Julio C. Ramirez-San-Juan

Impact of velocity distribution assumption on simplified laser speckle imaging equation.

Since blood flow is tightly coupled to the health status of biological tissue, several instruments have been developed to monitor blood flow and perfusion dynamics. One such instrument is laser speckle imaging. The goal of this study was to evaluate the use of two velocity distribution assumptions (Lorentzian- and Gaussian-based) to calculate speckle flow index (SFI) values. When the normalized autocorrelation function for the Lorentzian and Gaussian velocity distributions satisfy the same definition of correlation time, then the same velocity range is predicted for low speckle contrast (0 < C < 0.6) and predict different flow velocity range for high contrast. Our derived equations form the basis for simplified calculations of SFI values.

Linear response range characterization and in vivo application of laser speckle imaging of blood flow dynamics

Noninvasive blood flow imaging can provide critical information on the state of biological tissue and the efficacy of approaches to treat disease. With laser speckle imaging (LSI), relative changes in blood flow are typically reported, with the assumption that the measured values are on a linear scale. A linear relationship between the measured and actual flow rate values has been suggested. The actual flow rate range, over which this linear relationship is valid, is unknown. Herein we report the linear response range and velocity dynamic range (VDR) of our LSI instrument at two relevant camera integration times. For integration times of 1 and 10 ms, the best case VDR was 80 and 60 dB, respectively, and the worst case VDR was 20 and 50 dB. The best case VDR values were similar to those reported in the literature for optical Doppler tomography. We also demonstrate the potential of LSI for monitoring blood flow dynamics in the rodent dorsal skinfold chamber model. These findings imply that LSI can provide accurate wide-field maps of microvascular blood flow rate dynamics and highlight heterogeneities in flow response to the application of exogenous agents.

Time-resolved analysis of cavitation induced by CW lasers in absorbing liquids

We present novel results on thermocavitation using a CW medium-power near infrared laser (lambda=975 nm) focused into a saturated copper nitrate saline solution. Due to the large absorption coefficient at the laser wavelength, the solution can be heated to its superheat limit (T(sh) approximately 270-300 degrees C). Superheated water undergoes explosive phase transition around T(sh) producing approximately half-hemispheric bubbles (gamma approximately 0.5) in close contact with the substrate. We report the temporal dynamic of the cavitation bubble, which is much shorter than previously reported under similar conditions. It was found that the bubble radius and pressure wave amplitude emitted on bubble collapse decreases exponentially with the power laser. Thermocavitation can be a useful tool for the generation of ultrasonic waves and controlled ablation for use in high-resolution lithography.

Coherent-Mode Representation of Propagation-Invariant Fields

The general coherent-mode structure of a propagation-invariant field is found as the solution of the differential equation for propagation of the coherent modes of said field. It is shown that three kinds of the propagation-invariant fields exist. Examples of propagation-invariant fields of the three kinds are discussed.

Simple correction factor for laser speckle imaging of flow dynamics

One of the major constraints facing laser speckle imaging for blood-flow measurement is reliable measurement of the correlation time (τ(C)) of the back-scattered light and, hence, the bloods speed in blood vessels. In this Letter, we present a new model expression for integrated speckle contrast, which accounts not only for temporal integration but spatial integration, too, due to the finite size of the pixel of the CCD camera; as a result, we find that a correction factor should be introduced to the measured speckle contrast to properly determine τ(C); otherwise, the measured bloods speed is overestimated. Experimental results support our theoretical model.

Spatial versus temporal laser speckle contrast analyses in the presence of static optical scatterers

Abstract. Previously published data demonstrate that the temporal processing algorithm for laser speckle contrast imaging (LSCI) can improve the visibility of deep blood vessels and is less susceptible to static speckle artifacts when compared with the spatial algorithm. To the best of our knowledge, the extent to which the temporal algorithm can accurately predict the speckle contrast associated with flow in deep blood vessels has not been quantified. Here, we employed two phantom systems and imaging setups (epi-illumination and transillumination) to study the contrast predicted by the spatial and temporal algorithms in subsurface capillary tubes as a function of the camera exposure time and the actual flow speed. Our data with both imaging setups suggest that the contrast predicted by the temporal algorithm, and therefore the relative flow speed, is nearly independent of the degree of static optical scattering that contributes to the overall measured speckle pattern. Collectively, these results strongly suggest the potential of temporal LSCI at a single-exposure time to assess accurately the changes in blood flow even in the presence of substantial static optical scattering.

Self-calibrating common-path interferometry

A quantitative phase measuring technique is presented that estimates the object phase from a series of phase shifted interferograms that are obtained in a common-path configuration with unknown phase shifts. The derived random phase shifting algorithm for common-path interferometers is based on the Generalized Phase Contrast theory [pl. Opt.40(2), 268 (2001)10.1063/1.1404846], which accounts for the particular image formation and includes effects that are not present in two-beam interferometry. It is shown experimentally that this technique can be used within common-path configurations employing nonlinear liquid crystal materials as self-induced phase filters for quantitative phase imaging without the need of phase shift calibrations. The advantages of such liquid crystal elements compared to spatial light modulator based solutions are given by the cost-effectiveness, self-alignment, and the generation of diminutive dimensions of the phase filter size, giving unique performance advantages.

Effect of ambient humidity on light transmittance through skin phantoms during cryogen spray cooling

Cryogen spray cooling (CSC) is a technique employed to reduce the risk of epidermal damage during dermatologic laser surgery. However, while CSC protects the epidermis from non-specific thermal damage, it might reduce the effective fluence reaching the target chromophore due to scattering of light by the spray droplets and subsequent water condensation/freezing on the skin surface. The objective of this work was to study the effect of ambient humidity (omega) on light transmittance during CSC. An integrating sphere was employed to measure the dynamics of light transmittance through a deformable agar phantom during CSC. The study included two representative CSC spurt patterns studied using four omega: 57, 40, 20 and 12%. Results show that during CSC, as omega increased, light transmittance decreased. For the highest humidity level (57%) studied, light transmittance reached a minimum of 55% approximately 30 ms after spurt termination. In a controlled environment with omega = 12%, light transmittance reached a minimum of 87% approximately 30 ms after spurt termination. The reduced light transmittance immediately after spurt termination was most likely because of scattering of light caused by condensation of water vapour due to aggressive cooling of ambient air in the wake of the cryogen spurt.

Generation of light string and light capillary beams

Abstract Recently we reported a new modal theory of propagation-invariant optical fields [Opt. Commun. 195 (2001) 27–34]. Within the framework of this theory we predicted the existence of so-called light string and light capillary beams. Here, we propose an optical technique for generating these beams and demonstrate it through experimental results.

Fiber-based laser speckle imaging for the detection of pulsatile flow

In endodontics, a major diagnostic challenge is the accurate assessment of pulp status. In this study, we designed and characterized a fiber‐based laser speckle imaging system to study pulsatile blood flow in the tooth.