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53 A contradictory finding in human breast cancer was reported by Ambekar et al, in which they investigated the d22d31,d15d31, and d33d31 tensor components. In this study, abnormal fibers were defined as having d15d31 values outside the range of normal fibers. They found the percentage of abnormal fibers to be 10.4%, 19.1%, and 48.8% in hyperplastic, dysplastic, and malignant tissues, respectively.21 It is notable that, even within the malignant breast tissues, 52.2% of the fibers were considered ��normal��. Therefore, it is important to have a robust localized method to precisely detect these Duvelisib chemical structure changes, and we suggest that polarization analysis using the generic model could possibly resolve these differences. Still, despite these conflicting reports and experimental complexity, it is anticipated that polarization-resolved methods will gain increased interest in SHG imaging of cancers. Phase-matching considerations and SHG directionality Because SHG is a coherent process, phase-matching conditions between the excitation laser and SHG emission must be satisfied, and this can be exploited for structural analysis. The conditions specific to SHG creation in biological tissues are different from those of the more familiar uniaxial doubling crystals in which the SHG is perfectly phase-matched, where ideal phase-matching (eg, type I) can be observed, PRDX4 with ?k = k2�� �C 2k2�� = 0, where k2�� and k�� are the wave vectors for the SHG and incident photon, respectively. As the coherence length is 2��/?k, all the SHG propagates Bleomycin research buy in the forward direction in this scenario. However, this ideal phase matching is found only in a few uniaxial, birefringent crystals [eg, potassium dihydrogen phosphate (KDP) and beta barium borate (BBO)]. While collagen has been described as a nematic liquid crystal,54 it does not have type I phase-matching conditions. Although it is not necessary to have ?k = 0 for SHG creation, the momentum between the excitation and SHG waves must be conserved, and for ?k �� 0 this results in a distribution of forward and backward emitted components, which we have defined as FSHG/BSHG.55 We previously developed a model describing how more random structures relative to the size scale of ��SHG result in lower FSHG/BSHG values than aligned fibrillar assemblies.56 In addition to this emission directionality, the phase mismatch also has implications on the observed SHG intensity. The relative SHG intensity scales as sin(m?kL/2) (where m is an integer) and thus becomes less efficient for larger phase mismatch, ie, larger ?k values, which correspond to more random structures compared to the length scale of ��SHG.