Paramagnetic contrast agents are used to enhance MRI images and display areas of hypervascularity and associated pathology. They have their strongest effect in T1 weighted imaging because they predominantly alter the T1 relaxation time in the tissues in which they have accumulated.
Paramagnetic substances have at least one unpaired electron. The protons of paramagnetic substances align their net magnetization vector parallel to B0 when a magnetic field is applied, while their angular momentum is random in its absence. This is compared to diamagnetic and ferromagnetic substances 1.
MRI imaging relies upon the signal generated from the behavior of water protons. The relaxation of these water protons is affected by surrounding tissues and when a paramagnetic contrast agent has been absorbed in its vicinity, the relaxation of water protons is enhanced.
The paramagnetic contrast agents generate a magnetic field 1000 times stronger than water protons. The interaction between the contrast agent and the water proton is exactly the same as the corresponding interactions with other molecules, except that the magnitude of their magnetic interaction has a much greater effect on the relaxation time.
The primary class of paramagnetic contrast agents is gadolinium-based contrast agents. Others include ferric ammonium citrate and manganese-based contrast agents.