Endoluminal ultrasound defines anatomy of the anal canal and pelvic floor

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Abstract
The aims of this study were to determine whether endoluminal ultrasound (ELUS) could identify various layers of the normal anal canal and to evaluate whether a 10-MHz probe provided better image resolution than a 7-MHz probe. Sonographic anatomy of the anal canal on ELUS was directly correlated with anatomic dissection of various layers (mucosa-submucosa, internal anal sphincter, and external anal sphincter) in cadavers. Sonographic appearance of the anal sphincters was further evaluated in patients by “tagging” various layers using sonodense needles. A higher frequency 10-MHz ultrasound probe (focal length, 1-4 cm) provides improved sonographic images of the anal canal, compared with the 7-MHz probe (focal length, 2-5 cm). ELUS can also successfully identify various structures of the pelvic floor including the puborectalis, urethral sphincter, vagina, and outlines of the pelvis and ischiorectal fossae. Its role in the evaluation of anorectal disorders appears promising.