Toward understanding transverse relaxation in human brain through its field dependence

Area of Science:

• Magnetic Resonance Imaging
• Neuroimaging
• Biophysics

Background:

• Transverse-relaxation rate (R₂⁺) is a crucial MRI parameter reflecting tissue properties.
• Understanding R₂⁺ dependence on magnetic field strength (B(0)) is vital for quantitative MRI.
• Brain iron concentration and macromolecular content are known to influence R₂⁺.

Purpose of the Study:

• To measure R₂⁺ in the healthy human brain across a range of magnetic fields (1.5-7 T).
• To investigate the relationship between R₂⁺, magnetic field strength, regional iron concentration, and macromolecular mass fraction.
• To develop a model explaining regional R₂⁺ variations based on tissue composition.

Main Methods:

• Utilized a multiecho adiabatic spin-echo sequence for R₂⁺ measurements.
• Acquired data at five magnetic field strengths: 1.5, 1.9, 3, 4.7, and 7 T.
• Employed multivariable linear regression to fit R₂⁺ to iron concentration, macromolecular fraction, and a constant.

Main Results:

• R₂⁺ demonstrated a clear dependence on magnetic field strength (B(0)).
• Regional R₂⁺ was accurately modeled by a linear combination of iron concentration ([Fe]), macromolecular fraction (f(M)), and a region-independent factor (γ).
• The coefficient for iron ([Fe]), α, increased linearly with B(0), while the coefficient for macromolecular fraction (f(M)), β, showed a quadratic dependence on B(0).

Conclusions:

• The proposed model R₂⁺ = α[Fe] + βf(M) + γ successfully explains regional R₂⁺ variations in the brain.
• The magnetic field dependence of the coefficients α and β provides insights into underlying relaxation mechanisms.
• Findings support the role of non-heme iron and macromolecules in modulating R₂⁺ across different field strengths in neuroimaging.