2014 L'influence de l'inhomogénie spatiale du champ magnétique est étudiée, à l'aide des équations de Bloch, par la déformation produite sur une raie théoriquement lorentzienne. L'étude est effectuée pour un échantillon immobile dans différents cas de détection et sur le signal de précession libre dans le cas d'un échantillon tournant. La mise en évidence d'une fonction ne dépendant que de la forme de l'échantillon et que de l'inhomogénie du champ magnétique principal permet de proposer une méthode d'amélioration de la résolution des spectres de résonance magnétique nucléaire par transformée de Fourier. Cette méthode, valable pour des composés liquides dont les raies sont théoriquement lorentziennes, n'introduit pas de facteur empirique. Quelques exemples permettent d'en apprécier l'efficacité.Abstract. -The spatial inhomogeneous magnetic field is studied by means of its effects on a line shape which theoretically would be a lorentzian curve. The Bloch equations permit to solve this problem for a non-spinning sample, in various experiments, and also for a sample rotating around the axis of the field, in the case of a free induction decay signal. We show as an example, how one can theoretically obtain the line shape for some types of inhomogeneous fields. Then, through the definition of a complex function depending only on the inhomogeneity of the magnetic field and on the form of the sample, we propose a new method for the resolution enhancement of Fourier transform nuclear magnetic resonance. This method, suitable for lines with a theoretically lorentzian shape, does not use an empirical parameter. Few examples are presented to demonstrate the potential of this method.
The slotted cylinder, an inductive structure with low self-inductance, low electric field, has been studied as a probe for NMR imaging applications. A theoretical calculation allows us to map the magnetic field and to evaluate electrical parameters of the structure. Several implementations, including new designs, have been experimentally tested over a wide range of frequencies (4-40 MHz), and compared to a classical coil probe. This study demonstrates the efficiency of the slotted cylinder for NMR imaging. It is optimized for large conductive samples when imaging at high frequencies (for human head, above 20 MHz).
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