Designing and simulating realistic spatial frequency domain imaging systems using open-source 3D rendering software

Abstract

Spatial frequency domain imaging (SFDI) is a low-cost imaging technique that maps absorption and reduced scattering coefficients, offering improved contrast for important tissue structures such as tumours. Practical SFDI systems must cope with various imaging geometries including imaging planar samples ex vivo, imaging inside tubular lumen in vivo e.g. for endoscopy, and measuring tumours or polyps of varying morphology. There is a need for a design and simulation tool to accelerate design of new SFDI systems and simulate realistic performance under these scenarios. We present such a system implemented using open-source 3D design and ray-tracing software Blender that simulates media with realistic absorption and scattering in a wide range of geometries. By using Blender’s Cycles ray-tracing engine, our system simulates effects such as varying lighting, refractive index changes, non-normal incidence, specular reflections and shadows, enabling realistic evaluation of new designs. We first demonstrate quantitative agreement between Monte-Carlo simulated absorption and reduced scattering coefficients with those simulated from our Blender system, achieving 16% discrepancy in absorption coefficient and 18% in reduced scattering coefficient. However, we then show that using an empirically derived look-up table the errors reduce to 1% and 0.7% respectively. Next, we simulate SFDI mapping of absorption, scattering and shape for simulated tumour spheroids, demonstrating enhanced contrast. Finally we demonstrate SFDI mapping inside a tubular lumen, which highlighted a important design insight: custom look-up tables must be generated for different longitudinal sections of the lumen. With this approach we achieved 2% absorption error and 2% scattering error. We anticipate our simulation system will aid in the design of novel SFDI systems for key biomedical applications.

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

“…Because the absorption is consistently underestimated, this may be rectified using additional calibration steps, for example, phantom-based LUT generation 39 or realistic simulation of the fiber imaging setup. 32 …”

“…The noise has been partially addressed by varying the spatial frequency but could be further improved by normalizing for laser power, using AI 45 for noise-reduction reconstruction or building custom LUTs based on non-ideal projection patterns. 32 The underestimation of absorption at larger absorption levels can be up to 60%, though this too may be reduced by alternative calibration procedures. 39 This does not appear to significantly impact the quantitative maps of tissue produced and, more importantly, still permits a suitably high theoretical sensitivity and specificity.…”

“…This angle is smaller than conventional SFDI systems, 6 but it is more amenable to miniaturization. Previous work has shown that this angle can still produce high quality optical property maps 32 …”