A Perianal MRI is a focused MRI scan of the tissues around the anus. It is the best test for showing anal fistulas — small abnormal tunnels that form between the inside of the anal canal and the skin around it.
Doctors request this scan before surgery for an anal fistula or recurrent abscess. The findings tell the surgeon exactly where each tunnel runs, whether there are hidden branches, and how the tracks relate to the sphincter muscles that keep you continent. Operating without this map risks cutting the wrong muscle and causing leakage of stool.
Why would my doctor ask for a Perianal MRI?
Your doctor may recommend a Perianal MRI to look closely at the tissues around the anus, especially if you have:
- A confirmed or suspected anal fistula.
- A perianal abscess that keeps coming back.
- Anal fistulas as part of Crohn's disease.
- Symptoms after childbirth, such as discharge or pain that suggests a fistula.
- Recurrent symptoms after a previous fistula operation.
- A planned operation, where the surgeon needs a full map before cutting.
What can a Perianal MRI detect?
A Perianal MRI helps doctors see:
- The main fistula track and where it opens inside the anal canal.
- Any secondary branches, side tracks, or horseshoe extensions.
- Hidden abscesses that have not yet broken through to the skin.
- The relationship of the fistula to the internal and external anal sphincter muscles.
- Inflammation in the surrounding fat and muscle.
- Specific patterns of disease that suggest Crohn's involvement.
Do I need any special preparation for a Perianal MRI?
Preparation is usually minimal:
- A small enema may be given an hour or so before the scan to keep the area clean during imaging.
- You do not need to fast.
- Remove all jewellery, body piercings, and metal objects, and change into a hospital gown.
- Tell the team about any implants, especially pacemakers or surgical metal.
- If you have a draining seton or other device in place from previous surgery, let the team know — it will not be removed for the scan.
Will I be injected with dye for this scan?
Often, yes. Gadolinium contrast helps highlight active fistula tracks and small abscesses that might otherwise be missed. The dye is given through a small drip in your arm. A few centres can perform an adequate study without contrast, especially in children.
What does a Perianal MRI feel like?
The scan is painless. You will lie on your back on a padded table that slides into the MRI scanner.
A receiving device (a coil) will be placed over your pelvis. The scanner makes loud knocking and humming sounds, and earplugs or headphones will be provided. You will be asked to lie still and breathe normally — there are usually no breath-holds. The radiographer will speak to you between sequences.
If the area is currently painful, tell the radiographer before the scan starts; cushions can be added to help you stay comfortable.
Is it safe for pregnant women or children?
The scan uses no radiation and is safe in principle. In pregnancy, contrast is usually avoided, and the scan is performed only when the result will change immediate care. In children, it is used for complex or Crohn's-related fistulas and is preferred over repeated CT because it avoids radiation.
How long does a Perianal MRI take?
The scan usually takes 30 to 45 minutes. Including changing and any cannula placement, the whole appointment is usually about an hour.
When will I get my results?
A Radiologist with experience in pelvic and gastrointestinal imaging will review the images and prepare a detailed report. Results are typically ready within a few days. Your colorectal surgeon will use the report to plan the operation, and may go through the images with you at your next appointment.
Will this scan tell my surgeon exactly how to operate?
In most cases, yes. The scan gives the surgeon a precise map: where the track starts inside the anal canal, where it travels, where it ends, and which sphincter muscles it crosses. With that information, the surgeon can choose the safest operation — for example, a simple cut for a low fistula, or a seton for a high one that crosses the sphincter — and avoid causing incontinence. Without the scan, hidden branches and small abscesses are often missed, and patients come back with recurrence.
Why is MRI better than a CT or an ultrasound for this problem?
CT struggles to show the small fistula tracks and exposes you to radiation. Endoanal ultrasound is useful and can be done in the clinic, but it is operator-dependent and gives a narrower view. MRI shows the entire perianal area in fine detail, including high tracks and abscesses that lie above the reach of an ultrasound probe — and it does this without any radiation, which matters because some patients need repeated scans over many years.
What is the Parks classification and will it be in my report?
The Parks classification is a system surgeons use to describe anal fistulas by how they relate to the sphincter muscles — for example, intersphincteric, transsphincteric, suprasphincteric, or extrasphincteric. Most radiology reports will include this classification, because it is the language colorectal surgeons use to plan the operation. Your surgeon will explain what your specific classification means for your treatment.
What if I'm nervous about being inside the scanner?
It is normal to feel anxious — especially because the scan area is sensitive. The radiographer will be in contact with you the entire time and you can hold a squeeze ball to alert them if you need a break. If you are very claustrophobic, ask your doctor about a mild sedative beforehand.
Sources and further reading
- RadiologyInfo: MRI of the body
- ACR Manual on Contrast Media
- FDA: Gadolinium retention safety communication
Conclusion
A Perianal MRI is the standard test for mapping anal fistulas and abscesses before surgery. The detail it gives your surgeon directly protects the sphincter muscles that control continence — which is why most colorectal surgeons will not operate on a complex fistula without this scan. Tell the team about any previous fistula surgery so they know exactly what to look for.