In 1991, my colleagues and I at the Massachusetts Institute of Technology developed optical coherence tomography (OCT) to provide cross-sectional imaging of tissue with micrometer-level resolution. Since then, the speed and resolution of OCT has steadily improved. The latest leap in speed was provided by Fourier-domain OCT. This article summarizes how Fourier-domain OCT may improve clinicians' ability to diagnose and manage early glaucoma.
VISUALIZING THE RETINA
The difference in speed between Fourier-domain and classic time-domain OCT is like that between a jet airplane and an older-generation propeller plane (Figure 1). The first commercially available retinal scanner to use Fourier-domain technology was the RTVue (Optovue Inc., Fremont, CA). This system's scanning rate was 65 times faster than that of the fastest time-domain OCT system on the market (26,000 vs 400 axial scans per second).
The RTVue's higher scanning rate allows more detailed mapping of the retinal structures affected by glaucoma: the peripapillary nerve fiber layer and the macular ganglion cell complex. This development in turn leads to a more accurate diagnosis of glaucoma and more precise tracking of the disease's progression.
To take OCT beyond structural assessment, my colleagues and I have developed a new scanning protocol that may allow clinicians to use this technology to measure retinal blood flow.1
ASSESSING RETINAL CIRCULATION
Studies have shown that many of the leading causes of blindness, including diabetic retinopathy and age-related macular degeneration, are related to abnormal retinal blood flow.2,3 Unfortunately, current approaches to analyzing this functional parameter (eg, fluorescein angiography, ultrasound, and laser Doppler flowmetry) provide limited information. Some investigators have successfully used OCT to visualize retinal circulation,4,5 but they could not measure the relative angle between the OCT beam and the blood vessel (angle of incidence), which is necessary to calculate total retinal blood flow (Figure 2).
The dual-plane scanning technique my colleagues and I developed for Doppler OCT allows us to capture these missing elements and to measure the volume of blood flowing through the retinal vessels.1
First, we sample the retinal blood vessels with a double circular scan around the optic nerve head (Figure 3). The scanning pattern transects all retinal branch vessels four to six times each second, depending on the system used. The relative positions of blood vessels in the two Doppler OCT images are used to calculate the angle between the probe's beam and blood flow. We can then use the detected Doppler frequency shift to determine flow velocity. Next, we compare parallel cross-sectional scans from different sections of the same vessel to establish the direction in which the blood is flowing relative to the reference beam (Figure 4). This step allows us to differentiate between retinal veins and arteries.
Finally, we calculate total retinal blood flow by summing the volume of blood passing through the major retinal veins. We prefer to measure veins because the flow velocity in some arteries can exceed the detection range of Doppler OCT. The repeatability of total retinal blood flow measurement is approximately 10%.6
DETECTING ABNORMAL BLOOD FLOW
Using the double circular scan technique, we evaluated retinal circulation in 10 healthy human retinas. The normal total retinal blood flow was 45.6 ±3.8 µL/min and the average venous speed was 19.3 ±2.9 mm/sec.6 These values were within the range previously established by Doppler flowmetry. The flow speed was independent of vein caliber.
A comparison of blood flow in healthy and glaucomatous eyes showed a statistically significant difference between the two groups (40.8 to 52.9 µL/min in healthy controls vs 23.6 to 43.11 µL/min in glaucoma patients).7 This study also showed a correlation between the decrease in blood flow and the presence of severe visual field defects in glaucomatous eyes.7 Additional studies with the RTVue detected reduced blood flow in eyes with diabetic retinopathy (32.3 µL/min).8
CONCLUSION
Fourier-domain OCT already provides more information about the anatomy of the optic nerve and the retina than other advanced imaging modalities (Table 1). The addition of blood-flow analysis to this device will increase its utility for detecting early glaucomatous changes in the eye. Furthermore, the clinical assessment of retinal blood flow with Doppler Fourier-domain OCT may help clinicians better understand the role of perfusion in the causation and treatment of glaucoma and other optic neuropathies.
The double circular scan technique for Doppler OCT was licensed by the University of Southern California (where the author works) to Optovue, Inc. The technology has been implemented on the RTVue Fourier-domain OCT systems and is undergoing clinical studies at several academic eye centers.
