MRI applications: Classification according to their biodistribution

Abstract

MRI contrast agents are known for about 30 years. Most of clinical compounds are extracellular agents without tissue specificity. They are excreted by the kidneys (Gd-DTPA, Gd-DOTA, Gd-DTPA-BMA or Gd-HP-DO3A for example). More recently, intracellular agents (Gd-EOB-DTPA, Gd-BOPTA, … undergoing hepatocyte uptake), and blood pool contrast agent (MS-325, binding albumin) have been developed. These Gd complexes have an extended clearance time and allow longer contrast-enhanced imaging sessions. These systems are called “T1 or positive” agents because they increase the water signal of the tissue areas where they are concentrated. A second kind of contrast agents is made of superparamagnetic iron oxide nanoparticles (USPIO or SPIO). These nanosystems have a high efficiency and are generally “T2 or negative” agents, inducing a darkening of the region where they accumulate. Such nanoparticles with a diameter of approximately 20–200 nm can be used as contrast agents for liver, lymphatic system (because of macrophage uptake, mainly as negative agent), or blood vessels (because of low extravasation, mainly as positive agent). The different kinds of mechanisms through which they affect the nuclear magnetic relaxation time of the water are briefly described. Nowadays, researches are focused on the development of molecular imaging contrast agents, to target receptors which are overexpressed in pathologies.