Fluorescence-lifetime determination in tissues or other scattering media from measurement of excitation and emission kinetics

In vitro and in vivo phasor analysis of stoichiometry and pharmacokinetics using short-lifetime near-infrared dyes and time-gated imaging

We introduce a simple new approach for time-resolved multiplexed analysis of complex systems using near-infrared (NIR) dyes, applicable to in vitro and in vivo studies. We show that fast and precise in vitro quantification of NIR fluorophores' short (subnanosecond) lifetime and stoichiometry can be done using phasor analysis, a computationally efficient and user-friendly representation of complex fluorescence intensity decays obtained with pulsed laser excitation and time-gated camera imaging. We apply this approach to the study of binding equilibria by Förster resonant energy transfer using two different model systems: primary/secondary antibody binding in vitro and ligand/receptor binding in cell cultures. We then extend it to dynamic imaging of the pharmacokinetics of transferrin engagement with the transferrin receptor in live mice, elucidating the kinetics of differential transferrin accumulation in specific organs, straightforwardly differentiating specific from nonspecific binding. Our method, implemented in a freely-available software, has the advantage of time-resolved NIR imaging, including better tissue penetration and background-free imaging, but simplifies and considerably speeds up data processing and interpretation, while remaining quantitative. These advances make this method attractive and of broad applicability for in vitro and in vivo molecular imaging and could be extended to applications as diverse as image-guided surgery or optical tomography.

65-fs Yb-doped fiber laser system with gain-narrowing compensation

We report a broadband Yb-doped fiber laser system with a gain-narrowing compensator comprised of multiple dielectric layers. Utilizing this filter, we obtained broadband pulses over a bandwidth of 1020-1080 nm directly from the amplifier. After the dispersion compensation, the chirped pulse amplification system delivered 65-fs pulses with energies of 100 nJ and a repetition rate of 3 MHz.

Compressive diffuse optical tomography: noniterative exact reconstruction using joint sparsity

Diffuse optical tomography (DOT) is a sensitive and relatively low cost imaging modality that reconstructs optical properties of a highly scattering medium. However, due to the diffusive nature of light propagation, the problem is severely ill-conditioned and highly nonlinear. Even though nonlinear iterative methods have been commonly used, they are computationally expensive especially for three dimensional imaging geometry. Recently, compressed sensing theory has provided a systematic understanding of high resolution reconstruction of sparse objects in many imaging problems; hence, the goal of this paper is to extend the theory to the diffuse optical tomography problem. The main contributions of this paper are to formulate the imaging problem as a joint sparse recovery problem in a compressive sensing framework and to propose a novel noniterative and exact inversion algorithm that achieves the l(0) optimality as the rank of measurement increases to the unknown sparsity level. The algorithm is based on the recently discovered generalized MUSIC criterion, which exploits the advantages of both compressive sensing and array signal processing. A theoretical criterion for optimizing the imaging geometry is provided, and simulation results confirm that the new algorithm outperforms the existing algorithms and reliably reconstructs the optical inhomogeneities when we assume that the optical background is known to a reasonable accuracy.

Low-frequency wide-field fluorescence lifetime imaging using a high-power near-infrared light-emitting diode light source

Fluorescence lifetime imaging (FLi) could potentially improve exogenous near-infrared (NIR) fluorescence imaging, because it offers the capability of discriminating a signal of interest from background, provides real-time monitoring of a chemical environment, and permits the use of several different fluorescent dyes having the same emission wavelength. We present a high-power, LED-based, NIR light source for the clinical translation of wide-field (larger than 5 cm in diameter) FLi at frequencies up to 35 MHz. Lifetime imaging of indocyanine green (ICG), IRDye 800-CW, and 3,3(')-diethylthiatricarbocyanine iodide (DTTCI) was performed over a large field of view (10 cm by 7.5 cm) using the LED light source. For comparison, a laser diode light source was employed as a gold standard. Experiments were performed both on the bench by diluting the fluorescent dyes in various chemical environments in Eppendorf tubes, and in vivo by injecting the fluorescent dyes mixed in Matrigel subcutaneously into CD-1 mice. Last, measured fluorescence lifetimes obtained using the LED and the laser diode sources were compared with those obtained using a state-of-the-art time-domain imaging system and with those previously described in the literature. On average, lifetime values obtained using the LED and the laser diode light sources were consistent, exhibiting a mean difference of 3% from the expected values and a coefficient of variation of 12%. Taken together, our study offers an alternative to laser diodes for clinical translation of FLi and explores the use of relatively low frequency modulation for in vivo imaging.

