Ocular Blood Flow Measurements in Healthy White Subjects Using Laser Speckle Flowgraphy

Purpose. To assess the feasibility and reliability of Laser Speckle Flowgraphy (LSFG) to measure ocular perfusion in a sample of healthy white subjects and to elucidate the age-dependence of the parameters obtained. Methods: This cross-sectional study included 80 eyes of 80 healthy, non-smoking white subjects of Western European descent between 19 and 79 years of age. A commercial LSFG instrument was applied to measure ocular blood flow at the optic nerve head (ONH) three successive times before and after pharmacological pupil dilation. The mean blur rate (MBR), a measure of relative blood flow velocity, was obtained for different regions of the ONH. Eight parameters of ocular perfusion derived from the pulse-waveform analysis of MBR including blowout time (BOT) and falling rate (FR) were also recorded. Results Artifact-free LSFG images meeting the quality criteria for automated image analysis were obtainable in 93.8% without pupil dilation and in 98.8% with pharmacological pupil dilation. Measurements of MBR showed excellent repeatability with intraclass correlation coefficients >= 0.937 and were barely affected by pupil dilation. The majority of pulse-waveform derived variables exhibited equally high repeatability. MBR-related blood flow indices exhibited significant age dependence (p<0.001). FR (r = 0.747, p<0.001) and BOT (r = -0.714, p<0.001) most strongly correlated with age. Conclusions: LSFG represents a reliable method for the quantitative assessment of ocular blood flow in white subjects. Our data affirms that the LSFG-derived variables FR and BOT may be useful biomarkers for age-related changes in ocular perfusion

Effect of Diffuse Luminance Flicker Light Stimulation on Total Retinal Blood Flow Assessed With Dual-Beam Bidirectional Doppler OCT

Purpose: We assess the increase in total retinal blood flow (TRBF) induced by flicker stimulation of the human retina in vivo and investigate the flicker induced hyperemia by means of a vascular flow model of the retinal circulation to study neurovascular coupling (NC). Methods: In six healthy subjects, TRBF was measured before and during stimulation with diffuse luminance flicker. Blood flow velocities in retinal vessels were measured via dual-beam bidirectional Doppler Fourier-domain optical coherence tomography (FD-OCT), retinal vessel diameters were assessed based on FD-OCT phase data. This allowed for the calculation of TRBF before and during visual stimulation. Additionally, a mathematical flow model for the retinal vasculature was adapted to study the implications of diameter variations on retinal perfusion. Measured and simulated perfusion was compared to draw conclusions on the diameter variations in different layers of the vascular tree. Results: The measured mean baseline flow was 36.4 6.5 l/min while the mean flow during flicker stimulation was 53.4% 8.3 l/min. The individual increase in TRBF during flicker stimulation ranged between 34% and 66%. The average increase in TRBF over all measured subjects was 47.6% 12.6%. Conclusions: Dual-beam bidirectional Doppler FD-OCT allowed quantifying NC in the human retina in vivo and may be a promising method for monitoring alterations in NC caused by various pathologies. The comparison of the measured data with the results obtained in the simulated vasculature indicates that the vasodilation induced by NC is more pronounced in smaller vessels.(VLID)484318

Factors Determining Flicker-Induced Retinal Vasodilation in Healthy Subjects

PURPOSE. The purpose of this study was to analyze factors determining retinal arterial and venous responses to stimulation with diffuse luminance flicker in healthy subjects. METHODS. We retrospectively analyzed results obtained in 374 healthy subjects who had previously participated in clinical studies in our department. A total of 153 subjects underwent a protocol in which flicker stimulation was delivered through the fundus camera at 8 Hz (protocol 1), separating measurement and stimulation light depending on the wavelength, and 221 subjects underwent a protocol in which diffuse luminance flicker was delivered at 12.5 Hz with high modulation depth (protocol 2). We investigated whether sex, systemic blood pressure, baseline vessel size, blood plasma concentration of fasting glucose and hematocrit, and serum concentration of cholesterol, triglycerides, creatinine and Creactive protein influenced the retinal vascular response to flicker stimulation RESULTS. Flicker responses in arteries and veins were more pronounced in protocol 2 than in protocol 1 (P < 0.001, each). In both of the protocols the vascular response to stimulation with diffuse luminance flicker was larger in smaller vessels (P between 0.001 and 0.016). In protocol 2 the retinal arterial flicker response was negatively associated with cholesterol serum levels (P = 0.033); in protocol 1, only a tendency toward this effect was observed (P 0.056). CONCLUSIONS. The present analysis indicates that retinal arterial and venous responses to stimulation with diffuse luminance flicker depend on the way the stimulation is delivered through the ftndus camera. In addition, the flicker response varied with vessel size, that is, the smaller the vessel width, the larger the flicker response. Finally, our data indicate that, even within the normal range, higher cholesterol serum levels are associated with lower hyperemic flicker responses.KLI 250(VLID)311758

