Most injected MRI contrast agents are what type?

Most injected MRI contrast agents are gadolinium-based and categorized as non-specific extracellular agents. Gadolinium is a rare earth element that has paramagnetic properties, which enhances the clarity of MRI images. When gadolinium chelates are administered, they distribute within the extracellular space, allowing for improved differentiation of tissues based on their different relaxation properties in the presence of this contrast agent.

Non-specific means that these agents enhance all types of tissues in the extracellular space rather than targeting specific tissues or abnormalities. This broad enhancement is useful for assessing a variety of conditions, such as tumors, inflammation, or lesions in various organs. The primary mechanism of action is that gadolinium affects the T1 relaxation times of nearby water protons, improving the contrast in MRI images.

Overall, the properties of gadolinium-based contrast agents—being water-soluble and effective in enhancing signals without being specific—make them the most commonly used type in clinical MRI applications.