Endoscopic micro-optical coherence tomography with extended depth of focus using a binary phase spatial filter

Significance Statement

Optical coherence tomography has been widely applied in the medical field to acquire cross-sectional images of tissue microstructure with high spatial resolution of approximately 10 µm. This interferometric imaging technique offers a superior resolution that makes it a crucial diagnostic tool in clinical fields such as cardiology and ophthalmology. However, the optical coherence tomography’s typical resolution is not enough to visualize the cellular and subcellular features of biological samples. It has been found that a micro-optical coherence tomography that has an optimized binary phase spatial filter design which creates a compromise between the depth of focus and sensitivity can be used.

A team of researchers led by Professor Hongki Yoo at Hanyang University in South Korea developed a miniaturized side-view endoscopic micro-optical coherence tomography with an extended depth of focus using a binary phase spatial filter. They hoped to attain a reasonable imaging depth by extending the depth of focus when using a high numerical aperture lens. Their work is now published in the peer-reviewed journal Optics Letters.

The research team developed the imaging probe for the endoscopic micro-optical coherence tomography system based on a gradient index lens. The imaging probe was composed of a single mode fiber, a gradient index lens, a binary phase spatial filter  and a 41° gold coated mirror. The 41° gold coated mirror was used to observe the side of the probe and to reduce the direct reflection from the sample surface to avoid signal saturation. The beam from the single-mode fiber passing through the spacer was focused by the gradient index lens and reflected at 82° by the gold coated mirror and eventually guided to the sample. The used optical elements had 1mm outer diameter.

The researchers observed that the miniaturized endoscopic imaging probe achieved high resolution imaging ranging between 2.49-2.59 µm in both the axial and lateral directions respectively. The imaging depth was also noted to have been extended by a 1.91 factor. From the binary phase spatial filter pattern design and simulations, chromatic aberrations of the imaging probe was observed to induce chromatic focal shift that resulted in the further extension of the depth of focus. Effects of the broad wavelength on the extension of the depth of focus were found to be lower than 5% in a different simulation that was used to demonstrate the robustness of the design.

From the research discussed above, a novel technology that has enabled clear visualization of the microstructure of internal biological samples has been presented. High-spatial resolution of the micro-optical coherence tomography probe may in the near future enable the visualization of endothelial cell damage in vivo which is greatly related with arterial lesion formation. Micro-optical coherence tomography has also enabled a full assessment of the fibrous cap thickness of the vulnerable plaque when using the extended depth. It is therefore expected that this miniaturized endoscopic micro optical coherence tomography will be utilized in both clinical and pre-clinical studies in varying medical fields.

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About the author

Hongki Yoo is an Associate Professor in the Department of Biomedical Engineering, Hanyang University, Korea. Professor Yoo received the B.S. and Ph.D. in Mechanical Engineering from KAIST in 2001 and 2007, respectively. He has worked as a postdoctoral research fellow and an instructor in Wellman Center for Photomedicine at Harvard Medical School and Massachusetts General Hospital. He joined Hanyang University in 2012.

His research topics include multi-modal optical imaging, endoscopic imaging technology, optical coherence tomography, confocal microscopy, molecular imaging, and cardiovascular disease.  


Junyoung Kim1, Jingchao Xing1, Hyeong Soo Nam1, Joon Woo Song2, Jin Won Kim2, Hongki Yoo1. Endoscopic micro-optical coherence tomography with extended depth of focus using a binary phase spatial filter. Optics Letters Volume 42 Number 3 (February 1 2017) pages 379-382.

Show Affiliations
  1. Department of Biomedical Engineering, Hanyang University, Seoul 04763, South Korea
  2. Cardiovascular Center, Korea University Guro Hospital, Seoul 08308, South Korea


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