Radiation Risk of Medical Imaging During Pregnancy
Which radiology tests are absolutely contraindicated during pregnancy? No radiological examination is absolutely contraindicated during pregnancy. As there are no…Read more
Most gastroenterologists will investigate the upper and lower GI tract, often by endoscopy, before considering small bowel imaging. This may establish a diagnosis and so eliminate the need for the test. However, in established Crohn’s disease, this is not always so. In women, anaemia is best investigated by tests of the GU tract first, with small bowel investigation only if this is negative.
Pregnancy is not an absolute contraindication for an MRI, but if possible scanning in the first trimester should be avoided.
Serum creatinine and glomerular filtration rate are often required by MRI departments prior to administration of intravenous contrast media with MRI in order to reduce the risk of nephrogenic systemic fibrosis (see gadolinium contrast medium (MRI contrast agents)) in patients with severe renal impairment. MRI of the small bowel can still be done without gadolinium in a patient with renal failure.
At some scanners the patient may be given an injection to slow down the normal movements of the bowel (these contractions can blur the MRI pictures). The injection may be Buscopan (hyoscine) or glucagon. Some of these medications can worsen glaucoma, cause difficulty urinating in those with prostate disease, and worsen some heart rhythm disturbances.
If there is any doubt about whether an implant, medical device in the patient, or a foreign body might be a contraindication to MRI, please contact the MRI facility where you intend to refer your patient for advice.
The size of the hole in the MRI machine is smaller than that in CT machines and means very large patients cannot fit inside, particularly those with central obesity or large shoulders. For obese patients a “test table ride” can sometimes be helpful to determine whether scanning will be possible.
Claustrophobia may also prevent successful scanning but can generally be prevented with administration of oral or intravenous sedation at the MRI facility just prior to scanning. If you know your patient is claustrophobic, it is helpful to indicate this on the request form. If it is known the patient needs sedation it can be arranged with reduced wasted time and patient distress. Most practices recommend that patients who have had sedation do not drive themselves home immediately after the appointment.
Intellectually normal children can usually manage MRI from the age of 6-7 (depending on the child), but younger children will need a general anaesthetic.
Enteroclysis has few side effects; mild diarrhoea and feeling cold during the exam are the most likely. Symptoms may be reproduced. Nasojejunal tube positioning (if used) may cause minor damage to mucosa of the nose and throat.
See items on MRI and gadolinium contrast medium for general risks.
Patients who receive hyoscine/Buscopan (common practice to reduce artefacts from peristalsis) can experience worsening of glaucoma. They may also notice some dryness in the mouth and some visual blurring within the first hour of drug administration, which should resolve spontaneously. Very rarely, male patients with enlarged prostates may find it more difficult to void after hyoscine.
If your patient has either of these conditions, please indicate this on the referral.
All of these require radiation and cumulative dose is important, as many patients who have small bowel studies require repeated imaging to monitor chronic problems such as Crohn’s disease. This means that young patients requiring repeat examinations over time stand to gain the most from MRI examination in terms of radiation dose reduction. In patients over 60 years, when cumulative radiation dose over a lifetime is less of an issue than in children and adolescents, CT enterography is still a good alternative, in terms of diagnostic accuracy, especially if MRI is unavailable for any reason.
The distinction between the different soft tissues of the body is better on MRI than CT. Both MRI and CT give a cross section of the body, which may be easier for the radiologist to interpret than previous X-ray fluoroscopy investigations which demonstrate only the lumen of the bowel (these are called a “barium follow through” or “gastrografin follow through” or “small bowel enema”). These tests continue to be used in some centres and are still sometimes useful, particularly in the setting of suspected acute mechanical small bowel obstruction.
Page last modified on 26/7/2017.
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