Phase imaging of axonal integrity of cranial corticospinal tract in experimental spinal cord injury at 9.4 T

Abstract

Spinal cord injury (SCI) leads to neuronal cell death, axonal damage and demyelination. Both spinal cord and brain undergo anatomical changes following SCI. Recently MR phase imaging (PI) has shown promising application in visualizing demyelination or axonal damage. Therefore, in this study we explored the possibility to investigate integrity of cranial corticospinal tract (CST) in SCI using PI. Diffusion tensor imaging (DTI), and its metrics, e.g. axial diffusivity (λ∥) were also used to verify the axonal integrity and preclude the potential contribution of axonal injury to the observed decrease in frequency contrast. We also correlated the MRI findings to immunohistochemistry.Results show that the phase contrast decreased mainly in contralateral pyramid two weeks post-injury compared to pre-injury levels. Significant reduction in contralateral pyramid two weeks post-injury compared to pre-injury levels when quantified radial diffusivity (λ⊥) values. The λ∥ in all those regions did not significantly change 2 weeks post-injury compared to pre-injury levels.The reduction of frequency contrast along with no significant changed λ∥ was observed in the contralateral pyramids, suggestive of demyelination without obvious axonal damage in the CST white matter. These MRI findings were confirmed by immunohistological results, i.e. myelin basic protein staining lost but without obvious reduction in neurofilament staining after two weeks after injury. The reduced λ⊥ in the contralateral pyramids, which may be associated with activated astrocytes according to the increased expression of GFAP in those regions. In conclusion, PI is a potential endogenous biomarker for brain axonal integrity after spinal cord injury.