Impact of scattering phase function and polarization on the accuracy of diffuse and sub-diffuse spatial frequency domain imaging.

Significance: Although spatial frequency domain imaging (SFDI) has been well characterized under diffuse optical conditions, tissue measurements made outside the diffuse regime can provide new diagnostic information. Before such measurements can become clinically relevant, however, the behavior of sub-diffuse SFDI and its effect on the accuracy of derived tissue parameters must be assessed. Aim: We aim to characterize the impact that both the assumed scattering phase function (SPF) and the polarization state of the illumination light source have on the accuracy of SFDI-derived optical properties when operating under diffuse or sub-diffuse conditions, respectively. Approach: Through the use of a set of well-characterized optical phantoms, SFDI accuracy was assessed at four wavelengths (395, 545, 625, and 850 nm) and two different spatial frequencies (0.3 and 1.0 mm - 1 ), which provided a broad range of diffuse and sub-diffuse conditions, using three different SPFs. To determine the effects of polarization, the SFDI accuracy was assessed using both unpolarized and cross-polarized illumination. Results: It was found that the assumed SPF has a direct and significant impact on the accuracy of the SFDI-derived optical properties, with the best choice of SPF being dictated by the polarization state. As unpolarized SFDI retains the sub-diffuse portion of the signal, optical properties were found to be more accurate when using the full SPF that includes forward and backscattering components. By contrast, cross-polarized SFDI yielded accurate optical properties when using a forward-scattering SPF, matching the behavior of cross-polarization to attenuate the immediate backscattering of sub-diffuse reflectance. Using the correct pairings of SPF and polarization enabled using a reflectance standard, instead of a more subjective phantom, as the reference measurement. Conclusions: These results provide the foundation for a more thorough understanding of SFDI and enable new applications of this technology in which sub-diffuse conditions dominate (e.g., µ a ≮ µ s ' ) or high spatial frequencies are required.

Development of a Low-Cost Paper-Based Platform for Coffee Ring-Assisted SERS.

The need for highly sensitive, low-cost, and timely diagnostic technologies at the point of care is increasing. Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique that is an advantageous technique to address this need, as it can rapidly detect analytes in small or dilute samples with improved sensitivity compared to conventional Raman spectroscopy. Despite the many advantages of SERS, one drawback of the technique is poor reproducibility due to variable interactions between nanoparticles and target analytes. To overcome this limitation, coupling SERS with the coffee ring effect has been implemented to concentrate and localize analyte-nanoparticle conjugates for improved signal reproducibility. However, current coffee ring platforms require laborious fabrication steps. Herein, we present a low-cost, two-step fabrication process for coffee ring-assisted SERS, utilizing wax-printed nitrocellulose paper. The platform was designed to produce a highly hydrophobic paper substrate that supports the coffee ring effect and tested using gold nanoparticles for SERS sensing. The nanoparticle concentration and solvent were varied to determine the effect of solution composition on ring formation and center clearance. The SERS signal was validated using 4-mercaptobenzoic acid (MBA) and tested with Moraxella catarrhalis bacteria to ensure functionality for chemical and biological applications. The limit of detection using MBA is 41.56 nM, and the biochemical components of the bacterial cell wall were enhanced with low spectral variability. The developed platform is advantageous due to ease of fabrication and use, representing the next step toward implementing low-cost coffee ring-assisted SERS for point-of-care sensing.

Measurement of rat and human tissue optical properties for improving the optical detection and visualization of peripheral nerves.

Peripheral nerve damage frequently occurs in challenging surgical cases resulting in high costs and morbidity. Various optical techniques have proven effective in detecting and visually enhancing nerves, demonstrating their translational potential for assisting in nerve-sparing medical procedures. However, there is limited data characterizing the optical properties of nerves in comparison to surrounding tissues, thus limiting the optimization of optical nerve detection systems. To address this gap, the absorption and scattering properties of rat and human nerve, muscle, fat, and tendon were determined from 352-2500 nm. The optical properties highlighted an ideal region in the shortwave infrared for detecting embedded nerves, which remains a significant challenge for optical approaches. A 1000-1700 nm hyperspectral diffuse reflectance imaging system was used to confirm these results and identify optimal wavelengths for nerve imaging contrast in an in vivo rat model. Optimal nerve visualization contrast was achieved using 1190/1100 nm ratiometric imaging and was sustained for nerves embedded under ≥600 µm of fat and muscle. Overall, the results provide valuable insights for optimizing the optical contrast of nerves, including those embedded in tissue, which could lead to improved surgical guidance and nerve-sparing outcomes.

