Using Doppler FD-OCT, we measured total venous blood flow in normal human subjects. Since arterial and venous flows must be balanced, this is equivalent to measuring the total retinal flow. Measurements by laser Doppler flowmeter (LDF) had confirmed that the difference in the total retinal blood flow measured from arteries compared with veins did not exceed 9%.18
The average measured total venous flow in our study, 45.71 μl/min, was within the range of previously published values of 34.0 (6.3) μl/min19
to 64.9 (12.8) μl/min using LDF.20
In our study, the volumetric flow rate varied with the vessel diameter with a power coefficient of 1.97, which indicates that flow speed was not affected by vein diameter. This logarithmic slope value previously reported for LDF was 2.76~3.35,19 20
which indicated faster flows in larger veins. The difference might be due to the vessel diameter measurement methods. In our study, the inner diameters of the veins were measured from the OCT Doppler shift image. For LDF, vessel diameters were measured from a fundus camera image and might be slightly larger.
With regard to superior versus inferior retinal blood flow rates, we did not observe a significant difference. This agrees with other investigators who also found no statistical differences between superior and inferior retinal flows.18-20
We also observed that the total venous flow rate correlated with TVCA in normal subjects. This agrees with previous LDF results.19
The precision of total blood-flow measurement in this study was 10.5% as assessed by CV. The primary limitation to precision was the speed of OCT scanning. A faster FD-OCT system would decrease the motion error in incidence angle calculation and the sampling error due to cardiac pulsation. A faster system would also allow reliable measurement of the faster flow in retinal arteries by reducing phase wrapping and signal loss due to washout of interference fringes.
The measurement of total blood flow could provide an objective tool to monitor treatment approaches that target retinal vascular conditions. This has led many investigators to study retinal blood flow as a surrogate marker of retinal function.21-25
However, none of the previously studied blood flow imaging modalities has found its way to widespread clinical application, except for fluorescein angiography (FA), which cannot provide quantitative measurements of volumetric flow. LDF can provide quantitative blood-flow measurement.19 20 23-25
However, it is laborious to use because each blood vessel is measured one at a time, and LDF’s accuracy is limited by the lack of direct information on the flow-velocity profile and blood-vessel dimensions.
In Doppler OCT, flow profile can be characterised across both depth and transverse dimensions. Total flow is measured by integrating the flow profile over the vessel cross-section and does not require any assumptions regarding vessel shape or flow profile. The measured results in volume flow units can be compared for different subjects.
For clinical applications, the chair time is of great practical importance. Using a circular scanning method, total retinal venous blood flow can be calculated with the data sampled within 2 s. In our study, the scanning session for each subject was less than 10 min for eight measurements. This is much shorter than the reported 1 h session time for LDF.20
Finally, FD-OCT retinal scanners are becoming widely used in ophthalmology. Doppler blood-flow measurement only requires additional software for scan acquisition and analysis, and no additional hardware modification of the FD-OCT scanner is required. Thus, Doppler OCT may provide widely available clinical blood-flow evaluations with minimal additional cost.
In summary, we present the first clinical study of retinal blood flow using Doppler FD-OCT in a group of normal subjects. Volumetric flow in the branch veins around the optic nerve head was calculated with the data sampled within 2 s. The average total retinal blood flow in eight normal subjects was 45.6 (3.8) μl/min. This method can be used to measure total retinal blood flow without relying on any assumption for vessel sizes or flow profiles. The precision of total blood-flow measurement could be further improved using a faster FD-OCT system. Larger studies are needed to validate clinical application in health and disease.