T₂ contrast variation in human brain at 7 T and its potential contributors

Area of Science:

• Neuroimaging
• Magnetic Resonance Imaging (MRI)
• Biophysics

Background:

• High-field MRI (7T and above) excels at detecting brain iron and myelin using susceptibility-weighted imaging (T2*-weighted MRI).
• Macroscopic field perturbations in T2*-weighted MRI cause artifacts, complicating the interpretation of tissue iron and myelin content.
• T2-based contrast offers potential artifact reduction but faces technical challenges at high fields due to RF refocusing difficulties and power deposition limits.

Purpose of the Study:

• To evaluate the Gradient Echo Sampling of Spin Echo (GESSE) method for measuring R2 (=1/T2) variations in the healthy human brain at 7T.
• To assess the sensitivity of R2 contrast to tissue iron and myelin across different brain regions while minimizing susceptibility artifacts.
• To investigate R2 contrast contributions, including fiber bundle specificity and orientation dependence, in deep white matter.

Main Methods:

• Employed the Gradient Echo Sampling of Spin Echo (GESSE) technique at 7 Tesla magnetic field strength.
• Acquired R2 (1/T2) relaxation rate maps in healthy human brain subjects.
• Analyzed R2 variations across subcortical and cortical structures, white matter, and investigated correlations with microstructural properties.

Main Results:

• R2 measured by GESSE demonstrated preserved sensitivity to tissue iron and myelin contrast in cortical and subcortical areas, with reduced susceptibility artifacts compared to T2*-weighted methods.
• R2 contrast in deep white matter showed specificity to fiber bundles, correlating significantly with fiber diameter and orientation relative to the main magnetic field (B0).
• The study comprehensively mapped R2 contrast contributors across the entire brain at 7T, extending prior localized or lower-field investigations.

Conclusions:

• R2 mapping using GESSE at 7T is a promising method for quantifying brain tissue iron and myelin, complementing existing R2*- and susceptibility (χ)-based techniques.
• The findings highlight the importance of considering various contributors, including microstructural properties, for accurate quantitative interpretation of R2 contrast.
• GESSE-derived R2 offers a valuable tool for neuroimaging research and clinical applications requiring precise assessment of brain tissue composition.