Any abnormal reaction of a patient to an examination or procedure, like for example claustrophobia or side effects of MRI contrast agents.
A claustrophobic attack is MRI scanner dependent and more rare with an open MRI. An adverse reaction with magnetic resonance imaging contrast medium is very infrequent. In general, adverse reactions increase with the quantity of contrast media (usual dose of paramagnetic contrast agents is 0.1 mmol/kg) and also with the osmolarity of the compound.
Most frequently encountered adverse reactions are heat sensation, dizziness, nausea, hypotension due to vasodilatation, which can progress to hypotensive shock and anaphylactic reactions.
See also MRI Safety, Contrast Enhanced MRI, Breast MRI, and Cardiac MR imaging.

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.

The brain tissue is provided with a tight endothelial layer on vessels that acts as a filter for substances that reach the brain through the blood stream. This filter is called the blood brain barrier.
The blood brain barrier is responsible for the absence of contrast agent enhancement in normal brain tissue after administration of the iodinated or paramagnetic contrast media used in brain MRI and computed tomography (CT) diagnostic. The absence of contrast uptake in normal tissue provides the basis for differentiation from pathological brain tissue, which is conversely characterized by a disruption of the blood brain barrier.

See also Contrast Enhanced MRI, MRI Safety, Adverse Reaction.

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'

Intravenous contrast agents used in MRI are distributed in the extracellular spaces of the body before being excreted. In this sense they are similar to iodinated X-ray contrast media. However, contrast agents used for MRI are quite different from conventional radiographic contrast media in structure and function and there is no known cross sensitivity between these types of contrast agents. Common MRI contrast agents use metal ions (e.g., gadolinium or manganese) complexed with organic molecules.
Gd-DTPA, an ionic linear molecule complex (gadopentetate dimeglumine) was the first marketed MRI contrast agent. Although the osmolality of this substances can be relatively high (up to 1940 mOsm/kg H2O) compared with plasma, adverse reactions and side effects are very rare. The used doses are smaller compared with radiographic contrast media.

See also Nonionic Intravenous Contrast Agents, Dotarem®, and Magnevist®.