Suspected perianal fistula or perianal sepsis

RadioGraphics perianal fistula review; ECCO-ESGAR-ESP-IBUS guideline; NICE CG49

A small-field-of-view, high-resolution question that a general pelvic MRI cannot answer. The surgeon needs the track classified against the sphincter complex, and that requires slices angled to the anal canal, not to the patient.

Recurrent perianal discharge, a suspected or recurrent fistula before surgery, or perianal Crohn disease being assessed or monitored.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MRI Perianal — fistula mapping

MRI Perianal / Anal Fistula

What we'd amend, and why

Classification depends on resolving two thin muscle layers a few millimetres apart, so the study is planned from the axis of the anal canal rather than the axis of the body: oblique-axial and oblique-coronal high-resolution T2 with a small field of view show the internal and external sphincters and where the track crosses them. Fluid-sensitive sequences make an active, fluid-filled track and any collection bright against dark fibrosis and fat, and post-contrast fat-suppressed T1 separates an inflamed enhancing track from an inactive fibrotic one — which is the difference between draining sepsis and closing a fistula.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-gadolinium-renal — Kidney function and gadolinium-based contrast; checked by Radiographer at the scanner
rulerule-pregnancy-gadolinium — Gadolinium in known or possible pregnancy; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(2)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Febrile or septic perianal disease

Role	Study & protocol	Why this answers the question
First line	MRI Perianal / Anal Fistula MRI Perianal — fistula mapping usually appropriate	MRI remains the study, and in sepsis its value is a different one from elective mapping: the deliverable is the extent of the collection and specifically whether it is supralevator or horseshoe, because that is what a surgeon cannot reach through a simple perianal incision and is the commonest reason a drained abscess recurs. Fluid-sensitive sequences make the pus bright and the small field of view keeps the resolution to say what it is crossing. Two things travel with this. Imaging must not delay drainage — a fluctuant, clinically obvious abscess in a septic patient goes to theatre, and examination under anaesthesia with drainage is both treatment and a diagnostic study in its own right. And an endoprobe has no place here: an acutely inflamed, painful anal canal will not tolerate a rotating probe, and the predictable outcome is an abandoned study or, far worse, an incomplete one reported as negative.
Second line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	For the systemically septic patient in whom MRI cannot be delivered on the day, portal-venous CT finds and localises the drainable collection and shows supralevator, retroperitoneal and intra-abdominal extension, which is what decides whether this is a perineal incision or a laparotomy. It is promoted from problem-solving to a real second rung in this arm because that timeframe is the point. What it cannot do is resolve the internal and external sphincter as separate layers, so it directs drainage and never definitive fistula surgery — the mapping MRI still has to happen once the sepsis is controlled.
Problem solving	Ultrasound musculoskeletal / soft tissue Ultrasound musculoskeletal / soft tissue	Transperineal ultrasound is applied to the skin surface rather than inside the canal, so it stays available in a patient who could not tolerate an endoprobe, and in experienced hands it answers the immediate bedside question of whether there is superficial pus to release now. It cannot show supralevator extension or a high blind track, so a negative study in a septic patient escalates rather than reassures.

Endoanal ultrasound is deliberately off this ladder. It is a good study for sphincter integrity in a comfortable patient and it returns to the pathway once the sepsis has settled.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Perianal / Anal Fistula MRI Perianal — fistula mapping usually appropriate	Classification depends on resolving two thin muscle layers a few millimetres apart, so the study is planned from the axis of the anal canal rather than the axis of the body: oblique-axial and oblique-coronal high-resolution T2 with a small field of view show the internal and external sphincters and where the track crosses them. Fluid-sensitive sequences make an active, fluid-filled track and any collection bright against dark fibrosis and fat, and post-contrast fat-suppressed T1 separates an inflamed enhancing track from an inactive fibrotic one — which is the difference between draining sepsis and closing a fistula.
Reasonable alternative	Endoanal / endorectal ultrasound Endoanal ultrasound — sphincter and perianal assessment	The honest division of labour is that the two tests answer different halves of the question. MRI maps the track — its course, any secondary extensions, and precisely where it crosses the internal and external sphincters — over a field of view that includes the supralevator space. Endoanal ultrasound cannot see that far: its useful field stops a few centimetres from a rotating endoprobe, so supralevator extension, deep ischioanal collections and horseshoe tracks are the things it misses, and it is operator-dependent in a way MRI is not. What it does better than anything else is the sphincter itself, resolving the internal and external sphincter as discrete layers and quantifying a defect by clock face and craniocaudal extent — which is the deliverable when the question is continence after obstetric injury or previous laying-open rather than where the track runs. It is therefore the right study in two situations: MRI unavailable, unacceptably delayed or not tolerated, and a question that is about sphincter integrity rather than track mapping. Instilling hydrogen peroxide through the external opening opacifies the track and materially improves identification of the internal opening, which narrows the gap for simple, low disease.
Reasonable alternative	Ultrasound musculoskeletal / soft tissue Ultrasound musculoskeletal / soft tissue	Transperineal ultrasound in experienced hands localises a superficial track and a drainable collection quickly at the bedside, and is a reasonable first look when the question is simply whether there is pus to release now. It cannot reliably show supralevator extension or a high blind track, so a negative study in recurrent disease should not stop the pathway.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	CT is for the septic patient in whom supralevator or intra-abdominal extension must be excluded urgently. It shows a collection but not the sphincter anatomy, so it directs drainage rather than definitive surgery.

Pitfalls

Vetting this as a routine pelvic MRI. A standard large-field-of-view pelvis lacks the resolution and the angulation, and the study will have to be repeated.
Planning straight axial slices instead of oblique axials perpendicular to the anal canal, which is what makes a track appear to cross the sphincter when it does not.
Cropping the field of view above the levator plate and missing supralevator extension, which is the extension that changes the operation most.
Missing secondary tracks and a horseshoe extension because only the visible external opening was assessed.
Reporting perianal Crohn disease without commenting on the rectum: proctitis changes whether definitive fistula surgery is safe at all.
Reporting a limited or painful endoanal study as negative. Active sepsis makes the probe intolerable and the examination incomplete, and an incomplete endoanal study should escalate to MRI rather than close the question.
Over-calling the normal anterior sphincter defect of a woman, which is the commonest false positive of endoanal ultrasound and can send a continent patient towards a repair she does not need.

Priors — what to pull first

Get the operative record: previous laying-open, seton placement or advancement flap changes the anatomy and the appearances permanently.
In Crohn disease, compare with the previous fistula study — treatment response is judged as change in track signal and volume rather than as track disappearance.

What makes a good request

The operation depends on how much external sphincter the track crosses, because dividing too much of it costs continence. That is the question the scan exists to answer.
Say whether a seton is in place and whether there has been previous surgery; both change the appearances substantially.
Two different questions get written on the same request card. "Where does the track go and what does it cross" is a mapping question and belongs to MRI. "Is the sphincter intact" — after obstetric injury, after previous fistula surgery, or in a patient whose complaint is incontinence rather than discharge — is a sphincter question, and endoanal ultrasound resolves those two thin muscle layers better than any other test. State which one is being asked.

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

MRI Perianal — fistula mapping CT Abdomen and Pelvis — Portal Venous Phase Ultrasound musculoskeletal / soft tissue Endoanal ultrasound — sphincter and perianal assessment

Confirm locally

MRI Perianal — fistula mapping: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.