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WIREs Nanomed Nanobiotechnol
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Magnetic resonance relaxation properties of superparamagnetic particles

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Abstract Nanometric crystals of maghemite are known to exhibit superparamagnetism. Because of the significance of their magnetic moment, maghemite nanoparticles are exceptional contrast agents and are used for magnetic resonance imaging (of the liver, spleen, lymph nodes), for magnetic resonance angiography and for molecular and cellular imaging. The relaxivity of these agents depends on their size, saturation magnetization and magnetic field and also on their degree of clustering. There are different types of maghemite particles whose relaxation characteristics are suited to a specific MRI application. The relaxation induced by maghemite particles is caused by the diffusion of water protons in the inhomogeneous field surrounding the particles. This is well described by a theoretical model that takes magnetite crystal anisotropy and Néel relaxation into account. Another type of superparamagnetic compound is ferritin, the iron‐storing protein: it contains a superparamagnetic ferrihydrite core. Even if the resulting magnetic moment of ferritin is far smaller than for magnetite nanoparticles, its massive presence in different organs darkens T2‐weighted MR images, allowing the noninvasive estimation of iron content, thanks to MRI. The relaxation induced by ferritin in aqueous solutions has been demonstrated to be caused by the exchange of protons between bulk water protons and the surface of the ferrihydrite crystal. However, in vivo, the relaxation properties of ferritin are still unexplained, probably because of protein clustering. Copyright © 2009 John Wiley & Sons, Inc. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging

Inverse spinel structure of magnetite. (a) The front side of a cubic unit cell is illustrated. (b) Ferrimagnetic organization in magnetite; illustration of [1,1,1] plane.

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Outer sphere relaxation.

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Linear increase of r2 with the magnetic field for a ferritin aqueous solution. (Reprinted with Permission from Ref 41. Copyright 2000 Wiley Periodicals, Inc.).

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Evolution of (a) r1 and (b) r2 with magnetic field for an SPIO (AMI‐25—Endorem, reproduced from the data of Ref 34) and a USPIO (MION‐46L, reproduced from the data of Ref 36).

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(a) Ferrihydrite structure. (b) Diagram of antiferro‐ magnetic organization in ferrihydrite, with cation vacancies (V) and uncompensated magnetic moments (dashed arrows). (Reprinted with permission from Ref 11. Copyright 2007 AAAS).

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Illustration of Weiss domains in a large magnetite (or maghemite) crystal. (a) The small sphere represents a typical magnetite nanoparticle, whose size is much smaller than a Weiss domain. (b) Magnetization versus magnetic field for magnetite (or maghemite) nanoparticles (plain line) with a radius R = 5 nm and a saturation magnetization Msat = 110 Am2 kg−1(iron) and for ferritin (dashed line).

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Model of uniaxial anisotropy for a magnetite (or maghemite) nanoparticle. The graph shows the probability of alignment of the magnetic moment in one direction with respect to the angle between this direction and the anisotropy axis (for a sphere with a radius R = 5 nm and an anisotropy constant K = 13, 500 J m−3).

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