Our results demonstrate that scan circle location may add substantial variability to OCT measurements. It is interesting to note that regional RNFL thickness measurements were affected in different ways by scan displacement. For example, the maximum range of displacements when considering shift effect on mean RNFL and superior and inferior quadrant RNFL thicknesses showed a horizontally elongated elliptical shape, whereas the temporal quadrant showed a vertically elongated ellipse and the nasal quadrant showed a near-perfect circle (). The area within a given ellipse is dependent on our preset threshold levels: The nasal segment had the largest threshold and thus had the largest elliptical area.
Understanding the three-dimensional structural distribution of the RNFL, however, may help explain the horizontally and vertically elongated ellipses.29
Histomorphologic and imaging studies have shown that the maximum RNFL thickness appears in the superotemporal and inferotemporal regions.14,30,31
Any shift in the location of the scan circle that brings a sector closer to the disc margin causes a thickness increase in this region and a decrease in the opposite region. Any shift that brings the thickened RNFL bundles into the sector will also induce an increase in thickness. A downward displacement of the circle brings the scan circle closer to the disc margin in the superior quadrant, with subsequent thickening of RNFL measurements in that quadrant and thinning in the inferior quadrant. A nasal displacement of the scan brings the thickened bundles of the RNFL into the temporal region and causes an increased thickness in that region. As the superotemporal and inferotemporal bundles do not transect the nasal quadrant, changes in thickness in this region are only related to the proximity to the disc margin and thus result in the near-perfect circular pattern in .
The horizontally elongated ellipses representing the maximum range of displacements for the mean and the superior and inferior quadrants signified that a small vertical displacement of the circle had a substantially larger effect on RNFL measurements than did a horizontal shift (). In the temporal quadrant, horizontal shifts had a larger effect than did vertical shifts, as reflected by the ellipse elongated in the vertical direction. All the ellipses were shifted superotemporally, perhaps due to a tendency of the operator when defining all the disc margins.
Our results show that the distance between the two humps in the RNFL profile changes as the scan circle is displaced horizontally (). A vertical displacement did not affect the peak distance but altered the height of each peak ().
The results of this study should be taken into account when comparing consecutive OCT scans, as they may explain some of the variability observed between scans. This variability affects cross-sectional glaucoma detection as well as longitudinal glaucoma progression assessment. A clinical example showing how circle displacement can affect the classification of quadrant measurements is provided in . On the basis of our results, we hypothesize that it may be possible to predict the amount of change in RNFL measurements that correspond to a given shift amount, which would then allow an adjustment of measurements that are not well centered. Future studies to test this hypothesis are warranted.
FIGURE 6 An example of a StratusOCT scan showing circle placement errors and corresponding changes in peak distance. Left: A well-centered scan. Right: As the scan circle is displaced temporally, the peaks move closer together and superior quadrant and clock hours (more ...)
One of the limitations of this study is that the circle shift amount was limited to ±500 µm, whereas our threshold limits were more than 600 µm in some cases. The results are extrapolated based on the assumption that the data outside of the shifting range fit the same quadratic model. We restricted the range to ±500 µm to ensure that most of the resampled circles were within the 6 × 6-mm scanning window, regardless of the small disc center offset within the scanning window. Further investigation is needed to clarify the effect of a shift beyond 500 µmm.
Another limitation is that the prototype hsUHR-OCT device used in this study had a relatively long scanning duration (3.84 seconds) and uneven sampling density in x- and y-shift directions (512 × 180 sampling points). We excluded images with obvious eye motion, but there may have been small, undetected eye movements within the scans. We chose to use our prototype device in this study because its unique properties outweigh the limitations. The device that we used has better axial resolution than any commercially available OCT device, which gave us greater accuracy in our measurements. Further, none of the commercial spectral-domain OCT devices have been validated, nor do they have software capable of facilitating a study such as this. We created our own software to access these detailed data. However, because our findings are based on the ocular structure of the peripapillary region, our results can be generalized to any type of OCT circumpapillary scan.
The method presented in this article for resampling the 3.4-mm diameter circle after defining the disc margin and disc center may provide a way to have consistent sampling as patients are observed longitudinally by hsUHR-OCT. Although hsUHR-OCT does not eliminate eye motion, this method would lead to more precise registration from image to image, provided there was little eye motion or a method of correcting eye motion. This resampling method could be used with other scan patterns in addition to raster scanning. If a method for marking the disc margin could be developed for the StratusOCT, the disc center could be calculated and used to improve registration. One way to accomplish this would be to mark the disc margin on the live fundus image. With the current iteration of StratusOCT, however, the exact location of the scan circle cannot be determined.
In conclusion, the location of the OCT scan circle affects RNFL thickness measurements. Accurate registration of OCT scans is therefore important for measurement reproducibility and longitudinal examination and should be taken into account when comparing scans over time.