Editorial Feature

Polarization-Resolved Imaging in Biophotonics

Biophotonics is a scientific field that deals with the interaction of light with biological materials such as cells and tissues. Polarization-resolved imaging is a technique that focuses on understanding and analyzing the polarization properties of light as it interacts with biological substances. This article provides an overview of polarization-resolved imaging and its application in the clinical and biomedical sciences.

Imaging in Biophotonics, Polarization-Resolved Imaging

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Importance of Biophotonics

Biophotonics embraces all light-based technologies applied in medicine and life sciences. The controlled use of light has revolutionized various aspects of life. It is a convenient tool that allows the rapid and robust exploration of cellular structures and functions with high precision and sensitivity.

Biophotonics assists in photochemical modification of cellular functions and tissue removal by photothermal or photomechanical processes. Their biomedical and clinical applications include:

Thermal contact: The impact of light in this application is photothermal because the heat produced from high-energy laser light disrupts tissues. Here, the extent of the increase in temperature through the laser light and the water content in the tissue determine the response of the target tissue to the laser light.

Bioimaging: This is a non-invasive imaging technique that helps visualize biological processes in real-time. This imaging technique helps quantify the levels of metabolites or ions during molecular processes. Fluorescence resonance energy transfer (FRET) and two-photon excitation fluorescence (TPEF) microscopy are the latest bioimaging techniques that are widely used in biomedical applications.

Photobiostimulation: This is the process of activating organisms or live cells using laser radiation. Owing to its low power, the laser used for photobiostimulation does not generate heat or disrupt biological tissues. However, their ability to penetrate deep tissues promotes phytochemical curing at the tissue level.

Optical coherence tomography (OCT): This is a label-free, high-resolution optical imaging technique that is commonly used to generate images of histological sections through optical biopsies. OCT is analogous to the ultrasound technique, where light is replaced by sound in ultrasound imaging.

Polarization in Optics and Biomedicine

The fundamental properties of light include intensity, wavelength, phase, and polarization. While the first three are scalar quantities (having only magnitude but no direction), polarization is a vector quantity that has both magnitude and direction.

Hence, the application of polarization involves advanced optical and computational approaches. Polarization optics has helped in advancing numerous research areas, including fundamental research, material characterization, biomedical studies, and clinical applications.

Multiple scattering processes alter the degree and state of polarization of the incident light beam. While this alteration in polarization provides insightful structural information about biomedical samples, such as cells and tissues, it may also introduce uncertainty in the expected photon properties, hindering the accurate information analysis of biomedical samples. To this end, polarization-resolved imaging techniques can provide more systematic information.

Polarization-Resolved Imaging of Biological Material

The interaction between electromagnetic waves (light) and the medium (such as cells or tissues) depends on the angle between the polarization axis of the light and the axis of the dipole oscillation, that is, the angle at which the light waves hit cells or tissues. Hence, polarization-resolved optical imaging allows the study of non-isotropic (irregular) organization, revealing the molecular orientation in cells and tissues.

The light waves are randomly oriented without distinct polarization signatures during absorption; consequently, regularly ordered molecular structures produce birefringence. Here, a  birefringence quantifying parameter, termed retardance, helps quantify the presence of ordered molecular structures. Polarization-resolved optical imaging techniques have many applications ranging from disease diagnosis to optical data storage.

Recent Studies

Collagen organization is crucial to maintaining the structural integrity of cells and determining tissue function. An article published in Light: Science & Applications highlighted the importance of polarization-sensitive optical coherence tomography (PSOCT), a non-invasive three-dimensional (3D) imaging tool, for mapping in vivo collagen organization.

The multiple polarization inputs of PSOCT system helped visualize collagen organization in deeper tissues, which was otherwise incompletely demonstrated when single-input polarization states were used. The adopted method was tested ex vivo on both healthy and infarcted rodent hearts to visualize depth-resolved myocardial architecture.

This study will be useful for monitoring the healing response to an infarct event at various times from injury or in the remodeling of grafts in the heart. It could also help in understanding many important physiological diseases, such as the biomechanical origin of arrhythmia.

Additionally, the method was utilized for the in vivo structural study of collagen in skin tension lines at different anatomical locations on the face of a healthy human volunteer, suggesting that the non-invasive PSOCT imaging can be integrated with other cosmetic procedures to increase the contrast and diagnostic capability.

Another study published in the Journal of Biophotonics used two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) on 5 μm thick skin paraffin sections obtained from 15-day-, 1-month- and 21-month-old rats to investigate the biophotonic properties of collagen, the main skin component that changes with age.

Polarization-resolved SHG (PSHG) images revealed changes in collagen fibers with aging in rats, which is consistent with the results obtained via light and electron microscopy. This study suggests that slight changes in the collagen structure due to damage from processes such as oxidative stress can be detected using this technique.

Conclusion

Polarization-resolved imaging is a transformative tool in the field of biophotonics. This aids in the resolution of the previously unsolved complexity of biological tissues. Its capacity to identify minute structural differences based on the behavior of light waves when interacting with biological materials has opened new avenues for biomedical research. Analysis of the characteristics of polarized light during its interaction with biological materials reveals a plethora of knowledge, illuminating the internal mechanisms of molecules, tissues, and cells.

Polarization-resolved imaging provides a route towards earlier and more precise diagnosis by helping to comprehend the minute changes in collagen fibers brought on by aging processes and identifying minute changes suggestive of illnesses. Its capacity to reveal tissue composition and molecular orientation in real-time improves our understanding of biological systems and creates new opportunities for creative therapeutic approaches.

Further advancements in technology and methodology will undoubtedly increase the scope and precision of polarization-resolved imaging. Integrating this technique with other modalities can revolutionize medical imaging and potentially lead to more personalized and targeted healthcare solutions.

 

References and Further Readings

Biomedical Applications of Biophotonics. Accessed on December 8 2023

Tang, P., Kirby, M. A., Le, N., Li, Y., Zeinstra, N., Lu, G. N., Wang, R. K. (2021). Polarization sensitive optical coherence tomography with single input for imaging depth-resolved collagen organizations. Light: Science & Applications, 10(1), 237. https://doi.org/10.1038/s41377-021-00679-3

Miler, I., Rabasovic, M. D., Aleksic, M., Krmpot, A. J., Kalezic, A., Jankovic, A., Korac, A. (2021). Polarization‐resolved SHG imaging as a fast screening method for collagen alterations during aging: Comparison with light and electron microscopy. Journal of Biophotonics, 14(3). https://doi.org/10.1002/jbio.202000362

He, C., He, H., Chang, J., Chen, B., Ma, H., Booth, M. J. (2021). Polarization optics for biomedical and clinical applications: a review. Light: Science & Applications, 10(1), 194.

https://doi.org/10.1038/s41377-021-00639-x

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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