Biphasic oral contrast agents may produce either high or low signal intensities depending on the pulse sequence used, for example low signal intensity on T1 weighted MR images and high signal intensity on T2 weighted images. The combination of different oral contrast agents can generate a macroscopic cancellation of negative and positive magnetic susceptibility, thereby eliminating susceptibility artifacts.
Possible combinations are e.g., ferric ammonium citrate and corn oil, or ferrous sulfate emulsified with baby formula. Paramagnetic agents combined with oil emulsion may be used in MRI as positive abdominal contrast agents. The combination of diamagnetic barium sulfate and superparamagnetic iron oxide (SPIO) in one suspension may be a useful negative contrast agent.

See also Gastrointestinal Paramagnetic Contrast Agents, Gastrointestinal Superparamagnetic Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Gastrointestinal Imaging.

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.

Types of oral contrast agents with positive signal enhancement:
Ideal oral contrast agents are immiscible with water, biologically inert, have a low viscosity and surface tension. Oral positive contrast agents may improve the separation of bowel loops, the detection of polyps in colon MRI or the assessment of inflammatory bowel in the small intestine. Several positive oral contrast agents are available and they are safe to use, for example gadolinium solution, ferric ammonium citrate, different oil emulsions and pediatric formula.
Unfavorably motion artifacts caused by respiration and peristalsis may be increased in MR imaging. In addition, the signal of the positive contrast medium may decrease caused by dilution in gastrointestinal (GI) secretions. With the use of contrast agents that are immiscible with water, no dilution and accompanying signal loss occur even when the contrast agent is in contact with the intraluminal contents of the GI tract. Another disadvantage may be residual substances in the bowel, resembling masses when enclosed by bright signal. In addition, positive contrast agents may have a similar signal as bright masses, which make their (e.g. lipoma) detection difficult.

See also Gastrointestinal Paramagnetic Contrast Agents, Combination Oral Contrast Agents, Gastrointestinal Diamagnetic 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.