A dual-modal optical system combining depth-sensitive laser induced fluorescence (LIF) spectroscopy and optical coherence tomography (OCT) for analyzing layered biological tissue

There is a significant current interest in combining multiple modalities in a single optical system for a more complete characterization of tissue for improved diagnosis. We report development of a dual-modal optical system combing the depth-sensitive laser induced fluorescence (LIF) spectroscopy and optical coherence tomography (OCT) for analysis of layered biological tissues. A time-domain, real-time OCT system, assembled in-house, was used as the platform for developing the dual-modal system. It involved appropriate modification of the sample arm of the OCT for making the LIF excitation beam (337 nm) collinear with the OCT illumination beam (1310 nm) and implementation of a confocal fluorescence configuration for rejecting the out-of-focus fluorescence light. The system was first validated using a layered non-biological phantom prepared by placing a thin layer of tissue paper pasted over a green dye card of much larger thickness. A good correlation was observed between the OCT images and the fluorescence signal. The system was also used to record OCT images and fluorescence spectra from a chicken leg tissue covered with a thin layer of epithelial membrane. While OCT images showed the presence of two distinct layers, fluorescence spectra measured from these layers confirmed the biochemical difference between the two. In contrast to the dual-modal LIF-OCT systems reported earlier, the present system can sequentially record fluorescence signal and the OCT image from the same depth of a tissue sample thereby showing its potential for obtaining at once both morphological as well as biochemical information of the tissue layer from that depth.