Adequate distension of the bowel lumen is mandatory in MRI, as it facilitates demonstration of morphological changes caused by Crohn's disease and allows identification of subtle abnormalities. Collapsed bowel loops can hide lesions or mimic disease by mimicking pathologically thickened bowel wall in collapsed segments . Moreover, collapsed normal bowel loops can exhibit enhancement that is similar to diseased segments, after administration of an intravenous contrast medium .
Various kinds of intraluminal contrast agents have been proposed for MR imaging [7, 18-26] and are classified as positive, negative, or biphasic. Positive agents produce high intraluminal signal, and negative agents produce little or no intraluminal signal regardless of the applied pulse sequence. Biphasic contrast agents may produce either a high or low signal depending on the pulse sequence used, usually demonstrating low signal intensity on Tl-weighted MR images and high signal intensity on T2-weighted images. Negative or biphasic contrast agents seem to be more suitable for assessing the small bowel [7,19]. Polyethylene glycol has been proposed by several authors as a suitable biphasic contrast medium; it is not absorbable, remains unmodified in the small bowel, is easily prepared and administered, and provides adequate bowel distension. Furthermore, transit time is fast, allowing for small-bowel distension within 30 min. However, undesirable side effects (e.g., motion artifacts or severe diarrhea) can occur due to the prokinetic action of the solution. Theoretically, water would be a perfect biphasic contrast medium, but in many patients, the water is reabsorbed before it has reached the terminal ileum. Moreover, it does not optimally distend the bowel (Fig. 1).
An anti-peristaltic agent is injected to minimize potential artifacts caused by bowel movement or contraction. Although many authors reporting on MR enteroclysis administer anti-peristaltic drugs to reduce motion artifacts, reflex atony is induced by high flow rates, theoretically allowing images (almost) to be free of motion artifacts.
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