[show abstract][hide abstract] ABSTRACT: The accuracy of the commonly used diffusion approximation as used in diffuse optical tomography is known to be limited in cases involving strong absorption and in these situations a higher ordered approximation is necessary. In this study, a light transport model has been developed based upon the three-dimensional frequency-domain simplified spherical harmonics (SP(N)) approximation for orders up to N = 7. The SP(N) data are tested against a semi-infinite multi-layered Monte Carlo model. It has been shown that the SP(N) approximation for higher orders (N >1) provides an increase in accuracy over the diffusion equation specifically near sources and at boundaries of regions with increased optical absorption. It is demonstrated that the error of fluence calculated near the sources between the diffusion approximation and the SP(N) model (N = 7) can be as large as 60%, therefore limiting the use of the diffusion approximation for small animal imaging and in situations where optical changes near sources are critical for tomographic reconstructions.
Physics in Medicine and Biology 04/2009; 54(8):2493-509. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Monte Carlo simulations were used to predict optimal fiber-optic probe configurations for selectively interrogating autofluorescence from the superficial (50 µm) middle layer of a tissue-engineered construct with a different native fluorophore in each layer.
[show abstract][hide abstract] ABSTRACT: Tissue engineered constructs can be employed to graft wounds or replace diseased tissue. Non-invasive methods are required to assess cellular viability in these constructs both pre- and post-implantation into patients. In this study, Monte Carlo simulations and fluorescence experiments were executed on ex vivo produced oral mucosa equivalent (EVPOME) constructs to investigate the fluorescence emitted at 355 nm excitation from these constructs. Both simulations and experiments indicated the need to investigate alternative excitation wavelengths in order to increase the cellular fluorescence from these constructs, while decreasing contributions from extra-cellular fluorophores.
[show abstract][hide abstract] ABSTRACT: Photon transport in complex biological tissues is most accurately predicted via Monte Carlo (MC) modeling methods, which often require lengthy computation times. In this report, a semi-analytical technique (henceforth referred to as PI-scaling) was derived that combines MC simulation, absorption scaling, and path integrals (PI) to rapidly reconstruct time-resolved reflectance from the surface of bilayered epithelial tissue models. Comparisons to forward MC simulations indicated that the PI-scaling method was accurate to better than 10% for tissue models where the optical properties of the top layer did not greatly influence the time-resolved reflectance. Employing such a method should provide a novel solution to the first step of the problem of rapid simulation of time-resolved reflectance of photons in layered tissues.
[show abstract][hide abstract] ABSTRACT: Tissue-simulating phantoms are widely used for controlled studies of photon transport in turbid media. Here, we describe how polystyrene microspheres, which are often used to simulate optical scattering in such phantoms, can reduce fluorophore quantum yield via collisional quenching. We report studies on UV-visible (fluorescein-based) and NIR (IR125-based) phantoms with differing fluorophore and scatterer concentrations, as well as differing microsphere sizes. Results consistent with the Stern-Volmer relation suggest that the fluorophore intrinsic excited-state lifetime decreased due to collisional quenching from polystyrene microspheres and that the quenching efficiency was dependent on the concentration ratio of fluorophores to microspheres. Lifetime decreases ranging from 10-35% (20%) were measured for fluorescein-based (IR 125-based) phantoms. Since polystyrene microspheres are commonly used in tissue-simulating phantoms for quantitative studies of fluorescence light propagation, their quenching effects on fluorescence intensities may be difficult to separate from intensity losses attributed to optical absorption and scattering in the phantom unless fluorescence lifetime measurements are performed simultaneously.
[show abstract][hide abstract] ABSTRACT: A method to non-invasively and quantitatively characterize thick biological tissues by combining both experimental and computational approaches in tissue optical spectroscopy was developed and validated on fifteen porcine articular cartilage (AC) tissue samples. To the best of our knowledge, this study is the first to couple non-invasive reflectance and fluorescence spectroscopic measurements on freshly harvested tissues with Monte Carlo computational modeling of time-resolved propagation of both excitation light and multi-fluorophore emission. For reflectance, quantitative agreement between simulation and experiment was achieved to better than 11%. Fluorescence data and simulations were used to extract the ratio of the absorption coefficients of constituent fluorophores for each measured AC tissue sample. This ratio could be used to monitor relative changes in concentration of the constituent fluorophores over time. The samples studied possessed the complexity and variability not found in artificial tissue-simulating phantoms and serve as a model for future optical molecular sensing studies on tissue engineered constructs intended for use in human therapeutics. An optical technique that could non-invasively and quantitatively assess soft tissue composition or physiologic status would represent a significant advance in tissue engineering. Moreover, the general approach described here for optical characterization should be broadly applicable to quantitative, non-invasive molecular sensing applications in complex, three-dimensional biological tissues.
