Ultra-miniature dual-wavelength spatial frequency domain imaging for micro-endoscopy

Abstract

Significance: There is a need for a cost-effective, quantitative imaging tool that can be deployed endoscopically to better detect early stage gastrointestinal cancers. Spatial frequency domain imaging (SFDI) is a low-cost imaging technique that produces near-real time, quantitative maps of absorption and reduced scattering coefficients, but most implementations are bulky and suitable only for use outside the body.

Aim: We aim to develop an ultra-miniature SFDI system comprising an optical fiber array (diameter 0.125 mm) and a micro camera ($1\times 1\mathrm{mm}$ package) to displace conventionally bulky components, in particular, the projector.

Approach: First, we fabricated a prototype with an outer diameter of 3 mm, although the individual component dimensions could permit future packaging to a $<1.5\mathrm{mm}$ diameter. We developed a phase-tracking algorithm to rapidly extract images with fringe projections at three equispaced phase shifts to perform SFDI demodulation.

Results: To validate the performance, we first demonstrate comparable recovery of quantitative optical properties between our ultra-miniature system and a conventional bench-top SFDI system with an agreement of 15% and 6% for absorption and reduced scattering, respectively. Next, we demonstrate imaging of absorption and reduced scattering of tissue-mimicking phantoms providing enhanced contrast between simulated tissue types (healthy and tumour), done simultaneously at wavelengths of 515 and 660 nm. Using a support vector machine classifier, we estimate that sensitivity and specificity values of $>90\%$ are feasible for detecting simulated squamous cell carcinoma.

Conclusions: This device shows promise as a cost-effective, quantitative imaging tool to detect variations in optical absorption and scattering as indicators of cancer.

Keywords: miniaturization; optical fibers; optical properties; spatial frequency domain imaging.