GABA, glutamatergic dynamics and BOLD contrast concurrently assessed using functional MR spectroscopy during a cognitive task
Authors: Craven Alexander R. , Dwyer Gerard , Ersland Lars , Kazimierczak Katarzyna , Noeske Ralph , Sandøy Lydia Brunvoll , Johnsen Erik , Hugdahl Kenneth 
Published: May 2023

A recurring issue in functional neuroimaging is how to link task-driven hemodynamic BOLD-fMRI responses to underlying neurochemistry at the synaptic level. Glutamate and GABA, the major excitatory and inhibitory neurotransmitters respectively, are typically measured with MR spectroscopy (MRS) sequences in a resting state, in the absence of a task. We propose to resolve this disconnect by applying a novel method to concurrently acquire BOLD, Glx and GABA measurements from a single voxel, using a locally adapted MEGA-PRESS sequence implementation which incorporates unsuppressed water reference signals at a regular interval, allowing continuous assessment of BOLD-related linewidth variation for fMRS applications.

Healthy subjects (N = 81) performed a cognitive task (Eriksen Flanker) which was presented visually in a task-OFF, task-ON block design, with individual event stimulus timing varied with respect to the MRS readout. BOLD data acquired with the adapted MEGA-PRESS sequence were correlated with data acquired using a standard fMRI EPI sequence as a means of validating the concurrent approach: a significant (although moderate) correlation was observed, specific to the fMRS-targeted region of interest. We additionally present a novel linear model for extracting modelled spectra associated with discrete functional stimuli, building on well-established processing and quantification tools. Behavioural outcomes from the Flanker task, and activation patterns from the BOLD-fMRI sequence, were as expected from the literature. fMRS-assessed BOLD response correlated strongly with slowing of response time in the incongruent Flanker condition. Moreover, there was a significant increase in measured Glx levels (~8.8%), between task-OFF and task-ON periods. These findings verify the efficacy of our functional task and analysis pipelines for the simultaneous assessment of BOLD and metabolite fluctuations in a single voxel. As well as providing a robust basis for further work using these techniques, we also identify a number of clear directions for further refinement in future studies.

Highlights:

Concurrent measurement of temporally resolved metabolite estimates and local BOLD-related signal changes is demonstrated

In-vivo, GABA-edited functional1H-MRS data were collected from 81 healthy subjects whilst they performed a cognitive task

Robust task-related increases in measured Glutamate+Glutamine were observed in the Anterior Cingulate Cortex

Moderate correlation was observed between BOLD contrast strength as assessed by fMRS and fMRI techniques, specific to the prescribed region

A novel technique for extraction of block- or event-related sub-spectra is demonstrated

Graphical Abstract:

A novel sequence adaption for concurrent measurement of GABA-edited spectroscopy and functional BOLD changes is demonstrated on N=81 healthy subjects. Significant changes in measured Glutamate+Glutamine concentration are found, along with the expected behavioural and BOLD effects in response to a Flanker task.