Monte Carlo algorithm for efficient simulation of time-resolved fluorescence in layered turbid media

We present an efficient Monte Carlo algorithm for simulation of time-resolved fluorescence in a layered turbid medium. It is based on the propagation of excitation and fluorescence photon bundles and the assumption of equal reduced scattering coefficients at the excitation and emission wavelengths. In addition to distributions of times of arrival of fluorescence photons at the detector, 3-D spatial generation probabilities were calculated. The algorithm was validated by comparison with the analytical solution of the diffusion equation for time-resolved fluorescence from a homogeneous semi-infinite turbid medium. It was applied to a two-layered model mimicking intra- and extracerebral compartments of the adult human head.

Fluorescence diffuse optical tomography with functional and anatomicala prioriinformation: feasibility study

Fluorescence diffuse optical tomography (FT) is an emerging molecular imaging technique that can spatially resolve both fluorophore concentration and lifetime parameters. In this study, we investigate the performance of a frequency-domain FT system for small inclusions that are embedded in a heterogeneous background. The results demonstrate that functional and structural a priori information is crucial to be able to recover both parameters with high accuracy. The functional a priori information is defined by the absorption and scattering maps at both excitation and emission wavelengths. Similarly, the boundaries of the small inclusion and different regions in the background are utilized as the structural a priori information. Without a priori information, the fluorophore concentration of a 5 mm inclusion in a 40 mm medium is recovered with 50% error, while the lifetime cannot be recovered at all. On the other hand, when both functional and structural information are available, the true lifetime can be recovered and the fluorophore concentration can be estimated only with 5% error. This study shows that a hybrid system that can acquire diffuse optical absorption tomography (DOT), FT and anatomical images in the same setting is essential to be able to recover the fluorophore concentration and lifetime accurately in vivo.

Fluorescence molecular imaging

There is a wealth of new fluorescent reporter technologies for tagging of many cellular and subcellular processes in vivo. This imposed contrast is now captured with an increasing number of available imaging methods that offer new ways to visualize and quantify fluorescent markers distributed in tissues. This is an evolving field of imaging sciences that has already achieved major advances but is also facing important challenges. It is nevertheless well poised to significantly impact the ways of biological research, drug discovery, and clinical practice in the years to come. Herein, the most pertinent technologies associated with in vivo noninvasive or minimally invasive fluorescence imaging of tissues are summarized. Focus is given to small-animal imaging. However, while a broad spectrum of fluorescence reporter technologies and imaging methods are outlined, as necessary for biomedical research, and clinical translation as well.

Measurements of FRET in a Glucose-sensitive Affinity System with Frequency-domain Lifetime Spectroscopy

We report measurements of fluorescence resonance energy transfer (FRET) for glucose sensing in an established concanavalin A-dextran affinity system using frequency-domain lifetime spectroscopy. A dextran (MW 2,000,000) labeled with a small fluorescent donor molecule, Alexa Fluor 568, was used to competitively bind to a sugar-binding protein, concanavalin A, labeled with acceptor molecule, Alexa Fluor 647, in the presence of glucose. The FRET-quenching kinetics of the donor were analyzed from frequency-domain measurements as a function of both glucose and acceptor-protein concentrations using a Förster-type decay kinetics model. The results show that the frequency-domain measurements and donor decay kinetics can quantitatively indicate changes in the competitive binding of 0.09 microM dextran to labeled concanavalin A at a solution concentration of 10.67 microM in the presence of glucose at concentrations ranging from 0 to 224 mg/dL.

A comparison of exact and approximate adjoint sensitivities in fluorescence tomography

Many approaches to fluorescence tomography utilize some form of regularized nonlinear least-squares algorithm for data inversion, thus requiring repeated computation of the Jacobian sensitivity matrix relating changes in observable quantities, such as emission fluence, to changes in underlying optical parameters, such as fluorescence absorption. An exact adjoint formulation of these sensitivities comprises three terms, reflecting the individual contributions of 1) sensitivities of diffusion and decay coefficients at the emission wavelength, 2) sensitivities of diffusion and decay coefficients at the excitation wavelength, and 3) sensitivity of the emission source term. Simplifying linearity assumptions are computationally attractive in that they cause the first and second terms to drop out of the formulation. The relative importance of the three terms is thus explored in order to determine the extent to which these approximations introduce error. Computational experiments show that, while the third term of the sensitivity matrix has the largest magnitude, the second term becomes increasingly significant as target fluorophore concentration or volume increases. Image reconstructions from experimental data confirm that neglecting the second term results in overestimation of sensitivities and consequently overestimation of the value and volume of the fluorescent target, whereas contributions of the first term are so low that they are probably not worth the additional computational costs.

