When laser light illuminates a diffuse object, it produces a random interference effect known as a speckle pattern. If there is movement in the object, the speckles fluctuate in intensity. These fluctuations can provide infor- mation about the movement. A simple way of accessing this information is to image the speckle pattern with an exposure time longer than the shortest speckle fluctuation time scale—the fluctuations cause a blurring of the speckle, leading to a reduction in the local speckle contrast. Thus, velocity distributions are coded as speckle con- trast variations. The same information can be obtained by using the Doppler effect, but producing a two-dimen- sional Doppler map requires either scanning of the laser beam or imaging with a high-speed camera: laser speckle contrast imaging (LSCI) avoids the need to scan and can be performed with a normal CCD- or CMOS-camera. LSCI is used primarily to map flow systems, especially blood flow. The development of LSCI is reviewed and its lim- itations and problems are investigated. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.

/pdf/laser-speckle-contrast-imaging-theoretical-and-practical-17q04l6pa3.pdf

Laser speckle contrast imaging: theoretical and practical limitations

When a biological tissue is illuminated with coherent light, an interference pattern will be formed at the detector, the so-called speckle pattern. Laser speckle contrast imaging (LSCI) is a technique based on the dynamic change in this backscattered light as a result of interaction with red blood cells. It can be used to visualize perfusion in various tissues and, even though this technique has been extensively described in the literature, the actual clinical implementation lags behind. We provide an overview of LSCI as a tool to image tissue perfusion. We present a brief introduction to the theory, review clinical studies from various medical fields, and discuss current limitations impeding clinical acceptance.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-24/issue-8/080901/Clinical-applications-of-laser-speckle-contrast-imaging-a-review/10.1117/1.JBO.24.8.080901.pdf

Clinical applications of laser speckle contrast imaging: a review.

Laser speckle imaging (LSI) based on the speckle contrast analysis is a simple and robust technique for imaging of heterogeneous dynamics. LSI finds frequent application for dynamical mapping of cerebral blood flow, as it features high spatial and temporal resolution. However, the quantitative interpretation of the acquired data is not straightforward for the common case of a speckle field formed by both by moving and localized scatterers such as blood cells and bone or tissue. Here we present a novel processing scheme, we call dynamic laser speckle imaging (dLSI), that can be used to correctly extract the temporal correlation parameters from the speckle contrast measured in the presence of a static or slow-evolving background. The static light contribution is derived from the measurements by cross-correlating sequential speckle images. In-vivo speckle imaging experiments performed in the rodent brain demonstrate that dLSI leads to improved results. The cerebral hemodynamic response observed through the thinned and intact skull are more pronounced in the dLSI images as compared to the standard speckle contrast analysis. The proposed method also yields benefits with respect to the quality of the speckle images by suppressing contributions of non-uniformly distributed specular reflections.

https://doc.rero.ch/record/13203/files/scheffold_dls.pdf

Dynamic laser speckle imaging of cerebral blood flow

Laser speckle is a complex interference phenomenon that can easily be understood, in concept, but is difficult to predict mathematically, because it is a stochastic process. The use of laser speckle to produce images, which can carry many types of information, is called laser speckle imaging (LSI). The biomedical applications of LSI started in 1981 and, since then, many scientists have improved the laser speckle theory and developed different imaging techniques. During this process, some inconsistencies have been propagated up to now. These inconsistencies should be clarified in order to avoid errors in future works. This review presents a review of the laser speckle theory used in biomedical applications. Moreover, we also make a review of the practical concepts that are useful in the construction of laser speckle imagers. This study is not only an exposition of the concepts that can be found in the literature but also a critical analysis of the investigations presented so far. Concepts like scatterers velocity distribution, effect of static scatterers, optimal speckle size, light penetration angle, and contrast computation algorithms are discussed in detail.

/pdf/laser-speckle-imaging-to-monitor-microvascular-blood-flow-a-5a5su6l5eb.pdf

Laser Speckle Imaging to Monitor Microvascular Blood Flow: A Review

Laser speckle contrast imaging (LSCI) provides a rapid characterization of cortical flow dynamics for functional monitoring of the microcirculation. The technique stems from interactions of laser light with moving particles. These interactions encode the encountered Doppler phenomena within a random interference pattern imaged in widefield, known as laser speckle. Studies of neurovascular function and coupling with LSCI have benefited from the real-time characterization of functional dynamics in the laboratory setting through quantification of perfusion dynamics. While the technique has largely been relegated to acute small animal imaging, its scalability is being assessed and characterized for both chronic and clinical neurovascular imaging.

https://foil.bme.utexas.edu/media/papers/10.1038/jcbfm.2015.84.pdf

Expanding applications, accuracy, and interpretation of laser speckle contrast imaging of cerebral blood flow.

