Investigating Cervical Spinal Cord Structure Using Axial Diffusion Tensor Imaging

Abstract

This study describes a new technique for Diffusion Tensor Imaging (DTI) that acquires axial (transverse) images of the cervical spinal cord. The DTI images depict axonal fiber orientation, enable quantification of diffusion characteristics along the spinal cord, and have the potential to demonstrate the connectivity of cord white matter tracts. Because of the high sensitivity to motion of diffusion-weighted magnetic resonance imaging and the small size of the spinal cord, a fast imaging method with high in-plane resolution was developed. Images were acquired with a single-shot EPI technique, named ZOOM-EPI (zonally magnified oblique multislice echo planar imaging), which selects localized areas and reduces artefacts caused by susceptibility changes between soft tissue and the adjacent vertebrae. Cardiac gating was used to reduce pulsatile flow artefacts from the surrounding cerebrospinal fluid. Voxel resolution was 1.25 × 1.25 mm2 in-plane with 5-mm slice thickness. Both the mean diffusivity (MD) and the fractional anisotropy (FA) indices of the cervical spinal cord were measured. The FA index demonstrated high anisotropy of the spinal cord with an average value of 0.61 ± 0.05 (highest value of 0.66 ± 0.03 at C3), comparable to white matter tracts in the brain. The diffusivity components parallel and orthogonal to the longitudinal axes of the cord were λ∥ = (1648 ± 123) × 10−6 mm2s−1 and λ⊥ = (570 ± 47) × 10−6 mm2 s−1, respectively. The high axial resolution allowed preliminary evaluation of fiber connectivity using the fast-marching tractography algorithm, which generated traces of fiber paths consistent with the well-known cord anatomy.