Basic principles of MR contrast.

The use of intravenous contrast media is well established in magnetic resonance (MR) for improved diagnosis. MR differs from other imaging modalities in the complexity of signal and contrast dependence, with the method of measurement having great impact on tissue contrast. Unlike computed tomography, in which contrast depends solely on x-ray density, the signal intensity on MR is determined by differences in spin density, T1 and T2 relaxation times, diffusion, perfusion, and magnetic susceptibility. The only contrast agents in widespread clinical use at this time are paramagnetic metal ion chelates. These compounds influence tissue contrast by enhancing T1 and T2 relaxation. The effectiveness of any one agent is dependent on a number of basic parameters, including concentration, number of coordination sites, magnetic moment, distance between the ion and water protons, and correlation times. An understanding of general contrast mechanisms, principles of contrast agent design, and basic MR imaging techniques is important for proper clinical implementation and medical diagnosis.