Fast-field cycling magnetic resonance imaging FFC-MRI Earth-field MRI Pre-polarised MRI Free-radical imaging Magnetisation-transfer contrast Mots-clés : IRM en champ cyclé FFC-IRM IRM en champ magnétique terrestre IRM prépolarisée Imagerie par radicaux libres Contraste par transfert de magnetisationMagnetic resonance imaging (MRI) and fast field-cycling (FFC) NMR are both welldeveloped methods. The combination of these techniques, namely fast field-cycling magnetic resonance imaging (FFC-MRI) is much less well-known. Nevertheless, FFC-MRI has a number of significant applications and advantages over conventional techniques, and is being pursued in a number of laboratories. This article reviews the progress in FFC-MRI over the last two decades, particularly in the areas of Earth's field and prepolarised MRI, as well as free radical imaging using field-cycling Overhauser MRI. Different approaches to magnet design for FFC-MRI are also described. The paper then goes on to discuss recent techniques and applications of FFC-MRI, including protein measurement via quadrupolar cross-relaxation, contrast agent studies, localised relaxometry and FFC-MRI with magnetisation-transfer contrast. r é s u m é L'imagerie par Résonance Magnétique (IRM, ou MRI en anglais) et la RMN avec cyclage de champ rapide (« fast field cycling », FFC) sont toutes deux des méthodes bien développées. La combinaison de ces techniques, c'est-à-dire l'imagerie par résonance magnétique avec cyclage de champ rapide (FFC-MRI en anglais) est beaucoup moins bien connue. Cependant, la FFC-MRI a un nombre d'applications et d'avantages significatifs par rapport aux techniques conventionnelles, et son étude est poursuivie dans un certain nombre de laboratoires. Cet article passe en revue les progrès de la FFC-MRI au cours des deux dernières décennies, en particulier dans les domaines de l'imagerie en champ terrestre avec pré-polarisation, de même que l'imagerie des radicaux libres en utilisant la FFC-MRI avec effet Overhauser. Diverses approches à la conception des aimants pour FFC-MRI sont également décrites. L'article continue en discutant des techniques et applications récentes de la FFC-MRI, telles que la mesure des protéines par relaxation croisée quadrupolaire, les études d'agents de contraste, la relaxométrie localisée et la FFC-MRI avec contraste par transfert d'aimantation.
Fast field-cycling (FFC) MRI allows switching of the magnetic field during an imaging scan. FFC-MRI takes advantage of the T(1) dispersion properties of contrast agents to improve contrast, thus enabling more sensitive detection of the agent. A new contrast agent designed specifically for use with FFC was imaged using both a homebuilt FFC-MRI system and a 3 T Philips clinical MRI scanner. T(1) dispersion curves were obtained using a commercial relaxometer which showed large changes in relaxation rate between fields. A model of magnetization behaviour was used to predict optimum evolution times for the maximum T(1) contrast between samples at each field. Images were processed and analysed to create maps of R(1) values using a set of images at each field. The R(1) maps produced at two different fields were then subtracted from each other in order to create a map of ΔR(1) in which pixel values depend on the change in R(1) of the sample between the two fields. The dispersion properties of the agent resulted in higher contrast in a ΔR(1) image compared with a standard T(1)-weighted image.
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