Etdrs Position - Zeiss CIRRUS HD-OCT 500 User Manual

8-7
Posterior Segment
CIRRUS software identifies the Fovea location automatically by looking for the reduced
reflectivity below the retina. You can also change the Fovea location manually (see
"ETDRS Position" on page 8-7), which will update the data table and the ETDRS grid
thickness measurements.
Fovea Not Found
If the Fovea cannot be identified from the scan the message "Fovea not found" will appear
on the screen. In this case, the center of the scan is used for the initial placement (position
256 and 64 for 512x128 scans and position 100 and 100 for 200x200 scans).
It is also possible for the algorithm to find a depression in the reflectivity around the ILM
that is not related to the fovea. In such a case, the reported fovea will be wrong. In both of
these cases, the user can set the fovea manually.
The most common pathologic conditions that can cause failure of the fovea–finding
algorithm are those that cause the greatest disturbance of the foveal architecture, such as
AMD, and macular edema, as well as epiretinal membranes and other vitreoretinal
interface disorders where the vitreoretinal interface becomes distorted.
If the fovea is very far from the center, the algorithm may also fail to find it. In order to
ensure that the fovea is within a reasonable distance of the center, it helps to use the
alignment tool during acquisition (see Figure 6-17).

ETDRS Position

When ETDRS position is selected from the overlay menu, a small red circle appears
centered around the CIRRUS–calculated fovea position, as shown on the left. This
calculated ETDRS Grid position can be repositioned by clicking and dragging the circle
using the mouse.
CIRRUS HD-OCT User Manual 2660021169012 Rev. A 2017-12