Oral contrast agents for magnetic resonance imaging of the bowel

The development of fast imaging sequences, which provide the ability to acquire motion-free T1- and T2-weighted images of static fluids, has greatly increased the interest in magnetic resonance imaging of the small bowel. Luminal distension is a necessary prerequisite for small bowel imaging methods because collapsed bowel loops can hide even large lesions and may mimic wall thickening. Poor distension of normal bowel loops in basal conditions has led researchers to study different oral contrast media to optimally distend the bowel lumen. Several MR oral contrast agents with various signal properties are available. According to these signal properties, agents are classified as positive ("bright" lumen), negative ("dark" lumen), or biphasic ("bright" lumen on T1 and "dark" on T2, or conversely "dark" lumen on T2 and "bright" on T1). Positive contrast agents cause a reduction in T1 relaxation time; consequently, these agents act on T1-weighted images by increasing the signal intensity of the bowel lumen. Negative contrast agents are based on superparamagnetic particles and act by inducing local field inhomogeneities, which results in shortening of both T1 and T2 relaxation times. Using superparamagnetic contrast agents, T2-weighted effects are predominant. Biphasic contrast agents are substances that have different signal intensities on different sequences, depending on the concentration at which they are administered. The choice of a single agent presents advantages and disadvantages; thus, the radiologist should choose the appropriate contrast medium according to the clinical setting, MR experience, availability of the agent, and patient tolerance.