Pilot Study of Optical Coherence Tomography Measurement of Retinal Blood Flow in Retinal and Optic Nerve Diseases

The development of noninvasive methods such as magnetic resonance angiography1 and functional magnetic resonance imaging (MRI)2 to measure cerebral hemodynamics has greatly enhanced the study of neurologic diseases and functional neuroanatomy. Such methods would also be very helpful in the eye because the leading causes of blindness in the industrialized world—diabetic retinopathy, macular degeneration, and glaucoma—are all related to abnormal retinal3,4 or optic nerve blood flow.5 Unfortunately, MRI resolution is too coarse for quantitative imaging of retinal blood vessels, which have a very fine caliber. Although several techniques are being used for retinal blood flow evaluation, they all have serious limitations. Ultrasound color Doppler imaging has sufficient resolution to measure only the larger retrobulbar vessels.6 It can measure blood velocity but not vessel diameter; therefore, volumetric blood flow cannot be determined. Several types of laser Doppler techniques are able to measure flow in individual retinal vessels7–9 or capillary beds.10 Although it is possible to measure total retinal blood flow by adding measurements from individual vessels, this requires many measurements over a long session.8 These specialized instruments are generally available only in major research centers because they are expensive. Fluorescein and indocyanine green angiographies are widely used to visualize retinal and choroidal circulations. However, they do not provide quantitative measurements of blood flow and require the intravenous injection of dyes that have potential side effects.11 Optical coherence tomography (OCT)12 is commonly used in the diagnosis and management of retinal diseases.13–16 It has the requisite resolution to image retinal blood vessels.17 Because it is a coherent detection technique, OCT can detect the Doppler frequency shift of back-scattered light, which provides information on blood flow velocity.18,19 With the development of high-speed Fourier-domain OCT,20–22 it has become possible to capture the pulsatile dynamics of blood flow.23,24 Using Doppler Fourier-domain OCT, we developed a double circular scanning pattern (Fig. 1) that measures flow in all the blood vessels around the optic nerve head four to six times per second.25 Total retinal blood flow could be calculated with the data sampled within 2 seconds. We have demonstrated that flow measurements in normal subjects26 and in a patients with diabetic retinopathy27 can be reproducibly obtained. In this study, we used this new technique in a systematic investigation of blood flow abnormalities in retinal and optic nerve diseases. Figure 1. (a) Fundus photograph showing the double circular pattern of the OCT beam scanning retinal blood vessels emerging from the optic disc. (b) The relative position of a blood vessel in the two OCT cross-sections is used to calculate the Doppler angle θ ...