Diamagnetism occurs only by a substance in the presence of an externally applied magnetic field. Diamagnetic contrast agents are complexes in which the metal ion (e.g., Zn, Bi and Ca) is diamagnetic.
Potential diamagnetic materials in gastrointestinal MRI:
A suspension of clay minerals (Kaopectate with kaolin, a common over the counter drug) can be used as a negative oral contrast agent caused by the diamagnetic properties. By using this preparation as a gastrointestinal contrast agent e.g., in pancreas MRI or MRCP, the absence of signal is clearly visible in the stomach and duodenum. Barium sulfate commonly used as an X-ray contrast agent has also been tested for use in abdominal imaging. The diamagnetic properties of the barium particles are caused by a susceptibility effect around them, the resulting signal loss is strengthening by a replacement of water protons with barium.

See also Diamagnetism.

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.

Perflubron® is a perfluorochemical for use as an oral contrast agent. Due to its insolubility in water it does not mix with intestinal secretions; thus bowel lumina appear homogeneously dark on MR images when Perflubron® replaces bowel contents. Filled bowel loops appear black with all pulse sequences because the contrast agent lacks mobile protons.
It is commercially available as Imagent GI. Because rapid transit through the gastrointestinal tract it reaches the rectum within 30 to 40 minutes in most patients. MR imaging of the upper abdominal region should begin within 15 minutes and of the pelvic region 15 to 60 minutes after ingestion of perflubron.

See also Classifications, Characteristics, etc.

Short name: CMC 001, central moiety: Mn2+
CMC 001 is an orally administered MRI contrast agent in development for enhancement of the liver, bile ducts and gastrointestinal tract. Applications are screening of liver tumors/metastases, imaging of the biliary tract and it could be suitable for liver function studies by following the metabolism and excretion of manganese in the liver cells. Several advantages like better images, higher safety, lower costs and less inconvenience for the patient can be expected.
Animal experiments and clinical studies in humans Phase I and Phase II A, have shown excellent contrast enhancing properties with only minimal adverse effects. CMC 001 is under development by the Medicon Valley based CMC Contrast AB to receive a cost effective, safe and highly diagnostic MR contrast medium for abdominal MR imaging. Clinical studies in Phase III are started.

Short name: PFOB (C8F17Br), generic name: Perfluoroctylbromide.
Perflubron® is a negative oral contrast agent consisting of perfluorochemicals used in MRI scans to darken the bowel. In gastrointestinal MR imaging the differentiation between intestines, adjacent structures and pathologic masses are improved.
Not commercially available in Canada.

See also Classifications, Characteristics, etc.