Gastrointestinal (GI) superparamagnetic contrast agents are used in MRI to improve the visualization of e.g., the intestinal tract, the pancreas (see MRCP), etc. Disadvantages are susceptibility artifacts e.g., dependent on delayed imaging or large volumes resulting in artifacts in the colon and distal small bowel loops related to higher concentration of the particles and absorption of the fluid.
Different types of MRI gastrointestinal superparamagnetic contrast agents:
Usually gastrointestinal superparamagnetic contrast media consist of small iron oxide crystals (ferrites), which produce a signal reduction in the stomach and bowel after oral administration. The T2 shortening caused by these particles is produced from the local magnetic field inhomogeneities associated with the large magnetic moments of superparamagnetic particles. Ferrites are iron oxides of the general formula Fe203.MO, where M is a divalent metal ion and may be mixed with Fe3O4 in different preparations. Ferrites can produce symptoms of nausea after oral administration, as well as flatulence and a transient rise in serum iron. Embedding in inert substances reduce side effects by decreasing the absorption and interaction with body tissues. Combining these contrast materials with polymers such as polyethylene glycol or cellulose, or with sugars such as dextrose, results in improved T1 and/or T2 relaxivity compared with that of the contrast agent alone.

See also Negative Oral Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Relaxivity, and Combination Oral Contrast Agents.

Magnevist® enteral (Gadopentetate) is a Gd-DTPA solution for use as a gastrointestinal contrast agent to visualize the bowel in MRI scans.

See also Positive Oral Contrast Agents and Gadopentetate Gastrointestinal.

(Gadolite® oral Suspension) An oral paramagnetic MRI contrast agent to enhance delineation of the bowel. Zeolites are hydrated aluminosilicates with large holes that can provide a temporary home for guest molecules like gadolinium. Zeolites modified with Gd (III) are effective positive MRI contrast agents for gastrointestinal application.

(GastroMARK®, Lumirem®) A superparamagnetic MRI contrast agent for the gastrointestinal tract.

See also Oral Contrast Agents.

Categories of negative oral contrast agents:
Negative oral contrast media are usually based on superparamagnetic particles and act by inducing local field inhomogeneities, which results in shortening of both T1 and T2 relaxation times. Superparamagnetic contrast agents have predominant T2 weighted effects. Biphasic contrast media are agents that have different signal intensities on different sequences, depending on the concentration at which they are used.
Suitable materials for oral contrast agents should have little or no absorption by the stomach or intestines, complete excretion, no motion or susceptibility artifacts, affordability, and uniform marking of the gastrointestinal tract. Benefits of negative oral contrast agents are the reduction of ghosting artifacts caused by the lack of signal. Superparamagnetic iron oxides produce also in low concentrations a noticeable signal loss; but can generate susceptibility artifacts especially in gradient echo sequences. Perfluorochemicals do not dilute in the bowel because they are not miscible with water.
High cost, poor availability, and limited evaluations of side effects are possible disadvantages.
Negative oral contrast agents are used e.g., in MRCP, where the ingestion of 600-900 ml of SPIO cancels out the signal intensity of the lumen (in addition after the injection of a gadolinium-based contrast medium, the enhancement of the inflammatory tissues is clearer seen), and in MR abdominal imaging of Crohn's disease in combination with mannitol.

Blueberry or pineapple juices are useable for examinations of the pancreas (MRCP, upper abdominal imaging) as cheep contrast agents, because of the content of magnetic substances (e.g. manganese).

See also Ferristene, Ferumoxsil, Oral Magnetic Particles, Gastrointestinal Imaging.