Inferring diameters of spheres and cylinders using interstitial water

Area of Science:

• Neuroimaging
• Biophysics
• Diffusion MRI

Background:

• Early magnetic resonance imaging (MRI) methods for axon diameter distribution inference were limited to larger axons (> 5 μm).
• Oscillating gradient (OG) sequences offer potential for studying smaller axons, crucial for cortical connections.
• Previous simulations suggested a breakdown of constant extra-axonal diffusion models at OG frequencies.

Purpose of the Study:

• To experimentally validate the use of OG sequences for inferring axon diameter distributions.
• To test a time-varying interstitial apparent diffusion coefficient model.
• To assess the accuracy of OG diffusion spectroscopy in phantoms.

Main Methods:

• Diffusion spectra were acquired using OG sequences in phantoms containing water-filled beads (3, 6, 10 μm) and tubes (151 μm).
• Bead diameters and tube surface-to-volume ratios were measured.
• Interstitial models were applied to infer structural parameters.

Main Results:

• Inferred bead pore radii ranged from 0.54±0.06 μm to 1.0±0.1 μm, corresponding to bead diameters of 2.6±0.3 μm to 9±1 μm.
• The estimated surface-to-volume ratio for the tubes was 0.06±0.02 μm^-1, indicating a diameter of 180±70 μm.
• The OG diffusion spectroscopy accurately estimated the dimensions of the phantoms.

Conclusions:

• Interstitial models utilizing OG successfully inferred bead diameters (3-10 μm) and tube diameters (151 μm).
• The findings support the utility of OG diffusion spectroscopy for characterizing small axonal structures.
• This technique holds promise for advancing the study of brain microstructure and connectivity.