[show abstract][hide abstract] ABSTRACT: A method for noninvasive, quantitative characterization of tissues using molecular
fluorescence was applied to porcine knee cartilage. Experimental and computational results agreed
to within 5% and were reproducible with statistical significance across multiple biological
[show abstract][hide abstract] ABSTRACT: Noninvasive characterization of ex-vivo produced oral-mucosal equivalent (EVPOME) tissues was simulated using a Monte Carlo code to predict spatially-resolved fluorescence in a multi-layered tissue model. Relative contributions to surface fluorescence from endogenous fluorophores were quantified.
[show abstract][hide abstract] ABSTRACT: Absorption-scaling methods were applied to Monte Carlo simulations of two-layered tissues by incorporating a path integral (PI) formalism. Times spent by photons in the top-layer can be significantly underestimated when determined using PI.
[show abstract][hide abstract] ABSTRACT: In this article, we describe a novel Monte Carlo code for time-integrated and time-resolved photon migration simulations of excitation and fluorescent light propagation (with reabsorption) in bi-layered models of biological tissues. The code was experimentally validated using bi-layered, tissue-simulating phantoms and the agreement between simulations and experiment was better than 3%. We demonstrate the utility of the code for quantitative clinical optical diagnostics in epithelial tissues by examining design characteristics for clinically compatible waveguides with arbitrarily complex source-detector configurations. Results for human colonic tissues included a quantitative comparison of simulation predictions with time-resolved fluorescence data measured in vivo and spatio-temporal visualizations of photon migration characteristics in tissue models in both two- and three-dimensions for source-detector configurations, including variable waveguide spacing, numerical aperture, and diameter. These results were then extended from surface point spectroscopy to imaging modalities for both time-gated (fluorescence lifetime) and steady-state (fluorescence intensity) experimental conditions. To illustrate the flexibility of this computational approach, time-domain results were extended to simulate predictions for frequency-domain instrumentation. This work is the first demonstration and validation of a time-domain, multi-wavelength photon transport model with these capabilities in layered turbid-media.
[show abstract][hide abstract] ABSTRACT: Numerical simulations of time-resolved light transport in inhomogeneous tissues reveal quantitative, 3-D-distributions of excitation and fluorescent light. Visualizations generated can assist the optimization of endoscopy-compatible fiber-optic probes and optical imaging systems.
[show abstract][hide abstract] ABSTRACT: Fluorescence spectroscopy and imaging methods, including fluorescence lifetime sensing, are being developed for noninvasive tissue diagnostics. The purpose of this study was to identify and quantify those factors affecting the accurate recovery of fluorophore lifetimes from inhomogeneous tissues in vivo. A Monte Carlo code was developed to numerically simulate time-resolved fluorescence measurements on layered epithelial tissues. Simulations were run with experimental parameters matching previously reported clinical studies in the gastrointestinal tract. The results demonstrate that variations in fluorescence decay time as large as those detected clinically between normal and premalignant tissues (approximately 2 ns) could be simulated by variations in tissue morphology or biochemistry, even when intrinsic fluorophore lifetimes were held constant.
[show abstract][hide abstract] ABSTRACT: Near-infrared (NIR) tomography is a technique used to measure light propagation through tissue and generate images of internal optical property distributions from boundary measurements. Most popular applications have concentrated on female breast imaging, neonatal and adult head imaging, as well as muscle and small animal studies. In most instances a highly scattering medium with a homogeneous refractive index is assumed throughout the imaging domain. Using these assumptions, it is possible to simplify the model to the diffusion approximation. However, biological tissue contains regions of varying optical absorption and scatter, as well as varying refractive index. In this work, we introduce an internal boundary constraint in the finite element method approach to modelling light propagation through tissue that accounts for regions of different refractive indices. We have compared the results to data from a Monte Carlo simulation and show that for a simple two-layered slab model of varying refractive index, the phase of the measured reflectance data is significantly altered by the variation in internal refractive index, whereas the amplitude data are affected only slightly.
Physics in Medicine and Biology 09/2003; 48(16):2713-27. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Tissue-simulating phantoms that replicate intrinsic optical properties in a controlled manner are useful for quantitative studies of photon transport in turbid biological media. In such phantoms, polystyrene microspheres are often used to simulate tissue optical scattering. Here, we report that using polystyrene microspheres in fluorescent tissue-simulating phantoms can reduce fluorophore quantum yield via collisional quenching. Fluorescence lifetime spectroscopy was employed to characterize quenching in phantoms consisting of a fluorescein dye and polystyrene microspheres (scattering coefficients
100-600cm–1). For this range of tissue-simulating phantoms, analysis using the Stern-Volmer equation revealed that collisional quenching by polystyrene microspheres accounted for a decrease in fluorescence intensity of 6-17% relative to the intrinsic intensity value when no microspheres (quenchers) were present. The intensity decrease from quenching is independent of additional, anticipated losses arising from optical scattering associated with the microspheres. These results suggest that quantitative fluorescence measurements in studies employing such phantoms may be influenced by collisional quenching.
