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Scientists Develop Multimodal Nonlinear Endomicroscopic Imaging Probe for Tissue Diagnostics

The diagnostic gold standard in most surgeries is the extraction of biopsies and their histopathological examination to confirm the tumor and the tumor borders. The histopathological examination is carried out by means of rapid sections of non-contrasted tissue sections during the surgery and also on tissue sections of fixed material.

This procedure is time-consuming, investigator-dependent, and dependent on the size, number and quality of the removed tissue samples. Thus, in tumor surgery there is a great need for new technologies that are able to precisely localize the tumor so that surgeons can remove it as completely as possible, because a reliable detection of tumor margins is the key to effective tumor treatment.

In this context multimodal nonlinear imaging combining the three different nonlinear imaging techniques – namely coherent anti-Stokes Raman scattering (CARS), two-photon excited autofluorescence (TPEF), and second harmonic generation (SHG) – offers the potential to reliably assess tissue and the success of surgery or endoscopy.

However, the in vivo application of these methods during endoscopic or surgical procedures is challenging requiring (i) robust ultrafast lasers, (ii) low loss laser delivery and signal collection fibers maintaining the pulse shape, (iii) compact, fast and precise scanners as well as (iv) high-performance endomicroscopic objectives.

In a new paper published in Light Science & Application, a team of scientists from Jena, Germany, led by Prof. Dr. Juergen Popp of the Leibniz Institute of Photonic Technology and Friedrich Schiller University Jena and Dr. Bernhard Messerschmidt of the company Grintech introduces a novel all-fiber based endoscopic set-up for multimodal non-linear endoscopy allowing to record tissue images displaying both morphological and biochemical information.

The scientists have developed a CARS/SHG/TPEF endoscopy platform, in which they have custom designed all abovementioned key components for best performance, i.e., the portable fiber laser, a new type of solid fiber for guiding the excitation laser wavelengths in two separate cores and collecting the signal in an outer collection cladding, a resonant fiber scanner, and a custom endomicroscopic objective for laser recombination and color corrected for the delivery lasers.

As such, their novel multimodal image probe allows to record tissue images comparable to images acquired with a commercially available bulky laser scanning microscope. The reported unique fiber probe concept will open new possibilities for label free tissue diagnostics during endoscopy or surgery e.g. in terms of tumor margin detection. This could lead to an improved patient care and cost savings by e.g. avoiding expensive follow-up treatments.

The novel flexible ultracompact endoscopy approach is centered around a specially tailored double-core double-clad fiber and focus-combining micro-optical concept allowing for a background free, low-loss, high peak power laser delivery, and an efficient signal collection in backward direction. The scientists summarize the operational principle of their endoscopic approach:

"The heart of this fiber-scanning endoscope is an in-house custom-designed, new type of optical fiber for delivery of the CARS fiber laser, namely a single mode, double clad, double core (DCDC) pure silica optical fiber. This type of fiber avoids the generation of a disturbing four-wave-mixing (FWM) background contribution by separately guiding the CARS pump and Stokes laser pulses in individual cores enabling single mode operation. The DCDC fiber has been produced by stack-and-draw technology at the Leibniz Institute of Photonic Technology" says Prof. Popp.

"In addition to the DCDC fiber, the second key part of the endoscopic platform is the specially designed endomicroscopic objective of 0.55 numerical aperture and 180 µm field of view. Here, a linear diffractive optical grating overlays the pump and Stokes laser foci across the full field of view, such that diffraction-limited performance is demonstrated for tissue imaging at one frame per second with sub-micron spatial resolution and at a high transmission of 65% from the laser to the specimen using a distal resonant fiber scanner." adds Dr. Bernhard Messerschmidt.

"This interplay of a tailored optical fiber design with a smart and ultra-compact optical concept leads to an all-fiber based endoscopic set-up for multimodal non-linear endoscopy representing a promising design for routine clinical imaging applications such as surgical guidance and in vivo diagnostics" the scientist forecast.

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