Dynamic contrast-enhanced MR imaging of perianal fistulas.

OBJECTIVE: The objective of this study was to prospectively compare dynamic contrast-enhanced MR imaging with MR sequences previously described for assessing perianal fistulas in order to determine the best MR protocol for their evaluation. SUBJECTS AND METHODS: MR examinations of 42 consecutive patients with clinically suspected perianal fistulas were independently evaluated by two experienced observers blinded to the 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-weighted, short inversion time inversion recovery, and dynamic contrast-enhanced MR imaging in the coronal plane; and spin-echo T2-weighted imaging in the axial plane. Surgical findings were accepted as the gold standard and were recorded independently by the surgeon, who was unaware of the findings of the MR assessment. MR findings were subsequently correlated with digital rectal examination before surgery and with clinical follow-up.
RESULTS: MR imaging correctly allowed our blinded observers to predict the surgical anatomy of perianal disease in 37 of the 42 patients (accuracy, 88%). For detection of the presence and site of an enteric fistulous entry, MR imaging had a sensitivity of 97%, a specificity 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 complex fistulas. MR imaging revealed all 14 perianal abscesses and fluid collections 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 fistulas, dynamic contrast-enhanced MR imaging ranked as the best sequence for 22 of 34 fistulas. The short inversion time inversion recovery sequence, which was unable to distinguish small abscesses from perianal inflammation and showed spurious high signal in old fibrotic tracks, led our observers to misdiagnose five cases. In four patients for which initial surgery did not confirm enteric entry sites that our observers 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 than digital rectal examination before surgery in detecting complex features of perianal fistulas. MR imaging is noninvasive, is highly accurate, and has low interobserver variability. With MR imaging, observers may better predict outcome than with initial surgical exploration. MR assessment that includes dynamic contrast-enhanced MR imaging and axial T2-weighted sequences (examination time, 20 min) provides the anatomic and pathologic information required to guide surgical management.