How effective/accurate are MRI scans of the small intestine for diagnosing Crohn's disease?
As far as I know MRI scans are mainly used to assess the location and extent of crohn's disease manifestations and crohn's associated complications (strictures, fistulas, wall thinkening). MRI is usefull to determine the phenotype of someones crohn's (e.g. localized terminal ileitis, perianal disease).
An MRI of course is not suitable as sole instrument to establish a crohn's disease diagnosis. In my case diagnosis was established by ileocolonoscopy, examination of the biopsies collected thereby by a phatologist, stool markers and full blood count. MRI was used as additional diagnosis instrument since ileoscopy raised the gastrologists strong suspicion of a crohn's, the phatologist examination of the biopsies was inconclusive. Blood count showed the typical crohn's associated malnutrition signs and all the signs of an immune system actively fighting an inflammation. In addition a bacterology of the stool sample showed no evidence of yersinia enteritis.
The ecco guidelines thoroughly describe the establishment of a crohn's diagnosis [ecco guidelines part definition and diagnosis].