**4. Pre and per-operative evaluation**

The decision to deploy an intracranial stent is taken after considering the feasibility of performing the treatment without it (e.g. aneurysm coiling to be safely treatable using balloon remodeling techniques), or the possibility of not completing the treatment due to technical difficulties such as poor navigability. The diameter and length of each device is chosen according to the diameter of the native vessel and the extension of the pathological segment.

Important issues for treatment planning are: exact aneurysm anatomical location, parent artery morphology and presence of side branches and perforators. These factors are studied on CT, MRI or DSA images before the operative procedure. The size and shape of the aneurysm, as well as the diameter of the neck, are recorded. The diameter of the parent artery is then measured, as well as the segment of the artery that will be covered by the stent. The operator will then be able to choose the adequate diameter and length of the device to use so that adequate covering of the neck can be assured.

The tortuosity of the parent artery and the technique for coiling (e.g. jailing, semi-jailing, 'X' and Y' stents, etc.) also influences the type of stent used (open cell versus closed cell, selfexpandable versus balloon-mounted, etc). It is particularly important to detect potential irregularities due to other vascular pathologies such as atherosclerosis or fibromuscular dysplasia. Part of the assessment of feasibility of the stent-assisted treatment is the study of branches presenting with sharp angle of bifurcation or incorporation of its origin into the neck of the aneurysm. Such vessels may be very difficult to catheterize. If it needs to be stented, this may result in a longer and more laborious procedure. If the progression of a microguidewire and a microcatheter inside a recurrent branch is impossible after numerous attempts, other treatment modalities (e.g. surgical) must be considered. As a consequence, the patient must be properly informed before the endovascular procedure that his or her treatment presents elements of technical complexity, and that endovascular treatment may not be feasible.
