Statistical detection and imaging of objects hidden in turbid media using ballistic photons

We exploit recent advances in active high-resolution imaging through scattering media with ballistic photons. We derive the fundamental limits on the accuracy of the estimated parameters of a mathematical model that describes such an imaging scenario and compare the performance of ballistic and conventional imaging systems. This model is later used to derive optimal single-pixel statistical tests for detecting objects hidden in turbid media. To improve the detection rate of the aforementioned single-pixel detectors, we develop a multiscale algorithm based on the generalized likelihood ratio test framework. Moreover, considering the effect of diffraction, we derive a lower bound on the achievable spatial resolution of the proposed imaging systems. Furthermore, we present the first experimental ballistic scanner that directly takes advantage of novel adaptive sampling and reconstruction techniques.     

Attenuation of energy in time-gated transillumination imaging: numerical results

Numerical simulations based on a semianalytic Monte Carlo procedure have been developed to investigate the feasibility of a time-gated transillumination imaging system that could be useful for breast cancer screening. Numerical results showed that attenuation of earlier received photons strongly increases when the gating time decreases and strongly depends on single-scattering properties of the medium. For gating times shorter than ~ 5 ps, attenuation of scattered received energy approaches the exponential attenuation expected for unscattered photons. For typical optical properties of a healthy breast the use of gating times shorter than 100 ps seems to be questionable because of the low level of received energy. The image resolution expected with gating times longer than lOOps is of the order of 10mm. A comparison with predictions or the diffusion equation showed the inadequacy of this theory to describe the dependence of earlier photons on the single-scattering properties of the medium. However, thepredictions of the diffusion equation are within the range of values obtained from numerical simulations for the different scattering properties investigated.     

Simulating the effects of multiple scattering on images of dense sprays and particle fields

Most optical measurements in turbid media (including sprays, fogs, particulate and colloidal suspensions) assume single scattering of the detected photons. Multiple scattering introduces error, which has been quantified in very few systems. To quantify this error, we have written a flexible Monte Carlo photon transport simulation code capable of handling any three-dimensional geometry. Simulations of planar laser spray imaging with large, nonabsorbing particles show that up to 50% of the photons reaching the camera are multiply scattered. Because forward scattering dominates, the image is affected little. For particles with more absorption or with size closer to the wavelength of the light than those we have simulated, the effects are expected to be more serious.     

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.     

Effects of polarization state and scatterer concentration on optical imaging through scattering media

The imaging resolution in turbid media is severely degraded by light scattering. Resolution can be improved if the unscattered or weakly scattered light is extracted. Here the state of polarization of the emerging light is used to discriminate photon path length, with the more weakly scattered photons maintaining their original polarization state. It is experimentally demonstrated that over a wide range of scatterer concentrations there exist three distinct imaging regimes. It is also shown that within the intermediate regime one of two distinct imaging techniques is appropriate, depending on the particle size.     

Thermal neutron imaging detectors combining novel composite foil convertors and gaseous electron multipiers

The potential of a thermal neutron imaging system based on a composite neutron convertor foil combined with a low-pressure, multistep avalanche chamber is demonstrated. Neutron-induced charged particles from a primary convertor element induce multiple low-energy electrons escaping from a second thin film of high electron-emissive material. We investigated the performance of detectors with Gd- and Li-based primary convertors coated with CsI as a secondary electron emitter. It is shown, that the detector can be operated with high stability at a sufficiently high gain to detect all escaping particles. A localisation resolution of 0.4 mm (fwhm) was obtained. With Li-based convertors a very low γ-ray sensitivity was established. The good imaging performance, free of parallax errors in divergent neutron beams, fast time resolution, low occupation time and high count rate capability, make this instrument an excellent tool for time-resolved neutron scattering experiments and for neutron radiography and tomography.     

Raman and luminescence probes for the study of compound semiconductors

Optical characterization using local probes with submicrometric spatial resolution is very useful for many problems concerning compound semiconductors and devices. In particular, micro-Raman spectroscopy and cathodoluminescence spectrum imaging are very powerful analytical techniques that manage good signal/noise ratios allowing to acquire images including spectral information and slightly submicrometric spatial resolution in a short time. The main analytical and spatial resolution aspects concerning both techniques are addressed. Several examples are presented in order to illustrate the capabilities of micro-Raman and cathodoluminescence spectrum imaging for the characterization of compound semiconductors and the devices based on them.     

