MRI Scan of the Rectum

Authors: Dr Nick Ferris*
                            Dr Helen Moore *

What are the prerequisites for having an MRI scan of the rectum done?

The test cannot be done if there are contraindications to MRI such as the presence of a cardiac pacemaker. While most implants and prosthetics are safe for MRI examination, the site where the MRI is to be done will appreciate as much detail as possible about any implants the patient may have. The test cannot safely proceed until all such implants have been satisfactorily identified and characterised.

Intravenous contrast agents are not generally required for MRI examinations of the rectum, so renal function testing is not routinely required beforehand. However, if recent data are available they may be helpful in the unlikely event that a contrast scan is required.

What are the absolute contraindications for an MRI scan of the rectum?

In patients who do not have metallic and/or electronic implants, there are no absolute contraindications to MRI of the rectum. The most common implants that prevent an MRI examination are:

• cardiac pacemakers;
• ferromagnetic intracranial aneurysm clips (and clips of unknown composition); and
• Cochlear implants.

Other implants (including ferromagnetic material in the eye) can also be problematic, please supply as much detail as is possible where there is any doubt.

What are the relative contraindications for an MRI scan of the rectum?

Large patients (>130 kg) may have difficulty fitting within the bore of standard MRI systems, and many system tables are not rated for accurate performance above this weight. Larger-bore systems are available at a few sites.

Severely claustrophobic patients may need sedation to enable them to complete the examination.

Patients who are unable to lie still for at least several minutes at a time have to be examined with accelerated MRI sequences which give poorer quality images. Patients who are unable to lie supine can often be scanned lying on their sides, but again this will likely reduce image quality.

What are the adverse effects of an MRI scan of the rectum?

• The MRI examination itself is quite noisy – patients routinely receive ear protectors to minimise the noise that they are subjected to. The interaction of the radio waves with the patient causes very small amounts of tissue warming (< 1 ‘ °C), rarely noticed by patients. There are no after-effects of the examination itself.

Patients who receive hyoscine/Buscopan (common practice to reduce artefacts from peristalsis) may notice some dryness in the mouth and some visual blurring within the first hour of drug administration, this should resolve spontaneously. Very rarely, male patients with enlarged prostates may find it more difficult to void after hyoscine.

The main hazards arise from metallic and electronic implants, and from foreign bodies inadvertently brought into the scan room.

Metal implants that are strongly attracted by magnetic fields, like some older aneurysm clips, can exert dangerous forces on tissues such as blood vessels, when placed inside an MRI system.

Patients confirmed to have such implants will generally not be able to have an MRI scan.

Electronic devices (e.g. mini infusion pumps) may be permanently damaged by the magnetic field. It may be possible to remove or de-activate these for the period of the scan.

Some metal wires and bands placed around the patient can be quite rapidly heated by radio waves used in MRI, and patients have received significant burns as a result. At-risk wires and bands will be removed before the scan.

Are there alternative imaging tests, interventions or surgical procedures to an MRI scan of the rectum?

MRI gives the best definition of the tissues around the rectum, for local staging of the disease. Multi-detector row CT soft tissue detail is significantly inferior to that of MRI. Endorectal ultrasound performs a little better than MRI for early-stage lesions (ones which may not have breached the muscular wall of the rectum), but is not as good for staging more advanced disease; it is not widely available.

The referring clinician may prefer either endorectal ultrasound or MRI as the staging test for a given case ; in general multi-detector-row CT is only used when MRI is not feasible.

Useful websites about MRI scans of the rectum:

• Royal Marsden Hospital:
  www.royalmarsden.nhs.uk/cancer-information/detection-diagnosis/pages/mri-scan.aspx
• Diagnostic Imaging Pathways:
  www.imagingpathways.health.wa.gov.au

*The author has no conflict of interest with this topic.

Page last modified on 26/7/2017.