Spontaneous retinal venous pulsations (SRVP) describe rhythmic caliber oscillations of one or multiple major retinal veins at the site of the optic nerve head (ONH). This phenomenon is reported to possibly enable non-invasive intracranial pressure (ICP) assessment besides its potential significance for major ocular diseases such as glaucoma or diabetic retinopathy. In this work, we illustrate the advantages of optical coherence tomography (OCT) imaging for investigation of SRVP. Using conventional intensity based OCT as well as the functional extension Doppler OCT (DOCT), the pulsatile changes in venous vessel caliber are analyzed qualitatively and quantitatively. Single-channel and double-channel line scanning protocols of our time-encoded multi-channel OCT prototype are employed to investigate venous caliber oscillations as well as venous flow pulsatility in the eyes of healthy volunteers. A comparison to recordings of scanning laser ophthalmoscopy – a standard en-face imaging modality for evaluation of SRVP – is provided, emphasizing the advantages of tomographic image acquisition. To the best of our knowledge, this is the first quantitative time-resolved investigation of SRVP and associated retinal perfusion characteristics using OCT.
Investigations on retinal vasculature and blood flow are of interest for understanding and diagnostics of numerous ocular diseases. Conventional OCT systems use various scan patterns like linear B-scans, circular scans around the optic nerve head, or raster scans for 3D data acquisition. However, for some studies it is preferable to have customized scan patterns that can, e.g., follow the trace of an arbitrary linear structure in the retina, such as a vessel. In this work, we present an OCT instrument with an integrated retinal tracker that allows repeated scans along an arbitrary trace, whereby ocular motions are corrected by the retinal tracker. The setup comprises an OCT system and a line scanning laser ophthalmoscope (LSLO). The OCT subsystem operates at a center wavelength of 860 nm, with a bandwidth of 60 nm and an A-scan rate of 70kHz. The LSLO system operates at 790 nm and at a frame rate of 60 Hz. This system was used for reflectivity and Doppler imaging along retinal vessels. In a first step, the vessel is manually marked on the LSLO image. Then, repeated scans along the vessel trace are performed (2048 A-scans per scan along trace, up to 500 scans along the trace). The intensity images show a clear delineation of the vessel walls, the phase difference (Doppler) tomograms allow for a time-resolved analysis of blood flow along the vessel over the cardiac cycle.
We introduce the approach of variable time encoding for multichannel optical coherence tomography (OCT). High-speed fiber optical switches are applied for sequential sample arm switching to enable quasisimultaneous image acquisition from three different orientation angles. In comparison with previous multichannel OCT (using simultaneous sample illumination), time-encoded multichannel OCT has no need for division of illumination power among the respective channels to satisfy laser safety requirements. Especially for ophthalmic applications—in particular retinal imaging, which the presented prototype was developed for—this advantage strongly influences image quality through an enhanced sensitivity. Nevertheless, time encoding comes at the cost of a decrease in imaging speed due to sequential channel illumination. For the typical multichannel OCT modality Doppler OCT, this results in a reduction of the maximum unambiguously determinable Doppler velocity. However, we demonstrate that this drawback can be overcome by adaptation of the illumination channel switching scheme. Thus, a re-extension of the maximum unambiguously determinable Doppler frequency to the full A-scan rate of the tunable light source is presented. The performance of the technique is demonstrated by flow phantom experiments and measurements of retinal blood flow in the eyes of healthy human volunteers.
A sequential multi-channel OCT prototype featuring high-speed fiber optical switches to enable inter A-scan (A-scan rate: 100 kHz) sample arm switching was developed and human retinal image data is presented.