Novel NIR Endoscopic Techniques for Medical Imaging – An Investigation Considering both LED and LASER Excitation Sources
The aim of this project is to build and test an illumination and detection system for fluorescence endoscopy in order to image the fluorophore Indocyanine Green. The project was developed through collaboration with clinicians at Imperial College London, the company partner Cymtec Ltd and the University of South Wales for imaging cancerous tissue in the breast via minimally invasive surgical procedures. The use of Indocyanine Green as a fluorescent marker at Near Infrared excitation wavelengths is well established in clinical imaging. Typical systems comprise multiple LED sources, or even Xenon bulbs, for optimal imaging which can result in unnecessary energy transfer to patients and contribute to tissue damage. The use of LASER as an illumination source is not well established nor is the possibility of using a single camera for the simultaneous imaging of fluorescence and bright-field components. Furthermore, there are gaps in knowledge regarding the optical properties of Indocyanine Green for endoscopic applications. An experimental setup comprising a 780nm excitation channel generating up to 10mW of optical power is used so as to determine if there is potential to exploit the optical properties of ICG, in order to reduce the total excitation power through pulsing. We demonstrate in this work that a single 1.6 Megapixel CMOS camera, with quantum efficiency less than 30%, is appropriate to capture both fluorescent and non-fluorescent landmarks at Near Infrared wavelengths. Aqueous solutions containing Indocyanine Green in varying concentrations are imaged under three illumination conditions. Experimental results verify that all solutions tested, yielded detectable fluorescence and that degradation of fluorescence intensity over time is complex. The intensity of fluorescence and image quality is analysed to confirm the viability of a single camera system with LED or LASER as the preferred illumination source. With a single detector, the presence of ICG within a sample can be verified without the need for filtering, this has the potential to impact on the commercial market for these systems. Results suggest that there is no profound difference between illumination source which is valuable for future system design and manufacture. This study signifies that current technologies are not well adapted to focus on patient outcomes, complex filtered systems require image merging which reduces the overall image quality. This research contributes to knowledge with regards to the optical properties of Indocyanine Green while proposing significant changes to system design which would maintain image quality while enhancing patient safety.
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