Single scattering by red blood cells

Optical discrimination of pathological red blood cells

Fast diagnostic methods are crucial to reduce the burden on healthcare systems. Currently, detection of diabetes complications such as neuropathy requires time-consuming approaches to observe the correlated red blood cells (RBCs) morphological changes. To tackle this issue, an optical analysis of RBCs in air was conducted in the 250-2500 nm range. The distinct oscillations present in the scattered and direct transmittance spectra have been analyzed with both Mie theory and anomalous diffraction approximation. The results provide information about the swelling at the ends of RBCs and directly relate the optical data to RBCs morphology and deformability. Both models agree on a reduction in the size and deformability of RBCs in diabetic patients, thus opening the way to diabetes diagnosis and disease progression assessment.

Dragonfly algorithm-support vector machine approach for prediction the optical properties of blood

Knowledge of the optical properties of blood plays important role in medical diagnostics and therapeutic applications in laser medicine. In this paper, we present a very rapid and accurate artificial intelligent approach using Dragonfly Algorithm/Support Vector Machine models to estimate the optical properties of blood, specifically the absorption coefficient, and the scattering coefficient using key parameters such as wavelength (nm), hematocrit percentage (%), and saturation of oxygen (%), in building very highly accurate Dragonfly Algorithm-Support Vector Regression models (DA-SVR). 1000 training and testing sets were selected in the wavelength range of 250-1200 nm and the hematocrit of 0-100%. The performance of the proposed method is characterized by high accuracy indicated in the correlation coefficients (R) of 0.9994 and 0.9957 for absorption and scattering coefficients, respectively. In addition, the root mean squared error values (RMSE) of 0.972 and 2.9193, as well as low mean absolute error values (MAE) of 0.2173 and 0.2423, this result showed a strong match with the experimental data. The models can be used to accurately predict the absorption and scattering coefficients of blood, and provide a reliable reference for future studies on the optical properties of human blood.

Non-invasive method for assessment of inflammation

This article explores the potential of non-invasive measurement for elevated levels of erythrocyte aggregation in vivo, which have been correlated with a higher risk of inflammatory processes. The study proposes utilizing a dynamic light scattering approach to measure aggregability. The sensor modules, referred to as "mDLS," comprise VCSEL and two photodiodes. Two of these modules are placed on an inflatable transparent cuff, which is then fitted to the subject's finger root, with one sensor module positioned on each side. By temporarily halting blood flow for one minute using over-systolic inflation of the cuff, signals from both sensors are recorded. The study involved three distinct groups of subjects: a control group consisting of 65 individuals, a group of 29 hospitalized COVID-19 patients, and a group of 34 hospitalized patients with inflammatory diseases. Through experimental results, significant differences in signal kinetic behavior were observed between the control group and the two other groups. These differences were attributed to the rate of red blood cell (RBC) aggregation, which is closely associated with inflammation. Overall, the study emphasizes the potential of non-invasive diagnostic tools in evaluating inflammatory processes by analyzing RBC aggregation.

LED based large field of view off-axis quantitative phase contrast microscopy by hologram multiplexing

In this manuscript, we describe the development of a single shot, self-referencing wavefront division, multiplexing digital holographic microscope employing LED sources for large field of view quantitative phase imaging of biological samples. To address the difficulties arising while performing interferometry with low temporally coherent sources, an optical arrangement utilizing multiple Fresnel Biprisms is used for hologram multiplexing, enhancing the field of view and increasing the signal to noise ratio. Biprisms offers the ease of obtaining interference patterns by automatically matching the path length between the two off-axis beams. The use of low temporally coherent sources reduces the speckle noise and the cost, and the form factor of the setup. The developed technique was implemented using both visible and UV LEDs and tested on polystyrene microspheres and human erythrocytes.

Leveraging lifetime information to perform real-time 3D single-particle tracking in noisy environments

A microscopy platform that leverages the arrival time of individual photons to enable 3D single-particle tracking of fast-moving (translational diffusion coefficient of ≃3.3 µm²/s) particles in high-background environments is reported here. It combines a hardware-based time-gating module, which enables the rate of photon processing to be as high as 100 MHz, with a two-photon-excited 3D single-particle tracking confocal microscope to enable high sample penetration depth. Proof-of-principle experiments where single quantum dots are tracked in solutions containing dye-stained cellulose, are shown with tracking performance markedly improved using the hardware-based time-gating module. Such a microscope design is anticipated to be of use to a variety of communities who wish to track single particles in cellular environments, which commonly have high fluorescence and scattering background.

Using elastic scattering to determination of diseases via urine samples

<abstract> <p>In diagnosing the urinary tract and related diseases, the problem of light scattering from human urine was examined on the basis of classical electromagnetic theory. Numerical calculations were made for the designed cylindrical model with the help of optical parameters in the literature obtained from the laboratory test results of urine samples. In the designed model, the changes of the scattered intensity of the light from the urine solution according to the size parameter of the particles and the angular distribution of the system (including forward, side and back scattering) in the equatorial plane were obtained, in both transverse electric (TE) and transverse magnetic (TM) of the polarization states of the light. It was observed that the molecular density changes caused by the materials in the urine sample changed primarily the optical parameters and indirectly the intensity distribution of the scattered light. Thus, with the contribution of standard data provided as a result of light scatter calculations from urine samples taken from people with normal and different diseases, it will be easier to diagnose diseases that will be encountered later.</p> </abstract>

Deep learning-based cell identification and disease diagnosis using spatio-temporal cellular dynamics in compact digital holographic microscopy

