MRI Rectum — primary staging

From at least the level of the sacral promontory (higher for a proximal tumour, to include the origin of the inferior mesenteric artery where nodal assessment demands it) to below the anal verge, including the whole mesorectum and both pelvic sidewalls.

Typical, not policy

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.

Hyoscine butylbromide (Buscopan), or glucagon where unavailable — where locally used

Suppression of rectal and small bowel peristalsis. The high-resolution oblique-axial T2 is a long acquisition at roughly 0.6 mm in-plane resolution, and that resolution is the only reason the study can call the muscularis propria and the mesorectal fascia; motion during it blurs exactly the interface the staging decision rests on.

Dose and route: Typically 20 mg intravenously, or about 20 mg intramuscularly where intravenous access is not being placed. Glucagon roughly 0.25-1 mg intravenously where hyoscine is contraindicated or unavailable.
When: Immediately before the high-resolution T2 sequences it is meant to cover. The effect lasts only a few minutes against an examination of 20-30 minutes, so some centres redose part-way through; where a single dose is given, it should be timed to the oblique-axial acquisition rather than to the start of the study.
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 genuinely varies. ESGAR consensus favours a spasmolytic where it is available and not contraindicated, but it is not universally given, and a number of departments run rectal staging without it and report acceptable image quality — confirm the local protocol rather than assuming either way.
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 — a point routinely omitted in an outpatient staging list where people have driven themselves in.
If unsuitable: Proceed without it: an unparalysed rectal staging study is degraded rather than worthless, and repeating the oblique-axial acquisition is often enough. Substitute glucagon where the contraindication is specific to hyoscine.

When to use it

Newly diagnosed rectal adenocarcinoma requiring local staging before treatment selection.
Assessment of mesorectal fascia involvement and the predicted circumferential resection margin.
Identification of extramural venous invasion and tumour deposits.
Assessment of low rectal tumours in relation to the sphincter complex and levator plate.
Nodal assessment within the mesorectum and along the pelvic sidewall.

Technique

Sagittal T2 first, used to plan everything else.
High-resolution oblique-axial T2 angled strictly perpendicular to the tumour axis, with thin slices of the order of 3 mm, a small field of view and no fat suppression — in-plane resolution of roughly 0.6 mm is the published target.
Oblique-coronal T2 parallel to the anal canal for low tumours, to assess the sphincter complex and intersphincteric plane.
Diffusion-weighted imaging acquired in the same orientation as the oblique-axial T2 so the two can be compared directly.
A large field-of-view axial T2 covering the whole pelvis for sidewall nodes and incidental findings.

Where it goes wrong

Axial slices angled to the patient rather than perpendicular to the tumour, which is the defining technical failure: it artefactually blurs the muscularis propria and overcalls extramural spread.
Fat suppression applied to the high-resolution T2, which destroys the fat-plane contrast the whole staging assessment depends on.
Slice thickness above about 3 mm, or field of view too large, losing the resolution needed to see the mesorectal fascia.
Superior coverage stopping too low for a proximal tumour, missing the relevant nodal territory.
Motion from bowel peristalsis where no antiperistaltic was given.

Clinical questions that reach this study

Rectal cancer — local staging and restaging

Contrast

None

Intravenous gadolinium is not required for rectal cancer staging and does not improve T-staging or mesorectal fascia assessment.
An antiperistaltic agent is commonly given; rectal filling practice varies and is contested.

Acquisition

Breathing: Free breathing with an antiperistaltic agent; the high-resolution sequences are long and bowel motion is the main enemy.
Preparation: Give the distance of the tumour from the anal verge and the endoscopy findings; the radiographer needs it to angle the oblique planes. Local practice on micro-enemas and rectal filling varies and is genuinely contested; follow the local protocol rather than improvising. An antiperistaltic agent is commonly used to limit motion; screen as for other pelvic studies.

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