Imaging retinal capillaries using ultrahigh-resolution optical coherence tomography and adaptive optics

Abstract

Purpose: Ultrahigh-resolution optical coherence tomography (UHR-OCT) with adaptive optics (AO) provides micrometer-scale 3D resolution that is attractive for imaging the retinal microvasculature. Such imaging may be useful for early detection of pathologic changes as in diabetic retinopathy. Here the authors investigate this potential for detecting individual capillaries in healthy subjects.

Methods: UHR-AO-OCT volumes centered on the fovea were acquired from seven subjects (age range, 25-61 years) with three preselected with no foveal avascular zone (FAZ). Images were compared with entoptic diagrams using the capillaries at the rim of the FAZ. Methods of comparison were testing for the presence of a FAZ, noting distinct features in the capillary pattern, and measuring the size of the FAZ. Additional analysis included measurements of capillary diameter and depth range with retinal eccentricity.

Results: UHR-AO-OCT results are consistent with entoptic observations for all three methods of comparison. FAZ diameters measured by UHR-AO-OCT and entoptic imaging are strongly correlated (R(2) = 0.86). Average capillary diameter near the FAZ rim is 5.1 (4.6) ± 1.4 μm, with the value in parentheses accounting for axial image blur. This is consistent with histology (average, ~4.7 μm). Depth range of the capillaries increases monotonically with eccentricity (0°-1.25°) and is larger and more variable for subjects without FAZ.

Conclusions: UHR-AO-OCT permits observation of many of the capillaries proximal to the FAZ, including those of average size based on published histology. This supports the view that the vast majority of capillaries in the retina are likely detectable with UHR-AO-OCT.

Figure 1.

UHR-AO-OCT volumes (extracted capillary subregion) showing the foveal region in two subjects, one with a well-defined FAZ (a) and the other without (b). Dimensions of the volumes are 895 × 100 × 100 voxels (width × length × depth) that correspond to 891 × 900 × 180 μm. The volumes subtend slightly less than 3° × 3° because of image cropping (typically, 10 A-scans were removed). Optimal focus for vessel clarity was 0.35 D (a) and 0.25 D (b), realized with the AOptix DM. Central bright spot in (b) is a residual of the fovea reflex that was not completely removed in postprocessing when the capillary subregion was extracted. As evident, the UHR-AO-OCT volumes contained visible image artifacts. The high-magnification image—necessary for viewing microscopic details at the level of individual capillaries—exacerbates the effect of speckle noise (which is intrinsic to the interferometric nature of OCT) and the 9-μm gap between consecutive B-scans. Both noise and gap are comparable to the diameter of the capillaries. Speckle noise reduces the contrast of capillaries, and the gaps between B-scans disrupt the continuity of individual capillaries in the volume. Although both result in obvious image degradation, the volumes nevertheless retain the relevant information needed for the comparison study and for extracting quantitative information (FAZ size; capillary diameter; capillary depth range). The supplementary videos (Supplementary Movies S1 and S2) can be downloaded at http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.10-6424/-/DCSupplemental.

Figure 2.

UHR-AO-OCT images (a, c) and entoptic drawings (b, d) of the FAZ in two subjects: subject 3 (top) and subject 1 (bottom). Central bright spot in volumes is a residual of the fovea reflex that was not completely removed in postprocessing. (b) Subject 3 has a well-defined FAZ in addition to an adjacent rectangular region that is free of capillaries (distinct capillary feature) immediately nasal to the FAZ. (a) The UHR-AO-OCT volume is consistent with these entoptic observations, also suggesting a well-defined FAZ and a capillary free sub-region nasal to the FAZ. The drawing in (d) shows subject 1 also has a well-defined FAZ. The superior and inferior edges of the FAZ, however, are notably different from those of subject 3. The superior edge appears arch-like, and the inferior edge appears tortuous. (c) The UHR-AO-OCT volume image is consistent, showing a well-defined FAZ, a smooth superior capillary edge, and a tortuous inferior one.

Figure 3.

Correlation between UHR-AO-OCT and entoptic measurements of FAZ diameter. The plot contains horizontal and vertical measurements for the four subjects with a FAZ. Solid line: linear regression fit, which as a point of caution depends strongly on the two data points at approximately 3°. The FAZ for the entoptic measurements had an average diameter of 2.16° ± 0.60° and a range of 1.57° to 3.22°. For UHR-AO-OCT, the average was 2.09° ± 0.55° and the range 1.4° to 2.9°. The average absolute difference (percentage error) between the two methods was 9.3%. On average, UHR-AO-OCT undermeasured the FAZ size by 2.8% relative to entoptic viewing.

Figure 4.

Average diameter of retinal capillaries near the FAZ as measured with UHR-AO-OCT in subject 2. (a) UHR-AO-OCT volume shows location of 10 capillary points (white lines with numbers) that were selected for diameter measurements. (b) The 10 measurements were registered, averaged, and plotted as shown. Error bars in the plot represent ±1 SD. FWHM diameter, defined at 3 dB below maximum, is 4.6 μm.

Figure 5.

Average diameters of retinal capillaries in subjects with (left) and without (right) a FAZ. Measurements were taken at the FAZ edge (left) and across the fovea (right). The average ± SD of capillary diameter is 5.4 ± 1.3 μm across the four subjects with a FAZ and 4.7 ± 1.7 μm across the three subjects without it. The average ± SD across all seven subjects is 5.1 ± 1.4 μm.

Figure 6.

Capillary depth range (shallowest to deepest) with retinal eccentricity and grouped by subjects (a) with and (b) without FAZ. Error bars represent ± 1 SD of the capillary bed thickness, as measured at different angular positions (45° intervals) for each retinal eccentricity. Error bars are based on the depth values at different orientations. Note subject 4 is not shown because the FAZ edge lies outside the plot range (>1.4°).