Consideration of a spread-out source in problems of near-infrared optical tomography

When the light propagates in media where absorption is not negligible and/or scattering is weak, a contribution to the energy density coming from ballistic photons cannot be neglected. A point source effectively spreads out over a scattering volume and its spatial distribution is described by the source function. We consider a boundary value problem of light propagation in half-space for such a source on the basis of the telegraph equation. A solution is found by convolution of Green's function with the source function. The final result shows a significant difference in the behavior of the radiant energy density between the solution obtained for a distributed source and the diffusion approximation. Our results agree well with the Monte Carlo simulations over a broad range of scattering and/or absorption conditions. The obtained results are of practical importance in luminescence optical tomography because an erroneous shape of the energy density function may lead to an incorrect estimate of the light source depth after image reconstruction. The range of applications of the diffusion approximation is also discussed.     

Anisotropic scattering of light in random media: incoherent backscattered spotlight

We discuss the anisotropic scattering of unpolarized light in optically dense random media and the flux analysis of an incoherent backscattered spotlight. We present a classic statistical approach based on the photon-diffusion approximation and Monte Carlo simulations to describe the anisotropic propagation of ballistic and long-path photons in a semi-infinite random medium with internal reflections. An imagery technique with high gray-level resolution is used to measure the surface flux density in the incoherent backscattered spotlight. We investigated light scattering from homogeneous suspensions of nonspherical alumina particles in water. We analyzed the particle volume fraction and the particle-size dependence of the surface flux density to determine the transport mean free path and the optical properties of scatterers from scaling laws that account for short-path photons and internal reflections.     

Quantification of glistenings in intraocular lenses using a ballistic-photon removing integrating-sphere method

An alternative method for quantification of glistenings in intraocular lenses (IOLs) using an integrating sphere with an adjustable back aperture to remove ballistic photons is presented. Glistenings in soft IOLs have been known for more than a decade; however, their severity and visual impact are still under investigation. A number of studies have been made to quantitatively describe glistenings in IOLs. Quantization and precise grading of IOLs will provide needed information to evaluate the severity and visual impact of glistenings in patients. We investigated the use of a simple modification of an integrating-sphere method to eliminate ballistic photons to quantitatively measure scattered light from glistenings in IOLs. The method described in this paper provides a simple and effective way to quantitatively characterize glistenings in vitro. It may be especially useful to quantify scattering associated with low-grade glistenings where the density of the scattering centers is low. Finally, the modified integrating-sphere method may also be generally applicable to quantitatively characterize scattering from other optical media.     

Near-infrared laser tomographic imaging with right-angled scattered coherent light using an optical heterodyne-detection-based confocal scanning system

We demonstrate, what is to the best of our knowledge, a novel optical tomographic method for the visualization of the inner structure of scattering media such as biological tissue in the near-infrared region. We constructed a scanning confocal imaging system with a cross-axes arrangement using optical fibers. This system is based on the optical heterodyne technique and enables the detection of very weak coherence photons that are generated in the spatially restricted confocal region and scattered laterally (90 degrees ) against an incident beam. To evaluate the fundamental imaging capabilities of the system, we assessed measurements from scattering phantoms composed of an Intralipid suspension with varying volume concentrations. The results of this study demonstrate that the right-angled scattered light adheres to the Lambert-Beer law and that the present system can detect light propagating through a distance of approximately 31l of the mean free path. An inclusion as small as 100 microm can be discriminated in a scattering media with an optical thickness of 4. We investigated the potential of the proposed system for imaging biological tissues in preliminary experiments using samples of chicken breast tissue.     

Streak camera: a multidetector for diffuse optical tomography

We describe an experimental setup for time-resolved diffuse optical tomography that uses a seven-channel light guide to transmit scattered light to a streak camera. This setup permits the simultaneous measurement of the time profiles of photons reemitted at different boundary sites of the objects studied. The instrument, its main specifications, and detector-specific data analysis before image reconstruction are described. The instrumentation was tested with phantoms simulating biological tissue, and the absorption and reduced scattering images that were obtained are discussed.     

