In vivo optical microscopy of peripheral nerve myelination with polarization sensitive-optical coherence tomography

Abstract

Assessing nerve integrity and myelination after injury is necessary to provide insight for treatment strategies aimed at restoring neuromuscular function. Currently, this is largely done with electrical analysis, which lacks direct quantitative information. In vivo optical imaging with sufficient imaging depth and resolution could be used to assess the nerve microarchitecture. In this study, we examine the use of polarization sensitive-optical coherence tomography (PS-OCT) to quantitatively assess the sciatic nerve microenvironment through measurements of birefringence after applying a nerve crush injury in a rat model. Initial loss of function and subsequent recovery were demonstrated by calculating the sciatic function index (SFI). We found that the PS-OCT phase retardation slope, which is proportional to birefringence, increased monotonically with the SFI. Additionally, histomorphometric analysis of the myelin thickness and g-ratio shows that the PS-OCT slope is a good indicator of myelin health and recovery after injury. These results demonstrate that PS-OCT is capable of providing nondestructive and quantitative assessment of nerve health after injury and shows promise for continued use both clinically and experimentally in neuroscience.

Fig. 1

Schematic of polarization sensitive-optical coherence tomography (PS-OCT) system (pbs: polarizing beam splitter, pm: polarization modulator, oc: optical circulator, ndf: neutral density filter, pc: polarization controller, g: grating, pbs: polarizing beam splitter, pol: polarizer, and lsc: line scan camera).

Fig. 2

Volume render of three-dimensional PS-OCT data (covering a $5\times 5\times 2{\mathrm{mm}}^3$ region) with the area corresponding to the central 50% of the sciatic nerve used to quantify the average phase retardation outlined in red for (a) control, (b) nerve transection, (c)–(f) 1 to 4 weeks postnerve crush injury.

Fig. 3

Normal sciatic nerve: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.4328\mathrm{deg}/\mu \mathrm{m}$), and (d) corresponding $400\times$ histology.

Fig. 4

Sciatic nerve, 7 days postinjury: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.0872\mathrm{deg}/\mu \mathrm{m}$), and (d) corresponding $400\times$ histology.

Fig. 5

Sciatic nerve, 14 days postinjury: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.0851\mathrm{deg}/\mu \mathrm{m}$), and (d) corresponding $400\times$ histology.

Fig. 6

Sciatic nerve, 21 days postinjury: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.1261\mathrm{deg}/\mu \mathrm{m}$), and (d) corresponding $400\times$ histology.

Fig. 7

Sciatic nerve, 28 days postinjury: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.3080\mathrm{deg}/\mu \mathrm{m}$), and (d) corresponding $400\times$ histology.

Fig. 8

Sciatic nerve, 14 days following transection: (a) normal and (b) polarization-sensitive OCT images, (c) plot of average phase retardation as a function of depth (slope of $0.0554\mathrm{deg}/\mu \mathrm{m}$) and (d) corresponding $400\times$ histology.

Fig. 9

Sciatic function index (SFI) versus PS-OCT slope. SFI and PS-OCT slope increase with time postinjury.

Fig. 10

Myelin thickness versus PS-OCT slope. Myelin degeneration occurred through week 2, followed by myelin regeneration.

Fig. 11

$g$-ratio versus PS-OCT slope.