4 - Contrast Agents for Magnetic Resonance Imaging: An Overview

Abstract

This chapter introduces the major classes of magnetic resonance imaging (MRI) contrast agents, the physical parameters that influence their efficacy, and their comparative advantages and disadvantages for in vivo imaging. There are three classes of contrast agents. The first class includes relaxation agents that work by influencing the relaxation times T1 and T2 and are the most widely used contrast agents. The second, relatively new, class of agents is based on an effect called chemical exchange-dependent saturation transfer (CEST). This effect involves a spin preparation technique that reduces the MRI signal detected in the presence of a CEST agent. The third class of contrast agents contains spin-1/2 nuclei other than protons and is designed to be directly imaged by the MRI hardware. Acceleration of relaxation rates by paramagnetic contrast agents is due to their interactions with spin-1/2 nuclei in solvent that contribute directly to the image. The majority of T1 agents in clinical or laboratory use today are small molecule complexes incorporating paramagnetic metals. Gd(III) is the most commonly used metal because of its long electronic relaxation time and seven unpaired electrons. Mn(II) and Fe(III) have a lower spin number but are also used in effective T1 agents. Polydentate chelators form tight, kinetically stable, and soluble complexes with the metals, rendering them tolerable for in vivo use. Superparamagnetic iron oxide nanoparticles (SPIO) are the principal T2 agents used in MRI today. SPIO contrast agents are colloidal suspensions, sometimes called ferrofluids, in which each particle contains one or more nanometer-scale crystals of magnetite or its more stable oxidation product, maghemite.