Basic principles of magnetic resonance contrast agents.

The use of contrast agents in MRI is well established as a means to improve diagnosis. MRI differs from other imaging modalities because signal and contrast are multiparametric in both the properties of the tissue and the method of measurement. Contrast depends on differences in proton-spin density, magnetic susceptibility, molecular diffusion and perfusion, and T1 and T2 relaxation times. Relaxivity contrast agents, those that focus on shortening relaxation times, are most commonly employed in the form of paramagnetic chelates and depend on a variety of mechanisms, including concentration, number of ion-coordination sites, spin quantum number, magnetic moment, ion-to-proton distance, and correlation time constants characteristic of the chemical and molecular structure. A sound understanding of the principles of general contrast mechanisms, contrast agent design, and MRI techniques used in conjunction with contrast agents is vital to ensure proper enhancement and optimal diagnostic results.