No abstract

We analyse the diffusion problem in the traditional Fe(II/III) agarose gel system employed in MRI studies of radiation dosimetry. The diffusion coefficient is measured using an inversion recovery null-point imaging method in a model gel/water phantom. The diffusion coefficient of Fe(III) in 1% agarose gel at pH 1.1 is D = 2.7 +/- 0.3 x 10(-6) cm2 s-1. The diffusion coefficient of Fe(II) is D = 3.3 +/- 0.5 x 10(-6) cm2 s-1. Measurement of the diffusion coefficients permits simulation of the MRI signal intensity from phantoms with model radiation dose distributions. We allow for diffusion of both Fe(II) and Fe(III) in our simulations as well as the effect of both relaxation agents on the local spin-lattice relaxation time T1. We also analyse the effects of the physical penumbra on the diffusion problem.

SynopsisThe transverse NMR relaxation times of hydrogen nuclei of water absorbed in white spruce sapwood [Picea glauca (Meunch) Voss] were measured for moisture contents in the range from 5 to 176%.The spin echo amplitudes resulting from the Carr-Purcell sequence decay nonexponentially suggesting the possibility of a t least two different relaxation times for water in wood. A simplified structural model of the wood-water mixture is used to estimate the rates of chemical exchange at room temperature of hydrogen nuclei between various sites in the system. The high-resolution NMR line shape is discussed briefly in terms of this proposed model. INTRODUCTIONIt is well known that the macroscopic properties of cellulose and similar polymers are influenced profoundly by the presence of ~a t e r . l -~ Thus, measurement of the moisture content of wood is of considerable importance, and numerous methods for determining the mass fraction of water in wood and pulp have been p r o p o~e d .~ Measurement of the nuclear magnetic resonance signal of hydrogen nuclei in cellulose-water mixtures is one possible t e~h n i q u e .~-~ Magnetic resonance also has played a significant role in understanding the wood-water relationship,a12 because the technique is sensitive to the microscopic environment and dynamics of molecules containing spin-bearing nuclei such as hydrogen. The wide-line NMR absorption spectrum of hydrogen nuclei in wood-water systems exhibits a relatively narrow line attributable to water, superimposed upon a much broader line resulting from hydrogen nuclei within the wood fiber.1°J2 The intensity of the narrow absorption line gives an accurate measurement of the amount of water present in a given ample,^ but the line shape of this component was found to be much broader than that of pure water, possibly implying that water in wood is much less mobile than in the liquid phase.The NMR evidence indicates that water in cellulosic materials takes at least two different formsg: water that is associated intimately with wood cell walls and water that is relatively free to move about in the cell cavities. Pulsed nuclear magnetic resonance techniques are quicker, easier, and more readily analyzed in terms of models for the molecular dynamics than steady-state absorption methods, but in spite of this, few such studies have been undertaken for watercellulose systems. Therefore, as part of an ongoing program to investigate the properties of water and oils in wood, we have used pulsed NMR methods to measure the transverse relaxation time T2 as a function of the moisture content of white spruce sapwood. EXPERIMENTALCylindrical samples 0.5 cm in diameter and 1.5 cm long were cut from a block of green white spruce [Picea glauca (Meunch) Voss], dried for varying lengths of time, and sealed in glass tubes with picine vacuum wax.7 After the magnetic resonance experiments, moisture contents of the specimens were determined by weighing before and after drying at 105°C for 12 hr. The moisture contents ranged from 5 to 176% on an oven dry...

In wet hen egg white lysozyme (HEWL), the molecular dynamics at the lysozyme–water interface was studied using a proton NMR line-shape-relaxation correlation approach that employed selective inversion of the proton magnetization. The intrinsic lysozyme proton spin-lattice relaxation rate, the intrinsic water proton spin-lattice relaxation rate, and the lysozyme proton – water proton cross-relaxation rate were determined. The lysozyme proton – water proton intermolecular interaction couples these protons and contributes to spin-lattice relaxation as well. The results suggest that a minimum of three different correlation times are needed to characterize the water molecule dynamics in wet HEWL.

No abstract