Exogenous contrast agents include artificial substances, usually administered intravenously, to modify the signal of the vessels and organs.
These agents are considered as drug and are subject to the same development and prescription requirements (FDA).
MRI contrast agents are not directly visible. The modification in contrast is due to their effect of shortening the relaxation time T1 and/or T2 of the hydrogen nuclei located in their vicinity.
If the contrast agent reduces time T1 (paramagnetic contrast agents), we observe a T1 hypersignal. On the other hand, if it shortens T2 (superparamagnetic contrast agents), there will be a reduction in the T2 and T2* signal.
The effectiveness of the contrast agent depends on its relaxivity (i.e. its capacity to modify relaxation times).
Two main classes of contrast agent can be distinguished:
Within each of these classes, certain contrast agents may be organ-specific (liver, ganglion, vascular remanence).