We demonstrate a successful deep learning strategy for cell identification and disease diagnosis using spatio-temporal cell information recorded by a digital holographic microscopy system. Shearing digital holographic microscopy is employed using a low-cost, compact, field-portable and 3D-printed microscopy system to record video-rate data of live biological cells with nanometer sensitivity in terms of axial membrane fluctuations, then features are extracted from the reconstructed phase profiles of segmented cells at each time instance for classification. The time-varying data of each extracted feature is input into a recurrent bi-directional long short-term memory (Bi-LSTM) network which learns to classify cells based on their time-varying behavior. Our approach is presented for cell identification between the morphologically similar cases of cow and horse red blood cells. Furthermore, the proposed deep learning strategy is demonstrated as having improved performance over conventional machine learning approaches on a clinically relevant dataset of human red blood cells from healthy individuals and those with sickle cell disease. The results are presented at both the cell and patient levels. To the best of our knowledge, this is the first report of deep learning for spatio-temporal-based cell identification and disease detection using a digital holographic microscopy system.

Beyond diffuse correlations: deciphering random flow in time-of-flight resolved light dynamics

Diffusing wave spectroscopy (DWS) and diffuse correlation spectroscopy (DCS) can assess blood flow index (BFI) of biological tissue with multiply scattered light. Though the main biological function of red blood cells (RBCs) is advection, in DWS/DCS, RBCs are assumed to undergo Brownian motion. To explain this discrepancy, we critically examine the cumulant approximation, a major assumption in DWS/DCS. We present a precise criterion for validity of the cumulant approximation, and in realistic tissue models, identify conditions that invalidate it. We show that, in physiologically relevant scenarios, the first cumulant term for random flow and second cumulant term for Brownian motion alone can cancel each other. In such circumstances, assuming pure Brownian motion of RBCs and the first cumulant approximation, a routine practice in DWS/DCS of BFI, can yield good agreement with data, but only because errors due to two incorrect assumptions cancel out. We conclude that correctly assessing random flow from scattered light dynamics requires going beyond the cumulant approximation and propose a more accurate model to do so.

Generative optical modeling of whole blood for detecting platelets in lens-free images

In this paper, we consider the task of detecting platelets in images of diluted whole blood taken with a lens-free microscope. Despite having several advantages over traditional microscopes, lens-free imaging systems have the significant challenge that the resolution of the system is typically limited by the pixel dimensions of the image sensor. As a result of this limited resolution, detecting platelets is very difficult even by manual inspection of the images due to the fact that platelets occupy just a few pixels of the reconstructed image. To address this challenge, we develop an optical model of diluted whole blood to generate physically realistic simulated holograms suitable for training machine learning models in a supervised manner. We then use this model to train a convolutional neural network (CNN) for platelet detection and validate our approach by developing a novel optical configuration which allows collecting both lens-free and fluorescent microscopy images of the same field of view of diluted whole blood samples with fluorescently labeled platelets.

Time-of-flight resolved light field fluctuations reveal deep human tissue physiology

Red blood cells (RBCs) transport oxygen to tissues and remove carbon dioxide. Diffuse optical flowmetry (DOF) assesses deep tissue RBC dynamics by measuring coherent fluctuations of multiply scattered near-infrared light intensity. While classical DOF measurements empirically correlate with blood flow, they remain far-removed from light scattering physics and difficult to interpret in layered media. To advance DOF measurements closer to the physics, here we introduce an interferometric technique, surmounting challenges of bulk motion to apply it in awake humans. We reveal two measurement dimensions: optical phase, and time-of-flight (TOF), the latter with 22 picosecond resolution. With this multidimensional data, we directly confirm the unordered, or Brownian, nature of optically probed RBC dynamics typically assumed in classical DOF. We illustrate how incorrect absorption assumptions, anisotropic RBC scattering, and layered tissues may confound classical DOF. By comparison, our direct method enables accurate and comprehensive assessment of blood flow dynamics in humans.

Non-mydriatic chorioretinal imaging in a transmission geometry and application to retinal oximetry

The human retina is typically imaged in a reflection geometry, where light is delivered through the pupil and images are formed from the light reflected back from the retina. In this configuration, artifacts caused by retinal surface reflex are often encountered, which complicate quantitative interpretation of the reflection images. We present an alternative illumination method, which avoids these artifacts. The method uses deeply penetrating near-infrared (NIR) light delivered transcranially from the side of the head, and exploits multiple scattering to redirect a portion of the light towards the posterior eye. This unique transmission geometry simplifies absorption measurements and enables flash-free, non-mydriatic imaging as deep as the choroid. Images taken with this new transillumination approach are applied to retinal oximetry.

Measurement of blood-oxygen saturation using a photoacoustic technique in the rabbit hypoxemia model

The golden standard method to obtain accurate blood oxygen saturation is blood gas analysis that needs invasive procedure of blood sampling. Photoacoustic technique enables us to measure real-time blood oxygen saturation without invasive procedure. The aim of this study is to use the photoacoustic technique, an optical method, for accurately determining oxygen saturation in vivo. We measured induced photoacoustic signals of arterial blood in the rabbit model of stable hypoxemia after irradiation at 750 and 800 nm. Oxygen saturation was calculated from the photoacoustic signals using two calibration curves. Calibration curve 1 is a conventional curve derived from the absorbance coefficient of hemoglobin, whereas calibration curve 2 is derived from the photoacoustic signals obtained from the original blood vessel model. Simultaneously, blood-gas analysis was performed to obtain the reference standard of oxygen saturation. Regression analysis and Bland-Altman analysis were performed to assess the accuracy of oxygen saturation obtained using the two methods. The oxygen saturation calculated using calibration curves 1 and 2 showed strong correlations with the reference standard in regression analysis (R = 0.965, 0.964, respectively). The Bland-Altman analysis revealed better agreement and precision with calibration curve 2, whereas there was significant underestimation of values obtained using calibration curve 1. Photoacoustic measurement of oxygen saturation using calibration curve 2 provided an accurate estimate of oxygen saturation, which was similar to that obtained using a portable blood-gas analyzer.

