Contrast agent with a preferential intracellular distribution.
Intracellular agents (such as manganese derivatives and ultrasmall superparamagnetic iron oxide), exhibit a flow- and metabolism-dependent uptake. These properties may allow delayed imaging, similar to isotopic methods.
Phospholipid liposomes are rapidly sequestered by the cells in the reticuloendothelial system (RES), primarily in the liver. For imaging of the liver, liposomes may be labeled with MR contrast medium, both positive (T1-shortening) paramagnetic media, and negative (T2-shortening) superparamagnetic media.
Several other nonliposome MR contrast media are also taken up by the RES, e.g.:
Other MR contrast agents accumulate selectively in the hepatocytes, e.g.:

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.

Contrast agents are chemical substances introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by altering the relaxation times. MRI contrast agents are classified by the different changes in relaxation times after their injection.
The design objectives for the next generation of MR contrast agents will likely focus on prolonging intravascular retention, improving tissue targeting, and accessing new contrast mechanisms. Macromolecular paramagnetic contrast agents are being tested worldwide. Preclinical data shows that these agents demonstrate great promise for improving the quality of MR angiography, and in quantificating capillary permeability and myocardial perfusion.
Ultrasmall superparamagnetic iron oxide (USPIO) particles have been evaluated in multicenter clinical trials for lymph node MR imaging and MR angiography, with the clinical impact under discussion. In addition, a wide variety of vector and carrier molecules, including antibodies, peptides, proteins, polysaccharides, liposomes, and cells have been developed to deliver magnetic labels to specific sites. Technical advances in MR imaging will further increase the efficacy and necessity of tissue-specific MRI contrast agents.

See also Adverse Reaction and Nephrogenic Systemic Fibrosis.

See also the related poll result: 'The development of contrast agents in MRI is'

This superparamagnetic oral contrast agent consists of large iron oxide particles, coated with insoluble material. Abdoscan® particles have a mean diameter no less than 300 nm. Gastrointestinal superparamagnetic contrast agents are used for negative bowel enhancement.

Abdoscan® was approved in Europe but was taken off the market in 2000, and all sales stopped by the end of 2002.

Blood pool agents (intravascular contrast agents) remain in the blood for a prolonged time compared with conventional contrast agents, which diffuse quickly into the interstitial space. Magnetic resonance angiography (MRA), cardiovascular imaging, or contrast enhanced MRIs are possible over an hour or more. This advantage over conventional MRI contrast media allows also higher resolution MRA of several territories using respiratory or cardiac gating techniques with a single contrast bolus.
Different types of blood pool contrast agents:
Blood pool MRI contrast agents with their longer intravascular circulation can be designed to be targeted to necrotic myocardium, to assess myocardial viability, or tumor directed to provide better diagnostic information for various tumors. A disadvantage of the use of blood pool agents for MRA is that the separation of arteries and veins is more difficult because they are present in both and the overlapping of those vessels is disturbing. This can be solved by e.g. different MIP segmentation algorithms.

See also Necrosis Avid Contrast Agent, Tumor Specific Agents, Feruglose, Gadofosveset Trisodium (Vasovist), Ultrasmall Superparamagnetic Iron Oxide and Contrast Medium.