Monte Carlo model of the penetration depth for polarization gating spectroscopy: influence of illumination-collection geometry and sample optical properties

Interactions of Linearly Polarized and Unpolarized Light on Kiwifruit Using Aquaphotomics

Near infrared (NIR) spectroscopy is an important tool for predicting the internal qualities of fruits. Using aquaphotomics, spectral changes between linearly polarized and unpolarized light were assessed on 200 commercially grown yellow-fleshed kiwifruit (Actinidia chinensis var. chinensis ‘Zesy002’). Measurements were performed on different configurations of unpeeled (intact) and peeled (cut) kiwifruit using a commercial handheld NIR instrument. Absorbance after applying standard normal variate (SNV) and second derivative Savitzky–Golay filters produced different spectral features for all configurations. An aquagram depicting all configurations suggests that linearly polarized light activated more free water states and unpolarized light activated more bound water states. At depth (≥1 mm), after several scattering events, all radiation is expected to be fully depolarized and interactions for incident polarized or unpolarized light will be similar, so any observed differences are attributable to the surface layers of the fruit. Aquagrams generated in terms of the fruit soluble solids content (SSC) were similar for all configurations, suggesting the SSC in fruit is not a contributing factor here.

Polarimetric imaging in backscattering for the structural characterization of strongly scattering birefringent fibrous media

Interpreting the polarimetric data from fiber-like macromolecules constitutive of tissue can be difficult due to strong scattering. In this study, we probed the superficial layers of fibrous tissue models (membranes consisting of nanofibers) displaying varying degrees of alignment. To better understand the manifestation of membranes' degree of alignment in polarimetry, we analyzed the spatial variations of the backscattered light's Stokes vectors as a function of the orientation of the probing beam's linear polarization. The degree of linear polarization reflects the uniaxially birefringent behavior of the membranes. The rotational (a-)symmetry of the backscattered light's degree of linear polarization provides a measure of the membranes' degree of alignment.

Study of polarization memory's impact on detection range in natural water fogs

The influence of the initial polarization state of a source on the detection range of a system probing through natural dense water fog is analyzed. Information about the source is conveyed by ballistic, snake, and highly scattered photons. During propagation, the polarization state of ballistic and snake photons is not altered. It is shown that though circular polarization is not altered by simple direction changes during scattering, and has thus a tendency to be preserved longer in the highly scattered photons, it does not necessarily convey more useful information about the source than linear polarization or even an unpolarized beam. It is also shown that in any forward propagating system that can be described by the small-angle approximation the impact of polarization memory can be neglected.

Experimental study on the sensitive depth of backwards detected light in turbid media

In the recent past, optical spectroscopy and imaging methods for biomedical diagnosis and target enhancing have been widely researched. The challenge to improve the performance of these methods is to know the sensitive depth of the backwards detected light well. Former research mainly employed a Monte Carlo method to run simulations to statistically describe the light sensitive depth. An experimental method for investigating the sensitive depth was developed and is presented here. An absorption plate was employed to remove all the light that may have travelled deeper than the plate, leaving only the light which cannot reach the plate. By measuring the received backwards light intensity and the depth between the probe and the plate, the light intensity distribution along the depth dimension can be achieved. The depth with the maximum light intensity was recorded as the sensitive depth. The experimental results showed that the maximum light intensity was nearly the same in a short depth range. It could be deduced that the sensitive depth was a range, rather than a single depth. This sensitive depth range as well as its central depth increased consistently with the increasing source-detection distance. Relationships between sensitive depth and optical properties were also investigated. It also showed that the reduced scattering coefficient affects the central sensitive depth and the range of the sensitive depth more than the absorption coefficient, so they cannot be simply added as reduced distinct coefficients to describe the sensitive depth. This study provides an efficient method for investigation of sensitive depth. It may facilitate the development of spectroscopy and imaging techniques for biomedical diagnosis and underwater imaging.

Effects of collection geometry variations on linear and circular polarization persistence in both isotropic-scattering and forward-scattering environments

We present simulation and experimental results showing circular polarization is more tolerant of optical collection geometry (field of view and collection area) variations than linear polarization for forward-scattering environments. Circular polarization also persists superiorly in the forward-scattering environment compared to linear polarization by maintaining its degree of polarization better through increasing optical thicknesses. In contrast, both linear and circular polarizations are susceptible to collection geometry variations for isotropic-scattering (Rayleigh regime) environments, and linear polarization maintains a small advantage in polarization persistence. Simulations and measurements are presented for laboratory-based environments of polystyrene microspheres in water. Particle diameters were 0.0824 μm (for isotropic-scattering) and 1.925 μm (for forward-scattering) with an illumination wavelength of 543.5 nm.