Wavefront division digital holographic microscopy

Digital holographic microscopy is the state of the art quantitative phase imaging of micro-objects including living cells. It is an ideal tool to image and quantify cell thickness profiles with nanometer thickness resolution. Digital holographic techniques usually are implemented using a two-beam setup that may be bulky and may not be field portable. Self-referencing techniques provide compact geometry but suffer from a reduction of the field of view. Here, we discuss the development of a wavefront division digital holographic microscope providing the full field of view with a compact system. The proposed approach uses a wavefront division module consisting of two lenses. The developed microscope is tested experimentally by measuring the physical and mechanical properties of red blood cells.

Confocal Laser Endomicroscopy and Optical Coherence Tomography for the Diagnosis of Prostate Cancer: A Needle-Based, In Vivo Feasibility Study Protocol (IDEAL Phase 2A)

BACKGROUND

Focal therapy for prostate cancer has been proposed as an alternative treatment to whole-gland therapies in selected men to diminish side effects in localized prostate cancer. As nowadays imaging cannot offer complete prostate cancer disease characterization, multicore systematic biopsies are recommended (transrectal or transperineal). Optical imaging techniques such as confocal laser endomicroscopy and optical coherence tomography allow in vivo, high-resolution imaging. Moreover, they can provide real-time visualization and analysis of tissue and have the potential to offer additive diagnostic information.

OBJECTIVE

This study has 2 separate primary objectives. The first is to assess the technical feasibility and safety of in vivo focal imaging with confocal laser endomicroscopy and optical coherence tomography. The second is to identify and define characteristics of prostate cancer and normal prostate tissue in confocal laser endomicroscopy and optical coherence tomography imaging by comparing these images with the corresponding histopathology.

METHODS

In this prospective, in vivo feasibility study, needle-based confocal laser endomicroscopy and optical coherence tomography imaging will be performed before transperineal template mapping biopsy or radical prostatectomy. First, confocal laser endomicroscopy and optical coherence tomography will be performed in 4 patients (2 for each imaging modality) undergoing transperineal template mapping biopsy to assess the feasibility and safety of confocal laser endomicroscopy and optical coherence tomography. If proven to be safe and feasible, confocal laser endomicroscopy and optical coherence tomography will be performed in 10 patients (5 for each imaging modality) undergoing radical prostatectomy. Confocal laser endomicroscopy and optical coherence tomography images will be analyzed by independent, blinded observers. Confocal laser endomicroscopy- and optical coherence tomography-based qualitative and quantitative characteristics and histopathology will be compared. The study complies with the IDEAL (Idea, Development, Exploration, Assessment, Long-term study) stage 2a recommendations.

RESULTS

At present, the study is enrolling patients and results and outcomes are expected in 2019.

CONCLUSIONS

Confocal laser endomicroscopy and optical coherence tomography are promising optical imaging techniques that can visualize and analyze tissue structure, possible tumor grade, and architecture in real time. They can potentially provide real-time, high-resolution microscopic imaging and tissue characteristics of prostate cancer in conjunction with magnetic resonance imaging or transrectal ultrasound fusion-guided biopsy procedures. This study will provide insight into the feasibility and tissue-specific characteristics of confocal laser endomicroscopy and optical coherence tomography for real-time optical analysis of prostate cancer.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03253458; https://clinicaltrials.gov/ct2/show/NCT03253458 (Archived by WebCite at http://www.webcitation.org/6z9owM66B).

REGISTERED REPORT IDENTIFIER

RR1-10.2196/9813.

Sickle cell disease diagnosis based on spatio-temporal cell dynamics analysis using 3D printed shearing digital holographic microscopy

We present a spatio-temporal analysis of cell membrane fluctuations to distinguish healthy patients from patients with sickle cell disease. A video hologram containing either healthy red blood cells (h-RBCs) or sickle cell disease red blood cells (SCD-RBCs) was recorded using a low-cost, compact, 3D printed shearing interferometer. Reconstructions were created for each hologram frame (time steps), forming a spatio-temporal data cube. Features were extracted by computing the standard deviations and the mean of the height fluctuations over time and for every location on the cell membrane, resulting in two-dimensional standard deviation and mean maps, followed by taking the standard deviations of these maps. The optical flow algorithm was used to estimate the apparent motion fields between subsequent frames (reconstructions). The standard deviation of the magnitude of the optical flow vectors across all frames was then computed. In addition, seven morphological cell (spatial) features based on optical path length were extracted from the cells to further improve the classification accuracy. A random forest classifier was trained to perform cell identification to distinguish between SCD-RBCs and h-RBCs. To the best of our knowledge, this is the first report of machine learning assisted cell identification and diagnosis of sickle cell disease based on cell membrane fluctuations and morphology using both spatio-temporal and spatial analysis.

Reflectivity and transmissivity of a surface covered by a disordered monolayer of large and tenuous particles: theory versus experiment

The main objective of this paper is to endorse a recently derived theoretical model for the coherent reflectance and transmittance from a surface supporting a disordered monolayer of large and tenuous particles by comparison with experimental measurements. The model is based on the so-called anomalous-diffraction approximation and is assumed to be valid for small and moderate angles of incidence. We prepared disordered monolayers of spherical polystyrene particles of 1.8 μm diameter and of human red blood cells on glass microscope slides. In both cases, particles were immersed in a liquid of refractive index close to that of the particle. We measured the relative reflectivity and transmissivity of the samples versus the wavelength of light at normal incidence and the reflectivity and transmissivity versus the angle of incidence at a fixed wavelength, and compared with predictions of the anomalous-diffraction approximation model. For the polystyrene particle samples, we also compare the results with another available theoretical model developed some years ago to deal with disordered monolayers of highly scattering particles. In the case of red blood cell monolayers, we also present measurements with a hemolyzed sample and a disordered multilayer film for comparison. We find that the anomalous diffraction model can be fitted very well to the experimental curves, in some cases, even for high angles of incidence.

