Endoscopic fiber probe for non-linear spectroscopic imaging
In collaboration with GRINTECH, a fiber probe was developed for the first time at IPHT, which uses an imaging fiber to generate multimodal images of biological tissue. Using a clever optical design, the 8 mm probe eliminates the need for moving parts and live leads, greatly simplifying clinical approval and reducing costs.
By: Alexander Lukic // Sebastian Dochow // Hyeonsoo Bae // Tobias Meyer // Gregor Matz // Ines Latka // Bernhard Messerschmidt // Michael Schmitt // Jürgen Popp
The average age of the population is rising steadily: Aging and a decline in the birth rate will have an increasing impact on society in the coming decades. Along with this, the demands on medical care are growing. As a result, the number of cancers will rise even further within the future since the tumor incidence increases with age and is already one of the leading causes of death worldwide. New diagnostic and therapeutic approaches are therefore needed to ensure optimal medical care in the future and to limit the associated costs. This can best be achieved by identifying and treating cancers as early as possible using ideally non-invasive research methods, as time plays a key role in the successful treatment of cancer. In this context, it is particularly important to differentiate between benign and malignant tissue or between normal and pathophysiologically altered cells with high local precision, in particular to be able to recognize the tumor margin. Spectroscopic imaging techniques are able to meet these conditions. These imaging techniques provide in-depth insights into both the morphology and molecular composition and function of the tissue, offering the potential for objective online medical diagnostics. In order to extend the diagnostic potential of the method, it has been proven to be very advantageous to combine several spectroscopic contrast mechanisms in a multimodal approach. In recent years, the Leibniz IPHT demonstrated the great potential of combining CARS (coherent anti-Stokes-Raman scattering), SHG (second harmonic generation), and TPEF (two-photon-excited fluorescence) microscopy to characterize large ex vivo tissue structures and visualization of tumor borders.
In order to apply the abovementioned multimodal microscopic approach in vivo for the examination of difficult-to-access organs, it is important to study the implementation of the above-mentioned micro-spectroscopic procedures for morphochemical tissue characterization in flexible endoscopes. The Leibniz IPHT has succeeded in realizing a novel CARS / SHG / TPEF imaging fiber probe in a compact endoscopic design. For this purpose, an endoscopic probe was developed in collaboration with GRINTECH. The heart of the concept is a so-called „imaging fiber“ consisting of 10,000 individual cores, which preserve the spatial relationship between input and output of the fiber. Thus, the laser scanning can be moved from the distal to the proximal end of the fiber probe, so that in the probe head contains neither moving parts nor electrical cables in order to realize multimodal in vivo endoscopy. Together with GRIN lenses, filters and diffractive correction elements, a very compact 8 mm diameter probe head has been developed (see Figure 1).1
To create multimodal images, the distal end of the imaging fiber is placed in the focal plane of a Laser Scanning Microscope (LSM). The fiber surface is then scanned line by line by the excitation lasers and the scan pattern being coherently transmitted to the proximal end of the fiber to the probe head. The integrated optics focus the excitation laser on the tissue sample while keeping the probe in direct contact with it. The generated signals are collected in the reverse direction and directed by means of a collection fiber to a multi-channel detection setup. Figure 2 shows a schematic representation of the laser incoupling and signal detection.
With the probe, multi-modal images with the modalities CARS, SHG and TPEF of 0.15 mm² field of view and with a resolution of <10 μm can be recorded within a few seconds, so that the hand-held fiber probe can record multimodal images of unprepared, human skin tissue in real time. The resolution is dictated by the pixelated structure or pitch of the imaging fiber, but is sufficient to represent both the morphology and the molecular composition of a tissue structure. Figure 3 compares an LSM image of healthy, human skin tissue (a) with a fiber probe image (Figure 3 b, CARS contribution red, SHG signals blue, TPEF green).
Overall, Figure 3 shows a strong correlation between multimodal LSM images and images taken with the probe. The morphology is displayed sufficiently well despite reduced optical resolution. The probe thus demonstrates its applicability as an endomicroscopic tool for nonlinear spectroscopic imaging of biological tissue.
Compared to existing endoscopic approaches, the present probe design impresses with its simplicity and robustness. It completely eliminates any moving parts and power in the probe head, reducing both patient risks and hurdles to clinical approval. This contrasts with the hitherto widespread use of piezo elements for imaging with nonlinear fiber probes and offers the advantage of a very robust design with small probe head diameters as well as simpler sterilization and recycling capabilities. The goal is to make diagnoses significantly faster and more precise in the future in order to ensure the earliest possible and personalized treatment.
Funded by: EU, TMWWDG, TAB, BMBF, DFG, FCI, Carl-Zeiss Foundation
Optical design of the probe head with imaging fiber (black), GRIN lenses (white), long pass filter (green), diffractive correction element (red), prism mirror (blue) and collection fiber (blue / gray). [Courtesy of Optical Society of America]
Coupling of the excitation laser to the imaging fiber using an LSM (left), signal collection using a multimode fiber and three detection channels (right). [Courtesy of Optical Society of America]
a) Multimodal LSM image of human skin tissue with CARS (red), SHG (blue) and TPEF (green). (b) The same tissue region was recorded with the fiber probe, the contributions of the individual modalities can be seen below. [Courtesy of Optical Society of America] Fictitious application of the probe for the investigation of melanomas.