Summary, discussion and future perspectives 171 Discussion and future perspectives The research performed in this thesis underlines the potential of real‐time, intraoperative optical tools facilitating improved anatomical navigation. Innovative optical techniques might ultimately be implemented during routine surgery as an adjunct to current imaging modalities or even (partly) replace conventional imaging methods, such as conventional intraoperative cholangiography in laparoscopic cholecystectomy. Tissue differentiation beyond the limits of the human eye As already highlighted in the Introduction of this thesis (Chapter 1), in vivo optical techniques can be based on exogenous or endogenous contrasts. The research described in this thesis studied both types of optical techniques. Current state‐of‐the‐art fluorescence imaging of tissues is based on exogenous contrast agents active in the near‐infrared (NIR) spectral range (wavelength range: 700 – 900 nm). Tissue auto‐fluorescence, absorbance and scattering are minimized in this wavelength range, making NIR fluorophores advantageous over optical dyes which are fluorescent in the visible range, below the NIR range3,4. NIR imaging has already been applied for various clinical applications in surgery, for both anatomical guidance (Chapter 2) and intraoperative cancer detection5‐8. Wide‐band (wavelength range: 350 – 1830 nm) diffuse reflectance spectroscopy is based on endogenous tissue contrasts. In the so‐called extended NIR spectral range (wavelength range: 1100 – 1400 nm) particularly the endogenous tissue chromophores water and lipid generate different spectra. The different spectra are due to the fact that the chemical tissue composition differs per tissue type. For example, adipose tissue contains 11.4 to 30.5% water and 61.4 to 87.3% lipid; blood constitutes for 79.0% of water and only for 0.6% of lipid9. This “second near‐infrared window” 10 is a relatively undiscovered field in biomedical imaging. This is mainly due to the spectral response limit of silicon (Si) cameras that are blind beyond 1100 nm. With the availability of indium gallium arsenide (InGaAs: ≥900 nm) sensor camera chips next to standard silicon (Si: ≤1000 nm) sensors11, this extended spectral window now becomes applicable for affordable optical imaging. In the extended NIR spectral range, deep tissue imaging is very well possible, given an even further minimized auto‐fluorescence and scattering compared to the “first NIR window”. Cao et al have demonstrated that multispectral imaging in the extended NIR range provides new opportunities for label‐free imaging, i.e. without administration of exogenous contrast agents9.
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