# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 3 PS-OCT Applications
### PS-OCT in Nerves and Brain  
Nerve fibers exhibit birefringence, an optical property that has been exploited for quantitative measurements in the retinal nerve fiber layer (see Section 3.1). Also nerve fibers in cerebral white matter or in peripheral nerves may be imaged by PS-OCT based on their birefringence [223].  
Aside from neural structures in the central nervous system, peripheral nerves have also been imaged by PS-OCT. Improved delineation of the sciatic nerve boundaries to muscle and adipose  
Figure 11: Dental PS-OCT. (**a**–**c**) PS-OCT tomograms of a carious lesion in enamel at the distal surface of a human molar (adapted with permission from [209], Karger Publishers, 1999). (**a**) Reflectivity image; (**b**) Phase retardation image; (**c**) Corresponding histologic section. (**d**–**e**) PS-OCT and transverse microradiography (TMR) of tooth demineralization (adapted with permission from [213], John Wiley and Sons, 2010). (**d**) PS-OCT image showing the cross-polarized signal; (**e**) Corresponding TMR. The area of demineralization has a strong cross-polarized signal in (**d**) and appears darker in (**e**).  
tissues as well as quantitative birefringence measurements were enabled by the additional polarization contrast [224]. In an experimental nerve crush model, decreasing birefringence was observed in parallel to a loss of myelination (Figure 12a-d) [225]. The prostatic nerves--indiscernible from surrounding tissue by standard, intensity based OCT--were identified in prostates of rats and humans, thereby indicating the feasibility of PS-OCT as a method for intrasurgical imaging [226].  
In the brain, PS-OCT was not only used to enhance the contrast of birefringent structures but also to trace white matter structures as shown in Figure 12e-h. PS-OCT based tractography provides images encoding the orientation of fiber tracts in different colors and can be used to verify diffusion tensor based MRI tractography images with micrometer scale resolution [51; 227; 228; 229]. Lately, PS-OCT was also demonstrated for imaging a hallmark of [PERSON]'s disease, namely neuritic amyloid-beta plaques as well as amyloidosis in cerebral vasculature [230]. PS-OCT images of birefringent neuritic plaques are shown in Figure 12i-k.  
Figure 12: PS-OCT of neural structures. (**a–d**) PS-OCT of rat sciatic nerve two weeks post nerve crush (adapted with permission from [225], [PERSON], 2015). (**a**) Intensity image; (**b**) PS-OCT retardation image; (**c**) Plot of average phase retardation per depth computed within the red lines shown in (**b**); (**d**) Histologic image. Toluidine blue staining for assessing the myelination state; (**e–h**) En face PS-OCT optic axis orientation maps quantitatively depict in-plane fiber orientations in the medulla (from [229] with permission by Elsevier). The color wheel shows the orientation values ranging between \(-90^{\circ}\) and \(90^{\circ}\). The brightness of colors in the images is determined by the en face retardance values. (**i–k**) PS-OCT imaging of neuritic plaques in post mortem cerebral cortex of an Alzheimer’s disease patient. (**i**) Rendering of 3D retardation data showing increased retardation in plaques. Color map range: 7–4\({}^{\circ}\) (reproduced with permission from [230], [PERSON] et al., 2017). (**j**) Reflectivity B-scan image; (**k**) Retardation B-scan image. The location of one plaque is marked by an orange arrow.