Halo-free Phase Contrast Microscopy

We present a new approach for retrieving halo-free phase contrast microscopy (hfPC) images by upgrading the conventional PC microscope with an external interferometric module, which generates sufficient data for reversing the halo artifact. Acquiring four independent intensity images, our approach first measures haloed phase maps of the sample. We solve for the halo-free sample transmission function by using a physical model of the image formation under partial spatial coherence. Using this halo-free sample transmission, we can numerically generate artifact-free PC images. Furthermore, this transmission can be further used to obtain quantitative information about the sample, e.g., the thickness with known refractive indices, dry mass of live cells during their cycles. We tested our hfPC method on various control samples, e.g., beads, pillars and validated its potential for biological investigation by imaging live HeLa cells, red blood cells, and neurons.

Collimated light reflection and transmission of a surface partially covered by large and tenuous particles

We derive simple approximate expressions for the reflectivity and transmissivity of light from disordered monolayers of tenuous particles of dimensions larger than the wavelength and supported by a flat interface. The expressions derived can be used for different particle shapes and for moderate angles of incidence. We then investigate the effects of particle shape and orientation on reflectivity and transmissivity spectra of a monolayer of tenuous particles containing an optical chromophore in a solution in their interior. We also simulate the effects of a particle's shape and orientation on the angle dependence of the optical reflectivity and transmissivity. In our examples, we consider disordered monolayers of particles analogous to some biological cells.

Using electron microscopy to calculate optical properties of biological samples

The microscopic structural origins of optical properties in biological media are still not fully understood. Better understanding these origins can serve to improve the utility of existing techniques and facilitate the discovery of other novel techniques. We propose a novel analysis technique using electron microscopy (EM) to calculate optical properties of specific biological structures. This method is demonstrated with images of human epithelial colon cell nuclei. The spectrum of anisotropy factor g, the phase function and the shape factor D of the nuclei are calculated. The results show strong agreement with an independent study. This method provides a new way to extract the true phase function of biological samples and provides an independent validation for optical property measurement techniques.

Development of a photon-cell interactive monte carlo simulation for non-invasive measurement of blood glucose level by Raman spectroscopy

Turbidity variation is one of the major limitations in Raman spectroscopy for quantifying blood components, such as glucose, non-invasively. To overcome this limitation, we have developed a Raman scattering simulation using a photon-cell interactive Monte Carlo (pciMC) model that tracks photon migration in both the extra- and intracellular spaces without relying on the macroscopic scattering phase function and anisotropy factor. The interaction of photons at the plasma-cell boundary of randomly oriented three-dimensionally biconcave red blood cells (RBCs) is modeled using geometric optics. The validity of the developed pciMCRaman was investigated by comparing simulation and experimental results of Raman spectroscopy of glucose level in a bovine blood sample. The scattering of the excitation laser at a wavelength of 785 nm was simulated considering the changes in the refractive index of the extracellular solution. Based on the excitation laser photon distribution within the blood, the Raman photon derived from the hemoglobin and glucose molecule at the Raman shift of 1140 cm(-1) = 862 nm was generated, and the photons reaching the detection area were counted. The simulation and experimental results showed good correlation. It is speculated that pciMCRaman can provide information about the ability and limitations of the measurement of blood glucose level.

Evaluation of erythrocyte flow at a bearing gap in a hydrodynamically levitated centrifugal blood pump

We have developed a hydrodynamically levitated centrifugal blood pump for extracorporeal circulatory support. In the blood pump, a spiral groove bearing was adopted for a thrust bearing. In the spiral groove bearing, separation of erythrocytes and plasma by plasma skimming has been postulated to occur. However, it is not clarified that plasma skimming occurs in a spiral groove bearing. The purpose of this study is to verify whether plasma skimming occurs in the spiral groove bearing of a hydrodynamically levitated centrifugal blood pump. For evaluation of plasma skimming in the spiral groove bearing, an impeller levitation performance test using a laser focus displacement meter and a microscopic visualization test of erythrocyte flow using a high-speed microscope were conducted. Bovine blood diluted with autologous plasma to adjust hematocrit to 1.0% was used as a working fluid. Hematocrit on the ridge region in the spiral groove bearing was estimated using image analysis. As a result, hematocrits on the ridge region with gaps of 45 μm, 31 μm, and 25 μm were calculated as 1.0%, 0.6%, and 0.3%, respectively. Maximum skimming efficiency in this study was calculated as 70% with a gap of 25 μm. We confirmed that separation of erythrocyte and plasma occurred in the spiral groove bearing with decrease in bearing gap in a hydrodynamically levitated centrifugal blood pump.

Optical signature of erythrocytes by light scattering in microfluidic flows

A camera-based light scattering approach coupled with a viscoelasticity-induced cell migration technique has been used to characterize the morphological properties of erythrocytes in microfluidic flows. We have obtained the light scattering profiles (LSPs) of individual living cells in microfluidic flows over a wide angular range and matched them with scattering simulations to characterize their morphological properties. The viscoelasticity-induced 3D cell alignment in microfluidic flows has been investigated by bright-field and holographic microscopy tracking, where the latter technique has been used to obtain precise cell alignment profiles in-flow. Such information allows variable cell probability control in microfluidic flows at very low viscoelastic polymer concentrations, obtaining cell measurements that are almost physiological. Our results confirm the possibility of precise, label-free analysis of individual living erythrocytes in microfluidic flows.

