arXiv subjects

Vadim Backman

Publications and source records attributed to Vadim Backman.

9 recordsLinked to original sources

arXiv2024

RN-SDEs: Limited-Angle CT Reconstruction with Residual Null-Space Diffusion Stochastic Differential Equations

Computed tomography is a widely used imaging modality with applications ranging from medical imaging to material analysis. One major challenge arises from the lack of scanning information at certain angles, resulting in distortion or artifacts in the reconstructed images. This is referred to as the Limited Angle Computed Tomography (LACT) reconstruction problem. To address this problem, we propose the use of Residual Null-Space Diffusion Stochastic Differential Equations (RN-SDEs), which are a variant of diffusion models that characterize the diffusion process with mean-reverting (MR) stochastic differential equations. To demonstrate the generalizability of RN-SDEs, we conducted experiments with two different LACT datasets, ChromSTEM and C4KC-KiTS. Through extensive experiments, we demonstrate that by leveraging learned MR-SDEs as a prior and emphasizing data consistency using Range-Null Space Decomposition (RNSD) based rectification, we can recover high-quality images from severely degraded ones and achieve state-of-the-art performance in most LACT tasks. Additionally, we present a quantitative comparison of RN-SDE with other networks, in terms of computational complexity and runtime efficiency, highlighting the superior effectiveness of our proposed approach.

eess.IV↗

arXiv2023

Local Volume Concentration, Packing Domains and Scaling Properties of Chromatin

We propose the Self Returning Excluded Volume (SR-EV) model for the structure of chromatin based on stochastic rules and physical interactions. The SR-EV rules of return generate conformationally-defined domains observed by single cell imaging techniques. From nucleosome to chromosome scales, the model captures the overall chromatin organization as a corrugated system, with dense and dilute regions alternating in a manner that resembles the mixing of two disordered bi-continuous phases. This particular organizational topology is a consequence of the multiplicity of interactions and processes occurring in the nuclei, and mimicked by the proposed return rules. Single configuration properties and ensemble averages show a robust agreement between theoretical and experimental results including chromatin volume concentration, contact probability, packing domain identification and size characterization, and packing scaling behavior. Model and experimental results suggest that there is an inherent chromatin organization regardless of the cell character and resistant to an external forcing such as Rad21 degradation.

physics.bio-ph↗

arXiv2015

Enhancing the sensitivity of mesoscopic light reflection statistics in weakly disordered media by interface reflections

Reflection statistics have not been well studied for optical random media whose mean refractive indices do not match with the refractive indices of their surrounding media. Here, we theoretically study how this refractive index mismatch between a one dimensional (1D) optical sample and its surrounding medium affects the reflection statistics in the weak disorder limit, when the fluctuation part of the refractive index (dn) is much smaller than the mismatch as well as the mean refractive index of the sample (dn << ). In the theoretical derivation, we perform a detailed calculation that results in the analytical forms of mean and standard deviation (STD) of the reflectance in terms of disorder parameters (dn and lc) in an index mismatched backscattering system. Particularly, the orders of disorder parameters in STD of the reflectance for index mismatched systems is shown to be lower ( ( lc )^1/2 ) than that of the matched systems ( lc). By comparing STDs of the reflection coefficient of index matched and mismatched systems, we show that reflectance at the sample boundaries in index mismatched systems can enhance the signal of the STD to the disorder parameters of the reflectance. In terms of biophotonics applications, this result can lead to potential techniques that effectively extract the sample disorder parameters by manipulating the index mismatch conditions. Potential applications of the technique for enhancement in sensitivity of cancer detection at single cell level are also discussed.

physics.optics↗

arXiv2015

Reflection statistics of weakly disordered optical medium when its mean refractive index is different from an outside medium

