# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 3 PS-OCT Applications  
### PS-OCT in the Eye  
OCT is most established in ophthalmology, where it has become a standard diagnostic method in everyday clinical routine [130]. Also PS-OCT has been successfully applied for ophthalmic imaging using experimental prototypes [131]. The eye features a variety of tissues exhibiting birefringence or depolarization, which enable PS-OCT to provide additional contrast for discerning, segmenting, and quantifying ocular structures. Birefringence can be found in fibrous tissues such as the retinal nerve fiber layer (RNFL), the sclera (i.e., the white outer shell of the eye), the cornea, as well as in extraorbital muscles and tendons. Depolarization is pronounced in structures containing melanin pigments such as the retinal pigment epithelium (RPE), the choroid, and the pigment epithelium of the iris. Other structures such as the photoreceptor layer, conjunctive tissue, and the stroma of the iris are rather polarization preserving and do not markedly influence the polarization state of light. The RNFL consists of the axons of the retinal ganglion cells. Since the RNFL is damaged in glaucoma--the second leading cause of blindness worldwide [132]--and since RNFL birefringence is connected to layer integrity [133; 134], the polarization properties of the RNFL were investigated as potential diagnostic markers for glaucoma. PS-OCT based assessment of the RNFL's birefringent properties might be particularly interesting since it was shown that polarization changes in experimental glaucoma can be observed earlier than RNFL thickness changes [135]. Peripapillary RNFL thickness is currently a key OCT parameter for glaucoma diagnostics in state-of-the-art clinical routine [136]. In the vein of earlier scanning laser polarimetry approaches [137; 138; 139], PS-OCT was applied to investigate the RNFL using PS-OCT. After initial experiments in the primate retina [140], RNFL birefringence was measured in vivo in the human eye by performing circular scans around the optic nerve head [79; 141]. Faster Fourier domain PS-OCT later enabled 2D mapping of RNFL birefringence and retardation as well as comparisons to scanning laser polarimetry [142; 143; 144; 73; 145]. Exemplary PS-OCT fundus images mapping reflectivity and RNFL retardation in a human eye are shown in Figure 5. Also in preclinical research, PS-OCT was used to investigate the birefringence properties of the RNFL and their relation to the intraocular pressure, which is an important parameter for glaucoma, in animals [146; 147; 148; 135]. Aside from measuring their birefringence, PS-OCT was also demonstrated for tracing nerve fiber bundles in the RNFL [149]. PS-OCT images of the human retina exhibit strong depolarization in pigmented structures such as the RPE [150; 151]. In the RPE, this depolarization is most pronounced around the fovea [152] and correlates with the pigmentation status, i.e., it is reduced or even absent in albino patients [153; 25]. Comparative measurements of PS-OCT and histology in rat eyes have revealed a correlation between DOPU and the density of melanin pigments in the RPE and choroid (Figure 5e) [26]. Depolarization has proven a particularly useful contrast for the assessment of the RPE in clinical cases, where it is often hard to distinguish ocular structures in pathological eyes [154; 155; 20; 103]. Based on DOPU images, algorithms were developed to assess areas and volumes of lesions quantitatively [121]. In age-related macular degeneration (AMD), PS-OCT was not only used to distinguish drusen characteristics but also to quantify the area and volume of drusen during disease progression (Figure 5f-h) [122; 156; 152]. In late stage non-exudative (dry) AMD, PS-OCT enables the assessment of atrophic areas lacking RPE (Figure 5a-d) [157; 121; 158]. In exudative diseases such as wet AMD, central serous chorioretinopathy, and diabetic macular edema, PS-OCT was demonstrated for imaging and identifying fibrotic scars, hard exudates, as well as pigment epithelial features [159; 160; 161; 162; 123].