Spectral domain optical coherence tomography (SDOCT) offered for the first time comprehensive volumetric in-vivo imaging due to its achievable high acquisition speeds [1
]. So far the most successful field of application is ophthalmology [5
]. Here OCT profits from the transparent tissue of the eye and OCT’s unique possibility of acquiring 3D volumes of the cornea and retina. CMOS detector technology pushed the speed limit for spectrometer based SDOCT systems further up to reported 312.500 A-scans per second opening new directions for structural as well as functional imaging [8
]. Such high imaging speed reduces motion artifacts to a minimum. That allows resolving microscopic structures, which otherwise would have been blurred. It was already shown with en-face time domain OCT that fast transverse scanning modes reveal microscopic details such as the photoreceptor cone mosaic of the human retina even in the absence of adaptive optics [11
]. Of course the eccentricity at which individual photoreceptors are visible is larger than for adaptive optics supported OCT systems [12
]. Nevertheless, it gives new exciting perspectives for studying retinal physiology on cellular level [14
]. An important retinal microstructure for the early diagnosis of several retinal diseases, including diabetes, is the capillary network [15
]. At an early stage such diseases manifest as changes of capillary physiology [17
]. Changes may be occlusions, the complete degradation of parts of the capillary network, as well as neo-vascularization. Hence the visualization of this structure has great diagnostic potential. A natural candidate for extracting blood flow is Doppler OCT [18
]. However the flat vascular bed, together with slow flow values leave this task challenging. Recent work showed that with the help of dedicated scanning protocols the sensitivity of those methods is strongly enhanced and capillary flow is accessible [19
]. Nevertheless any Doppler method needs oversampling in order to keep proper signal correlations. Large volumes become therefore time consuming and are prone to motion artifacts. Another way to contrast the flow of the retinal capillary network, using a dual beam setup was presented by Makita et. al. [21
]. However, even with alternative scanning protocols or sophisticated double beam configurations, highly phase sensitive flow extraction suffers from phase decorrelation below vessels that lead to axially extended Doppler shadows.
shows a data set, imaged with a 1050 nm swept source OCT system in our lab, operating at 100 kHz A-scan frequency. In this case the capillary network was extracted by calculating the phase variance of 9 successive B-scans. In we present the maximum intensity projection of the inner retinal layers. On the right hand side in a representative B-scan of the same phase variance volume is shown. One can already tell from the B-scan, that a 3D visualization of such a volume will suffer from shadowing artifacts, due to the decorrelation tails below each vessel. Hence, typically only en-face projections of the inner retinal layers are presented. This is especially disappointing, as the 3D information represents the major advantage compared to other imaging techniques such as scanning laser ophthalmoscopy or high resolution fundus fluorescence angiography [24
]. These methods provide high resolution 2D fundus photographs and videos with high contrast of the smallest parafoveal capillaries. However, they do not provide depth information comparable to OCT. Axially oriented vessel structures are hardly observable in these data sets. Resolving such oriented vessels is important for a number of retinal diseases such as vessels breaking through the retinal pigment epithelium in age-related macular degeneration or capillaries growing down towards the photoreceptors in telangiectasia. We aimed for the extraction of capillary structure on a pure intensity basis. We demonstrate in the present work strategies for micro-vessel extraction as well as methods of analyzing and characterizing the integrity and structure of the retinal capillary network using fractal characterization. Those parameters are accessible through the introduction of our high-speed ultra-high resolution SDOCT system.
Fig. 1 The human parafoveal capillary network, extracted using phase variance analysis with swept-source OCT at 1050 nm. (a) Fundus projection taken from the inner retinal layers. (b) Phase variance tomogram across the blue line in (a). The arrow indicates typical (more ...)