Development of perturbation Monte Carlo methods for polarized light transport in a discrete particle scattering model

We present a polarization-sensitive, transport-rigorous perturbation Monte Carlo (pMC) method to model the impact of optical property changes on reflectance measurements within a discrete particle scattering model. The model consists of three log-normally distributed populations of Mie scatterers that approximate biologically relevant cervical tissue properties. Our method provides reflectance estimates for perturbations across wavelength and/or scattering model parameters. We test our pMC model performance by perturbing across number densities and mean particle radii, and compare pMC reflectance estimates with those obtained from conventional Monte Carlo simulations. These tests allow us to explore different factors that control pMC performance and to evaluate the gains in computational efficiency that our pMC method provides.

Fiber-bundle microendoscopy with sub-diffuse reflectance spectroscopy and intensity mapping for multimodal optical biopsy of stratified epithelium

Early detection of structural or functional changes in dysplastic epithelia may be crucial for improving long-term patient care. Recent work has explored myriad non-invasive or minimally invasive "optical biopsy" techniques for diagnosing early dysplasia, such as high-resolution microendoscopy, a method to resolve sub-cellular features of apical epithelia, as well as broadband sub-diffuse reflectance spectroscopy, a method that evaluates bulk health of a small volume of tissue. We present a multimodal fiber-based microendoscopy technique that combines high-resolution microendoscopy, broadband (450-750 nm) sub-diffuse reflectance spectroscopy (sDRS) at two discrete source-detector separations (374 and 730 μm), and sub-diffuse reflectance intensity mapping (sDRIM) using a 635 nm laser. Spatial resolution, magnification, field-of-view, and sampling frequency were determined. Additionally, the ability of the sDRS modality to extract optical properties over a range of depths is reported. Following this, proof-of-concept experiments were performed on tissue-simulating phantoms made with poly(dimethysiloxane) as a substrate material with cultured MDA-MB-468 cells. Then, all modalities were demonstrated on a human melanocytic nevus from a healthy volunteer and on resected colonic tissue from a murine model. Qualitative in vivo image data is correlated with reduced scattering and absorption coefficients.

Monte Carlo model of the depolarization of backscattered linearly polarized light in the sub-diffusion regime

We present a predictive model of the depolarization ratio of backscattered linearly polarized light from spatially continuous refractive index media that is applicable to the sub-diffusion regime of light scattering. Using Monte Carlo simulations, we derived a simple relationship between the depolarization ratio and both the sample optical properties and illumination-collection geometry. Our model was validated on tissue simulating phantoms and found to be in good agreement. We further show the utility of this model by demonstrating its use for measuring the depolarization length from biological tissue in vivo. We expect our results to aid in the interpretation of the depolarization ratio from sub-diffusive reflectance measurements.

Effect of probe geometry and optical properties on the sampling depth for diffuse reflectance spectroscopy

The sampling depth of light for diffuse reflectance spectroscopy is analyzed both experimentally and computationally. A Monte Carlo (MC) model was used to investigate the effect of optical properties and probe geometry on sampling depth. MC model estimates of sampling depth show an excellent agreement with experimental measurements over a wide range of optical properties and probe geometries. The MC data are used to define a mathematical expression for sampling depth that is expressed in terms of optical properties and probe geometry parameters.

Algorithm for automated selection of application-specific fiber-optic reflectance probes

Several optical techniques and fiber-optic probe systems have been designed to measure the optical properties of tissue. While a wide range of options is often beneficial, it poses a problem to investigators selecting which method to use for their biomedical application of interest. We present a methodology to optimally select a probe that matches the application requirements. Our method is based both on matching a probe's mean sampling depth with the optimal diagnostic depth of the clinical application and on choosing a probe whose interrogation depth and path length is the least sensitive to alterations in the target medium's optical properties. Satisfying these requirements ensures that the selected probe consistently assesses the relevant tissue volume with minimum variability. To aid in probe selection, we have developed a publicly available graphical user interface that takes the desired sampling depth and optical properties of the medium as its inputs and automatically ranks different techniques in their ability to robustly target the desired depth. Techniques investigated include single fiber spectroscopy, differential path length spectroscopy, polarization-gating, elastic light scattering spectroscopy, and diffuse reflectance. The software has been applied to biological case studies.