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Cover Picture: MRI Thermometry Based on Encapsulated Hyperpolarized Xenon (ChemPhysChem 16/2010)
The cover picture shows a new magnetic resonance imaging (MRI) method using encapsulated hyperpolarized xenon to map temperature, described on p. 3529 by F. Schilling, A. Pines et al. This method allows unprecedented absolute temperature accuracy of 0.1 °C at low and ultralow sensor concentrations. The method is based on the temperature-dependent chemical shift of hyperpolarized xenon in a cryptophane-A cage (shown in the upper left corner). This shift can be detected by MRI and is linear with a slope of 0.29 ppm °C−1 as can be seen in the graph. Three distinct NMR signals appear in a xenon NMR spectrum (see figure at the bottom): the signal of xenon dissolved in water at around 190 ppm, the xenon-in-cage signal at around 60 ppm, and the peak of gaseous xenon at 0 ppm. The xenon-in-cage peak is used for direct temperature mapping by relating its chemical shift to temperature based on the linear gauge curve. This way, dynamic heating procedures can be observed as for example the build-up of a temperature gradient (see figure in the upper right corner). Thermometry based on hyperpolarized xenon sensors improves the accuracy of currently available MRI thermometry methods, potentially giving rise to biomedical applications that could include monitoring of thermotherapy in cancer treatment or the detection of hot arteriosclerotic plaques.