Efficacy of diffusion tensor anisotropy indices and tractography in assessing the extent of severity of spinal cord injury: an in vitro analytical study in calf spinal cords

Abstract

Background context: Signal intensity changes observed in magnetic resonance imaging (MRI) do not reveal the actual severity of axonal damage incurred in spinal cord injuries. Diffusion tensor imaging (DTI) is an imaging technique with a potential to track individual nerve fiber tracts.

Purpose: We explored the use of DTI in quantifying the extent of spinal cord injury and differentiated areas of injured spinal cord from regions with intact fiber tracts in a cadaveric animal model.

Study design: Radiological study on cadaveric calf spinal cord specimens.

Methods: Diffusion tensor imaging was performed in six freshly acquired spinal cord specimens of Indian calves (Bos primigenius indicus). An axial single-shot echo planar parallel diffusion-weighted imaging sequence was done with a 1.5-tesla MRI. Through fixed seed points, tracking of white matter tracts was done. Diffusion tensor imaging anisotropy indices, fractional anisotropy (FA), apparent diffusion coefficient (ADC), relative anisotropy (RA), volume ratio (VR), and eigenvector (E1) values were acquired at each region of interest. Various injuries were then inflicted in the spinal cord, and DTI was repeated. The tractography images and DTI anisotropy indices were compared with those of the uninjured specimens.

Results: The mean FA, ADC, VR, RA, and E1 values of normal spinal cord were 0.849±0.025, 0.52±0.073 μm(2)/ms, 0.250±0.04 μm(2)/ms, 0.889±0.027, and 1.178±0.22 μm(2)/ms, respectively. The average FA and RA of injured spinal cord regions were significantly decreased being 0.651±0.024 and 0.704±0.041, respectively, at the sites of mild compression. The mean VR values showed significant increase being 0.502±0.027 and 0.628±0.031 μm(2)/ms at the sites of mild and severe compression. Fractional anisotrophy, RA, and VR values showed a progressive alteration as the severity of compression increased. In contrast, the ADC and E1 values did not show significant changes at the sites of pathology. Tractography of injured spinal cord revealed that the white matter was disrupted at the injury site but preserved on the intact regions clearly differentiating regions of partial and complete transection and varying degrees of compression from normal spinal cord.

Conclusion: The study shows that DTI tractography is useful for structural imaging of the spinal cord. Fractional anisotrophy, RA, and VR parameters were found to be more sensitive than ADC and E1 values in assessing the severity of compression.