Final answer:
The contrast medium used in MRI procedures is a gadolinium-based contrast, which enhances the imaging of soft tissues.
Step-by-step explanation:
When undergoing a magnetic resonance imaging (MRI) test, the contrast medium typically used is a gadolinium-based contrast agent. This type of contrast agent works by altering the magnetic properties of water in cells, providing enhanced contrast for the MRI images of soft tissues, such as organs. Gadolinium, represented as Gd³⁺, is paramagnetic due to its seven unpaired electrons, which enhances the images produced by the MRI scan. However, because free gadolinium ions are quite toxic, they are bound in a very stable complex with ligands like DTPA³⁻ (diethylene thiamine pentametric acid) to prevent dissociation and allow for safe excretion from the body after the scan.