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Abstract:
Introduction: In the human brain, magnetic resonance spectroscopy is able to measure various metabolites of interest, visualized as peaks along a spectrum, which can be related to metabolism and functional processes. It is also able to measure chemical exchange at equilibrium, without disrupting the system. In this study, proton exchange between water and urea has been measured for the first time in human brain in vivo, at 9.4 tesla, the highest magnetic field strength human scanner available worldwide.
Materials and Methods: Magnetic resonance spectroscopy data were acquired on a 9.4T magnet in a white matter region of the brain in eleven healthy volunteers and in a grey matter region of the brain in a further ten volunteers. Exchange with water was measured using an inversion transfer experiment, which involves specifically perturbing (inverting) the water proton magnetization, and waiting for certain delay times before data acquisition in order to measure varying amounts of exchange between the water protons and the other metabolites such as urea. The data were averaged and a model was developed to fit the fourteen visible peaks in the averaged spectra, including urea, at the various delay times.
Results and discussion: The inversion transfer average series visualizes peaks that are strongly modulated in intensity by exchanging magnetization with the inverted water signal. In particular, the fast-exchanging peaks include urea at 5.8ppm and amide protons (related to proteins) in the 8.2-8.5ppm range. The preliminary fitting model captures most of the peaks very well, and the improved peak separation at ultra-high field has allowed for more peaks to be included in the model compared to the ones used at lower magnetic field strengths such as 3T (clinical strength) or 7T. Exchange rates for the different peaks will be obtained by Bloch McConnell simulations, which take both magnetization and exchange into account.