Development of a platform for broadband, spectra-fitted, tissue optical phantoms.

Significance: Current methods of producing optical phantoms are incapable of accurately capturing the wavelength-dependent properties of tissue critical for many optical modalities. Aim: We aim to introduce a method of producing solid, inorganic phantoms whose wavelength-dependent optical properties can be matched to those of tissue over the wavelength range of 370 to 950 nm. Approach: The concentration-dependent optical properties of 20 pigments were characterized and used to determine combinations that result in optimal fits compared to the target properties over the full spectrum. Phantoms matching the optical properties of muscle and nerve, the diffuse reflectance of pale and melanistic skin, and the chromophore concentrations of a computational skin model with varying oxygen saturation ( StO 2 ) were made with this method. Results: Both optical property phantoms were found to accurately mimic their respective tissues' absorption and scattering properties across the entire spectrum. The diffuse reflectance phantoms were able to closely approximate skin reflectance regardless of skin type. All three computational skin phantoms were found to have emulated chromophore concentrations close to the model, with an average percent error for the StO 2 of 4.31%. Conclusions: This multipigment phantom platform represents a powerful tool for creating spectrally accurate tissue phantoms, which should increase the availability of standards for many optical techniques.

Simultaneous Exposure to Intracellular and Extracellular Photosensitizers for the Treatment of Staphylococcus aureus Infections.

Staphylococcus aureus is a serious threat to public health due to the rise of antibiotic resistance in this organism, which can prolong or exacerbate skin and soft tissue infections (SSTIs). Methicillin-resistant S. aureus is a Gram-positive bacterium and a leading cause of SSTIs. As such, many efforts are under way to develop therapies that target essential biological processes in S. aureus. Antimicrobial photodynamic therapy is an effective alternative to antibiotics; therefore we developed an approach to simultaneously expose S. aureus to intracellular and extracellular photosensitizers. A near infrared photosensitizer was conjugated to human monoclonal antibodies (MAbs) that target the S. aureus iron-regulated surface determinant (Isd) heme acquisition proteins. In addition, the compound VU0038882 was developed to increase photoactivatable porphyrins within the cell. Combinatorial photodynamic treatment of drug-resistant S. aureus exposed to VU0038882 and conjugated anti-Isd MAbs proved to be an effective antibacterial strategy in vitro and in a murine model of SSTIs.

Optimization of optical parameters for improved photodynamic therapy of Staphylococcus aureus using endogenous coproporphyrin III.

BACKGROUND: It has recently been shown that endogenous photosensitization of Gram-positive bacteria is achieved through the accumulation of the heme precursor coproporphyrin III and not protoporphyrin IX, as was previously assumed. As previous studies have operated under this assumption, the efficacy of optimal targeting of the absorption peaks of coproporphyrin III has not been explored. METHODS: Staphylococcus aureus was endogenously photosensitized through the addition of either the small molecule VU0038882, aminolevulinic acid, or both. The efficacy of five different LEDs whose wavelengths target different coproporphyrin III absorption peaks were determined in vitro. Based on these in vitro measurements, the effectiveness of utilizing these LEDs to treat a skin infection was predicted using a Monte Carlo simulation to estimate the fluence rates and resulting bacterial reductions as a function of depth. RESULTS: Optimal targeting of the Soret band provided a 4.7-log improvement as compared to previously utilized wavelengths. Activation of the Q-bands was found to provide similar cytotoxic effects but required significantly larger doses of light. Despite near sterilization in vitro, it was predicted that Soret band targeted light would only provide at least a 2 log-reduction up to 430 µm into the skin while Q-band targeted light could remain effective up to 1 mm in depth. Multiplexing these different wavelengths was found to provide a further 0.5-1.0 log-reduction in bacterial viability. CONCLUSIONS: Accurate targeting of coproporphyrin III has shown that endogenous photodynamic therapy has the potential to be further developed into an effective treatment of skin and soft tissue infections caused by Gram-positive bacteria.