# Polarization Sensitive Optical Coherence Tomography: A Review of Technology and Applications
## 3 PS-OCT Applications
### PS-OCT in Muscles, Tendons, Cartilage, and Bone  
Tendon was the first biological tissue imaged by PS-OCT [8]. Being collagen-rich structures, tendons and muscles exhibit strong birefringence, which enables an easy discrimination from surrounding supportive tissue by PS-OCT.  
Since the integrity of collagen is an indicator for structural stability and pathologic state, PS-OCT was suggested for collagen assessment in tendons and ligaments [189]. Consequently, PS-OCT was used to visualize the evolution of the collagen fiber alignment via birefringence in tissue-engineered tendons in response to varying growth environments and to investigate degenerative changes related to rupture in Achilles tendons [190; 191]. Lately, the influence of proteoglycans--which are essential components of the tendon extracellular matrix associated with tendinopathies--on the optical  
Figure 8: PS-OCT of cancerous tissue. (**a–d**) PS-OCT of basal cell carcinoma in human skin (adapted with permission from [24], Optical Society of America, 2016). (**a**) Reflectance B-scan image. BCC indicates the basal cell carcinoma tumor position; (**b**) Linear retardance along the \(x\)–\(y\) direction; (**c**) Linear retardance along the \(\pm 45^{\circ}\) directions; (**d**) Normalized differential depolarization index image. Scale bar is 0.5(\(x\)) \(\times\) 0.3(\(z\)) mm. (**e–j**) Needle-based PS-OCT of breast cancer (reproduced with permission from [112], [PERSON] et al., 2016). Imaging was performed in grade 1 invasive ductal carcinoma (**e–g**) and grade 2 invasive ductal carcinoma (**h,j**). (**e,h**) Structural intensity (**I**), (**f,i**) overlay of tissue birefringence and intensity (I, \(\Delta n\)), and (**g,j**) matching histological section stained with hematoxylin and eosin. Scale bar in (**e**) is 1 mm and applies to panels (**e–j**).  
properties of tendons have been studied by PS-OCT [192]. In skeletal muscle of genetically-altered (mdx) mice, exercise-induced ultrastructural changes were detected by PS-OCT in in vivo animals [193]. Compared to wildtype controls, the highly birefringent properties of skeletal muscles markedly decreased in mdx mice, thus suggesting a relationship between the degree of birefringence detected using PS-OCT and the sarcomeric ultrastructure present within skeletal muscle. PS-OCT was also shown to be capable of detecting muscle necrosis in dystrophic mdx mice [194].  
In cartilage, PS-OCT can detect areas of enhanced or reduced birefringence in hyaline cartilage (mostly composed of type-II collagen) and fibrocartilage (predominantly type-I collagen) related to degeneration and repair mechanisms [195]. In an in vivo study on human knee joints prior to partial or total joint replacement treatment, reduced birefringence was found in degenerated cartilage [196]. PS-OCT images of one proximal joint surface of bovine tibia are shown in Figure 9d [188]. PS-OCT using variable incidence angles was further used to investigate the 3D architecture of the collagen fiber network in cartilage [197; 198]. Due to its high sensitivity to cartilage disorder, PS-OCT proved a promising tool for imaging cartilage in osteoarthritis in both humans and animal models [199; 200].