This educational course covers the technical challenges and solutions related to non-proton MR, focusing on the nuclei 31P and 13C. Topics are dedicated RF hardware (particularly multiply tuned RF coils), pulse sequences and quantification methods, as well as methods and devices for stimulation of the organ under investigation.
The challenges arising with 31P and 13C MRS and MRI are due to the lower resonance frequency and sensitivity (intrinsic NMR sensitivity as well as metabolite concentration and/or natural abundance) and higher chemical shift between resonances, as compared with 1H.Relaxation times and their B0 dependence have to be considered for choosing an appropriate acquisition scheme and its parameters, as well as homo-nuclear and heteronuclear coupling of resonances. The reward for overcoming these hurdles is a non-invasive view on metabolism and its kinetics, hard or impossible to be obtained in any other way.
This educational course covers the technical challenges and solutions related to non-proton MR, focusing on the nuclei 31P and 13C.
Topics are dedicated RF hardware (particularly multiply tuned RF coils), pulse sequences and quantification methods, as well as methods and devices for stimulation of the organ under investigation.
The challenges arising with 31P and 13C MRS and MRI are due to the lower resonance frequency and sensitivity (intrinsic NMR sensitivity as well as metabolite concentration and/or natural abundance) and higher chemical shift between resonances, as compared with 1H.Relaxation times and their B0 dependence have to be considered for choosing an appropriate acquisition scheme and its parameters, as well as homo-nuclear and heteronuclear coupling of resonances. The reward for overcoming these hurdles is a non-invasive view on metabolism and its kinetics, hard or impossible to be obtained in any other way.
Hardware components discussed:
An overview over the properties of 31P and 13C MR spectra and the information content of MR images is given: For example, following the time course of phosphocreatine concentration in skeletal muscle which is depleted during an exercise scheme and recovering towards its equilibrium value allows us to study ATP synthesis and its various contributions like PCr splitting, glycolysis and oxidative phosphorylation, while the pH value can be measured from the chemical shift between inorganic phosphate, obtained from the same time-resolved 31P spectra. With directly detected 13C MRS it is possible to quantify e.g. glycogen at natural abundance, while administration of labelled substances enables the analysis of the metabolisation of substrates in the tricarboxylic acid cycle.
Properties and challenges of such measurements have implications on the acquisition schemes: Generally, the spatial resolution is relatively low, and sometimes an unlocalised acquisition is the method of choice. Nevertheless, recent developments of hardware (RF coils and ultra-high field magnets) and pulse sequences have opened the possibility to localise the NMR signal to regions of interest or to acquire metabolite-specific or full spectroscopic images. The application of these spatially resolved dynamic methods has revealed heterogeneity of metabolic activity, with e.g. individual muscles exercising differently, according to the exact stimulation paradigm or even within one single muscle, which justifies the effort of advanced acquisition schemes.
Acquisition schemes discussed: