Combining Optical Coherence Tomography with Fluorescence Imaging

1.1 Overview of optical coherence tomography Optical coherence tomography (OCT) is a promising imaging technology which can provide subsurface imaging of biological tissues (Huang et al., 1991). OCT images are similar to the ultrasound images, but with significantly higher resolution (5-10 times) than the clinical ultrasound. OCT enables imaging of tissue microstructure with micron-level resolution, approaching that of standard excisional biopsy and histopathology, except that imaging can be performed in real time without the removal of a tissue specimen. The optical configuration of OCT is the same as that of a low coherence interferometer (LCI). Low-coherence interferometry is a type of optical interferometry based on low-coherent light sources (Mandel & Wolf, 1995). This technique is capable of measuring depth-resolved (axial, z) tissue structure, birefringence, flow (Doppler shift), and spectra at a micrometerlevel resolution by precisely measuring the amplitude and the relative phase of reflected or backscattered light. Optical interferometer has been invented for 120 years. But most of the time, it was just laboratory equipments for physics research until 1970s. In the early 1980s the emergence of high brightness semiconductor broadband light sources and single mode optical fibers stimulated the development of LCI. The availability of superluminescent diodes (SLD) made low-coherence reflectometry practical in a fiber-optic system (Wang et al., 1982). SLD could be used in LCI to achieve a distance resolution of about 10 μm. Another innovation was the development of fiber optic devices such as couplers which are important in the development of fiber optic based designs. These technological developments are the prerequisites of current OCT system used for noninvasive optical imaging of living tissues in vivo. In 1980s, the LCI technique was mainly developed by several groups for reflection measurements in telecommunications devices with micron resolution (Danielson & Whittenberg, 1987; Youngquist et al., 1987). The potential of LCI for three-dimensional imaging in biological tissue was first realized in 1991 (Huang et al., 1991). Since the original work, a large number of papers have been published regarding various aspects of OCT. OCT has been successfully translated to various clinical applications including ophthalmology (Schuman et al., 2004), cardiology (Jang et al., 2005), gastroenterology (Bouma et al., 2000; Li et al., 2000; Sivak et al., 2000; Chen et al., 2007a), dermatology (Welzel