Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Water diffusion and exchange as they influence contrast enhancement.

Abstract: The contrast-enhanced magnetic resonance imaging (MRI) signal is rarely a direct measure of contrast concentration; rather it depends on the effect that the contrast agent has on the tissue water magnetization. To correctly interpret such studies, an understanding of the effects of water movement on the magnetic resonance (MR) signal is critical. In this review, we discuss how water diffusion within biological compartments and water exchange between these compartments affect MR signal enhancement and therefore our ability to extract physiologic information. The two primary ways by which contrast agents affect water magnetization are discussed: (1) direct relaxivity and (2) indirect susceptibility effects. For relaxivity agents, for which T1 effects usually dominate, the theory of relaxation enhancement is presented, along with a review of the relevant physiologic time constants for water movement affecting this relaxation enhancement. Experimental issues that impact accurate measurement of the relaxation enhancement are discussed. Finally, the impact of these effects on extracting physiologic information is presented. Susceptibility effects depend on the size and shape of the contrast agent, the size and shape of the compartment in which it resides, as well as the characteristics of the water movement through the resulting magnetic field inhomogeneity. Therefore, modeling of this effect is complex and is the subject of active study. However, since susceptibility effects can be much stronger than relaxivity effects in certain situations, they may be useful even without full quantitation.

Dielectric Relaxation of Polar Liquids

Abstract: The statistical theory of the dielectric relaxation of polar liquids is developed using the fluctuation‐dissipation approach to linear dissipative phenomena, and an expression is derived relating the complex dielectric constant to a time‐dependent microscopic correlation function. It is found that a finite number of microscopic relaxation times leads to an equal number of macroscopic decay times, and, in the case of a single relaxation time τ0, the decay time is given by T0=[3e0/(2e0+e∞)]τ0, e0 being the static dielectric constant, and e∞ being the high frequency dielectric constant. Relaxation times are also determined for systems having two decay times, and for systems characterized by the circular‐arc and skewed‐arc distribution functions.

A monometallic lanthanide bis(methanediide) single molecule magnet with a large energy barrier and complex spin relaxation behaviour

Abstract: We report a dysprosium(III) bis(methanediide) single molecule magnet (SMM) where stabilisation of the highly magnetic states and suppression of mixing of opposite magnetic projections is imposed by a linear arrangement of negatively-charged donor atoms supported by weak neutral donors. Treatment of [Ln(BIPMTMS)(BIPMTMSH)] [Ln = Dy, 1Dy; Y, 1Y; BIPMTMS = {C(PPh2NSiMe3)2}2−; BIPMTMSH = {HC(PPh2NSiMe3)2}−] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPMTMS)2][K(18C6)(THF)2] [Ln = Dy, 2Dy; Y, 2Y]. AC magnetic measurements of 2Dy in zero DC field show temperature- and frequency-dependent SMM behaviour. Orbach relaxation dominates at high temperature, but at lower temperatures a second-order Raman process dominates. Complex 2Dy exhibits two thermally activated energy barriers (Ueff) of 721 and 813 K, the largest Ueff values for any monometallic dysprosium(III) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex 2Dy has rich magnetic dynamics; field-cooled (FC)/zero-field cooled (ZFC) susceptibility measurements show a clear divergence at 16 K, meaning the magnetic observables are out-of-equilibrium below this temperature, however the maximum in ZFC, which conventionally defines the blocking temperature, TB, is found at 10 K. Magnetic hysteresis is also observed in 10% 2Dy@2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground 6H15/2 multiplet of 2Dy are essentially pure, well-isolated |±15/2〉, |±13/2〉 and |±11/2〉 states quantised along the CDyC axis. Thermal relaxation occurs via the 4th and 5th doublets, verified experimentally for the first time, and calculated Ueff values of 742 and 810 K compare very well to experimental magnetism and luminescence data. This work validates a design strategy towards realising high-temperature SMMs and produces unusual spin relaxation behaviour where the magnetic observables are out-of-equilibrium some 6 K above the formal blocking temperature.

Water-Soluble GdF3 and GdF3/LaF3 NanoparticlesPhysical Characterization and NMR Relaxation Properties

Abstract: Nanoparticles consisting of either a solid core of GdF3 or an 80/20 mixture of GdF3 and LaF3 have been prepared for use as NMR and MRI relaxation agents. To obtain high aqueous solubilities, the particles were coated with either citrate (cit) groups (in the case of GdF3 nanoparticles), giving the nanoparticle a negatively charged surface, or 2-aminoethyl phosphate (AEP) groups (in the case of GdF3/LaF3 = 80/20), giving the nanoparticle a positively charged surface at physiological pH. In the presence of the 80/20 GdF3/LaF3:AEP, the paramagnetic contribution to the water spin−lattice relaxation rate was observed to be 7.5 s-1 at a nanoparticle concentration of 9.0 nM (0.78 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Similarly, paramagnetic rates of 10.5 s-1 were observed for water using the GdF3:cit nanoparticles at a nanoparticle concentration of 0.55 nM (0.77 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Relaxivity measurements confirmed the potential of the particles for applications as contrast agents at...