Journal of Fluorescence 12/2002; 13(1):105-108. · 1.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: A Monte Carlo model developed to simulate time-resolved fluorescence propagation in a semi-infinite turbid medium was validated against previously reported theoretical and computational results. Model simulations were compared to experimental measurements of fluorescence spectra and lifetimes on tissue-simulating phantoms for single and dual fibre-optic probe geometries. Experiments and simulations using a single probe revealed that scattering-induced artefacts appeared in fluorescence emission spectra, while fluorescence lifetimes were unchanged. Although fluorescence lifetime measurements are generally more robust to scattering artefacts than are measurements of fluorescence spectra, in the dual-probe geometry scattering-induced changes in apparent lifetime were predicted both from diffusion theory and via Monte Carlo simulation, as well as measured experimentally. In all cases, the recovered apparent lifetime increased with increasing scattering and increasing source-detector separation. Diffusion theory consistently underestimated the magnitude of these increases in apparent lifetime (predicting a maximum increase of approximately 15%), while Monte Carlo simulations and experiment were closely matched (showing increases as large as 30%). These results indicate that quantitative simulations of time-resolved fluorescence propagation in turbid media will be important for accurate recovery of fluorophore lifetimes in biological spectroscopy and imaging applications.
Physics in Medicine and Biology 10/2002; 47(18):3387-405. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: This investigation explores the effect of index of refraction, as an optical property, on light transport through optically turbid media. We describe a model of light propagation that incorporates the influence of refractive index mismatch at boundaries within a domain. We measure light transmission through turbid cylindrical phantoms with different distributions of refractive index. These distributions approximate the heterogeneous, layered nature of biological tissue. Finite element method model calculations of diffuse transmittance through these phantoms show good agreement with the trends observed experimentally. We see that phase measurements of light that propagates through approximately 90 (mm) of scatter-dominated media may vary by 10 degrees depending upon the values of refractive index of the medium. Amplitude measurements are not as sensitive to this parameter as phase. Model calculations of diffuse reflectance from a semi-infinite slab geometry with different layers also shows good agreement with Monte Carlo simulations. We conclude that incorporating refractive index into light transport models may be worthwhile. Applying such a model in tomographic image reconstruction may improve the estimation of optical properties of biological tissues.
[show abstract][hide abstract] ABSTRACT: We present a computational code capable of simulating time-resolved fluorescence emission from multi-layered biological tissues, and apply this code to model tissue fluorescence emission data acquired in vivo during clinical endoscopy. The code for multi-layered media is based on a Monte Carlo model we developed previously to simulate time-resolved fluorescence propagation in a semi-infinite turbid medium. Here, the code is applied to simulate data acquired from measurements on tissues in the lower gastrointestinal tract. Clinical data were obtained in vivo during endoscopy using a portable time-resolved fluorescence spectrometer employing a single fiber-optic probe for excitation and detection. Tissue was modeled as a two-layered medium consisting of a mucosal layer of finite thickness above a sub-mucosal layer. The emitted fluorescence was considered as arising from mucosal epithelial cells, due to the presence of nicotinamide dinucleotide as the constituent fluorophore (lifetime tau = 1.5 ns), and from sub-mucosal structural proteins (collagen, lifetime tau = 5.2 ns). Simulations modeled changes in tissue pathology as a function of independently changing the mucosal layer thickness, the fluorophore absorption coefficients and the fluorescence quantum yields. It was observed that the emanating fluorescence from the mucosal layer changes by ~50-60% with these changes resulting in appreciable differences of ~2 ns in the average lifetimes. These simulations indicate that it may be possible to quantify the fluorescence observed from tissue based on both biochemical and histological criteria. The simulations may also be used to provide a useful method for designing and testing the efficacies of different fiber-probe geometries.
[show abstract][hide abstract] ABSTRACT: Monte Carlo (MC) simulations are considered the "gold standard" for mathematical description of photon transport in tissue, but they can require large computation times. Therefore, it is important to develop simple and efficient methods for accelerating MC simulations, especially when a large "library" of related simulations is needed. A semi-analytical method involving MC simulations and a path-integral (PI) based scaling technique generated time-resolved reflectance curves from layered tissue models. First, a zero-absorption MC simulation was run for a tissue model with fixed scattering properties in each layer. Then, a closed-form expression for the average classical path of a photon in tissue was used to determine the percentage of time that the photon spent in each layer, to create a weighted Beer-Lambert factor to scale the time-resolved reflectance of the simulated zero-absorption tissue model. This method is a unique alternative to other scaling techniques in that it does not require the path length or number of collisions of each photon to be stored during the initial simulation. Effects of various layer thicknesses and absorption and scattering coefficients on the accuracy of the method will be discussed.