Optical coherence angiography for the eye
Poor blood supply is one of the main causes of several retinal diseases. Vascular disorders and impaired circulation are observed in major eye diseases that cause blindness, including age-related macular degeneration (AMD)1 and glaucoma.2 A noninvasive, 3D imaging tool for major vascular systems of the eye might be helpful for understanding and diagnosing eye diseases. Circulation abnormalities are typically diagnosed using fluorescence angiography, in which injected fluorescent dye is detected. However, this technique is invasive, may have side effects, and cannot be used for patients who are allergic to the dye. In addition, axial resolution—in the light's direction of travel—is poor. This is significant when distinguishing between vascular systems in the retina and the choroid (a layer of blood-rich tissue behind the retina), or recording details of the fine and complex 3D network of choroidal vessels.
Other existing eye-imaging techniques have similar problems. Ultrasound Doppler imaging can be used to visualize the cross-sectional flow distribution in the eye. However, its limited acquisition speed prevents 3D in vivo imaging while its axial resolution is also insufficient. Scanning-laser Doppler flowmetry is noninvasive and can quantify the microvascular blood flow but it cannot distinguish between the choroidal and retinal vascular systems. We have developed 3D angiography for the eye—optical coherence angiography (OCA)—based on optical coherence tomography (OCT). OCT is widely used in clinical ophthalmology to visualize the retina's 3D microstructure.3
OCA uses high-speed Fourier-domain OCT for 3D in vivo imaging of the human eye. To enhance the choroid's contrast, we use light sources with a central wavelength of around 1μm.4 Scattering OCA uses differences between the optical properties of blood and the surrounding tissue to increase the contrast in the eye vasculature,5 while Doppler OCA uses phase-resolved Doppler analysis with OCT6 to detect blood flow.7
We developed clinical prototypes of 1μm Fourier-domain OCT, which we used to scan the eyes of healthy human volunteers in vivo. The 3D data sets obtained were processed with scattering and/or Doppler OCA.
Since rich pigments are present in choroidal tissue, the backscattered-light intensity from this tissue type is larger than that from blood. Unfortunately, it also causes OCT signal decay in the choroid. This attenuation must be considered in scattering OCA when attempting to obtain a clear 3D choroidal vascular image. The retinal pigment epithelium (RPE), a monocellular layer between retina and choroid, is segmented in OCT cross-sectional images. We extracted choroidal images sliced at equal distances from the RPE and applied segmentation based on image statistics. Figure 1 shows a volumetric rendering of the structures of the retina and choroid, and the choroidal vasculature.8 AMD can alter the complex choroidal vasculature9 and so its progress can be monitored in vivo through its effect on the 3D choroidal vasculature.
The blood flow of the retinal and choroidal vessels can be visualized using 1μm OCT.10 The RPE is segmented and the retinal and choroidal vasculature can be distinguished. Figure 2 shows the 3D blood-flow distribution of the human eye. The retinal and choroidal vessels are displayed in different colors (yellow and green, respectively). Around the optic nerve head (ONH), even a vessel beneath the choroid is visible: see Figure 2(c), white arrow. In the Doppler OCA image—see Figure 2(b)—corresponding vessels have a particular structure that partially encircles the optic disc (white arrow). These vessels might be short posterior ciliary arteries. These are the main vessels supplying blood to the ONH and are of considerable interest in the investigation of glaucoma.
In summary, we developed a new technique that can be used noninvasively to visualize the vasculature and blood flow at the posterior part of the eye. This is a step towards developing new techniques to evaluate eye diseases. We are now working on methods to quantify the blood flow in the complex choroidal vasculature and investigating its relationship to eye abnormalities. In addition, Doppler OCA, which is faster and more sensitive, may enable comprehensive investigation of blood flow in the eyeball.
This research is supported by the Japan Society for the Promotion of Science (KAKENHI 18360029) and the Japan Science and Technology Agency.
Shuichi Makita is a postdoctoral researcher in the computational optics group at the University of Tsukuba, which granted him a PhD degree in 2007 for work on spectral-domain OCT.
Yoshiaki Yasuno leads the computational optics group. He received his PhD from the same university in 2001 for work on spatio-temporal optical computing. His main research interest is OCT and ophthalmic imaging.
