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

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 mm 2 /s [median], .43−.91 [range]) and axial (1.47 × 10 −3 mm 2 /s, .58−1.8) diffusivity were lower compared to grades 2 (1.003, .68−1.36; P = .02 and 1.81 × 10 −3 mm 2 /s, 1.36−2.12; P < .001, respectively) and 3 (1.07 × 10 −3 mm 2 /s, .77−1.5; P = .04 and 1.92 × 10 −3 mm 2 /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 mm 2 /s, .77−1.36 vs. .83 × 10 −3 mm 2 /s, .64−1.5; P = .03) and radial diffusivity (.75 × 10 −3 mm 2 /s, .38−1.04 vs. .44 × 10 −3 mm 2 /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.