Does loss of gray- and white-matter contrast in injured spinal cord signify secondary injury? In vivo longitudinal MRI studies.

Longitudinal in vivo magnetic resonance imaging (MRI) of moderately injured rat spinal cord was performed to investigate the transient loss in gray matter (GM) and white matter (WM) contrast. In addition, neurobehavioral scores (based on open field walking, inclined plane test, and grid walking) and tissue cholesterol concentrations at pre-determined time points were measured. On MRI, a loss in contrast between GM and WM in sections 2-4 mm caudal to the site of injury was consistently observed 2 days after injury. A substantial recovery in the contrast was observed within 1 week after injury. A strong correlation between the loss of GM-WM contrast and the cholesterol concentration was also observed. The neurobehavioral scores improved with the return of the GM-WM contrast. These serial MRI studies indicate that the loss of contrast in the cord is mainly due to edema accumulated in the WM. The resolution of edema seems to occur around the same time as the restoration of cholesterol in the cord tissue. It is speculated that the compromised membrane stability due to the loss of cholesterol may be one of the factors leading to the accumulation of edema in WM.