What is venous access? In radiology, venous access generally refers to central venous access, which is the insertion of a…Read more
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.
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:
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.
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.
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.
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.
Page last modified on 26/7/2017.
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