Magnetic resonance imaging in Crohn’s disease

Plain abdominal radiograph

Plain abdominal radiography may help identify complications related to Crohn’s disease, such as bowel obstruction or perforation, but it has a limited role assessing inflammatory activity.

Barium examinations

Radiological imaging of the small bowel has traditionally relied on barium examinations. Barium examinations, however, result in a considerable number of false negative results.1 A barium examination may also overlook extramural complications such as fistulas, sinuses, and extraintestinal abscesses.

Enteroclysis

Conventional enteroclysis is performed after placement of a naso-jejunal catheter and infusion of a barium and methylcellulose solution, which provides double contrast images of the small intestine. The technique produces excellent visualisation of the mucosa, although extraintestinal detail is limited. One of the disadvantages of enteroclysis is discomfort experienced by the patient during intubation. The effective radiation dosages on barium and enteroclysis examinations range from 2 mSv to 4 mSv.

Fistulography

Fistulography may be performed to outline fistulous tracks and their communication with the bowel. Occasionally, fistulography may not demonstrate a fistula in its entirety, as a chronic fistula may have a fibrotic or stenosed lumen. Furthermore, fistulography may not accurately demonstrate the extent of bowel involvement or outline internal (inter-loop) fistulas.

Ultrasound

A meta-analysis has shown that ultrasonography of the bowel has high sensitivity in diagnosing inflammatory bowel disease, particularly of the terminal ileum. Sensitivity and specificity are 75-94% and 67-100%, respectively.2 Sonographic findings in inflammatory bowel disease include increased bowel wall thickening, increased bowel vascularity, and associated mesenteric lymphadenopathy. Crohn’s related complications such as fistulas and abscesses can also be detected on sonography. Comprehensive examination of the entire bowel is often limited, however, because of the presence of excessive bowel gas and overlapping bowel loops. Furthermore, bowel ultrasonography usually requires specialist experience that may not be widely available.

Computed tomography

Comparative studies of computed tomography enterography and enteroclysis have reported sensitivities and specificities of 73-82% and 70-98%, respectively.3 The current generation of computed tomography scanners can acquire images rapidly—within one breath-hold—and are also widely available.

Computed tomography of the abdomen carries high radiation dosages, however, which is worrying because young patients with Crohn’s disease may have several examinations during the disease process. The effective radiation dosages of computed tomography of the abdomen and pelvis typically range from 7 mSv to 10 mSv.4

Magnetic resonance imaging

Enteroclysis and enterography with magnetic resonance imaging can be used in the diagnostic investigation of patients with Crohn’s disease.5 6 Enteroclysis is performed after instillation of a naso-jejunal catheter and infusion of 1.5-2 litres of iso-osmotic enteral contrast. This technique provides optimal bowel distension, which can be useful in identifying early mucosal disease and subacute or partial strictures. These advantages have to be counterbalanced by the complexity of the procedure and the patient’s discomfort.

Enterography is performed after oral ingestion of 1.5-2 litres of enteral contrast, which helps to distend the bowel. Enteral contrast agents used in magnetic resonance imaging enterography include superparamagnetic iron particles, polyethylene glycol, locust bean gum, and mannitol solution. Enterography is less uncomfortable for the patient but may not produce the uniform bowel distension achieved by enteroclysis.

A recent prospective randomised study found that enterography and enteroclysis had similar rates of diagnostic accuracy, and recommended enterography for follow-up of established Crohn’s disease.7 Comparative and prospective magnetic resonance imaging studies have reported a sensitivity of 88-98% and specificity of 78-100%.789

Magnetic resonance imaging has several advantages over conventional barium examinations. It provides extraintestinal detail and delineates complications more accurately than barium examinations or conventional enteroclysis. The important advantages of magnetic resonance imaging compared with computed tomography (table⇓) include better tissue contrast, absence of exposure to radiation, and higher sensitivity for intestinal and extraintestinal changes in Crohn’s disease.10 Dynamic real time examination of bowel movements and peristalsis can also be performed on magnetic resonance imaging, and this may be useful in identifying contrast flow across stenotic lesions. The excellent tissue contrast obtained on magnetic resonance imaging can also be used to differentiate between fibrotic and acute inflammatory disease. This differentiation is clinically useful, as fibrotic disease may need surgery, whereas inflammatory disease may benefit from medical treatment.