MRI Contrast Agent - Types

Types

Most clinically used MRI contrast agents work through shortening the T1 relaxation time of protons located nearby. T1 shortens with an increase in rate of stimulated emission from high energy states (spin anti-aligned with the main field) to low energy states (spin aligned). Thermal vibration of the strongly magnetic metal ions in the contrast agent creates oscillating electromagnetic fields at frequencies corresponding to the energy difference between the spin states (via E = hν), resulting in the requisite stimulation.

MRI contrast agents may be administered by injection into the blood stream or orally, depending on the subject of interest. Oral administration is well suited to G.I. tract scans, while intravascular administration proves more useful for most other scans. A variety of agents of both types enhance scans routinely.

MRI contrast agents can be classified in many ways, including by their:

1. chemical composition
2. administration route
3. magnetic properties
4. effect on the image
5. presence and nature of metal atoms
6. biodistribution and applications:
   1. Extracellular fluid agents (also known as intravenous contrast agents)
   2. Blood pool agents (also known as intravascular contrast agents)
   3. Organ specific agents (i.e.Gastrointestinal contrast agents and hepatobiliary contrast agents)
   4. Active targeting/cell labeling agents (i.e. tumor-specific agents)
   5. Responsive (also known as smart or bioactivated) agents
   6. pH-sensitive agents