MRI Rectum — restaging after neoadjuvant therapy

Suppression of rectal and small bowel peristalsis, as at primary staging. Restaging adds a reason of its own: the study is read side by side against the pre-treatment images, so motion on one of the pair degrades the comparison rather than just that one series, and the residual abnormality being judged is often only a few millimetres of wall thickening within fibrosis.

Dose and route

Typically 20 mg intravenously, or about 20 mg intramuscularly where no cannula is being sited. Glucagon roughly 0.25-1 mg intravenously where hyoscine is contraindicated or unavailable.

When

Immediately before the high-resolution oblique T2 sequences, and ideally covering the diffusion-weighted acquisition too, since diffusion carries more of the restaging decision than it does at primary staging. Where practice is to give a single dose it should be matched to whichever acquisition is most decisive locally.

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.
• The same variation in practice applies as at primary staging — a spasmolytic is favoured where available but is not given everywhere, and it should be treated as the local protocol rather than a requirement.
• 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.

If unsuitable

Proceed without it, and be explicit in the report that motion limits the comparison if it does — a restaging study called equivocal for motion is a different clinical message from one called equivocal for fibrosis, and in a watch-and-wait pathway the distinction matters.

• Beets-Tan RGH et al. MRI for clinical management of rectal cancer: updated ESGAR consensus. Eur Radiol 2018 — restaging technique and spasmolysis.
• Royal College of Radiologists — guidance on the use of hyoscine butylbromide in imaging, including contraindications and driving advice.