MRI Perianal — fistula mapping
From above the levator plate to below the anal verge, including both ischioanal fossae, the supralevator space and the perineal skin.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
Premedication
Given to make the acquisition work, not to treat the patient. Doses are typical — confirm against your local protocol and prescribe within your own governance.
Suppression of rectal and pelvic bowel peristalsis during the long high-resolution oblique T2 acquisitions. The case for it is weaker here than for rectal staging or enterography and should be stated as such: the anal canal, sphincter complex and ischioanal fossae are relatively fixed structures, and much of the motion that degrades a perianal study is whole-patient movement from discomfort rather than peristalsis. Where it helps is at the upper end of the field of view, keeping supralevator extension and any adjacent loop-related motion from blurring.
- Dose and route
- Where used, typically 20 mg intravenously, or about 20 mg intramuscularly in an unenhanced study where no cannula is otherwise needed. Glucagon roughly 0.25-1 mg intravenously as the substitute.
- When
- Immediately before the oblique high-resolution T2 series. If contrast is being given, the practical option is to use the same cannula and dose immediately before the post-contrast fat-suppressed T1.
- Do not give if
- If glucagon is used instead of hyoscine, it carries its OWN absolute contraindications rather than inheriting a clean slate: phaeochromocytoma, where it provokes catecholamine release and hypertensive crisis, and insulinoma or glucagonoma, where it causes rebound hypoglycaemia. It also raises blood glucose transiently, which matters in diabetes, and commonly causes nausea and vomiting.
- MHRA Drug Safety Update (February 2017), issued after nine reported deaths mostly from cardiac arrest: in patients with cardiac disease, monitor the patient and ensure resuscitation equipment AND staff trained to use it are readily available before giving it. This is an availability requirement, not a caution to note and move past.
- Practice varies more here than anywhere else in this section. Published perianal protocols differ on whether a spasmolytic is given at all, and many centres omit it routinely — this entry describes an option, not a standard, and the local protocol governs.
- Contraindicated in untreated angle-closure glaucoma, myasthenia gravis, megacolon, and significant tachyarrhythmia.
- Caution in prostatic enlargement with urinary retention, and in significant cardiac disease.
- Warn the patient about transient blurred vision and that they must not drive until it resolves.
- It does nothing for the commonest cause of a motion-degraded perianal study, which is pain from active sepsis. Analgesia and positioning address that; an antispasmodic does not.
- If unsuitable
- Omit it. A perianal fistula study is not routinely dependent on spasmolysis, and where motion is the limiting factor the more productive interventions are analgesia, comfortable positioning with knee support, and shortening or repeating the offending sequence.
When to use it
- Suspected or known perianal fistula before surgery, to classify the track and detect secondary extensions.
- Recurrent fistula after previous surgery.
- Perianal Crohn disease at diagnosis and for monitoring response to biological therapy.
- Suspected perianal or supralevator abscess where clinical examination is limited.
- Assessment of sphincter integrity before any procedure that risks continence.
Technique
- Straight sagittal T2 acquired first to define the axis of the anal canal.
- Oblique-axial and oblique-coronal high-resolution T2 planned perpendicular and parallel to that axis, with slices of roughly 3-3.5 mm, a small field of view of about 18-20 cm and a high matrix.
- Fat-suppressed T2 or STIR in the same oblique planes for fluid, oedema and collections.
- Fat-suppressed post-contrast T1 in the oblique planes; diffusion-weighted imaging is increasingly added and is useful where contrast is withheld.
Where it goes wrong
- Standard axial and coronal planes rather than planes aligned to the anal canal, so the relationship of the track to the sphincters cannot be determined.
- Field of view too large, losing the resolution needed to trace a thin track.
- Coverage stopping below the levator plate and missing supralevator extension.
- Post-contrast T1 without fat suppression, in which an enhancing track is lost against ischioanal fat.
- Assuming a seton is inert: it must be declared at MR safety screening and it also distorts local signal.
Clinical questions that reach this study
Contrast
Macrocyclic gadolinium agent at a typical single dose of about 0.1 mmol/kg, with fat-suppressed T1 after injection. Some centres omit contrast where fluid-sensitive sequences are diagnostic.
- Contrast mainly helps separate an active inflamed track and a drainable abscess from a fibrotic, inactive track.
Acquisition
- Breathing
- Free breathing; the patient must remain still for several long high-resolution acquisitions.
- Preparation
- No routine bowel preparation. Seton sutures should be declared — they are useful landmarks but are also a device-screening item. Say whether the patient has Crohn disease: it changes the expected pattern and the value of contrast. Rectal contrast and fistula-track injection are not part of the standard MR protocol.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- MR safety screening for implants and foreign bodies· radiographer at scan
- Gadolinium in known or possible pregnancy· radiographer at scan