Perianal Fistula MRI on Radiology Reports: What It Means

A perianal fistula is a small tunnel that forms between the anal canal and the skin near the anus. An MRI of the pelvis maps this tunnel system in detail so your care team can plan the best treatment and reduce the risk of the fistula coming back.

What is a perianal fistula?

A perianal fistula usually starts after a tiny gland near the anus gets blocked and infected. The infection can drain to the skin and leave behind a persistent tract (tunnel). People may notice pain, swelling, discharge, or a small opening near the anus. Fistulas are common in general practice and also in Crohn’s disease. The key to successful treatment is finding the exact pathway of the tract and any hidden branches or pockets of infection. That’s where MRI shines.

Why MRI is the preferred test

MRI is the gold-standard imaging test for perianal fistulas because it shows soft tissues clearly without radiation. It can:

• Find the internal opening where the tract connects to the anal canal.

• Show the primary tract and any secondary branches or extensions.

• Detect abscesses (pockets of pus) that may need drainage.

• Map involvement of the sphincter muscles, which is important for protecting continence during surgery.

• Track inflammation vs scar tissue on follow-up.

This information helps surgeons choose the right procedure.

How the MRI is done

A perianal fistula MRI is a specialized pelvic MRI focused on the anal canal and the tissues around it.

• Position: You’ll lie on your back. The scan usually takes 20–30 minutes.

• Coils: A surface coil over the pelvis improves detail.

• No prep: Typically no bowel prep. You can eat and drink normally unless told otherwise.

• Contrast: Gadolinium contrast is often used. It helps show active inflammation and abscess walls but may be omitted in certain cases. Tell your team about kidney issues, allergies, or pregnancy.

• Sequences: Radiologists use different sequences or ways of looking at the tissues:

  • T2-weighted images to highlight fluid-filled tracts and abscesses.

  • T1-weighted images for anatomy and to spot blood or fat.

  • Post-contrast T1-weighted images to show enhancing active tracts and abscess rims.

  • Diffusion-weighted imaging (DWI) is sometimes added to help identify pus.

What your report will describe

MRI reports answer surgical planning questions in a structured way. Expect wording like this:

• Internal opening (IO): The connection to the anal canal, often described by a “clock-face” (e.g., 6 o’clock posterior midline).

• Primary tract path: Whether it is intersphincteric, transsphincteric, suprasphincteric, or extrasphincteric—terms that describe how the tract relates to the internal and external sphincter muscles.

• Secondary extensions: Side branches, horseshoe components (wrap around), and any extensions into the ischioanal fossa.

• Abscesses: Size, location, and whether they cross spaces or muscles.

• Sphincter involvement: Which parts of the internal or external sphincter are crossed—key to choosing a sphincter-sparing approach.

• Activity: Words like “active” suggest inflammation; “fibrosis” or “scar” suggests an inactive tract.

Decoding common report terms

• Intersphincteric: The tract runs between the internal and external sphincter muscles and may open on the skin near the anus.

• Transsphincteric: The tract passes through the sphincters into the fat outside it

• Suprasphincteric: The tract loops above the external sphincter before descending to the skin.

• Extrasphincteric: The tract bypasses the sphincter complex entirely (rare; think trauma, surgery, or Crohn’s).

• Horseshoe extension: The tract curves around the back (posterior) or front (anterior) in a semicircle.

• Ischioanal fossa: The fat-filled space adjacent to the anal canal—important when infections spread.

What “active” vs “fibrotic” means

• Active fistula: Bright on T2 fat-suppressed images, often enhances after contrast, and may restrict on DWI. Symptoms (pain, discharge) are more likely.

• Fibrotic/inactive fistula: Darker on T2, minimal or no enhancement. This can reflect healing or prior treatment. Surgeons may treat differently depending on activity.

How MRI guides treatment

The surgeon needs a “map.” MRI provides:

• Exact site of the internal opening to target during surgery.

• Extent across sphincter muscles to decide surgical procedure.

• Abscess detection to plan drainage and antibiotics.

• Recurrent or complex disease assessment—especially in Crohn’s disease or prior operations.

Special situations

Crohn’s disease

Patients with Crohn’s often have multiple, branching tracts and recurrent abscesses. MRI helps distinguish active inflammation from scar and monitors response to therapy.

Recurrent fistula after surgery

Scar can hide small residual tracts. MRI clarifies whether symptoms are due to a new tract, a missed side branch, or just scarring.

Pregnancy and MRI

MRI without contrast is generally considered when needed. Decisions are individualized; always tell your team if you are pregnant or breastfeeding.

What to ask your care team

• Where is the internal opening?

• Does the tract cross the external sphincter?

• Are there abscesses or horseshoe extensions?

• Is the tract active or mostly fibrotic?

• What procedure is safest for me, and how does the MRI guide that choice?

• If I have Crohn’s, how will imaging be used to monitor healing?

Safety and comfort

MRI is noninvasive and uses magnets, not X-rays. Most people tolerate it well. Let the team know about implants, pacemakers, or metal in the body. If you are claustrophobic, ask about comfort strategies or light sedation. Gadolinium contrast is generally safe for people with normal kidney function.

Bottom line

Perianal fistula MRI creates a clear map of tunnels and any pockets of infection around the anus. Your report focuses on the internal opening, the path of the tract, involvement of the sphincter muscles, and the presence of abscesses. This information directly shapes your surgeon’s plan and improves outcomes.

References

https://radiopaedia.org/articles/perianal-fistula

https://my.clevelandclinic.org/health/diseases/14466-anal-fistula

https://insightsimaging.springeropen.com/articles/10.1007/s13244-010-0022-y