Abstract
The MEGA-PRESS method is the most common method used to measure γ-aminobutyric acid (GABA) in the brain at 3T. It has been shown that the underestimation of the GABA signal due to B0 drift up to 1.22 Hz/min can be reduced by post-frequency alignment. In this study, we show that the underestimation of GABA can still occur even with post frequency alignment when the B0 drift is up to 3.93 Hz/min. The underestimation can be reduced by applying a frequency shift threshold. A total of 23 subjects were scanned twice to assess the short-term reproducibility, and 14 of them were scanned again after 2–8 weeks to evaluate the long-term reproducibility. A linear regression analysis of the quantified GABA versus the frequency shift showed a negative correlation (P < 0.01). Underestimation of the GABA signal was found. When a frequency shift threshold of 0.125 ppm (15.5 Hz or 1.79 Hz/min) was applied, the linear regression showed no statistically significant difference (P > 0.05). Therefore, a frequency shift threshold at 0.125 ppm (15.5 Hz) can be used to reduce underestimation during GABA quantification. For data with a B0 drift up to 3.93 Hz/min, the coefficients of variance of short-term and long-term reproducibility for the GABA quantification were less than 10% when the frequency threshold was applied.
Highlights
Due to the low concentration of GABA, the voxel size of MEGA-point resolved spectroscopy (PRESS) is usually set as large as possible to cover the region of interest, with the number of measurements usually over 256 to ensure that the signal-to-noise ratio (SNR) is in the editing spectrum
When the B0 drift is above this level, it is possible that an underestimation of the quantified GABA signal can occur even with post frequency alignment
The typical shapes of the N-Acetyl Aspartate (NAA), Cr, and choline spectra can be clearly identified on the edit-off spectrum and the saturated NAA peak can be found on the edit-on spectrum
Summary
Due to the low concentration of GABA, the voxel size of MEGA-PRESS is usually set as large as possible to cover the region of interest, with the number of measurements usually over 256 to ensure that the signal-to-noise ratio (SNR) is in the editing spectrum. B0 field drift may occur during successive scans, especially after scans involving the heavy use of gradients, such as EPI19 This can lead to errors in the quantification of the GABA signal. Even more than 30 min after fMRI scanning, a B0 drift of − 0.5 Hz/min was found In these previous studies, the underestimation of the GABA+ signal due to B0 drift could be reduced by post frequency alignment. When the B0 drift is above this level, it is possible that an underestimation of the quantified GABA signal can occur even with post frequency alignment. The underestimation of GABA still occurred even when subtraction artifacts were minimized by the frequency alignment. The level of underestimation in the GABA+ quantification was evaluated by a linear regression analysis of the quantified GABA+ versus the frequency shift. The short-term and long-term reproducibility of GABA+ quantification using the optimized frequency shift threshold were determined
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