MRI for Perianal Fistula

What Is a Perianal Fistula?

A perianal fistula is an abnormal tunnel-like track connecting the anal canal to the skin near the anus. It typically develops after a perianal abscess — a collection of pus near the anus that either drains spontaneously or is surgically drained. The abscess cavity doesn't fully heal, and the remaining track becomes a fistula.

The anatomy of the anal canal makes this area particularly prone to fistula formation. Around the anal canal sit the internal and external anal sphincter muscles (responsible for continence) and, above them, the levator ani muscles of the pelvic floor. Between the sphincters are the intersphincteric glands — these can become infected, forming an abscess that may drain in various directions, creating fistula tracks that follow complex anatomical pathways.

Perianal fistulas affect approximately 1-2 per 10,000 people per year in the UK, with men affected roughly twice as often as women. Peak incidence is between ages 20 and 40. The most common cause is cryptoglandular (from infected anal glands), accounting for about 90% of cases. Crohn's disease is the other major cause — up to 30-50% of people with Crohn's disease will develop perianal fistulas at some point, and these tend to be more complex and harder to treat.

Why MRI Is Used for Perianal Fistulas

MRI has become the single most important investigation before perianal fistula surgery. Here's why:

• Mapping the fistula track: MRI shows the exact course of the fistula — where it starts (internal opening), where it goes (through or between the sphincter muscles), and where it ends (external opening on the skin or secondary extensions). This information is not reliably obtained by clinical examination alone.
• Detecting hidden extensions and abscesses: Studies have shown that MRI reveals secondary fistula tracks, extensions, and occult abscesses that were missed on clinical examination in 10-25% of cases. Missing these at surgery leads to recurrence.
• Classifying the fistula: The Parks classification (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric) determines the surgical approach. An intersphincteric fistula can be safely laid open; a transsphincteric fistula involving significant sphincter muscle cannot be simply cut without risking incontinence.
• Assessing sphincter integrity: Before any surgical intervention, MRI shows whether the sphincter muscles are intact. Previous obstetric injury, surgery, or muscle atrophy affects the amount of sphincter that can safely be divided.
• Crohn's disease assessment: In Crohn's-related fistulas, MRI reveals the often complex, branching pattern of disease, associated rectal inflammation, and helps guide both medical (biologics) and surgical management.
• Monitoring treatment response: Follow-up MRI after medical treatment (particularly anti-TNF therapy for Crohn's fistulas) shows whether tracks are healing — more reliably than clinical examination, as the external opening may close while deep tracks persist ("superficial healing" without deep healing).

MRI Sequences Used

• T2-weighted axial (perpendicular to the anal canal): The primary diagnostic sequence. Fistula tracks appear as high-signal (bright) linear structures if they contain fluid or granulation tissue. The sphincter muscles appear as distinct dark rings — inner (internal sphincter) and outer (external sphincter) — against which the fistula course is mapped.
• T2-weighted coronal (parallel to the anal canal): Shows the vertical extent of fistula tracks, extensions into the ischiorectal fossa or supralevator space, and the relationship to the levator ani muscles.
• STIR axial and coronal: Fat suppression makes the fluid-filled fistula tracks and associated inflammation stand out even more brightly against the suppressed perianal fat.
• T1-weighted axial: Provides anatomical reference. Active tracks appear low signal (dark) on T1. Collections of pus show intermediate-to-low signal.
• T1 post-gadolinium with fat saturation: Enhancing tissue (granulation tissue, abscess walls) lights up. This helps distinguish active, inflamed tracks from inactive, fibrotic ones — particularly relevant for Crohn's disease monitoring.
• DWI: Pus and active inflammation restrict diffusion (bright on DWI). Useful for detecting small abscesses that might be subtle on other sequences.

What Radiologists Look For

• Internal opening: Where the fistula originates in the anal canal — usually at the level of the dentate line (the junction between anal canal and rectum). Most internal openings are posterior midline or lateral. The clock-face position (e.g. 6 o'clock) is documented.
• Parks classification:
  • Intersphincteric (most common, ~45%): Track runs between the internal and external sphincters.
  • Transsphincteric (~30%): Track passes through both internal and external sphincters into the ischiorectal fossa.
  • Suprasphincteric (~5%): Track passes over the top of the external sphincter, through the levator plate.
  • Extrasphincteric (rare): Track passes outside the sphincter complex entirely, from the rectum directly to the perineal skin.
• St James's University Hospital (SJUH) classification: Some centres use this MRI-specific grading (Grade 1-5) that incorporates secondary extensions and abscesses, providing a more detailed surgical roadmap than Parks alone.
• Secondary tracks and extensions: Horseshoe extensions (connecting across the midline), supralevator extensions, ischiorectal extensions. These must be addressed surgically or they'll lead to recurrence.
• Abscesses: Undrained collections of pus — appearing as bright areas on T2/STIR with surrounding enhancement. Their size, location, and relationship to the sphincters are documented.
• Sphincter involvement: How much of the external sphincter is traversed by the track? This directly determines whether fistulotomy (laying open) is safe or whether a sphincter-preserving approach is needed.
• Rectal inflammation: In Crohn's disease, associated proctitis (rectal inflammation) affects management — active rectal disease usually needs to be treated medically before fistula surgery.

