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

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. We present an ultra-miniature SFDI system comprised of an optical fiber array (diameter $0.125$ mm) and a micro camera ($1\times1$ mm package) displacing conventionally bulky components, in particular the projector. The prototype has outer diameter $3$ mm, but the individual components dimensions could permit future packaging to $<1.5$ mm diameter. We develop a phase-tracking algorithm to rapidly extract images with fringe projections at $3$ equispaced phase shifts in order to perform SFDI demodulation. To validate performance, we first demonstrate comparable recovery of quantitative optical properties between our ultra-miniature system and a conventional bench-top SFDI system with 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 $515$ nm and $660$ nm. This device shows promise as a cost-effective, quantitative imaging tool to detect variations in optical absorption and scattering as indicators of cancer.