Speckle structured illumination endoscopy: A novel technique for super resolution imaging with wide FOV and large DOF

Super resolution imaging has been a highly discussed topic in the field of microscopy for many years. While significant progress has been made in microscopic imaging, there remains a significant gap between microscopy and endoscopic imaging.

Two crucial imaging parameters that need to be addressed to bridge this gap are image acquisition and processing with a wide field of view (FOV) and a large depth of field (DOF). These factors often pose challenges when attempting to achieve super resolution in images. One method commonly used in microscopy to achieve a wide FOV with high temporal resolution and low phototoxicity is structured illumination microscopy (SIM). By employing SIM, the spatial resolution can be improved up to twice the diffraction limit of an optical system.

However, SIM is limited by its small depth of field, necessitating precise focusing distance control, which poses practical limitations for microscopy applications. Conversely, endoscopic imaging requires an exceptionally wide FOV and large DOF due to the nature of the samples being imaged and explored. Therefore, exploring the possibility of achieving super resolution in endoscopic images with a wide FOV and large DOF is of great interest.

A recent technique called speckle structured illumination endoscopy (SSIE) has been developed to address these challenges. In this study, published in Opto-Electronic Advances, the authors introduce two fibers into a standard white light endoscope (WLE) to deliver high-resolution speckles for illuminating the object. Random speckle patterns are generated by the interference between laser lights from the two fibers. Multiple images with standard resolution are captured by the WLE camera and then processed using an image reconstruction algorithm to generate a single super resolution image.

By carefully positioning the optical light sources, namely the multimode fibers carrying the random illumination patterns from the laser, the researchers achieve a wide FOV, large DOF, and super resolution. The orientation of the fibers enables them to cover a wide FOV and DOF while also creating large angle interference between the illumination beams, contributing to the achievement of super resolution in imaging. The study demonstrates the effectiveness of SSIE on both planar and non-planar surfaces, showcasing its ability to image at a large DOF.

The study also explores the theoretical aspects of the technique, revealing that the field of view (FOV) and depth of field (DOF) can be extended to the limits allowed by a white light endoscope (WLE). Unlike structured illumination microscopy (SIM), SSIE does not require precise control of illumination patterns, calibration protocols, or focusing optics, simplifying the experimental setup significantly.

The experimental results presented in the study demonstrate a notable 2 to 4.5 times improvement in resolution at a wide FOV and DOF compared to the systemic limit of a standard WLE. These findings highlight the potential of SSIE as a unique approach to achieving super resolution in endoscopic imaging with wide FOV and DOF, which could greatly benefit clinical endoscopy. Moreover, this imaging technique can be adopted in other domains such as biomedical research, medical imaging, and camera-based systems where high resolution at wide FOV and DOF is crucial.

The authors propose and validate a novel method called speckle structured illumination endoscopy for achieving super resolution in endoscopic images. This technique utilizes random optical illumination patterns generated from a coherent light source, such as a laser, directed onto the sample under investigation.

The significance of this work lies primarily in enhancing image resolution while maintaining optimal imaging parameters of a wide FOV and large DOF, pushing the boundaries of what conventional high-resolution endoscopy can achieve, as it is often limited by a narrow FOV and limited DOF. High resolution typically comes at the cost of compromised FOV or DOF, as these parameters are typically inversely related.

Therefore, this study tackles the major challenge of achieving super resolution in endoscopic imaging while also addressing the inherent trade-off between FOV and DOF. Notably, the proposed system does not rely on specific properties of the sample, making it applicable to a wide range of specimens and expanding its potential impact. The findings of this study can have significant implications for the endoscopic imaging community, offering improved imaging capabilities without requiring complex equipment or strict imaging controls during data acquisition and processing.

The versatility of the speckle structured illumination endoscopic system makes it highly applicable to a variety of imaging fields beyond endoscopy. It can be easily translated and adopted in imaging modalities that utilize incoherent imaging methods, particularly those involving fluorescent dyes for sample staining. Additionally, the demonstrations of speckle structured endoscopic illumination are not dependent on the internal structure, type, or specifications of the scope or probe used. This means that the imaging technique can be implemented in any white light endoscopic modality, offering similar resolution improvement factors. This broad applicability holds true whether the application is in a clinical or industrial setting, as the underlying working principle remains the same.

Moreover, in practical imaging scenarios, the sample under study is often non-planar. This study specifically explores the imaging of three-dimensional non-planar surfaces using random optical illuminations. Therefore, the imaging concept developed in this study can be readily applied to other imaging fields such as biomedical research, medical imaging, or camera-based imaging systems with minimal modifications.

In a broader context, any imaging system equipped with a camera that can incorporate the generation and routing of random pattern-based optical illuminations onto the sample, as demonstrated in this study, has the potential to achieve super resolution while maintaining optimal imaging parameters. This technique is particularly beneficial in systems that require a large depth of field, such as endoscopy, depth-based imaging in camera systems, microscopy, and other related fields.

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