Fluorescence image-guided surgery

Fluorescence guided surgery (FGS), (also called 'Fluorescence image-guided surgery', or in the specific case of tumor resection, 'fluorescence guided resection') is a medical imaging technique used to detect fluorescently labelled structures during surgery. Similarly to standard image-guided surgery, FGS has the purpose of guiding the surgical procedure and providing the surgeon of real time visualization of the operating field. When compared to other medical imaging modalities, FGS is cheaper and superior in terms of resolution and number of molecules detectable. As a drawback, penetration depth is usually very poor (100 μm) in the visible wavelengths, but it can reach up to 1–2 cm when excitation wavelengths in the near infrared are used.

FGS is performed using imaging devices with the purpose of providing real time simultaneous information from color reflectance images (bright field) and fluorescence emission. One or more light sources are used to excite and illuminate the sample. Light is collected using optical filters that match the emission spectrum of the fluorophore. Imaging lenses and digital cameras (CCD or CMOS) are used to produce the final image. Live video processing can also be performed to enhance contrast during fluorescence detection and improve signal-to-background ratio. In recent years a number of commercial companies have emerged to offer devices specializing in fluorescence in the NIR wavelengths, with the goal of capitalizing upon the growth in off label use of indocyanine green (ICG). However commercial systems with multiple fluorescence channels also exist commercially, for use with fluorescein and protoporphyrin IX (PpIX).

Fluorescence excitation is accomplished using various kind of light sources.Halogen lamps have the advantage of delivering high power for a relatively low cost. Using different band-pass filters, the same source can be used to produce several excitation channels from the UV to the near infrared. Light-emitting diodes (LEDs) have become very popular for low cost broad band illumination and narrow band excitation in FGS. Because of their characteristic light emission spectrum, a narrow range of wavelengths that matches the absorption spectrum of a given fluorophore can be selected without using a filter, further reducing the complexity of the optical system. Both halogen lamps and LEDs are suitable for white light illumination of the sample. Excitation can also be performed using laser diodes, particularly when high power over a short wavelength range (typically 5-10 nm) is needed. In this case the system has to account for the limits of exposure to laser radiation.

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