OBJECTIVETo evaluate the value of magnetic resonance imaging (MRI) in the diagnosis of complex anal fistula.

METHODSThe preoperative digital examination and MRI with the phased-array coil were implemented for 28 patients who were clinically suspected with complex anal fistula. The final diagnosis were based on surgical findings. Outcomes of MRI and digital examination were compared with surgical results.

RESULTSTwenty-five patients were diagnosed as complex anal fistula, 1 presacral cyst and 2 chronic anorectal fistula combined with perianal mucinous adenocarcinoma. All the patients were correctly diagnosed by MRI,while the patients with presacral cyst and perianal mucinous adenocarcinoma could not be diagnosed correctly by digital examination. According to the Parks classification, 3 patients were suffered from trans-sphincteric fistula, 10 intersphincteric, 5 supra- sphincteric and 7 extra-sphincteric. The diagnosis rates of the internal opening with digital examination and MRI were 48% and 84%, the rates of the primary tract were 76% and 100%, and the rates of the secondary extensions were 57.9% and 94.7% respectively. The differences in detection of internal opening, primary tract and secondary extensions between MRI and digital examination were significant (P<0.01).

CONCLUSIONMRI with the phased-array coil can correctly orient the internal opening and direction of the complex anal fistula, and find the relationship between anorectal sphincters and the complex fistula.