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Biomedical subjects

Vadim Backman

Publications and source records attributed to Vadim Backman.

At least 37 records · Page 2Linked to original sources

Low-coherent backscattering spectroscopy for tissue characterization.

Although the phenomenon of coherent backscattering (CBS) in nonbiological media has generated substantial research interest, observing CBS in biological tissue has been extremely difficult. Here we show that the combination of low-spatial-coherence, broadband illumination, and low-temporal-coherence, spectrally resolved detection significantly facilitates CBS observation in biological tissue and other random media with long-transport mean-free path lengths, which have been previously beyond the reach of conventional CBS investigations. Furthermore, we demonstrate that depth-selective, speckle-free, low-coherent backscattering spectroscopy has the potential to diagnose the earliest, previously undetectable, precancerous alterations in the colon by means of probing short light paths.

Animals↗

Exact solution of Maxwell's equations for optical interactions with a macroscopic random medium: addendum.

This Addendum provides a revised set of figures containing converged numerical data for total scattering cross section (TSCS), replacing the figures in our recent publication [Opt. Lett. 29, 1393 (2004)]. Due to the use of an overly large time step, our original TSCS data exhibited a systematic, nonphysical diminution above 150 THz for all cases studied. We have determined that numerical convergence in the temporal sense for the pseudospectral time-domain (PSTD) algorithm employed previously requires limiting the time step to no more than 1/60th of the sinusoidal period at the maximum frequency of interest, which in the previous case was 300 THz. This is an important point that we hereby report to future users of PSTD simulations in electrodynamics and optics. Note that all our original conclusions remain valid.

Algorithms↗

Spectral markers in preneoplastic intestinal mucosa: an accurate predictor of tumor risk in the MIN mouse.

BACKGROUND: We have reported recently that microarchitectural analysis of the histologically normal mucosa using a novel optics technology, four-dimensional elastic light scattering fingerprinting (ELF), provided unprecedented sensitivity for early detection of colon carcinogenesis. In the present study, we explored the ability of four-dimensional ELF to identify an inherited predisposition to colorectal cancer, an issue of considerable importance for optimizing population screening strategies. METHODS: We used the MIN mouse, a model whose germ line adenomatous polyposis coli truncation leads to spontaneous intestinal tumorigenesis, thus replicating the human syndrome, familial adenomatous polyposis. Spectral markers were assessed by four-dimensional ELF analysis in MIN mice at preneoplastic time points and compared with age-matched controls (C57BL6 mice with wild-type adenomatous polyposis coli). To assess the responsiveness of spectral markers to chemopreventive agents, a subset of MIN mice was supplemented with celecoxib 1,500 ppm. RESULTS: Spectral slope, fractal dimension, and principal component 3 were dramatically altered in the uninvolved MIN mouse mucosa at the earliest time points. Furthermore, alteration in spectral variables increased over time, consonant with the microarchitectural underpinnings of subsequent tumorigenesis. Additionally, these markers spatially correlated with future adenoma development (small intestine > colon). Short-term treatment with the potent chemopreventive agent, celecoxib, resulted in near normalization of fractal dimension and principal component 3. CONCLUSIONS: We report, for the first time, that spectral markers, assayed by four-dimensional ELF, were able to sensitively identify a genetic predisposition for intestinal tumorigenesis before the occurrence of phenotypic manifestations. Moreover, the reversal of spectral markers by celecoxib treatment supports the neoplastic relevance.

Animals↗

Equiphase-sphere approximation for light scattering by stochastically inhomogeneous microparticles.

We report the development and validation of the equiphase-sphere (EPS) approximation for calculating the total-scattering cross-section (TSCS) spectra of inhomogeneous microparticles having complex interior structures. We show that this closed-form, analytical approximation can accurately model the TSCS of randomly inhomogeneous spherical particles having internal refractive index variations with geometrical scales spanning from nanometers (i.e., subwavelength) to microns (i.e., suprawavelength). Moreover, we derive an easy-to-use criterion for the range of validity of the EPS approximation in modeling TSCS of inhomogeneous particles. The work discussed in this paper may positively impact tissue optical imaging and diagnostic applications.

Journal Article↗

Coherent backscattering spectroscopy.

Coherent backscattering (CBS) of light in random media has been previously investigated by use of coherent light sources. Here we report a novel method of CBS measurement that combines low spatial coherence, broadband illumination, and spectrally resolved detection. We show that low spatial coherence illumination leads to an anomalously broad CBS peak and a dramatic speckle reduction; the latter is further facilitated by low temporal coherence detection. Thus CBS can be observed in biological tissue and other media that previously were beyond the reach of conventional CBS measurements. We also demonstrate, for the first time to our knowledge, spectroscopic analysis of CBS. CBS spectroscopy may find important applications in probing random media such as biological tissue in which depth-selective measurements are crucial.

