Comparison of fast field-cycling magnetic resonance imaging methods and future perspectives

ABSTRACT

Fast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents.     

Emulsifier-free emulsion copolymerization of styrene and butyl acrylate. I. Kinetic studies in the absence of surfactant

Soapless emulsion copolymerization of styrene (S) and n-butyl acrylate (BuA) has been investigated using two types of initiator and different comonomer feed mixtures. When using K₂S₂O₈ as initiator, the particle size and size distribution of the final latexes (500 nm and 1.003, respectively) is not significantly affected by the comonomer feed composition, whereas the molecular weight and surface characteristics were found to sharply change at high butyl acrylate content. Based on the most probable particle nucleation mechanism and type of chain termination in the monomer swollen particles, a tentative explanation of these results has been proposed. Replacing persulfate by a carboxylic initiator (4-4′-azobiscyanopentanoic acid) results in the formation of stable particles as α observed with the persulfate, provided the aqueous phase pH is fixed in between 6 and 7. Results on the initiator residue location as a function of the conversion point out that the particle flocculation mechanism is strongly significant in the preparation of such latexes.     

Temperature evolution of the umbrella structure in holmium iron garnet

Using neutron diffraction data, the thermal evolution of the magnetic structure of Ho₃Fe₅O₁₂ has been studied. Below the compensation temperature (T _comp=137 K), the umbrella structure parameters which fit the spontaneous ferrite magnetizations have been determined. AboveT _comp, the compound behaves as a Néel type ferrimagnet. Particular attentions have been given to the magnetic form factors of both iron sublattice on one hand and on the other hand to a rhombohedral distorsion fromI a3d toR below 30 K and in first approximation toR c forT>30 K.     

A refined study of the [111] magnetoelastic properties of terbium iron garnet: Relation to the cotton mouton birefringence

Accurate measurement of the ^{, 2} magnetostriction constant of a single crystal of TbIG has been performed from 90 K up to the Neel temperature. Both spontaneous and forced magnetostriction differ markedly from the one ion model predictions. Moreover, the forced magnetostriction modulus is much stronger below the compensation temperature than expected from extrapolating the high temperature variations. The spontaneous [111] linear magnetic birefringence and its field derivative exhibit thermal variations proportional respectively to those of the spontaneous and forced magnetostriction from 80 to 200 K.Loboratoire associé à l'Université Scientifique et Médicale de Grenoble.     

Magnetic properties on holmium iron garnet in high magnetic field. Optical absorption spectra in the infra-red region

On single crystals of holmium iron garnet (HoIG), magnetic properties have been studied in magnetic field up to 150kOe applied parallel to the main crystallographic directions in the 4.2–300K temperature range. Above 130 K, the magnetization is isotropic and linear magnetic field dependent as previously found in polycrystals and predicted by Néel's ferrimagnetic theory. Nevertheless the paramagnetic Curie temperature is much higher than the polycrystal value. Below 130K, due to the onset of the umbrella structure, the ferrite magnetization presents a non linear field variation with [111] as easy direction. The field evolutions of the anisotropy constants (K ₁ andK ₂) were calculated. Optical absorption measurements of both 5₈5I ₆ and 5I ₈5I ₇ transitions are reported and compared to the results of the literature in terms of inequivalent magnetic sites.     

Internal Transfer Reactions in the Copolymerization of Vinyl Acetate with Chlorinated Monomers

In the copolymerization of vinyl acetate (A) with either vinyl chloride (C) or vinylidene chloride (V), an internal transfer (backbiting) reaction—of the C- or V-ended radi-cals on an antepenultimate A unit—is proposed to be responsible for the deviation of the copolymerization kinetics from the Lewis and Mayo theory. The deviations disappear if A is replaced by isopropenylacetate [I_p], Then one gets, for the I_p -C copolymerization. r_{I p} =0.35 and :r_c=2.4, and for I -V copolymerization, r_{I p}=0.13 and r_v=5.9. The internal transfer reaction causes the formation of branches which may be evidenced by NMR analysis of constant composition suspension A-C copolymers. A kinetic scheme is proposed and the corresponding reactivity ratios derived r_A=0.29, r_c=1.60, r=0.3 (radical resulting from the transfer reaction), and k_T=1500 (rate constant of the transfer reaction at 50°C). The distribution of branches is calculated together with the sequence distribution functions for the .A. or Cunits.     

Unraveling σ and π Effects on Magnetic Anisotropy in cis‐NiA4B2 Complexes: Magnetization,HF‐HFEPR Studies,First‐Principles Calculations,and Orbital Modeling

By using complementary experimental techniques and first‐principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C_2v, has been investigated. Four complexes have the general formula [Ni(bpy)X₂]ⁿ⁺ (bpy=2,2′‐bipyridine; X₂=bpy ( 1 ), (NCS^?)₂ ( 2 ), C₂O₄^2? ( 3 ), NO₃^? ( 4 )). In the fifth complex, [Ni(HIM₂‐py)₂(NO₃)]⁺ ( 5 ; HIM₂‐py=2‐(2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM₂‐py. Analysis of the high‐field, high‐frequency electronic paramagnetic resonance (HF‐HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from ?1 to ?10 cm^?1. First‐principles SO‐CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3 , on one hand, and 4 and 5 , on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and g_i) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the e_g‐like orbitals and is due to the difference in the σ‐donor strength of NO₃^? and bpy or HIM₂‐py, whereas the difference in anisotropy between the two compounds is due to splitting of the t_2g‐like orbitals; and 2) the anisotropy of complexes 1 – 3 arises from the small splitting of the t_2g‐like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.     

A Multifunctional Photoswitch: 6π Electrocyclization versus ESIPT and Metalation

A terthiazole‐based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady‐state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time‐dependent photochemical outcome and displays efficient ESIPT‐based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.     

Direct Oxidative Coupling of N‐Acetyl Indoles and Phenols for the Synthesis of Benzofuroindolines Related to Phalarine

Inspired by the biogenetic synthesis of benzofuroindoline‐containing natural products, we designed an oxidative coupling between phenol and N‐acetyl indoles. This straightforward and direct radical process, mediated by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone and FeCl₃ allowed the regioselective synthesis of benzofuro[3,2‐b]indolines, whose structure is found in the natural product phalarine.