Longitudinal In Vivo Diffusion Magnetic Resonance Imaging Remote from the Lesion Site in Rat Spinal Cord Injury

Abstract

Diffusion tensor imaging (DTI) has demonstrated success as a biomarker of spinal cord injury (SCI) severity as shown from numerous pre-clinical studies. However, artifacts from stabilization hardware at the lesion have precluded its use for longitudinal assessments. Previous research has documented ex vivo diffusion changes in the spinal cord both caudal and cranial to the injury epicenter. The aim of this study was to use a rat contusion model of SCI to evaluate the utility of in vivo cervical DTI after a thoracic injury. Forty Sprague-Dawley rats underwent a thoracic contusion (T8) of mild, moderate, severe, or sham severity. Magnetic resonance imaging (MRI) of the cervical cord was performed at 2, 30, and 90 days post-injury, and locomotor performance was assessed weekly using the Basso, Bresnahan, and Beattie (BBB) scoring scale. The relationships between BBB scores and MRI were assessed using region of interest analysis and voxel-wise linear regression of DTI, and free water elimination (FWE) modeling to reduce partial volume effects. At 90 days, axial diffusivity (AD_FWE), mean diffusivity (MD_FWE), and free water fraction (FWF_FWE) using the FWE model were found to be significantly correlated with BBB score. FWE was found to be more predictive of injury severity than conventional DTI, specifically at later time-points. This study validated the use of FWE technique in spinal cord and demonstrated its sensitivity to injury remotely.

Keywords: diffusion tensor imaging; free water elimination; spinal cord injury.

FIG. 1.

Image analysis pipeline. Schematic representation of image preprocessing, including phase correction, and registration to template space for group-level analysis.

FIG. 2.

Echo planar imaging (EPI) artifact phase correction. Ghosting artifacts present in multi-shot diffusion-weighted EPI were evident in both non–diffusion-weighted and diffusion-weighted images (top). The automated phase correction had a negligible effect on artifact free images (A-A′), reduced artifacts caused by motion (B-B′ and D-D′) and corrected phase artifact likely due to motion occurring during the pre-scan EPI calibration (C-C′ and D-D′). Four spatial suppression bands placed outside the cord reduced the contamination from non-cord tissues.

FIG. 3.

Mean parameter maps. Registered parameter maps from C4 across all animals demonstrated a reduction of cerebrospinal fluid (CSF) partial volume effects in the free water elimination (FWE) maps compared with diffusion tensor imaging (DTI). The radial diffusivity (RD) map, in particular, showed an improvement in the delineation between the CSF and white matter boundary. The coefficient of variation is more uniform across all animals in the FWE compared with DTI. Color image is available online.

FIG. 4.

Diffusion tensor imaging (DTI) and free water fraction (FWF) injury group means at C4 at 90 days. Axial diffusivity using the free water elimination (FWE) model (AD_FWE) and mean diffusivity (MD_FWE) appear to decrease with injury severity, while FWF_FWE increases. DTI parameters do not show a strong relationship to injury severity. Color image is available online.

FIG. 5.

Correlations with Basso, Bresnahan, and Beattie (BBB) score. At 90 days post-injury, axial diffusivity using the free water elimination (FWE) model (AD_FWE), mean diffusivity (MD_FWE), and free water fraction (FWF_FWE) had significant correlations with BBB score. All significantly correlated pixels (corrected p < 0.05) are overlaid onto a template with axial view at C4 (top row) and coronal view of the cervical cord C1-C7 (bottom row). Positive correlations are illustrated in red hues and negative correlations are in blue. MD_FWE had a significant positive correlation with BBB score in several regions in the cord. In white matter, a prominent association between BBB score and AD_FWE was evident, whereas in the gray matter BBB score was predominantly associated with FWF_FWE. Multiple comparison correction was performed at the voxel level by controlling for the false discovery rate. Color image is available online.

FIG. 6.

Percentage of water loss. Average weight of vertebral spinal cord segments before dehydration (left) and mean percentage of water loss (right) at 90 days post-injury. Injured thoracic segments exhibited lower weight than sham samples, but no significant differences in water content were observed. Color image is available online.