In the last few casebooks, we have outlined how a sensible approach for a first (or screening) OCT assessment would be to undertake a line scan (horizontal and vertical), a volume scan and a scan around the disc.
Case 1
HH is a healthy, 28-year-old myope (around -3.00/-050 each eye). Other points of relevance here are that HH is a soft lens wearer, has had one episode of microbial keratitis two years ago, and has some dry eye symptoms. As we will see in a later casebook, poor tear flow has an influence on OCT accuracy, and also anterior OCT is a useful way of looking at corneal scarring. For this routine appointment, however, we followed our ‘routine’ protocol.
Lines scans
Horizontal (figures 1 and 2) and vertical line scans were taken. Tracking was used to maximise the resolution of the final image and the line was set to pass from the disc directly through the fovea.
Figure 2
Volume scan
The instrument made 61 raster sweeps across the posterior pole and three are shown in figure 3. Figure 3b shows the scan through the fovea.
Figure 3a
Figure 3b
Figure 3c
RNFL scan
An annular scan was made around each disc, after changing the fixation target, as shown in figure 4.
Figure 4
Results
The complete scanning of each eye takes about two minutes with a compliant patient. Once completed, the scans are saved and are visible on a summary screen (figure 5). At this point, it is possible to delete scans of poor quality or are surplus to requirements. It is also at this point where, prior to further analysis, a couple of important extra clicks are useful:
- Change analysis; one of the most important functions of OCT is to detect change over time. Next time I see HH, I want to be able to refer the later scans to those taken here. To do this, I right click on a scan and click ‘progression’ and then ‘set reference’ (figure 6). The scan will then show a small red symbol to indicate this has been done (figure 7).
- Symmetry analysis; the Spectralis is able to analyse the thickness of structures, for example after a volume scan, and relate it to specific individual layers of the retina. So, for example, it might be useful to know if a layer such as the ganglion cell complex has thinned over time, or is thinner above and below the horizontal raphe as might be the case in glaucoma. To do this, I repeat the right click as before, but this time select ‘segmentation’ (figure 6 again).
Figure 5
Figure 6
Figure 7
At this point, I am now ready to analyse the data and, if required, select a particular display for either issuing to the patient or, perhaps, for sending information to another clinician as required. Next casebook, we will look at how this is done and what the typical displays from these scans are and how to interpret them. In the case of HH, I was happy to continue the routine assessment after the initial acquisition knowing that, next time I see her, I will be able to look for changes in her retinal structure with accuracy.
The material presented in these casebooks is based upon use of the Heidelberg Spectralis but is designed to cover generic principles. Individual details of scanning and data presentation will differ depending on your OCT. The author and colleagues have no commercial interests in Heidelberg Engineering.