Third harmonic generation (THG) is a nonlinear optical process in which light of three times the frequency of the incident radiation is produced. In particular, this process is sensitive to changes in refractive index, making it attractive as a tool for high resolution imaging in specimens where the refractive index changes rapidly over small length scales. For this reason, THG is well-suited to probing neuroanatomy, where features such as long, slender axons sheathed in myelin (i.e. several changes in refractive index in the space of a few microns) match these criteria. For optically thin samples, differential interference contrast (DIC) imaging is an effective tool for visualizing these features. However, in vivo, no efficient dye-free analog of this tool yet exists, making THG a promising candidate. One major objective of my project is to demonstrate that THG is an effective tool for in vivo imaging of morphological changes brought about due to various neuropathologies. The neuropathology I will be investigating is spinal cord injury (SCI). Unlike the peripheral nervous system (PNS), the central nervous system (CNS) contains a number of axonal growth inhibitors, meaning that damage done during SCI does not fully repair. Consequently, individuals who have suffered an SCI often find themselves with lifelong disabilitiesâ€”notably paraplegia and quadriplegiaâ€”which result in a reduction in quality of life. As such, the ability to rapidly assess the outcomes of employing various growth promoting agents in vivo is essential to developing effective treatments that may result in either partial or full recovery in SCI victims.