Diffusivity Measures Detect Changes in Cellular Organisation in the Cortex Associated with Elevated Glutamate in ASD: A 7T MRS and Ex Vivo DTI Study

Background:  

Minicolumns of cells are altered in cortical regions in ASD. Minicolumns have been found to be wider in primary and associative cortex. New MR imaging measurements derived from analysis of high resolution DTI of cerebral cortex show promise as an index of minicolumn alterations.

Objectives:  

The present study sought to confirm these DTI measures as potential biomarkers for ASD by investigating: (i) the correspondence between histology and DTI in selected cortical regions and in the arcuate fasciculus that connects them; (ii) their diagnostic sensitivity for detection of ASD in vivo.

Methods:  

2 experiments were conducted to confirm the utility of these biomarkers. The newly developed ‘CHIPS’ (Cortical High-Intensity Profile Segmentation) software was used to calculate several unique measures by comparing diffusivity data with the predicted minicolumn vertical axis derived from structural MRI of cortical grey matter.

i) Post-mortem MRI was conducted to validate the DTI signal as an index of minicolumn alteration by comparison of histological measurements from the identical brain regions. The 3T MRI acquisition protocol on formalin-fixed tissue included:  Modified spin-echo sequence with 3D segmented-EPI, and Structural 3D balanced steady state free precession (BSSFP) sequence.  Data processing used the FMRIB software library (FSL). ROIs were Brodmann areas 9, 40, 41, 42 . Fiber tracts were estimated via Bayesian Estimation of Diffusion Parameters (Bedpostx). The arcuate fasciculus was obtained and thresholded. The mean FA, MD, Drad and Dax were extracted from the tracts.

ii) In vivo 7T DTI and magnetic resonance spectroscopy (MRS) were conducted to investigate the relationship of the DTI signal to neurochemical markers of elevated excitation in ASD. A sample of 11 ASD and 10 TD individuals were MRI scanned and MRS was used to measure brain metabolites in primary visual and medial prefrontal cortex. Data were collected with the semi-LASER sequence and the LCModel. A VBM analysis was also performed on these structural scans.

Results:  

i) The post-mortem DTI confirmed the model’s predictions indicating a clear correlation between the mean diffusivity and minicolumn width and significant differences in the components of the principal diffusion direction between TD and ASD in brain regions with wider minicolumns.

ii) The high resolution in vivo DTI and MRS found significantly elevated glutamate in ASD and correlations between increased prefrontal glutamate and a diffusion marker of wider minicolumns.

The inferior frontal gyrus also had significantly reduced grey matter density in ASD compared to TD individuals. The relationship between glutamate and the diffusion signal in this region was different from other brain regions and contrasted with TD controls.

Conclusions:  

Diffusivity parameters in the cerebral cortex correspond to variation in the minicolumnar organisation of neurons. Minicolumns are more widely spaced in several cortical regions in ASD and this can be detected using novel diffusivity measures.

There is a link between elevated excitation (glutamate level) and these microstructural changes in ASD, consistent with the excitation/inhibition imbalance hypothesis. Greater excitation appears to be associated with wider minicolumns in ASD which may have a toxic effect causing reduced grey matter in some regions.