Fig. 4

Perianal tomographic ultrasound imaging (TUI) of the anal canal. TUI slices with a width of 2 mm. a Midsaggital plane with marked TUI slices, followed by the 5 slices in coronal plane, beginning with the lowermost slice (b) where external (EAS) and internal sphincter muscles (IAS) have been both visible completely for the first time starting from the anocutaneous transition zone moving in cranial direction. c Slice 2 mm cranial of b. d 2 mm cranial of c, slice where all measurements were done. e 2 mm cranial of d, f 2 mm cranial of e, most cranial slice where EAS and IAS are still both seen completely