Theory of 1/T₁ and 1/T₂ NMRD profiles of solutions of magnetic nanoparticles

Abstract

Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.

Octapod iron oxide nanoparticles as high-performance T2 contrast agents for magnetic resonance imaging

“…However, the spherical iron oxide nanoparticles with large size would show ferri/ferromagnetic properties at room temperature, resulting in interparticle agglomeration even in the absence of external magnetic field 12 . On the basis of the quantum mechanical outer sphere theory, the T 2 relaxivity is highly dependent on both the M s value and the effective radius of typically superparamagnetic core [19][20][21] . In the motional average regime 22,23 , the relaxivity r 2 is given by (where all of the nanoparticle contrast agents were simulated as spheres) 24 r 2 ¼ 256p 2 g 2 =405 À Á kM 2 s r 2 =Dð1 þ L=rÞ ð 1Þ…”

Relaxation of protons by radicals in rotationally immobilized proteins

“…In most cases, the relaxation efficiency of these compounds is limited by the short electron spin-lattice relaxation times in the range of tens to hundreds of picoseconds, which in turn defines the concentration range where these compounds may be used as practical contrast agents [4][5][6][7][8][9][10][11]. MRI contrast agents currently in use are generally soluble extracellular and blood-pool agents; however, targeting to provide specific anatomical or biochemical information will necessarily involve binding of an agent to a cell surface site or a specific macromolecular matrix [4,12,13].…”

Water relaxation by SPM particles: Neglecting the magnetic anisotropy? A caveat

“…At any rate, for water magnetic relaxation induced by SPM particles, there is no "usual theory": anisotropy is in fact ignored in Ref. 6, while it is fully considered in another work dealing with the same subject (11) (Kellar et al's Ref. 23), which extends to nuclear magnetic relaxation the conclusion of the classical studies quoted above: "the critical influence of the anisotropy energy on the shape of Nuclear Magnetic Relaxation Dispersion profiles of superparamagnetic colloids is clearly shown, which completely explains the severe attenuation (for Ultra Small Particles of Iron Oxide) and the vanishing (for Small Particles of Iron Oxide) of the low-field dispersion, in agreement with the experimentally observed NMRD curves.…”

“…6, in which it had been implicitly assumed to be zero. Our theoretical study (11) allowed a full understanding of the role of anisotropy energy, leading us to qualify Ref.…”