Magnetic resonance imaging of the anal sphincter using an internal coil

A surface coil placed within the anal canal was used to image the anal sphincter and determine normal anatomy and contrast enhancement patterns as well as appearances in disease. Sixteen normal volunteers and 24 patients were examined. Imaging was performed on a 0.5-T Picker Asset and a 1.0-T Picker HPQ Vista MRI scanner. T1-weighted and T2-weighted spin-echo, T1-weighted gradient-echo, STIR images transverse to the sphincter, and T1-weighted spin-echo images parallel to the sphincter in the coronal oblique plane were obtained. Intravenous gadopentetate dimeglumine (0.1 mmol/kg) was given to two normal subjects and 10 patients. The coil was easy to insert and well-tolerated and provided high spatial resolution. The internal sphincter had a higher signal intensity than the external sphincter on all sequences but particularly on STIR images. Brisk contrast enhancement of the internal sphincter was seen. Sphincteric abscesses and fistulous tracks were identified in three patients and confirmed at surgery. Sphincter defects were seen in three patients with obstetric trauma, and these were confirmed at surgery. Sphincter atrophy was seen in four patients with idiopathic fecal soiling. High resolution magnetic resonance imaging with a dedicated endoanal coil provides excellent visualisation of normal anatomy and pathology in and around the anal sphincter and may be of considerable value in diagnosis.