Highly stable digital holographic microscope using Sagnac interferometer

Interferometric microscopy has grown into a very potent tool for quantitative phase imaging of biological samples. Among the interfermetric methods, microscopy by digital holography is one of the most effective techniques, especially for studying dynamics of cells. Imaging of cell fluctuations requires digital holographic setups with high temporal stability. Common path setups in which the object and the reference beams encounter the same set of optical elements provide better temporal stability compared to two-beam setups. Here, we present a compact, easy-to-implement, common path digital holographic microscope based on Sagnac interferometer geometry. The microscope is implemented using a diode laser module employing a CCD array or a webcam sensor to record holograms. The system was tested for three-dimensional imaging capability, numerical focusing ability, and temporal stability. Sub-nanometer temporal stability without external vibration isolation components was obtained in both cases. The higher temporal stability makes the microscope compatible to image cell fluctuations, which is demonstrated by imaging the oscillation of the cell membrane of human red blood cells.

Pathway to Retinal Oximetry

Events and discoveries in oxygen monitoring over the past two centuries are presented as the background from which oximetry of the human retina evolved. Achievements and the people behind them are discussed, showing parallels between the work in tissue measurements and later in the eye. Developments in the two-wavelength technique for oxygen saturation measurements in retinal vessels are shown to exploit the forms of imaging technology available over time. The last section provides a short summary of the recent research in retinal diseases using vessel oximetry.

Probing the in vitro cytotoxicity of the veterinary drug oxytetracycline

The study investigated the effect of oxytetracycline (OTC) on the anti-oxidative defense system, the structure (hemolysis rate and morphology) and function (ATP enzyme activity) of human red blood cells (hRBCs) to investigate the possible toxic mechanism of OTC to hRBCs. The experimental results indicate that OTC can cause a decline in the function of the antioxidant defense system of hRBCs, resulting in oxidative stress. OTC can bring about morphological changes to hRBCs, and further leads to hemolysis, when the concentration of OTC is over 8×10(-5) M (about 164 µg/ml). At a low OTC concentration, below 4×10(-5) M (82 µg/ml), OTC can enhance the activity of ATP enzyme of hRBCs, known as hormesis. However, at a high concentration, above 4×10(-5) M (about 82 µg/ml), the ATP enzymatic activity was inhibited, affecting the function of hRBCs. The estalished mechanism of toxicity of OTC to hRBCs can facilitate a deeper understanding of the toxicity of OTC in vivo.

Laser-photophoretic migration and fractionation of human blood cells

Laser photophoretic migration behavior of human blood cells in saline solution was investigated under the irradiation of Nd:YAG laser beam (532 nm) in the absence and the presence of the flow in a fused silica capillary. Red blood cells (RBC) were migrated faster than white blood cells (WBC) and blood pellets to the direction of propagation of laser light. The observed photophoretic velocity of RBC was about 11 times faster than those of others. This was understood from the larger photophoretic efficiency of RBC than that of WBC, which was simulated based on the Mie scattering theory. Furthermore, it was found that, during the photophoretic migration, RBCs spontaneously orientated parallel to the migration direction so as to reduce the drag force. Finally, it was demonstrated that RBC and WBC were separated in a micro-channel flow system by the laser photophoresis.

Retinal oximetry

ABSTRACT.:

PURPOSE

Malfunction of retinal blood flow or oxygenation is believed to be involved in various diseases. Among them are retinal vessel occlusions, diabetic retinopathy and glaucoma. Reliable, non-invasive technology for retinal oxygen measurements has been scarce and most of the knowledge on retinal oxygenation comes from animal studies. This thesis describes human retinal oximetry, performed with novel retinal oximetry technology. The thesis describes studies on retinal vessel oxygen saturation in (1) light and dark in healthy volunteers, (2) central retinal vein occlusion, (3) branch retinal vein occlusion, (4) central retinal artery occlusion, (5) diabetic retinopathy, (6) patients undergoing glaucoma surgery and (7) patients taking glaucoma medication.

METHODS

The retinal oximeter (Oxymap ehf., Reykjavik, Iceland) is based on a fundus camera. An attached image splitter allows the simultaneous capture of four images of the same area of the fundus. Two images are used for further analysis, one acquired with 586 nm light and one with 605 nm light. Light absorbance of retinal vessels is sensitive to oxygen saturation at 605 nm but not at 586 nm. Measurement of reflected light at these wavelengths allows estimation of oxygen saturation in the main retinal vessels. This is performed with custom-made analysis software.

