Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm (1502.02775v2)

Abstract: Compared to imaging in the visible and near-infrared regions below 900 nm, imaging in the second near-infrared window (NIR-II, 1000-1700 nm) is a promising method for deep-tissue high-resolution optical imaging in vivo mainly due to the reduced scattering of photons traversing through biological tissues. Herein, semiconducting single-walled carbon nanotubes with large diameters were used for in vivo fluorescence imaging in the long-wavelength NIR region (1500-1700 nm, NIR-IIb). With this imaging agent, 3-4 um wide capillary blood vessels at a depth of about 3 mm could be resolved. Meanwhile, the blood-flow speeds in multiple individual vessels could be mapped simultaneously. Furthermore, NIR-IIb tumor imaging of a live mouse was explored. NIR-IIb imaging can be generalized to a wide range of fluorophores emitting at up to 1700 nm for high-performance in vivo optical imaging.

• The paper demonstrates that NIR-IIb imaging using laser-vaporized SWNTs achieves deep-tissue resolution of capillaries as small as 3-4 μm at depths up to 3 mm in live mouse models.
• The paper reveals that the signal-to-background ratio in NIR-IIb imaging significantly outperforms traditional NIR-I and NIR-II windows, recording ratios of 4.50 compared to 1.19-2.01.
• The paper introduces video-rate imaging for dynamic blood flow mapping and tumor detection, highlighting its potential for real-time vascular and cancer diagnostics.

Overview of "Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm"

This research article presents significant advancements in the field of in vivo fluorescence imaging by leveraging the second near-infrared window (NIR-II, 1000—1700 nm), particularly focusing on the long-wavelength region from 1500 to 1700 nm (NIR-IIb). The primary innovation lies in the use of semiconducting single-walled carbon nanotubes (SWNTs) synthesized through laser vaporization to achieve high-resolution deep-tissue imaging surpassing traditional NIR imaging techniques.

The paper addresses a prevalent challenge in fluorescence-based optical imaging: the depth penetration and resolution constraints posed by photon absorption and scattering in biological tissues. Prior imaging methodologies focusing on the visible and NIR-I (750–900 nm) windows encountered limitations due to higher photon scattering and inadequate tissue penetration. The transition to NIR-IIb, facilitated by SWNT emitters, attempts to mitigate these issues by exploiting a region characterized by minimal photon scattering and a local minimum in water absorption spectrum.

Key Findings

1. Enhanced Imaging Capabilities: The authors successfully demonstrate that NIR-IIb imaging allows the resolution of capillary blood vessels as narrow as 3-4 μm at depths close to 3 mm in live mouse models. This depth of penetration with preserved resolution significantly surpasses conventional capabilities in other NIR windows.
2. Improved Signal-to-Background Ratio (SBR): The research shows that the SBR is notably superior in the NIR-IIb window compared to NIR-I and NIR-II (4.50 in NIR-IIb vs. 2.01 in NIR-II and 1.19 in NIR-I), suggesting notable benefits in imaging signals against a biological background.
3. Blood Flow Mapping: The application of video-rate NIR-IIb fluorescence imaging to resolve blood-flow speeds in multiple vessels represents a novel and adept method for dynamic vascular studies in small animal models.
4. Tumor Imaging Exploration: The efficiency of NIR-IIb imaging for tumor detection is evaluated and affirmed through detailed in vivo studies, highlighting the potential for improved cancer imaging strategies.

Methodological Advances

The SWNTs employed were meticulously synthesized through the laser vaporization method to achieve a specific diameter distribution optimal for NIR-IIb emission. The authors detail an exhaustive characterization of these fluorophores, emphasizing their enhanced brightness in comparison to HiPCO SWNTs that predominantly emit below 1400 nm. The research provides a comprehensive methodological framework, including surfactant-aided separation and biocompatible functionalization of the SWNTs, ensuring efficient systemic circulation and targeted imaging efficacy.

Practical and Theoretical Implications

The evidence presented substantiates the potential of NIR-IIb imaging as a transformative technique for deep-tissue imaging in biological research and clinical diagnostics. The reduced photon scattering characteristic of this window enables unprecedented resolution and depth penetration, pivotal for complex cancer imaging and real-time vascular monitoring. This advancement opens avenues for future exploration of other biocompatible fluorophores with higher quantum yields, aiming to further enhance imaging performance in this domain.

Future Directions

Work stemming from this paper could evolve along multiple vectors, including:

• Expanding the library of fluorophores compatible with NIR-IIb imaging to include materials with superior quantum efficiencies.
• Investigating the incorporation of these advancements in clinical imaging technologies, particularly for cancer diagnostics and intraoperative applications.
• Developing real-time imaging capabilities and algorithms to extend dynamic biological process visualization.

In conclusion, the insights and methodologies presented in this article offer a noteworthy contribution to the field of optical bioimaging, providing a solid foundation for further exploration and application of long-wavelength NIR imaging techniques. This holds promise for advancing precision in in vivo diagnostic imaging and broadening the scope of biomedical investigation.