Using in-tube extraction and slice selective NMR experiments allow imaging via statistical analysis of metabolic profiles

Imaging the distribution of metabolites is very powerful in diagnostics but it is also employed in fundamental research. Although NMR spectroscopy is well established for determining metabolic profiles of biological samples, its application is limited to magnetic resonance imaging that can produce images of larger structures, but the number of detectable metabolites is very low. Mass spectrometry imaging on the other hand is well established with pixel sizes in the μm range. This limits the analysis of larger structures like tissue sections and detection of metabolites depends on their ionization properties. High resolution NMR metabolomics could complement these methods. However, this is prevented due to time consuming extraction procedures. To overcome these limitations, the following protocol was established and applied to two different ham slices: sampling is directly done into the NMR tube and after extraction of polar and non-polar metabolites in the NMR tube, slice selective NMR spectra are acquired. Multivariate analysis (PCA) of the NMR-spectra and subsequent visualization of the differences correlate well with structures visible in the ham slices. The proposed protocol can be used for metabolic imaging and could complement other imaging methods.