The microstructural correlates of T1 in white matter

Area of Science:

• Neuroimaging
• Biophysics
• Quantitative MRI

Background:

• Strong correlations exist between myelin content and T1 relaxation times in the brain.
• T1 is proposed as a method to estimate myelin content.
• Micro-anatomical features, like compartment size, can also affect longitudinal relaxation rates.

Purpose of the Study:

• To investigate the relationship between T1 relaxation rates and axon size in white matter.
• To determine if micro-anatomical features, independent of myelin, influence T1.

Main Methods:

• T1 measurements were acquired.
• Axon size measurements were obtained or sourced from published data.
• Data analyzed across white matter tracts in rat spinal cord, rat brain, and human brain (ex vivo and in vivo).

Main Results:

• Significant correlations were observed between the relaxation rate (1/T1) and axon size.
• These correlations were evident in rat spinal cord regions with consistent myelin content.
• Findings were consistent in both ex vivo and in vivo experimental conditions.

Conclusions:

• Myelination is a primary determinant of T1 in white matter.
• White matter microstructure variations, independent of myelin volume fraction, contribute to T1 differences.
• T1 measurements may reflect more than just myelin content, including axon size and other micro-structural properties.