Long-term Follow-up of Anal Fistulae in Crohn's Disease Treated With Anti-TNFalpha and Interest of New MRI Sequences

Main purposes of this study are : * Analysis of relapse risk of anal fistulae with prospective follow-up of patients undergoing pelvic MRI every 6 months for one year. This monitoring also determines if the presence of path of fistulae visible on MRI during inclusion visit is associated to increased risk of perianal abscess under infliximab or adalimumab treatment. * Evaluation of interest of new magnetic resonance imaging sequences for detection of persistent anal fistulae under infliximab or adalimumab treatment. Secondary purposes are: * Description of patients with one or more paths of fistulae visible with MRI after at least one year of treatment with infliximab or adalimumab administered for anal fistulae and showing a complete healing of fistulae at clinical examination of inclusion visit. * Identification of predictive factors (included the presence of paths of fistulae visible with MRI during inclusion visit) of relapse of anal fistulae during the 1-year-follow-up after inclusion visit.