Hyperpolarized gases

Fig. 5 shows the image produced by flowing hyperpolarized gas in a phantom consisting of a hollow Vycor cylinder filled with NaY zeolite. The spectrum (fig.5, bottom left) shows lines characteristic of the gas near 0 ppm, the xenon in Vycor at 76.1 ppm and... 

Polarization transfer from hyperpolarized gas to H, etc. holds great promise for sensitivity enhancement of solution-state NMR. Leawoods et al. ... 

MRI measurement of hyperpolarized gas diffusion is another important source of information about lung morphology and disease. The diffusion of gas is restricted by the microstructure of the lung—its small airways and alveolar walls—making diffusion sensitive to microstructural features on an alveolar distance scale, well below the spatial—resolution limits of conventional MRI. These properties can be quantified as a gas apparent diffusion coefficient (ADC), complementing information about the uniformity of gas transport derived from ventilation images. 

Chen XJ, Moller HE, Chawla MS, Cofer GP, Driehuys B, Hedlund LW, Johnson GA. Spatially resolved measurements of hyperpolarized gas properties in the lung in vivo. Part I diffusion coefficient. Magn Reson Med 1999 42(4) 721-728. 

Hyperpolarized 129Xe NMR Spectroscopy, MRI and Dynamic NMR Microscopy for the In Situ Monitoring of Gas Dynamics in Opaque Media Including Combustion Processes... 

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.

Fig. 5.3.3 (A) NMR spectrum of hyperpolarized 129Xe in NaX zeolites. (B) 2D slice in the flow direction of a 3D chemical shift selective MRI of gas in the zeolite pellets. (C) 2D slice perpendicular to the flow direction of the same 3D chemical shift selective MRI as in (A). Adapted from Ref. [14].

Some less frequently encountered nuclides, such as 57Fe and 59Co, have chemical shift ranges of more than 10,000 ppm, hence encompass a very large frequency range in spite of smaller magnetogyric ratios. The chemical shift of monatomic 129Xe gas is very sensitive to its environment, and hyperpolarized xenon (Section 2.5) can be used to probe adsorption sites in solids. 

Adenosine binds to adenosine receptors (AD-Rs) (subtypes Ah A2A, A2b and A3). Ap and Ap R activation gives Gai-mediated inhibition of adenylyl cyclase (resulting in decreased cAMP) and Gai/Gao-mediated activation of a K+ channel (with a de-excitatory hyperpolarizing effect). A2A and A2B activation gives Gas-mediated stimulation of adenylate cyclase (resulting in elevated cAMP). Adenosine acting via particular receptors variously has cardioprotective, neuroprotective, sedative, anticonvulsant, soporific, vasodilatory and bronchoconstrictive effects. The plant-derived methylxanthines theophylline and caffeine are adenosine A1 and A2 receptor antagonists (Table 5.1). 

Hyperpolarized e has been detected in the rat brain following the injection of the gas, dissolved in a lipid emulsion, into the carotid artery. Spectra were recorded at 2.35 T over 51 s with a repetition time of 253 ms. Two peaks were detected and were assigned to intravascular xenon (194 1 ppm) and xenon dissolved in brain tissue (199 1 ppm)." ... 

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