Animals↗

Equiphase-sphere approximation for analysis of light scattering by arbitrarily shaped nonspherical particles.

We extend the previously proposed concept of equiphase sphere (EPS) to analyze light-scattering properties of arbitrarily shaped particles. Our analyses based on the Wentzel-Kramers-Brillouin technique and numerical studies based on the finite-difference time-domain method demonstrate that a wide range of irregularly shaped particles can be approximated as their equivalent equiphase ellipsoids to determine their total scattering cross-section (TSCS) spectra. As a result, a simple expression given by the EPS approximation can be used to calculate the TSCS spectra of these particles. We find that the accuracy of the EPS approximation is influenced by both the magnitude and the geometric scale of the surface perturbation of the particle, and we derive validity conditions of the EPS approximation to guide the practical application of this method.

Journal Article↗

Exact solution of Maxwell's equations for optical interactions with a macroscopic random medium.

We report what we believe to be the first rigorous numerical solution of the two-dimensional Maxwell equations for optical propagation within, and scattering by, a random medium of macroscopic dimensions. Our solution is based on the pseudospectral time-domain technique, which provides essentially exact results for electromagnetic field spatial modes sampled at the Nyquist rate or better. The results point toward the emerging feasibility of direct, exact Maxwell equations modeling of light propagation through many millimeters of biological tissues. More generally, our results have a wider implication: Namely, the study of electromagnetic wave propagation within random media is moving toward exact rather than approximate solutions of Maxwell's equations.

Computer Simulation↗

Analytical techniques for addressing forward and inverse problems of light scattering by irregularly shaped particles.

Understanding light scattering by nonspherical particles is crucial in modeling the transport of light in realistic structures such as biological tissues. We report the application of novel analytical approaches based on modified Wentzel-Kramers-Brillouin and equiphase-sphere methods that facilitate accurate characterization of light scattering by a wide range of irregularly shaped dielectric particles. We also demonstrate that these approaches have the potential to address the inverse-scattering problem by means of a spectral analysis of the total scattering cross section of arbitrarily shaped particles.

Algorithms↗

Spectroscopic diagnosis and imaging of invisible pre-cancer.

The theme of this paper is the use of optical spectroscopy to diagnose invisible pre-cancer in patients undergoing endoscopy and similar medical procedures. We describe three techniques that provide diagnostic information and two instruments to implement them, the FastEEM for studying small regions of tissue and the LSS (light scattering spectroscopy) imaging system for wide-area surveillance. The FastEEM is an optical fiber clinical device that collects spectra of reflected light and fluorescence at multiple excitation wavelengths from the tissue, all in a fraction of a second. Quantitative information is obtained in real time, without removing the tissue and without the need for staining and fixation. Three types of spectral information are extracted intrinsic fluorescence, diffuse reflectance and elastic light scattering. Each of the three analyses is based on a biophysical model, and each provides complementary quantitative physical and chemical information about cellular/tissue structures. This information is used to make a combined spectral diagnosis, a method we call tri-modal spectroscopy (TMS). Promising clinical studies are being carried out on patients undergoing routine pre-cancer surveillance in the oral cavity, the uterine cervix and the gastrointestinal tract. The LSS imaging system provides wide-area spectroscopic images of the epithelium, typically 2 cm in each dimension, depicting the size distribution and chromatin content of the cell nuclei, which are key parameters in diagnosing pre-cancer. This instrument is in preclinical stages of development, although a laboratory prototype has been used to create diagnostic images in resected colon polyp samples. The combination of the TMS/FastEEM and LSS imaging instrument will constitute a powerful new diagnostic tool, with LSS imaging to provide wide area surveillance and the TMS probe to provide detailed information on suspect tissue sites.

Cervix Uteri↗

Four-dimensional elastic light-scattering fingerprints as preneoplastic markers in the rat model of colon carcinogenesis.

BACKGROUND & AIMS: Identification of preneoplastic changes in histologically normal epithelium (the "field effect") could provide a powerful screening tool for colorectal cancer. However, to date, reliable detection has not been possible. We have recently developed a new generation of optical technology, 4-dimensional elastic light-scattering fingerprinting (4D-ELF), which enables us to probe the nanoscale/microscale architecture of living cells. We therefore investigated whether 4D-ELF would be able to identify preneoplastic changes in the colonocytes of the azoxymethane (AOM)-treated rat model of colon carcinogenesis. METHODS: Forty-eight Fisher 344 rats were randomized to either 2 weekly injections of AOM or saline. Animals were killed 2-20 weeks after the second injection of AOM. Colons were removed and subjected to 4D-ELF analysis, with a subset undergoing assessment of aberrant crypt foci (ACF). All AOM-treated animals were compared with age-matched saline-treated controls. RESULTS: AOM-induced ACF became apparent at approximately 4-6 weeks and continued to increase over time. ACF were predominantly located in the distal colon. At 2 weeks (before development of ACF), there were marked changes in a number of 4D-ELF signatures. The relevance to carcinogenesis of these 4D-ELF-detected microarchitectural abnormalities is supported by their spatial and temporal correlation with subsequent development of ACF. All changes reported were highly statistically significant. CONCLUSIONS: We show that probing the nanoscale cellular architecture with 4D-ELF provided an unprecedented tool for detecting the earliest stages of colon carcinogenesis. Future studies are necessary to explore the clinical applicability of this technology and elucidate the biological determinants of these microarchitectural changes.

