Non-contrast pelvic magnetic resonance (MR) protocol for imaging of complex perianal fistulizing disease

Deirdre C Kelleher, MD, M. Sophia Villanueva, MD, Kirthi Kolli, MD, James D McFadden, MD, Anjali S Kumar, MD, MPH

MedStar Washington Hospital Center – Section of Colon and Rectal Surgery, Department of Surgery and Department of Radiology

Objective: Magnetic resonance (MR) is emerging as the technology of choice for evaluating complex perianal fistulas. MR precisely displays the anatomy of the sphincter muscles in an operator-independent fashion, without exposing the patient to ionizing radiation or uncomfortable probes. Transrectal ultrasound offers a limited field of view (absence of a coronal plane) and requires an anal transducer which can be painful for patients with perianal disease. CT fistulography is also of limited utility as fistula tracts, fibroses, and sphincter muscles often have similar attenuations, preventing clear anatomical delineations.

Description: With the advancement of MR technology, imaging of pelvic musculature can be obtained without the need for endolumnial coils or gadolinium dye. External phased-array surface coils can depict the external anal sphincter as hypointense on T1W, T2W, and fat-suppressed T2W images. Axial fat-saturated T2W imaging shows the internal sphincter as relatively hyperintense, bounded laterally by the hypointense external sphincter. There is no need for the addition of gadolinium, or other contrast agent, as the T2W images (TSE and fat-suppressed) provide good contrast between the hyperintense tract fluid and the hypointense fibrous fistula wall. Fat uptake of gadolinium would create a bright-on-bright appearance obscuring the view of the anal sphincter layers and the fistula tract. The combination of views provided by MR (axial, radial, coronal, and sagittal) also aids in the distinction between fistula types. Axial images best depict the internal opening of the fistula and the location of the primary tract (ischioanal or intersphincteric). Coronal imaging depicts the levator plane for distinguishing between supralevator and infralevator infections. Disruption of the external anal sphincter on any series differentiates transsphincteric and intersphincteric fistulas.

Preliminary Results: In an exploratory study, we sought to understand the current use of MR in fistula management at our institution. First, we retrospectively reviewed records of patients who underwent a fistula procedure for the past two years to determine the frequency of pelvic MR use prior to surgical management. We then reviewed radiology records from the same time period to ascertain which patients underwent medical therapy following MR imaging. We found that of 185 patients who underwent at least one procedure for a perianal fistula at our institution, 20 underwent MR imaging prior to their procedure. From the radiologic data, we found 47 patients with fistula protocol MRs, 37 of which were positive for a fistula. Comparing the data, 16 patients had both an MR and a procedure at our institution.

Future directions: MR imaging is a useful tool for determining the anatomy of complex perianal fistulas, providing excellent images without the need for uncomfortable devices, the use of ionizing radiation or contrast dyes. A comparison of the MR fistula description with that of the operative reports will aid further in our understanding the utility of this technology. We will also look more closely at those who were found to have a fistula but did not undergo a procedure at our institution to determine how the results of the MRI were used in patient management.

Session: Poster Presentation

Program Number: ETP048

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