Trace every tract on T2 fat-saturated images from the internal opening (clock position) to the skin, classify it against the sphincters (Parks), and report extensions and abscesses — the surgical map.
Orient first
- Parks classification: intersphincteric, transsphincteric, suprasphincteric, extrasphincteric; superficial tracts lie outside it.
- Active tracts are fluid-filled and bright on T2 fat-saturated images; fibrotic tracts are dark.
- Crohn disease produces complex, branching fistulas; MAGNIFI-CD and similar scores grade activity (verify the one your surgeons use).
Acquire the study
- Pelvic MRI with a phased-array coil: oblique axial and oblique coronal T2 (with and without fat saturation) aligned to the anal canal, DWI, and post-gadolinium fat-saturated T1.
The manoeuvre
- Oblique axial T2 fat-saturated: find the internal opening and give it as a clock position in the lithotomy convention.
- Follow the primary tract: which sphincters it crosses — internal only (intersphincteric) or both (transsphincteric) — and at what level of the external sphincter.
- Oblique coronal T2: relation to the levator ani (supralevator extension) and the ischioanal fossa.
- Secondary tracts and horseshoe extension across the midline behind the anal canal.
- Abscesses: fluid collections with rim enhancement on post-contrast T1, size in cm; state the distance from the anal verge.
What confirms it
- A T2-hyperintense tract with enhancing wall connecting an internal opening in the anal canal to the perianal skin or a blind end.
What licenses you to exclude it
- No T2-bright tract or collection on oblique planes aligned to the canal excludes an active fistula; fibrotic healed tracts remain dark.
The classic misread
- Imaging in true axial instead of oblique planes — the tract-sphincter relation becomes ambiguous.
- Missing a supralevator extension, which changes surgery.
- Mistaking a seton track or post-operative scar for active disease — compare with post-contrast images.