Animals↗

Concept of the equiphase sphere for light scattering by nonspherical dielectric particles.

We introduce the concept of the equiphase sphere for light scattering by nonspherical dielectric particles. This concept facilitates the derivation of a simple analytical expression for the total scattering cross section of such particles. We tested this concept for spheroidal particles and obtained a bound on the minor-to-major axis ratio for the valid application of this technique. We show that this technique yields results that agree well with the rigorous numerical solution of Maxwell's equations obtained with the finite-difference time-domain method. The new technique has the potential to be extended to the study of light scattering by arbitrarily shaped convex dielectric particles.

Journal Article↗

Down-regulation of SNAIL suppresses MIN mouse tumorigenesis: modulation of apoptosis, proliferation, and fractal dimension.

OBJECTIVES: Emerging evidence implicates the SNAIL family of transcriptional repressors in cancer development; however, the role of SNAIL in colorectal cancer has not been established. To investigate the importance of SNAIL in colorectal carcinogenesis, we examined the phenotypic and cellular consequences of SNAIL down-regulation in the MIN mouse. METHODS: Twenty-eight male MIN mice were randomized to treatment with an antisense phosphorodiamidate morpholino oligomer (AS-PMO) to SNAIL, saline, or a scrambled sequence control for 6 weeks. Tumors were scored and the molecular/cellular effects of anti-SNAIL treatment were evaluated through immunohistochemical analysis of the uninvolved intestinal mucosa for SNAIL and E-cadherin levels along with rates of apoptosis and proliferation. Furthermore, microarchitectural alterations were determined through measurement of fractal dimension. RESULTS: In the uninvolved mucosa, SNAIL AS-PMO treatment moderately decreased SNAIL protein when compared with saline-treated animals (immunohistochemistry scores 3.0 +/- 0.8 versus 2.1 +/- 0.6, respectively; P=0.01) with a concomitant increase in E-cadherin expression (1.8 +/- 0.6 versus 2.4 +/- 0.5; P < 0.05). Anti-SNAIL PMO, but not scramble control, resulted in a significant decrease in both total tumor number and incidence of tumors >2 mm (22% and 54%, respectively; P < 0.05). Furthermore, this was accompanied by an increased apoptosis rate (2-fold), decreased proliferation (3-fold), and normalization of the fractal dimension in the uninvolved intestinal mucosa. CONCLUSIONS: We show, for the first time, that SNAIL overexpression is important in intestinal tumorigenesis. While this PMO regimen afforded modest SNAIL suppression and hence tumor reduction, this provides compelling evidence for the role of SNAIL overexpression in colonic neoplasia.

Animals↗

Equivalent volume-averaged light scattering behavior of randomly inhomogeneous dielectric spheres in the resonant range.

Finite-difference time-domain numerical experiments and supporting analyses demonstrate that the spectral dependence of the total scattering cross sections of randomly inhomogeneous dielectric spheres of sizes in the resonant range closely resemble those of their homogeneous counterparts that have a volume-averaged refractive index. This result holds even for the extreme case in which the refractive index within an inhomogeneous sphere varies randomly over the range 1.0-2.0.

Journal Article↗

Spectroscopic detection and evaluation of morphologic and biochemical changes in early human oral carcinoma.

