DYNAMIC CONTRAST-ENHANCED MR-IMAGING OF PERIANAL FISTULAS

OBJECTIVE. The objective of this study was to prospectively compare dy namic contrast-enhanced MR imaging with MR sequences previously descri bed for assessing perianal fistulas in order to determine the best MR protocol for their evaluation. SUBJECTS AND METHODS. MR examinations o f 42 consecutive patients with clinically suspected perianal fistulas were independently evaluated by two experienced observers blinded to t he findings of digital rectal examination. The observers' evaluations occurred before definitive surgical exploration. All patients had body -coil MR imaging examinations, including the following sequences that were ranked for anatomic and pathologic information: spin-echo T1-weig hted, short inversion time inversion recovery, and dynamic contrast-en hanced MR imaging in the coronal plane; and spin-echo T2-weighted imag ing in the axial plane. Surgical findings were accepted as the gold st andard and were recorded independently by the surgeon, who was unaware of the findings of the MR assessment. MR findings were subsequently c orrelated with digital rectal examination before surgery and with clin ical follow-up. RESULTS. MR imaging correctly allowed our blinded obse rvers to predict the surgical anatomy of perianal disease in 37 of the 42 patients (accuracy, 88%). For detection of the presence and site o f an enteric fistulous entry, MR imaging had a sensitivity of 97%, a s pecificity of 67%, a positive predictive value of 88%, and a negative predictive value of 89%. On MR imaging examination, eight patients had no fistula, 12 had simple intersphincteric fistulas, and 22 had compl ex fistulas. MR imaging revealed all 14 perianal abscesses and fluid c ollections found at surgery. Digital rectal examination before surgery failed to reveal abscesses or important secondary tracks in eight of the 22 complex fistulas. For anatomic and pathologic depiction of fist ulas, dynamic contrast-enhanced MR imaging ranked as the best sequence for 22 of 34 fistulas. The short inversion time inversion recovery se quence, which was unable to distinguish small abscesses from perianal inflammation and showed spurious high signal in old fibrotic tracks, l ed our observers to misdiagnose five cases. In four patients for which initial surgery did not confirm enteric entry sites that our observer s had predicted by MR imaging, follow-up has confirmed the observers' diagnoses. The observers' evaluations of the MR examinations agreed in 37 (88%) of the 42 cases. CONCLUSION. MR imaging is more accurate tha n digital rectal examination before surgery in detecting complex featu res of perianal fistulas. MR imaging is noninvasive, is highly accurat e, and has low interobserver variability. With MR imaging, observers m ay better predict outcome than with initial surgical exploration. MR a ssessment that includes dynamic contrast-enhanced MR imaging and axial T2-weighted sequences (examination time, 20 min) provides the anatomi c and pathologic information required to guide surgical management.