Retinal imaging

In an article sponsored by ISOPT Clinical, Gisèle Soubrane MD, PhD, FEBO 
says analysis of images by experienced ophthalmologists is crucial

Gisèle Soubrane

Posted: Saturday, October 1, 2016

New imaging can have two meanings – either new information from the recently available imaging systems, or imaging systems resulting from 
new concepts. When fluorescein angiography became popular in the late 1960s, one of the indirect advantages was that ophthalmologists started to collect more documents which markedly improved the quality of the follow-up and thus the knowledge of natural history and treatment results.

We now have numerous techniques at our disposal: fluorescein angiography, indocyanine green (ICG) angiography, optical coherence tomography (OCT) with its various modalities, autofluorescence imaging, ultrasonography… And the latest toy that retinologists have received is OCT angiography (OCTA).
Although not yet in current clinical use, this new tool in our diagnostic armamentarium has already provided us with new information on the clinical features, as well as on the pathogenesis of diseases. For example, OCTA has confirmed that retinal angiomatous proliferations (RAPs) are located in the outer retinal layers. OCTA has unequivocally demonstrated their origin from the deep retinal capillary plexus.
Of course, multimodal imaging had already suggested a retinal origin for RAP, but OCTA demonstrated it. In addition, it also confirmed the two different ways of progression of the disease, towards the retinal pigment epithelium (RPE) or towards the choroid. Finally, it added clues to evaluate the results of anti-vascular endothelial growth factor treatment in confrontation with OCT B-scans. However, the preliminary results of these clinical studies must be confirmed in 
larger series.

It has become mandatory to compare the results of the various imaging techniques available, at least initially to come to a proper diagnosis. In case of doubt during follow-up, classic imaging, such as fluorescein and ICG angiography, remains essential. The inclusion of OCTA into modern multimodal imaging systems seems reasonable.

However, one has to take into account its limitations. Although OCTA is based on motion of blood flow, it does not provide information on haemodynamic characteristics or on vascular permeability changes. Its major advantage is to specify the morphology and in-depth location and extent of vascular lesions, without the need of dye injections. My feeling is that OCTA will be largely used for post-treatment follow-up and will complete the information obtained with OCT B-scans. This new imaging method is only at its beginning, and no doubt the meticulous analysis of all slabs will open further insight in the 
pathogenesis of some well-known diseases.

Already now, better use can be made of combining different imaging modalities. A good example is central serous chorioretinopathy (CSC). Enhanced depth imaging OCT (EDI-OCT) permits a more precise study of the choroid. It has been shown that in CSC the choroid is thickened. ICG angiography has demonstrated sectorial choroidal permeability changes.
Combining the results of EDI-OCT and ICG angiography has permitted a better understanding of the underlying phenomenon in CSC, but also an improvement in monitoring of treatment. When one suspects the presence of active new vessels in CSC, with OCTA it is possible to extract the picture of a network from the background, thus rectifying the diagnosis and guiding to an adapted treatment.

Ophthalmologists have long dreamed of a handy and easy clinical tool permitting them to visualise the morphology of a lesion together with its functional repercussion. This could allow a cellular identification of the initial dysfunction and possibly lead to preclinical diagnosis.
My additional personal dream is to be able to visualise and to assess the function of cells in the human eye, in health and disease, especially of the functional skeleton of the retina, the Müller cells. This is not utopic. These dreams will certainly be fulfilled relatively rapidly.

Techniques such as autofluorescence imaging have permitted an insight in the biochemical mechanisms in a number of diseases. Stargardt disease is an excellent example. The dark choroid on fluorescein angiography was described in patients with Stargardt disease. Histologically, the accumulation of lipofuscin was demonstrated in RPE cells of Stargardt patients.

Autofluorescence permits a rapid and noninvasive study of the distribution of lipofuscin, improving an early diagnosis and allowing a better follow-up. The amount of information contained within the various imaging modalities is staggering. However, only a relatively small portion is extracted. The impressive improvements in information technology should permit a more thorough analysis of the various data and also a better correlation of the results from the different techniques. This will permit a better understanding of the physiopathology of diseases, of their natural history and also of their response to treatment.

To conclude, without doubt images are essential, but even more important is the analysis of these images by experienced ophthalmologists. Images are a means to allow progress of our understanding of pathogenesis of retinal diseases and to help the progress of our knowledge.

Gisèle Soubrane was Professor of Ophthalmology and Chair of the Department of Créteil, Paris. Currently she is Professor of Ophthalmology at Hotel Dieu,University Paris V. She has a passion for imaging and retinal neovascularisation, both clinically and in basic research.