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 m odalities in the complexity of signal and contrast dependence, with th e method of measurement having great impact on tissue contrast. Unlike computed tomography, in which contrast depends solely on x-ray densit y, the signal intensity on MR is determined by differences in spin den sity, T1 and T2 relaxation times, diffusion, perfusion, and magnetic s usceptibility. The only contrast agents in widespread clinical use at this time ate paramagnetic metal ion chelates. These compounds influen ce tissue contrast by enhancing T1 and T2 relaxation. The effectivenes s of any one agent is dependent on a number of basic parameters, inclu ding concentration, number of coordination sites, magnetic moment, dis tance between the ion and water protons, and correlation times. An und erstanding of general contrast mechanisms, principles of contrast agen t design, and basic MR imaging techniques is important for proper clin ical implementation and medical diagnosis.