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Abstract
Magnetic resonance imaging (MRI) has developed at an exponential rate over the last decades, and the development of contrast agents to enhance the visualization of organs has followed the same trend. Meanwhile, magnetic nanoparticles that generate either “positive” or “negative” contrast in MRI have become one of the most important biomedical applications of nanotechnology. Indeed, superparamagnetic iron oxide nanoparticles, as negative contrast agents for T2/T2*-weighted imaging, have found numerous applications in preclinical and clinical MRI (cell labeling, vascular contrast, lymph node imaging, liver contrast). In addition to this, paramagnetic and antiferromagnetic nanoparticles based on the elements Gd3+ and Mn2+ have mainly been exploited in vascular procedures and targeted imaging, for their capacity to enhance the MR signal of blood and of molecular signatures of endovascular disease. They are commonly referred to as “positive” contrast agents for T1-weighted imaging.
The present chapter is an introduction to the fundamental principles of nanoparticle-based MRI contrast agents. It addresses the main considerations guiding the relaxometric characterization of aqueous suspensions of magnetic nanoparticles, based on the elements iron, manganese, and gadolinium (Fe, Mn, Gd). The relaxivity of MRI contrast agents depends on their nanoparticulate structure, on their magnetic properties, on the distance between water molecules and their surface, and on the kinetics and rotational rate of the compound in biological fluids and in tissues. Among the main parameters guiding the relaxation time of water protons in the vicinity of contrast agents, figure the number of water molecules bound to the contrast agent, the size of the nanocrystals, the total hydrodynamic diameter of nanoparticles, their rotational correlation time, and the exchange rate between the water and the nanoparticle surface.
The general magnetic and relaxometric characteristics of the major classes of nanoparticles used as MRI contrast agents will be reviewed. Examples of nuclear magnetic relaxation dispersion profiles (NMRD), revealing the relaxometric potential of magnetic particles at increasing magnetic field strengths, are also presented and discussed.
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