Temperature dependence of water diffusion pools in brain white matter

Area of Science:

• Neuroscience
• Biophysics
• Magnetic Resonance Imaging

Background:

• Water diffusion in brain tissue provides insights into cellular microstructure, like axonal orientation.
• Diffusion in white matter is non-Gaussian, indicating multiple water compartments.
• Magnetic resonance (MR) techniques enable detailed investigation of water diffusion.

Purpose of the Study:

• To test the hypothesis that white matter contains two slowly exchanging water pools with different diffusion properties.
• To characterize water diffusion in porcine brain white matter across a range of temperatures.
• To investigate the microstructure of brain tissue using advanced MR methods.

Main Methods:

• Utilized a 125-MHz MR spectrometer (3T) with high magnetic field gradients (up to 35,000 mT/m).
• Employed a Stejskal-Tanner diffusion sequence with short gradient pulses and high b-factors (up to 18.8 ms/μm²).
• Measured water diffusion in excised porcine white matter samples at temperatures from -14 to +19 °C.

Main Results:

• Diffusion attenuation dependence on b-factor was fitted by a biexponential function, indicating two distinct water pools.
• Comparable volume fractions were observed for each water diffusion component.
• Diffusion coefficients followed Arrhenius behavior with significantly different activation energies for each pool.

Conclusions:

• The findings support a model of three water-containing compartments in white matter: cytoplasmic/extracellular, glial, and membrane-associated water.
• The activation energy suggests a Grotthuss mechanism for water diffusion at membrane surfaces.
• MR diffusion measurements offer a powerful tool for understanding brain tissue microstructure and water dynamics.