Origin of low-coherence enhanced backscattering

Coherent backscattering cone shape depends on the beam size

Coherent backscattering (CBS) is a beautiful physical phenomenon that takes place in a highly scattering medium, which has potential application in noninvasive optical property measurement. The current model that explains the CBS cone shape, however, assumes the incoming beam diameter is infinitely large compared to the transport length. In this paper, we evaluate the effect of a finite scalar light illumination area on the CBS cone, both theoretically and experimentally. The quantitative relationship between laser beam size and the CBS cone shape is established by using two different finite beam models (uniform top hat and Gaussian distribution). A series of experimental data with varying beam diameters is obtained for comparison with the theory. Our study shows the CBS cone shape begins to show distortion when beam size becomes submillimeter, and this effect should not be ignored in general. In biological tissue where a normal large beam CBS cone is too narrow for detection, this small beam CBS may be more advantageous for more accurate and higher resolution tissue characterization.

Numerical investigation on polarization characteristics of coherent enhanced backscattering using SLPSTD

We investigate the polarization characteristics of coherent enhanced backscattering (EBS) using the pseudo-spectral time domain method implemented on staggered grid and local Fourier basis (SLPSTD) [Opt. Express 18, 9236 (2010)]. The studies are focused on Mie scatterers with findings profound to the understanding of polarization evolution in the scattering process. For linear polarization studies, the low-order scattering component of EBS is azimuthally anisotropic. A relationship between the degree of anisotropy and the photon's penetration depth is established to characterize the depolarization progress. For circular polarization, exact numerical solutions disclose the origin of polarization memory effect and the helicity-flipping phenomenon. The region responsible for helicity-flipping is identified. Our numerical technique can be potentially applied to subsurface imaging that explores polarization memory effect.

Microscopic imaging and spectroscopy with scattered light

Optical contrast based on elastic scattering interactions between light and matter can be used to probe cellular structure, cellular dynamics, and image tissue architecture. The quantitative nature and high sensitivity of light scattering signals to subtle alterations in tissue morphology, as well as the ability to visualize unstained tissue in vivo, has recently generated significant interest in optical-scatter-based biosensing and imaging. Here we review the fundamental methodologies used to acquire and interpret optical scatter data. We report on recent findings in this field and present current advances in optical scatter techniques and computational methods. Cellular and tissue data enabled by current advances in optical scatter spectroscopy and imaging stand to impact a variety of biomedical applications including clinical tissue diagnosis, in vivo imaging, drug discovery, and basic cell biology.

Enhancement factor in low-coherence enhanced backscattering and its applications for characterizing experimental skin carcinogenesis

We experimentally study potential mechanisms by which the enhancement factor in low-coherence enhanced backscattering (LEBS) can probe subtle variations in radial intensity distribution in weakly scattering media. We use enhanced backscattering of light by implementing either (1) low spatial coherence illumination or (2) multiple spatially independent detections using a microlens array under spatially coherent illumination. We show that the enhancement factor in these configurations is a measure of the integrated intensity within the localized coherence or detection area, which can exhibit strong dependence on small perturbations in scattering properties. To further evaluate the utility of the LEBS enhancement factor, we use a well-established animal model of cutaneous two-stage chemical carcinogenesis. In this pilot study, we demonstrate that the LEBS enhancement factor can be substantially altered at a stage of preneoplasia. Our animal result supports the idea that early carcinogenesis can cause subtle alterations in the scattering properties that can be captured by the LEBS enhancement factor. Thus, the LEBS enhancement factor has the potential as an easily measurable biomarker in skin carcinogenesis.

Staggered-grid PSTD on local Fourier basis and its applications to surface tissue modeling

We introduce a high performance parallelization to the PSTD solution of Maxwell equations by employing the fast Fourier transform on local Fourier basis. Meanwhile a reformatted derivative operator allows the adoption of a staggered-grid such as the Yee lattice in PSTD, which can overcome the numerical errors in a collocated-grid when spatial discontinuities are present. The accuracy and capability of our method are confirmed by two analytical models. In two applications to surface tissue optics, an ultra wide coherent backscattering cone from the surface layer is found, and the penetration depth of polarization gating identified. Our development prepares a tool for investigating the optical properties of surface tissue structures.

Optical coherence tomography - development, principles, applications

This paper presents a review of the development of optical coherence tomography (OCT), its principles and important applications. Basic OCT systems are described and the physical foundations of OCT signal properties and signal recording systems are reviewed. Recent examples of OCT applications in ophthalmology, cardiology, gastroenterology and dermatology outline the relevance of this advanced imaging modality in the medical field.