BACKGROUND: Understanding the development and progression of head and neck squamous cell carcinoma is key in the quest for the early diagnosis and prevention of this type of malignancy. The current study correlated early biochemical and histologic changes in oral tissue with spectral features in fluorescence, reflectance, and light scattering spectra acquired in vivo to diagnose early stages of oral malignancies. METHODS: A total of 91 tissue sites from 15 patients with varying degrees of malignancy (normal, dysplastic, and cancerous sites) and 8 healthy volunteers were analyzed with 3 spectroscopic techniques. Direct biochemical information regarding oral tissue native fluorophores was obtained with intrinsic fluorescence spectroscopy by fitting a linear combination of collagen and the reduced form of nicotinamide adenine dinucleotide (NADH) fluorescence spectra to the intrinsic tissue fluorescence spectra excited with 337 nanometer (nm) and 358-nm laser light. Diffuse reflectance spectroscopy was used to provide information regarding tissue absorption and structure, such as hemoglobin concentration and stroma density, by measuring the wavelength-dependent absorption and scattering coefficients. By subtracting the diffusely reflected component from the measured reflectance, light scattering spectroscopy (LSS) information resulting from single backscattering from epithelial cell nuclei was obtained. LSS provides information concerning the size distribution of cell nuclei. RESULTS: These optically extracted tissue parameters provide biochemical or structural information in vivo without the need for tissue excision, and can be used to diagnose tissue abnormalities. By combining the information provided by the three techniques, a method known as trimodal spectroscopy, a sensitivity and specificity of 96% and 96%, respectively, in distinguishing cancerous/dysplastic (mild, moderate, and severe) from normal tissue was achieved. In addition, the authors were able to distinguish dysplastic from cancerous tissue with a sensitivity of 64% and a specificity of 90%. CONCLUSIONS: The results of the current study demonstrated that Trimodal spectroscopy is a highly sensitive and specific technique with which to diagnose tissue abnormalities.

Carcinoma↗

Cellular organization and substructure measured using angle-resolved low-coherence interferometry.

We measure the organization and substructure of HT29 epithelial cells in a monolayer using angle-resolved low-coherence interferometry. This new technique probes cellular structure by measuring scattered light, as in flow cytometry, but offers an advantage in that the structure can be examined in situ, avoiding the need to disrupt the cell monolayer. We determine the size distribution of the cell nuclei by fitting measured light-scattering spectra to the predictions of Mie theory. In addition, we obtain information about the cellular organization and substructure by examining the spatial correlations within the monolayer. A remarkable finding is that the spatial correlations over small length scales take the form of an inverse power law, indicating the fractal nature of the packing of the subcellular structures. We also identify spatial correlations on a scale large compared with the size of a cell, indicating an overlying order within the monolayer.

Biophysics↗

Trimodal spectroscopy for the detection and characterization of cervical precancers in vivo.

OBJECTIVE: The objective of this study was to assess the potential of 3 spectroscopic techniques (intrinsic fluorescence, diffuse reflectance, and light scattering) individually and in combination (trimodal spectroscopy) for the detection of cervical squamous intraepithelial lesions. STUDY DESIGN: The study was conducted with 44 patients who underwent colposcopy for the evaluation of an abnormal Papanicolaou smear. Fluorescence and reflectance spectra were collected from colposcopically normal and abnormal sites and analyzed to extract quantitative information about tissue biochemistry and morphologic condition. This information was compared with histopathologic classification, and diagnostic algorithms were developed and validated with the use of logistic regression and cross-validation. RESULTS: Diagnostically significant differences exist in the composition of fluorescing biochemicals, the scattering properties, and the epithelial cell nuclear morphology of cervical squamous intraepithelial lesions and non-squamous intraepithelial lesions. Trimodal spectroscopy is a superior tool for the detection of cervical squamous intraepithelial lesions than any 1 of the techniques alone. CONCLUSION: Trimodal spectroscopy has the potential to improve the in vivo detection of precancerous cervical changes.

Diagnosis, Differential↗

Determination of particle size by using the angular distribution of backscattered light as measured with low-coherence interferometry.

We employ a novel interferometer to measure the angular distribution of light backscattered by a turbid medium. Through comparison of the measured data with the predictions of Mie theory, we are able to determine the size of the scatterers comprising the medium with subwavelength precision. As the technique is based on low-coherence interferometry, we are able to examine the evolution of the angular distribution of scattered light as it propagates into the medium. The effects of multiple scattering as a function of penetration depth in the medium are analyzed. We also present various considerations for extending this technique to determining structural information in biological tissues, such as the effects of a distribution of particle sizes and the need to average out speckle contributions.

Interferometry↗

Optimal design of structured nanospheres for ultrasharp light-scattering resonances as molecular imaging multilabels.

Optical molecular imaging could potentially enable noninvasive high-resolution characterization and diagnosis of living tissue. The capability to image multiple molecular targets simultaneously is particularly important. Currently this task cannot be achieved using conventional optical contrast agents, due to their broad spectral responses (approximately 80 to 200 nm). Developments in research on semiconductor nanocrystals (quantum dots) provide one possible solution. We describe a different concept of multilabel molecular imaging that utilizes resonant light-scattering spectroscopy of multilayered nanospheres to achieve tunable ultrasharp resonance peaks with widths as narrow as 10 nm. Our theoretical study demonstrates that dozens of molecular targets can potentially be imaged simultaneously using this approach.

Equipment Design↗