Based on the difference between mean background of an optical sample refractive index n_0 and an outside medium, n_out, different than n_0, we study the reflection statistics of a one-dimensional weakly disordered optical medium with refractive index n(x)=n_0+dn(x). Considering dn(x) as color noise with the exponential spatial correlation decay length l_c and k as the incident wave vector, our results show that for the small correlation length limit, i.e. k*l_c<1, the average value of reflectance, r, follows a form that is similar to that of the matched refractive-index case n_0=n_out, i.e., proportional to l_c. However, the standard deviation of r is proven to be std(r(dn,l_c)) proportional to sqrt( l_c), which is different from the matched case. Applications to light scattering from layered media and biological cells are discussed

physics.optics↗

arXiv2009

Spectral oscillations in backscattering of light from a biological cell

Possible origins of the experimentally observed beat-like signal in elastic light backscattering from an epithelial biological cell are identified. The source of the beat-like signal is most likely the cellular nucleus, which is a spheroidal particle with an optically sharp edge, low relative index of refraction, and a size parameter in the resonant range; another possible source is an agglomeration of cell organelles of a similar size. Using Mie theory computer simulations for a dielectric sphere with biological nucleus-like parameters, we have shown that backscattering as a function of size parameter has a beat-like pattern. The high frequency periodic oscillation is the single scattering contribution, while multiple scattering events give slowly varying oscillatory contributions. With increasing contrast, the backscattering signal changes from a pure sinusoid to that of a beat signal. The beat-like pattern in scattering from a low contrast particle does not depend on any particular features of the sphere; a similar pattern is observable in scattering from star-like non-spherical objects with a sharp optical contrast and smooth shape; in this case, the high frequency of oscillations depends on the object orientation relative to incident light

physics.optics↗

arXiv2007

Penetration depth of low-coherence enhanced backscattered light in sub-diffusion 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 (sub-diffusion) 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 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 πL_sc^2 ]^1/3).

cond-mat.mes-hall↗

arXiv2006

Circular polarization memory effect in enhanced backscattering of light under partially coherent illumination

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

cond-mat.mes-hall↗

arXiv2006

Probing minimal scattering events in enhanced backscattering of light using low-coherence induced dephasing

We exploit low spatial coherence illumination to dephase time-reversed partial waves outside its finite coherence area, which virtually creates a controllable coherence volume and isolates the minimal number of scattering events from higher order scattering in enhanced backscattering (EBS, also known as coherent backscattering) of light. We report the first experimental evidence that the minimum number of scattering events in EBS is double scattering in discrete random media, which has been hypothesized since the first observation of EBS of light. We discuss several unique characteristics and potential applications of low-coherence EBS in weakly scattering random media.

cond-mat.mes-hall↗

arXiv2005

Photon random walk model of low-coherence enhanced backscattering (LEBS) from anisotropic disordered media: a Monte Carlo simulation

Constructive interference among coherent waves traveling time-reversed paths in a random medium gives rise to the enhancement of light scattering observed in directions close to backscattering. This phenomenon is known as enhanced backscattering (EBS). According to diffusion theory, the angular width of an EBS cone is proportional to the ratio of the wavelength of light $λ$ to the transport mean free path length ls* of a random medium. In biological media, large ls* ~ 0.5-2 mm >> $λ$ results in an extremely small (~0.001 $^\degree$) angular width of the EBS cone making the experimental observation of such narrow peaks difficult. Recently, the feasibility of observing EBS under low spatial coherence illumination (spatial coherence length Lsc<<ls*) was demonstrated. Low spatial coherence behaves as a spatial filter rejecting longer path-lengths and, thus, resulting in more than 100 times increase in the angular width of low coherence EBS (LEBS) cones. However, conventional diffusion approximation-based model of EBS has not been able to explain such dramatic increase in LEBS width. Here we present a photon random walk model of LEBS using Monte Carlo simulation to elucidate the mechanism accounting for the unprecedented broadening of LEBS peaks. Typically, the exit angles of the scattered photons are not considered in modeling EBS in diffusion regime. We show that small exit angles are highly sensitive to low order scattering, which is crucial for accurate modeling of LEBS. Our results show that the predictions of the model are in excellent agreement with experimental data.

cond-mat.mes-hall↗