Observation of true and pseudo NOE signals using CEST-MRI and CEST-MRS sequences with and without lipid suppression

Purpose To investigate the characteristics of nuclear Overhauser enhancement (NOE) imaging signals in the brain at 7T. Methods Fresh hen eggs, as well as six healthy, and six C6 glioma-bearing Wistar rats were scanned using chemical exchange saturation transfer-magnetic resonance imaging (CEST-MRI) and chemical exchange saturation transfer-magnetic resonance spectroscopy (CEST-MRS) sequences (saturation duration 3 s, power 1.47 μT) with and without lipid suppression. CEST data were acquired over an offset range of -6 to +6 ppm relative to the water resonance in 0.5 ppm steps. Results The water signals were not disrupted by other protons during the CEST-MRS sequences, and true NOE signals could be observed. Using the CEST-MRI sequence without lipid suppression, pseudo NOE imaging signals were observed in the lipid-containing regions (egg yolk, scalp, and even white matter). These pseudo NOE signals were almost (but incompletely) removed with the lipid suppression. Egg yolk results indicated the presence of the NOE to olefinic protons overlapping with the water signal. In vivo experiments showed that the amide proton transfer signal was larger in the tumor, whereas the NOE signal was larger in the normal white matter. Conclusions True NOE signals can be detected using MRS sequences, and considerable pseudo NOE imaging signals may be observed using MRI sequences.