Combined Endoscopic Optical Coherence Tomography and Laser Induced Fluorescence
Optical coherence tomography (OCT) and laser induced fluorescence (LIF) are promising modalities for tissue characterization in human patients and animal models. OCT detects coherently backscattered light whereas LIF detects fluorescence emission of endog
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26.1 Introduction Optical coherence tomography (OCT) and laser induced fluorescence (LIF) are promising modalities for tissue characterization in human patients and animal models. OCT detects coherently backscattered light whereas LIF detects fluorescence emission of endogenous biochemicals such as reduced nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FAD), collagen, and fluorescent proteins, or exogenous substances such as cyanine dyes. Given the complimentary mechanisms of contrast for OCT and LIF, the combination of the two modalities could potentially provide more sensitive and specific detection of disease than either modality alone. Sample probes for both OCT and LIF can be implemented using small diameter optical fibers, suggesting a particular synergy for endoscopic applications. In this chapter, the mechanisms of contrast and diagnostic capability for both OCT and LIF are briefly examined. Evidence of complimentary capability is described. Three published combined OCT-LIF systems are reviewed, and existing and potential endoscope designs are illustrated.
26.2 Background on Optical Coherence Tomography OCT provides high-resolution, depth resolved images of scattering tissues. With micron scale resolution and millimeter depth of imaging, OCT is ideal for examining superficial and optically accessible tissues. Most commonly, the amplitude of the backscattered signal is displayed to yield structural images. In this case, signal originates from index of refraction mismatches. Functional variants described in elsewhere this handbook include polarization sensitive OCT, which is sensitive to tissue birefringence and optical axis orientation [1–5]; Doppler OCT, which measures the velocity of moving scatterers, [6–9]; phase contrast OCT, which is sensitive to small changes in optical pathlength [10–12]; and molecular contrast OCT (MCOCT) [13, 14], which
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includes measurement of changes in the source spectrum due to attenuation by endogenous or exogenous substances [15, 16], as well as detection of processes that yield coherent radiation, e.g. coherent anti-Stokes Raman scattering [17] and second harmonic generation [18, 19]. Several exogenous scattering agents have also been proposed to selectively enhance the OCT signal [20–23]. These additional mechanisms of contrast may augment the diagnostic capability of OCT. MCOCT, similar to LIF, has the potential to identify and quantify the presence of certain endogenous and targeted or untargeted exogenous molecules. 26.2.1 Diagnostic accuracy of Optical Coherence Tomography OCT has demonstrated promise for providing accurate diagnoses in a variety of organ sites. Several clinical applications are described in detail in this handbook. Diagnosis and management of retinal disorders, early cancer detection (especially in epithelial tissues), and detection of vulnerable plaque are perhaps the most widely studied applications. The number of large scale clinical studies performed to assess diagnostic accuracy of OCT is currently modest
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