The ability of magnetic compounds to increase the relaxation rates of the surrounding water proton spins.
Relaxivity is used to improve the contrast of the image, and to study tissue specific areas where the contrast agent better diffuses or to perform functional magnetic resonance imaging.
The relaxivity of MRI contrast agents depends on the molecular structure and kinetic of the complex. To increase the number of water molecules that are in the inner sphere of the complex, or to slow down the molecular rotational correlation time, are possibilities to improve the water relaxivity.
Relaxivity units (r1, r2) are mM-1 * sec-1 (at varying temperatures).

ABLAVAR™ (formerly named Vasovistâ„¢) is a blood pool agent for magnetic resonance angiography (MRA), which opens new medical imaging possibilities in the evaluation of aortoiliac occlusive disease (AIOD) in patients with suspected peripheral vascular disease.
ABLAVARâ„¢ binds reversibly to blood albumin, providing imaging with high spatial resolution up to 1 hour after injection, due to its high relaxivity and to the long lasting increased signal intensity of blood.
As with other contrast media: the possibility of serious or life-threatening anaphylactic or anaphylactoid reactions, including cardiovascular, respiratory and/or cutaneous manifestations, should always be considered.

WARNING:
NEPHROGENIC SYSTEMIC FIBROSIS Gadolinium-based contrast agents increase the risk for nephrogenic systemic fibrosis (NSF) in patients with acute or chronic severe renal insufficiency (glomerular filtration rate less than 30 mL/min/1.73m²), or acute renal insufficiency of any severity due to the hepato-renal syndrome or in the perioperative liver transplantation period.

See also Cardiovascular Imaging, Adverse Reaction, Molecular Imaging, and MRI Safety.

(Gd) Gadolinium is a Lanthanide element that is paramagnetic in its trivalent state.
This paramagnetic substance is used for MR imaging because of the effect of strongly decreasing the T1 relaxation times of the tissues to which gadolinium has access. When injected during magnetic resonance imaging, gadolinium will tend to change signal intensities by shortening the T1 time in its surroundings.
The relaxivity of gadolinium is an important measure of its efficacy, which is dependent on the chemical properties of the complex. The gadolinium ion cannot be used in its chloride, sulfate, or acetate forms because of poor tolerance and low solubility in water in the neutral pH range. Although toxic by itself, gadolinium can be given safely in a chelated form such as DTPA, that still retains much of its strong effect on relaxation times (relaxivity).

See also Dotarem®, Gadovist®, MultiHance®, Omniscan®, OptiMARK®, and Contrast Agents, the info sheet gives an overview and more in-dept information about different types of MRI contrast agents.

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.

MultiHance® is a paramagnetic contrast agent for use in diagnostic magnetic resonance imaging (MRI) of the liver and central nervous system. MultiHance® is a small molecular weight chelate, which tightly binds the Gd atom. The substance is excreted partly by the kidneys, partly by the biliary system, which is especially unique.
MultiHance® is indicated, for the detection of focal liver lesions in patients with known or suspected primary liver cancer (e.g. hepatocellular carcinoma) or metastatic disease.
MultiHance® is also indicated in brain MRI and spine MRI where it improves the detection of lesions and provides diagnostic information additional to that obtained with unenhanced MRI.
Gd-BOPTA-enhanced MRA can provide superior vascular signal intensity and SNR, as compared with Gd-DTPA, due to its higher relaxivity, even at lower doses.
1 ml of solution MultiHance® contains: (0.5M) gadobenate dimeglumine 529 mg = gadobenic acid 334 mg + meglumine 195 mg. Viscosity at 37°C: 5.3 mPa

WARNING: NEPHROGENIC SYSTEMIC FIBROSIS Gadolinium-based contrast agents increase the risk for nephrogenic systemic fibrosis (NSF) in patients with acute or chronic severe renal insufficiency (glomerular filtration rate less than 30 mL/min/1.73m²), or acute renal insufficiency of any severity due to the hepato-renal syndrome or in the perioperative liver transplantation period.