Therapies in ophthalmology (eye care) are controlled on the basis of regular monitoring of the course of the disease. The success of previous treatments is reviewed and the need for further treatment is determined. The validity of collected information, its appropriate processing and target-oriented quantification thus play a central role in therapy control.
The retinal vasculature is involved in the progression of diabetic retinopathy as well as age-related macular degeneration (AMD). Optical Coherence Tomography Angiography (OCTA), which has been commercially available for a few years, allows a fast, non-invasive visualization of these blood vessels. The acquired images contain indicators of the disease stage and the need for treatment.
In the present project, the origin of the OCT angiography signal is investigated. In addition, information processing tailored to clinical applications is part of the project. This is done by quantifying image information acquired by commercial devices using modern image processing algorithms. Collected metrics are discussed in the context of disease progression with respect to their informative value. This systematic and detailed exploration of the diagnostic chain forms the basis for objective, reliable monitoring in clinical application.
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.
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.
Conference Contributions (3)
S. Hoffmann, A. Naber, and W. Nahm. Towards Quantitative ICG Angiography: Fluorescence Monte Carlo Multi Cylinder. In Current Directions in Biomedical Engineering, vol. 7(2) , pp. 264-267, 2021