Optimized multi-voxel TE-averaged PRESS for glutamate detection in the human brain at 3T

The Goldilocks Zone for 3-T MRS Studies Using Semi-LASER: Determining the Optimal Balance Between Repetition Time and Scan Time

1H-MR spectroscopy studies often use a short TR to reduce scan time. However, this causes significant T1-weighting (T1w), which can alter metabolite estimates due to acquisition factors rather than biochemistry. Our goal was to determine the optimal balance between scan time and TR that minimizes T1w effects in semi-LASER MRS at 3 T. Spectra were acquired in the posterior cingulate cortex of five healthy volunteers (2 male/3 female, mean age 25 ± 2 years) and analyzed using FSL-MRS. The SNR and metabolite estimates of five metabolites were compared at TR = 2, 5, and 8 s, under conditions of "similar scan time" with varying acquisition numbers and "constant number of acquisitions" with varying scan times. T1 relaxation times derived from metabolite estimates were compared to literature. With a 25% longer scan time, SNRTR = 5s was 34% higher and SNRTR = 2s was 29% higher than SNRTR = 8s for data with "similar scan times." Using a TR = 5 s or longer, the SNR per minute is consistent for metabolites with T1s less than 2 s. Metabolite estimate trends were similar for the two different scenarios of "similar scan time" and "same number of acquisitions," where all metabolite estimates were obtained without metabolite T1 correction. The largest metabolite estimates were found at TR = 8 s, they were 10-15% lower at TR = 5 s and 15-30% lower at TR = 2 s. T1 values agreed with literature values. At TR = 2 s, SNR per minute and metabolite estimates were lower due to reduced signal availability via T1w effects. TR = 8 s had the least amount of T1w effects, but results in lower SNR per minute. TR = 5 s had enough signal recovery to be robust to T1w effects, and yielded the largest SNR for similar scan times, with a clinically feasible scan time of 5 m 40 s. Using semi-LASER MRS with a TR = 5 s is recommended to improve the sensitivity of MRS to changes in metabolite estimates.

Neurometabolite differences in Autism as assessed with Magnetic Resonance Spectroscopy: A systematic review and meta-analysis

1H-Magnetic Resonance Spectroscopy (MRS) is a non-invasive technique that can be used to quantify the concentrations of metabolites in the brain in vivo. MRS findings in the context of autism are inconsistent and conflicting. We performed a systematic review and meta-analysis of MRS studies measuring glutamate and gamma-aminobutyric acid (GABA), as well as brain metabolites involved in energy metabolism (glutamine, creatine), neural and glial integrity (e.g. n-acetyl aspartate (NAA), choline, myo-inositol) and oxidative stress (glutathione) in autism cohorts. Data were extracted and grouped by metabolite, brain region and several other factors before calculation of standardised effect sizes. Overall, we find significantly lower concentrations of GABA and NAA in autism, indicative of disruptions to the balance between excitation/inhibition within brain circuits, as well as neural integrity. Further analysis found these alterations are most pronounced in autistic children and in limbic brain regions relevant to autism phenotypes. Additionally, we show how study outcome varies due to demographic and methodological factors , emphasising the importance of conforming with standardised consensus study designs and transparent reporting.