Monte Carlo study of a 3D Compton imaging device with GEANT4

In this paper we investigate, with a detailed Monte Carlo simulation based on Geant4, the novel approach of Lenti (2008) [1] to 3D imaging with photon scattering. A monochromatic and well collimated gamma beam is used to illuminate the object to be imaged and the photons Compton scattered are detected by means of a surrounding germanium strip detector. The impact position and the energy of the photons are measured with high precision and the scattering position along the beam axis is calculated. We study as an application of this technique the case of brain imaging but the results can be applied as well to situations where a lighter object, with localized variations of density, is embedded in a denser container. We report here the attainable sensitivity in the detection of density variations as a function of the beam energy, the depth inside the object and size and density of the inclusions. Using a 600 keV gamma beam, for an inclusion with a density increase of 30% with respect to the surrounding tissue and thickness along the beam of 5 mm, we obtain at midbrain position a resolution of about 2 mm and a contrast of 12%. In addition the simulation indicates that for the same gamma beam energy a complete brain scan would result in an effective dose of about 1 mSv.     

Imaging time-resolved electrothermal atomization laser-excited atomic fluorescence spectrometry for determination of mercury in seawater

In this study, direct determination of mercury at the nanogram per liter level in the complex seawater matrix by imaging time-resolved electrothermal atomization laser-excited atomic fluorescence spectrometry (ITR-ETA-LEAFS) is described. In the case of mercury, the use of a nonresonant line for fluorescence detection with only one laser excitation is not possible. For measurements at the 253.652 nm resonant line, scattering phenomena have been minimized by eliminating the simultaneous vaporization of salts and by using temporal resolution and the imaging mode of the camera. Electrothermal conditions (0.1 M oxalic acid as matrix modifier, low atomization temperature) have been optimized in order to suppress chemical interferences and to obtain a good separation of specific signal and seawater background signal. For ETA-LEAFS, a specific response has been obtained for Hg with the use of time resolution. Moreover, an important improvement of the detection limit has been obtained by selecting, from the furnace image, pixels collecting the lowest number of scattered photons. Using optimal experimental conditions, a detection limit of 10 ng L(-1) for 10 μL of sample, close to the lowest concentration level of total Hg in the open ocean, has been obtained.     

Single-shot, two-dimensional ballistic imaging through scattering media

Imaging through scattering materials is an important research area that is generally limited to medical diagnostic applications. Published techniques typically use a method of time- or coherence-gating of ballistic photons that separates these early photons in order to acquire an image without the large background created by the later-arriving diffuse light. Because of the limited number of ballistic photons and the typically low signal-to-noise ratios of these schemes, a large number of averages or scans is necessary. If the desired image is changing rapidly, however, single images of this transient are required. We have therefore evaluated a two-dimensional, single-shot method that can be used for imaging rapid transients in scattering environments.     

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.     

Microscopic imaging through a turbid medium by use of annular objectives for angle gating

We report a new method for microscopic imaging of an object embedded in a turbid medium. The new method is based on the angle-gating mechanism achieved by the use of polarized annular objectives in the illumination and collection paths of a microscopic imaging system. A detailed experimental study is presented of the effects of the size of annular obstructions on image quality when turbid media, including polystyrene microspheres and milk suspensions, are imaged. Images of 22-mum polystyrene microspheres embedded in the turbid media show that misinterpretation can occur when circular objectives are used, because of the detection of mainly multiply scattered photons (i.e., diffusing photons). However, when annular objectives are employed, diffusing photons from a turbid medium can be efficiently suppressed; thus image contrast appears correctly, and image resolution is increased.     

Imaging of highly turbid media by the absorption method

The results of a study on imaging that is based on the absorption method are presented. This method is based on attenuation measurements carried out in the presence of a sufficiently high absorption coefficient by the use of a continuous-wave source. The benefit of absorption on image quality comes from the strong attenuation of photons traveling along long trajectories. When the absorption coefficient is increased, the received energy decreases, but the mean path length of received photons decreases. The effect of increasing the absorption coefficient is similar to that of decreasing the gating time when the time-gating technique is used. Experimental results showed that the spatial resolution obtained with the absorption technique is similar to that obtained with the time-gating technique.     

