Repeatability of 31P MRSI in the human brain at 7 T with and without the nuclear Overhauser effect

• Published in: NMR in biomedicine Vol. 29; no. 3; pp. 256 - 263
• Main Authors: Lagemaat, Miriam W., van de Bank, Bart L., Sati, Pascal, Li, Shizhe, Maas, Marnix C., Scheenen, Tom W. J.
• Format: Journal Article
• Language: English
• Published: Blackwell Publishing Ltd 01.03.2016
• Subjects: 7 T; MRS; repeatability; reproducibility; ultrahigh field
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.3455

An often‐employed strategy to enhance signals in 31P MRS is the generation of the nuclear Overhauser effect (NOE) by saturation of the water resonance. However, NOE allegedly increases the variability of the 31P data, because variation is reported in NOE enhancements. This would negate the signal‐to‐noise (SNR) gain it generates. We hypothesized that the variation in NOE enhancement values is not caused by the variability in NOE itself, but is attributable to measurement uncertainties in the values used to calculate the enhancement. If true, the expected increase in SNR with NOE would improve the repeatability of 31P MRS measurements. To verify this hypothesis, a repeatability study of native and NOE‐enhanced 31P MRSI was performed in the brains of seven healthy volunteers at 7 T. The repeatability coefficient (RC) and the coefficient of variation in repeated measurements (CoVrepeat) were determined for each method, and the 95% limits of agreement (LoAs) between native and NOE‐enhanced signals were calculated. The variation between the methods, defined by the LoA, is at least as great as that predicted by the RC of each method. The sources of variation in NOE enhancements were determined using variance component analysis. In the seven metabolites with a positive NOE enhancement (nine metabolite resonances assessed), CoVrepeat improved, on average, by 15%. The LoAs could be explained by the RCs of the individual methods for the majority of the metabolites, generally confirming our hypothesis. Variation in NOE enhancement was mainly attributable to the factor repeat, but between‐voxel effects were also present for phosphoethanolamine and (glycero)phosphocholine. CoVrepeat and fitting error were strongly correlated and improved with positive NOE. Our findings generally indicate that NOE enhances the signal of metabolites, improving the repeatability of metabolite measurements. Additional variability as a result of NOE was minimal. These findings encourage the use of NOE‐enhanced 31P MRSI. Copyright © 2015 John Wiley & Sons, Ltd. A repeatability study of native and nuclear Overhauser effect (NOE)‐enhanced 31P MRS brain imaging at 7 T was performed in healthy volunteers. The results showed that variations in NOE enhancements per metabolite could almost completely be explained by the degree of repeatability of native and NOE‐enhanced 31P MRSI. The signal enhancement generated by NOE improved the relative repeatability of brain 31P MRSI, which is a favorable effect for this low signal‐to‐noise technique.