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Abstract

In this MRI study, we investigated multi-compartment T₂ relaxation times of fresh, fixed, and washed pig spinal cord white matter (WM) and their dependence on tissue orientation relative to the main magnetic field. Multi-echo spin echo scans from eight pig spinal cord samples were acquired at 9.4 T with their axes parallel to the main magnetic field B₀. Four of these spinal cord segments were imaged at six different orientations with respect to B₀. After fixation, short fraction T₂ times and myelin water fraction (MWF) increased, while long fraction T₂ times decreased. After washing, short and long fraction T₂ times increased relative to fixed tissue, and MWF decreased relative to fresh tissue. We found a considerable orientation dependence in the short fraction relaxation rate R₂ (=1/T₂) in fresh tissue, which is reduced in washed tissue, and absent in fixed tissue. A similar, but reduced, orientation dependence was observed in long fraction R₂. This work presents a direct comparison of orientation dependence in a WM tissue sample in the fresh, fixed, and washed state. Understanding the changes caused by fixation and tissue orientation can aid in the correction of fixation and orientation-induced alterations of MRI metrics for more accurate and reliable measurements.