Neurophotonics beyond the Surface: Unmasking the Brain's Complexity Exploiting Optical Scattering

Abstract

The intricate nature of the brain necessitates the application of advanced probing techniques to comprehensively study and understand its working mechanisms. Neurophotonics offers minimally invasive methods to probe the brain using optics at cellular and even molecular levels. However, multiple challenges persist, especially concerning imaging depth, field of view, speed, and biocompatibility. A major hindrance to solving these challenges in optics is the scattering nature of the brain. This perspective highlights the potential of complex media optics, a specialized area of study focused on light propagation in materials with intricate heterogeneous optical properties, in advancing and improving neuronal readouts for structural imaging and optical recordings of neuronal activity. Key strategies include wavefront shaping techniques and computational imaging and sensing techniques that exploit scattering properties for enhanced performance. We discuss the potential merger of the two fields as well as potential challenges and perspectives toward longer term in vivo applications.

Keywords: brain probing; complex media; computational imaging; neurophotonics; wavefront shaping.

Figure 1.

Overview of Diverse Brain Probing Techniques: (a) Microscopy: Traditional imaging with direct optical access to the brain. (b) Multimode Fiber: Flexible approach using a fiber optic cable for light delivery and signal collection. (c) GRIN (Gradient Refractive Index) Lens: Minimally invasive imaging through a small-diameter lens. (d) Head Fixed: Apparatus for stable imaging with restrained subject movement. (e) Freely Moving: Setup allowing for natural behavior during imaging with a mobile recording system. Figures (a-e) adapted with BioRender.com.

Figure 2.. Representative advances from tools commonly used in the complex media community to address challenges in optical probing of the brain.

(a) Depth: scattering and aberration compensation using computational techniques to enhance reflectance imaging of cortical myelin through the skull in the living mouse brain. Before: conventional reflectance microscopy through the mouse skull. Left panel: After: computational conjugated adaptive optical corrected reflectance microscopy of cortical myelin in the mouse brain through skull. Right panel: 3D reconstruction of label-free structural information through skull. Scale bar: 40 μm. (b) Speed: Fast 3D volumetric imaging with targeted illumination of neurons in the mouse cortex labelled with a calcium indicator (GCaMP6f) to increase signal-to-noise of recorded neurons. Before: conventional volumetric calcium imaging with electrically tunable lens and extracted traces after deconvolution. After: illumination-targeted volumetric calcium imaging and extracted traces after deconvolution. Scale bar 50 μm. (c) Biocompatibility: upper panel: enhanced signal given the same laser power enabled by adaptive optics. Before: low signal-to-background of fluorescence-labelled neurons in the hippocampus around 1 mm depth imaged transcranially by conventional three-photon fluorescence microscopy. After: high signal-to-background neurons in the hippocampus imaged by adaptive optics. Scale bar: 20 μm. Lower panel: brain imaging of deep subcortical neurons labeled with a genetically-encoded calcium indicator GCaMP6s using a multimode fiber-based endoscope combined with wavefront shaping for minimally invasive imaging. Scale bar: 30 μm. (d) Field of view: enlarged field of view with diffraction-limited high-resolution imaging enabled by computational conjugated adaptive optics (after) compared with computational adaptive optics without conjugation (before, white boxes) Left: Image of myelin. Right: Phase pattern for aberration correction. SLM: spatial light modulator, DMD: Digital Micromirror Devices, MMF: multimode fibers. Panel (a) adapted from ref under license CC-BY 4.0. Panel (b) adapted from the ref under license CC-BY 4.0. Panel (c) the top images adapted from ref and the bottom images adapted from ref under license CC-BY 4.0. Panel (d) adapted from ref under license CC-BY 4.0.

Figure 3.

Optical access to the mouse brain through a scattering medium: (a) Schematic of a live mouse highlighting the brain area; (b) Inhomogeneous structures within the mouse brain that can cause optical scattering; (c) Multimode fiber (MMF), a frequently studied complex scattering medium in complex media field, is also often utilized for optical access to the brain; (d) Scattering-induced wavefront distortion; (e-i) Various memory effects: (e) Tilt/angular memory effect; (f) Lateral shift memory effect; (g) Axial shift memory effect; (h) Temporal memory effect; (i) Chromatic memory effect; (j) Representative quantitative correlation of wavefront correction pattern for achieving diffraction-limited focusing/imaging in highly scattering brain tissue, demonstrating that the range of the memory effect (defined by the full width at half maximum of the correlation curve) is substantially narrower compared to less scattering scenarios as shown in (k). The patterns for correcting wavefront distortion in highly scattering media (j) are more complex than in weakly scattering media (k). Note in (j,k), ζ could be any of the types of memory effect above in (e-i), but for the illustrative example we chose ζ = Δx (shift). Figures (a-b, d-i) adapted with BioRender.com.