RESULTS

LIGHT AND DARK: After 30 min in the dark, oxygen saturation in retinal arterioles of healthy volunteers was 92 ± 4% (mean ± SD, n = 15). After 5 min in 80 cd/m(2) light, the arteriolar saturation was 89 ± 5%. The decrease was statistically significant (p = 0.008). The corresponding values for retinal venules were 60 ± 5% in the dark and 55 ± 10% in the light (p = 0.020). Similar results were found after alternating 5 min periods of darkness and light. In a second experiment (n = 19), a significant decrease in retinal vessel oxygen saturation was found in 100 cd/m(2) light compared with darkness but 1 and 10 cd/m(2) light had no significant effect. CENTRAL RETINAL VEIN OCCLUSION: In patients with central retinal vein occlusion, the mean saturation in affected retinal venules was 49 ± 12%, while the mean value for venules in the fellow eye was 65 ± 6% (mean ± SD, p = 0.003, n = 8). The retinal arteriolar saturation was the same in affected (99 ± 3%) and the unaffected (99 ± 6%) eyes. The venous oxygen saturation showed much variation between affected eyes. BRANCH RETINAL VEIN OCCLUSION: Median oxygen saturation in venules affected by branch retinal vein occlusion was 59% (range, 12-93%, n = 22), while it was 63% (23-80%) in unaffected venules in the affected eye and 55% (39-80%) in venules in the fellow eye. The difference was not statistically significant (p > 0.05). There was a significant difference between affected arterioles (median 101%; range, 89-115%) and unaffected arterioles (95%, 85-104%) in the affected eye (p < 0.05, n = 18). CENTRAL RETINAL ARTERY OCCLUSION: In a patient with a day's history of central retinal artery occlusion due to temporal arteritis, the mean arteriolar saturation was 71 ± 9% and 63 ± 9% in the venules. One month later, after treatment with prednisolone, the mean arteriolar saturation was 100 ± 4% and the venous saturation 54 ± 5%. DIABETIC RETINOPATHY: When compared with healthy volunteers (n = 31), patients with all categories of diabetic retinopathy had on average 7-10 percentage points higher saturation in retinal arterioles (p < 0.05 for all categories, n = 6-8 in each category). In venules, the saturation was 8-12 percentage points higher (p < 0.05 for all categories). GLAUCOMA SURGERY: Oxygen saturation in retinal arterioles increased by 2 percentage points on average (p = 0.046, n = 19) with surgery, which lowered intraocular pressure from 23 ± 7 mmHg (mean ± SD) to 10 ± 4 mmHg (p < 0.0001). No other significant changes were found (p ≥ 0.35). DORZOLAMIDE: A significant reduction of 3 percentage points was found in arterioles (p < 0.01) and venules (p < 0.05) when patients with glaucoma or ocular hypertension changed from dorzolamide-timolol combination eye drops to timolol alone (n = 6). No change was found in patients, who started on timolol and switched to the combination therapy (p > 0.05, n = 7).

CONCLUSIONS

Dual wavelength oximetry can be used to non-invasively measure retinal vessel oxygen saturation in health and disease. The results indicate that retinal vessel oxygen saturation is (1) increased in the dark, (2) lower in venules affected by central retinal vein occlusions, (3) variable in branch retinal vein occlusion, (4) lower in retinal arterioles in central retinal artery occlusion, (5) increased in diabetic retinopathy, (6-7) mildly affected by glaucoma surgery or dorzolamide.

Estimation of scattering phase function utilizing laser Doppler power density spectra

A new method for the estimation of the light scattering phase function of particles is presented. The method allows us to measure the light scattering phase function of particles of any shape in the full angular range (0°-180°) and is based on the analysis of laser Doppler (LD) power density spectra. The theoretical background of the method and results of its validation using data from Monte Carlo simulations will be presented. For the estimation of the scattering phase function, a phantom measurement setup is proposed containing a LD measurement system and a simple model in which a liquid sample flows through a glass tube fixed in an optically turbid material. The scattering phase function estimation error was thoroughly investigated in relation to the light scattering anisotropy factor g. The error of g estimation is lower than 10% for anisotropy factors larger than 0.5 and decreases with increase of the anisotropy factor (e.g. for g = 0.98, the error of estimation is 0.01%). The analysis of influence of the noise in the measured LD spectrum showed that the g estimation error is lower than 1% for signal to noise ratio higher than 50 dB.

Quantitative phase-contrast imaging with compact digital holographic microscope employing Lloyd's mirror

Digital holographic microscopy (DHM) is one of the most effective techniques used for quantitative phase imaging of cells. Here we present a compact, easy to implement, portable, and very stable DHM setup employing a self-referencing Lloyd's mirror configuration. The microscope is constructed using a diode laser source and a CMOS sensor, making it cost effective. The reconstruction of recorded holograms yields the amplitude and phase information of the object. The temporal stability of the presented technique was found to be around 0.9 nm without any vibration compensation, which makes it ideal for studying cell profile changes. This aspect of the technique is demonstrated by studying membrane fluctuations of red blood cells.

Lateral shearing digital holographic imaging of small biological specimens

A lateral shearing interferometer is used for direct holographic imaging of microorganisms. This is achieved by increasing the shear to be larger than the object size and results in a very simple and inexpensive common-path imaging device that can be easily coupled to the output of an inverted microscope. The shear is created by reflections from the front and back surface of a glass plate. Stability measurements show a standard deviation of the phase measurements of less than 1nm over 8 min. without any vibration compensation. The setup is applied to imaging both microorganisms in a microfluidic channel and red blood cells and reconstructions are presented.

Nanometer-scale sizing accuracy of particle suspensions on an unmodified cell phone using elastic light scattering

We report on the construction of a Fourier plane imaging system attached to a cell phone. By illuminating particle suspensions with a collimated beam from an inexpensive diode laser, angularly resolved scattering patterns are imaged by the phone's camera. Analyzing these patterns with Mie theory results in predictions of size distributions of the particles in suspension. Despite using consumer grade electronics, we extracted size distributions of sphere suspensions with better than 20 nm accuracy in determining the mean size. We also show results from milk, yeast, and blood cells. Performing these measurements on a portable device presents opportunities for field-testing of food quality, process monitoring, and medical diagnosis.

