relaxivity

Relaxivity is a property of MRI contrast agents that describes how efficiently they accelerate the relaxation of nearby water protons. It is reported as longitudinal relaxivity r1 and transverse relaxivity r2, with units of s−1 mM−1. Relaxivity quantifies how the relaxation rates 1/T1 and 1/T2 increase with agent concentration, expressed as 1/Ti = 1/Ti,0 + ri [CA], where Ti is the relaxation time in the presence of the agent, Ti,0 is the relaxation time of the pure solvent, and [CA] is the agent concentration.

Relaxivity informs how agents influence image contrast. Higher r1 values indicate more effective T1 shortening and brighter signal on T1-weighted images, while higher r2 values indicate stronger T2 shortening and darker signal on T2-weighted images. The ratio r2/r1 helps determine whether an agent is more suitable for T2 or T1 contrast at a given field strength.

Relaxivity depends on several factors, including field strength and temperature. It is influenced by molecular properties such as the number of inner-sphere water molecules (q), the residence time of coordinated water, the rotational correlation time (affected by molecule size or binding to macromolecules), and the electronic relaxation of the paramagnetic center. The local environment, viscosity, and pH can also affect relaxivity.

In practice, relaxivity is used to compare MRI contrast agents. Common examples include gadolinium-based agents, which primarily enhance T1 contrast, and iron oxide particles, which provide strong T2 effects. Encapsulation or conjugation can modify relaxivity by altering water access and molecular motion.