This proposal aims to continue studies to better understand the factors that affect the NMR relaxation and other properties of protons in tissues which determine contrast in MR images, and the changes that occur in disease. Contrast in conventional MRI arises primarily from the heterogeneous distribution of tissue relaxation properties but no adequate model exists to quantitatively account for the relaxation rates even of normal tissues. Contrast produced by special MR sequences may also arise from variations in rates of magnetization transfer, from differences in water diffusion, and from subtle effects that occur in coupled spins in lipids. New imaging techniques have recently evolved that highlight the need for an improved understanding of such contrast mechanisms and relaxation effects, while a better understanding will likely point the way towards improved techniques and new applications. In the next phase of this project we will concentrate on (a) studies of cross relaxation, to better understand the structural factors that affect cross relaxation, magnetization transfer and spin diffusion in tissues (b) studies of the role of spin coupling effects and coherence transfer in lipids in the appearance of fat (c) studies of diffusion in tissues, and the factors that may affect apparent diffusion . We will quantify the contributions of individual interactions and mechanisms that affect contrast using a variety of high resolution spectral techniques as well as T1 and T2 measurements. We will measure (a) hydrodynamic effects, or the effects on water intramolecular dipolar interactions, using measurements of deuterium correlation times.(b) cross relaxation between water and macromolecular protons, using proton relaxation in deuterated samples, transient Overhauser effects and magnetization transfer (MT) (c) coherence transfer in hydrocarbon chains using CPMG and other sequences (d) apparent diffusion coefficients, restricted diffusion and translational displacement spectra in tissues using pulse gradient spin echo sequences. We will verify that present concepts of MT are valid and agree with estimates of cross relaxation by other methods. We will assess the relative importance of different surface groups on cross relaxation, particularly amino and amide groups, and of matrix rigidity and chain length on spin diffusion. We will study a selected group of proteins, polymers and gels, in different conditions; in free solution or cross-linked and immobilized; in different solvents and buffers wherein the surface character and affinity and proton exchange will be affected. The parameters that determine the signal from alkyl chains containing coupled nuclei in multiecho sequences will be determined using density matrix calculations and experiments on simple hydrocarbons. Diffusion in tissues will be documented in detail using displacement profile imaging, and the effects of changes in compartment size seen in stroke and seizure will be quantified. We will assess the influence of changes in the osmotic permeability of membranes and of background susceptibility changes on apparent diffusion. The overall project should provide many new insights into tissue proton NMR properties to aid in the better understanding of the origin of contrast in NMR images.
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