Quantitative analysis of optical properties of flowing blood using a photon-cell interactive Monte Carlo code: effects of red blood cells' orientation on light scattering

Optical properties of flowing blood were analyzed using a photon-cell interactive Monte Carlo (pciMC) model with the physical properties of the flowing red blood cells (RBCs) such as cell size, shape, refractive index, distribution, and orientation as the parameters. The scattering of light by flowing blood at the He-Ne laser wavelength of 632.8 nm was significantly affected by the shear rate. The light was scattered more in the direction of flow as the flow rate increased. Therefore, the light intensity transmitted forward in the direction perpendicular to flow axis decreased. The pciMC model can duplicate the changes in the photon propagation due to moving RBCs with various orientations. The resulting RBC's orientation that best simulated the experimental results was with their long axis perpendicular to the direction of blood flow. Moreover, the scattering probability was dependent on the orientation of the RBCs. Finally, the pciMC code was used to predict the hematocrit of flowing blood with accuracy of approximately 1.0 HCT%. The photon-cell interactive Monte Carlo (pciMC) model can provide optical properties of flowing blood and will facilitate the development of the non-invasive monitoring of blood in extra corporeal circulatory systems.

Versatile multispectral microscope based on light emitting diodes

We describe the development of a novel multispectral microscope, based on light-emitting diodes, capable of acquiring megapixel images in thirteen spectral bands from the ultraviolet to the near infrared. The system captures images and spectra in transmittance, reflectance, and scattering modes. We present as examples of applications ground truth measurements for remote sensing and parasitology diagnostics. The system is a general purpose scientific instrument that could be used to develop dedicated simplified instruments with optimal bands and mode selection.

Evaluation of algorithms for microperfusion assessment by fast simulations of laser Doppler power spectral density

In classical laser Doppler (LD) perfusion measurements, zeroth- and first-order moments of the power spectral density of the LD signal are utilized for the calculation of a signal corresponding to the concentration, speed and flow of red blood cells (RBCs). We have analysed the nonlinearities of the moments in relation to RBC speed distributions, parameters of filters utilized in LD instruments and the signal-to-noise ratio. We have developed a new method for fast simulation of the spectrum of the LD signal. The method is based on a superposition of analytically calculated Doppler shift probability distributions derived for the assumed light scattering phase function. We have validated the method by a comparison of the analytically calculated spectra with results of Monte Carlo (MC) simulations. For the semi-infinite, homogeneous medium and the single Doppler scattering regime, the analytical calculation describes LD spectra with the same accuracy as the MC simulation. The method allows for simulating the LD signal in time domain and furthermore analysing the index of perfusion for the assumed wavelength of the light, optical properties of the tissue and concentration of RBCs. Fast simulations of the LD signal in time domain and its frequency spectrum can be utilized in applications where knowledge of the LD photocurrent is required, e.g. in the development of detectors for tissue microperfusion monitoring or in measurements of the LD autocorrelation function for perfusion measurements. The presented fast method for LD spectra calculation can be used as a tool for evaluation of signal processing algorithms used in the LD method and/or for the development of new algorithms of the LD flowmetry and imaging. We analysed LD spectra obtained by analytical calculations using a classical algorithm applied in classical LD perfusion measurements. We observed nonlinearity of the first moment M₁ for low and high speeds of particles (v < 2 mm s⁻¹, v > 10 mm s⁻¹). It was also noted that the first moment M(1) is less sensitive to the change of the mean RBC speed for flat speed distributions. The low-pass filter frequency f₂ implemented in the LD instrument has a significant influence on the first moment of the spectrum. In particular, for a cut-off frequency lower than 10 kHz the M₁ value is strongly underestimated.

Using elastic light scattering of red blood cells to detect infection of malaria parasite

To develop noninvasive diagnosis of malaria, we studied the light-scattering pattern of healthy and 2% Plasmodium falciparum malaria parasitized red blood cells. By measuring the wavelength-dependent scattering at discrete angles of both forward and backward directions, we found that the signal can clearly distinguish between healthy and ring stage infected red blood cells. In particular, we demonstrated the first backward scattering measurement, which enables elastic light scattering as a promising noninvasive diagnostic tool for malaria.

Measurement of elastic light scattering from two optically trapped microspheres and red blood cells in a transparent medium

Optical tweezers can be used to manipulate small objects and cells. A trap can be used to fix the position of a particle during light scattering measurements. The places of two separately trapped particles can also be changed. In this Letter we present elastic light scattering measurements as a function of scattering angle when two trapped spheres are illuminated with a He-Ne laser. This setup is suitable for trapping noncharged homogeneous spheres. We also demonstrate measurement of light scattering patterns from two separately trapped red blood cells. Two different illumination schemes are used for both samples.

Hypochromicity in red blood cells: an experimental and theoretical investigation

Multiwavelength UV-visible transmission spectrophotometry is a useful tool for the examination of micron-size particle suspensions in the context of particle size and chemical composition. This paper reports the reliability of this method to characterize the spectra of purified red blood cells both in their physiological state and with modified hemoglobin content. Previous studies have suggested the contribution of hypochromism on the particle spectra caused by the close electronic interaction of the encapsulated chromophores. Our research shows, however, that this perceived hypochromism can be accounted for by considering two important issues: the acceptance angle of the instrument and the combined scattering and absorption effect of light on the particles. In order to establish these ideas, spectral analysis was performed on purified and modified red cells where the latter was accomplished with a modified hypotonic shock protocol that altered the hemoglobin concentration within the cells. Moreover, the Mie theory was used to successfully simulate the spectral features and trends of the red cells. With this combination of experimental and theoretical exploration, definition of hypochromism has been extended to two subcategories.

