Endogenous chemical exchange saturation transfer (CEST) effects of
protons resonating near to water
protons are always diluted by competing effects such as direct water saturation and semi-solid magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that contaminate the observed CEST signal. Furthermore, all CEST effects appear to be scaled by the T1 relaxation time of the mediating water pool. As MT, T1 and T2 are also altered in
tumor regions, a recently published correction algorithm yielding the apparent exchange-dependent relaxation AREX, is used to evaluate in vivo CEST effects. This study focuses on CEST effects of
amides (3.5ppm) and Nuclear-Overhauser-mediated saturation transfer (NOE, -3.5ppm) that can be properly isolated at 7T. These were obtained in 10
glioblastoma patients, and this is the first comprehensive study where AREX is applied in human brain as well as in human
glioblastoma. The correction of CEST effects alters the contrast significantly: after correction, the CEST effect of
amides does not show significant contrast between contrast enhancing
tumor regions and normal tissue, whereas NOE drops significantly in the
tumor area. In addition, new features in the AREX contrasts are visible. This suggests that previous CEST approaches might not have shown pure CEST effects, but rather water relaxation shine-through effects. Our insights help to improve understanding of the CEST effect changes in
tumors and correlations on a cellular and molecular level.