Evidence for biexponential glutamate T₂ relaxation in human visual cortex at 3T: A functional MRS study

Area of Science:

• Neuroimaging
• Biophysics
• Metabolomics

Background:

• Functional magnetic resonance spectroscopy (fMRS) measures metabolite concentration changes during neural activity.
• The biophysical basis for observed metabolite changes, particularly glutamate, is not fully understood.
• A leading hypothesis involves glutamate movement between cellular compartments with differing T2 relaxation times.

Purpose of the Study:

• To investigate the presence of short T2 components for glutamate (Glu), total creatine (tCr), and total N-acetylaspartate (tNAA).
• To examine if neural stimulation induces task-related changes in the distribution of these metabolites between cellular compartments.
• To assess the biophysical basis of fMRS signals by analyzing metabolite T2 relaxation properties.

Main Methods:

• Acquired MRS data from the visual cortex of 14 healthy participants using a range of echo times (9.3-280 ms).
• Collected data during rest and short blocks of visual stimulation (flickering checkerboards).
• Fitted monoexponential and biexponential T2 relaxation curves for Glu, tCr, and tNAA, using Akaike information criterion for model selection.

Main Results:

• Identified a significant short T2-component for Glu (10 ms, 0.35 volume fraction), tCr (26 ms, 0.25 volume fraction), and tNAA (15 ms, 0.34 volume fraction) at rest.
• No statistically significant changes in metabolite signals or short-T2 component volume fractions were detected during neural stimulation across all echo times.
• Power calculations indicated the study was adequately powered to detect a 2% shift in Glu between compartments.

Conclusions:

• This study provides robust evidence for the existence of short T2-components for glutamate, total creatine, and total N-acetylaspartate.
• The findings do not support the hypothesis that task-induced changes in fMRS signals are driven by glutamate redistribution between cellular compartments with distinct T2 relaxation times.
• The results highlight the importance of considering short T2 components in the interpretation of fMRS data.