Three-dimensional retinal imaging with high-speed ultrahigh-resolution optical coherence tomography

Abstract

Purpose: To demonstrate high-speed, ultrahigh-resolution, 3-dimensional optical coherence tomography (3D OCT) and new protocols for retinal imaging.

Methods: Ultrahigh-resolution OCT using broadband light sources achieves axial image resolutions of approximately 2 microm compared with standard 10-microm-resolution OCT current commercial instruments. High-speed OCT using spectral/Fourier domain detection enables dramatic increases in imaging speeds. Three-dimensional OCT retinal imaging is performed in normal human subjects using high-speed ultrahigh-resolution OCT. Three-dimensional OCT data of the macula and optic disc are acquired using a dense raster scan pattern. New processing and display methods for generating virtual OCT fundus images; cross-sectional OCT images with arbitrary orientations; quantitative maps of retinal, nerve fiber layer, and other intraretinal layer thicknesses; and optic nerve head topographic parameters are demonstrated.

Results: Three-dimensional OCT imaging enables new imaging protocols that improve visualization and mapping of retinal microstructure. An OCT fundus image can be generated directly from the 3D OCT data, which enables precise and repeatable registration of cross-sectional OCT images and thickness maps with fundus features. Optical coherence tomography images with arbitrary orientations, such as circumpapillary scans, can be generated from 3D OCT data. Mapping of total retinal thickness and thicknesses of the nerve fiber layer, photoreceptor layer, and other intraretinal layers is demonstrated. Measurement of optic nerve head topography and disc parameters is also possible. Three-dimensional OCT enables measurements that are similar to those of standard instruments, including the StratusOCT, GDx, HRT, and RTA.

Conclusion: Three-dimensional OCT imaging can be performed using high-speed ultrahigh-resolution OCT. Three-dimensional OCT provides comprehensive visualization and mapping of retinal microstructures. The high data acquisition speeds enable high-density data sets with large numbers of transverse positions on the retina, which reduces the possibility of missing focal pathologies. In addition to providing image information such as OCT cross-sectional images, OCT fundus images, and 3D rendering, quantitative measurement and mapping of intraretinal layer thickness and topographic features of the optic disc are possible. We hope that 3D OCT imaging may help to elucidate the structural changes associated with retinal disease as well as improve early diagnosis and monitoring of disease progression and response to treatment.

Figure 1

Schematic of high-speed ultrahigh-resolution optical coherence tomography system using spectral/Fourier domain detection (a). Echo time delays and magnitudes of backscattered or backreflected light are detected by measuring the spectrum of the interferometer output. A femtosecond laser light source generates broad bandwidths necessary to achieve ultrahigh axial image resolutions. The bandwidth of the light source is 150 nm (b), achieving an axial resolution of 2 μm in the retina (c). CCD = charge coupled device; FWHM = full width at half maximum; Ti:Sa = titanium:sapphire.

Figure 2

Comparison of normal optic nerve head imaged with different optical coherence tomography (OCT) technologies. a, Standard-resolution OCT image with axial resolution of ~10 μm, 512 transverse pixels (axial scans), acquired in ~1.3 seconds. b, Ultrahigh-resolution (UHR) OCT image with axial resolution of ~3 μm, 600 transverse pixels, acquired in ~4 seconds. c, High-definition image using high-speed UHR OCT with axial resolution of ~2 μm, 2048 transverse pixels, acquired in 0.13 seconds. High-speed imaging enables raster scan patterns for comprehensive 3-dimensional mapping of the retina (3D OCT). Examples of 2 different scan patterns are shown: (d) 10 cross-sectional images with 2048 axial scans (transverse pixels) each for high-definition imaging, (e) 170 images with 512 axial scans each for 3D OCT imaging, (f–h), representative high-definition OCT images of the macula, (i) representative cross-sectional images along orthogonal planes of the optic disc generated from the 3D OCT data set, and (j) volume rendering of the macula from the 3D OCT data. ELM = external limiting membrane; GCL = ganglion cell layer; INL = inner nuclear layer; IPL = inner plexiform layer; IS/OS = boundary between the inner and outer segments of the photoreceptors; NFL = nerve fiber layer; OPL = outer plexiform layer; RPE = retinal pigment epithelium.

Figure 3

a–c, An optical coherence tomography (OCT) fundus image can be generated directly from 3-dimensional OCT data by summing the signal along the axial direction. d, The OCT fundus image provides an en face view that is equivalent to a fundus photograph. The OCT fundus image enables individual OCT images to be registered precisely to fundus features because they are generated from the same data set. Optical coherence tomography fundus images can also be generated by displaying individual retinal layers such as (e) the nerve fiber layer or (f) retinal pigment epithelium.

Figure 4

Comparison of false-color macular thickness maps obtained using (a) the commercial optical coherence tomography (StratusOCT) and (b, c) high-speed ultrahigh-resolution (UHR) 3-dimensional OCT. StratusOCT uses 6 intersecting 6.0-mm OCT images in a radial pattern centered on the fovea. Using OCT images with 512 transverse pixels, this corresponds to 3072 different transverse points on the retina. Three-dimensional OCT using high-speed UHR OCT maps the retinal thickness using a raster scan with 87 000 points. Retinal thickness maps are divided into 9 Early Treatment Diabetic Retinopathy Study–type regions, and the average thickness for each region is displayed. Ultrahigh-resolution OCT can distinguish the junction between the inner and outer segments of the photoreceptors (IS/OS) separately from the retinal pigment epithelium (RPE). Therefore, it is possible to measure an effective retinal thickness from the IS/OS as in StratusOCT (b) versus the actual retinal thickness from the RPE (c).

Figure 5

Three-dimensional optical coherence tomography enables mapping of the thickness of individual intraretinal layers: (a) combined thickness of ganglion cell layer, inner plexiform layer, and nerve fiber layer; (b) distance from external limiting membrane to retinal pigment epithelium (RPE); (c) distance from the photoreceptor inner segment/outer segment junction to the RPE; and (d) outer nuclear layer thickness. Maps b– d are useful for quantitative measurement of photoreceptor changes.

Figure 6

Comparison between retinal nerve fiber layer (RNFL) analysis obtained using GDx VCC, StratusOCT, and 3-dimensional optical coherence tomography (3D OCT): (a, g) false-color maps of RNFL thickness from GDx and 3D OCT; (b, e, h) plots of RNFL thickness on a 3.4-mm diameter circumpapillary ring from GDx, StratusOCT3, and 3D OCT; (c) GDx RNFL deviation map; (d) StratusOCT fundus photograph; (f) StratusOCT circumpapillary image; and (i) virtual circumpapillary image reconstructed from 3D OCT. INF = inferior; NAS = nasal; SUP = superior; TEMP = temporal; UHR = ultrahigh resolution.

Figure 7

Comparison of optic nerve head analysis obtained by HRT, StratusOCT, and 3-dimensional optical coherence tomography (3D OCT) using high-speed ultrahigh-resolution (UHR) OCT imaging: (a, e) topographic maps of the optic nerve head from HRT and 3D OCT; (b, c, f) disc and cup contours from HRT, StratusOCT, and 3D OCT; (d, g) individual cross-sectional OCT images from StratusOCT and high-speed UHR OCT. inf = inferior; sup = superior.