Optimized diffusion-weighted spectroscopy for measuring brain glutamate apparent diffusion coefficient on a...

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging (MRI)
• Metabolomics

Background:

• Diffusion-weighted imaging (DWI) is vital for understanding brain microstructure.
• Measuring specific metabolite diffusion, like glutamate, presents technical challenges.
• Accurate quantification of apparent diffusion coefficients (ADCs) is essential for in vivo neuroscience research.

Purpose of the Study:

• To implement and validate a diffusion-weighted stimulated echo acquisition mode (DW-STEAM) sequence for measuring glutamate ADC in the macaque monkey brain.
• To optimize sequence parameters (TE, TM) for maximal glutamate signal intensity.
• To assess the impact of post-processing techniques on ADC quantification.

Main Methods:

• Utilized a whole-body 3 Tesla (T) MRI system with a DW-STEAM sequence.
• Adjusted echo time (TE) and mixing time (TM) to enhance glutamate signal.
• Acquired data from 5.8 mL voxels containing gray and white matter in macaque monkeys.
• Evaluated the critical role of post-processing, including individual scan phasing and macromolecule subtraction.

Main Results:

• Optimized DW-STEAM sequence yielded measurable ADC values for brain metabolites.
• Individual scan phasing corrected ~25% bias, and macromolecule subtraction corrected ~15% bias in glutamate ADC.
• Obtained ADC values: glutamate (0.21 ± 0.03 µm²/ms), NAA+NAAG (0.15 ± 0.04 µm²/ms), creatine (0.12 ± 0.03 µm²/ms), choline (0.11 ± 0.05 µm²/ms), and myo-inositol (0.18 ± 0.04 µm²/ms).

Conclusions:

• The implemented DW-STEAM sequence is effective for in vivo measurement of glutamate ADC in the primate brain.
• Post-processing steps are critical for accurate ADC quantification, significantly reducing systematic errors.
• The study provides reliable ADC values for key brain metabolites, valuable for future neuroscience and clinical applications.