographic images obtained via two-dimensional (2D) x-ray, computed tomography or magnetic resonance modalities.

Endovascular embolization is a way to treat intracranial aneurysms. It is performed under guidance from X-ray angiography images. It involves the insertion of wire coils or glue inside the aneurysm with the intention of forming a blood clot inside the aneurysm and relieving the pressure against the aneurismal wall. For some aneurysms, especially those with a large neck, an endovascular stent is used together with coils in order to maintain flow inside the parent artery and prevent the coils from protruding and forming a clot. Endovascular aneurysm embolization is performed under guidance of X-ray fluoroscopy images and 2D X-ray digital subtraction angiography (DSA) acquired via an angiographic C-arm system. 3-dimensional (3D) DSA is computed via reconstruction from 2D DSA projections of the blood vessels obtained via the rotation of the angiographic C-arm system around the patient’s head. 3D DSA provides an accurate image of 3D geometry of the blood vessels, free from background structures -- a feature which, along with the high spatial resolution offered, makes 3D DSA the image modality of choice for planning endovascular aneurysm treatment.

Vascular segmentation of 3D angiographies can provide patient-specific 3D vascular models. This has a significant role in endovascular treatment planning. Nevertheless, even if a vascular model is obtained, measuring the clinical parameters of interest still remains a problem. Patient-specific variables that a physician needs to take into account while planning endovascular aneurysm embolization include the aneurysm neck width, neck length, maximum diameter, dome height, aspect ratio, and parent artery diameter. Today, these parameters are measured manually off a 3D DSA image rendered on a 2D screen of an angiographic workstation. Additional parameters that are suggested in the literature for aneurysm treatment planning and rupture risk assessment include aneurysm volume, aneurysm area, and the ratio of aneurysm area to the neck area. These measurements would be easier to take if the aneurismal sac is demarcated from its parent arteries. As a result, accurate quantification of these patient-specific 3D geometric parameters requires segmentation and patient-specific modeling of the aneurysm and the blood vessel/vessels to which it is attached.

In segmentation of cerebral vasculature special considerations should be given to the diversity and complexity of the data: the structure of the vascular tree is complicated, consisting of hundreds of braches; the vessels are curvy, tortuous, and occasionally overlapped; the topology and morphology of the vasculature may vary according to different subjects and different sampling times; some vessels have very small diameters, etc.