Virtual pinhole-scanning spectroscopic imaging platform using low-coherence enhanced backscattering

We present that multiple mutually independent coherence areas can be used for simultaneous spatial filtering in an imaging platform as effective as pinhole scanning. In this imaging platform, the unique combination of low-spatial-coherence illumination and differential angle imaging allows us to take advantage of low-coherence enhanced-backscattering (LEBS) phenomenon to permit self-generated optical sectioning to the subsurface in a relatively large area. We further demonstrate that LEBS spectroscopic imaging substantially minimizes cross talk among adjacent pixels, rejects the background light caused by out-of-plane scattered light, and thereby enhances image contrast and resolution.

Low-coherence enhanced backscattering and its applications

The phenomenon of enhanced backscattering (EBS) of light, also known as coherent backscattering, has been the object of intensive investigation in non-biological media over the last two decades. However, there have been only a few attempts to explore EBS for tissue characterization and diagnosis. We have recently made progress in the EBS measurements in tissue by taking advantage of low spatial coherence illumination, which has led us to the development of low-coherence enhanced backscattering (LEBS) as a technique to characterize living tissue. In this paper, we review the current state of research on LEBS. In particular, we show that LEBS spectroscopy enables detection of early microarchitectural changes in tissue associated with carcinogenesis prior to the development of histologically-detectable alterations as well as any other known markers of neoplasia. Thus, LEBS may offer insights into initial events in carcinogenesis.

Penetration depth of low-coherence enhanced backscattered light in subdiffusion regime

The mechanisms of photon propagation in random media in the diffusive multiple scattering regime have been previously studied using diffusion approximation. However, similar understanding in the low-order (subdiffusion) scattering regime is not complete due to difficulties in tracking photons that undergo very few scatterings events. Recent developments in low-coherence enhanced backscattering (LEBS) overcome these difficulties and enable probing photons that travel very short distances and undergo only a few scattering events. In LEBS, enhanced backscattering is observed under illumination with spatial coherence length L{sc} less than the scattering mean free path l{s}. In order to understand the mechanisms of photon propagation in LEBS in the subdiffusion regime, it is imperative to develop analytical and numerical models that describe the statistical properties of photon trajectories. Here we derive the probability distribution of penetration depth of LEBS photons and report Monte Carlo numerical simulations to support our analytical results. Our results demonstrate that, surprisingly, the transport of photons that undergo low-order scattering events has only weak dependence on the optical properties of the medium (l{s} and anisotropy factor g) and strong dependence on the spatial coherence length of illumination, L{sc} relative to those in the diffusion regime. More importantly, these low-order scattering photons typically penetrate less than l{s} into the medium due to the low spatial coherence length of illumination and their penetration depth is proportional to the one-third power of the coherence volume (i.e., [l{s}piL{s}{2}]1/3) .

Circular polarization memory effect in low-coherence enhanced backscattering of light

We experimentally study the propagation of circularly polarized light in the subdiffusion regime by exploiting enhanced backscattering [(EBS), also known as coherent backscattering] of light under low spatial coherence illumination. We demonstrate for the first time, to the best of our knowledge, that a circular polarization memory effect exists in EBS over a large range of scatterers' sizes in this regime. We show that low-coherence EBS signals from the helicity preserving and orthogonal helicity channels cross over as the mean free path length of light in media varies, and that the cross point indicates the transition from multiple to double scattering in EBS.

Low-coherence enhanced backscattering: review of principles and applications for colon cancer screening

The phenomenon of enhanced backscattering (EBS) of light, also known as coherent backscattering (CBS) of light, has been the object of intensive investigation in nonbiological media over the last two decades. However, there have been only a few attempts to explore EBS for tissue characterization and diagnosis. We have recently made progress in the EBS measurements in tissue by taking advantage of low spatial coherence illumination, which has led us to the development of low-coherence enhanced backscattering (LEBS) spectroscopy. In this work, we review the current state of research on LEBS. After a brief discussion of the basic principle of EBS and LEBS, we present an overview of the unique features of LEBS for tissue characterization, and show that LEBS enables depth-selective spectroscopic assessment of mucosal tissue. Then, we demonstrate the potential of LEBS spectroscopy for predicting the risk of colon carcinogenesis and colonoscopy-free screening for colorectal cancer (CRC).