The Impact of Artificial Tear Viscosity on the Results of Optical Coherence Tomography

Abstract Background In earlier studies, an irregular tear film could be responsible for artifacts in imaging procedures in ophthalmology. As a result, this study will investigate the influence of hyaluronic acid-containing tear substitutes of different viscosities on the measurement results of optical coherence tomography and on the non-invasive tear film break-up time. Methods The study included three sessions in which three hyaluronic acid tear substitutes of different viscosities (Hylo-Vision 0.1%, 0.2%, 0.3%, OmniVision GmbH, Puchheim, Germany) were applied to each subject. In 20 healthy volunteers, the non-invasive tear film break-up time was measured before and after the application of the tear substitutes using Keratograph 5M (Oculus GmbH, Wetzlar, Germany) and optical coherence tomography using 3D OCT-2000 (Topcon, Hamburg, Germany). Results The median age of the volunteers was 28.5 years. 11 women (55%) and 9 men (45%) were examined. The non-invasive tear film break-up time improved significantly (p = 0.027) after the application of 0.3% hyaluronic acid. In the study, no significant difference was found in all OCT parameters used before and after application of the different viscous hyaluronic acid-containing tear substitutes. Conclusion In this study, no influence of tear substitutes containing different viscosities of hyaluronic acid on the measurement results of optical coherence tomography could be determined. Therefore, 0.1%, 0.2% and 0.3% hyaluronic acid can be applied to the patient to improve the corneal surface before the examination with optical coherence tomography, without influencing the measurement results of optical coherence tomography.</jats:p

Shock/Shock Interaction Experiments in the High Enthalpy Shock Tunnel Göttingen

Shock/shock-interaction studies have been conducted in the High Enthalpy Shock Tunnel Göttingen /HEG). A cylinder of radius 45mm with a span of 500mm and a 10 degree wedge are used. Two different freestream conditions with burst pressures of around 50 and 100 MPa are available. Freestream total enthalpies are around 20 MJ/kg and Mach numbers are approximately 9 for both conditions; air is used as the test gas. The cylinder is equipped with 17 pressure transducers and 17 thermocouples. For all tests holographic interferograms are obtained and, for some tests Schlieren pictures are also available. The experimental results are presented and compared to computations. Additionally the ideal dissociating gas model of Lighthill has been used to predict pressure and heat transfer rates on the body surface. A comparison between experimental, numerical and theoretical pressure and heat transfer loads is presented. Fairly good agreement is achieved. The influence of the chemical dissociation reactions are mainly seen to reduce the length scales of the flow. The experiments as well as the calculations of the Edney type IV interaction show unsteady flow behaviour. An approximate Strouhal number of Sh = 0.79 for the experiment and Sh = 1.02 for the calculation is determined