Myelin water imaging depends on white matter fiber orientation in the human brain

Area of Science:

• Neuroimaging
• Biophysics
• Magnetic Resonance Imaging

Background:

• Cerebral white matter MRI signals exhibit multi-exponential T2 decay.
• This decay can be separated into myelin water (short T2) and other water components (long T2).
• Myelin water imaging (MWI) is a technique used to quantify myelin content.

Purpose of the Study:

• To investigate the influence of white matter fiber orientation on the apparent myelin water fraction (MWF).
• To determine the degree to which the angle between white matter fibers and the magnetic field affects MWF estimation.

Main Methods:

• Acquisition of apparent MWF maps using multi-echo Carr-Purcell-Meiboom-Gill and gradient-echo spin-echo sequences.
• Acquisition of Carr-Purcell-Meiboom-Gill sequences at varying repetition times (TR): 1073 ms, 1500 ms, and 2000 ms.
• Mapping of white matter fiber orientation using diffusion tensor imaging (DTI).
• Angle-wise pooling of voxels to generate orientation-dependent apparent MWF curves.

Main Results:

• The apparent MWF demonstrated variability, ranging from 25% to 35% across different fiber orientations.
• The choice of repetition time (TR) was found to influence the apparent MWF measurements.

Conclusions:

• White matter fiber orientation introduces a significant systematic bias in apparent MWF estimation.
• This finding necessitates careful consideration for future MWI research and re-evaluation of previously published MWI results.