Abstract:

Optical Coherence Tomography (OCT) is a stan- dard imaging procedure in ophthalmology. OCT Angiography is a promising extension, allowing for fast and non-invasive imaging of the retinal vasculature analyzing multiple OCT scans at the same place. Local variance is examined and highlighted. Despite its introduction in the clinic, unanswered questions remain when it comes to signal generation. Multi- phase fluids like intralipid, milk-water solutions and human blood cells were applied in phantom studies shedding light on some of the mechanisms. The use of hydrogel beads allows for the generation of alternative blood models for OCT and OCT Angiography. Beads were produced in Hannover, their size was measured and their long term stability was assessed. Then, beads were shipped to Karlsruhe, where OCT imaging resulted in first insights. The hydrogel acts as a diffusion barrier, which enables a clear distinction of bead and fluid when scattering particles were added. Further on, the scattering medium be- low the bead showed increased signal intensity. We conclude that the inside of the bead structure shows enhanced transmis- sion compared to the plasma substitute with dissolved TiO2 surrounding it. Beads were found clumped and deformed af- ter shipping, an issue to be addressed in further investigations. Nevertheless, hydrogel beads are promising as a blood model for OCT Angiography investigations, offering tunable optical parameters within the blood substitute solution.

Abstract:

Purpose: To evaluate the impact of lens opacity on the reliability of optical coherence tomog- raphy angiography metrics and to find a vessel caliber threshold that is reproducible in cataract patients.Methods: A prospective cohort study of 31 patients, examining one eye per patient, by applying 33mm macular optical coherence tomography angiography before (18.94±12.22days) and 3 months (111 ± 23.45 days) after uncomplicated cataract surgery. We extracted superficial (SVC) and deep vascular plexuses (DVC) for further analysis and evaluated changes in image contrast, vessel metrics (perfusion density, flow deficit and vessel-diameter index) and foveal avascular area (FAZ). Results: After surgery, the blood flow signal in smaller capillaries was enhanced as image contrast improved. Signal strength correlated to average lens density defined by objective measurement in Scheimpflug images (Pearson’s r: –.40, p: .027) and to flow deficit (r1⁄4 –.70, p<.001). Perfusion density correlated to the signal strength index (r1⁄4.70, p<.001). Vessel metrics and FAZ area, except for FAZ area in DVC, were significantly different after cataract surgery, but the mean change was approximately 3–6%. A stepwise approach in extracting vessels according to their pixel caliber showed a threshold of > 6 pixels caliber ($20–30 mm) was comparable before and after lens removal.Conclusion: In patients with cataract, OCTA vessel metrics should be interpreted with caution. In addition to signal strength, contrast and pixel properties can serve as supplementary quality met- rics to improve the interpretation of OCTA metrics. Vessels with $20–30 mm in caliber seem to be reproducible.

Abstract:

During cerebral revascularization surgery, it is imperative to examine the perfusion of the treated region to preserve patients from fatal consequences, done by measuring the volume flow in single blood vessels. Weichelt et al. suggested to quantify the volume flow from contact free recorded fluorescence angiography video data, multiplying the vessel cross section by the observed fluorophor velocity. Compared to reference measurements, the method overestimates the volume flow. Depending on the vessel diameter d the deviations range from 7% (given as k = 1,07,d = 1,6mm) to 58% (given as k = 1,58,d = 4mm) [1]. The observed deviations are investigated in recent research. There is a flow velocity profile over the vessel cross section. There are varying amounts of intensity contributing to the video data coming from different depths within the vessel due to radiative transfer in turbid media. These varying amounts should be considered in an optic probability density function. So, one approach integrates the local relative blood velocity, weighted by the optic probability density function over the vessel cross section to approximate k. If the deviations can be explained by a combination of information depth and local blood velocity, the approximated k match the observed ones. In previous work, the optical weighting was obtained applying a Monte Carlo Multi Layer model, analyzing the deepest penetration depth of each photon. This implies many assumptions, especially regarding model geometry, information source and illumination modelling. The approximated k do not match the observations. [2] This work investigates the influence of use of optical weights from a Fluorescence Multi Cylinder Monte Carlo simulation instead of a Monte Carlo Multi Layer to assess the validity of assumptions made by using the optical weighting factors from Monte Carlo Multi Layer. Three aspects of the optic model were reimplemented to obtain the optic weights: 1. the fluorescence location of each photon was assumed to be the source of information given by this photon instead of the deepest penetration location 2. the Multi Layer geometry was changed to a Multi Cylinder geometry 3. homogeneous illumination was simulated instead of single point illumination It was found, that there are clear differences in approximated k-factors, obtained from optical weights from the Fluorescence Multi Cylinder Monte Carlo model, compared to the optical weights from Monte Carlo Multi Layer model. The deviations coming from model geometry, information source interpretation and illumination show a Root Square Mean Error of up to 38%. The assumptions made in previous work are not met.