Near-infrared fluorescence contrast-enhanced imaging with area illumination and area detection: the forward imaging problem

Fluorescence frequency-domain photon migration measurements were acquired from tissue phantoms, each containing a fluorescent target, by means of area illumination and area detection on the same surface and for the first time, to our knowledge, compared with predictions computed with a numerical solution to the coupled photon diffusion equations. We accomplished area illumination and area detection using a planar, intensity-modulated excitation light source and a gain-modulated intensified charge-coupled device camera, respectively. A 1-ml vessel containing 1-microm solution of Indocyanine Green in 1% Liposyn was immersed 1 cm deep in each 512-ml tissue phantom. For most tissue phantoms, the background surrounding the 1-ml target was composed of Liposyn solution containing Indocyanine Green or 3,3'-Diethylthiatricarbocyanine Iodide such that the target-to-background ratio of fluorescence yield was > or = 10:1. Measurements of fluorescence modulation amplitude and phase were predicted with a mean error ranging from 10.1% to 13.6% and 0.56 degrees to 1.72 degrees, respectively. These numbers are similar to those obtained by use of single-pixel frequency-domain photon migration techniques and validate the potential use of area illumination and area detection for biomedical imaging of tissues. Results also demonstrate that target-to-background ratios of fluorescence yield and fluorescence lifetime significantly affect target detectability.

Quantitative fluorescence lifetime spectroscopy in turbid media: comparison of theoretical, experimental and computational methods

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.

Fluorescence lifetime spectroscopy in multiply scattering media with dyes exhibiting multiexponential decay kinetics

To investigate fluorescence lifetime spectroscopy in tissue-like scattering, measurements of phase modulation as a function of modulation frequency were made using two fluorescent dyes exhibiting single exponential decay kinetics in a 2% intralipid solution. To experimentally simulate fluorescence multiexponential decay kinetics, we varied the concentration ratios of the two dyes, 3,3-diethylthiatricarbocyanine iodide and indocynanine green (ICG), which exhibit distinctly different lifetimes of 1.33 and 0.57 ns, respectively. The experimental results were then compared with values predicted using the optical diffusion equation incorporating 1) biexponential decay, 2) average of the biexponential decay, as well as 3) stretched exponential decay kinetic models to describe kinetics owing to independent and quenched relaxation of the two dyes. Our results show that while all kinetic models could describe phase-modulation data in nonscattering solution, when incorporated into the diffusion equation, the kinetic parameters failed to likewise predict phase-modulation data in scattering solutions. We attribute the results to the insensitivity of phase-modulation measurements in nonscattering solutions and the inaccuracy of the derived kinetic parameters. Our results suggest the high sensitivity of phase-modulation measurements in scattering solutions may provide greater opportunities for fluorescence lifetime spectroscopy.

In situ time-resolved fluorescence spectroscopy in the frequency domain in capillary electrochromatography

In situ time-resolved fluorescence spectroscopy for capillary electrochromatography (CEC) is described in the frequency domain. Fluorescence decay of the solute molecules is collected directly in the packed stationary phase of the CEC capillary. The fluorescence lifetime profile of the solute molecules reveals the microenvironments they experience in the C18 chromatographic interface. A quartz flow cell and experimental optimization of the signal-to-noise ratio are described that enable the collection of high-quality decay data and subsequent calculation of fluorescence lifetime profiles of the solute molecules. The distribution of pyrene (PY), 1-pyrenemethanol (PY-MeOH), and 1-pyrenebutanol (PY-BuOH) into the C18 stationary phase and the solute-C18 phase interactions are probed, under separation conditions for CEC. All three molecules display a Gaussian distribution of lifetimes, consistent with an ensemble of heterogeneous microenvironments in the C18 stationary phase. The least polar molecule PY diffuses deeply into and interacts extensively with the C18 phase, experiencing high hydrophobicity and significant heterogeneity of microenvironments. The retention order of PY-MeOH, PY-BuOH, and PY in CEC is determined by their interactions with the stationary phase, revealed by their fluorescence lifetime distributions.

