Cross-scanner and cross-protocol diffusion MRI data harmonisation: A benchmark database and evaluation of algorithms.

Diffusion MRI is being used increasingly in studies of the brain and other parts of the body for its ability to provide quantitative measures that are sensitive to changes in tissue microstructure. However, inter-scanner and inter-protocol differences are known to induce significant measurement variability, which in turn jeopardises the ability to obtain 'truly quantitative measures' and challenges the reliable combination of different datasets. Combining datasets from different scanners and/or acquired at different time points could dramatically increase the statistical power of clinical studies, and facilitate multi-centre research. Even though careful harmonisation of acquisition parameters can reduce variability, inter-protocol differences become almost inevitable with improvements in hardware and sequence design over time, even within a site. In this work, we present a benchmark diffusion MRI database of the same subjects acquired on three distinct scanners with different maximum gradient strength (40, 80, and 300 mT/m), and with 'standard' and 'state-of-the-art' protocols, where the latter have higher spatial and angular resolution. The dataset serves as a useful testbed for method development in cross-scanner/cross-protocol diffusion MRI harmonisation and quality enhancement. Using the database, we compare the performance of five different methods for estimating mappings between the scanners and protocols. The results show that cross-scanner harmonisation of single-shell diffusion data sets can reduce the variability between scanners, and highlight the promises and shortcomings of today's data harmonisation techniques.

Structural and neurochemical correlates of individual differences in gamma frequency oscillations in human visual cortex.

Neuronal oscillations in the gamma frequency range play an important role in stimulus processing in the brain. The frequency of these oscillations can vary widely between participants and is strongly genetically determined, but the cause of this variability is not understood. Previous studies have reported correlations between individual differences in gamma frequency and the concentration of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), as well as with age and primary visual cortex (V1) area and thickness. This study assessed the relationships between all of these variables in the same group of participants. There were no significant correlations between gamma frequency and GABA+ concentration, V1 area or V1 thickness, although the relationship with GABA+/Cr approached significance. Considering age as a covariate further reduced the strength of all correlations and, in an additional dataset with a larger age range, gamma frequency was strongly inversely correlated with age but not V1 thickness or area, suggesting that age modulates gamma frequency via an additional, as yet unknown, mechanism. Consistent with other recent studies, these findings do not demonstrate a clear relationship between gamma frequency and GABA+ concentration. Further investigation of additional variables and the interactions between them will be necessary in order to more accurately determine predictors of the frequency of gamma oscillations.

Impact of frequency drift on gamma-aminobutyric acid-edited MR spectroscopy.

PURPOSE: To investigate the quantitative impact of frequency drift on Gamma-Aminobutyric acid (GABA+)-edited MRS of the human brain at 3 Tesla (T). METHODS: Three sequential GABA+-edited MEGA-PRESS acquisitions were acquired in fifteen sessions; in ten of these, MRS was preceded by functional MRI (fMRI) to induce frequency drift, which was estimated from the creatine resonance at 3.0 ppm. Simulations were performed to examine the effects of frequency drift on the editing efficiency of GABA and co-edited macromolecules (MM) and of subtraction artifacts on GABA+ quantification. The efficacy of postprocessing frequency correction was also investigated. RESULTS: Gradient-induced frequency drifts affect GABA+ quantification for at least 30 min after imaging. Average frequency drift was low in control sessions and as high as -2 Hz/min after fMRI. Uncorrected frequency drift has an approximately linear effect on GABA+ measurements with a -10 Hz drift resulting in a 16% decrease in GABA+, primarily due to subtraction artifacts. CONCLUSION: Imaging acquisitions with high gradient duty cycles can impact subsequent GABA+ measurements. Postprocessing can address subtraction artifacts, but not changes in editing efficiency or GABA:MM signal ratios; therefore, protocol design should avoid intensive gradient sequences before edited MRS Magn Reson Med 72:941-948, 2014. © 2013 Wiley Periodicals, Inc.

Marked reductions in visual evoked responses but not γ-aminobutyric acid concentrations or γ-band measures in remitted depression.

BACKGROUND: Magnetic resonance spectroscopy (MRS) studies have consistently demonstrated reduced cortical γ-aminobutyric acid (GABA) concentrations in individuals with major depression. However, evidence for a persistent deficit during remission, which would suggest that GABA dysfunction is a possible trait marker of depression, is equivocal. Although MRS measures total concentration of GABA, magneto-encephalography provides direct measures of neural activity, with cortical γ oscillations shaped by the activity of GABAergic inhibitory interneurons. In this study we investigated whether γ oscillations and GABA concentrations would differ in individuals with remitted depression (RD) compared with never depressed control subjects (ND). METHODS: Thirty-seven healthy, unmedicated female volunteers (n = 19 RD, and n = 18 ND) were recruited. The γ oscillation frequencies and amplitudes in the visual cortex, induced by simple grating stimuli, were quantified with time-frequency analyses. Distinct GABA/glutamate + glutamine MRS peaks were resolved from MEGA-PRESS difference spectra in prefrontal, occipital, and subcortical volumes. RESULTS: The RD and ND individuals did not differ in the frequency of subclinical depressive symptoms. The ND were slightly older (mean = 23 years vs. 21 years), but age did not correlate with dependent measures. There were no group differences in GABA levels or induced cortical γ measures, but RD individuals had markedly reduced M80 (C1) components of the pattern-onset evoked response (46% reduction, Cohen's d = 1.01, p = .006). CONCLUSIONS: Both MRS and magneto-encephalography measures of the GABA system are normal in RD. However, the early visual evoked response is a potential trait marker of the disorder.