Nonexponential T₂ decay in white matter.

Area of Science:

• Neuroimaging
• Biophysics
• Magnetic Resonance Imaging

Background:

• Visualizing myelin in the human brain is crucial for studying neurological diseases such as multiple sclerosis.
• Previous MRI studies utilized T(1) and T(2) relaxation contrast to detect water pools indicative of myelin content.
• T(2) contrast shows promise for myelin imaging, especially at high magnetic fields, but interpretation is complex.

Purpose of the Study:

• To investigate T(2) relaxation behavior in the human brain at 3 and 7 Tesla.
• To analyze deviations from mono-exponential decay and understand the underlying mechanisms of T(2) contrast for myelin visualization.

Main Methods:

• Studied T(2) relaxation in the human brain at 3 T and 7 T.
• Acquired multiple gradient echoes to capture the full decay curve.
• Analyzed data for deviations from mono-exponential behavior and assessed frequency shifts.

Main Results:

• Confirmed a distinct, rapidly relaxing signal component (T(2) ~ 6 ms), attributed to myelin water.
• Observed significant resonance frequency shifts in this component, up to 36 Hz at 7 T in the corpus callosum.
• Found that the myelin water signal's amplitude and frequency shift depend on magnetic field strength and tissue fiber orientation.

Conclusions:

• The findings support the hypothesis of a myelin water pool detectable via T(2) relaxation.
• Observed frequency shifts are consistent with magnetic susceptibility effects, influenced by myelin and tissue structure.
• High-field T(2) contrast is potentially highly sensitive to myelin content, but requires modeling of susceptibility-induced shifts for accurate interpretation.