Analysis of heterogeneous fluorescence decays. Distribution of pyrene derivatives in an octadecylsilane layer in capillary electrochromatography

The distribution of solute molecules in the stationary phase in capillary electrochromatography (CEC) has been investigated with time-resolved fluorescence in the frequency domain. The analysis of fluorescence decay poses a challenging problem for the complex decay kinetics of heterogeneous systems such as the C18 stationary phase. The nonlinear least-squares (NLLS) method selects the decay model by minimizing the chi2 value. The chi2 criterion, in conjunction with the requirement that the residues should be randomly distributed around zero, frequently leads to a feasible set of multiple decay models that can all fit the data satisfactorily. The maximum entropy method (MEM) further chooses a unique model from the group of feasible ones by maximizing the Shannon-Jaynes entropy. The unique model, however, is not necessarily the most probable one. In this paper, the best model for the fluorescence decays of solute molecules is selected with NLLS using the chi2 statistics, the stability of the fit, and the consistency within replicate experiments. In addition, the recovered lifetime parameters of the true model should display the same trend as the fluorescence decay profiles when an experimental condition is varied. Using these criteria, a Gaussian distribution of fluorescence lifetimes satisfactorily fits the data under all experimental conditions. An additional minor component with a discrete lifetime is attributed to the systematic errors in the measurements. The distribution is a manifestation of an ensemble of heterogeneous microenvironments in the stationary phase of CEC. MEM is not suitable for the modeling of CEC data because of its inaccuracy in recovering broad fluorescence lifetime distributions and its lack of consistency in the replicate measurements in the studies of high-voltage effects.

Noninvasive functional imaging of human brain using light

Analysis of photon transit time for low-power light passing into the head, and through both skull and brain, of human subjects allowed for tomographic imaging of cerebral hemoglobin oxygenation based on photon diffusion theory. In healthy adults, imaging of changes in hemoglobin saturation during hand movement revealed focal, contralateral increases in motor cortex oxygenation with spatial agreement to activation maps determined by functional magnetic resonance imaging; in ill neonates, imaging of hemoglobin saturation revealed focal regions of low oxygenation after acute stroke, with spatial overlap to injury location determined by computed tomography scan. Because such slow optical changes occur over seconds and co-localize with magnetic resonance imaging vascular signals whereas fast activation-related optical changes occur over milliseconds and co-localize with EEG electrical signals, optical methods offer a single modality for exploring the spatio-temporal relationship between electrical and vascular responses in the brain in vivo, as well as for mapping cortical activation and oxygenation at the bedside in real-time for clinical monitoring.

Biomedical optical tomography using dynamic parameterization and bayesian conditioning on photon migration measurements

Stochastic reconstruction techniques are developed for mapping the interior optical properties of tissues from exterior frequency-domain photon migration measurements at the air-tissue interface. Parameter fields of absorption cross section, fluorescence lifetime, and quantum efficiency are accurately reconstructed from simulated noisy measurements of phase shift and amplitude modulation by use of a recursive, Bayesian, minimum-variance estimator known as the approximate extended Kalman filter. Parameter field updates are followed by data-driven zonation to improve the accuracy, stability, and computational efficiency of the method by moving the system from an underdetermined toward an overdetermined set of equations. These methods were originally developed by Eppstein and Dougherty [Water Resources Res. 32, 3321 (1996)] for applications in geohydrology. Estimates are constrained to within feasible ranges by modeling of parameters as beta-distributed random variables. No arbitrary smoothing, regularization, or interpolation is required. Results are compared with those determined by use of Newton-Raphson-based inversions. The speed and accuracy of these preliminary Bayesian reconstructions suggest the near-future application of this inversion technology to three-dimensional biomedical imaging with frequency-domain photon migration.

Role of higher-order scattering in solutions to the forward and inverse optical-imaging problems in random media

From analytical and numerical solutions that predict the scattering of diffuse photon density waves and from experimental measurements of changes in phase shift theta and ac amplitude demodulation M caused by the presence of single and double cylindrical heterogeneities, we show that second- and higher-order perturbations can affect the prediction of the propagation characteristics of diffuse photon density waves. Our experimental results for perfect absorbers in a lossless medium suggest that the performance of fast inverse-imaging algorithms that use first-order Born or Rytov approximations might have inherent limitations compared with inverse solutions that use iterative solutions of a linear perturbation equation or numerical solutions of the diffusion equation.

