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

Decreased axial (lambda(parallel to)) and increased radial (lambda(perpendicular to)) 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 A(parallel to) and lambda(perpendicular to). The results show that at 3 days after retinal ischemia, a 33% decrease in vivo and a 38% decrease ex vivo in lambda(parallel to) without change in lambda(perpendicular to) 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 lambda(perpendicular to) 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 All (33% decrease) was not as clearly detected by ex vivo lambda(parallel to) (17% decrease). The current findings suggest that ex vivo lambda(perpendicular to) is comparable to in vivo lambda(perpendicular to) in detecting myelin injury. However, the structural changes resulting from axonal damage causing the decreased in vivo All may not be preserved ex vivo in the fixed tissues. Despite the accurate depiction of the pathology using lambda(parallel to) and lambda(perpendicular to) in vivo, the use of ex vivo lambda(parallel to) to extrapolate the status of axonal injury in vivo would require further investigation. (c) 2006 Elsevier Inc. All rights reserved.