The paper presents a brief historical overview of the field of the measurement of diffusion in zeolites. The focus will be on macroscopic measurements, i.e. when diffusion through entire crystals is studied. While this may appear to be a simple task, we will see that there are difficulties in obtaining reliable results. Two case studies, the volumetric/piezometric experiment and the Transient Analysis of Products (TAP) apparatus are discussed and ways to improve either the experiments or the way in which the results are analyzed are presented. The author’s perspective into what are the current challenges in the field concludes the paper.

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Nuclear magnetic resonance (NMR) spectroscopy was used to study exchange of three distinctly different molecules across the membranes of human red blood cells (RBCs). In studying water, t-butanol, and dimethyl sulfoxide we exploited the marked differences in the apparent diffusion coefficients of these species inside and outside the cells in suspensions. The measurements were made with diffusion ordered spectroscopy (DOSY) with a domain of ‘diffusion times’ of 20 - 100 ms. In attempting to make the DOSY spectra quantitative we identified time domains for each of the three molecules in which the spectra showed well resolved peaks, and those in which only a single peak was evident for two of the species. The apparent mean resident times for water and t-butanol in the RBCs estimated by the methodology were ~17 ms, while the DMSO exchange was too slow, on the NMR timescale, to be quantified by this method. However, it is very clear that other methods that are based on regression analysis with a prescribed fitting function provide more reliable estimates of exchange rate constants. Also, to make the DOSY analysis more generally accessible we implemented a Laplace transform inversion method (the Post-Widder algorithm), and a procedure for enhancing the resolution of the resulting diffusion spectra by using standard functions in Mathematica.

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This dissertation describes the development, implementation, validation, optimization, and application, of a noninvasive and quantitative method for measuring cerebral blood volume changes with functional magnetic resonance imaging (fMRI) for mapping of neural activity changes. Since its inception over twenty years ago, the field of fMRI has grown in usage, sophistication, range of applications, and impact. Nevertheless it has yet to exploit its full potential regarding, spatiotemporal resolution, signal specificity, and quantifiability of hemodynamic changes. By utilization of a new MR pulse sequence, new concepts of radio frequency pulses, and high magnetic fields (7 T), a novel fMRI method named SS-SI VASO is presented here that overcomes sensitivity limitations of other noninvasive quantitative imaging methods. In order to validate that its signal represents changes in cerebral blood volume without other contaminations, SS-SI VASO is implemented in animal models for a close comparison with established, but invasive methods. A good agreement of blood volume sensitivity has been found with the new method compared to the established ones. After its validation, the SS-SI VASO method and its unprecedented sensitivity was used to localize and quantify hemodynamic changes in applications where conventional oxygenation based fMRI methods are limited. (A) SS-SI VASO was used to investigate biophysical aspects of actively controlled arteries and passive balloon-like veins during activity induced hemodynamic changes. (B) SS-SI VASO was used to provide insights whether the interplay of neural activity and resultant vascular response are the same for tasks that increase neural activity compared to tasks that suppress neural activity. (C) SS-SI VASO was used to calibrate conventional oxygenation based fMRI to quantify local changes in oxygen metabolism. (D) The high sensitivity of SS-SI VASO was further used to obtain sub-millimeter resolutions and estimate activity changes between cortical layers. This enables to address questions not only where the brain is activated but also how and whereby this activity is evoked. The implementation and application of this new SS-SI VASO fMRI method is a major step forward for the field of imaging neuroscience; it demonstrates that the current limitations of fMRI can be even overcome with respect to quantifiability, spatial specificity and distinguishing between vascular and neuronal phenomena.

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