Enhancing Imaging Depth and Sensitivity in Reflectance Mode Near Infrared Optical Imaging with Scatter Reducing Agents

TL;DR

The paper tackles the depth-sensitivity limitation of CW NIR diffuse optical imaging caused by strong scattering. It proposes Tartrazine-induced optical clearing to create a superficial refractive-index gradient and tests this with layered chicken tissue and uniform intralipid phantoms, supported by diffusion-based analytical models and Monte Carlo simulations. Findings show that reducing superficial scattering redistributes photon trajectories toward deeper tissue, enhancing interaction with subsurface absorbers and improving weak-signal detectability by up to about fivefold at depths around $2$–$3\ \mathrm{cm}$, without increasing total throughput. The results suggest a practical, low-complexity method to extend CW NIRS depth sensitivity and motivate improved layered phantoms and in vivo validation for deep-tissue sensing applications.

Abstract

We investigate the role of scatter reducing agents in a continuous wave (CW) near infrared (NIR)reflectance mode imaging setting. We use food-grade dye Tartrazine as a scatter reducing agent to enhance depth sensitivity and weak-absorber detectability in CW diffuse reflectance measurements. We found that reflectance signal was enhanced when the dye was applied on chicken breast phantom. However, we saw reduced reflectance sensitivity when the dye was uniformly dissolved in intralipid phantom which is a commonly used for NIR imaging studies. This shows that the gradient of refractive index modulation created as the dye diffuses from the top layer allows increased reflectance signal sensitivity of optical photons. However, when the scatter reduction is uniform throughout the phantom (like in intralipid phantom), the improved reflectance sensitivity was not observed. Our study points to significant redistribution of photons with scatter modulation with Tartrazine dye. We show significant improvement in sensitivity to signals with reflectance imaging. To elucidate the underlying mechanism of dye induced scatter reduction in tissue, analytical diffusion models and Monte Carlo simulations were employed. Modeling results show the impact of refractive index gradient created due to dye diffusion in enhancing reflectance sensitivity. These findings demonstrate that dye induced scatter reduction provides a practical, low-complexity approach to improving depth sensitivity in CW diffuse reflectance measurements and extend the functional capabilities of CW-NIRS systems for deep-tissue sensing applications. Our preliminary studies shows up to five fold enhancement in signal sensitivity for signals between two and three cm depth.

Figures (10)

• Figure 1: Conceptual illustration of Tartrazine-induced optical clearing. Topical application reduces superficial scattering and redistributes photons toward deeper tissue represented by the pink region, enhancing subsurface signal detection.
• Figure 2: Schematic of the experimental setup for CW diffuse reflectance measurements. A syringe pump delivers controlled ink injections to create a subsurface absorbing inclusion within the tissue phantom. Optodes are placed on the phantom surface to record diffuse reflectance intensity before, during, and after ink infusion, enabling assessment of depth-dependent signal detectability under varying optical clearing conditions.
• Figure 3: Layered tissue model used in Monte Carlo simulations before and after optical clearing.
• Figure 4: Optical property changes induced by Tartrazine in a silica particle suspension (1--5 $\mu$m) used as a controlled scattering medium. (a) Transmittance spectra, (b) molar absorption spectra, and (c) refractive-index shift as a function of wavelength for increasing Tartrazine concentrations. The systematic refractive-index increase across the visible--NIR range leads to reduced scattering despite absorption features at shorter wavelengths.
• Figure 5: CW diffuse reflectance measurements at a 6 cm source--detector separation for (a) chicken tissue phantom and (b) uniform intralipid phantom under no-dye and increasing dye layer conditions. Left panels show time-domain voltage traces during ink infusion; right panels show the corresponding extracted signal contrast.