Development and Application of an NMR Metabolic Imaging Technique based on CEST/NOE

For a thorough investigation of metabolic changes in cases of increased activity or disease, a high-resolution imaging modality would be of high-value. Conventional MR-based metabolite mapping techniques suffer from low spatial resolution and poor sensitivity to low concentrated metabolites. In this work, a novel optimization approach for high-resolution metabolic imaging by means of chemical exchange saturation transfer (CEST) and nuclear Overhauser effect (NOE) will be introduced. The general term of saturation transfer comprises CEST, NOE and magnetization transfer (MT). It describes the transfer of saturated magnetization from low concentrated solute metabolites to bulk water, after frequency selective RF irradiation was applied. The strong enhancement of attenuation effects—observed in a Z-spectrum—can be used to detect changes in metabolite concentrations. The vast complexity of in vivo tissue heavily distorts the baseline in Z-spectra. With a background spin system, modeled from a scout acquisition, saturation pulse amplitudes for each frequency offset were modulated, such that a specific target signal intensity was achieved. Further, a framework for a retrospective look-up correction of magnetic field inhomogeneities and background spin system deviations was developed. The approach of a prospective baseline enhancement (PROBE) was investigated by means of sensitivity and specificity in vitro. It was subsequently translated into a clinical setting at 3 T. A high-resolution mapping of elevated lactate in ischemic regions of acute stroke patients became feasible