Formalin tissue fixation biases myelin-sensitive MRI.

Area of Science:

• Neuroimaging
• Biophysics
• Histology

Background:

• Chemical fixatives like formalin alter tissue properties by forming protein cross-links.
• These alterations can affect magnetic resonance imaging (MRI) parameters, including relaxation times and diffusion properties.
• Myelin-sensitive MRI techniques may be susceptible to fixation-induced biases, impacting myelin density measurements.

Purpose of the Study:

• To evaluate potential fixation-induced biases in myelin-sensitive MRI methods.
• To assess the impact of formalin fixation on quantitative magnetization transfer (qMT) and myelin water imaging (MWI).
• To investigate changes in deuterium oxide-exchanged zero TE imaging after formalin fixation.

Main Methods:

• Human spinal cord tissue was imaged using qMT, MWI, and deuterium oxide-exchanged zero TE imaging at 9.4 Tesla.
• Measurements were repeated on unfixed tissue and after 1 day and 31 days of formalin fixation.
• Specimen cross-sectional area was measured to account for shrinkage.

Main Results:

• Formalin fixation significantly increased the qMT bound pool fraction (by 30.7% after 1 day, 42.6% after 31 days).
• Myelin water fraction increased (by ~40%) and its T2 nearly doubled after fixation.
• Deuterium oxide-exchanged zero TE signal intensity showed minimal change after fixation, especially after correcting for shrinkage.

Conclusions:

• Formalin fixation substantially increases myelin density estimates from qMT and MWI, potentially due to myelin sheath alterations.
• Deuterium oxide-exchanged imaging is less affected by fixation, but changes in lipid solid-state T1 may occur.
• Comparisons between fixed and unfixed tissues require careful consideration due to fixation-induced artifacts.