Endoluminal MR Imaging of the Rectum and Anus: Technique, Applications, and Pitfalls

Abstract

Anorectal diseases (eg, fecal incontinence, perianal and anovaginal fistulas, anorectal tumors) require imaging for proper case management. Endoluminal magnetic resonance (MR) imaging has become an important part of diagnostic work-up in such cases. Optimal endoluminal MR imaging requires careful attention to patient preparation, imaging protocols, and potential pitfalls in interpretation. Comfortable positioning and the use of an antiperistaltic drug are vital for adequate patient preparation. Selected sequences and imaging planes are used in imaging protocols tailored for specific diseases. In fecal incontinence, three-dimensional sequences allow detailed demonstration of the anal anatomy and related defects. In perianal and anovaginal fistulas, longitudinal imaging planes help determine the superior extent of the abnormality. In anorectal tumors, T1-weighted turbo spin-echo MR imaging can help detect extension into the perirectal fat and T2-weighted turbo spin-echo MR imaging is used to optimize contrast between tumor and the rectal wall. Off-axis and radial imaging planes are used in all anorectal diseases to minimize partial volume effects. Potential pitfalls include various parts of the normal anal anatomy mimicking sphincter defects, veins and hemorrhoids mimicking fistulas and abscesses, and overhanging tumor mimicking more extensive tumor. Adequate patient preparation combined with proper technique and a knowledge of potential pitfalls will allow optimal endoluminal MR imaging of the rectum and anus. MR, 757.121411, 757.121413 • Rectum, neoplasms, 757.30 RadioGraphics 1999; 19:383-398 1 From the Departments of Radiology (J.S., E.R., A.W.Z., J.S.L.) and Surgery (W.R.S.), University Hospital Rotterdam Dijkzigt, Erasmus University Rotterdam, the Netherlands, and the Department of Radiology, Academic Medical Center, P.O. Box 22700, 1100 DE Amsterdam, the Netherlands (J.S., J.S.L.). Recipient of a Certificate of Merit award for a scientific exhibit at the 1997 RSNA scientific assembly.

MRI Anatomy of the Anal Region

“…13 The results of 3-dimensional positioning in the current study indicate that this thick muscular layer is indeed the vertical levator (Fig. 2B) and that the vertical levator was mistaken for the LRM muscle in the study by Rociu et al 2,3,12 New MRI signs in the current study show that the caudal end of the vertical levator becomes a series of tendinous-like cords below the plane of the deep sphincter (Fig. 4B), so that the complete levator ani muscle is composed of the tendinous arch, a muscle belly, the anococcygeal raphe, 14 and these tendinous cords (Fig.…”

“…5C). [1][2][3]11 Our current MRI images demonstrated that the inner space is an important anatomical plane which subdivides the anal region into 2 portions, the "anal portion" and the "perianal portion." The anal portion is equivalent to the "imaging anal canal," which consists of the mucosa, submucosa, and the anal smooth muscle.…”

“…1 For example, endoanal coil MRI studies by Rociu and colleagues 2,3 suggested that the levator ani muscle has only a transverse portion and that the EAS muscle is composed of only a SC and a superficial portion. However, body coil MRI studies by Guo and Li 4 suggested that the levator ani muscle also has a vertical portion (vertical levator), and body coil MRI studies by Hsu et al 5 suggested that the EAS muscle has 3 separate components.…”

Mri Evaluation of Perianal Fistulas

“…[5] T2W images provide good contrast between the tract and the fibrous wall of the fistula, providing good delineation of the layers of the anal sphincter. [6], [7] With MRI providing excellent anatomical details and relations of fistulous tracts and their associated abscesses, an MR-based classification is now used that categorises perianal fistulas into five grades. Grade 1 fistulas are simple linear intersphincteric fistulas that traverse the intersphincteric plane to the perianal skin, completely confined by the external sphincter.…”

Evaluating Patients with Anorectal Incontinence

“…85 Lesions of the external anal sphincter, such as sphincter defects and scar tissue, have been shown to have an accuracy of 90%-95%. [85][86][87][88][89] Endoluminal MRI is superior to endoluminal ultrasonography because of its multiplanar capability and the higher inherent contrast resolution of the former, especially in detecting defects of the external anal sphincter (accuracy 91% versus 73%). 90 Endoluminal MRI can easily identify atrophy of the external anal sphincter, and it might predict negative outcomes of anterior anal repair.…”