Validation of UHR-AO-OCT involved three parts: testing for the presence of a FAZ, noting distinct features in the capillary pattern near the FAZ rim, and measuring the size of the FAZ. To facilitate the first part, 3 of the 7 subjects were preselected with no FAZ. All volumes acquired with the UHR-AO-OCT showed clear evidence of the presence or absence of a FAZ, and this correctly matched the entoptic results. The two UHR-AO-OCT images in
Figure 1 demonstrate how apparent it is to distinguish the two FAZ conditions. In short, UHR-AO-OCT was found just as effective as entoptic viewing at detecting microvasculature that protruded into the normally empty FAZ.
The second part took advantage of the fact that the FAZ vascular pattern varies significantly between eyes, making each pattern effectively unique. This was clearly evident in the four eyes that exhibited a FAZ. UHR-AO-OCT images for three of the eyes are shown in
Figures 2 and
3. Note the obvious vascular differences among the three UHR-AO-OCT images; it would be difficult to mistake these as being from the same eye. Furthermore, distinct vascular features, such as the rectangular subregion free of capillaries of
Figure 2a and the tortuous capillary pattern at the bottom of
Figure 2c, are also suggestive in the entoptic drawings (
Figs 2b,
2d). Although these observations are qualitative, the consistent finding of the same general capillary pattern across UHR-AO-OCT image and entoptic drawing provides further evidence that UHR-AO-OCT detects many of the capillaries proximal to the FAZ, many of which should be 4 to 5 μm in diameter.
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The third part was the most quantitative: correlating FAZ size measurements from UHR-AO-OCT and entoptic viewing. As shown in
Figure 3, a strong correlation was found (
R 2 = 0.862). Interestingly, one of the subjects had a noticeably larger (∼3°) FAZ that was correctly detected with UHR-AO-OCT and barely fit into the instrument's field of view. Note that the regression slope and intercept (
y = 0.845
x + 0.257) deviates somewhat from the ideal
y =
x line, indicating systematic errors in the data. These errors likely did not originate from noise in the UHR-AO-OCT data because the capillaries could be readily identified in the images, and variations in repeated measurements were negligible relative to the average absolute difference of 9.3% between the two methods. Furthermore, UHR-AO-OCT did not consistently overestimate or underestimate the FAZ size, indicating a system calibration error. UHR-AO-OCT overestimated two subjects and underestimated the other two. A plausible explanation, which points to a potential weakness of our test, is the use of a circular diaphragm in the entoptoscope to measure the FAZ. Irregularities in the FAZ shape are commonplace and distort the shape from circular, in some cases noticeably (up to 20%). Thus, though the subjects demonstrated good precision (well below the 9.3% average absolute difference) in repeated adjustments of the circular diaphragm, obvious judgment by the subject was required to determine the best match between diaphragm and FAZ in the horizontal and vertical meridians. Consistent with this, the subject with the most distorted FAZ gave the worst agreement between the two methods. Given this uncertainty plus the striking differences in how these two methods work (psychophysics compared with optical imaging), an average absolute difference of 9.3% should not be surprising.
In summary, UHR-AO-OCT yielded results consistent with those on entoptic viewing. UHR-AO-OCT correctly detected the three subjects missing a FAZ, delineated unique features in the FAZ boundary that qualitatively matched those produced by entoptic viewing, and yielded FAZ dimensions (width and height) that were strongly correlated with entoptic viewing. All evidence points to the conclusion that UHR-AO-OCT can detect the small capillaries that define the rim of the FAZ.