Abstract
Magnetic resonance spectroscopy (MRS) is a research tool for measuring the concentration of metabolites such as γ‐aminobutyric acid (GABA) and glutamate in the brain. MEGA‐PRESS has been the preferred pulse sequence for GABA measurements due to low physiological GABA concentrations, hence low signal. To compensate, researchers incorporate long acquisition durations (7–10 min) making functional measurements of this metabolite challenging. Here, the acquisition duration and sample sizes required to detect specific concentration changes in GABA using MEGA‐PRESS at 3 T are presented for both between‐groups and within‐session study designs. 75 spectra were acquired during rest using MEGA‐PRESS from 41 healthy volunteers in 6 different brain regions at 3 T with voxel sizes between 13 and 22 cm3. Between‐group and within‐session variance was calculated for different acquisition durations and power calculations were performed to determine the number of subjects required to detect a given percentage change in GABA/NAA signal ratio. Within‐subject variability was assessed by sampling different segments of a single acquisition. Power calculations suggest that detecting a 15% change in GABA using a 2 min acquisition and a 27 cm3 voxel size, depending on the region, requires between 8 and 93 subjects using a within‐session design. A between‐group design typically requires more participants to detect the same difference. In brain regions with suboptimal shimming, the subject numbers can be up to 4‐fold more. Collecting data for longer than 4 min in brain regions examined in this study is deemed unnecessary, as variance in the signal did not reduce further for longer durations.
Highlights
Magnetic resonance spectroscopy is a valuable tool used to study brain metabolism in vivo in clinical and healthy human populations
We examine glutamate and γ-aminobutyric acid (GABA)-edited data collected from 6 different brain regions using MEGA-PRESS at 3 T to test whether longer acquisition durations are warranted to increase the signal-to-noise ratio (SNR) and so improve precision
83 GABA-edited spectra were acquired during rest using MEGA-PRESS from 41 healthy volunteers (20 female, 21 male, age range 20–55 years old, mean age 29.4 ± 9.2) who all gave written informed consent in accordance with procedures approved by University of Manchester Ethics Committee (Refs 04/Q1405/66, 12049, 14/NW/0298, 08/H1004/93)
Summary
Magnetic resonance spectroscopy is a valuable tool used to study brain metabolism in vivo in clinical and healthy human populations. There have been several studies measuring changes in the concentration of metabolites as a function of time, either in response to neural activation (Ip et al, 2017; Lin et al, 2012), or immediately pre- and post-intervention (Antonenko et al, 2017; Bachtiar, Near, Near, Johansen-Berg, & Stagg, 2015), which for the purpose of this paper we term functional magnetic resonance spectroscopy (fMRS) Progress in this field is hampered by the long acquisition durations required to achieve reasonable spectrum signal-to-noise ratio (SNR). The within-session subject numbers will inform the design of fMRS studies and the between-group analyses will be useful for more general MRS research
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