MR imaging of fistula in ano: are endoanal coils the gold standard?

Objective: It has been suggested that fistula in ano is most accurately assessed using endoanal receiver coils because they provide superior spatial resolution. We aimed to determine their advantage by prospective comparison with conventional body coil imaging.

Subjects And Methods: Thirty consecutive unselected patients with a diagnosis of anorectal sepsis were examined by MR imaging with an endoanal coil. Imaging with a body coil followed. Imaging was independently evaluated by two radiologists who classified fistulas according to the coil used and then compared their findings, which were validated surgically.

Results: Five patients could not tolerate coil insertion. In the remaining 25 patients, endoanal imaging revealed no abnormalities in three patients in whom the body coil image correctly showed Crohn's disease, a sinus, and a transsphincteric fistula. Imaging with both coils revealed sepsis in 16 patients, allowing radiologists to make correct primary track classification in 13 patients on endoanal imaging compared with 15 patients on body coil imaging. Endoanal imaging revealed 10 secondary extensions in eight patients, but further extensions in two of these patients and in a third patient were undetected. All these extensions were seen on body coil imaging. Overall, surgical concordance was 68% for endoanal imaging compared with 96% for conventional body coil imaging.

Conclusion: Due to field-of-view limitations, endoanal imaging is less accurate than conventional body coil imaging for preoperative assessment of complex anal fistulas.