High-resolution endoluminal sonography of the anal sphincter complex

The ability of a high-frequency miniature ultrasonographic transducer to image the anatomy of the anal sphincter complex was investigated in 20 normal subjects. A 9 Fr catheter containing a 12.5 MHz transducer was inserted into the anal canal. Cross-sectional ultrasonographic images of the canal were acquired as the catheter was advanced through the canal into the distal rectum. Three analog images were digitally stored on a computer system, and measurements of the anal sphincters were made from an eight octant grid by two independent investigators. In all 20 subjects the hypoechoic mucosa, hyperechoic submucosa, and hypoechoic internal anal sphincter (IAS) were identified. The first band of the external anal sphincter (EAS) was imaged in 12 of 20 subjects. The mean width of the IAS was 3.5 +/- 0.5 mm (range, 2.6-4.3 mm). The mean width of the first band of the EAS was 2.3 +/- 0.5 mm (range, 1.6-3.2 mm). High-resolution sonography of the anal canal allows detailed evaluation of anatomic structures that cannot be achieved by conventional imaging technology.