Magnetic Properties
The magnetic properties of a material are the basis of their applications. Specifically, the contrast agents that will be developed in Chaps. 4 and 5 use their magnetic properties to play their role in magnetic resonance imaging. It is thus important to
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Magnetic Properties
Abstract The magnetic properties of a material are the basis of their applications. Specifically, the contrast agents that will be developed in Chaps. 4 and 5 use their magnetic properties to play their role in magnetic resonance imaging. It is thus important to describe briefly the different magnetisms.
The magnetic properties of a material are characterized by its magnetic susceptibility v, which describes the ability of the material or substance to be magnetized by an external magnetic field H. M ¼ vH The magnetization, M, represents the net magnetic moment per unit volume that is aligned parallel to the external magnetic field. The magnetism of the material comes from either electrons (electronic magnetism) or atomic nuclei (nuclear magnetism). However, the magnetization created by one electron being 657 higher than that created by one proton, electronic magnetism masks the nuclear one. According to the behavior exhibited in the presence of a magnetic field, several classes of substances can be distinguished.
2.1
Diamagnetic Substances
The majority of atoms constituting tissues of the body are diamagnetic because they do not have unpaired electrons. The global electronic magnetic moment of each atom is zero. However, an external magnetic field Bo may slightly alter the orbital paths of the electrons in motion and low magnetization can then be detected in a direction opposite to the external field direction B0. Diamagnetic agents are not used as contrast agents for their effects on T1 and T2 relaxation time. The consequence of diamagnetism is a very weak and negative susceptibility.
© The Author(s) 2017 S. Laurent et al., MRI Contrast Agents, Nanotheranostics, DOI 10.1007/978-981-10-2529-7_2
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2.2
2 Magnetic Properties
Paramagnetic Substances
Ions or molecules possessing unpaired electrons tend to move in the direction of the magnetic field and increase its effect, these compounds are called paramagnetic substances. The more unpaired electrons, the greater is the electron magnetic moment and the more important will be the paramagnetic properties. All transition metals (Cr2+, Cr3+, Mn2+, Mn3+, Fe2+, Fe3+) and lanthanides (Gd3+, Dy3+) have a significant number of unpaired electrons. The gadolinium ion, with seven free electrons, has a large magnetic moment. It was first chosen as the base of paramagnetic MRI contrast agents.
2.3
Ferromagnetic Substances
While paramagnetism is a property from the isolated ion, ferromagnetism is a property based on the cooperation of thousands of atoms in a paramagnetic crystal structure. Ferromagnetic compounds have regions or domains where there is an overall magnetization. Before being subjected to an external field, a ferromagnetic compound has no spontaneous magnetization: the different magnetic domains are oriented randomly. After application of an external field, the magnetic fields are oriented along the axis of the field and have a bulk magnetization describing a hysteresis curve (Fig. 2.1a). When the magnetic field B reaches a certain value, the sat
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