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Medical Photonics

ABSTRACT

Endoscope is a powerful tool for medical diagnosis and procedures, particularly in observing small curved tracts and conducting minimally invasive surgery (MIS). In MIS, small incisions reduce the time of wound healing, associated pain, and the risk of infection, whose size depends on the diameter of an endoscope. 


In our laboratory, we have successfully established a novel ultra-compact endoscopic imaging system, which uses a miniature CMOS sensor (O.D. ~1.4mm) and a multimode fiber (ϕ ~ 400μm) for light delivery. Critically, the illumination is realized by coupling the output of a supercontinuum into a multimode fiber. As a result, the overall diameter (~2.5 mm) of the endoscope is smaller than the currently used models in endoscopy. 


To uniformly illuminate the target within the field of view of the CMOS sensor, a miniature diffuser is prepared and attached to the tip of the light guide fiber to broaden the illumination cone from 10° to more than 70°, which is sufficient to cover the field of view (FOV) of generally used endoscopes. Additionally, bandpass filters are used to reduce the laser power and adjust the spectrum to optimize the illumination conditions. The illuminance, optical power, and image quality of the supercontinuum are also characterized and benchmarked with standard medical white LED.


The illumination parameters of supercontinuum can reach CRI~72% with CCT=5200K from CRI~97% with CCT = 3100K, when driving level is set at 95% of the maximum value. Note that white LEDs have CRI~76% with CCT=6500K. Many in vivo images are also obtained with the ultra-slim endoscope, showing supercontinuum as a viable light source for future endoscopy.


Recently, we are implementing a RGB laser module for such a purpose, which is far more cost effective than a supercontinuum one. RGB laser system can offer basic white light and combine or change different color easily to capture special image such as narrow band image which use to observe detail of mucosa. We have also developed efficient techniques to remove the presence of speckle effects due to the very different spectral profile of RGB laser.


PRINCIPLE 

Schematic of fiber illumination

EXPERIMENTAL SETUP 


The schematic on the setup of supercontinuum illumination through fiber coupling. PA is the test point for recording the illuminance, illumination angle, CRI, CCT, and optical power. 'TA Color checker' and 'Heart of Nude mice' represent the specimen and the experimental animal (nude mice) used for imaging characterization, respectively.

APPLICATION 

The nude mice’ thoracic cavity (top) and the color checker illuminated by (a) Supercontinuum (SC) without diffuser (b) SC with diffuser but without  filter (c) SC with both diffuser and filter (d) White LED lamp

Related publications 

  1. M. K. Lu, H. Y. Lin, C. C. Hsieh, and Fu-Jen KaoSupercontinuum as a light source for miniaturized endoscopes, Biomedical Optics Express, Vol. 7, Issue 9, pp. 3335-3344 (2016) (doi: 10.1364/BOE.7.003335).