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Frequency domain photon migration (FDPM) measurements were employed to accurately quantify optical properties of both the suspending fluid and particles within dense polystyrene suspensions of 143- or 226-nm mean diameter at varying concentrations (5-30% by volume). The measured absorption coefficients varied linearly with particle volume fraction whereas the isotropic scattering coefficients varied nonlinearly in agreement with the prediction that utilizes the hard-sphere structure factor model. These results validate the interference approximation of light scattering to describe light propagation accurately within dense suspensions. Furthermore, owing to the accuracy of FDPM absorption measurements, the imaginary refractive indices for both particles and their suspending fluid were determined and were found to compare favorably with literature values.

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.

The development of near-infrared (NIR) optical imaging for biomedical optical imaging is hampered by the computational intensiveness of large-scale three-dimensional (3-D) image reconstruction and the potential lack of endogenous contrast for detection of relevant tissue features. In this contribution the inverse optical imaging problem is formulated in three dimensions in a noncompressive geometry as a simple-bound constrained minimization problem in order to recover the interior fluorescence properties of exogenous contrast agent from frequency-domain photon migration measurements at the boundary. The solution of the forward optical diffusion problem for the frustum shape containing fluorescence inclusions of 10:1 contrast is accomplished by use of the Galerkin finite-element formulation. The inverse approach employs the truncated Newton method with trust region and a modification of automatic reverse differentiation to speed the computation of the optimization problem. The image-reconstruction results confirm that the constrained minimization may offer a more logical approach for the 3-D optical imaging problem than unconstrained optimization.

A method is presented to determine fluorescence decay lifetimes within tissuelike scattering media. Fluorescence lifetimes are determined for micromolar concentrations of the dyes 3,3'-Diethylthiatricarbocyanine Iodide and Indocyanine Green by frequency-domain investigations of light propagating in turbid media. Dual-wavelength photon-migration measurements that use intensity-modulated sources at excitation and emission wavelengths of the fluorophores provide optical parameters of the media as well as fluorescence properties of the dyes. The deduction of fluorescence lifetimes requires no calibration with reference fluorophores, and the results are shown to be independent of dye concentration.

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.

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.

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.

Measurements of nanosecond and subnanosecond fluorescence lifetimes are restricted to dilute, nonscattering systems since excitation and emission photon times of flight significantly affect measured fluorescent decay kinetics. We provide the theoretical rationale for frequency-domain measurements of phase-shift and amplitude demodulation made at excitation and emission wavelengths for direct determination of lifetimes in tissues and other scattering media. We confirm our analytical expressions using standard laser dyes such as 3,3'-diethylthiatricarbocyanine iodide, IR- 125, and IR- 140 in polystyrene suspensions with similar scattering properties as tissues. Our results have significant implication for lifetime-based spectroscopy in tissues and other scattering media.