Ultrahigh-resolution OCT imaging of the human cornea

We present imaging of corneal pathologies using optical coherence tomography (OCT) with high resolution. To this end, an ultrahigh-resolution spectral domain OCT (UHR-OCT) system based on a broad bandwidth Ti:sapphire laser is employed. With a central wavelength of 800 nm, the imaging device allows to acquire OCT data at the central, paracentral and peripheral cornea as well as the limbal region with 1.2 µm x 20 µm (axial x lateral) resolution at a rate of 140 000 A-scans/s. Structures of the anterior segment of the eye, not accessible with commercial OCT systems, are visualized. These include corneal nerves, limbal palisades of Vogt as well as several corneal pathologies. Cases such as keratoconus and Fuchs’s endothelial dystrophy as well as infectious changes caused by diseases like Acanthamoeba keratitis and scarring after herpetic keratitis are presented. We also demonstrate the applicability of our system to visualize epithelial erosion and intracorneal foreign body after corneal trauma as well as chemical burns. Finally, results after Descemet’s membrane endothelial keratoplasty (DMEK) are imaged. These clinical cases show the potential of UHR-OCT to help in clinical decision-making and follow-up. Our results and experience indicate that UHR-OCT of the cornea is a promising technique for the use in clinical practice, but can also help to gain novel insight in the physiology and pathophysiology of the human cornea.Published versio

Retinal oxygen extraction in individuals with type 1 diabetes with no or mild diabetic retinopathy

Aims/hypothesis The aim of this study was to compare retinal oxygen extraction in individuals with diabetes with no or mild non-proliferative diabetic retinopathy and healthy age- and sex-matched volunteers. Methods A total of 24 participants with type 1 diabetes and 24 healthy age- and sex-matched volunteers were included in this cross-sectional study. Retinal oxygen extraction was measured by combining total retinal blood flow measurements using a custom-built bi-directional Doppler optical coherence tomography system with measurements of oxygen saturation using spectroscopic reflectometry. Based on previously published mathematical modelling, the oxygen content in retinal vessels and total retinal oxygen extraction were calculated. Results Total retinal blood flow was higher in diabetic participants (46.4 7.4 l/min) than in healthy volunteers (40.4 5.3 l/min, p = 0.002 between groups). Oxygen content in retinal arteries was comparable between the two groups, but oxygen content in retinal veins was higher in participants with diabetes (0.15 0.02 ml O2/ml) compared with healthy control participants (0.13 0.02 ml O2/ml, p < 0.001). As such, the arteriovenous oxygen difference and total retinal oxygen extraction were reduced in participants with diabetes compared with healthy volunteers (total retinal oxygen extraction 1.40 0.44 vs 1.70 0.47 l O2/min, respectively, p = 0.03). Conclusions/interpretation Our data indicate early retinal hypoxia in individuals with type 1 diabetes with no or mild diabetic retinopathy as compared with healthy control individuals. Further studies are required to fully understand the potential of the technique in risk stratification and treatment monitoring. Trial registration: ClinicalTrials.gov NCT01843114.(VLID)355032

Calculation of the pulse-waveform parameters.

<p>(a) Blowout time (BOT) is defined as the ratio of the half width (i.e. the time that the waveform is higher than half of the mean of the minimum and maximum signal) to the duration of one complete cardiac cycle. High BOT is an indicator of well-maintained perfusion in between two heartbeats. (b) Similarly, blowout score (BOS) is considered as an index of the blood flow that is maintained between heartbeats and is calculated from the difference of the maximum and the minimum MBR as well as the average MBR. High BOS indicates a high constancy of blood flow during the cardiac cycle. (c) Skew serves as a measure of the asymmetry of the waveform distribution. It was developed as an indicator of the condition of the systemic circulatory system. A skew value of 0 describes a perfectly symmetrical waveform shape. Skew is positive if the distribution is leftward and negative if the distribution is rightward. As the blood flow in arteries rises more quickly than in veins, arterial skew is greater than venous skew. Skew also increases with a steeper decline of the waveform curve after the peak indicating a more rapid drop-off in blood flow after the peak. (d) Acceleration time index (ATI) is defined as the ratio of time before the pulse-wave peak value is reached to the duration of the entire heartbeat. (e) The indices rising rate (RR) and (f) falling rate (FR) characterize the steepness of the ascending and, respectively, descending portion of the waveform curve. Higher values indicate a more sudden increase, or decrease, of MBR. (g) The flow acceleration index (FAI) represents the highest increment in MBR between two frames. (h) The resistivity index (RI) is the ratio of the difference between maximum and minimum MBR to the maximum MBR.</p

LSFG parameters obtained before and after pupil dilation.

<p>LSFG parameters obtained before and after pupil dilation.</p

Sex dependence of LSFG-derived parameters.

<p>Boxplots comparing Laser Speckle Flowgraphy (LSFG)-derived parameters between male and female subjects.</p

Laser Speckle Flowgraphy (LSFG) scan of the optic nerve head (ONH) area.

<p>(a) The ONH margin is to be delineated using an ellipsoid region of interest with variable size and radii. (b) The selected area representing the entire optic disc is taken into account for further analysis using the “vessel extraction” function. (c) The “vessel extraction” function distinguishes between areas of visible surface vessels (white) and ONH tissue areas (black). (d) Typical pulse-waveform curve (green) with a steep incline during the systolic phase and a flatter decline during the diastolic phase. The red line indicates the mean level of mean blur rate (MBR).</p

Age dependence of LSFG-derived parameters.

<p>Correlations between Laser Speckle Flowgraphy (LSFG)-derived parameters and age. Dashed lines indicate the 95% confidence intervals of the linear regression line (solid). Pearson’s r as well as the computed p-value is given for each correlation.</p