Quantitative pulsed CEST MR imaging

Abstract

Chemical Exchange Saturation Transfer (CEST) experiments enable the indirect detection of small metabolites, e.g. creatine, and proteins in living tissue by means of magnetic resonance imaging. Selective RF saturation of solute protons in chemical exchange with water leads to an accumulation of saturation in the water magnetization. The resulting reduction of the water signal depends on physiological properties, e.g. pH, temperature and solute concentration, but also on the saturation scheme. In a clinical setup, the latter is limited to a series of short RF-pulses to obey safety regulations. Pulsed saturation is diffcult to describe theoretically, thus, the quantitative determination of physiological parameters via CEST experiments is a challenging task. In this thesis, a new analytical model for CEST is proposed, which extends a former interleaved saturation-relaxation approach. This model enables the analytical calculation of Z-spectra yielding deeper insight into the physics of pulsed CEST experiments. Furthermore, it enables for the first time in the case of pulsed saturation the separate and independent determination of the exchange rate k and the relative proton concentration f. The validity of this approach was tested by simulations and verified in measurements of model solutions containing creatine on a 7-Tesla whole-body MR tomograph. Finally, the obtained knowledge was used to quantitatively investigate pH and absolute creatine concentration in the human calf muscle under resting conditions and during exercise.