How to Prepare for the Scan

• No special bowel preparation is usually required for pelvic MRI for fistula assessment
• Some centres use a micro-enema 1-2 hours before the scan to empty the rectum — this reduces artefact from rectal contents but isn't universal
• Eat and drink normally
• If you have a seton in place (a loose thread placed through the fistula track by a surgeon), this is fine — it doesn't affect the MRI and can actually help identify the track
• If you have an active abscess causing significant pain, let the department know — lying still may be difficult, and pain management before the scan may be needed
• An anti-spasmodic injection (Buscopan) may be given to reduce rectal movement during the scan

What Happens During the Scan

You'll lie on your back with a surface coil over your pelvis. The scan takes approximately 25-35 minutes. The images are acquired in planes oriented relative to the anal canal (not the body), which the radiographer angles based on initial scout images.

The scan is painless, though lying on your back may cause some discomfort if you have an active perianal abscess. You can communicate with the radiographer through the intercom at any time.

Gadolinium contrast is not always used. For straightforward fistula mapping, non-contrast MRI is often sufficient. Contrast is more likely to be used in Crohn's disease (to assess activity) or when distinguishing active from healed tracks on follow-up scans.

Some centres perform endoanal MRI using a small endorectal coil inserted into the anal canal — this provides extremely high-resolution images of the sphincters and immediately adjacent fistula anatomy. However, body-coil or phased-array external coil MRI is now considered to provide equivalent diagnostic accuracy for most clinical questions and is far more comfortable.

What MRI Can't Show

• Very superficial fistulas: Short, shallow subcutaneous fistulas near the skin surface may be below MRI resolution or appear indistinct against the perianal subcutaneous fat. Clinical examination is usually sufficient for these simple fistulas.
• Active vs. inactive with certainty (without contrast): On non-contrast MRI, it can be difficult to distinguish an active, fluid-filled track from a fibrotic, healed track — both can appear dark on T1. Post-gadolinium imaging helps, but even this has limitations for assessing deep healing.
• Epithelialisation: Whether a fistula track has become lined with epithelium (skin-like cells) — which makes spontaneous healing unlikely — isn't visible on MRI. This is determined during surgical examination.
• Functional sphincter assessment: MRI shows sphincter anatomy (thickness, integrity, defects) but doesn't measure sphincter function. Anorectal manometry (pressure testing) and endoanal ultrasound provide complementary functional information.

Treatment Pathways After Diagnosis

• Simple fistula — fistulotomy: For intersphincteric and low transsphincteric fistulas involving minimal sphincter muscle, the track is laid open (fistulotomy) and allowed to heal from the base. Success rate over 90%. MRI confirms the track is suitable for this approach.
• Complex fistula — seton insertion: A loose seton (a thread or loop) is placed through the fistula track to keep it draining and prevent abscess recurrence. This may be the definitive treatment (draining seton) or preparation for a later definitive procedure (cutting seton or LIFT/advancement flap).
• LIFT procedure (Ligation of Intersphincteric Fistula Track): The track is divided and ligated in the intersphincteric space, preserving sphincter muscle. Good results for transsphincteric fistulas.
• Advancement flap: Tissue is mobilised from inside the rectum to cover the internal opening. Preserves the sphincter completely. Success rates of 50-80%.
• VAAFT (Video-Assisted Anal Fistula Treatment): A fistuloscope is inserted into the track, allowing direct visualisation and treatment of the internal opening and any secondary tracks.
• Crohn's fistula — medical management first: Anti-TNF therapy (infliximab, adalimumab) combined with antibiotics and seton drainage. Medical treatment aims to control the underlying Crohn's inflammation before or instead of definitive surgical repair. Follow-up MRI monitors deep healing.
• Abscess drainage: If MRI identifies an undrained abscess, surgical drainage (examination under anaesthesia — EUA) is the priority before any definitive fistula treatment.

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