When more in-depth images are required, indocyanine green (ICG) and fluorescein dyes are introduced. There are various considerations, one of which is which lens to use.
The 30° lens gives the best view of the corneal shoulder and the limbus, but for work on the cornea itself the blue OCT lens is best due to its ability to flatten the image, bringing more of the vessels into focus.
It is also important to get the focus right, which can of course be difficult because the cornea is completely transparent. Mr Hodson gets the best results by setting the focus to 34 D.
Lastly, it is important to ensure the eyelids do not interfere with the picture. Many patients find it hard to keep their eyes open when confronted with a bright light and it is likely that it will be necessary for the technician to physically hold the eyelids out of the way.
Once the dye has been injected and has been seen to appear within the eyelids, a 30-second video is started.
Figure 1 compares images taken using the different techniques. There is much more detail apparent using ICG compared with the (good) gold standard colour image, because use of the dye in conjunction with infrared light allows one to see past the corneal scarring.
The ‘early’ (30 seconds) and ‘late’ (about 2–3 minutes) fluoroscein images allow the age of the vessel to be estimated: older vessels, which have better-developed walls, will show leakage in the mid- to late-stage images, whereas younger vessels will leak between 30 seconds and 1 minute.
The clarity of images taken using these new techniques are such that it is starting to become possible to identify the feeder vessel from a mass of vessels. If the feeder vessel can be identified, fine-needle therapy can be used to block it. Similarly, the Eye Unit is starting to look at vessel structure in papillomas and carcinoma in situ.
In summary, Dr Hodson reiterated that it is important, every time you go to the camera, to remember that every patient is different so your technique might be slightly different. You should keep an open mind how the camera will be positioned, and remember that most patients cannot focus on the bright light. He is now seeing five to six patients every week and demand for the technique is rising.