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 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.