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.