title: Quantitative separation of CEST effect from magnetization transfer and spillover effects by Lorentzian-line-fit analysis of z-spectra.
abstract: Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.
authors.affiliation: Department of Medical Physics in Radiology, Deutsches Krebsforschungszentrum (DKFZ, German Cancer Research Center), Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany. m.zaiss@dkfz.de
authors.forename: Moritz
authors.initials: M
authors.lastname: Zaiss
authors.forename: Benjamin
authors.initials: B
authors.lastname: Schmitt
authors.forename: Peter
authors.initials: P
authors.lastname: Bachert
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D003287:Contrast Media / Q000737:chemistry; D003627:Data Interpretation, Statistical; D018130:Diamond; D006863:Hydrogen-Ion Concentration; D016018:Least-Squares Analysis; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009010:Monte Carlo Method; D011522:Protons; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D003287:Contrast Media; D011522:Protons; D014867:Water; D018130:Diamond