Fluorescence and absorption contrast mechanisms for biomedical optical imaging using frequency-domain techniques

The ability to optically image or detect diseased tissue volumes located deep within tissues depends upon the degree of contrast provided by differences in local optical properties. In this report, we show that the exogenous contrast offered by fluorescent compounds is superior to that provided by nonfluorescing, light-absorbing compounds when time-dependent measurements are employed. In addition, we show that the induced contrast is not only moderated by the preferential uptake of fluorescent agents into diseased tissue volumes of interest but also by the fluorescent optical properties and the fluorescence dynamics in the specific tissue volume. Using tissue phantom studies, we demonstrated experimentally that near-infrared-absorbing and fluorescent dyes such as indocyanine green can provide detection of diseased tissue volumes from fluorescence measurements made at the periphery of tissue when there is perfect, 100-fold and 10-fold partitioning in diseased tissues over that in surrounding normal tissues. Experimental results of common laser dyes show the contrast is also mediated by the quantum yield and lifetime parameters that may be dependent upon the local tissue environment.

Imaging of fluorescent yield and lifetime from multiply scattered light reemitted from random media

The feasibility of employing fluorescent contrast agents to perform optical imaging in tissues and other scattering media has been examined through computational studies. Fluorescence lifetime and yield can give crucial information about local metabolite concentrations or environmental conditions within tissues. This information can be employed toward disease detection, diagnosis, and treatment if noninvasively quantitated from reemitted optical signals. However, the problem of inverse image reconstruction of fluorescence yield and lifetime is complicated because of the highly scattering nature of the tissue. Here a light propagation model employing the diffusion equation is used to account for the scattering of both the excitation and fluorescent light. Simulated measurements of frequency-domain parameters of fluorescent modulated ac amplitude and phase lag are used as inputs to an inverse image-reconstruction algorithm, which employs the diffusion model to predict frequency-domain measurements resulting from a modulated input at the phantom periphery. In the inverse image-reconstruction algorithm, a Newton-Raphson technique combined with a Marquardt algorithm is employed to converge on the fluorescent properties within the medium. The successful reconstruction of both the fluorescence yield and lifetime in the case of a heterogeneous fluorophore distribution within a scattering medium has been demonstrated without a priori information or without the necessity of obtaining absence images.

Frequency domain modeling of reradiation in highly scattering media

We present a straightforward procedure for frequency domain modeling of reradiation in a highly scattering medium with an arbitrary, finite three-dimensional geometry. We use a finite difference numerical solver to determine the fluence distribution at the excitation wavelength, which is then coupled to the emission wavelength with an array of equivalent reradiating sources. We then calculate the fluence distribution at the emission wavelength with a second, independent numerical simulation with new optical parameters appropriate to the emission wavelength, using the distributed reradiating sources as the excitation. We compare three-dimensional simulations of a fluorophore distributed in a scattering medium with experimental data. We also compare simulations of the Raman reradiation of small diamonds in a scattering medium with experiment.

Experimental verification of a theory for the time-resolved fluorescence spectroscopy of thick tissues

Fluorescence spectroscopy provides potential contrast enhancement for near-infrared tissue imaging and physiologically correlated spectroscopy. We present a fluorescence photon migration model and test its quantitative predictive capabilities with a frequency-domain measurement that involves a homogeneous multiple-scattering tissue phantom (with optical properties similar to those of tissue in the near infrared) that contains a fluorophore (rhodamine B). After demonstrating the validity of the model, we explore its ability to recover the fluorophore's spectral properties from within the multiple-scattering medium. The absolute quantum yield and the lifetime of the fluorophore are measured to within a few percent of the values measured independently in the absence of scattering. Both measurements are accomplished without the use of reference fluorophores. In addition, the model accurately predicts the fluorescence emission spectrum in the scattering medium. Implications of these absolute measurements of lifetime, quantum yield, concentration, and emission spectrum from within multiple-scattering media are discussed.

Quantitative optical spectroscopy for tissue diagnosis

The interaction of light within tissue has been used to recognize disease since the mid-1800s. The recent developments of small light sources, detectors, and fiber optic probes provide opportunities to quantitatively measure these interactions, which yield information for diagnosis at the biochemical, structural, or (patho)physiological level within intact tissues. However, because of the strong scattering properties of tissues, the reemitted optical signal is often influenced by changes in biochemistry (as detected by these spectroscopic approaches) and by physiological and pathophysiological changes in tissue scattering. One challenge of biomedical optics is to uncouple the signals influenced by biochemistry, which themselves provide specificity for identifying diseased states, from those influenced by tissue scattering, which are typically unspecific to a pathology. In this review, we describe optical interactions pursued for biomedical applications (fluorescence, fluorescence lifetime, phosphorescence, and Raman from cells, cultures, and tissues) and then provide a descriptive framework for light interaction based upon tissue absorption and scattering properties. Finally, we review important endogenous and exogenous biological chromophores and describe current work to employ these signals for detection and diagnosis of disease.