ABSTRACT
Second harmonic generation (SHG) imaging microscopy has great potential for visualization of disease states where there is change in collagen structure. is is because SHG, as a second-order nonlinear optical process, requires a noncentrosymmetric environment, and tissue alterations modify the overall symmetry of the collagen architecture. Specically, SHG is an exquisitely sensitive probe of the brillar structure in tissues as it directly visualizes the supramolecular assembly, over the size scale of collagen brils to bers, that is, from ~50 nm to a few microns. While there are approximately 20 isoforms of collagen, type I collagen (also known as col I) is the most abundant one and in fact is the most abundant protein in the body. It is either the primary component or at least a component of the structure in the matrix in diverse tissues such as tendon [1-3], skin [4,5], cornea [6,7], blood vessels [8], and bone and also in internal organs such as lung [9], liver [10], and kidney [11]. Given this range of tissues, it is possible that SHG could be used to image collagen changes in a wide range of pathologies. For example, many connective tissue disorders including osteogenesis imperfecta (OI) and scleroderma are characterized by abnormal collagen assembly and SHG may reveal dierences in the morphology of diseased bers not possible by other optical methods [12]. In addition, it is becoming increasingly documented that extracellular matrix (ECM) changes occur in most cancers. Similarly, brosis, that is, an increase in collagen, secretion is associated with several diseases and is further associated with poor prognoses.
