Magnetization transfer from inhomogeneously broadened lines: A potential marker for myelin

Purpose: To characterize a new approach to magnetization transfer (MT) imaging with improved specificity for myelinated tissues relative to conventional MT.

Methods: Magnetization transfer preparation sequences were implemented with all radiofrequency power centered on a single frequency and also with power evenly divided between positive and negative frequencies. Dual frequency saturation was achieved both with short, alternating frequency pulses and with sinusoidal modulation of continuous irradiation. Images following preparation were acquired with a single shot fast spin echo sequence. Single and dual frequency preparation should achieve similar saturation of molecules except for those with inhomogenously broadened lines. Inhomogenous MT (IHMT) images were generated by subtraction of dual from single frequency prepared images. IHMT imaging was performed with different power and frequency in the brains of normal volunteers.

Results: The IHMT method demonstrated a greater white/gray matter ratio than conventional MT and virtual elimination of signal in scalp and other unmyelinated tissues. IHMT exceeded 5% of the fully relaxed magnetization in white matter. A broad frequency spectrum and signs of axonal angular dependence at high frequency were observed that are consistent with dipolar broadening.

Conclusion: IHMT shows promise for myelin-specific imaging. Further study of physical mechanisms and diagnostic sensitivity are merited.