Time-resolved ring structure of circularly polarized beams backscattered from forward scattering media

Micro-relief characterization of benign and malignant skin lesions by polarization speckle analysis in vivo

BACKGROUND/PURPOSE

A recent direction in skin disease classification is to develop quantitative diagnostic techniques. Skin relief, colloquially known as roughness, is an important clinical feature. The aim of this study is to demonstrate a novel polarization speckle technique to quantitatively measure roughness on skin lesions in vivo. We then calculate the average roughness of different types of skin lesions to determine the extent to which polarization speckle roughness measurements can be used to identify skin cancer.

METHODS

The experimental conditions were set to target the fine relief structure on the order of ten microns within a small field of view of 3 mm. The device was tested in a clinical study on patients with malignant and benign skin lesions that resemble cancer. The cancer group includes 37 malignant melanomas (MM), 43 basal cell carcinomas (BCC), and 26 squamous cell carcinomas (SCC), all categories confirmed by gold standard biopsy. The benign group includes 109 seborrheic keratoses (SK), 79 nevi, and 11 actinic keratoses (AK). Normal skin roughness was obtained for the same patients (301 different body sites proximal to the lesion).

RESULTS

The average root mean squared (rms) roughness ± standard error of the mean for MM and nevus was equal to 19 ± 5 μm and 21 ± 3 μm, respectively. Normal skin has rms roughness of 31 ± 3 μm, other lesions have roughness of 35 ± 10 μm (AK), 35 ± 7 μm (SCC), 31 ± 4 μm (SK), and 30 ± 5 μm (BCC).

CONCLUSION

An independent-samples Kruskal-Wallis test indicates that MM and nevus can be separated from each of the tested types of lesions, except each other. These results quantify clinical knowledge of lesion roughness and could be useful for optical cancer detection.

Spatial helicity response metric to quantify particle size and turbidity of heterogeneous media through circular polarization imaging

Backscattered circularly polarized light from turbid media consists of helicity-flipped and helicity-preserved photon sub-populations (i.e., photons of perpendicular and parallel circular handedness). Their intensities and spatial distributions are found to be acutely sensitive to average scatterer size and modestly sensitive to the scattering coefficient (medium turbidity) through an interplay of single and multiple scattering effects. Using a highly sensitive intensified-CCD camera, helicity-based images of backscattered light are captured, which, with the aid of corroborating Monte Carlo simulation images and statistics, enable (1) investigation of subsurface photonic pathways and (2) development of the novel 'spatial helicity response' metric to quantify average scatterer size and turbidity of tissue-like samples. An exciting potential application of this work is noninvasive early cancer detection since malignant tissues exhibit alterations in scatterer size (larger nuclei) and turbidity (increased cell density).

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.

Role of scattering and birefringence in phase retardation revealed by locus of Stokes vector on Poincaré sphere

SIGNIFICANCE

Biological tissues are typically characterized by high anisotropic scattering and may also exhibit linear form birefringence. Both scattering and birefringence bias the phase shift between transverse electric field components of polarized light. These phase alterations are associated with particular structural malformations in the tissue. In fact, the majority of polarization-based techniques are unable to distinguish the nature of the phase shift induced by birefringence or scattering of light.

AIM

We explore the distinct contributions of scattering and birefringence in the phase retardation of circularly polarized light propagated in turbid tissue-like scattering medium.

APPROACH

The circularly polarized light in frame of Stokes polarimetry approach is used for the screening of biotissue phantoms and chicken skin samples. The change of optical properties in chicken skin is accomplished by optical clearing, which reduces scattering, and mechanical stretch, which induces birefringence. The change of optical properties of skin tissue is confirmed by spectrophotometric measurements and second-harmonic generation imaging.

RESULTS

The contributions of scattering and birefringence in the phase retardation of circularly polarized light propagated in biological tissues are distinguished by the locus of the Stokes vector mapped on the Poincaré sphere. The phase retardation of circularly polarized light due to scattering alterations is assessed. The value of birefringence in chicken skin is estimated as 0.3  ×  10^-3, which agrees with alternative studies. The change of birefringence of skin tissue due to mechanical stretch in the order of 10^-6 is detected.

CONCLUSIONS

While the polarimetric parameters on their own do not allow distinguishing the contributions of scattering and birefringence, the resultant Stokes vector trajectory on the Poincaré sphere reveals the role of scattering and birefringence in the total phase retardation. The described approach, applied independently or in combination with Mueller polarimetry, can be beneficial for the advanced characterization of various types of malformations within biological tissues.

Transient polarized radiative transfer in cloud layers: numerical simulation of imaging lidar returns

In this paper, we investigate the time-dependent backscattering halo of a pulsed light beam in a layer of scattering medium. We start from numerical simulations of polarized radiative transfer in the layer, which immediately reveals the effect under investigation. Then we analyze time-dependent structure of the light field using the simulation results. From the radiation field, we extract two principal components, immediately forming the halo structure. For each diffuse and ballistic component, we use the proper theoretical model, yielding a convenient analytic description of the time-dependent behavior of the radiation field. From the theory developed by us, we derive a simple numerical criterion of visibility of the halo. Finally, we validate our theory against Monte Carlo radiative transfer simulations. Thus, we propose a quantitative explanation of the studied effect.

