The protocol has many assets. A prospective nationwide recruitment allows for the inclusion of a large cohort of patients with UIA. It will combine accurate clinical phenotyping and comprehensive imaging with CAWE screening. Besides, it will enable to exploit metadata and to explore new pathophysiological pathways of interest by crossing clinical, genetic, biological, and imaging information.

Management of small (<7mm) unruptured intracranial aneurysms (UIA) remains controversial. Previous retrospective studies have suggested that post gadolinium arterial wall enhancement (AWE) of unruptured intracranial aneurysms on MR imaging may reflect aneurysm wall instability, and hence may highlight a higher risk of UIA growth over time. This multicentric prospective cohort aims at exploring vessel wall imaging findings of UIAs with consecutive follow-up to substantiate these assumptions.The objective is to develop diagnostic and predictive tools for the risk of IA evolution. The aim is to demonstrate in clinical practice the predictive value of (AWE) for UIA growth. The growth will be determined by any modification of the UIA measurement. Both UIA growth and the UIA wall enhancement will be assessed in consensus by two expert neuroradiologists.The French prospective UCAN project is a non-interventional international wide and multicentric prospective cohort. UIA of bifurcation between 3 and 7 mm for whom a clinical and imaging follow-up without occlusion treatment was scheduled by local multidisciplinary staff will be included. Extensive clinical, biological and imaging data will be recorded during a 3 years follow-up (visits at 1 and 3 years after inclusion).

Inclusion Criteria :

• Subject carrying unruptured, asymptomatic and untreated typical IA of bifurcation, measured on conventional imaging (MRI, CTA or DSA) between 3 and 7 mm of larger diameter.
• Ability to be followed-up during 3 years decided in consensus multidisciplinary gathering.
• Age > 18 years old.

Exclusion Criteria :

• A failure to obtain informed consent
• Contraindications for undergoing an MRI scan include : (heart pacemaker, a metallic foreign body (metal sliver) in their eye, or aneurysm clip in their brain, severe claustrophobia)
• Contraindications for a gadolinium contrast medium injection (:eGFR below 30 mL/min/1.73 m2, Previous or pre-existing nephrogenic systemic fibrosis, Previous anaphylactic/anaphylactoid reaction to gadolinium containing contrast agent, Acutely deteriorating renal function, Pregnancy and breast-feeding)
• A mycotic, fusiform-shaped, or dissecting IA, an IA in relation with an arteriovenous malformation
• A family history of polycystic kidney disease, Ehlers-Danlos syndrome, Marfan's syndrom, fibromuscular dysplasia, or Moya Moya disease
• Intra-cavernous UIA because the sinus cavernous that is fulfilled with venous blood precluded a reliable assessment of aneurysmal wall enhancement (AWE)

The recruited population is composed of subjects carrying unruptured asymptomatic typical IA of bifurcation for whom a clinical, a biological and an imaging follow-up, without occlusion treatment, was scheduled by local multidisciplinary staff.