Edite Figueiras

Laser Speckle Imaging to Monitor Microvascular Blood Flow: A Review

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.

Optical projection tomography as a tool for 3D imaging of hydrogels

An Optical Projection Tomography (OPT) system was developed and optimized to image 3D tissue engineered products based in hydrogels. We develop pre-reconstruction algorithms to get the best result from the reconstruction procedure, which include correction of the illumination and determination of sample center of rotation (CoR). Existing methods for CoR determination based on the detection of the maximum variance of reconstructed slices failed, so we develop a new CoR search method based in the detection of the variance sharpest local maximum. We show the capabilities of the system to give quantitative information of different types of hydrogels that may be useful in its characterization.

Effect of static scatterers in laser speckle contrast imaging: an experimental study on correlation and contrast

Laser speckle contrast imaging (LSCI) is a non-invasive microvascular blood flow assessment technique with good temporal and spatial resolution. Most LSCI systems, including commercial devices, can perform only qualitative blood flow evaluation, which is a major limitation of this technique. There are several factors that prevent the utilization of LSCI as a quantitative technique. Among these factors, we can highlight the effect of static scatterers. The goal of this work was to study the influence of differences in static and dynamic scatterer concentration on laser speckle correlation and contrast. In order to achieve this, a laser speckle prototype was developed and tested using an optical phantom with various concentrations of static and dynamic scatterers. It was found that the laser speckle correlation could be used to estimate the relative concentration of static/dynamic scatterers within a sample. Moreover, the speckle correlation proved to be independent of the dynamic scatterer velocity, which is a fundamental characteristic to be used in contrast correction.

Laser speckle contrast analysis for pulse waveform extraction

The present paper shows a method for pulse waveform extraction using laser speckle contrast analysis. An experimental apparatus was assembled, using a coherent light source and a digital video camera to record time varying speckle patterns emitted from the radial artery. The speckle data were analysed by computing the speckle pattern contrast on a sequence of video frames. The speckle pulse wave signal was then compared with a photoplethysmographic signal both time and frequency domain. A total of thirty data-sets were acquired from 10 individuals. Subjects heart rate was identified with a root mean square error of 1.3 beats per minute. Signals similarity was evaluated using spectral coherence with an overall mean coherence of 0.63. Speckle contrast analysis is a newly commercialized technique to monitor microvascular blood flow. However, these results demonstrate the ability of the same technique to extract pulse waveform information. The inclusion of this feature in the current speckle devices is only associated with a slightly change in the signal processing techniques and video acquisition parameters but can be very useful in clinical context.

Finite Element Mapping for Efficient Image Reconstruction in Rotational Electrical Impedance Tomography

Electrical impedance tomography (EIT) is a label free harmless imaging method capable of imaging differences in electrical conductivity of a sample. In EIT, a low frequency current is injected into the sample, voltage differences on sample surface are measured, and from these measurements, interior conductivity distribution is reconstructed. To increase the accuracy of reconstruction, rotational EIT (rEIT) has been proposed where independent measurements are taken from multiple rotational positions around the sample. However, the benefit of conventional electrode configurations are limited to small number of rotational positions. We have presented an approach called Limited Angle Full Revolution rEIT (LAFR-rEIT) that uses a small number of electrodes and large number of rotational measurement position measurements over 360°. The results are comparable to previous rotational EIT implementations, and furthermore, the limited EIT boundary access provides space for simultaneous attachment of other measurement modalities. On the other hand, the increased number of measurement positions cause an increase in computational complexity, and optimization is required until 3D applications are feasible. This work presents modifications into finite element mesh presentation of the imaging domain and outlines an optimization, that enables sufficiently light rotation for 3D LAFR-rEIT computations.

Performance Analysis of Spatial Laser Speckle Contrast Implementations.

This work presents an analysis of the performances for four different implementations used to compute laser speckle contrast on images. Laser speckle contrast is a widely used imaging technique for biomedical applications. These implementations were tested using synthetic laser speckle patterns with different resolutions, speckle sizes, and contrasts. From the applied methods, three implementations are already known in the literature. A new alternative is proposed herein, which relies on two-dimensional convolutions, in order to improve the image processing time without compromising the contrast assessment. The proposed implementation achieves a processing time two orders of magnitude lower than the analytical evaluation. The goal of this technical manuscript is to help the developers and researchers in computing laser speckle contrast images.

Laser Based Sensors for Hemodynamic Parameters Measurement

This short note presents an overview of some laser-baser methods used to extract hemodynamic parameters. The sensors and methods have been divided in two application fields, microcirculation and macrocirculation. For further readings about this subject please consult the cited references.

Computational model for multifocal imaging in optical projection tomography and numerical analysis of all-in-focus fusion in tomographic image reconstruction

Optical projection tomography (OPT) is a non-invasive 3D imaging method that has been used to study small biological samples. In OPT samples can be mounted in hydrogel scaffold mimicking real life extracellular matrix, and hence grown in all natural dimensions. In optical imaging systems, focusing lenses are required for image acquisition. Due to these lenses, particles at a certain distance from objectives — in the focal plane of the lens — are captured accurately and the further a particle is from the focal plane the blurrier it is captured in the resulting image. To compensate this limitation, multifocal OPT is implemented, where images from each angle are taken with multiple focal planes at different distances. From these images, parts in focus are detected and combined into a single image using all-in-focus fusion algorithm. In this work we present computational way of modeling multifocal imaging and use the presented model to assess the performance of two different all-in-focus fusion methods.

Texture-property relations of bioamine crosslinked gellan gum hydrogels

Gellan gum is a hydrogel with potential for soft tissue engineering but a quick and thorough method is needed for screening of different possible compositions for more extensive studies. Here optical projection tomography in bright field mode was used to image nearly transparent hydrogels to record their optical texture in 3D. The gained Haralick’s textural features were then analyzed with multiple discriminant analysis and combined with data from mechanical testing and neuronal cell culturing. We show the usefulness of optical texture analysis in screening of hydrogel compositions when aiming for tissue engineering applications.

Optical Projection Tomography Imaging of Single Cells in 3D Gellan Gum Hydrogel

3D cell culturing has become attractive in biology and tissue engineering laboratories as it mimics the natural environment for the cells to grow, differentiate and interact in all directions. To study cells and cellular interactions within 3D, cell culture requires a non-invasive, non-toxic, and high resolution imaging technique. The existing imaging techniques face challenges to image cells in 3D macro-scale environment because of the sample size, photo-bleaching or resolution requirements. Optical projection tomography (OPT) is a non-invasive 3D imaging technique for samples in the range of 1-10 mm. It works in both emission and transmission modes for fluorescence and bright-field imaging, respectively. Here, we demonstrate the use of OPT for imaging of cells and cellular materials in 3D gellan gum hydrogel. Fluorescence projection images showed alive and dead human lung fibroblast cells encapsulated in hydrogel. The mineralized extracellular matrix secreted by the human adipose stem cells in the hydrogel was evenly distributed throughout the sample and analyzable in 3D volume.