Semiquantitative assessment of optic nerve injury using manganese-enhanced MRI

To evaluate the capability of manganese (Mn2+)-enhanced MRI (MEMRI) in a continuously semiquantitative assessment of rat optic nerve (ON) injury. Forty rats were divided into three groups: (I) a control group that was submitted to MEMRI or to fluorescent labeling of retinal ganglion cells (RGCs) (n = 10); (II) an ON injury group that was submitted to MEMRI (n = 15); (III) an ON injury group that was submitted to fluorescent labeling of RGCs (n = 15). Groups II and III were examined at 3, 7, and 14 days post-lesion (dpl), when the contrast-to-noise ratio (CNR) of the retina and ON was measured on MEMRI images and the RGCs were counted by fluorescence microscopy and compared between the groups. In the control group, the intact visual pathway from the retina to the contralateral superior colliculus was visualized by MEMRI. In group II, continuous Mn2+ enhancement was seen from the retina to the lesion site of the optic nerves at 3, 7, and 14 dpl. However, no Mn2+ enhancement was observed distal to the lesion site at those time points. The observed Mn2+ enhancement proximal to the ON lesion site declined between 7 and 14 dpl. The decrease in Mn2+-enhanced signal intensity at these sites at 7 and 14 dpl when compared to that at 3 dpl was significant (P < 0.05). The RGC density dropped by 6.84, 45.31, and 72.36 % at 3, 7, and 14 dpl, respectively. MEMRI can be used to evaluate the structural changes after optic nerve injury.