Compton back-scattering imaging

A Compton back-scatter tomographical imaging model with minimal constraints was imposed as a potential contender for on-line portable imaging. The line-scan method based on scattered photon energy spectra can obtain a fairly high scan speed and spatial resolution. As such, radiation scattering imaging (or density reconstruction) is viewed as a nonlinear inverse problem. A multicriteria optimization-based density reconstruction from computed compton scattering data illustrates the feasibility of this method. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 410–414, 1999     

Influence of optical properties on two-photon fluorescence imaging in turbid samples

A numerical model was developed to simulate the effects of tissue optical properties, objective numerical aperture (N.A.), and instrument performance on two-photon-excited fluorescence imaging of turbid samples. Model data are compared with measurements of fluorescent microspheres in a tissuelike scattering phantom. Our results show that the measured two-photon-excited signal decays exponentially with increasing focal depth. The overall decay constant is a function of absorption and scattering parameters at both excitation and emission wavelengths. The generation of two-photon fluorescence is shown to be independent of the scattering anisotropy, g, except for g > 0.95. The N.A. for which the maximum signal is collected varies with depth, although this effect is not seen until the focal plane is greater than two scattering mean free paths into the sample. Overall, measurements and model results indicate that resolution in two-photon microscopy is dependent solely on the ability to deliver sufficient ballistic photon density to the focal volume. As a result we show that lateral resolution in two-photon microscopy is largely unaffected by tissue optical properties in the range typically encountered in soft tissues, although the maximum imaging depth is strongly dependent on absorption and scattering coefficients, scattering anisotropy, and objective N.A..     

Spectral holography for imaging through scattering media

Absorbers were imaged through a highly scattering material by application of spectral holography. A broad-spectrum source forms the spectral hologram of light transmitted by a diffuser, and computer processing reconstructs a series of images of the diffuser as it would have appeared with scattered light that traveled different path lengths. One of these reconstructed images is certain to be an image formed with first-arriving light and is therefore an image of the diffuser as seen with the least-scattered light. Experimental results present an image of an absorber hidden by a diffuser in which the gating was done by computer.     

Mass production and dynamic imaging of fluorescent nanodiamonds

Fluorescent nanodiamond is a new nanomaterial that possesses several useful properties, including good biocompatibility, excellent photostability and facile surface functionalizability. Moreover, when excited by a laser, defect centres within the nanodiamond emit photons that are capable of penetrating tissue, making them well suited for biological imaging applications. Here, we show that bright fluorescent nanodiamonds can be produced in large quantities by irradiating synthetic diamond nanocrystallites with helium ions. The fluorescence is sufficiently bright and stable to allow three-dimensional tracking of a single particle within the cell by means of either one- or two-photon-excited fluorescence microscopy. The excellent photophysical characteristics are maintained for particles as small as 25 nm, suggesting that fluorescent nanodiamond is an ideal probe for long-term tracking and imaging in vivo, with good temporal and spatial resolution.     

Optical/UV and X-Ray Microwave Kinetic Inductance Strip Detectors

Microwave Kinetic Inductance Detectors (MKIDs) are superconducting detectors that sense the change in the surface impedance of a thin superconducting film when Cooper Pairs are broken by using a high quality factor resonant circuit. We are developing strip detectors that have aluminum MKID sensors on both ends of a rectangular tantalum strip. These devices can provide one dimensional spatial imaging with high quantum efficiency, energy resolution, and microsecond time resolution for single photons from the IR to the X-ray. We have demonstrated X-ray strip detectors with an energy resolution of 62 eV at 6 keV, and hope to improve this substantially. We will also report on our progress towards optical arrays for a planned camera for the Palomar 200″ telescope.     

Early-photon fluorescence tomography of a heterogeneous mouse model with the telegraph equation

In this study, we investigate the performance of early-photon fluorescence tomography based on a heterogeneous mouse model. The telegraph equation is used to accurately describe the propagation of light in tissues at short times. The optimal time gate for early photons is determined by singular value analysis at first. Then, fluorescent targets located in different organs of the mouse model are investigated. The simulation results demonstrate that the reconstructed tomographic images based on early photons yield improvement in spatial resolution and quantification than the quasi-CW measurements. Meanwhile, compared with the homogeneous model, the use of the heterogeneous model can improve the accuracy of fluorescence distribution and quantification in early-photon fluorescence tomography.