Cross-sectional imaging anatomy of the anal sphincters

BACKGROUND: To describe the cross-sectional anatomy of the anal sphincter mechanism relevant to magnetic resonance imaging (MRI) and ultrasound cross-sectional images. METHOD: Axial, sagittal, and coronal 5-mm sections of female pelves were reviewed from six cadaver specimens (ages 24-72 years). Fetal anatomy was studied in plastinated histologic sections from 19 and 26 weeks' gestation. Images of the anal sphincter were obtained by MRI in six and by ultrasound using an exoanal technique in 12 nulliparous volunteers. EXPERIENCE: The internal anal sphincter is clearly visible in anatomic sections central to the external sphincter and is visible in MRI and ultrasound images. The external anal sphincter can be subdivided into a subcutaneous and a deep portion. On anatomic sections and on MRI, the subcutaneous part shows as two parallel muscle strips in the axial plane; the deep portion presents with a characteristic teardrop form in the section perpendicular to the axis of the anal canal. The puborectalis muscle and the external anal sphincter form a "double bump" in the sagittal section. The longitudinal muscle can be identified by its fiber orientation in anatomic sections but is not clearly visible in imaging studies. CONCLUSION: This information should make it possible to identify accurately anal sphincter anatomy in two-dimensional sectional images of the anal sphincter.