Differential sensitivity of in vivo and ex vivo diffusion tensor imaging to evolving optic nerve injury in mice with retinal ischemia

Decreased axial (λ||) and increased radial (λ⊥) diffusivity have been shown to reflect axonal and myelin injury respectively. In the present study, evolving white matter injury within the optic nerves of mice with retinal ischemia was examined by in vivo and ex vivo measurements of λ|| and λ⊥. The results show that at 3 days after retinal ischemia, a 33% decrease in vivo and a 38% decrease ex vivo in λ|| without change in λ⊥ was observed in the injured optic nerve compared to the control, suggestive of axonal damage without myelin injury. At 14 days, both in vivo and ex vivo measured λ⊥ increased significantly to 220–240% of the control level in the injured optic nerve suggestive of myelin damage. In contrast, the axonal injury that was clearly detected in vivo as a significantly decreased λ|| (33% decrease) was not as clearly detected by ex vivo λ|| (17% decrease). The current findings suggest that ex vivo λ⊥ is comparable to in vivo λ⊥ in detecting myelin injury. However, the structural changes resulting from axonal damage causing the decreased in vivo λ|| may not be preserved ex vivo in the fixed tissues. Despite the accurate depiction of the pathology using λ|| and λ⊥ in vivo, the use of ex vivo λ|| to extrapolate the status of axonal injury in vivo would require further investigation.