Finally, as mentioned, polarized light has been shown to effectively detect ex vivo tissue disorders of internal organs, such as the heart, cervix, and bladder.32–35 Reaching these organs deep within the body can enable in vivo deployment; therefore, polarimetric optical delivery/pick-up methods not based on free-space optics must be developed. These include various varieties of rigid or flexible waveguides, fibers, and catheters. This is challenging in that these optical delivery vehicles often distort the polarization states of the light they transmit; this will produce large signal artifacts that will likely overwhelm the weak tissue polarization signals these delivery vehicles are meant to relay. One promising solution to avoid this problem is the use of distal polarization components, which has been suggested by several researchers.74,75 Using distal polarizers facilitates accurate generation of polarized illumination and collection of the polarized light backscattered from the tissue. Using this concept, our group is in the process of fabricating a fiber optic probe with distal polarizing components for measuring the full tissue Mueller matrix (manuscript in preparation). This approach of using distal polarizing components has also been used in wide-field endoscopes; for instance, as shown in Figs. 5(a) and 5(b), Qi et al. incorporated moving polarizers at the proximal and distal ends of a commercially available laparoscope (1 cm outer diameter, rigid construction) to measure the linear part of the Mueller matrix.76 The first row and first column of the Mueller matrix are, thus, not measured, often due to difficulties in generating and transmitting circularly polarized light. As illustrated in Figs. 5(c)–5(e), this modified laparoscope was used to measure the depolarization and the retardance of different organs in an open rat abdomen in vivo.76 As expected, the organs have different scattering, absorption, and morphological characteristics and, thus, can be differentiated with specific ranges of retardances and depolarizations. Another alternative is to use polarization maintaining fibers for delivery/collection of light and a PSA and a PSG at the proximal ends to modulate the polarization. For example, in a very recent report, Vizet et al. demonstrated full Mueller matrix measurements through a single polarization maintaining fiber. To correct for random changes in the fiber optical axis, which changes the polarization at the sample compared with the proximal end, a switchable calibration mirror was used.77 This is similar to the calibration procedure applied in some PS-OCT systems that corrects the axis with the signal reflected back from the tissue surface. The details are not described in peer-reviewed publications yet, but are likely forthcoming. These developments will eventually enable in vivo polarimetry of deep-seated tissue pathologies in the near future, most likely in the cystoscopy/endoscopy/bronchoscopy settings.