Retinal Blood Flow Measurement by FD-OCT

The FD-OCT system was coupled to a retinal scanning setup mounted on a slit-lamp biomicroscope. The chin and forehead of the subject rested on a frame in front of the objective lens. A green flashing cross target was used to fix the subject's gaze. The FD-OCT probe beam was scanned in circular patterns on the retina around the optic nerve head at radii r₁ and r₂ (Fig. 4) using a pair of high-precision optical scanners (6810P, Cambridge Technology, Inc., Cambridge, Massachusetts). Sinusoidal voltage drive signals were generated by InVivoVue imaging software (Bioptigen, Inc., Research Triangle Park, North Carolina) and were triggered to start the scan at the beginning of an FD-OCT frame acquisition. As circular scans are used, the duty cycle of the drive signals is 99.3%, with the scanners returning to their original position at the end of one scan. Shifts between radii were conducted automatically between scans. The scanning radii of 1.8 and 2.0 mm were chosen so that the probe beam incidence angle onto the retinal vessels was slightly off-perpendicular, making the Doppler signal for all the retinal branch veins within the the detection range. There were 3000 A-lines sampled in each circle. The phase differences for every three A-lines were calculated to get the Doppler frequency shift. Thus, each frame consisted of 1000 vertical lines. Data was acquired, processed, and streamed to disk for real-time display of Doppler FD-OCT at 4.2 frames per second (VC++ software, Bioptigen). There were four pairs of Doppler FD-OCT images sampled for each flow measurement. Four were sampled at radius r₁, and four were from radius r₂. The total recording time for the eight-image set was approximately 2 s. The sampled Doppler FD-OCT images were saved for further offline data processing.
Blood flow in the vessels was calculated in a semiautomatic fashion. The locations of the vessels were selected by a human operator (coauthor Y.W.) and then the flows were automatically calculated according to Eqs. (1)–(5). The distance h from the nodal point N to the retina surface was assumed to be 17 mm according to a standard eye model.25 The speed profile of a single vessel in the eight Doppler images was calculated. Peak velocity in the eight flow profiles was normalized to the maximum one and plotted against time to show the flow pulsation. This curve was integrated as the pulsation term k in Eq. (5). The maximum flow speed profile of the eight analyzed flow profiles was used in Eq. (5) as A_p to calculate the retinal blood flow (F). For some veins, the Doppler flow signal was too weak for accurate reading at diastole, the minimum flow portion of the cardiac cycle. For those occasions, the pulsation factor k in the adjacent veins was used for flow calculation.