A new method for quantifying APT and NOE(-3.5) using chemical exchange saturation transfer with double saturation powers (DSP-CEST)

Abstract

PurposeQuantifications of amide proton transfer (APT) and nuclear Overhauser enhancement (NOE(-3.5)) mediated transfer with high specificity are challenging since their signals measured in a Z-spectrum are overlapped with confounding signals from direct water saturation (DS), semi-solid magnetization transfer (MT) and chemical exchange saturation transfer (CEST) of fast-exchange pools. In this study, based on two canonical CEST acquisitions with double saturation powers (DSP), a new data-postprocessing method is proposed to specifically quantify the effects of APT and NOE. MethodsFor CEST imaging with relatively low saturation powers [Formula], both the fast-exchange CEST effect and the semi-solid MT effect increase linearly with [Formula] whereas the slow-exchange APT/NOE(-3.5) effect has no such a dependence on [Formula], which is exploited to isolate the APT and NOE effects from the confounding signals in this study. After a mathematical derivation for the establishment of the proposed method, numerical simulations based on Bloch equations are then performed to demonstrate its specificity to detections of the APT and NOE effects. Finally, an in vivo validation of the proposed method is conducted using an animal tumor model at a 4.7-T MRI scanner. ResultsThe simulations show that DSP-CEST can quantify the effects of APT and NOE and substantially eliminate the confounding signals. The in vivo experiments demonstrate that the prosed DSP-CEST method is feasible for the imaging of tumors. ConclusionThe data-postprocessing method proposed in this study can quantify the APT and NOE effects with considerably increased specificities and a reduced cost of imaging time.