Furthermore, the anal sphincter complex and levator ani are not visualized fluoroscopically; as such, the relationship of the fistula to these structures cannot be defined. Pelvic MRI with a multichannel phased array body coil has become the standard for imaging perianal fistula, especially those associated with CD, because they are more frequently complex with clinically occult tracts. Imaging with a body coil is better tolerated and is not limited by the field of view compared with MRI with an endoanal coil. The surgical concordance with fistula detection has been shown to be better with a body coil (96%) compared with an endoanal coil (68%) [20]. In multiple studies, MRI has been shown to have high sensitivity and specificity for the evaluation of perianal fistula. The meta-analysis by Zbar and Armitage [21] showed MRI had a sensitivity ranging from 81% to 100% and specificity from 67% to 100%, with accuracy for identification of the internal opening of 74% to 97%, and delineation of horseshoe fistula of 97% to 100% [21]. Sahni et al [22] showed that, for diagnosis of the primary fistula in CD patients, MRI had an accuracy of 64% to 100%. Comparing MRI to examination under anesthesia with or without endoanal US, the sensitivity and specificity for discriminating complex from simple perianal fistula was 97% and 96%, 92% and 85% for MRI, and 75% and 64% for endoanal US. Though fistula can be readily identified on MRI without IV contrast as hyperintense tracts on FSE T2-weighted sequences and short-tau inversion recovery sequences, the use of IV contrast facilitates visualization because tracts with active inflammation will avidly enhance and small associated abscesses will show ring enhancement around a central fluid collection. IV contrast enables the differentiation of inactive tracts containing granulation tissue, which diffusely enhance from active tracts that have ring or rim-like enhancement [18].
Anorectal Disease. Furthermore, the anal sphincter complex and levator ani are not visualized fluoroscopically; as such, the relationship of the fistula to these structures cannot be defined. Pelvic MRI with a multichannel phased array body coil has become the standard for imaging perianal fistula, especially those associated with CD, because they are more frequently complex with clinically occult tracts. Imaging with a body coil is better tolerated and is not limited by the field of view compared with MRI with an endoanal coil. The surgical concordance with fistula detection has been shown to be better with a body coil (96%) compared with an endoanal coil (68%) [20]. In multiple studies, MRI has been shown to have high sensitivity and specificity for the evaluation of perianal fistula. The meta-analysis by Zbar and Armitage [21] showed MRI had a sensitivity ranging from 81% to 100% and specificity from 67% to 100%, with accuracy for identification of the internal opening of 74% to 97%, and delineation of horseshoe fistula of 97% to 100% [21]. Sahni et al [22] showed that, for diagnosis of the primary fistula in CD patients, MRI had an accuracy of 64% to 100%. Comparing MRI to examination under anesthesia with or without endoanal US, the sensitivity and specificity for discriminating complex from simple perianal fistula was 97% and 96%, 92% and 85% for MRI, and 75% and 64% for endoanal US. Though fistula can be readily identified on MRI without IV contrast as hyperintense tracts on FSE T2-weighted sequences and short-tau inversion recovery sequences, the use of IV contrast facilitates visualization because tracts with active inflammation will avidly enhance and small associated abscesses will show ring enhancement around a central fluid collection. IV contrast enables the differentiation of inactive tracts containing granulation tissue, which diffusely enhance from active tracts that have ring or rim-like enhancement [18].