Magnetization transfer and diffusion tensor imaging in dogs with intervertebral disk herniation

J Vet Intern Med. 2020 Nov;34(6):2536-2544. doi: 10.1111/jvim.15899. Epub 2020 Oct 2.

Abstract

Background: Quantitative magnetic resonance imaging (QMRI) techniques of magnetization transfer ratio (MTR) and diffusion tensor imaging (DTI) provide microstructural information about the spinal cord.

Objective: Compare neurologic grades using the modified Frankel scale with MTR and DTI measurements in dogs with thoracolumbar intervertebral disk herniation (IVDH).

Animals: Fifty-one dogs with thoracolumbar IVDH.

Methods: Prospective cohort study. Quantitative MRI measurements of the spinal cord were obtained at the region of compression. A linear regression generalized estimating equations model was used to compare QMRI measurements between different neurological grades after adjusting for age, weight, duration of clinical signs, and lesion location.

Results: Grade 5 (.79 × 10-3 mm2 /s [median], .43-.91 [range]) and axial (1.47 × 10-3 mm2 /s, .58-1.8) diffusivity were lower compared to grades 2 (1.003, .68-1.36; P = .02 and 1.81 × 10-3 mm2 /s, 1.36-2.12; P < .001, respectively) and 3 (1.07 × 10-3 mm2 /s, .77-1.5; P = .04 and 1.92 × 10-3 mm2 /s, 1.83-2.37;P < .001, respectively). Compared to dogs with acute myelopathy, chronic myelopathy was associated with higher mean (1.02 × 10-3 mm2 /s, .77-1.36 vs. .83 × 10-3 mm2 /s, .64-1.5; P = .03) and radial diffusivity (.75 × 10-3 mm2 /s, .38-1.04 vs. .44 × 10-3 mm2 /s, .22-1.01; P = .008) and lower MTR (46.76, 31.8-56.43 vs. 54.4, 45.2-62.27; P = .004) and fractional anisotropy (.58, .4-0.75 vs. .7, .46-.85; P = .02). Fractional anisotropy was lower in dogs with a T2-weighted intramedullary hyperintensity compared to those without (.7, .45-.85 vs. .54, .4-.8; P = .01).

Conclusion and clinical relevance: Mean diffusivity and AD could serve as surrogates of severity of spinal cord injury and are complementary to the clinical exam in dogs with thoracolumbar IVDH.

Keywords: axial diffusivity; fractional anisotropy; intervertebral disk disease; magnetic resonance imaging; magnetization transfer ratio; mean diffusivity; radial diffusivity; spinal cord injury.