title: Magnetic resonance imaging and 3-dimensional analysis of external anal sphincter anatomy.
abstract: OBJECTIVE
  To use magnetic resonance images of living women and 3-dimensional modeling software to identify the component parts and characteristic features of the external anal sphincter (EAS) that have visible separation or varying origins and insertions.
  
  
  METHODS
  Detailed structural analysis of anal sphincter anatomy was performed on 3 pelvic magnetic resonance imaging (MRI) data sets selected for image clarity from ongoing studies involving nulliparous women. The relationships of anal sphincter structures seen in axial, sagittal, and coronal planes were examined using the 3-D Slicer 2.1b1 software program. The following were requirements for sphincter elements to be considered separate: 1) a clear and consistently visible separation or 2) a different origin or insertion. The characteristic features identified in this way were then evaluated in images from an additional 50 nulliparas for the frequency of feature visibility.
  
  
  RESULTS
  There were 3 components of the EAS that met criteria as being "separate" structures. The main body (EAS-M) is separated from the subcutaneous external anal sphincter (SQ-EAS) by a clear division that could be observed in all (100%) of the MRI scans reviewed. The wing-shaped end (EAS-W) has fibers that do not cross the midline ventrally, but have lateral origins near the ischiopubic ramus. This EAS-W component was visible in 76% of the nulliparas reviewed.
  
  
  CONCLUSIONS
  Three distinct external anal sphincter components can be identified by MRI in the majority of nulliparous women.
authors.affiliation: Pelvic Floor Research Group, Division of Gynecology, Department of Obstetrics and Gynecology, University of Michigan, Ann Arbor, Michigan 48109-0276, USA. yvonneh@med.umich.edu
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Hsu
authors.forename: Dee E
authors.initials: DE
authors.lastname: Fenner
authors.forename: William J
authors.initials: WJ
authors.lastname: Weadock
authors.forename: John O L
authors.initials: JO
authors.lastname: DeLancey
mesh_terms: D000328:Adult; D000367:Age Factors; D001003:Anal Canal / Q000033:anatomy & histology*; D015331:Cohort Studies; D005260:Female; D020073:Gravidity; D006801:Humans; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008875:Middle Aged; D011247:Pregnancy; D012307:Risk Factors; D012680:Sensitivity and Specificity