Simultaneous Detection of GABA and Glycine Using MEGA-PRESS With TE Optimization at 3T.

Area of Science:

• Neuroscience
• Magnetic Resonance Imaging (MRI)
• Metabolomics

Background:

• Simultaneous quantification of gamma-aminobutyric acid (GABA) and glycine (Gly) is crucial for understanding brain function and neurological disorders.
• The MEGA-PRESS sequence is a common tool for GABA detection, but optimizing it for simultaneous Gly detection requires careful consideration of echo time (TE).
• Previous studies have used varying TEs, potentially impacting the accuracy and reliability of metabolite quantification.

Purpose of the Study:

• To determine the optimal echo time (TE) for the MEGA-PRESS sequence to simultaneously acquire GABA and Gly signals at 3 Tesla (3T).
• To evaluate the impact of the optimized TE on the sensitivity and reliability of GABA and Gly quantification.
• To assess the influence of Gly on the accuracy of other metabolite measurements, such as myo-inositol (mI) and glucose.

Main Methods:

• MEGA-PRESS simulations were conducted across a range of TEs (60-88 ms) to identify the optimal TE for Gly detection.
• In vivo MEGA-PRESS data were acquired from the occipital lobe of 6 healthy subjects at TEs of 64 ms and 68 ms.
• Quantification of GABA+, Gly, mI, and glucose was performed, and fit reliability was assessed using coefficient of variation (CV) and Cramer-Rao Lower Bounds (CRLB). Spectral fits were analyzed with and without Gly in the basis set.

Main Results:

• Simulations identified TE = 64 ms as optimal for Gly detection with minimal overlap from mI and maximal GABA sensitivity.
• In vivo experiments at TE = 64 ms showed no reduction in GABA+ sensitivity compared to the commonly used TE = 68 ms.
• At TE = 64 ms, Gly quantification exhibited significantly lower between-acquisition and inter-subject variability, along with improved CRLBs, compared to TE = 68 ms. Excluding Gly from the basis set at TE = 64 ms significantly increased CRLBs and residuals for mI and glucose.

Conclusions:

• A MEGA-PRESS sequence TE of 64 ms enables the simultaneous and reliable quantification of GABA+ and Gly at 3T.
• This optimized TE improves the accuracy of metabolite quantification and reduces variability compared to longer TEs.
• The inclusion of Gly in the spectral fitting process at TE = 64 ms is essential for accurate quantification of other metabolites like mI and glucose.