Sensitivity of Diffusion Tensor Imaging for Assessing Injury Severity in a Rat Model of Isolated Diffuse Axonal Injury: Comparison with Histology and Neurological Assessment

Abstract

Diffuse axonal brain injury (DAI) is a common, debilitating consequence of traumatic brain injury, yet its detection and severity grading remain challenging in clinical and experimental settings. This study evaluated the sensitivity of diffusion tensor imaging (DTI), histology, and neurological severity scoring (NSS) in assessing injury severity in a rat model of isolated DAI. A rotational injury model induced mild, moderate, or severe DAI in male and female rats. Neurological deficits were assessed 48 h after injury via NSS. Magnetic resonance imaging, including DTI metrics, such as fractional anisotropy (FA), relative anisotropy (RA), axial diffusivity (AD), mean diffusivity (MD), and radial diffusivity (RD), was performed prior to tissue collection. Histological analysis used beta amyloid precursor protein immunohistochemistry. Sensitivity and variability of each method were compared across brain regions and the whole brain. Histology was the most sensitive method, requiring very small groups to detect differences. Anisotropy-based MRI metrics, especially whole-brain FA and RA, showed strong correlations with histology and NSS and demonstrated high sensitivity with low variability. NSS identified injury but required larger group sizes. Diffusivity-based MRI metrics, particularly RD, were less sensitive and more variable. Whole-brain FA and RA were the most sensitive MRI measures of DAI severity and were comparable to histology in moderate and severe groups. These findings support combining NSS and anisotropy-based DTI for non-terminal DAI assessment in preclinical studies.

Keywords: axonal injury; diffusion tensor imaging; histology; magnetic resonance imaging; neurological assessment; traumatic brain injury.

Figure 1

Quantification of β-APP-positive axonal profiles across different brain regions in sham-operated, mild, moderate, and severe DAI groups. Graphs show the number of β-APP-positive axons per mm² in the (a) thalamus, (b) hypothalamus, (c) hippocampus, (d) neocortex, and (e) corpus callosum. All DAI groups exhibited significantly increased β-APP staining compared to the sham group (p < 0.01). Axonal pathology increased with injury severity across all regions. Bars represent mean ± standard deviation.

Figure 2

Diffusion MRI metrics measured 48 h after DAI across injury severity groups compared to sham-operated controls. (a) FA, (b) RA, (c) AD, (d) MD, and (e) RD were quantified from whole-brain regions of interest. FA, RA, AD, and MD values significantly declined with increasing injury severity. No significant group differences were observed for RD. Bars represent mean ± standard error. NS = not significant.

Figure 3

Experimental timeline. NSS—neurological severity score; DAI—diffuse axonal brain injury; MRI—magnetic resonance imaging.