Gas phase MRI

Hyperpolarized 129Xe not only allows for sufficient signal intensity for chemical shift selective gas phase MRI, it also provides the means for a unique type of contrast. The imaging contrast derived from the transport of hyperpolarized gases into the material can be utilized to obtain snapshots of the gas flow and diffusion into porous samples. In this context, it is important to appreciate that Figures... 

Fig. 5.3.2 (A) NMR spectrum of hyperpolar- abundance of approximately 25% of the, 29Xe ized 129Xe from a sample that contains bulk gas isotope. (B) 2D slice of 3D chemical shift phase (0.3 ppm) and xenon occluded within selective MRI of the bulk gas phase. (C-E) 2D aerogel fragments (25 ppm). The gas mixture slices of 3D chemical shift selective MRI of the used for the experiment contained 100 kPa of 25 ppm region for various recycle times T.

The in situ monitoring of high temperature reactions by hpl29Xe magnetic resonance is still in its infancy. Although the previous work on gas phase dynamics in porous media has shown the feasibility of dynamic microscopy and M RI and the first in situ combustion NMR spectra have been collected, much more development remains to be done. To date, hpl29Xe NMR and MRI are currently the only techniques available to study gas dynamics in porous and opaque systems. 

Translational motion occurs in liquid and gas phases of water, but is virtually eliminated in the solid phase (ice). Translational motion of water molecules can be measured using NMR and magnetic resonance imaging (MRI) spectroscopy (Sun and Schmidt, 1995). 

See also Chemical Exchange Effects in NMR Diffusion Studied Using NMR Spectroscopy Gas Phase Applications of NMR Spectroscopy Liquid Crystals and Liquid Crystai Solutions Studied By NMR MRI Applications, Clinicai NMR in Anisotropic Systems, Theory NMR Microscopy NMR Relaxation Rates Nuclear Overhauser Effect Polymer Applications of IR and Raman Spectroscopy. 

The third basic technique for creating hyperpolarized nuclear spin magnetization makes use of spin exchange optical pumping. This method is commonly applied to hyperpolarize Xe (/= 1/2) in the gas phase. The prineiples of the technique as applied in the MRI eontextwere reviewed reeently by Palaniappan and co-workers, who put partieular emphasis on the CEST approach (called in this case HYPERCEST beeause of combination witb xenon hyperpolarization). Several papers deseribed different teebnique for preparation of byperpolarized Xe for bio-... 

Our very first experiments with the reactor depicted in Figure 5.4.1 were carried out with a 15% Pt-Y-Al203 single cylindrical catalyst pellet [10-12], The acquisition time of 2D images of an axial slice at that time was about 260 s. Despite this, the first direct MRI visualization of the operation of a model gas-liquid-solid reactor has revealed the existence of large gradients of the liquid phase content within the catalyst pellet upon imbibition of liquid a-methylstyrene (AMS) under conditions... 

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