High-resolution magnetic resonance imaging of the anal sphincter using a dedicated endoanal receiver coil

The use of a surface coil in MR imaging improves signal-to-noise ratio of a djacent tissues of interest. We therefore devised an endoanal receiver coil for imaging the anal sphincter. The probe is solid and re-usable: it compr ises a saddle geometry receiver with integral tuning, matching and decoupli ng. It is placed in the anal canal and immobilised externally. Both in vitr o and in vivo normal anatomy is identified. The mucosa is high signal inten sity, the submucosa low signal intensity, the internal sphincter uniformly high signal intensity and the external sphincter low signal intensity on T1 - and T2-weighted images. In females, the transverse perineal muscle bridge s the inferior part of the external sphincter anteriorly. In perianal sepsi s. collections and the site of the endoanal opening are identified. In earl y-onset fecal incontinence following obstetric trauma/surgery, focal sphinc ter defects are demonstrated; in late-onset fecal incontinence external sph incter atrophy is seen. In fecally incontinent patients with scleroderma, f orward deviation of the anterior sphincter musculature with descent of rect al air and feces into the anal canal is noted. The extent of sphincter inva sion is assessed in low rectal tumours. In children with congenital anorect al anomalies, abnormalities of the muscle components are defined using smal ler diameter coils. Such information is invaluable in the assessment and su rgical planning of patients with a variety of anorectal pathologies.