Measurement of the local optical properties of turbid media by differential path-length spectroscopy

We report on the development of an optical-fiber-based diagnostic tool with which to determine the local optical properties of a turbid medium. By using a single fiber in contact with the medium to deliver and detect white light, we have optimized the probability of detection of photons scattered from small depths. The contribution of scattered light from greater depths to the signal is measured and subtracted with an additional fiber, i.e., a collection fiber, to yield a differential backscatter signal. Phantoms demonstrate that, when photons have large mean free paths compared with the fiber diameter, single scattering dominates the differential backscatter signal. When photons have small mean free paths compared with the fiber diameter, the apparent path length of the photons that contribute to the differential backscatter signal becomes approximately equal to 4/5 of the fiber diameter. This effect is nearly independent of the optical properties of the sample under investigation.     

Picosecond magneto-optical phenomena in turbid media: toward magneto-optical characterization of highly scattering biological samples

The behavior of a Faraday-active turbid medium under ultrafast optical excitation is investigated. As the degree of polarization of early arriving photons is mostly preserved during the first 100 ps after the arrival of the ballistic component, the possibility of using the magneto-optical rotation of the light polarization as a new tool for tissue characterization is addressed. A technique is proposed for determining photon-scattering statistics in turbid biological media. The analysis is performed on the basis that photons trapped in multiple-scattering events leave the medium with larger induced rotation angles. A measurement of the magneto-optical rotatory power of turbid biological samples is possible if only the early arriving unscattered photons are probed.     

Collection efficiency of a single optical fiber in turbid media

If a single optical fiber is used for both delivery and collection of light, two major factors affect the measurement of collected light: (1) the light transport in the medium that describes the amount of light that returns to the fiber and (2) the light coupling to the optical fiber that depends on the angular distribution of photons entering the fiber. We focus on the importance of the latter factor and describe how the efficiency of the coupling depends on the optical properties of the medium. For highly scattering tissues, the efficiency is well predicted by the numerical aperture (NA) of the fiber. For lower scattering, such as in soft tissues, photons arrive at the fiber from deeper depths, and the coupling efficiency could increase twofold to threefold above that predicted by the NA.     

Nonlinear optical processes in optically trapped InP nanowires

We report on the observation of nonlinear optical excitation and related photoluminescence from single InP semiconductor nanowires held in suspension using a gradient force optical tweezers. Photoexcitation of free carriers is achieved through absorption of infrared (1.17 eV) photons from the trapping source via a combination of two- and three-photon processes. This was confirmed by power-dependent photoluminescence measurements. Marked differences in spectral features are noted between nonlinear optical excitation and direct excitation and are related to band-filling effects. Direct observation of second harmonic generation in trapped InP nanowires confirms the presence of nonlinear optical processes.     

Numerical simulation of a dual frequency all-optical flip-flop based on a nonlinear semiconductor distributed feedback laser structure

A new all-optical flip-flop generating light at two different wavelengths λ1 (state "a"), or λ2 (state "b") was suggested. It consists of an active layer and a nonlinear wave-guiding layer. Two parallel nonlinear gratings having different periods and periodic negative nonlinearities exist along the propagation direction in the wave-guiding layer. In state "a," the first grating provides the optical feedback for lasing, and the second grating is weak. In state "b," due to optical nonlinearity, the first grating weakens, and the second one provides the optical feedback for lasing. The refractive index nonlinearity is due to the direct absorption of photons at the Urbach tail. The device is triggered from state "a" to state "b" and vise versa by input optical pulses of wavelengths λ2 and λ1, respectively. The time domain simulations show switching dynamics in nanosecond time scale.     

Excitation with a focused, pulsed optical beam in scattering media: diffraction effects

To gain a better understanding of the spatiotemporal problems that are encountered in two-photon excitation fluorescence imaging through highly scattering media, we investigate how diffraction affects the three-dimensional intensity distribution of a focused, pulsed optical beam propagating inside a scattering medium. In practice, the full potential of the two-photon excitation fluorescence imaging is unrealized at long scattering depths, owing to the unwanted temporal and spatial broadening of the femtosecond excitation light pulse that reduces the energy density at the geometric focus while it increases the excitation energy density in the out-of-focus regions. To analyze the excitation intensity distribution, we modify the Monte Carlo-based photon-transport model to a semi-quantum-mechanical representation that combines the wave properties of light with the particle behavior of the propagating photons. In our model the propagating photon is represented by a plane wave with its propagation direction in the scattering medium determined by the Monte Carlo technique. The intensity distribution in the focal region is given by the square of the linear superposition of the various plane waves that arrive at different incident angles and optical path lengths. In the absence of scattering, the propagation model yields the intensity distribution that is predicted by the Huygens-Fresnel principle. We quantify the decrease of the energy density delivered at the geometric focus as a function of the optical depth to the mean-free-path ratio that yields the average number of scattering events that a photon encounters as it propagates toward the focus. Both isotropic and anisotropic scattering media are considered. Three values for the numerical aperture (NA) of the focusing lens are considered: NA = 0.25, 0.5, 0.75.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Dynamics of ultrashort light pulses in a semiconductor superlattice in the presence of a magnetic field