Quantitative cell imaging using single beam phase retrieval method

Quantitative three-dimensional imaging of cells can provide important information about their morphology as well as their dynamics, which will be useful in studying their behavior under various conditions. There are several microscopic techniques to image unstained, semi-transparent specimens, by converting the phase information into intensity information. But most of the quantitative phase contrast imaging techniques is realized either by using interference of the object wavefront with a known reference beam or using phase shifting interferometry. A two-beam interferometric method is challenging to implement especially with low coherent sources and it also requires a fine adjustment of beams to achieve high contrast fringes. In this letter, the development of a single beam phase retrieval microscopy technique for quantitative phase contrast imaging of cells using multiple intensity samplings of a volume speckle field in the axial direction is described. Single beam illumination with multiple intensity samplings provides fast convergence and a unique solution of the object wavefront. Three-dimensional thickness profiles of different cells such as red blood cells and onion skin cells were reconstructed using this technique with an axial resolution of the order of several nanometers.

Measurement and autocorrelation analysis of two-dimensional light-scattering patterns from living cells for label-free classification

We incorporate optics and an ICCD to record the two-dimensional angular optical scattering (TAOS) patterns retrieved from single aerosolized cells. We analyze these patterns by performing autocorrelations and demonstrate that we are able to retrieve cell size from the locations of the secondary maxima. Additional morphological information is contained in the autocorrelation functions and decay rate of the heights of the autocorrelation peaks. We demonstrate these techniques with C6 and Y79 cells, which are readily distinguishable. One key advantage of this methodology is that there is no requirement for antibody and fluorescent labeling molecules.

Photon-cell interactive Monte Carlo model based on the geometric optics theory for photon migration in blood by incorporating both extra- and intracellular pathways

A photon-cell interactive Monte Carlo (pciMC) that tracks photon migration in both the extra- and intracellular spaces is developed without using macroscopic scattering phase functions and anisotropy factors, as required for the conventional Monte Carlos (MCs). The interaction of photons at the plasma-cell boundary of randomly oriented 3-D biconcave red blood cells (RBCs) is modeled using the geometric optics. The pciMC incorporates different photon velocities from the extra- to intracellular space, whereas the conventional MC treats RBCs as points in the space with a constant velocity. In comparison to the experiments, the pciMC yielded the mean errors in photon migration time of 9.8±6.8 and 11.2±8.5% for suspensions of small and large RBCs (RBC(small), RBC(large)) averaged over the optically diffusing region from 2000 to 4000 μm, while the conventional random walk Monte Carlo simulation gave statistically higher mean errors of 19.0±5.8 ( p < 0.047) and 21.7±19.1% (p < 0.055), respectively. The gradients of optical density in the diffusing region yielded statistically insignificant differences between the pciMC and experiments with the mean errors between them being 1.4 and 0.9% in RBC(small) and RBC(larger), respectively. The pciMC based on the geometric optics can be used to accurately predict photon migration in the optically diffusing, turbid medium.

Assessment of ultrasound modulation of near infrared light on the quantification of scattering coefficient

PURPOSE

To assess the effect of ultrasound modulation of near infrared (NIR) light on the quantification of scattering coefficient in tissue-mimicking biological phantoms.

METHODS

A unique method to estimate the phase of the modulated NIR light making use of only time averaged intensity measurements using a charge coupled device camera is used in this investigation. These experimental measurements from tissue-mimicking biological phantoms are used to estimate the differential pathlength, in turn leading to estimation of optical scattering coefficient. A Monte-Carlo model based numerical estimation of phase in lieu of ultrasound modulation is performed to verify the experimental results.

RESULTS

The results indicate that the ultrasound modulation of NIR light enhances the effective scattering coefficient. The observed effective scattering coefficient enhancement in tissue-mimicking viscoelastic phantoms increases with increasing ultrasound drive voltage. The same trend is noticed as the ultrasound modulation frequency approaches the natural vibration frequency of the phantom material. The contrast enhancement is less for the stiffer (larger storage modulus) tissue, mimicking tumor necrotic core, compared to the normal tissue.

CONCLUSIONS

The ultrasound modulation of the insonified region leads to an increase in the effective number of scattering events experienced by NIR light, increasing the measured phase, causing the enhancement in the effective scattering coefficient. The ultrasound modulation of NIR light could provide better estimation of scattering coefficient. The observed local enhancement of the effective scattering coefficient, in the ultrasound focal region, is validated using both experimental measurements and Monte-Carlo simulations.

Computational study of scattering from healthy and diseased red blood cells

We present a comparative study of scattering from healthy red blood cells (RBCs) and diseased RBCs with deformed shapes. Scattering problems involving three-dimensional RBCs are formulated accurately with the electric and magnetic current combined-field integral equation and solved efficiently by the multilevel fast multipole algorithm. We compare scattering cross section values obtained for different RBC shapes and different orientations. In this way, we determine strict guidelines to distinguish deformed RBCs from healthy RBCs and to diagnose various diseases using scattering cross section values. The results may be useful for designing new and improved flow cytometry procedures.

A novel first principles approach for the estimation of the sieve factor of blood samples

Light may traverse a turbid material, such as blood, without encountering any of its pigment containing structures, a phenomenon known as sieve effect. This phenomenon may result in a decrease in the amount of light absorbed by the material. Accordingly, the corresponding sieve factor needs to be accounted for in optical investigations aimed at the derivation of blood biophysical properties from light transmittance measurements. The existing procedures used for its estimation either lack the flexibility required for practical applications or are based on general formulas that incorporate other light and matter interaction phenomena such as detour (scattering) effects. In this paper, a ray optics framework is proposed to estimate the sieve factor for blood samples. It employs a first principles approach to account for the distribution, orientation and shape of the cells that contain hemoglobin, the essential (oxygen-carrying) pigment found in human blood. Within this framework, ray-casting techniques are used to determine the probability that light can traverse a blood sample without encountering any of these cells. The predictive capabilities of the proposed framework are demonstrated through a series of in silico experiments. Its effectiveness is further illustrated by visualizations depicting the different blood parameterizations considered in the simulations.