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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
TL;DR
The paper addresses the challenge of imaging deep brain tissue with a minimally invasive probe, using a modular multimode-fibre system calibrated through transmission-matrix measurements and DMD beam shaping. In anaesthetized mice, the probe was inserted 1–4 mm into brain tissue toward visual cortex and hippocampus, with inhibitory neurons labelled for fluorescent imaging.
Problem
The study targets minimally invasive, high-resolution observation of neuronal activity in deep brain areas.
Method
A modular system measures the multimode fibre’s transmission matrix and uses DMD-based beam shaping, while controlled in vivo experiments image labelled neurons in anaesthetized mice.
Results
The probe was inserted 1–4 mm into brain tissue targeting deep cortical layers in V1 and ventrally through the hippocampus to the base of the brain.
Takeaways & Limitations
The experimental platform supports controlled multimode-fibre imaging procedures in anaesthetized mice targeting visual cortex and hippocampus.
Abstract
from arXiv · showhide
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 system combines modular optical components with transmission-matrix calibration and DMD-based beam shaping for multimode-fibre imaging. Experiments used labelled mice, anaesthesia, craniotomy, controlled fibre insertion, and post-mortem tract confirmation.
- Setup: The modular setup comprises laser, calibration, beam-shaping, and sample modules, with calibration measuring the fibre’s transmission matrix.The calibration module is subsequently replaced by the sample module for imaging the anaesthetized animal model.
- Setup: The reference and signal beams are combined and polarization-controlled to maximize interference-pattern signal-to-noise during calibration.A translation stage precisely adjusts the calibration plane relative to the fibre endface.
- Calibration methods: The calibration basis uses a 65×65 input grid, reduces coupled input modes to approximately 3000, and samples output modes on a 100×100 grid spaced about 0.5 µm apart.Binary amplitude gratings based on the Lee hologram approach operate the DMD in the off-axis regime.
- Animals: Data were acquired from five adult mice, with inhibitory SST or VIP neurons labelled by tdTomato in transgenic lines.Four mice carried labelled SST neurons and one carried labelled VIP neurons.
- Surgical procedures: Mice were anaesthetized, underwent an approximately 2×2 mm craniotomy over left primary visual cortex, and received analgesic and anti-inflammatory treatment.Isoflurane was used for induction, maintenance, surgery, and imaging.
- In vivo imaging: A motorized three-axis stage controlled animal positioning and fibre penetration 1–4 mm into brain tissue toward V1 and the hippocampus.Head fixation, facemask anaesthesia, and thermal support were used during imaging.
- Post-mortem confirmation: After imaging, brains were fixed, sectioned coronally at 50 µm thickness, and examined to confirm the fibre-tract location.Perfusion used phosphate-buffered saline followed by 4% paraformaldehyde after sodium-pentobarbital overdose.