There are two principal classes of MRI contrast agents such as paramagnetic and superparamagnetic compounds. Paramagnetic agents primarily shorten the T1 relaxation time of the tissue, in which they accumulate. It is based on metal ions with one or more unpaired electrons. Superparamagnetic agents consist of magnetic nanoparticles of iron oxide coated with a polymer matrix such as dextran. These MRI contrast agents work by affecting the time it takes for the hydrogen atoms to return to their original energy state. This increases the difference in the signal intensity from the two types of tissue (the one with and the other without the contrast agent), thus increasing the degree of contrast on the image.