Cardiac-Gated En Face Doppler Measurement of Retinal Blood Flow Using Swept-Source Optical Coherence Tomography at 100,000 Axial Scans per Second.

Measurement of retinal blood flow in vivo is an important research area in ophthalmic imaging since alterations in retinal perfusion are suggested to play a role in pathogenesis of ocular diseases such as glaucoma1–4 and diabetic retinopathy.5–8 Quantitative assessment of total retinal blood flow (TRBF) enables objective comparison of retinal circulation among different subjects and simplifies longitudinal tracking of changes associated with disease progression. Total retinal blood flow measurement can provide useful information for investigating ocular pathophysiology as well as for identifying potential early diagnostic markers. Multiple imaging modalities such as video fluorescein angiography, laser speckle flowgraphy, laser Doppler velocimetry, scanning laser Doppler flowmetry, and ultrasound color Doppler imaging can be used to investigate quantitative retinal blood flow.9–16 However, objective measurement of TRBF remains challenging. Video fluorescein angiography and laser speckle flowgraphy cannot directly measure blood flow velocity but yield indirect metrics such as arteriovenous passage time and mean blur rate. Scanning laser Doppler flowmetry (e.g., Heidelberg Retina Flowmeter; Heidelberg Engineering, Heidelberg, Germany) can map the flow velocity component along the probe beam direction over a two-dimensional scan area; however, the absolute flow velocity is required for quantitative flow rate measurement. Ultrasound color Doppler imaging can perform cross-sectional imaging and volumetric imaging of blood flow velocity but has ∼200-μm spatial resolution, which is insufficient to visualize detailed structure of the central retinal vasculature, complicating accurate blood flow calculation. Among these techniques, bidirectional laser Doppler velocimetry (BLDV) combined with fundus photography is one of the best-suited methods for quantitative TRBF measurement. This technique measures the maximum absolute flow velocity in a vessel by detecting the Doppler frequency shifts of the light backscattered in two different directions. The blood flow in the vessel is calculated as the product of the maximum absolute flow velocity and the effective vessel cross-sectional area, estimated from the vessel diameter measured in a fundus photograph. Total retinal blood blow is then calculated as the sum of the blood flow in major retinal vessels near the optic disc. Birectional laser Doppler velocimetry measurement of TRBF was validated in healthy subjects17–20 and used for investigating TRBF in diabetic patients.21–24 However, the calculation of effective vessel cross-sectional area relies on the assumption that the vessel shapes are circular and the blood flow velocity follows a parabolic profile. This assumption can be inaccurate in some subjects and may result in flow measurement errors. Moreover, measurement times can be preclusive because individual major retinal vessels must be measured separately, requiring 20 to 55 minutes per eye.25 Due to this limitation, the number of patients in most TRBF studies using BLDV has been limited. Fourier-domain optical coherence tomography (OCT) has enabled phase-sensitive imaging methods such as Doppler OCT, which promises simpler and more robust measurements of TRBF using depth-resolved information on vascular structure and Doppler velocity.26 The first demonstration of TRBF measurement with Fourier-domain Doppler OCT used multiple circumpapillary scans intercepting all of the retinal vessels around the optic disc in order to measure the flow velocity component along the OCT probe beam direction and the Doppler angle between the velocity vector and the OCT beam.27–30 However, the flow measurement was highly sensitive to errors in the Doppler angle, especially when the vessel is oriented near 90° to the OCT beam as in the optic disc margin. Eye motion artifacts as well as varying vessel geometries in different subjects can make angle measurements challenging and require reader assistance to analyze data. Another method, bidirectional Doppler OCT, is analogous to BLDV. This approach measures Doppler velocity in two probe beam directions and uses trigonometric relations to calculate the absolute flow velocity31 and blood flow.32 This method addresses some of the limitations in the circumpapillary method, but cannot be performed with commercial OCT instruments because a major hardware modification is required for dual-beam scanning. Recent improvements in OCT imaging speed enabled a new Doppler OCT approach, which avoids the need to measure the Doppler angle by analyzing blood flow in the en face plane. This method, en face Doppler OCT, was first demonstrated in brain sciences for cerebral blood flow measurement in small animals.33 In ophthalmology, TRBF can be measured by integrating the Doppler velocity over an en face plane in volumetric data of the optic disc. En face Doppler OCT does not require Doppler angle measurement because the detected flow velocity component is always perpendicular to the en face plane. This simplicity enables fully automatic analysis of TRBF. High imaging speed is crucial for en face Doppler OCT because a densely sampled volumetric image is required to measure TRBF and multiple volumetric images per cardiac cycle are necessary to accurately assess pulsatile blood flow. Our group has demonstrated en face Doppler measurements of human TRBF using swept-source/Fourier domain OCT (SS-OCT) at 200-kHz axial scan (A-scan) rate,34 spectral/Fourier domain OCT (SD-OCT) at 244-kHz A-scan rate,35 and SS-OCT at 400-kHz A-scan rate.36 Current commercial OCT instruments operate at A-scan rates of 100 kHz or less, which yield low volume rates, limiting the ability to sample the cardiac cycle. Therefore, in order to perform accurate pulsatile TRBF measurements with en face Doppler OCT, data acquisition should be cardiac gated so that the measurements at different cardiac phases can be distributed across multiple cardiac cycles. Cardiac gating is commonly used in other imaging modalities such as computed tomography37 and magnetic resonance imaging.38 In OCT, pulsatile blood flow measurements using cross-sectional Doppler with pulse oximetry cardiac gating were previously performed in a single choroidal vessel.39 Another demonstration of en face Doppler OCT partitioned multiple volumes based on the cardiac phase to reconstruct cardiac phase–coherent volumes and measure pulsatile blood flow in individual retinal vessels and TRBF.40 However, this implementation used long imaging sessions with 10 to 20 OCT acquisitions. In this paper, we present a new method for measuring TRBF using pulse oximetry–gated en face Doppler with SS-OCT at 100-kHz A-scan rate. Doppler SS-OCT at 1050-nm wavelength has several advantages over SD-OCT at 840 nm, such as larger measurable velocity ranges, improved visibility of deeper blood vessels, and absence of fringe washout. Pulsatile TRBF was measured using three OCT acquisitions and fully automatic postprocessing methods. Cardiac pulse information obtained from the pulse oximetry plethysmogram was used to trigger the OCT acquisition during imaging as well as to accurately integrate the pulsatile TRBF in post processing. Since commercial SS-OCT instruments operating at 100-kHz A-scan rates have been recently released (DRI OCT-1 Atlantis; Topcon Medical Systems, Oakland, NJ, USA), this technique can be applied in the clinic with a pulse oximeter and relatively simple instrument modifications. If sufficient reproducibility can be achieved, the short measurement time and the ability to automatically process data and obtain quantitative measurements should facilitate larger-scale studies of TRBF.