Maximized field-of-view deep-brain calcium imaging through gradient-index lenses
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eLife Assessment
This study presents a valuable and practical approach for one-photon imaging through GRIN lenses. By scanning a low numerical aperture (NA) beam and collecting fluorescence with a high NA, the method expands the usable field of view and yields clearer cellular signals. The evidence is solid overall, with strong qualitative demonstrations, but some claims would benefit from additional quantitative tests. The work will interest researchers who need simple, scalable tools for large‑area cellular imaging in the brain.
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Abstract
Advances in genetically encoded fluorescent indicators have enabled increasingly sensitive optical recordings of neural activity. However, light scattering in the mammalian brain tissue restricts optical access to deeper regions. To address this limitation, researchers often employ implanted gradient-index (GRIN) lenses to reach deep brain areas. Nevertheless, the severe optical aberrations of GRIN lenses significantly reduce the effective field of view (FOV). In this work, we present a simple and robust imaging approach that combines low-NA telecentric scanning (LNTS) of laser excitation with high-NA fluorescence collection to increase the FOV. This configuration effectively eliminates common aberrations such as astigmatism and field curvature, resulting in a FOV ∼100% as large as the GRIN lens facet area — corresponding to a ∼400% increase in imaging area compared with conventional approaches. We validate this method through both structural and functional in vivo imaging. The highly consistent imaging performance, fully maximized imaging FOV, and the very simple optical design make this method well-suited for broad dissemination in neuroscience research.
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eLife Assessment
This study presents a valuable and practical approach for one-photon imaging through GRIN lenses. By scanning a low numerical aperture (NA) beam and collecting fluorescence with a high NA, the method expands the usable field of view and yields clearer cellular signals. The evidence is solid overall, with strong qualitative demonstrations, but some claims would benefit from additional quantitative tests. The work will interest researchers who need simple, scalable tools for large‑area cellular imaging in the brain.
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Reviewer #1 (Public review):
Summary:
The manuscript reported a method for deep brain imaging with a GRIN lens that combines "low-NA telecentric scanning (LNTS) of laser excitation with high-NA fluorescence collection" to achieve a larger FOV than conventional approaches.
Strengths:
The manuscript presented in vivo structural images and calcium activity results in side-by-side comparison to wide-field epi fluorescence imaging through a GRIN lens and two-photon scanning imaging.
Weaknesses:
(1) Lack of sufficient technique information on the "high-NA (1.0) fluorescence collection". Is it custom-made or an off-the-shelf component? The only optical schematic, Figure 1, shows two lenses and a Si-PMT as the collection apparatus. There is no information about the lenses and the spacing between each component.
(2) There is no discussion about …
Reviewer #1 (Public review):
Summary:
The manuscript reported a method for deep brain imaging with a GRIN lens that combines "low-NA telecentric scanning (LNTS) of laser excitation with high-NA fluorescence collection" to achieve a larger FOV than conventional approaches.
Strengths:
The manuscript presented in vivo structural images and calcium activity results in side-by-side comparison to wide-field epi fluorescence imaging through a GRIN lens and two-photon scanning imaging.
Weaknesses:
(1) Lack of sufficient technique information on the "high-NA (1.0) fluorescence collection". Is it custom-made or an off-the-shelf component? The only optical schematic, Figure 1, shows two lenses and a Si-PMT as the collection apparatus. There is no information about the lenses and the spacing between each component.
(2) There is no discussion about the speed limitation of the LNTS method, which, as a scanning-based method, is limited by the scanner speed. At a 10 Hz frame rate, the LNTS, although it has a better FOV, is much slower than widefield fluorescence imaging. The 10 Hz speed is not sufficient for some fast calcium activities.
(3) Supplementary Figure 5 is irrelevant to the main claim of the manuscript. This is a preliminary simulation related to the authors' proposed future work.
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Reviewer #2 (Public review):
Summary:
This study introduces a simple optical strategy for one-photon imaging through GRIN lenses that prioritizes coverage while maintaining practical signal quality. By using low-NA telecentric scanned excitation together with high-NA collection, the approach aims to convert nearly the full lens facet into a usable field of view (FOV) with uniform contrast and visible somata. The method is demonstrated in 4-µm fluorescent bead samples and mouse brain, with qualitative comparisons to widefield and two-photon (2P) imaging. Because the configuration relies on standard components and a minimalist optical layout, it may enable broader access to large-area cellular imaging in the deep brain across neuroscience laboratories.
Strengths:
(1) This method mitigates off-axis aberrations and enlarges the usable FOV. …
Reviewer #2 (Public review):
Summary:
This study introduces a simple optical strategy for one-photon imaging through GRIN lenses that prioritizes coverage while maintaining practical signal quality. By using low-NA telecentric scanned excitation together with high-NA collection, the approach aims to convert nearly the full lens facet into a usable field of view (FOV) with uniform contrast and visible somata. The method is demonstrated in 4-µm fluorescent bead samples and mouse brain, with qualitative comparisons to widefield and two-photon (2P) imaging. Because the configuration relies on standard components and a minimalist optical layout, it may enable broader access to large-area cellular imaging in the deep brain across neuroscience laboratories.
Strengths:
(1) This method mitigates off-axis aberrations and enlarges the usable FOV. It achieves near full-facet usable FOV with consistent centre-to-edge contrast, as evidenced by 4-µm fluorescent bead samples (uniform visibility to the edge) and in vivo microglia imaging (resolvable somata across the field).
(2) The optical design is simple and supports efficient photon collection, lowering the barrier to adoption relative to adaptive optics (AO) or lens design-based correction. Using standard components and treating the GRIN lens as a high-NA (~1.0) light pipe increases collection efficiency for ballistic and scattered fluorescence. Figure annotations report the illumination energy required to reach a fixed detected-photon target (e.g., ~1000 detected photons per bead/cell for the 500-µm FOV condition), and under this equal-output criterion, the LNTS configuration achieves comparable or better image quality at lower illumination energy than conventional wide-field imaging, supporting improved photon efficiency and implying reduced bleaching and heating for equivalent signal levels.
(3) The in vivo functional recordings are stable and exhibit strong signals. In vivo calcium imaging shows high-SNR ΔF/F₀ traces that remain stable over ~30-minute sessions with only modest baseline drift reported, supporting physiological measurements without heavy denoising and enabling large-scale data collection.
(4) The low-NA excitation provides an extended focal depth, enabling more neurons to be tracked concurrently within a single FOV while maintaining practical signal quality. It reduces sensitivity to axial motion and minor misalignment and enhances overall experimental efficiency.
Weaknesses:
(1) Quantitative characterization is limited. Resolution and contrast are not comprehensively mapped as functions of field position and depth, and a clear, operational definition of "usable FOV" is not specified with threshold criteria.
(2) The claim of approximately 100% usable FOV is largely supported by qualitative images; standardized metrics (e.g., PSF/MTF maps, contrast-to-noise ratio profiles, cell-detection yield versus radius) are needed to calibrate expectations and enable comparison across systems.
(3) The trade-off inherent to low NA excitation, namely a broader axial PSF and possible neuropil/background contamination, is acknowledged qualitatively but not quantified. Analyses that separate in-focus from out-of-focus signal would help readers judge single-cell fidelity across the field.
(4) Generalizability remains to be established. Performance across multiple GRIN models (e.g., diameter, NA), wavelengths, is not yet demonstrated. Longer-session photobleaching, heating, and phototoxicity, particularly near the edge of the FOV, also require fuller evaluation.
Readers should view it as a coverage-first strategy that enlarges the FOV while accepting a modest trade-off in resolution due to the low-NA excitation and the extended axial PSF.
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