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High-fidelity multimode fibre-based endoscopy for deep-brain in vivo imaging

Sergey Turtaev, Ivo T. Leite, Tristan Altwegg-Boussac, Janelle M. P. Pakan, Nathalie L. Rochefort, Tomáš Čižmár

arXiv:1806.01654v1physics.med-phphysics.optics

TL;DR

Deep-brain in vivo imaging requires minimally invasive access alongside high resolution, while multimode-fibre endoscopy offers a way to overcome the trade-off between probe size and resolution. This paper uses holographic control through a multimode fibre for fluorescence imaging and demonstrates micron-scale imaging of neuronal cell bodies and processes in deep mouse brain.

  • Problem

    Deep-brain in vivo imaging needs minimally invasive access and high resolution, while optical-element size and attainable resolution involve a trade-off.

    Source: p. 2

  • Method

    The authors use holographic wavefront control through a multimode fibre to create diffraction-limited foci for fluorescence imaging at the fibre’s distal tip.

    Source: pp. 2–3

  • Results

    7-kilopixel images at 3.5 frames per second with micron-size spatial resolution enabled in vivo imaging of deep mouse-brain structures, including cell bodies and fine processes.

    Source: p. 5

  • Takeaways & Limitations

    The work provides a proof of concept for deep-brain in vivo fluorescence imaging through a single multimode-fibre probe with minimal tissue damage.

    Source: p. 5

    In dense volumetric samples, speckled background manifests as glare that reduces image contrast.

    Source: p. 4

Abstract

from arXiv · show

Progress in neuroscience constantly relies on the development of new techniques to investigate the complex dynamics of neuronal networks. An ongoing challenge is to achieve minimally-invasive and high-resolution observations of neuronal activity in vivo inside deep brain areas. A perspective strategy is to utilise holographic control of light propagation in complex media, which allows converting a hair-thin multimode optical fibre into an ultra-narrow imaging tool. Compared to current endoscopes based on GRIN lenses or fibre bundles, this concept offers a footprint reduction exceeding an order of magnitude, together with a significant enhancement in resolution. We designed a compact and high-speed system for fluorescent imaging at the tip of a fibre, achieving micron-scale resolution across a 50 um field of view, and yielding 7-kilopixel images at a rate of 3.5 frames/s. Furthermore, we demonstrate in vivo observations of cell bodies and processes of inhibitory neurons within deep layers of the visual cortex and hippocampus of anesthetised mice. This study forms the basis for several perspective techniques of modern microscopy to be delivered deep inside the tissue of living animal models while causing minimal impact on its structural and functional properties.

METHODS

The methods combine a modular multimode-fibre endoscope with interferometric calibration and in vivo imaging in anaesthetised mice. The probe was used to target deep cortical and hippocampal regions.

Optical setup

  • The modular system comprises laser, calibration, beam-shaping, and sample modules, with signal and reference beams coupled into polarisation-maintaining fibres.A 532 nm continuous-wave, linearly polarised laser supplies the beam.Source: METHODS · p. 6

Beam-shaping module

  • The 50 µm-core, 0.22-NA fibre sustains approximately 2100 propagation-invariant modes at 532 nm, while the beam-shaping optics match the input numerical aperture to 0.23.A DMD generates holograms, and an iris isolates their first diffraction order.Source: METHODS · p. 6

Calibration

  • Calibration used interferometry with an external phase reference; Lee holograms sampled ~3000 coupled inputs and ~7000 output foci from a 100×100 grid spaced ~0.5 µm apart.The output grid was restricted to a Ø50 µm circular area for image acquisition.Source: METHODS · pp. 6–7

Animals

  • Experiments used five adult mice: four with tdTomato-labelled SST neurons and one with tdTomato-labelled VIP neurons; both male and female animals were included.Animal procedures were approved by the University of Edinburgh animal welfare committee and conducted under a UK Home Office project license.Source: METHODS · p. 7

Surgical procedures

  • Mice were anaesthetised with isoflurane for surgery and imaging, and a ~2×2 mm craniotomy was made over left primary visual cortex.Analgesics and anti-inflammatory drugs were administered subcutaneously during preparation.Source: METHODS · p. 7

In vivo imaging

  • Images were acquired at ~3.5 frames per second as the fibre probe was inserted 1–4 mm into brain tissue, targeting deep V1 layers and the hippocampus.The imaging stage fixed the animal’s head, maintained isoflurane anaesthesia and body temperature, and enabled lateral and axial positioning.Source: METHODS · p. 8

CONFLICT OF INTERESTS

The authors report no conflicts of interest.

CONFLICT OF INTERESTS

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