In vivo magnetic resonance studies of glycogen and hyperpolarized lithium in the rat brain

Nuclear magnetic resonance (NMR) is used for a large array of applications, ranging from chemical characterization to oil drilling to medical imaging. In these fields NMR is used as an investigational tool, but new techniques and applications are continuously being developed as well. The latter is also the subject of this thesis. After an introduction to NMR theory, it focuses on the development of new methodologies for two separate areas of investigation: the metabolism of brain glycogen and hyperpolarization via dynamic nuclear polarization (DNP). Brain glycogen is the main storage form of glucose in the brain. However, because it is present in much lower concentrations in the brain than in other tissues, the significance of this storage and other roles are subject to debate. To better understand the roles of brain glycogen, it is important to quantify its metabolic parameters. The only currently available method to detect brain glycogen in vivo is 13C NMR spectroscopy. Incorporation of 13C-labeled glucose is necessary to allow glycogen measurement, but might be affected by changes in the turnover between glucose and glycogen. We therefore established a protocol to measure the glycogen absolute concentration in the rat brain by eliminating label turnover as a variable. The approach is based on establishing an increased, constant 13C isotopic enrichment (IE). 13C-glucose infusion is then performed at this IE of brain glycogen. As glycogen IE cannot be assessed in vivo, we validated that it can be inferred from the N-acetyl-aspartate (NAA) IE in vivo: after [1-13C]-glucose ingestion, glycogen IE was 2.2 ± 0.1 times that of NAA. Glycogen concentration measured in vivo by 13C NMR (5.8 ± 0.7 µmol/g) was in excellent agreement with post-mortem biochemistry (6.4 ± 0.6 µmol/g). A second glycogen NMR protocol was then implemented to measure concentration and turnover simultaneously. After reaching isotopic steady state for glycogen C1 using [1-13C]-glucose administration, [1,6-13C2]-glucose was infused such that isotopic steady state was maintained at the C1 position, but the C6 position reflected 13C label incorporation. To overcome the large chemical shift displacement error between the C1 and C6 resonances of glycogen, 2D gradient-based localization using the Fourier series window approach was implemented. The glycogen concentration of 5.1 ± 1.6 µmol/g measured from the C1 position was in excellent agreement with concomitant biochemical determinations, while glycogen turnover measured from the rate of label incorporation into the C6 position of glycogen in the α-chloralose anesthetized rat was 0.7 µmol/g/h. The contrast agent Omniscan was added to a formic acid-filled glass reference sphere in the abovementioned protocols in order to shorten its T1 relaxation time and to accelerate the calibration procedure it is used in; therefore it was established that Omniscan in pure 13C formic acid has a relaxivity of 2.9 mM-1s-1. The study was expanded, and th