γ-aminobutyric acid measurement in the human brain at 7 T: Short echo-time or Mescher–Garwood editing
AUTHORS: Lim SI, Xin L
NMR in Biomedicine, 2022(35): e4706, January 2022
The purposes of the current study were to introduce a Mescher–Garwood (MEGA)semi-adiabatic spin-echo full-intensity localization (MEGA-sSPECIAL) sequence withmacromolecule (MM) subtraction and to compare the test–retest reproducibility ofγ-aminobutyric acid (GABA) measurements at 7 T using the sSPECIAL and MEGA-sSPECIAL sequences. The MEGA-sSPECIAL editing scheme using asymmetric adia-batic and highly selective Gaussian pulses was used to compare its GABA measure-ment reproducibility with that of short echo-time (TE) sSPECIAL. Proton magneticresonance spectra were acquired in the motor cortex (M1) and medial prefrontal cor-tex (mPFC) using the sSPECIAL (TR/TE=4000/16 ms) and MEGA-sSPECIALsequences (TR/TE=4000/80 ms). The metabolites were quantified using LCModelwith unsuppressed water spectra. The concentrations are reported in institutionalunits. The test–retest reproducibility was evaluated by scanning each subject twice.Between-session reproducibility was assessed using coefficients of variation (CVs),Pearson’srcorrelation coefficients, and intraclass correlation coefficients (ICCs).Intersequence agreement was evaluated using Pearson’srcorrelation coefficientsand Bland–Altman plots. Regarding GABA measurements by sSPECIAL, the GABAconcentrations were 0.92 ± 0.31 (IU) in the M1 and 1.56 ± 0.49 (IU) in the mPFC.This demonstrated strong between-session correlation across both regions (r=0.81,p< 0.01; ICC=0.82). The CVs between the two scans were 21.8% in the M1 and10.2% in the mPFC. On the other hand, the GABA measurements by MEGA-sSPECIAL were 0.52 ± 0.04 (IU) in the M1 and 1.04 ± 0.24 (IU) in the mPFC. MEGA-sSPECIAL demonstrated strong between-session correlation across the two regions(r=0.98,p< 0.001; ICC=0.98) and lower CVs than sSPECIAL, providing 4.1% inthe M1 and 5.8% in the mPFC. The MEGA-editing method showed better reproduc-ibility of GABA measurements in both brain regions compared with the short-TEsSPECIAL method. Thus it is a more sensitive method with which to detect small changes in areas with low GABA concentrations. In GABA-rich brain regions, GABAmeasurements can be achieved reproducibly using both methods.