ABSTRACT

This chapter provides an overview of the different contrast mechanisms and the relevant chemistry and biophysics for each class of contrast agent. It assumes the reader has a basic knowledge of magnetic resonance imaging (MRI) and its terminology. The magnetic resonance (MR) image in clinical and biological systems is typically an image of the hydrogen atoms in water and fat. MRI contrast agents must be biocompatible pharmaceuticals in addition to nuclear relaxation probes. The paramagnetic ion and coordinated water molecule are essential to providing contrast. The gadolinium(III) ion has a high magnetic moment and a relatively slow electronic relaxation rate, properties that make it an excellent relaxer of water protons. The MS-325 example clearly shows the importance of chemical speciation on observed relaxation rates. Paramagnetism generally involves the magnetism of small isolated ions that only behave as local magnets in the presence of an external magnetic field.