MRI of the anal sphincter

Abstract

Objective: The anal sphincter was imaged with MR using an internal coil to demonstrate its anatomy, contrast enhancement patterns, and appearance in disease.

Materials and methods: A cylindrical saddle geometry coil was placed in the anal canal. Sixteen volunteers and 18 patients were examined. Imaging was performed on a 0.5 T Picker Asset MRI scanner in all the volunteers and nine patients and on a 1.0 T Picker HPQ Vista in nine patients. Then T1- and T2-weighted SE, T1-weighted GE and STIR images transverse to the sphincter, and T1-weighted SE images parallel to the sphincter in the coronal oblique plane were obtained. Intravenous gadopentetate dimeglumine (0.1 mmol/kg) was given to 2 normal subjects for dynamic studies and 10 patients for conventional postcontrast imaging.

Results: The coils were 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 past obstetric trauma, and these were also confirmed at surgery. Sphincter atrophy was seen in three patients with idiopathic fecal soiling.

Conclusion: MRI of the anal sphincter with an internal coil provides excellent visualization of normal anatomy and may be of considerable value in diagnosis.