A high definition Mueller polarimetric endoscope for tissue characterisation.

Ji Qi,Daniel S Elson

doi:10.1038/srep25953

Abstract

The contrast mechanism of medical endoscopy is mainly based on metrics of optical intensity and wavelength. As another fundamental property of light, polarization can not only reveal tissue scattering and absorption information from a different perspective, but can also provide insight into directional tissue birefringence properties to monitor pathological changes in collagen and elastin. Here we demonstrate a low cost wide field high definition Mueller polarimetric endoscope with minimal alterations to a rigid endoscope. We show that this novel endoscopic imaging modality is able to provide a number of image contrast mechanisms besides traditional unpolarized radiation intensity, including linear depolarization, circular depolarization, cross-polarization, directional birefringence and dichroism. This enhances tissue features of interest, and additionally reveals tissue micro-structure and composition, which is of central importance for tissue diagnosis and image guidance for surgery. The potential applications of the Mueller polarimetric endoscope include wide field early epithelial cancer diagnosis, surgical margin detection and energy-based tissue fusion monitoring, and could further benefit a wide range of endoscopic investigations through intra-operative guidance.

Highlights

Endoscopes are pivotal devices for non-invasive diagnostic investigation and minimally invasive image guided surgery, and novel image contrast mechanisms that can improve tissue characterization as well as specificity and sensitivity of detection are highly desirable
By imaging birefringent tissue mimicking phantoms and a porcine bladder, we show that this novel endoscopic imaging modality is able to provide a number of image contrast mechanisms besides traditional unpolarized radiation intensity, including linear depolarization, circular depolarization, cross-polarization, directional birefringence and dichroism
Besides a light source and an image sensor, a Mueller polarimetric imaging system consists of a polarization state generator (PSG) to generate the required SOPs of incident light corresponding to minimally four linear independent Stokes vectors and a polarization state analyser (PSA) to analyse SOPs of emergent light

Summary

Introduction

Endoscopes are pivotal devices for non-invasive diagnostic investigation and minimally invasive image guided surgery, and novel image contrast mechanisms that can improve tissue characterization as well as specificity and sensitivity of detection are highly desirable. Biological tissue is highly scattering and incident polarized photons that are singly scattered or undergo a small number of scattering events near the tissue surface maintain this polarization state, while those multiply scattered penetrate deeper and are depolarized This principle has been applied to select single-scattering spectra from multiply-scattered backgrounds for cell nucleus and cytoplasm size characterization[1] and for in situ in vivo early cancer diagnosis[2]. A few endoscopes that can resolve linear polarization states were designed and demonstrated[2,5,6] by using a pair of parallel and orthogonal linear polarizers for detection with respect to illumination polarization This type of approach can only be applied to isotropic tissues that do not demonstrate any birefringence, dichroism or anisotropic linear depolarization, because linear depolarization images for anisotropic tissues are dependent on the sample orientation and the incident polarization direction resulting in bad reproducibility. Mueller polarimetric imaging has not become an endoscopic imaging technique, to the best of our knowledge, due to the lack of a practical and clinically feasible design

Methods

Results

Conclusion