title: Endoanal magnetic resonance imaging versus endosonography.
abstract: BACKGROUND
  A major limitation of anal endosonography is the poor inherent contrast, which is the cause of the cumbersome identification of the longitudinal layer and external sphincter. Endoanal MRI was introduced to overcome this problem. The normal anatomy, perianal fistulas and sphincter defects as visualized by endoanal MRI as compared to endoanal sonography are discussed.
  
  
  METHODS
  Endoanal sonography was performed with an 18-mm diameter 7-MHz endoanal probe. For endoanal MRI a 19-mm diameter endoanal receiver coil was used at 0.5 T. Volunteers and patients with anal fistula or fecal incontinence were examined with both techniques.
  
  
  RESULTS
  Endoanal MRI demonstrates the internal sphincter but also the longitudinal layer and external sphincter in great detail. The external sphincter is the lower outer part of the sphincter, while the puborectal muscle is the upper outer part of the sphincter which is connected to the levator ani muscle. External sphincter defects are more often identified and better delineated with MRI. Endoanal MRI is superior in the identification and classification of perianal fistulas.
  
  
  CONCLUSIONS
  Endoanal MRI is an important new diagnostic tool in anal imaging. The anal sphincter anatomy and pathology are excellently demonstrated. Endoanal MRI is superior to endoanal sonography, especially in the identification of external sphincter defects and in the classification of tracks.
authors.affiliation: Department of Radiology, University Hospital Dijkzigt, Erasmus University, Rotterdam, The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Stoker
authors.forename: S M
authors.initials: SM
authors.lastname: Hussain
authors.forename: J S
authors.initials: JS
authors.lastname: Laméris
mesh_terms: D001003:Anal Canal / Q000000981:diagnostic imaging* / Q000473:pathology*; D005242:Fecal Incontinence / Q000000981:diagnostic imaging* / Q000473:pathology*; D005401:Fissure in Ano / Q000000981:diagnostic imaging* / Q000473:pathology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D014463:Ultrasonography