Electric field Monte Carlo simulations of focal field distributions produced by tightly focused laser beams in tissues

The focal field distribution of tightly focused laser beams in turbid media is sensitive to optical scattering and therefore of direct relevance to image quality in confocal and nonlinear microscopy. A model that considers both the influence of scattering and diffraction on the amplitude and phase of the electric field in focused beam geometries is required to describe these distorted focal fields. We combine an electric field Monte Carlo approach that simulates the electric field propagation in turbid media with an angular-spectrum representation of diffraction theory to analyze the effect of tissue scattering properties on the focal field. In particular, we examine the impact of variations in the scattering coefficient (µ(s)), single-scattering anisotropy (g), of the turbid medium and the numerical aperture of the focusing lens on the focal volume at various depths. The model predicts a scattering-induced broadening, amplitude loss, and depolarization of the focal field that corroborates experimental results. We find that both the width and the amplitude of the focal field are dictated primarily by µ(s) with little influence from g. In addition, our model confirms that the depolarization rate is small compared to the amplitude loss of the tightly focused field.

Contrast improvement by selecting ballistic-photons using polarization gating

In this paper a new approach to improve contrast in optical subsurface imaging is presented. The method is based on time-resolved reflectance and selection of ballistic photons using polarization gating. Numerical studies with a statistical Monte Carlo method also reveal that weakly scattered diffuse photons can be eliminated by employing a small aperture and that the contrast improvement strongly depends on the single-scattering phase function. A possible experimental setup is discussed in the conclusions.

On the accuracy of generalized Fokker-Planck transport equations in tissue optics

Forward-peaked and large-angle scattering approximations of the radiative transport equation give rise to generalized Fokker-Planck equations whose main feature is the replacement of the integral scattering operator with differential operators in the direction-space variables. Using the P(N) method, an appraisal of generalized Fokker-Planck equations due to González-Rodríguez and Kim [Appl. Opt.47, 2599-2609 (2008)], Leakeas and Larsen [Nucl. Sci. Eng.137, 236-250 (2001), and J. Opt. Soc. Am. A20, 92-98 (2003)], and Pomraning [Math. Models Meth. Appl. Sci.2, 21-36 (1992)] is carried out by computing the relative error between the backscattered and transmitted surface flux predicted by the generalized Fokker-Planck equations and the transport equation with Henyey-Greenstein phase function for anisotropies ranging from 0 to 1. Generalized Fokker-Planck equations whose scattering operators incorporate large-angle scattering and possess eigenvalues similar to the integral scattering operator with Henyey-Greenstein phase function are found to minimize the relative error in the limit of unit anisotropy.

Low-coherence enhanced backscattering beyond diffusion

An analytical theory for coherent backscattering (CBS) of low-coherence light is presented. An expression linking the CBS profile to the radial distribution of the incoherent backscattered light is derived when the incident light is partially spatially coherent. The backscattered snake light, which has experienced exactly two large-angle scatterings, is taken into account together with the diffuse light in the analysis. Monte Carlo simulations demonstrate that the model describes well the CBS profile as long as the spatial coherence length, L(c), of the incident beam is larger than the scattering mean free path of light in the medium. The intensity of the enhanced backscattered light in the exact backscattering direction and the width of the CBS cone are found to be proportional to L(c) and L(c)(-1), respectively, in the limit of small L(c).

Impact of large-angle scattering on diffusively backscattered halos

We discuss the impact of large-angle scattering events in highly forward-scattering media on the spatial distribution of the diffusively reflected light. We show that, even for highly forward-scattering media, the reflected light near the incident beam axis is strongly dependent on the small number of large-angle scattering events. Reliable modeling of near-axis reflection thus requires accurate knowledge of the scattering phase function's behavior at large angles.

Analytical cumulant solution of the vector radiative transfer equation investigates backscattering of circularly polarized light from turbid media

The backscattering of circularly polarized light pulses from an infinite uniform scattering medium is studied as a function of helicity of the incident light and size of scatterers in the medium. The approach considers a polarized short pulse of light incident on the scattering medium, and uses an analytical cumulant solution of the vector radiative transfer equation with the phase matrix obtained from the Mie theory to calculate the temporal profile of scattered polarized photons for any position and any angle of detection. The general expression for the scattered photon distribution function is an expansion in spatial cumulants up to an arbitrary high order. Truncating the expansion at the second-order cumulant, a Gaussian analytical approximate expression for the temporal profile of scattered polarized photons is obtained, whose average center position and half width are always exact. The components of scattered light copolarized and cross polarized with that of the incident light can be calculated and used for determining the degree of polarization of the scattered light. The results show that circularly polarized light of the same helicity dominates the backscattered signal when scatterer size is larger than the wavelength of light. For the scatterers smaller than the wavelength, the light of opposite helicity makes the dominant contribution to the backscattered signal. The theoretical estimates are in good agreement with our experimental results.