Evaluating the Influence of Injury-Induced Microstructural Variations on the Efficacy of a Diffusion MRI-Derived Axon Diameter Surrogate.

PURPOSE: To investigate the utility of a recently demonstrated diffusion MRI derived axon diameter surrogate measure (∆D⟂) under pathologic conditions induced by a spinal cord injury model.

METHODS

Contusion injuries were performed on the thoracic-lumbar spinal cord of adult male rats. Eight weeks post-injury, rats were sacrificed and their spinal cords imaged by MRI and then scanning electron microscopy (SEM). Relationships between the difference in radial diffusivity with diffusion time (∆D⟂) as well as longitudinal relaxometry based metrics and effective axon diameter from SEM were evaluated across regions of interest in slices 2-5 mm away from the injury epicenter.

RESULTS

∆D⟂ correlated strongly with histology derived effective axon diameter, while bound pool fraction and T1 of the free proton pool (T1f) had no significant correlations with effective axon diameter. A significant relationship was found between T1f and ∆D⟂ across slices.

CONCLUSION

In the context of contusion injured rat spinal cords, ∆D⟂ serves as a robust surrogate for axon diameter and provides complementary information to longitudinal relaxometry derived metrics.