Evaluation of laser-speckle contrast image analysis techniques in the cortical microcirculation of piglets

In laser speckle contrast analysis (LASCA) used for imaging of blood flow, besides the moving blood cells, the speckle pattern is also influenced by the imaging system and scattering properties of the laser-illuminated static surface. A latex microsphere (650 nm size) emulsion was covered with scattering semitransparent materials (Teflon foils, tracing paper). Speckle images were recorded with different exposure times (0.2 ms-500 ms), and correlation times were determined by parameterizing the theoretical contrast-exposure time function. The correlation times obtained for covered and uncovered microsphere emulsions were in good agreement. The possibility of obtaining comparable, setup-independent results in blood perfusion monitoring can contribute to better applicability of LASCA.

Real correlation time measurement in laser speckle contrast analysis using wide exposure time range images

Laser Speckle Contrast Analysis (LASCA) was introduced in 1981. Since then, several enhancements were applied to it. Nowadays, thetechnique can provide relatively high accuracy as well as high temporal and spatial resolution during the examination of ocular or cerebraltissues. However, in the case of skin, the results are highly affected by the intensive scattering on the skin surface, as the scattering onthe non-moving parts of the sample lead to the detrimental decrease of the accuracy. We present a LASCA method based on the use ofmultiple exposure times, combined with the switching-mode control of the light intensity and a special sampling technique to achieve nearto real-time measurement of the skin perfusion. The system based on our method is able to automatically handle the destructive effect ofthe skin surface and re-tune itself according to the changes of the sample, while it provides full-field perfusion maps with high accuracy,without the need of any precalibrations.

https://www.jeos.org/index.php/jeos_rp/article/download/13053/1052

Self-Tuning Laser Speckle Contrast Analysis Based on Multiple Exposure Times with Enhanced Temporal Resolution

The applicability of multiple exposure time based LASCA was demonstrated on skin blood perfusion monitoring. The method was compared to β-corrected and non-corrected single exposure time measurements while the data simultaneously recorded by a laser Doppler system were used as reference. The skin of the forearm was illuminated with the light of an 808 nm laser diode. Speckle images were recorded in the exposure time range of 1-100 ms and relative perfusion changes caused by ischemia were determined. The results showed an obvious nonlinear relationship between the data of the non-corrected single exposure time LASCA method and the laser Doppler system, while a good linearity was observed for the multiple exposure time method with a correlation factor of 0.83. Keywords-LASCA; Multiple Exposure Times; Skin Perfusion; Laser Doppler Flowmetry

Multiple Exposure Time Based Laser Speckle Contrast Analysis: Demonstration of Applicability in Skin Perfusion Measurements

Switching mode intensity modulation had been applied to the illumination of a LASCA system using multiple exposure times to substitute a variable neutral filter, which allows the variation of the exposure time from frame to frame. In most cases the direct current (DC) control of the laser intensity can lead to changes in the wavelength of regular laser diodes. The use of pulsed mode operation can avoid the mode hopping of the laser light while it offers the tuning of the average intensity in a wide range. While operating the camera at a constant integration time, the length of the exposition was varied from 2 ms to 100 ms by changing the duration of the illuminating laser pulse train. The pulse train was built up of 10 pulses, each having the length of 40 μs with variable separation time. The mode stability of the light source was monitored with a spectrometer, meanwhile speckle images of a static scattering surface were recorded as well. This paper demonstrates the usefulness of this technique in a multi-exposure LASCA system for monitoring the changes in the skin perfusion

Dániel Zölei

Papers

Evaluation of laser-speckle contrast image analysis techniques in the cortical microcirculation of piglets

Real correlation time measurement in laser speckle contrast analysis using wide exposure time range images

Self-Tuning Laser Speckle Contrast Analysis Based on Multiple Exposure Times with Enhanced Temporal Resolution

Multiple Exposure Time Based Laser Speckle Contrast Analysis: Demonstration of Applicability in Skin Perfusion Measurements

Laser power modulation with wavelength stabilization in multiple exposure laser speckle contrast analysis