Optical correlation techniques for the investigation of colloidal systems

This review aims to provide a simple introduction to the application of optical correlation methods in colloidal science. In particular, I plan to show that full appraisal of the intimate relation between light scattering and microscopy allows designing novel powerful investigation techniques that combine their powers. An extended version of this paper will appear in "ColloidalFoundations of Nanoscience", edited by D. Berti and G. Palazzo, Elsevier (ISBN 978-0-444-59541-6). I am very grateful to the publisher for having granted me the permission to post this preprint on arXiv.Comment: 19 pages, 5 figure

Molecular Contrast Optical Coherence Tomography: A Review

This article reviews the current state of research on the use of molecular contrast agents in optical coherence tomography (OCT) imaging techniques. After a brief discussion of the basic principle of OCT and the importance of incorporating molecular contrast agent usage into this imaging modality, we shall present an overview of the different molecular contrast OCT (MCOCT) methods that have been developed thus far. We will then discuss several important practical issues that define the possible range of contrast agent choice, the design criteria for engineered molecular contrast agent and the implementability of a given MCOCT method for clinical or biological applications. We will conclude by outlining a few areas of pursuit that deserve a greater degree of research and development

EFFECTS OF SCATTERING AND ABSORPTION ON LASER SPECKLE CONTRAST IMAGING

Laser Speckle Contrast Imaging (LSCI) is a real-time, non-invasive method in used to investigate blood flow and perfusion in biological tissues with high temporal and spatial resolution. A reduction in speckle contrast due to particle motion is the primary contrast mechanism in LSCI. Motion results in speckle fluctuations in time and reduces the contrast over a given camera integration period. There are a variety of parameters that effect contrast besides motion. The optical properties of the scattering medium are one of the parameters effecting LSCI values. Changes in blood hematocrit levels manifest as changes in optical properties. In this work, we explore the effects of different hematocrit levels on LSCI contrast values using fluid phantoms with varying optical properties. Herein, the combined effects of scattering and absorption coefficients on LSCI values are investigated using fluid phantoms. These fluid phantoms were designed to mimic the scattering and absorbing properties of blood with varying levels of hematocrit. The flow phantoms in our experiments contained different concentrations of glass microspheres (brand name Luxil) and India ink mixed with DI water. The different number of scatterers and absorbers in the phantoms mimic the scattering and absorption behaviors of blood with different number of red blood cells. An LSCI setup combined with a simple flow system was used in our experiments in order to investigate the effects of combined scattering and absorption coefficient of 121 samples with different concentrations of Luxil and India ink microspheres. The fluid phantoms were run in 2mm glass tubing on top of a plastic block using a mini peristaltic pump. An LSCI setup imaged the flow using a CCD camera. A MATLAB GUI controlled the pump and camera to provide near real-time contrast images of the flow. An 11x11 matrix of phantoms was created. Scattering coefficient was varied on the columns and absorption coefficient was varied on the rows such that the first element of the matrix is water and the last element contains the phantom with the maximum number of scatterers and absorbers. A hundred raw speckle images were recorded for each phantom experiment using the described optical setup. The experiments were conducted 3 times for each element of the matrix. The 11x11 results matrix displayed the average speckle image of all 300 raw speckle images. Additionally, the matrix was filled by the contrast images where contrast was defined as standard deviation of intensity over mean intensity. In order to compare the results numerically, we calculated the ratio of the contrast from the same size window of moving portion over the static portion of the phantoms. According to the results from LSCI experiments, an increase in scattering and absorption coefficients led to a reduction in contrast values of LSCI images. By increasing the number of scatterers and absorbers (equivalent to changing hematocrit level), the optical properties (scattering and absorption coefficient) increased, which led to a reduction in contrast value in the moving area. A negative slope linear curve describes the relationship between and scattering coefficient and between and absorption coefficient

Adaptive Speckle Imaging Interferometry: a new technique for the analysis of microstructure dynamics, drying processes and coating formation

We describe an extension of multi-speckle diffusing wave spectroscopy adapted to follow the non-stationary microscopic dynamics in drying films and coatings in a very reactive way and with a high dynamic range. We call this technique "Adaptive Speckle Imaging Interferometry". We introduce an efficient tool, the inter-image distance, to evaluate the speckle dynamics, and the concept of "speckle rate" (SR, in Hz) to quantify this dynamics. The adaptive algorithm plots a simple kinetics, the time evolution of the SR, providing a non-invasive characterization of drying phenomena. A new commercial instrument, called HORUS(R), based on ASII and specialized in the analysis of film formation and drying processes is presented.Comment: 11 pages, 4 figure

Influence of the absorber dimensions on wavefront shaping based on volumetric optoacoustic feedback

The recently demonstrated control over light distribution through turbid media based on real-time three-dimensional optoacoustic feedback has offered promising prospects to interferometrically focus light within scattering objects. Nevertheless, the focusing capacity of the feedback-based approach is strongly conditioned by the number of effectively resolvable optical modes (speckles). In this letter, we experimentally tested the light intensity enhancement achieved with optoacoustic feedback measurements from different sizes of absorbing microparticles. The importance of the obtained results is discussed in the context of potential signal enhancement at deep locations within a scattering medium where the effective speckle sizes approach the minimum values dictated by optical diffraction

Some comments on particle image displacement velocimetry

Laser speckle velocimetry (LSV) or particle image displacement velocimetry, is introduced. This technique provides the simultaneous visualization of the two-dimensional streamline pattern in unsteady flows as well as the quantification of the velocity field over an entire plane. The advantage of this technique is that the velocity field can be measured over an entire plane of the flow field simultaneously, with accuracy and spatial resolution. From this the instantaneous vorticity field can be easily obtained. This constitutes a great asset for the study of a variety of flows that evolve stochastically in both space and time. The basic concept of LSV; methods of data acquisition and reduction, examples of its use, and parameters that affect its utilization are described

Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Overcoming the acoustic diffraction limit in photoacoustic imaging by localization of flowing absorbers

The resolution of photoacoustic imaging deep inside scattering media is limited by the acoustic diffraction limit. In this work, taking inspiration from super-resolution imaging techniques developed to beat the optical diffraction limit, we demonstrate that the localization of individual optical absorbers can provide super-resolution photoacoustic imaging well beyond the acoustic diffraction limit. As a proof-of-principle experiment, photoacoustic cross-sectional images of microfluidic channels were obtained with a 15 MHz linear CMUT array while absorbing beads were flown through the channels. The localization of individual absorbers allowed to obtain super-resolved cross-sectional image of the channels, by reconstructing both the channel width and position with an accuracy better than $\lambda/10$. Given the discrete nature of endogenous absorbers such as red blood cells, or that of exogenous particular contrast agents, localization is a promising approach to push the current resolution limits of photoacoustic imaging