A system of equations that describes the propagation of ultrashort light pulses (optical solitons) in a semiconductor superlattice in the presence of a magnetic field is obtained using coupled Maxwell equations for the electromagnetic field and the Boltzmann equation written in the relaxation time approximation for the one-electron distribution function. It is shown that an initial linearly polarized light pulse induces a field with the orthogonal polarization in the sample. The dynamics of joint propagation of the initial and induced pulses in the sample is studied.     

Generating non-classical light inside an optical cavity by depositing photons

The creation of non-classical states of light is an interesting problem, that we solve sending atoms through an optical cavity. We show that it is possible to add or subtract many photons from a cavity field by interacting it resonantly with a two-level atom. The atom, after entangling with the field inside the cavity and exiting it, may be measured in one of the Schmidt states, producing a multiphoton process (in the sense that can add or annihilate more photons than a single transition allows), i.e. adding or subtracting several photons from the cavity field. By plotting the quadratures and the Husimi Q-function, we also show that the non-classical state produced by such measurements is a squeezed state.     

Concentration measurements of a light absorber localized in a scattering medium

Using a time-resolved technique, we have studied two methods of quantifying the concentration of a light absorber localized in a scattering medium. Because some of the transmitted photons pass through the absorber region many times because of complicated photon propagation in the scattering medium, we used the following methods. First, early-arriving photons passing through the absorber approximately once were selected. Using these photons, we quantified the concentration of the absorber by an ordinary spectroscopic method based on the Beer-Lambert law. Second, for later-arriving photons, after empirically determining the distribution of the number of passes through the absorber as a function of detection time, we obtained the concentration by applying this function to the Beer-Lambert law.     

Trends in optical biomedical imaging

Abstract

Recent progress in the development of optical techniques for non-invasive imaging in biology and medicine is reviewed. We start with some fundamental considerations of light propagation in random media. We then discuss imaging using coherent unscattered light and diffuse light. Microscopic techniques based on one- and two-photon fluorescence are described and several types of nonlinear and time-resolved microscopy are explained. We conclude with some application examples and an outlook.     

Generalized diffusion model in optical tomography with clear layers

We introduce a generalized diffusion equation that models the propagation of photons in highly scattering domains with thin nonscattering clear layers. Classical diffusion models break down in the presence of clear layers. The model that we propose accurately accounts for the clear-layer effects and has a computational cost comparable to that of classical diffusion. It is based on modeling the propagation in the clear layer as a local tangential diffusion process. It can be justified mathematically in the limit of small mean free paths and is shown numerically to be very accurate in two- and three-dimensional idealized cases. We believe that this model can be used as an accurate forward model in optical tomography.     

Ultrasound-modulated optical tomography with intense acoustic bursts

Ultrasound-modulated optical tomography (UOT) detects ultrasonically modulated light to spatially localize multiply scattered photons in turbid media with the ultimate goal of imaging the optical properties in living subjects. A principal challenge of the technique is weak modulated signal strength. We discuss ways to push the limits of signal enhancement with intense acoustic bursts while conforming to optical and ultrasonic safety standards. A CCD-based speckle-contrast detection scheme is used to detect acoustically modulated light by measuring changes in speckle statistics between ultrasound-on and ultrasound-off states. The CCD image capture is synchronized with the ultrasound burst pulse sequence. Transient acoustic radiation force, a consequence of bursts, is seen to produce slight signal enhancement over pure ultrasonic-modulation mechanisms for bursts and CCD exposure times of the order of milliseconds. However, acoustic radiation-force-induced shear waves are launched away from the acoustic sample volume, which degrade UOT spatial resolution. By time gating the CCD camera to capture modulated light before radiation force has an opportunity to accumulate significant tissue displacement, we reduce the effects of shear-wave image degradation, while enabling very high signal-to-noise ratios. Additionally, we maintain high-resolution images representative of optical and not mechanical contrast. Signal-to-noise levels are sufficiently high so as to enable acquisition of 2D images of phantoms with one acoustic burst per pixel.